Designing a Heat Treatment to Achieve Ductile Advanced High Strength Steels

Abdellatif LAARICH

Materials Engineering, master's level (120 credits) 2020

Luleå University of Technology Department of Engineering Sciences and Mathematics Abstract Heat treatment is a way to significantly change materials properties. When presented with materials that lack certain mechanical properties, it is possible to change its chemical properties and microstructures by applying heat. This can help achieve better yield strength, ductility and toughness. This project discusses the effects of multiple distinct heat treatment methods for several materials in order to improve ductility and elongation without diminishing strength. The materials in question are High Aluminum Steel and Strenx 700MC steel, the first being under development and the second being a commercially available steel. These steels show promise to be used as high ductility, high strength, and 3rd generation steels. The heat treatments can change the mechanical proprieties of the base materials in order to optimize these steels for applications in vertical access solutions. The heat treatments in this project were Quenching and Partitioning (QP), Quenching and Tempering (QT), Austempering (AUST), Intercritical Heat Treatment (IHT) and other usual heat treatments such as Double normalizing (D-Norm). First, the most beneficial type of the above mentioned heat treatments was selected for each steel and series of heat treatments were performed in order to identify and optimize the best method for each steel. Then, heat treated samples underwent a series of tests to numerically quantify their properties and compare them to the existing steels in Alimak’s applications. The results show that Quenching and Partitioning is the most promising heat treatment for optimizing strength and ductility in High Aluminum Steel, with elongation values up to 19% together with yield strengths of 700 MPa. For Strenx 700MC a combination of temperature and time was found that gave an elongation of above 25% with a yield strength of 450 MPa. The explanation for the good properties was partly grain refinement and phase transformations during heat treatments.

i Acknowledgements I am blessed to have great parents who taught me that hard work always pays off so for that, thank you dad Mr. El Arabi LAARICH and mom Mrs. Khaddouj El Omari. For guidance and help along the project I would like to thank: Marta-Lena Antti, my examiner and supervisor Farnoosh Forouzan, my co-supervisor Hadi Torkamani, my co-supervisor Dennis Johansson, my industrial supervisor from Alimak

I am also grateful to: Esa Vuorinen, for sharing his knowledge Lars Frisk for helping me with the tensile test experiments My friends from AMASE, (especially Chihab) for sharing good moments with me All my friends from the university.

شكرا ! Thanks!

Tack!

“Do not belittle any good deed, even meeting your

(ﷺ) brother with a cheerful face.” Prophet Muhammad

Abdellatif LAARICH Luleå, June 2020

ii Table of contents Abstract ...... i Acknowledgements ...... ii List of Figures ...... v List of Tables ...... vii 1. Introduction ...... 1 1.1. Steel ...... 2 1.1.1. Iron-carbon phase diagram ...... 2 1.1.2. Isothermal transformation diagram (TTT diagram) ...... 3 1.1.3. Advanced high strength steel ...... 5 1.2. Stress and strain ...... 5 1.2.1. Concepts of stress and strain ...... 5 1.2.2. Stress-strain curve ...... 6 1.3. Ductility and strength in steels ...... 7 1.3.1. Work hardening ...... 8 1.3.2. Grain size reduction ...... 8 1.3.3. Solid solution strengthening ...... 8 1.4. Heat treatments ...... 8 1.4.1. Quenching and Partitioning ...... 9 1.4.2. Quenching and Tempering ...... 10 1.4.3. Austempering ...... 11 1.4.4. Intercritical Heat Treatment ...... 13 2. Materials and Methodology ...... 14 2.1. Alimak steels ...... 14 2.2. Chosen steels ...... 14 2.2.1. High Aluminum Steel...... 14 2.2.2. Strenx 700MC ...... 15 2.3. Heat treatments ...... 15 2.3.1. Heat treatments performed on High Aluminum Steel ...... 16 Quenching and Partitioning ...... 16 Quenching and Tempering ...... 17 Austempering ...... 17 2.3.2. Heat treatments performed on Strenx 700MC ...... 18 Other usual heat treatments ...... 18

iii Intercritical Heat Treatment ...... 18 2.4. Microstructure characterization ...... 19 2.5. Phase identification ...... 20 2.6. Mechanical testing ...... 20 2.6.1. Sample preparation...... 20 2.6.2. Micro-hardness measurements ...... 20 2.6.3. Tensile testing ...... 21 2.6.4. Conversion of elongation A25 to A50 ...... 22 3. Results and Discussions ...... 23 3.1. Microstructure characterization ...... 23 3.1.1. As-received and heat treated High Aluminum Steel ...... 23 3.1.2. As-received and heat treated Strenx 700MC ...... 26 3.2. Phase identification ...... 28 3.3. Mechanical testing ...... 31 3.3.1. Microhardness results ...... 31 As-received and heat treated High Aluminum Steel ...... 31 As-received and heat treated Strenx 700MC ...... 33 3.3.2. Tensile test results ...... 34 Tensile test Results of Alimak steels ...... 34 Quenching and Partitioning of High Aluminum Steel ...... 35 Quenching and Tempering of High Aluminum Steel ...... 38 Austempering of High Aluminum Steel...... 39 Usual heat treatments of Strenx 700MC ...... 41 Intercritical Heat Treatment of Strenx 700MC ...... 43 4. Conclusions ...... 45 5. Future Work ...... 46 References ...... 47 Appendix ...... 51

iv List of Figures Figure 1: Iron-carbon phase diagram [8]...... 3 Figure 2: TTT diagram [13]...... 4 Figure 3: Advanced High Strength Steels classification [16]...... 5 Figure 4: High speed tensile tests experiment setup [18]...... 6 Figure 5: Stress-strain curve of steels [19]...... 7 Figure 6: Schematic view of the Quenching and Partitioning process [21]...... 9 Figure 7: Schematic view of Quenching and Tempering process[25]...... 10 Figure 8: Variation of the heat transfer mechanism during a quench [26] ...... 11 Figure 9: Schematic view of Austempering process [27]...... 12 Figure 10: Eutectoid portion of the Fe-Fe3C phase diagram illustrating the phases present during full austenization and during Intercritical Heat Treatment. (A) Quench and temper heat treatment (complete austenization; % Calloy = % Cϒ); (B) Intercritical Heat Treatment (% ϒ and %Cϒ depend upon intercritical temperature) [30]...... 14 Figure 11: a) Furnace Nabertherm GmbH N11/HR b) Salt bath...... 16 Figure 12: Quenching and Partitioning process of High Aluminum Steel...... 16 Figure 13: Quenching and Tempering (QT) process of High Aluminum Steel...... 17 Figure 14: Austempering process of High Aluminum Steel...... 17 Figure 15: Normalizing heat treatment of Strenx 700MC steel...... 18 Figure 16: Intercritical Heat Treatment process...... 19 Figure 17: the shape of tensile test samples of Strenx 700MC, High Aluminum and Alimak steels...... 21 Figure 18: The shape of tensile samples of Alimak steels...... 21 Figure 19: Microstructure of as-received High Aluminum Steel...... 23 Figure 20: Microstructure of quenched and partitioned High Aluminum Steel 1 (QP1)...... 24 Figure 21: Microstructure of Austempered High Aluminum Steel 1 (AUST1)...... 25 Figure 22: Microstructure of Quenched and Tempered High Aluminum steel 1 (QT1)...... 25 Figure 23: Microstructure of as-received Strenx 700MC...... 26 Figure 24: Microstructure of air cooled Strenx 700MC...... 27 Figure 25: Microstructure of Intercritical heat treated Strenx 700MC 1 (IHT1)...... 28 Figure 26: XRD results of Quenched and Tempered High Aluminum Steel 1 (QT1)...... 29 Figure 27: XRD results of Austempered High Aluminum Steel 1 (AUST1)...... 30

v Figure 28: XRD results of Quenched and Partitioned High Aluminum Steel 1 (QP1)...... 30 Figure 29: Microhardness results of as-received, Quenched and Partitioned, Quenched Tempered and Austempered steels...... 31 Figure 30: Microhardness results of as-received, furnace cooled, double normalized, water quenched and Intercritical heat treated Strenx 700MC...... 33 Figure 31: Stress-strain curves of Alimak Steels (S-355MC, S-420MC and S-650MC)...... 35 Figure 32: Stress-strain curves of as-received steel and 4 different Quenched and Partitioned High Aluminum Steels: QP1, QP2, QP3 and QP5...... 36 Figure 33: Stress-strain curves of as-received steel and 4 different Quenched and Partitioned High Aluminum Steels: QP4, QP6, QP7 and QP8...... 37 Figure 34: Stress-strain curves of as-received steel and two different Quenched and Tempered steels: QT1 and QT2...... 38 Figure 35: Stress-strain curves of as-received steel and two different Austempered High Aluminum Steels: AUST1, AUST2 and AUST3...... 40 Figure 36: Stress-strain curves of as-received steel and three different usual heat treated steels: Air Cooled (AC), Furnace Cooled (FC) and Double Normalized (D-Norm) steels...... 41 Figure 37: Stress-strain curves of 4 different samples of Water quenched Strenx 700MC...... 42 Figure 38: Stress-strain curves of three different intercritical heat treated (IHT) Strenx 700MC steels: IHT1, IHT2 and IHT3...... 43

vi List of Tables Table 1: Chemical composition of High Aluminum Steel (in wt.%)...... 15 Table 2: Chemical composition of Strenx 700MC steel (in wt.%) [31] ...... 15 Table 3: Quenching and partitioning conditions of High Aluminum Steels ...... 16 Table 4: Quenching and tempering conditions of High Aluminum Steel ...... 17 Table 5: Austempering conditions of High Aluminum Steel ...... 18 Table 6: Intercritical conditions of Strenx 700MC ...... 19 Table 7: Number of tensile tested samples ...... 22 Table 8: Ultimate tensile strength, upper and lower yield strength, and Elongations A50 of Alimak steels ...... 35 Table 9: Ultimate tensile strength, upper and lower yield strength and Elongations A50 of Quenched and Partitioned High Aluminum Steels ...... 36 Table 10: Ultimate tensile strength, yield strength and Elongations A50 of as-received two different Quenched and Partitioned High Aluminum Steels: QT1 and QT2 ...... 39 Table 11: Ultimate tensile strength, yield strength and Elongations A50 of as-received and three different Austemeperd steels High Aluminum Steels: AUST1, AUST2 and AUST3 ...... 40 Table 12: Ultimate tensile strength, yield strength and Elongations A50 of as-received Strenx 700MC and three different usual heat treated steels: Air Cooled (AC), Furnace Cooled (FC) and Double Normalized (D- Norm) steels ...... 41 Table 13: Ultimate tensile strength, yield strength and Elongations A50 of three different intercritical heat treated (IHT) Strenx 700MC steels: IHT1, IHT2 and IHT3 ...... 43

vii 1. Introduction

The main initiative to this project comes from the Swedish company Alimak. The purpose has been to develop steels that can be used in Alimak’s construction of vertical access solutions for their national- and international market. Alimak desires steel that can reach 22% elongation with yield strength above 350MPa. This task is a rather daunting one as most steels have difficulties reaching such high elongation values, and achieving this was the main goal of the project. The second aspect was to investigate selected heat treatment methods in order to see which one can be applicable in industry, as some of the heat treatment processes are time intensive, energy intensive or just cannot handle complex geometries. Heat treatment is a way to significantly change material´s properties. By applying heat, it is possible to change the chemical properties and the microstructure of materials in order to reach certain mechanical properties [1]. Desired properties such as: strength, ductility and toughness can be achieved by applying a heat treatment [2]. In this project, the effects of several selected heat treatment methods for two advanced high strength steels has been investigated in order to improve the ductility of the material losing its strength. The materials in question are High Aluminum Steel and Strenx 700MC steel. The first steel is under development from the Finnish SSAB Company, and the second is commercially available steel from the Swedish SSAB Company. These steels show promising to be used as high ductility, high strength steels and it is believed that. Heat treatments can change the base materials’ mechanical properties in order to further optimize these steels for their use in vertical solutions. The heat treatments in question are Quenching and Partitioning (QP), Quenching and Tempering (QT), Austempering (AUST), Intercritical Heat Treatment (IHT) and other usual heat treatments such as Double normalizing (D-Norm), Air cooling (AC), Furnace cooling (FC). First, the most beneficial type of the above mentioned heat treatments was selected for each steel and series of heat treatments were performed in order to identify and optimize the best method for each steel. Then, treated samples underwent a series of tests to numerically quantify how good they behaved. A numerical comparison will be held to see if these samples are viable to be used as elevator components, and if not assess other possible uses for them. Achievement of both strength and ductility is a crucial demand for structural engineering materials [ref], which could have impact on the ecological and economical concerns for weight loss and energy efficiency improvements. Since the foundation of the materials science field, the development of materials with both properties of high strength and high ductility has been a constant challenge [3]. This report consists of five sections. The first section, which is a background about the project, will contain a brief presentation of the materials used in the project. It will also consist of a thorough explanation of the heat treatments and their influence on the mechanical properties of advanced high strength steel. Besides, it will contain an analysis of previous student work on the same application. The second section, which is materials and methodology, consists of a presentation of high aluminum steel and Strenx 700MC which are the chosen steels that are used in this project. This part will also contain a description of the methodology and the experiments used in order to achieve good mechanical properties. In the third section, which is results and discussions, results from different experiments such as microhardness measurements, optical microscopy images, X-ray diffraction (XRD) and tensile test results will be presented. Furthermore, these results will be discussed and related to physical phenomenon. The fourth section, which is conclusions, consists of a brief summary and the conclusions extracted from the results and discussions part. Finally, specific recommendation will be posed that can be helpful for future work.

1 1.1. Steel Steel is an alloy of iron and carbon and other elements such as chromium, aluminum, copper…etc. These elements are vital chemical components of steel because most of these elements will change its mechanical properties[4]. Carbon is the most crucial element in steel. By increasing the carbon content, the hardness and the strength can increase [5]. However, the ductility and the toughness decrease as a result. In addition, carbon enhances the brittleness and reduces the weldability because of its ability to form martensite [6]. So, in order to optimize mechanical properties, an optimal amount of carbon should be achieved. For example, low carbon steels have good ductility while martensitic steel, on the other hand, containing higher amount of carbon, can be hardened to create stronger and harder steel. The chemical composition and the method of manufacturing, including the process during fabrication, are crucial factors that could define the properties of these structural steels [7].

1.1.1. Iron-carbon phase diagram The iron-carbon phase diagram is presenting phases present in steel alloys, up to 6.67% of carbon, see Figure 1. Iron-carbon phase diagram presents the phase compositions at different conditions of temperature, time, pressure and chemical composition. The transformation of phases is occurring in alloys during cooling or heating. As shown in Figure 1, by changing the temperature and percentage of carbon, different phases might exist in the microstructure of steel as listed below [8]. δ-ferrite: which is a solid solution of carbon in iron. The maximum concentration of carbon in δ-ferrite is 0.09% at 1493⁰C. The crystal structure is body centered cubic (BCC). Austenite: which is an interstitial solid solution of carbon in γ-iron. It has a high solubility of carbon up to 2.06% at 1147⁰C and the structure is face centered cubic (FCC). α-ferrite: which is a solid solution of carbon in α-iron. It exists at room temperature and has a BCC crystal structure. α-ferrite has a low solubility of carbon up to maximum 0.025% at 723⁰C. Cementite: which is an iron carbide that is considered an intermetallic compound. It has a fixed chemical composition of Fe3C. Cementite is hard and brittle because of the large amount of carbon. Pearlite: which comprises a combination between a lamellar arrangement of ferrite (which is a soft phase) and cementite (which is a brittle phase). The pearlite is a result of the decomposition of austenite by an eutectoid reaction of austenite. Pearlite does not form instantaneously, and the time required varies depending on the point of cooling below the 732 °C transformation temperature [9].

Figure 1: Iron-carbon phase diagram [8].

1.1.2. Isothermal transformation diagram (TTT diagram) Some phases do not appear in the phase diagram of iron-carbon as they are non-equilibrium phases. These phases can be seen in a so called isothermal transformation diagram, a TTT diagram. In order to understand the influence of the heat treatment on the phase transformation, a TTT diagram is presented in Figure 2. The phases can be formed depending on temperature and time. The austenite is formed on heating the material to a temperature above the so called A1 temperature (723-911C), which is the austenitization temperature. The transformation of austenite is strongly dependent on cooling rate [10] and to which temperature the cooling is performed. A fast cooling rate (called quench) will result in martensite where a slow cooling rate or a cooling to a temperature close to A1 will result in ferrite and pearlite. In order to get martensite it is needed to cool the austenite to a temperature below the martensite start temperature (Ms). Martensite has non-equilibrium body-centered tetragonal structure and high dislocation density, very fine grain size and a high supersaturation of solute atoms (carbon) [11]. By tempering, the martensite can transform to tempered martensite which has better mechanical properties in term of ductility and toughness. When cooling to a temperature that is above the Ms, other phases can form such us lower and upper bainite. Lower bainite is formed at a slightly lower temperature than upper bainite. The difference between the two types of bainite arises from competition between the diffusion rates of carbide precipitation from supersaturated ferrite and the rate of carbon rejection from supersaturated ferrite to austenite[12]. In uncompleted transformations, the austenite will not be fully transformed and can be stabilized with carbon by diffusion and is called retained austenite. All the mentioned phases have a significant effect on the mechanical properties of the steel. Martensite makes steel strong, brittle and hard, the lower and upper bainite have intermediate hardness and carbon can diffuse from the bainite into the retained austenite and stabilize it during heat treatment. Pearlite is a hard phase that is also increasing the strength of the steel. The retained austenite is a highly desirable phase for toughness and ductility in advanced high strength steels [13]. Bainite has the same appearance and mechanical properties in terms of tempered martensite.

Figure 2: TTT diagram [13].

The stabilization of the phases and the microstructure of steels can be affected by the chemical composition. Carbon in a steel can be used to create any of the phases that includes, but is not limited to, austenite, martensite, ferrite, bainite and carbide. A simple mass balance of the carbon shows us that the increase of one of these phases would result in a decrease of all the other. Thus, if there is carbon being consumed in carbide formation, there is less carbon to create useful phases such as austenite and martensite. Carbides are often unwanted as they have undesired mechanical properties [14]. There are several other alloying elements besides carbon that have roles in the formation of different phases as described below [14],[15]: Carbon: The carbon stabilizes austenite. When the amount of carbon increases, the Ms temperature decreases, so the retained austenite can be more stable. The carbon also increases the strength is steels. Silicon: The silicon prohibits carbide formation (such as cementite), retards bainite transformation and also stabilizes austenite during partitioning. High silicon content steels cannot be galvanized, which is important for the application of this work. Aluminum: High aluminum content prohibits carbide formation during partitioning but increases the bainite transformation. Aluminum is a good element for galvanization, which means High Aluminum Steels can be galvanized. Manganese: Manganese improves austenite stability and hardenability and can be used to retard ferrite, pearlite and bainite transformations. Nickel: An increase in nickel content increases austenite stability for a long time, prohibiting the carbide formation, retarding ferrite, pearlite and bainite transformations. Chromium: A high chromium concentration leads to an increase in hardenability. It also stabilizes austenite in a more effective way than nickel, and reduces the carbon diffusivity. Molybdenum: the main function of molybdenum is to stabilize austenite.

4 The stability of retained austenite is very important to improve the toughness. For that reason, the chemical composition can affect the retained austenite’s stability. For example the carbon content can affect the stability of retained austenite and its mechanical properties. In addition, the grain size and shape of austenite can influence its own stability. This can be attributed to blocky austenite being less stable than filmy austenite. The last factors that can affect the stability of retained austenite are the crystallographic factor, crystallographic orientation and dislocation density [14].

1.1.3. Advanced high strength steel Regarding the complex microstructure and chemical composition, advanced high strength steels (AHSS) have many families as shown in Figure 3. Dual Phase (DP), complex phase (CP), transformation-induced plasticity (TRIP), martensitic (MART) and high-strength low-alloy (HSLA) steels are the main groups of commercial steels because of their useful mechanical properties. Nowadays, the challenge of materials science is to improve the mechanical properties of steels in order to obtain a 3rd generation of these steels [16]. That means, the researchers are trying to increase the elongation of AHSSs without decreasing its tensile strength as shown in Figure 3. Similarly, the main objective of this project is to find a method which can improve the mechanical properties of AHSSs into that of the 3rd generation. This can be achieved by increasing the ductility (≈22 %) and increasing the tensile strength (≈700 MPa) for two different high strength steels: High Aluminum and Strenx 700MC steels.

Figure 3: Advanced High Strength Steels classification [16].

1.2. Stress and strain

1.2.1. Concepts of stress and strain In material science, the simplest questions that a design engineer can ask about the mechanical properties of structural materials are “how strong is the steel?” and “how much deformation must I expect given a certain load?” These two questions are answered by the tensile test experiment, which is presented in Figure 4. The annotations show the most important parts of a tensile testing equipment [17].

Figure 4: High speed tensile tests experiment setup [18].

The stress is a measure of the internal forces between the atoms inside the tested materials. In another way, the engineering stress, σ, is defined as the force per unit area as shown in Equation 1: 푃 휎 = (Eq. 1) A0 Where P is the instantaneous load applied on the sample and A0 is the initial cross-sectional area before any load is applied. The strain is the deformation of the sample after applying a load. It is defined as the ratio of the change in length. The engineering strain, ε, is defined according to Equation 2: 푙푖−푙0 ∆푙 휀 = = (Eq. 2) 푙0 푙0

In which l0 is the original length before any load is applied, li is the instantaneous length.

1.2.2. Stress-strain curve In the tensile test, a specimen is deformed, usually until it fractures, with gradually increasing the applied load. This can be observed in Figure 5, where there are different concepts that should be known [19].

Figure 5: Stress-strain curve of steels [19].

Yield strength: also called yield stress, is usually found by performing tensile test experiment. It is a material property and is the stress corresponding to the point at which the material starts to deform plastically. The yield strength is used to determine the maximum permissible load in a mechanical component, since it is the upper limit of the stress that can be applied without permanent deformation. Often, the offset yield point (or proof stress) is considered to be the stress at which 0.2 percent of the plastic deformation occurs. Ultimate strength: is the highest point of the stress-strain curve and crucial for brittle materials that do not experience yielding, such as alloys, ceramics, wood and composites. The ultimate strength is rarely used in design of ductile materials. Necking: is a mode of tensile deformation in which relatively large quantity of strain is predominantly located in a small area of the material. The pronounced decrease in cross-sectional local area provides the basis for the term "neck" and local strains in the neck are large. The necking is a form of plastic deformation associated with ductile materials, such as metals. The neck gradually develops into a fracture when adequate stress is applied. Fracture: is the separation of the material in two or more pieces under stress. The fracture of the material is usually caused by the development of certain displacement discontinuity surfaces within the solid. When the displacement develops perpendicular to the surface of the displacement, it is called a normal tensile crack or simply a crack; when the displacement develops tangentially to the surface of the displacement, it is called a shear crack, a slip band, or a dislocation. Ductility: is usually established by operating tensile experiments. It is a measure of the ability of material to undergo plastic deformation before rupture. Ductility is expressed as percent elongation or percent area reduction.

1.3. Ductility and strength in steels As stated before, the main aim of the project is to enhance the strength and ductility of a low-alloyed AHSS. The strengthening mechanisms are based on the connection between metal-plastic deformation and the movement of metal dislocations [14]. Indeed, the easier the dislocation movement, the easier the plastic deformation. However, the plastic deformation also results in the prevention of the dislocation movement. Therefore obstructing the dislocation movement leads to a stronger and harder material.

7 Nevertheless, the material's ductility is often reduced by the strengthening processes. The main reinforcement mechanisms are the strengthening of the solid solution, work hardening and the reduction of the grain size [14]. These methods are described briefly in the sections below.

1.3.1. Work hardening Work hardening, also called cold hardening or strain hardening, is the phenomenon that takes place when the hardness and strength of ductile metal increase with plastic deformation, during processes such as cold forging, cold rolling. This leads to the amount of the dislocation increasing during plastic deformation [14]. As the density of dislocation increases, dislocations are getting closer and closer to each other. The interactions of the dislocation-dislocation strains are repulsive, therefore, as a side effect the movement of the dislocation is impeded by the presence of other dislocations. So, the work hardening increases both strength and hardness. However, this comes at a final cost of a reduction in ductility [14].

1.3.2. Grain size reduction The grain size refinement is the best reinforcement mechanisms that might enhance ductility [14]. The grain boundary between two different grain regions forms a barrier for dislocation movement. It is necessary to change the direction of movement in order for dislocation to transfer from one grain to the next. In addition, the grain boundary regions are submitted to atomic disorder because of the discontinuity of the slip planes, that also obstructs the movement of dislocations. The relation between the size of the grain and the strength of the matter given by Hall-Petch formula:

−1/2 휎푦 = 휎0 + 푘푦푑 (Eq. 3)

Where, σ0 and ky are constants for particular material, d is the grain size diameter and σy is the yield strength.

1.3.3. Solid solution strengthening When strengthening via solid solution, doping elements can be added to the metal to reduce the movement of dislocations. The impurity elements occur directly in the lattice and develop a substitute or interstitial solid solution that sets lattice strains on the surrounding host atoms and thus impedes dislocation movement. An increase in the concentrations of impurities results in an increase in strength but also in a decrease in ductility [20].

1.4. Heat treatments Three distinct heat treatments will be discussed in the following sections, with attention to the effects on microstructure and mechanical properties. The basis of these experiments was based of values in literature, but the parameters were calculated individually for the specific materials in question.

8 1.4.1. Quenching and Partitioning A multiphase microstructure is created through Quenching and Partitioning process. In general, three main phases can be used as guidelines: tempered martensite (initial martensite), retained austenite, and untempered martensite (fresh martensite). The heat treatment is performed in four steps [14]. During the Quenching and Partitioning process, the three observable phases appear in different stages of the heat treatment. As shown in Figure 6, the Quenching and Partitioning treatment is divided into four steps. Firstly, a full or partial austenitization is performed by heating the material to above the austenitization temperature, normally between 723-900°C. Secondly, a quenching to a temperature between martensite starting temperature (Ms) and martensite finishing temperature (Mf) is carried out. This quenching temperature (QT) is determined depending on the desired quantity of martensite and austenite in the final steel. Thirdly, the partitioning step can be performed at two distinct temperatures. The partitioning temperature could either be chosen to be the same as the quenching temperature, which is called a one- step process, or be a higher temperature (above Ms), which is labeled as a two-step process. In the last step, the sample is quenched to room temperature [14].

Figure 6: Schematic view of the Quenching and Partitioning process [21].

During Quenching and Partitioning heat treatment, multiple phases can be formed. In the austenitization step, the steel microstructure transforms to austenite. During the quenching step, a big part of the austenite turns into fresh martensite when the temperature is dropped below the Ms temperature. The newly formed martensite has a large amount of carbon which causes the material to be hard and brittle [22] During the partitioning step, the carbon diffuses from the fresh martensite into the retained austenite and stabilizes [23]. This step is considered as the determining step in Quenching and Partitioning heat treatment. Due to carbon diffusion, the fresh martensite is transformed into tempered martensite because it loses a large quantity of carbon. The benefit of the carbon diffusion is that it retains the austenitic phase, which is unstable at room temperature. This makes it benefit from the mechanical properties of austenite while still having the structure of martensite. This combination exhibits better toughness without any major decrease in strength [23].

9 If the carbon content increases, the Ms decreases. Big part of austenite stabilizes by carbon diffusion. However, some part of the austenite which has lower amounts of carbon turns to fresh martensite during the final quench step. Fresh martensite is hard and brittle compared to tempered martensite because the fresh martensite has higher dislocation density compared to tempered martensite Furthermore, it is possible that a part of the austenite turns into bainite during partitioning step [22]. The stability of retained austenite is very important in order to improve the toughness [1]. The chemical composition can affect the retained austenite’s stability, for example the carbon content can affect the stability of retained austenite, and the consequent mechanical properties. The grain size and shape of austenite can influence the stability of that phase. This can be attributed to blocky austenite being less stable than filmy austenite. The last factors that can affect the stability of retained austenite are the crystallographic orientation, dislocation density and direction of the load.

1.4.2. Quenching and Tempering The as-quenched steels are strong, brittle, and present big internal stresses which can lead to cracks. Because of that, it is necessary to perform another heat treatment to improve mechanical properties of the steels and this heat treatment is called tempering. Tempering is the heat treatment performed to adjust ductility, strength and toughness by diminishing the internal stresses. This is achieved by heating the material to a temperature which does not cause a change in the phases, but exerts enough heat in order to alleviate the strain caused by dislocations [24]. The science behind the Quenching and Tempering processes and its benefits will be presented and discussed in this part. Figure 7 shows the Quenching and Tempering (QT) process, which can be divided into four steps. The first step is full or partial austenization which is similar to the Quenching and Partitioning process. The second step is the quenching of the sample to room temperature. This can be carried out using water or other fluids. In the third step, the material is heated until the material has reached enough tempered martensite. Finally, the sample is quenched to room temperature via air [25].

Figure 7: Schematic view of Quenching and Tempering process[25]. A sample's cooling rate during the quench is not constant. Once the sample is inside the fluid, the fluid can vaporize, especially if the fluid in question is water. Vapor films appear between the water and the sample resulting in a slowed cooling rate. The vapor layer around the sample dissipates after a while and

10 the cooling rate increases. When the temperature of the sample is lower than the boiling temperature of the fluid, the final step is taken and the heat transfer decreases as shown in Figure 8 [26]. When the material is cooled at a constant rate the quench is defined as gentle. This can be achieved using a fluid other than water. Examples of such processes are oil hardening, air-hardening and a fluid made of water and polymeric additives to cool down the sample. These types of quenching result in more uniform cooling compare to regular quenching. As the cooling is more uniform, the shape distortion effect is reduced [26]. Quenching can be a major cause in crack formation. Due to internal stresses, which can be thermal stresses or stresses on transformation, the appearance of cracks is more likely. The amount of thermal stresses can be increased due to a number of different reasons. The increase in the quenching speed can lead to more captivated stresses, while the material’s properties such as size, shape and surface area can also have an impact [26].

Figure 8: Variation of the heat transfer mechanism during a quench [26]

If the heat treatment process is performed poorly, the steel can have a drop in strength, thus it is important to optimize the quench and tempering process: to have a combination of good strength and ductility. To increase ductility and toughness it is important that the internal stresses are relieved, that crystal defects are eliminated or rearranged and that carbon is redistributed so it can precipitate properly. All these phenomena are thermally activated and encouraged by the tempering step. The tempering step is the heat treatment that consists of heating the sample between 250⁰C and 650⁰C for a specified time. Due to the addition of excess heat, the as-quenched martensite can transform itself into tempered martensite [26] .

1.4.3. Austempering Austempering is a heat treatment which results in the formation of bainite. Bainite is normally composed of cementite and ferrite. During austempering, the formation of carbide can be prohibited leading to the formation of Carbide free bainite, i.e. bainite without any cementite. During the holding step, the bainitic ferrite enriches the austenite by the transfer of carbon. This carbon enriched austenite is better retained,

11 thus is in a stable phase after the treatment. The austempering process is also composed of four steps which all affect the final mechanical properties. The Austempering heat treatment takes place in four steps. The first one is a partial or full austenitization of the sample. Then the sample is quenched at a temperature above the bainite-start temperature (Bs), to enable bainitic transformation. The third step consists of holding the sample to the quench temperature during a certain time. Finally, the sample is quenched at room temperature. The Figure 9 presents a diagram of the different steps of the treatment.

Figure 9: Schematic view of Austempering process [27].

Two different types of bainite can be formed: upper and lower bainite. That depends on the transformation temperature. The composition of both upper and lower bainite consists of ferrite and cementite but their microstructures are different. The lower bainite grows at lower temperature between 250⁰C and 400 ⁰C. The lower bainite microstructure has the cementite precipitates between the fine plate and ferrite. The upper bainite grows at higher temperature between 400 ⁰C and 500 ⁰C. The upper bainite microstructure consist a fine plates of ferrite separated by cementite. The carbide free bainite microstructure has several benefits. For high aluminum and silicon steels, the transformation into upper bainite leads to interesting microstructures. As mentioned before in Quenching and Partitioning heat treatment, the aluminum and silicon prevent the formation of carbide. So, the carbon diffuses into austenite which enriches in carbon, this carbon make austenite stable at room temperature. Thus, the final microstructure is composed of fine plates of bainitic ferrite separated by retained austenite. Because of the presence retained austenite instead of cementite, carbide free bainite steels have a good combination of strength and ductility [26]. As stated previously, all of the phases of steel tap into the same pool of carbon, and by doing a mass balance on the carbon the formation of each phase can be predicted. Since this time we have different phases we are interested in, it is more interesting to see the interaction between the austenite and bainite phases. The same presumptions apply, as we do not want carbide tainting our material, so it should be avoided by any means necessary [28].

12 1.4.4. Intercritical Heat Treatment Most conventional heat treatments start by heating the material above the A3 temperature, where 100% of austenite (ϒ) forms with the same carbon content as the alloy, as shown in Figure 10-A. During Intercritical Heat Treatment the temperature is held between the A1 and A3 temperatures, as shown in Figure 10-B. Heating the material into the intercritical region leads to formation of two phases; ferrite and austenite (α + ϒ). The fraction of phases can be calculated by the Lever rule. For example, for an alloy or mixture with two phases: α and β, which themselves contain two elements, A and B, the Lever rule states that the mass fraction of α phase is [29]:

훽 훼 푤퐵−푤퐵 푤 = 훼 훽 (Eq. 4) 푤퐵−푤퐵 Where: 훼 푤퐵 is the mass fraction of α phase 훽 푤퐵 is the mass fraction of β phase

For a given alloy, the amount of carbon inside the austenitic microstructure depends on the intercritical temperature. When the intercritical temperature increases, the percentage of austenite increases but its carbon content decreases. Upon cooling from the intercritical temperature, the ferrite region present in the duplex structure remains, while the austenite region transforms into martensite, bainite or ferrite + pearlite. These transformations depend on the cooling rate and on the intercritical temperature that controls the composition and therefore the austenite volume fraction. The thickness or fineness of the microstructure that forms as a result of Intercritical Heat Treatment depends on the original microstructure that controls the nucleation of the austenite at the intercritical temperature. Typical fine ferrite / tempered martensite microstructure forms after Intercritical Heat Treatment and subsequent tempering. This microstructure is significantly different from the ferrite / martensite microstructure (with considerable amounts of retained austenite) found in dual-phase steels. The hardness after Intercritical Heat Treatment is less than after rapid cooling during normal heat treatment where 100% of the austenite is transformed to martensite. Intercritical Heat Treatment leads to a smaller amount of martensite. The final temperature after Intercritical Heat Treatment must therefore be lower than that used for similar quench and temper heat treatments if the same final hardness levels are desired. These resulting microstructures can improve mechanical properties of the steels in terms of ductility and strength [30].

Figure 10: Eutectoid portion of the Fe-Fe3C phase diagram illustrating the phases present during full austenization and during Intercritical Heat Treametnt. (A) Quench and temper heat treatment (complete austenization; % Calloy = % Cϒ); (B) Intercritical Heat Treatment (% ϒ and %Cϒ depend upon intercritical temperature) [30].

2. Materials and Methodology

2.1. Alimak steels The project is in collaboration with the company Alimak Group, a global market leader in vertical access solutions supplying elevators, hoists and work platforms based on rack-and-pinion technology for industry and the construction sector. The company today uses three different grades of Strenx steel: Strenx 355MC, Strenx 420MC and Strenx 650MC, which are commercially available steels from SSAB. These steels has the required combination of ductility and strength. These steels will be used as reference and will be compared with the other steels chosen for this project.

2.2. Chosen steels In this project, two types of advanced high strength steels are chosen: High Aluminum Steel and Strenx 700MC and they will be described in the following sections.

2.2.1. High Aluminum Steel High Aluminum Steel is produced at the Finnish SSAB factory and is currently under development. This steel was chosen because of two different parameters: 1. High Aluminum Steel is a low carbon steel which has 0.158 % of carbon, and as mentioned before the ductility increases when the amount of carbon decreases. The chemical composition is shown in Table 1. 2. High Aluminum Steel has a significant amount of aluminum as seen in Table 1. The aluminum prevents the formation of carbides and facilitates galvanization which is a requirement from the company. The other alloying elements fulfill the role of improving the materials mechanical properties.

14 Table 1: Chemical composition of High Aluminum Steel (in wt.%)

C (%) Si (%) Mn (%) P (%) S (%) Al (%) Nb (%) V (%)

0.158 0.376 2.08 0.012 0.0005 1.03 0.001 0.005

Cu (%) Cr (%) Ni (%) N (%) Mo (%) Ti (%) Ca (%) B (%)

0.033 0.304 0.044 0.003 0.009 0.003 0.0003 0.0007

2.2.2. Strenx 700MC Strenx 700MC is a low carbon commercial steel from the Swedish SSAB company. As shown in Table 2, the Strenx 700MC has a small amount of carbon (0.12 %), together with some other elements in order to increase the ductility of the steel.

Table 2: Chemical composition of Strenx 700MC steel (in wt.%) [31]

C (%) Si (%) Mn (%) P (%) S (%) Al (%) Nb (%) V (%) Ti (%)

0.11 0.091 0.64 0.009 0.017 0.017 0.088 0.19 0.14

2.3. Heat treatments The experiments were performed in a furnace and a salt bath furnace as shown in Figure 11. For each condition, three samples were treated at the same time to maximize the accuracy of the results. The furnace Nabertherm GmbH N11/HR was used to perform the austenitization of samples, at 850 ⁰C for High Aluminum Steel and at 900 C Strenx 700MC. The Ms temperature depends on chemical composition and on the austenitization temperature. The Ms temperature used for all heat treatment of High Aluminum Steel was 203 C [32]. For Strenx 700MC, the Ms temperature was 464C. The samples were placed on a steel block in the furnace with one end sticking out from the block and by this the samples could easily be grabbed in this end. Tempering, partitioning, and holding steps were performed using a Therm Concept salt bath furnace. The quenching treatment was realized in water at room temperature after the austenitization. Then the samples were carefully dried before being tempered in a separate salt bath furnace at the desired temperature. All samples were cooled down in ambient air after the second salt bath treatment. In order to fully investigate how to achieve a ductile advanced high strength steel, five different types of heat treatments were chosen. Quenching and Partitioning, Quenching and Tempering and Austempering were the three heat treatments used for High Aluminum Steel. For Strenx 700MC, Intercritical Heat Treatment and a normalizing heat treatment were performed.

Figure 11: a) Furnace Nabertherm GmbH N11/HR b) Salt bath.

2.3.1. Heat treatments performed on High Aluminum Steel Quenching and Partitioning In Quenching and Partitioning, the austenitization occurred at 850 ⁰C throughout a 4 min time interval. Then the samples were quenched to two different quenching temperatures; 165 ⁰C and 175 ⁰C for 10 s. During the third step, two different partitioning temperatures (400 ⁰C and 450 ⁰C) were chosen with two different partitioning times of 50s and 100 s. Finally, the samples were cooled down in air. Figure 12 shows the heat- and time cycle for this heat treatment.

1000

800

600

400

200

TEMPERATURE(⁰C) 0 0 1 2 3 4 5 6 7 8 9 10 11 TIME (MIN) Figure 12: Quenching and Partitioning process of High Aluminum Steel.

In total 8 conditions were performed by various combinations of quenching temperatures, partitioning temperatures and partitioning times as shown in Table 3.

Table 3: Quenching and partitioning conditions of High Aluminum Steels Condition Quenching Partitioning Partitioning time name temperature (QT) temperature (PT) (Pt) QP1 175 ⁰C 450 ⁰C 100 s QP2 175 ⁰C 450 ⁰C 50 s QP3 175 ⁰C 400 ⁰C 100 s QP4 175 ⁰C 400 ⁰C 50 s QP5 165 ⁰C 450 ⁰C 100 s QP6 165 ⁰C 450 ⁰C 50 s QP7 165 ⁰C 400 ⁰C 100 s QP8 165 ⁰C 400 ⁰C 50 s

Quenching and Tempering In the Quenching and Tempering experiments, the austenitization step was chosen at 850 ⁰C for a duration of 4 min. Then, the samples were quenched to room temperature by using water. During the tempering step, two different temperatures were chosen; 400 ⁰C and 450 ⁰C with 5 min in tempering time, as shown in Table 4. Finally, these samples were cooled down in air. The Quenching and Tempering heat treatment cycle is shown in Figure 13.

1000 800 600 400 200 0 TEMPERATURE(⁰C) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 TIME (MIN) Figure 13: Quenching and Tempering (QT) process of High Aluminum Steel.

Table 4: Quenching and tempering conditions of High Aluminum Steel Condition Quenching Tempering Tempering time name temperature (QT) temperature (TT) (Pt) QT1 Room temp. 450 ⁰C 5 min QT2 Room temp. 450 ⁰C 5 min

Austempering In the Austempering heat treatment, the austenitization step was determined to be at 850 ⁰C for 4 min. After austenitization, the samples were cooled to three separate temperatures, named isothermal temperatures (IT): 230 ⁰C, 260 ⁰C and 270 ⁰C. In the last step, the samples were cooled down using air. The heat treatment cycle is shown in Figure 14. In total, three conditions were performed as shown in Table 5. 1000

800

600

400

200

TEMPERATURE (⁰C) TEMPERATURE 0 0 1 2 3 4 5 6 7 8 9 10 11 14 TIME (MIN) Figure 14: Austempering process of High Aluminum Steel.

Table 5: Austempering conditions of High Aluminum Steel Condition Isothermal Holding time (Ht) name temperature (IT) AUST1 230 ⁰C 5 min AUST2 250 ⁰C 5 min AUST3 270 ⁰C 5 min

2.3.2. Heat treatments performed on Strenx 700MC Other usual heat treatments During normalizing, the samples were heated to 900 ⁰C and held for 20 min as shown in Figure 15. Then, four different ways of cooling were performed: air cooling, water quenching, furnace cooling and double normalizing. In air cooling, the samples were cooled down using ambient air, and in water quenching the samples were cooled in a bucket of water to room temperature. During furnace cooling the samples were kept inside the furnace, and cooled by setting the cooling rate to 300⁰C/hour. Finally, in the double normalizing heat treatment the samples were cooled down in air, then heated again to 850 ⁰C for a duration of 20 min and cooled down again in air. During normalizing, the samples are heated to 900 ⁰C and held for 20 min as shown in. Then, four different ways of cooling were analyzed: air cooling, water quenching, and furnace cooling and double normalizing. First, in air cooling, the samples were cooled down using still ambient air. Second, in the water quenching method, the samples were cooled in a bucket of water to room temperature. Third, in the furnace cooling method, the samples were kept inside the furnace, and cooled by setting the cooling rate to 300⁰C/hour. Finally, in the double normalizing heat treatment, the samples were cooled down in air, then heated up again to 850 ⁰C for duration of 20 min and cooled down yet again in air.

1000

800 ⁰C) 600

400

200 TEMPERATURE ( TEMPERATURE 0 0 1 10 20 22 TIME (MIN) Figure 15: Normalizing heat treatment of Strenx 700MC steel.

Intercritical Heat Treatment In Intercritical Heat Treatment, the samples were heated to three different intercritical temperatures: 725⁰C, 750⁰C and 775⁰C. Then, the samples were quenched to room temperature with water, which was stored in buckets. After quenching, the samples were tempered at 500 ⁰C for 1 hour and then finally

18 quenched in air. The heat treatment cycle for the Intercritical Heat Treatment is shown in Figure 16. Three conditions were performed in Table 6.

1000

800 ⁰C)

600

400 TEMPERATURE ( TEMPERATURE 200

0 0 1 10 20 22 23 24 40 50 60 70 80 83 84 TIME (MIN)

Figure 16: Intercritical Heat Treatment process.

Table 6: Intercritical conditions of Strenx 700MC Condition Intercritical Quenching Tempering name temperature (IT) temperature temperature IHT1 230 ⁰C Room temp. 1 h IHT2 250 ⁰C Room temp. 1 h IHT3 270 ⁰C Room temp. 1 h

2.4. Microstructure characterization Optical microscopy was used in the analysis of the microstructure of all the steels. This was carried out with the use of a Nikon H550L microscope with a 10x ocular lens and 100x objective lens, giving in total a 1000x magnification. In total, 14 images were taken for the High Aluminum Steel: one for the as-received, 8 for different conditions of quenched and tempering, 3 images for Austempering and 2 for Quenched and Tempered samples. For the Strenx 700MC, 10 images were captured: one for as-received, 3 for usual heat treatments, 3 for water quenched and 3 for intercritical heat treated samples. First, the samples were cut in smaller pieces with a Struers Discotom-100 using a 40A30 Struers cut-off wheel. After that the samples were hot mounted by using SimpliMet 1000 machine. Once the samples were mounted into Epoxy, they were ground by using Buehler machine for 4 SiC papers, 120, 240, 600 and 1200. Then they were polished with a Buehler semi-automatic machine. The samples were etched with a solution of 3% nital (ethanol + HNO3) in order to see the microstructure of the samples. Different times were chosen to do the etching, but 5s was the best one to clearly see the microstructure of the High Aluminum and Strenx 700MC steels.

19 2.5. Phase identification The phase formation was investigated with X-ray diffraction (XRD). An Analytical Empyrean diffractometer was used for the XRD experiments. The procedure of an XRD measurement can be divided into three steps. The first step is to prepare an adequate sample of the materials to be studied. After the tensile tests, small pieces of samples were cut by using a Discotom-100 equipped with a 40A30 Struers cut-off wheel. In the second step the sample should be placed under an intense beam of X-rays, usually a single monochromatic X-ray beam. This gives rise to a regular pattern of reflections which are diffracted to multiple specific directions. These angles and intensities of diffracted X-rays are then analyzed by the machine and numerically computed. In the third step, the data is combined with the chemical information in order to produce a model of the crystallographic arrangement within the sample. In total 13 samples underwent these XRD measurements, 8 Quenched and Partitioned, 2 Quenched and Tempered, and 3 Austempered samples. After that, the samples were ground by using 4 different SiC- papers, which are SiC 120, SiC 240, SiC 600 and SiC 1200. They are then polished by using UltraPad 9 µm, 6 µm, 3 µm, 1 µm, 0.25 µm and Mastertex polishing mats.

2.6. Mechanical testing In order to investigate mechanical properties, all samples underwent tensile tests and hardness measurements. To maximize the accuracy of these measurements, some preparation was done before each test. First, the samples were prepared according to the specifications described below. Then tensile tests were carried out where each sample was loaded until the point of fracture. Hardness measurements were carried out on different rectangular samples. Finally, all data sets were analyzed and treated in a way that will be described in the following sections.

2.6.1. Sample preparation As oxides and surface defects can aﬀect the measurements, it is important to grind the surface before performing any test. The grip section was ground clean so that it did not contain any impurities from the heat treatment. To grind this surface, a die grinder was used with a P120 die to remove the top layer of oxide. Then the sample surface was ground by P600 and P1200 silicon-carbide papers. Preparation for tensile test was performed by the edges of the sample being ground with the same die grinder equipped with a P60 die. After that, P240 and P600 paper were used to grind the edges by hand.

2.6.2. Micro-hardness measurements Hardness measurements were performed directly on the tensile samples before any tensile test by a Mitsuzawa hardness tester equipped with a Vicker’s diamond. On each sample, 10-15 indents were carried out with a load of 500g/1000g and a hold time of 15 seconds. The length of the two diagonals of the indent was measured manually and the micro-hardness values were calculated using the instrument’s data program. The software calculated the relative hardness by taking the force over the area of the indent. These values were then exported to excel, to facilitate creation of hardness profiles. The average hardness values together with the standard deviations were calculated.

20 2.6.3. Tensile testing According to the ASTM8 standard [33], the sheets of Strenx 700MC and High Aluminum Steel were cut with different parameters as shown in Figure 17. Strenx 700MC and High Aluminum Steels share the same parameters except for thickness, where Strenx 700MC is 3 mm thick but High Aluminum Steel is 1.5 mm thick. Alimak steels have two different shapes with different parameters as shown in Figure 17 and Figure 18 with 3 mm in thickness.

Figure 17: the shape of tensile test samples of Strenx 700MC, High Aluminum and Alimak steels.

Figure 18: The shape of tensile samples of Alimak steels.

Tensile tests were performed with a strain rate of 0.0001/s. Two diﬀerent machines were used, because one machine could not apply the required load for the steel with higher parameters (Figure 18). An Instron 1272 machine was used to investigate High Aluminum steel, Strenx 700MC and Alimak steel with the parameters in Figure 17. Whereas the Alimak steels with the parameters in Figure 18 were tested in a Dartec machine. 3 samples per conditions were performed For High Aluminum steels, 2 samples per conditions for Strenx 700MC and 2 samples per dimension for Alimak steels. In total 71 samples were tested as shown in Table 7. The tensile test provided values for the load, position of the clamps and the position of the extensometer, alongside other values. These were used to calculate the stress and strain values by plotting a stress-strain graph in excel. Afterwards, the values for toughness, elongation, Young’s modulus, yield strength and tensile strength were determined via graphical analysis. These values were then tabulated and compared with each other in order to see what difference each heat treatment method had on the mechanical properties of each material.

21 Table 7: Number of tensile tested samples

Grade of steel Condition Number of samples Quenching and 24 Partitioning Quenching and 6 Tempering High Aluminum steel Austempering 9 As-received 2 Usual heat treatments 10 Intercritical Heat 6 Strenx 700MC Treatment As-received 2 Alimak steels - 12

2.6.4. Conversion of elongation A25 to A50 Alimak company and many of their customers use a definition of the elongation called A50, i.e. the gauge length is 50 mm long during tensile testing. The High Aluminum Steel and Strenx 700MC samples tested in this project were only 25 mm in gauge length, because there was not enough material for larger samples. Therefore it has been needed to transform the resulting A25 values into A50 in order to be able to compare the resulting elongations with the required elongation expressed as A50. The A50 elongations were calculated from the A25 results with the so called Oliver equation (Eq. 5).

(Eq. 5)

Where L0 is the gauge length, A0 is the cross section area and n is a constant depending on the material. For the low-carbon annealed sheets that were used in this project, the Oliver equation defined the constant n of this type of material to be equal to 0.3 [34].

22 3. Results and Discussions

3.1. Microstructure characterization

3.1.1. As-received and heat treated High Aluminum Steel Figure 19 shows the optical microscope image of the as-received High Aluminum Steel. The sample contains two different areas with two different colour appearance; white and black. The white area is likely to be the ferrite and the black area is likely to be the martensite. The white grains have a fine grain size with an average of 10 µm, which forms slightly in a rectangular fashion because of the cold rolling of the material in the past. The columnar structure can be observed with the grains being stretched towards the top and bottom. The martensitic phase is distributed more randomly, and has no specific shape or size. The size of the martensitic phase is much smaller than that of the ferritic phase, and just by observing Figure 19, it can be said that there is more ferritic phase in the material than martensitic.

Figure 19: Microstructure of as-received High Aluminum Steel.

The grain sizes of the phases are intermediate in size. This grain size is attributed to the processing of this material, where it underwent cold rolling, which impacted the grain size and orientation. This process was primarily used to get uniform thickness for these steels, but also work by promoting twinning and reducing the bimodality. The end result is a flat sheet with low amounts of defects and an even microstructure [35]. Figure 20 shows the microstructure of Quenched and Partitioned High Aluminum Steel 1 (QP1). The sample has two distinct areas (black and white). The white area, which appears in a fine grain structure is likely to be ferrite. The black area, according to Quenching and Partitioning heat treatment, is supposed to consist of 3 different phases; retained austenite, fresh martensite, and tempered martensite. This results was confirmed by the XRD result, as discussed in the next section 3.2. The black spots form fine grains, finer than that of ferrite, and they are spread out rather homogeneously.

Figure 20: Microstructure of Quenched and Partitioned High Aluminum Steel 1 (QP1).

The QP1 microstructure shows promise because of its fine microstructure, which is an indication of good mechanical properties that can lead to good ductility and strength. When compared to the as-received sample, the QP1 sample has crystals 1/10th of its original size. This increases the homogeneity and improves the physical and mechanical properties of the material. From these figures it can be deduced that by applying a heat treatment method the material properties will improve. The partitioning heat treatment is the determining step in the distribution of the phases in this material. The initial ferrite was first fully austenized. This then underwent a relatively short isothermal hold. Here the austenized material broke down to other components. The distribution of these components depends on the temperature and hold time. As the hold time was relatively short (100 s), it did not promote the production of big, long grains, so the resulting microstructure is looking as shown Figure 20 . In conclusion, the variation of the holding time affects the grain size of steels. In literature, higher austenization temperatures leads to the formation of smaller sized particles in the final product. However, this also implies that further experimentation can be conducted varying the austenization temperature in order to optimize the heat treatment method, and subsequently the material properties [36]. Figure 21 shows the microstructure of Austempered High Aluminum Steel 1 (AUST1). The sample has two different areas, colored dark and white. According to the theory about Austempering heat treatment, the white area is likely to be ferrite or retained austenite which consists of very fine-grained particles and the dark area is likely to be a combination of 3 different phases: fresh martensite, tempered martensite and ferritic bainite. It is difficult to differ between these phases by optical microscopy analysis as the grain size is too small to be distinguished using optical light. The presence of these phases will be further discussed in next section 3.2 by the use of the XRD results. Finally, the distribution of all these phases can be seen as homogenous, however there is no clear distinction of their grain boundaries.

Figure 21: Microstructure of Austempered High Aluminum Steel 1 (AUST1).

The AUST1 steel can be seen as a desirable product since its microstructure, its fine grain size and its homogeneous distribution can lead to good mechanical properties. These microstructural components (i.e. coexistence of fine ferrite beside martensite islands) are expected to improve the mechanical properties as they act like a composite. Ferrite provides ductility while martensite, as a hard phase, can enhance the strength of the steels) [37]. Figure 22 shows the microstructure of Quenched and Tempered High Aluminum Steel 1 (QT1). There are two main zones present in the image, the black area, and the white area. The white area is attributed to the presence of ferrite, whereas the black areas consist of fresh martensite and tempered martensite. Even though these two phases are formed due to different phenomena, their properties are the same and it is impossible to differentiate between these two martensitic phases using optical data.

Figure 22: Microstructure of Quenched and Tempered High Aluminum steel 1 (QT1).

25 The amount of martensite cause the material to be stronger and more brittle. This martensite also might reduce the ductility of QT1 when it is compared with AUST1 and QP1. Like Quenching and Partitioning, the Quenching and Tempering heat treatment method also relies on the temperature of its steps. Furthermore, they are both heavily affected by the isothermal holding step. The only main difference is how they change the chemical composition before the start of the isothermal holding step. During Quenching and Tempering, the tempering step is the key because the temperature and time will greatly affect the properties of the final product. These steps will control the produced grain size and the strength of the material in accordance to the Hall-Petch relation (Eq. 3) [38].

3.1.2. As-received and heat treated Strenx 700MC Figure 23 shows the OM images of the as-received Strenx 700MC. The microstructure of this sample consists of the distribution of small dark/black areas in a white matrix, which is believed to be ferrite. Because, according to chemical composition (carbon content) of this steel and considering the lever rule in the phase diagram, ferrite is the dominant phase in the microstructure of this steel. The Strenx 700MC steel is a low carbon steel which contains only 0.11 wt.% carbon. In fact, aimed at increasing the ductility and/or improving the weldability of the steel parts, their carbon content can be reduced to some extent. In microalloyed/HSLA steels, the reduction in the strength caused by the lower carbon content would be compensated by the addition of micro-alloying elements. Because, these elements can form stable carbides, nitrides and/or carbonitrides in steels, enhancing their strength [39]. Moreover, regarding the chemical composition of the steel under investigation (Table 2), one can see that the studied steel contains V, which can promote the precipitation hardening during the heat treatment regime, improving the mechanical properties of the steel.

Figure 23: Microstructure of as-received Strenx 700MC.

Comparing the microstructure of this steel with High-Aluminum steel, it can be apprehended that the microstructure of Strenx 700MC is finer than that of High Aluminum Steel. Besides, the as-received Strenx 700MC has a high fraction of ferrite compared to High Aluminum Steel. Also, the amount of martensite is lower when compared to High Aluminum Steel. This difference can make as-received Strenx 700MC likely to be more ductile than as-received High Aluminum Steel.

26 It should be mentioned that the as-received Strenx 700MC steel is in deformed condition. Therefore it can be anticipated to possess the high level of hardness and strength. It is also worth mentioning that compared to the heat treatment procedures, the deformation process (especially thermos-mechanically controlled processing, TMCP) is much more costly. This is the reason why we tried to obtain the desired final properties by applying the heat treatment cycles rather than the deformation processes. Figure 24 shows the microscope image of the heat treated and air cooled (AC) Strenx 700MC. The sample has white areas which can be ferrite. This phase has fine grains with different sizes with random directions. This heat treatment process is also called normalizing, as the hard phase (martensite) has not been formed through air cooling. The low carbon content of this steel leads to the formation of a very low fraction of cementite/pearlite [39], not being visible at this magnification.

Figure 24: Microstructure of air cooled Strenx 700MC.

Since ferrite is able to provide the steel with good ductility and formability, the resulting microstructure can be desirable in order to reach the required mechanical properties like ductility, but for strength the normalizing heat treatment is not very good because ferrite, as the dominant phase, is very soft. Air cooled Strenx 700MC microstructure has the same phase as the as-received steel, but it has less uniform microstructure compared to as-received Strenx 700MC. The microstructure of this steel in different conditions (different treatments, e.g. double normalizing, furnace cooling, or normalizing) might look similar. But, the fact is that their differences cannot be fully understood from comparing the optical images. For instance, as mentioned above, the as-received steel is in the deformed condition, which is believed to lead to very high dislocation density that is strengthening the steel. Besides, in addition to the state of dislocation density, to study the precipitation behavior of micro-alloying elements, it is essential to use transmission electron microscopes (TEM) [40]. Therefore, the mechanical properties obtained (see the subsection 3.3.2) might also be contributed to the features that cannot be studied by optical microscopy observations. Due to the low carbon content of the studied steel its hardenability is expected to be very low. Hence, very fast cooling is required to obtain a hard phase (martensite) in the microstructure.

27 Figure 25 shows the microstructure of Intercritical Heat Treated Strenx 700MC at 725°C. This steel contains two main phases; ferrite which occupy the majority of the microstructure (white region) and martensite (black region). The grains sizes of ferrite are as fine as the as-received sample. The martensite on the other hand, forms fine lines at the grain boundaries of ferrite.

Figure 25: Microstructure of Intercritical heat treated Strenx 700MC 1 (IHT1).

The grains sizes of ferrite is very fine, and have a random direction. Considering the coexistence of the two phases (ferrite + austenite) in the two phase (intercritical) region, there are grain boundaries of ferrite and austenite. Martensite likely form at the boundaries between ferrite and austenite during the cooling. The formation of martensite at such boundaries results in the microstructure shown in Figure 25. This structure (the combination of martensite and ferrite) is known as dual phase structure and the heat treated steel is then called Dual Phase (DP) steel. The microstructural components in the DP steels act like a composite, including a hard phase (martensite) dispersed in a soft matrix (ferrite). Therefore, such steels provide a good combination of strength and ductility [41].

3.2. Phase identification QT1 has two phases; ferrite and martensite. Ferrite has the larger percentage of 87.3% and martensite has a smaller percentage of 12.7%. This can be considered good as the ferritic phase can improve a material’s ductility, while the martensite can improve the yield strength. Since more ductility is desired in comparison to strength, the high amount of ferrite shows that the experiments are heading towards the right direction as shown in Figure 26. The amount of ferrite in this condition corresponds to the initial ferrite (it existed in the intercritical/two-phase region) plus tempered martensite. When the martensite phase is tempered, its tetragonality is reduced which leads to that it might be indexed as ferrite in XRD measurement. Regarding the fraction of fresh martensite, it is feasibly due to the formation of martensite through quenching from the tempering temperature. Because, after hardening, fresh martensite exists beside retained austenite in the microstructure. During the tempering stage, carbon diffuses out from retained austenite to form secondary carbides. In this way, the solute carbon content of austenite is reduced which in turn leads to an increase in the Ms temperature. Consequently, the retained austenite would transform to fresh martensite during cooling from tempering temperature. The microstructural constituents obtained in the QT1 sample are expected to fulfill the requirements dictated by the service condition (e.g. high ductility) since the ferrite phase provides the steel parts with good formability [42].

Figure 26: XRD results of Quenched and Tempered High Aluminum Steel 1 (QT1).

The most important aspect in austenitic steels is the carbon content. When calculating the distribution of phases, a carbon balance must be carried out. The carbon will be more likely to prefer stable phases unless there is a way to promote semi-stable ones. A great influence in the distribution of phases, such as the amount of austenite retained inside the sample, is the chemical composition, where alloying elements such as silicon and aluminum promote retained austenite[43]. AUST1 has a large amount of ferrite of around 83%. This quantity is relatively lower than that of QT1. AUST1 also has a large amount of martensite of around 16%. However, the largest difference is the presence of a third phase, which QT samples do not possess. AUST samples contain a small amount of austenite, with a fraction lower than 1% as shown in Figure 27. Yet, this will be crucial when the materials’ mechanical properties are taken into consideration. Figure 27 shows promising results as the high amounts of ferrite point towards a more ductile material. However, in comparison to QT1, it has larger amounts of martensite which may cause the material to be brittle. Which material that exhibits better mechanical properties will be further discussed later in the report. Quenching and Partitioning usually yields high austenitic contents, greater than 5%. This was not the case for our samples as the XRD results showed trace amounts of austenite, lower than 1%. This might be due to multiple reasons. Firstly, austenite is not a stable phase at room temperature. Any austenite on the surface layer would transform into martensite, reducing the amount of austenite observed. Secondly, the penetration of the X-ray beams might not have been enough. Austenite is usually retained in the center of the material, and the XRD beams can only penetrate a certain distance inside. However, these problems are relatively minor and would only decrease the phase fraction by a couple of percent. The main reason the XRD did not yield high austenite percentages is believed to be the transformation induced plasticity (TRIP) effect[44]. The TRIP effect is when the austenitic phase is subjected to external forces, and undergoes deformation. The austenite rather than behaving normally, transforms to martensite, which is due to the force exerted during sample preparation and it increases the plasticity of the overall material. This is important to consider when the XRD samples were cut from samples that had undergone tensile

29 testing. This implies that the samples experienced the TRIP effect before the XRD analysis, meaning some of the austenitic phase was lost [44].

Figure 27: XRD results of Austempered High Aluminum Steel 1 (AUST1).

Figure 28 shows the XRD result for QP1, where the highest concentration is that of ferrite, reaching about 87%. This is followed by a concentration of martensite of 12% and finally a low concentration of austenite, less than 1%.

Figure 28: XRD results of Quenched and Partitioned High Aluminum Steel 1 (QP1).

30 The graph in Figure 28 shows a similarity to AUST1, where both QP1 and AUST1 have similar amounts of each phases. This implies they will show similar mechanical properties. However, since the heat treatment methods are different where more time and energy is required for the Austempered sample, the partitioned sample might turn out to be more favorable. However, we should note that QP1 has a larger quantity of ferrite than AUST1. This implies that QP1 is more ductile than AUST1 because it contains more of this softer phase. On the other hand the amount of austenite in AUST1 is larger than QP1, but since the phase fraction is less than 1%, it might be negligible.

3.3. Mechanical testing

3.3.1. Microhardness results As-received and heat treated High Aluminum Steel For the microhardness investigation 14 experiments were carried out on High Aluminum Steels. These 14 experiments were divided among the different samples, and the data obtained was in form of an average of 10-15 indents made on the material. These indents were optically observed with microscopy, and the hardness of the steels were calculated. The same goes for the Strenx 700MC steel, except a lower number of samples were chosen, amounting to 8 in total.

Figure 29 shows that as-received High Aluminum Steel has an average hardness of 289 HV0,5. The Austempered samples have a microhardness with an average between 286 and 338 HV0,5. The Quenched and Partitioned steels have two different averages for microhardness, some have hardnesses over 300 HV0,5 that for QP1, QP2, QP4, QP5 and QP6 whereas others have a microhardness lower. QP3 and QP5 both have a lower microhardness of 251 HV0,5 and 219 HV0,5 respectively. As for the Quenched and Tempered samples, QT1 has a relative low hardness of 285 HV0,5 whereas QT2 has a relatively higher average of 326 HV0,5.

400 338 314 350 310 326 304 321 325 285 307 289 286 300 275 251 250 219 200

150

Microhardness HV0,5 100

0 As received Aust1 Aust2 Aust3 QT1 QT2 QP1 QP2 QP3 QP4 QP5 QP6 QP7 QP8

Figure 29: Microhardness results of as-received, Quenched and Partitioned, Quenched Tempered and Austempered steels.

31 In general, the heat-treated High Aluminum Steel has a higher hardness than its as-received counterpart. However, there appears to be 2 outliers, namely QP3 and QP7, which are softer than the as-received material. For Austempered steels, the difference between AUST1, AUST2 and AUST3 is the isothermal temperature which increases from AUST1 to AUST3 (230° C to 270° C). It is mainly due to the isothermal temperature used for Austempering. The hardness of bainite depends on the temperature at which it forms. The isothermal temperature can even decide what type of bainite (e.g. lower or upper bainite) that is going to form within the Austempering treatment. The lower the formation temperature, the higher the hardness level would be. This can explain the corresponding results of Austempered steels. For Quenched and Tempered steels, the difference between QT1 and QT2 is the tempering temperature, which was 400 °C to 450 °C respectively. Figure 29 shows that the microhardness of QT2 is higher than QT1, which means that when the tempering temperature increases the microhardness increases. This result is not expected and seems to contradict previous published reports. As known from literature, when the tempering temperature increases the microhardness is facing a decrease [45]. One of the reasons that higher tempering temperature leads to higher hardness is the occurrence of the secondary hardening in the presence of strong carbide forming elements (e.g. Cr). To explain this result, complementary experiments are required and this issue remains an open question in this work. For Quenched and Partitioned steels the highest microhardness is achieved using the conditions with QP1, QP2, QP4 and QP6, and the lowest with QP3, QP7 and QP8. QP3, QP5 and QP7 have the same tempering time (100s) and yield a lower microhardness compared to QP4, QP6 and Q8 (which have the same tempering time of 50s). This shows a correlation showing if the tempering time increases, the microhardness decreases. The samples QP1, QP2, QP5 and QP6 (which have the same quenching temperature 175 °C) have a higher microhardness than QP3, QP4, QP7 and QP8 (which have the same quenching temperature of 165 °C). This means that the microhardness decreases when the quenching temperature increases. Samples QP1, QP3 and QP4 (which have the same partitioning temperature 450 °C) have a higher microhardness than QP5, QP7 and QP8 (which have the same partitioning temperature 400 °C). This implies the microhardness decreases when the partitioning temperature increases. However, there is an exception for this case where QP2 has lower microhardness than QP6. In general, Austempering increases the hardness of steel [46] because it forms a bainitic phase, which is a hard phase. Decreasing the isothermal temperature has been linked to an increase in the hardness value due to the formation of large amount of fine grained lower bainite alongside martensite[47]. Quenching and Partitioning improves the hardness of the materials. The hardness increase is directly proportional to the partitioning temperature due to the increased tempered martensite and the decrease in supersaturation of martensite with carbon[48]. When the quenching temperature increases, the amount of martensite formed decreases by the end of the quenching step. This leads to the formation of a large amount of retained austenite. Because the steel only contains a low amount of carbon, a big amount of austenite will not be stabilized during the partitioning step. For that reason, most of the austenite will transform into fresh martensite. This leads to increase in the hardness, where the microhardness at 175 °C is higher than the microhardness at 165 °C.

32 As-received and heat treated Strenx 700MC Figure 30 shows that the water quenched steel has the highest microhardness, with an average of 357 HV1. Surprisingly, the second hardest steel is the as-received Strenx 700MC with an average of 269 HV1. After Intercritical Heat Treatment, the steel had an average hardness of 240 HV1. The average hardness for Double Normalized heat treatment is 188 HV1. Furnace-cooled and air-cooled steels have the lowest microhardness with an average below 160 HV1.

400 357 350

300 269 253 239 250 236 188 200 167 152 150

100

Microhardness HV1 50

AR FC D-Norm AC IHT1 IHT2 IHT3 WQ

Figure 30: Microhardness results of as-received, furnace cooled, double normalized, water quenched and Intercritical heat treated Strenx 700MC.

A comparison between the heat treatments show that water quenching results in the hardest material and furnace cooling in the lowest. The resulting microstructure depends on the cooling rate, which is one of the key points in the determination of the hardness of steel. The faster the cooling rate, the harder the steel[49]. Air-cooled, furnace-cooled and double normalized steels have lower microhardness than in general. This result in a creation of a tough material with relatively low strength. This is due to their ferritic microstructure present in air-cooled, furnace-cooled, and double normalized samples[49]. On the other hand, Intercritical Heat Treated steels, shown in Figure 30, have the same average hardness, with low deviation. There is a difference in the temperature between IHT1-IHT3, and that might be the cause for the variation of hardness, but even if that is the case, the variation is minimal. Yet it can be generalized that when the intercritical temperature increases, the amount of austenite increases. Afterwards, when the sample is heated at the first heating step this will lead to formation of large amounts of martensite at the quenching temperature. This causes the steel to harden at higher intercritical temperatures[50]. Water quenched samples show the greatest hardness because of the high cooling rate. It causes the austenite to quickly transform into martensite, and when we have more of the material favoring martensite; the material becomes hard and brittle. The annealing can reduce its brittleness, but the amount of martensite does not decrease [51]. The hardness of the three Intercritical Heat Treated samples seems to be similar which can be explained based on the austenite condition and the phenomena taken place during the intercritical treatment. From one side, increasing the intercritical temperature leads to a higher fraction of austenite, which is going to transform to martensite afterward. In this way, the hardness of steel is supposed to increase. From the

33 other side, according to the Lever rule (section Error! Reference source not found.), increasing the intercritical temperature would reduce the carbon content of austenite. The lower carbon content of austenite/martensite results in lower hardness level. Therefore, these two factors act in a different way to determine the steel hardness.

3.3.2. Tensile test results In this part, all tensile test results will be calculated, tabulated, and then compared. This is done by plotting stress-strain curves, which provide the values for elongations and strength. Using this data, the heat treatment methods for Strenx 700MC and High Aluminum Steels will be analyzed and compared with each other in order to determine the promising heat treatment method for both steels. Also, the effect of temperature and time on the mechanical properties and phases will be taken into consideration during the discussion. As mentioned before in subsection 2.6.4 all elongation values are converted to A50 by using Oliver equation. As the original data of the tensile test is A25, the reader is kindly referred to appendices section where the original stress-strain curves are shown. The average elongation and yield strength for as- received and heat treated Strenx 700MC steels were based on two tested samples, whereas, the average values for as-received and heat treated High Aluminum Steel was calculated by using three tested samples. Tensile test Results of Alimak steels Alimak steels were considered to be the baseline for this project. However, there was no material data sheet available, so their properties were unknown. That is why it was chosen to compare heat treated samples to non-treated samples, rather than comparing the mechanical properties of treated samples to values in literature. Thus, new stress-strain curves were created for the materials provided by Alimak. This is in the favor of this project as it allows a comparison of all the materials on-site and in the lab, reducing the amount of discrepancies due to difference in measuring equipment. Figure 31 shows three stress-stain curves of the Alimak steels, designated S-355MC, S-420MC and S- 650MC. As seen in the figure the steel S-355MC has a high elongation of 25%, but has relatively low upper and lower yield strength (yield phenomena) with an average of 450 MPa. The steel S-420MC has relatively lower elongation, with an average of 24%, compared to S-355MC. Yet it boosts slightly higher upper and lower yield strength, with an average of 480 MPa. The steel S-650MC has low ductility compared to the two previous steels with an average of 16%, but it has good yield strength of 770 MPa. More details can be found in Table 8. As can be seen in the tensile curve, both steels (S-355MC and S-420MC) undergo the so-called yield point phenomenon which is usually observed in low carbon steels. This phenomenon is due to the formation of the Luders bands in such steels (especially mild steels) which is attributed to locking the dislocations by solute interstitial atoms (e.g. carbon) and releasing them by the stress applied. Hence, the stress drops to a lower level, known as the lower yield point. The occurrence of this phenomenon in materials can be considered a drawback since it leads to nonuniform deformation throughout the steel parts. One part of the sample yields while the other part is still in the elastic area, then the yielded zones expand towards the unyielded area [52].

34 1200

1000

800

600

400 Stress(MPa)

200

0 0 10 20 30 S-355MC S-420MC S-650 MC Strain (%)

Figure 31: Stress-strain curves of Alimak Steels (S-355MC, S-420MC and S-650MC).

Table 8: Ultimate tensile strength, upper and lower yield strength, and Elongations A50 of Alimak steels

Ultimate Tensile Upper yield Lower yield Total Elongation Grade of steel Strength (MPa) Strength (MPa) Strength (MPa) A50 (%) S-355MC 510 ± 11 453 ± 9 444 ± 7,5 25± 0.2 S-420MC 553 ± 11 484 ± 10 473 ± 11 24 ±0.6 S-650MC 833 ± 12 770 ± 7 16 ± 0.7

Quenching and Partitioning of High Aluminum Steel Time and temperature of the Quenching and Partitioning process are important variables affecting the microstructure and consequently, the mechanical properties of the High Aluminum Steel under investigation. In this study, eight combinations of these variables were selected, and the heat treatment cycles were designed based on them. This was executed in order to achieve a good combination of ductility and strength, where the requirement determined by industry should also be met. Figure 32 shows stress-strain curves of as-received High Aluminum Steel and four different Quenched and Partitioned High Aluminum Steels (QP1, QP2, QP3 and QP5). As-received High Aluminum Steel has a good yield strength with an average of 905 MPa but has a low ductility of 7%. However, Quenched and Partitioned High Aluminum Steels QP1, QP2, QP3 and QP5 have good ductility when compared to as- received steels with ductility of 19, 18, 16.5 and 16 % respectively. Also, they have good yield strengths of 669, 674, 703 and 678 MPa. The samples QP1 and QP2 show the best ductility in comparison to all the other experiments. More details can be found in Table 9.

35 1200

1000

800

600

400 Stress(MPa) 200

0 0 5 10 15 20 25 30 QP1 QP2 QP3 Strain (%)

Figure 32: Stress-strain curves of as-received steel and 4 different Quenched and Partitioned High Aluminum Steels: QP1, QP2, QP3 and QP5.

Table 9: Ultimate tensile strength, upper and lower yield strength and Elongations A50 of Quenched and Partitioned High Aluminum Steels

Ultimate Tensile Total Elongation A50 Sample Yield Strength (MPa) Strength (MPa) (%)

As-received 934 ± 6 905 ± 7 7 ± 0.2

QP1 884 ± 13 669 ± 19 19 ± 0.4

QP2 893 ± 10 674 ± 12 18 ± 1.5

QP3 918 ± 8 703 ± 11 16.5 ± 2.4

QP4 911 ± 3 694 ± 17 15 ± 0.8

QP5 913 ± 7 678 ± 3 16 ± 2.8

QP6 945 ± 11 680 ± 19 15 ± 0.7

QP7 929 ± 19 669 ± 21 15 ± 0.6

QP8 957 ± 17 703 ± 9 13 ± 0.2

Figure 33 shows the rest of the results of Quenched and Partitioned High Aluminum Steel (the samples QP4, QP6, QP7 and QP8). On one hand, QP4, QP6, QP7 and QP8 have good combination of ductility and strengths as shown in Table 9. On the other hand, these quenched steels have lower ductility when compared to QP1 and QP2. They make up for this with their higher ultimate tensile strength and yield

36 strength. These parameters are more than twice what is demanded for the application, so these steels could be suitable also for other applications requiring high strength steels. Even though these experiments did not reach the 22% elongation desired by the company, we can see a clear increase in both ultimate tensile strength and yield strength, while still retaining a considerable elongation. These values are comparable to the currently used S-650MC, and the QP samples provide better elongation. And since the elongation values are close to that of 22%, it is possible to further change and optimize this process in the future.

1200

1000

800

600

400

Stress(MPa) 200

0 0 10 20 30 Strain (%) QP4 QP6 QP7 QP8 Figure 33: Stress-strain curves of as-received steel and 4 different Quenched and Partitioned High Aluminum Steels: QP4, QP6, QP7 and QP8.

In order to better understand the effect of the heat treatment on the mechanical properties, the quenching temperature of the samples are compared. One set of samples have a quenching temperature of 165 °C and the other is 175 °C (seen in section 2.3.1.1). It can be deduced that the elongation is directly proportional to the quenching temperature. Another aspect to consider is the partitioning temperature. Since there are two distinct conditions for the partitioning temperature, the effect of increasing the partitioning temperature can be observed. Looking at the data in Table 9, we can see that the elongation increases when the partitioning temperature increases. Finally, the partitioning time can be varied to observe its effects on the material. By increasing the partitioning time from 50s to 100s, the materials exhibited an increase in ductility. This shows that there is a direct relationship between energy applied during heat treatment and the overall ductility achieved. Hence, the process can be economically optimized to get the best treatment for the material without spending too much money or energy. The quantity of fresh martensite affects mechanical properties of steels. The elongation decreases when the quantity of martensite increases. However, the strength increases when the amount of martensite increases. The samples QP1 and QP3 can be compared, where the only difference between them is their quenching temperature. Sample QP1 has less martensite than QP3, and consequently the elongation at 175 ⁰C is higher than the elongation at 165 ⁰C. The link between the elongation of the material and the partitioning step can be scientifically explained by the thermodynamics of the system. The higher the energy input to the sample, the more energy the

37 austenitic phase has. This leads to the austenitic phase being more stable, and increases the chance that it will be retained [53]. Since the austenitic phase is comparatively softer than martensite, it is more desirable to get this phase in order to improve the final material ductility [54]. From the results demonstrated above, it is deduced that the Quenching and Partitioning can be a good heat treatment process that will result in a good combination of ductility and strength where it can fulfill the targeted properties requested by the industry. Even though the elongation of the Quenched and Partitioned samples is a bit lower than the targeted value, they already possess higher values than the steels used previously (seen in section 3.3.2.1). It is worth mentioning again that these properties (of Quenched and Partitioned steels) have been obtained through the heat treatment process which is believed to be so much cheaper than the deformation processes. This is a good option when strength to price ratio is concerned.

Quenching and Tempering of High Aluminum Steel Three tensile tests were performed for each different tempering condition of High Aluminum Steels. The stress-strain for the best curves are plotted in Figure 34. The three values have been averaged in order to get the numerical result shown in Table 11. In general, mechanical properties of Quenched and Tempered High Aluminum Steels were increased as a result of the heat treatment. Firstly, the ultimate tensile strength is higher when compared to the as-received steel. Furthermore, the ultimate tensile strength is higher when compared to Quenched and Partitioned High Aluminum Steels. The yield strength shows a similar result, where Quenching and Tempering gives better values when compared to Quenching and Partitioning. The tempering process also manages to increase the ductility of the samples. When compared to the as-received material, we can see a clear increase in ductility. However, when compared to Quenched and Partitioned steel, this increase is relatively low. Even though tempering can increase the ductility of a material from ~7% to ~10%, this increase is not enough for the desired application. The ductility fails to reach half the set value and thus steels in this condition must be reconsidered for other use. Nevertheless, this heat treatment shows an increase in both ductility and strength. More details on the change in these properties will be given below.

1200

1000

800

600

400 Stress(MPa) 200

0 0 10 20 30 As received QT1 QT2 Strain (%) Figure 34: Stress-strain curves of as-received steel and two different Quenched and Tempered steels: QT1 and QT2.

38 Table 10: Ultimate tensile strength, yield strength and Elongations A50 of as-received two different Quenched and Partitioned High Aluminum Steels: QT1 and QT2

Ultimate Tensile Total Elongation A50 Sample Yield Strength (MPa) Strength (MPa) (%) As-received 934 ± 6 905 ± 7 7.09 ± 0,2 QT1 1070 ± 2 871 ± 4 9.61 ± 0,3 QT2 1015 ± 5 872 ± 3 10.59 ± 0,3

In literature, it can be seen that by increasing the tempering temperature higher ductility can be achieved together with lower tensile strength [55]. This is due to the transformation of fresh martensite into tempered martensite [56]. The higher temperature helps the diffusion of carbon, which promotes the transformation process. However, since the tempering time that was chosen for this project was limited to 5 minutes, it is possible that the process was not carried out to its full extent. It is advised to further investigate the addition of longer tempering time, as it can improve the ductility of the material. This can be used when creating a material which meets the company’s specifications in the future. In the case that retained austenite exists in the microstructure of the quenched part, diffusion of carbon or precipitation of carbides (during tempering) would lower the carbon content of retained austenite. In this way, the Ms temperature of austenite increases. Therefore, when the steel is cooled from the tempering temperature, the retained austenite would feasibly transform to martensite and reduce the ductility of the steels. On a final note, the properties of Quenched and Tempered materials are rather similar to that of Dual Phase DP steels [57]. They exhibit high strength with relatively good ductility and are composed of the same iron allotropes. Even though these experimental values fail to meet the demands of the company, they might find use in other branches, such as the automotive industry.

Austempering of High Aluminum Steel For the Austempering process, three sets of conditions were tested with three samples in each. This was then compared to the as-received High Aluminum Steel, and the best of each set was plotted in Figure 35 in form of a stress-strain curve. The average for each condition was calculated and plotted in Table 11. Figure 35 shows the tensile test results of three conditions of Austempered High Aluminum Steels: AUST1, AUST2 and AUST3. AUST1 has a lower ductility of 13%, but a relatively high yield and tensile strength. Aust3 has the highest ductility together with the lowest strength. As mentioned before the difference between AUST1, AUST2 and AUST3 is the isothermal hold temperature. It can be deduced that an increase in isothermal hold temperature would increase the ductility of the material, while sacrificing its strength. In general, Austempered steels have a good combination of strength and ductility. In Figure 35 the mechanical properties of Austempered steels are higher than the base material. They also show a higher elongation when compared to Quenched and Tempered steels. However, the maximum ductility achieved at the end of the experiments is still lower than that of Quenched and Partitioned steels. It can also be observed that there is a large difference between the yield strength and tensile strength. This value is commonly denoted as the yield to tensile ratio. This value is important when considering the structural integrity of the material [58]. Elastic deformation is preferred over plastic deformation, thus it is better to

39 have a material where the yield to tensile ration is equal to 1. However, the overall improvement in the mechanical properties is attributed to the composition of retained austenite.

1200

1000

800

600

400

Stress Stress (MPa) 200

0 0 5 10 15 20 25 30 As received Aust 1 Aust 2 Aust 3 Strain (%)

Figure 35: Stress-strain curves of as-received steel and two different Austempered High Aluminum Steels: AUST1, AUST2 and AUST3.

Table 11: Ultimate tensile strength, yield strength and Elongations A50 of as-received and three different Austemeperd steels High Aluminum Steels: AUST1, AUST2 and AUST3

Ultimate Tensile Yield Strength Total Elongation Sample Strength (MPa) (MPa) A50 (%) As-received 934 ± 6 905 ± 7 7 ± 0.2 AUST1 1058 ± 13 640 ± 14 13 ± 1 AUST2 1063 ± 3 608 ± 4 14 ± 1 AUST3 1027 ± 4 581 ± 4 15 ± 1

Through the experiments, it can be seen that the strength and the fracture toughness of the material decreases when the austempering temperature increases. On the other hand, there is an increase in ductility which is more important for the company. This is achieved by a twofold process during Austempering. Firstly, the increase in austempering temperature affects the recrystallization process, where a finer microstructure can be produced. Secondly, the addition of energy helps the redistribution of the carbon atoms, which can be used to promote the creation and retention of the austenitic phase. This phase can desirable as it increases both ductility and tensile strength of the final material. Austenite has a face centered cubic (FCC) structure and is softer than martensite. The presence of a larger volume fraction of austenite will reduce the strength and increase the ductility of the material [59].

40 Usual heat treatments of Strenx 700MC The heat treatment of the Strenx 700MC samples is separated into two sections. Usual heat treatments and Intercritical Heat Treatment. The Intercritical Heat Treatment will be described in next section. Each of these processes have their own conditions that can be found in sections 2.3.2.1 and 2.3.2.2 respectively. As mentioned in materials in section 2.3.2, a total of 8 samples were tested: as-received, air-cooled, furnace-cooled, and double normalized Strenx 700MC steels. The temperatures and times for the quench were considered equal, with the only thing being different is the medium used to cool it down. After tensile test experiments, the results of stress-strain curves are plotted in Figure 36. The figure shows elongations, and strengths of as-received, air cooled, furnace cooled and double normalized Strenx 700MC. As-received Strenx 700 has a good combination of ductility and strength due to its microstructure. This is due to the prior work hardening during the cold rolling of the product. Air cooled, furnace cooled and double normalized Strenx 700MC show good elongation values of 28, 30 and 26% respectively. Also the yield strength is reasonable with values of 372, 434 and 462 MPa. These materials show promise as they fulfill Alimak’s requirements to be used in the construction of elevators. The numerical values for the mechanical properties can be seen in Table 12 .

1200

1000

800

600

400 Stress(MPa) 200

0 0 5 10 15 20 25 30 As received AC FC D-Norm Strain (%)

Figure 36: Stress-strain curves of as-received steel and three different usual heat treated steels: Air Cooled (AC), Furnace Cooled (FC) and Double Normalized (D-Norm) steels.

Table 12: Ultimate tensile strength, yield strength and Elongations A50 of as-received Strenx 700MC and three different usual heat treated steels: Air Cooled (AC), Furnace Cooled (FC) and Double Normalized (D-Norm) steels

Ultimate Tensile Yield Strength Total Elongation Sample Strength (MPa) (MPa) A50 (%) As-received 816 ± 8 747 ± 1 20 ± 0.1 D-Norm 541 ± 0.2 462 ± 1 26 ± 0.2 FC 549 ± 9 434 ± 1 30 ± 1 AC 498 ± 3 372 ± 1 28 ± 2

41 As mentioned before in the microstructure characterization part, these steels consist of a mainly ferritic microstructure. Ferrite is a soft phase can be attributed to good ductility and toughness [60]. Soft allotropes of iron usually come at a sacrifice in strength. Ductility is the most crucial property for the design specification and the ductility achieved in this work is higher than the company’s currently used steel. So air-cooling, furnace-cooling, and double normalizing treatments can be used in the company in order to create the new desired product. Furthermore, the overhead cost for these heat treatments are relatively low, and would not have such a big impact. It should be mentioned that the studied steel (in the heat treated conditions) underwent the yield point phenomenon (Figure 36). As was described above, this is a usual phenomenon taken place in low carbon and low alloy steels which is due to the formation of the Luders bands. The ones who search for uniform deformation of the steel part, this steel and the regarded heat treatment should be selected with caution. The formation of Luders bands brings about the nonuniformity in deformation; a part of the sample faces the yield point while there are still unyielded parts. The deformed area expands towards the unyielded part covering all the sections of the sample [52]. Among these heat treatment cycles, it seems that double normalizing results in better properties. It is due to the refinement of the grains in the first step of austenitization and quenching. For the same reason, the regarded sample possesses a proper elongation (Table 12). Figure 37 shows the results of tensile tests for the water-quenched samples. The graphs presented in this figure cannot be considered as standard/normal tensile curves. This is likely due to the improper water quenching procedure executed. Due to the shape of the steel samples (sheet), they should be vertically quenched in the quenching medium. Otherwise the steel sheet would be distorted because of the nonuniform stresses in the sides of the sheet. Therefore, when applying the tensile load, the shear stresses would build up in the sample, causing the strange tensile behavior of the steel parts. All the tensile test samples broke at different locations, and therefore no trend can be found among the samples and the results cannot be discussed further.

800 700 600 500 400 300 Stress(MPa) 200 100 0 0 1 2 3 4 5 6 7 WQ1-S2 WQ2-S1 WQ2-S2 WQ1-S1 Strain (%)

Figure 37: Stress-strain curves of 4 different samples of Water quenched Strenx 700MC.

42 Intercritical Heat Treatment of Strenx 700MC The tensile tests for intercritical heat treated samples were performed on a total of 6 different times in order to assess its mechanical properties. These include three main conditions for the Intercritical Heat Treatment method. Figure 38 shows stress-strain curves of the best of the three intercritical heat treated Strenx 700MC samples; IHT1, IHT2 and IHT3. The data is collected in Table 13. The samples IHT1 and IH3 have a good ductility with an average of 20%. These materials can also reach relatively good yield strength higher than 600MPa. However, IHT2 has the lowest amount of ductility among these conditions and an intermediate tensile and yield strength. The yield strength decreases from 701MPa (IHT1) to 625MPa (IHT3) when the intercritical temperature increases from 725⁰C to 775⁰C. This shows a reverse correlation between the intercritical heat treatment temperature and the material’s strength. However, the elongation doesn’t seem to be affected as much by these changes in temperature.

1200

1000

800

600

400 Stress(MPa) 200

0 0 10 20 30 IHT1 IHT3 IHT2 Strain (%)

Figure 38: Stress-strain curves of three different intercritical heat treated (IHT) Strenx 700MC steels: IHT1, IHT2 and IHT3.

Table 13: Ultimate tensile strength, yield strength and Elongations A50 of three different intercritical heat treated (IHT) Strenx 700MC steels: IHT1, IHT2 and IHT3

Ultimate Tensile Yield Strength Total Elongation Sample Strength (MPa) (MPa) A50 (%) IHT1 782 ± 6 701 ± 0.7 19 ± 0.4 IHT2 736 ± 13 668 ± 3 17 ± 0.1 IHT3 687 ± 0.4 624 ± 8 20 ± 1

During Intercritical Heat Treatment, the increase in temperature can lead to an increase in the martensite phase formation. This leads to higher strengths while decreasing the ductility [61]. The changes in the mechanical properties can be predicted from the amount of ferrite through the use of iron-carbon phase diagrams. Even though these values would not give numerical solutions, they will still provide an estimate for the ductility.

43 The yield strength decreases from 701MPa (IHT1) to 625MPa (IHT3) when the intercritical temperature increases from 725⁰C to 775⁰C. This was not expected since an increase in the intercritical temperature should lead to an increase in the austenite, which during quenching step transforms to martensite and thereby the strength would be expected to increase. The reason for the decrease in yield strength is probably due to a decrease in the martensite hardness (or strength) when higher intercritical temperature has been employed. It is believed that the carbon content of austenite/martensite would decrease as the intercritical temperature increases. The decrease in yield strength can also be due to the heat treatment procedure performed on this steel as well as the phenomena taking place during the tempering stage. It could be suggested to not perform tempering treatment for the future work, since the steel under investigation is a low carbon steel and there is a chance of auto-tempering of martensite. The intercritical heat treatment is known to eliminate the yield point phenomenon from the tensile curve as it induces free dislocations in the microstructure, which in turn would surpass the yield point phenomenon. It is another reason why, for future work, we suggest not to perform tempering on intercritically heat treated steels; during tempering stage, the dislocations are again pinned by the interstitial atoms (e.g. carbon) and the yield point phenomenon would appear in the tensile test afterwards. Besides, the hardenability of such steels is very low, therefore, a delay (not accelerated) in quenching would lead to different results from what have been expected. Compared to the previous treatments applied on this steel (e.g. normalizing), it seems that the DP steels produced have a good combination of elongation and strength. This is attributed to the good formability as well as good work hardenability of ferrite coexisting with martensite [62]. Apart from the factors mentioned above, one can say that this steel is micro-alloyed by V and Nb [63]. Therefore, the precipitation behavior is supposed to affect the mechanical properties significantly. The intercritical temperature would change this behavior as it might cause a partial dissolution of micro- alloying precipitates. To study the micro-alloying precipitation, a deeper study by means of transmission electron microscopy seems to be essential.

44 4. Conclusions

In QP, when the quenching temperature increases from 165 ⁰C to 175 ⁰C, the ductility of High Aluminum Steel increases. This is a desirable outcome because it can be used to construct elevator parts which have good yield strength and also contain a relatively good ductility. This is important because this structural steel increases the safety of the components by being above the allotted margin for mechanical properties, whilst it can be treated to be corrosion resistant, temperature resistant and even fracture resistant. In QP, when the tempering temperature increases from 400 ⁰C to 450 ⁰C, the ductility of High Aluminum Steel increases. This proves there is a direct relation between the heat treatment temperature and the mechanical properties that we get. This is also enforcing the previous point in Quenching and Partitioning. To improve this hypothesis, extra experiments can be made to find the fall-off point and even find a cost- benefit correlation. In QP, when the tempering time increases from 50 s to 100 s, the ductility of High Aluminum Steel increases. This can be attributed to the material having less carbon being dissipated in the martensitic phase. The martensitic phase can be considered a hard and brittle phase, and even though it can add a lot to the structural strength, when talking about ductility it is not wanted. During stress brittle materials absorb a low amount of energy making it easily breakable, and brittle fracture is not desired as a failure method of structural materials. The best heat treatment for High Aluminum Steel is Quenching and Partitioning, because it takes less time that the other heat treatments and also consumes less energy. This makes it seem like it can be used in industry and this aspect is very promising. Even if the material specifications are different, it could possibly be used as an economically viable substitute. Furthermore, this heat treatment resulted in a multiphase material, where the austenitic and martensitic phases work in conjunction to give the best mechanical properties of each one of them. Good ductility with good yield strength were found by using Intercritical Heat Treatment at 775⁰C and 725 ⁰C. With this heat treatment, the resulting material showed better properties than the base material. This can be attributed the materials’ fine structure which improved its properties. Since this steel is commercially available, it is possible to take this material and improve its properties, and finally, sell it with a profit. Air cooled, furnace cooled, and double normalized steels have achieved the desired objectives, of 22 % in ductility and more than 350 MPa in yield strength. Since these steels fit the criterion given by Alimak, these steels can be the answer for the company’s search for new steels. Since these experiments were held in lab scale, some consideration should be taken on how to scale up this process. Pilot plants can be established in the factories where if the material produced the same result, it can be sold off as a new product. Since these steels are ductile, it is relatively easy to shape and form them, meaning the processes required to mold and to work these materials could be considerably shorter. On the other hand, these steels present a discontinuous yielding (yield point phenomena) which can lead to industrially undesirable problems, where the material changes its microstructure and can have adverse effects. That means showing a localized heterogeneous transition from elastic to plastic deformation that might affect the mechanical properties of theses steels during the manufacturing process. Good ductility and yield strength can be achieved by using the following condition of: QT = 175 ⁰C, TT = 450 ⁰C, Tt = 100 s, for Quenching and Partitioning. These steels perform better than those of Alimak (S- 650 MC) in terms of ductility, and even better results for tensile and yield strength. Furthermore, due to its chemical composition it can be galvanized, which gives better resistance to weathering effects.

45 Quenching and Partitioning heat treatment might be considered the best heat treatment that SSAB can use to improve the mechanical properties of High Aluminum Steel, which is currently under development. This is because this heat treatment can improve ductility, which is very important in metalworking. These aspects include, mold ability and workability, and ductile materials can be further hardened via work hardening to increase their strength, if desired. Furthermore, malleable materials require softer tools which have to be replaced at longer intervals which can result in economic benefit. Finally, Quenched and Tempered materials generally result in less ductile materials when compared to the other processes. This is not the recommended properties, thus it is better to avoid of this heat treatment method in future projects, as the material which undergoes this process can be seen as inferior compared to all the other heat treatment methods presented in this project.

5. Future Work

The suggested work for future projects for Alimak’s application involves further development of the Quenching and Partitioning heat treatment for High Aluminum Steels as it showed promising results. The suggestion for the QP heat treatment are the following: 1. Increasing quenching temperature (between 175 ⁰C and 204 ⁰C). 2. Increasing partitioning temperature (more than 450 ⁰C). 3. Increasing partitioning time (more than 100s). It would also be of interest to try more Austempering heat treatments for High Aluminum Steel, especially with increased isothermal temperature (more than 270 ⁰C) to increase the ductility further. The holding time of the Austempering should also be prolonged. For the steel Strenx 700MC a suggestion to improve the strength is to use water quenching instead of air- or furnace cooling. This will increase the amount of martensite and consequently improving the strength of the steel. In order to investigate the microstructure more in-depth the use of electron microscopy (transmission electron microscopy and high-resolution scanning electron microscopy) is recommended together with electron back-scatter diffraction (EBSD). Fractography of the tensile tested samples would be interesting in order to describe the crack propagation in the heat treated samples.

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50 Appendix Stress-strain curves with elongation A25 Stress-strain curves of Alimak steels

600

500

400

300

200

Stress Stress (MPa) 100

0 0 10 20 30 40 S355MC-S1 S355MC-S2 Strain (%) S355MC

600

500

400

300

200

Stress Stress (MPa) 100

0 0 5 10 15 20 25 30 35 S420MC-S1 S420MC-S2 Strain (%) S420 MC

900 800 700 600 500 400

Stress Stress (MPa) 300 200 100 0 0 5 10 15 20 25 S650MC-S1 S650MC-S2 Strain (%) S650 MC

Stress-strain curves of High Aluminum Steel

1000 900 800 700 600 500 400

300 Stress Stress (MPa) 200 100 0 0 2 4 6 8 As-recieved-S1 As-recieved-S2 As-recieved-S3 Strain (%) As-received

1000 900 800 700 600 500 400 300

Stress Stress (MPa) 200 100 0 0 5 10 15 20

QP1-S1 QP2-S2QP1-S2 QPQP3-S31-S3 Strain (%) QP1

1000

800

600

400 Stress Stress (MPa)

200

0 0 5 10 15 20 25 QP2-S1 QP2-S2 QP2-S3 Strain (%) QP2

1000 900 800 700 600 500 400 300

Stress Stress (MPa) 200 100 0 0 5 10 15 20 QP3-S1 QP3-S2 QP3-S3 Strain (%) QP3

1000 900 800 700 600 500 400

300 Stress Stress (MPa) 200 100 0 0 5 10 15 20 QP4-S1 QP4-S2 QP4-S3 Strain (%) QP4

1000 900 800 700 600 500 400 300

Stress Stress (MPa) 200 100 0 0 5 10 15 20 QP5-S1 QP5-S2 QP5-S3 Strain (%) QP5

1200

1000

800

600

400 Stress Stress (MPa) 200

0 0 5 10 15 20 QP6-S1 QP6-S2 QP6-S3 Strain (%) QP6

1200

1000

800

600

400

Stress Stress (MPa) 200

0 0 5 10 15 20 Strain (%) QP7-S1 QP7-S2 QP7-S3 QP7

1200

1000

800

600

400

Stress Stress (MPa) 200

0 0 2 4 6 8 10 12 14 QP8-S1 QP8-S2 QP8-S3 Strain (%) QP8

54 1200

1000

800

600

Stress Stress (MPa) 400

200

0 0 2 4 6 8 10 12 Strain (%) QT1-S1 QT1-S2 QT1-S3 QT1

1200

1000

800

600

400

Stress Stress (MPa) 200

0 0 2 4 6 8 10 12 QT2-S1 QT2-S2 QT2-S3 Strain (%) QT2

55 1200

1000

800

600

400 Stress Stress (MPa)

200

0 0 5 10 15 AUST1-S1 AUST1-S2 AUST1-S3Strain (%) AUST1

1200

1000

800

600

400 Stress Stress (MPa)

200

0 0 5 10 15 20 AUST2-S1 AUST2-S2 Strain (%) AUST2

1200

1000

800

600

400 Stress Stress (MPa) 200

0 0 5 10 15 20 AUST3-S1 AUST3-S2 AUST3-S3 Strain (%) AUST3

Stress-strain curves of Strenx 700MC

900 800 700 600 500 400

Stress(MPa) 300 200 100 0 0 5 10 15 20 25 As-received-S1 Strain (%) As-received

600

500

400

300

Stress(MPa) 200

100

0 0 5 10 15 20 25 30 35 D-NORM-S1 D-NORM-S2 Strain (%) D-Norm

600

500

400

300

Stress(MPa) 200

100

0 0 10 20 30 40 FC-S1 FC-S2 Strain (%) Furnace cooling

600

500

400

300 Stress(MPa) 200

100

0 0 10 20 30 40

AC-S1 AC-S2 Strain (%) Air cooling

900

800

700

600

500

400

Stress(MPa) 300

200

100

0 0 5 10 15 20 IHT1-S1 IHT1-S2 Strain (%) IHT1

58 800

700

600

500

400

Stress(MPa) 300

200

100

0 0 5 10 15 20 IHT2-S1 IHT2-S2 Strain (%) IHT2

800

700

600

500

400

300 Stress(MPa)

200

100

0 0 5 10 15 20 25 IHT3-S1 IHT3-S2 Stain (%) IHT3