Development of Test Methods for Detection of Hydrogen Embrittlement in High-Strength Steel

2009:169 CIV MASTER'S THESIS

Mattias Lindvall

Luleå University of Technology MSc Programmes in Engineering Engineering Physics Department of Applied Physics and Mechanical Engineering Division of Engineering Materials

2009:169 CIV - ISSN: 1402-1617 - ISRN: LTU-EX--09/169--SE PREFACE

This master’s thesis is the final part of my master of science in physics engineering degree from Luleå University of Technology. The project has been conducted in cooperation with the company Gestamp HardTech AB during 2009.

First of all I would like to thank my supervisors at Gestamp HardTech – Katarina Lindström and Håkan Andersson – and express my gratitude for their valuable guidance during the project. In addition, I would also like to thank my examiner Esa Vuorinen for all the fruitful discussions that we have had. Last but not least I would like to thank everybody at Gestamp HardTech and Luleå University of Technology who have put time and energy to facilitate my work.

Luleå, November 2009 Mattias Lindvall Mattias Lindvall

ABSTRACT

A number of new mechanical test methods are developed and evaluated with the aim of detecting hydrogen embrittlement in press-hardened martensitic steel. The test methods are uncomplicated in the sense that they are simple to conduct, does not require advanced equipment and the results are easily interpreted. Evaluation of the test methods is done by testing specimens with different hydrogen contents. Thus, it is possible to determine hydrogen’s role in the results and draw conclusions whether hydrogen embrittlement has occurred. Among the evaluated methods, the triangular wedge test method and the conical wedge test method detect hydrogen embrittlement successfully within a week. Both methods are based on that a wedge is pressed into a hole in the specimen which creates tensile stresses in the specimen. The load is maintained for 4-6 days which normally leads to delayed failure for specimens with moderate and high hydrogen contents while the specimens with low hydrogen contents remain intact.

SAMMANFATTNING

Ett antal nya testmetoder har under projektets gång utvecklats och utvärderats med målsättningen att detektera väteförsprödning i presshärdat martensitiskt stål. Metoderna är okomplicerade i den meningen att de är enkla att utföra, inte kräver avancerad utrustning och att resultaten är lätta att tolka. Utvärdering av metoderna görs genom att testa prover med olika vätehalter. Därigenom kan vätes roll i resultaten bestämmas och slutsatser kan dras om huruvida väteförsprödning har skett. Bland de utvärderade testmetoderna lyckades det triangulära kiltestet samt det koniska kiltestet detektera väteförsprödning inom en vecka. De både metoderna är baserade på att en kil intrycks i ett hål i provet vilket orsakar dragspänningar i materialet. Belastningen behålls under 4-6 dagar vilket normalt sett leder till fördröjt brott för prover med medelhöga och höga halter av väte, medan prover med låg vätehalt inte spricker.

III

TABLE OF CONTENTS

PREFACE ...... I ABSTRACT ...... II SAMMANFATTNING ...... III TABLE OF CONTENTS ...... IV 1 INTRODUCTION ...... 1 1.1 Background ...... 1 1.2 Purpose ...... 1 1.3 Methodology ...... 1 1.4 Limitations ...... 1 2 THEORY ...... 2 2.1 Hydrogen Embrittlement ...... 2 2.1.1 Conditions ...... 2 2.2 Types of Hydrogen Embrittlement ...... 3 2.3 Embrittling Models ...... 4 2.3.1 Hydrogen Enhanced De-Cohesion (HEDE) ...... 4 2.3.2 Hydrogen Enhanced Localized Plasticity (HELP) ...... 4 2.4 Hydrogen Absorption ...... 4 2.5 Prevention of Hydrogen Embrittlement ...... 6 2.6 Stress Corrosion Cracking ...... 6 2.7 Cathodic Potential ...... 7 2.8 Mechanical Test Methods ...... 8 2.8.1 Constant Load Test ...... 8 2.8.2 The Cantilever Beam Test ...... 9 2.8.3 Constant Strain-Rate ...... 9 2.8.4 Bending Tests ...... 10 2.8.5 The Wedge Opening Load Test ...... 11 2.8.6 The Disk-Pressure Testing Method ...... 11 2.8.7 C-Ring Test ...... 12 2.8.8 Strip Test ...... 13

2.8.9 Stressed O-Ring Test ...... 13 2.8.10 Bar-Bending Test ...... 14 2.8.11 Stepwise Increased Load ...... 14 2.9 Chemical Test Methods ...... 15 2.9.1 Hydrogen Extraction ...... 15 2.9.2 Electrochemical Method ...... 15 2.9.3 Electrochemical Hydrogen Probe ...... 15 2.10 Aggressive Environment ...... 16 3 METHODS ...... 19 3.1 Material ...... 19 3.1.1 Hydrogen Contents ...... 19 3.1.2 Micro-Structure ...... 20 3.1.3 Hardness ...... 20 3.2 Test Methods ...... 20 3.2.1 Stamped Hole ...... 20 3.2.2 The Curved Coin Test ...... 22 3.2.3 The Triangular Wedge Test ...... 22 3.2.4 The Conical Wedge Test ...... 23 3.2.5 Reference Tests ...... 25 3.3 Fracture Mode Characterisation ...... 26 4 RESULTS ...... 27 4.1 Material ...... 27 4.1.1 Hydrogen Contents ...... 27 4.1.2 Micro-Structure ...... 28 4.1.3 Hardness ...... 28 4.2 Test Methods ...... 29 4.2.1 Stamped Hole ...... 29 4.2.2 Curved Coin ...... 30 4.2.3 Triangular Wedge ...... 30 4.2.4 Conical Wedge ...... 31 4.2.5 Reference Tests ...... 32 4.3 Fracture Mechanism Characterisation ...... 33 DISCUSSION ...... 35

CONCLUSIONS ...... 38 FUTURE WORK ...... 39 REFERENCES ...... 40 APPENDICES ...... 43

Chapter 1 INTRODUCTION

1.1 Background The demand for high-strength steel is increasing thanks to its high performance in combination with relatively low weight. The material is commonly used in cars, bridges and other fields where a good strength to weight ratio is desired. However, the service life of many high-strength steel components is reduced by unexpected failure caused by the degrading phenomena hydrogen embrittlement.

Gestamp HardTech AB is one of the world leaders in producing safety components for the automotive industry. The company is located in Luleå and has other press-hardening production facilities in USA, Germany, Spain and China. Gestamp Automoción, which is a global group with focus on components and systems for the automotive industry, acquired HardTech in 2005. At the end of year 2007, Gestamp HardTech had 691 employees and a turnover of 1.219.800.000 SEK. The produced safety components are manufactured from high-strength steel which is formed and hardened in a process called press-hardening; a technique originally developed and patented by the company. During unfavourable conditions at the press-hardening process, hydrogen diffuses into the steel products. As a consequence, the products may experience degradation of mechanical properties due to hydrogen embrittlement. From a quality perspective it is essential to utilize accurate test methods to ensure that the products are not embrittled.

1.2 Purpose The aim of this thesis is to develop a test method which can be used for determining when there is risk for delayed fracture caused by hydrogen embrittlement in the produced safety components. Preferably, two test methods are developed; one that is quick and make a coarse detection and one that is more precise in detection but with longer duration.

1.3 Methodology The first step of this project was to conduct a literature review in order to learn about hydrogen embrittlement and also to get a picture of the existing test methods. From that point, several new test methods were developed and evaluated.

1.4 Limitations The theoretical part of this report mainly treats hydrogen embrittlement in high-strength steel since that is the material used by Gestamp HardTech. The development of test methods were limited as the methods should be easy to conduct, not require advanced equipment, and with simply interpreted results. Furthermore, the test methods’ results must be obtained within a week because that is the time that the products are stored before being sent to customers.

Chapter 2 THEORY

A literature review was conducted to gain knowledge about hydrogen embrittlement as well as the existing test methods which detect the phenomena. The findings from the review are presented in this chapter.

2.1 Hydrogen Embrittlement Hydrogen embrittlement is a term for several different types of fracture phenomena caused by hydrogen. It is most encountered in high-strength steels, but also occurs in other metal alloys such as stainless steel, nickel, titanium, aluminium and copper. The effects of the embrittling process are degradation of mechanical properties or delayed failure which can occur due to internal residual stresses or applied external stresses [1]. The most apparent mechanical degradation is that normally ductile materials become brittle. In addition, small changes are observed in the elastic behaviour as well as the yield strength [2].

Hydrogen embrittlement cannot be identified through fractography since the fracture mode can be either transgranular or intergranular. However, for delayed failure in steels the fracture mode is typically intergranular [3]. In martensitic high-strength steels, cracks are often observed along prior austenite grain boundaries [4]. The degradation of mechanical properties is enhanced by slow strain rates which suggest time-dependent diffusion as a controlling factor. Moreover, embrittlement is usually most severe at room temperature. At higher temperatures the hydrogen diffuses away from the material [5].

2.1.1 Conditions Three conditions must be fulfilled for the degradation of mechanical properties caused by hydrogen embrittlement:

• A material that is susceptible for hydrogen embrittlement. • Hydrogen either pre-existing in the alloy or in the service environment. • Tensile stresses.

In addition, the process is time-dependent since the embrittling is associated with hydrogen diffusion [6]. Furthermore hydrogen embrittlement is temperature dependent since the hydrogen mobility changes with temperatures; it is reduced as the temperature decreases and vice versa [3].

The risk for hydrogen embrittlement is controlled by several material factors. It rises with increasing levels of the material’s strength and hardness. General minimum threshold values for which steels can be embrittled are tensile strength of 800 MPa and hardness of 235 HB

[7]. However, the susceptibility to hydrogen embrittlement differs by alloy composition and microstructure for steels of the same strength level [4].

For hydrogen embrittlement to take place, the material must contain hydrogen. Hydrogen can be absorbed into the steel during service or during processing steps like melting, pickling, plating and welding. Furthermore, corrosion processes can embrittle steels as hydrogen can enter the metal lattice by any corrosive reaction involving hydrogen ion or water reduction as one of the cathodic reactions [8].

Static tensile stresses are required for the occurrence of hydrogen embrittlement. The stresses can be either residual or applied externally. With increasing stress magnitude, the risk for hydrogen embrittlement rises. High stress-concentrations arising from poor geometric design enhance the residual stresses and consequently also the risk of hydrogen embrittlement [6].

2.2 Types of Hydrogen Embrittlement Hydrogen embrittlement is classified into three principally different types:

• Internal Reversible Embrittlement (IHE) is the most common type of hydrogen embrittlement. Hydrogen is already present in the material as it has been absorbed during any process step. Typically the steel fractures when under static tensile stress. The mechanical effects may range from loss of ductility to delayed cracking. The embrittlement is reversible since the material’s original properties can be restored by removing the hydrogen through heat treatment provided that no internal micro-cracks have already been initiated. Internal reversible hydrogen embrittlement is also known as slow strain rate embrittlement and delayed failure [5]. • Hydrogen Environment Embrittlement (HEE) occurs in hydrogen-free materials utilized in hydrogen containing environments. Molecular hydrogen is adsorbed to the metal’s surface, and atomic hydrogen can be absorbed into the lattice [9]. • Hydrogen Reaction Embrittlement (HRE) takes place when hydrogen reacts chemically with the metal material. The reactions result in hydrides which precipitate in the matrix and degrade the mechanical properties of the material. This type of embrittlement does not occur in steels [10].

2.3 Embrittling Models Several different embrittling models are established, but neither fully explain the degradation mechanism itself. Instead, a combination of different models describes hydrogen embrittlement. At present, the HEDE and HELP models are the most established [12]. The mechanisms of hydrogen embrittlement have been studied extensively during the years, but the concept is yet not fully understood. Some reasons behind the analysis difficulties are that the dominating mechanisms differ for different materials, hydrogen’s low detection sensitivity, and the high mobility of hydrogen [11].

2.3.1 Hydrogen Enhanced De-Cohesion (HEDE) The de-cohesion theory is based on that absorbed hydrogen lowers the binding force of the metal atoms in the lattice. The damaging effects are believed to arise by that electrons from the hydrogen atoms enter the metallic atoms’ d-band which increases the electron concentration. Thus, repulsive forces are created between the metallic atoms and the lattice’s cohesive strength is lowered. Consequently less energy is needed for crack initiation and propagation [13].

2.3.2 Hydrogen Enhanced Localized Plasticity (HELP) Hydrogen accumulates in areas in the structure where stress-fields are present. The enhanced levels of hydrogen causes increased dislocation mobility in these areas. Consequently, the plastic flow rate associated with crack propagation is raised [6].

2.4 Hydrogen Absorption Hydrogen is present in molecular form in the most environments. Due to its relatively large size, the molecular hydrogen cannot dissolve in steel. However, hydrogen atoms are small enough to diffuse into the metal lattice. Thus, for hydrogen absorption the molecules must first dissociate to atomic form at the metal’s surface. The efficiency of the dissociation process depends on the properties of the metal surface. For example, the dissociation ability is significantly reduced by the presence of thin oxide films at the surface [3]. Normally a large fraction of the atomic hydrogen at the metal’s surface recombines to molecular hydrogen and leaves the material’s surface [1].

Hydrogen can diffuse into a metal lattice during processing steps such as melting, surface treatment, or during service in hydrogen containing environments [12]. Very small amounts of hydrogen are sufficient for embrittlement in high-strength steels. The critical hydrogen amount required for the embrittling process gets lower with increasing levels of material strength. For a material, there is a lower threshold limit of hydrogen content below which embrittlement does not occur. If the hydrogen content exceeds the lower limit, the material properties are proportionally degraded for further additions of hydrogen [14].

The driving force for hydrogen diffusion is a chemical potential gradient which originates from hydrogen concentration, temperature, electric field, and hydrostatic component of an elastic stress field. Hydrogen diffuses from areas with high chemical potential to areas with lower chemical potential. In addition, hydrogen tends to diffuse to regions with tensile stress fields [15]. The diffusion process is described by Fick’s first law:

= C (2.1)

𝐽𝐽 −𝐷𝐷 ∙ ∇ With: J = Diffusion flow, D = Diffusion coefficient, C = Hydrogen concentration [16].

Absorbed hydrogen can be located at two possible types of locations in the metal; either at interstitial sites or at extraordinary sites such as grain boundaries, vacancies, inclusions, and precipitates. Hydrogen at interstitial sites is termed “diffusible hydrogen”, while hydrogen at extraordinary sites is called “trapped hydrogen” [3]. For martensitic high-strength steels, the primary hydrogen trapping sites are at prior austenite grain boundaries [17]. The solubility of hydrogen in iron is extremely low at room temperature and under atmospheric pressure [11]. At these conditions, the hydrogen solubility is only 2×10-8 in atomic ratio. The solid solubility of hydrogen in iron, in atomic fraction C0, is found by Sievert’s law:

0 = 0,00185 exp( 3400 T) (2.2)

𝐶𝐶 ∙ 𝑃𝑃 ∙ − ⁄ With: √ P = external hydrogen pressure in 105 Pascal, T = temperature in Kelvin [4].

2.5 Prevention of Hydrogen Embrittlement Hydrogen embrittlement can be prevented, or at least limited, by considering some critical factors:

• Preventing hydrogen absorption during processing steps by minimizing the hydrogen content in the process environment. • Removal of the material’s hydrogen content by heat treatment at low temperatures (about 200 °C) which is a process known as baking. For high strength steels, baking normally takes 10-22 hours. The heat treatment should be done as soon as possible to prevent the creation of micro-cracks. • Minimize residual stresses in the metal. • Avoidance of cathodic cleaning. • Usage of mechanical methods (tumbling, sand blasting, et cetera) for oxide and scale removal, rather than doing that by pickling [1]. • Material selection since hydrogen embrittlement susceptibility generally increases with increasing tensile strength and hardness. • By alloying steels with strong hydride-forming elements like titanium, molybdenum, and vanadium the hydrogen embrittlement susceptibility is lowered [13].

2.6 Stress Corrosion Cracking Stress corrosion cracking (SCC) is a closely related phenomenon to hydrogen embrittlement. They have in common that normally ductile materials experience brittle failure. A corrosive environment in combination with tensile stresses cause stress corrosion cracking. The tensile stress can be either externally applied or residual, and the magnitude can be significantly lower than the material’s tensile strength. Hydrogen enter the metal through corrosive reactions. Crack initiation and propagation can occur with little outside evidence of corrosion. Usually the crack initiates at surface flaws, which can be pre-existing or created during service by corrosion or wear. The fracture mode can be either transgranular or intergranular [18]. However, crack propagation is typically intergranular in high-strength steels [19]. For aluminium alloys, stress corrosion cracking is characterized by secondary intergranular, branching cracks. This behaviour also happens in high-strength steels, but not always, which makes it difficult to separate stress corrosion cracking from hydrogen embrittlement by metallographic examination [20]. Hydrogen embrittlement and stress corrosion cracking can be distinguished from each other by the behaviour when a material is exposed to electric current. Cathodic protection prevents stress corrosion cracking but promotes hydrogen embrittlement [21].

Materials that are susceptible to stress corrosion cracking are for example stainless steels, high-strength steels, aluminium alloys, titanium alloys, and magnesium alloys. Parameters of the corrosive environments that influence the process are the type of solution, concentration, temperature, pressure, pH, and electrochemical potential. The corrosive medium is to some extent specific to the concerned metal; chloride solutions for austenitic stainless steels and nitrate solutions for carbon steels. But it is known that other substances may cause SCC under

6 unfavourable conditions [18]. Sufficient amounts of atomic hydrogen can be found in acids, neutral solutions, or by water dissociation in alkaline solutions [19].

Several models are established for the explanation of stress corrosion cracking. The models are categorized into anodic or cathodic stress corrosion cracking. External appearances are similar for the two models, but the mechanisms differ. For the anodic model, the protective film in the vicinity of the crack tip is ruptured by localized plastic flow. An electrolytic cell is created at the crack tip where the bare metal acts as anode and the unbroken surface film acts as cathode. The bare metal is rapidly dissolved which leads to crack initiation and propagation. Cathodic stress corrosion cracking occurs as a result of hydrogen generation at the metal surface. The hydrogen is later absorbed into the metal lattice which embrittles the material. Minor corrosion reactions at the steel surface from otherwise harmless humidity conditions like distilled water and humid air can provide sufficient amounts of hydrogen for crack initiation and propagation. The fracture models for cathodic stress corrosion are the same as for hydrogen embrittlement; HEDE, HELP, et cetera [22].

2.7 Cathodic Potential Cathodic potential has a strong influence on hydrogen embrittlement of high strength steels. Slow strain rate tensile tests were conducted in boric acid/borax aqueous buffer solution of pH 10,0 to find the dependence of hydrogen absorption characteristic on applied potential. It turned out that specimens tested at low cathodic potential (-0,36 V) showed similar properties as specimens tested in air. For the specimens exposed to high cathodic potential (-0,91 V) a significant decrease of ductility could be observed [23].

Figure 2.1 Effect of applied potential on susceptibility to hydrogen embrittlement [23].

2.8 Mechanical Test Methods Most of the existing hydrogen embrittlement test methods are mechanical. By applying constant, step-wise increased, or dynamic load the susceptibility to degradation of mechanical properties due to hydrogen embrittlement can be determined. Tests are conducted both for materials with pre-existing hydrogen contents and for hydrogen free materials in hydrogen environments. It is possible to shorten the test time by utilizing V-notched specimens and aggressive environments which enhance the embrittlement effects [15]. However, the difficulty of reproducing notch tip geometry tends to give rather scattered test results [2]. Hydrogen embrittlement evaluation is most often done by comparing the time to failure for specimens in a certain test condition or by examining the fraction of specimens that have failed during a predetermined test time. Conventional tensile and impact tests are usually not used because the test times are too short for the hydrogen diffusion processes to take place. Testing should be conducted at room temperature with regards to the hydrogen diffusion [14].

2.8.1 Constant Load Test Constant load tests are conducted by stressing the specimens with a constant load until fracture occurs. Test times are usually in the range from 200 hours up to 5000 hours. The hydrogen embrittlement evaluation is most commonly made by conclusions from time to failure analysis [15].

Kim et al performed a constant load test on hydrogen pre-charged tempered martensite and full pearlite specimens. The specimens are subjected to a static, uni-axial tensile stress until failure occurs. By analyzing the specimens’ time to failure, it was possible to determine a minimum hydrogen content for which hydrogen embrittlement occurs. The results show that the time to failure decreases for increasing hydrogen content, and also that there is a hydrogen content limit below which hydrogen embrittlement does not occur within 100 hours [17].

Figure 2.2 Time to failure for two different steels in a constant load test [17].

2.8.2 The Cantilever Beam Test Lever effect is achieved by attaching the specimen to a long loading arm. Load is applied at the end of the loading arm which creates a bending moment to the specimen. Constant load is maintained until failure occurs. Different environments can be adapted to the specimen by enclosure in a chamber. Susceptibility to hydrogen embrittlement is determined by time to failure analysis. The test should last for at least 200 hours, but it might be necessary with test times up to 5000 hours [18].

Figure 2.3 Experimental set for the cantilever beam test [18].

2.8.3 Constant Strain-Rate The test is conducted by applying a slow and constant strain-rate to the specimen until fracture occurs. Hydrogen embrittlement susceptibility is determined by comparing the fracture stress for materials with different hydrogen contents. Test times are relatively short for this method. Great demands are put on the equipment since the results are very sensitive to the strain rate [24]. A disadvantage with slow strain rate test is that the results tend to overstate the hydrogen embrittlement susceptibility because of the enforced plastic deformation which provides a very severe test condition [2].

Kim el al tested two different steels, tempered martensite and full pearlite, at a constant strain rate of 3×10-3 s-1. The steels were pre-charged with various amounts of hydrogen. It turned out that the fracture stress decreased significantly for the two steel types for increasing hydrogen contents [17].

Figure 2.4 Fracture stresses for two different steels in a constant strain-rate test [17]. 9

Tsay conducted slow strain rate tensile tests in a saturated hydrogen sulphide (H2S) solution to evaluate the effects of hydrogen embrittlement in ultra-high strength steels. The degradation of mechanical properties was evaluated by comparing the tensile stress of notched specimens tested in air and hydrogen sulphide acid. The results show that by doing the tensile test in hydrogen sulphide solution, the tensile strength is decreased significantly. By lowering the strain rate, the tensile strength is decreased even more which is due to prolonged time for interaction between the steel and the hydrogen [25].

Figure 2.5 Tensile stresses of four types of specimens in a constant strain-rate test [25].

2.8.4 Bending Tests Specimens are tested by being bent, where the load is applied symmetrically in a three or four point bending equipment. Constant load or constant displacement can both be used in this test method. Hydrogen embrittlement evaluation is usually conducted by time to failure analysis [18].

Figure 2.6 Experimental apparatus for three and four point bend tests [26].

2.8.5 The Wedge Opening Load Test A constant crack-opening displacement is applied to the specimen with the use of a bolt. The crack propagation is observed either by strain gages or by microscope. As the crack grows, the stress intensity decreases which slows down the crack propagation rate. Susceptibility of hydrogen embrittlement is determined by the stress intensity threshold value which is found when the crack growth stops or is too low to be measured. In order to establish the stress intensity threshold, test times up to 5 000 hours might be necessary [18].

Figure 2.7 Experimental set-up for the wedge opening test method [18].

2.8.6 The Disk-Pressure Testing Method By using high-pressure gas, materials’ susceptibility to hydrogen embrittlement can be evaluated. The specimens are thin and disk-shaped with diameter of about 6 centimetres and a thickness in the range of 0,20 to 1,50 millimetres. During the test, the specimens act as membrane in the test-equipment. Typically, the maximum pressure in disk-pressure tests is 1600 bars.

Figure 2.8 Apparatus for the disk pressure test method [27].

The test method is conducted by testing two types of specimens in the same environment, which can be for example helium. To evaluate susceptibility to hydrogen embrittlement, the first test is made on an unembrittled specimen and the second test is made on a specimen that 11 may be embrittled. The gas pressure is gradually increased until rupture. The hydrogen embrittlement susceptibility is evaluated through the ratio between the rupture pressure of the unembrittled specimen and the specimen that may be embrittled. If the critical pressure is lower for the latter specimen, hydrogen embrittlement has taken place [27].

A material’s susceptibility to hydrogen embrittlement can be determined by the disk pressure test. First, the test is conducted in inert environment such as helium. Secondly, the same type of material is tested in hydrogen environment. In the inert environment the failure will occur only due to mechanical overload, but for the hydrogen environment the effects from hydrogen embrittlement also may affect the failure. The failure pressure will be lower for the hydrogen environment ( 2 ) than for the inert environment ( ) if the material is susceptible for hydrogen embrittlement. The failure-pressure ratio ( ) is used to evaluate the 𝑃𝑃𝐻𝐻 𝑃𝑃𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 2 susceptibility: 𝑆𝑆𝐻𝐻

2 = (2.3) 𝑃𝑃𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼2 𝑆𝑆 𝐻𝐻 𝑃𝑃𝐻𝐻 If:

2 1 , no susceptibility. 1 < < 2 , moderate susceptibility. 𝑆𝑆𝐻𝐻 ≤ 2 2 < , high susceptibility [18]. 𝑆𝑆𝐻𝐻2

𝑆𝑆𝐻𝐻 2.8.7 C-Ring Test C-formed specimens are used for this test method. The load is applied to the specimen by a bolt and the stress magnitude is estimated by strain gauges attached to the specimen. If failure occurs within 200 hours the material is considered to be affected by hydrogen embrittlement [28].

Figure 2.9 Experimental equipment for the C-ring test [28].

2.8.8 Strip Test Thin, rectangular specimens are attached to a frame which bends the specimens. The load is maintained by keeping the specimens in their bow-shaped position until fracture occurs. By studying the time to failure for the evaluated specimens, it can be determined if hydrogen embrittlement has taken place [14].

Figure 2.10 Experimental set-up for the strip test [29].

2.8.9 Stressed O-Ring Test Specimens in this method are O-shaped with diameter of 6 cm. The stress is applied by inserting a bar into the ring-formed specimen. The bar is slightly longer than the specimen’s inner diameter which creates stresses in the specimen. By using bars of different lengths, the O-shaped specimens experience different stress magnitudes. If failure does not occur within 168 hours the specimen is considered to be unembrittled. Strain-gages, photostress measurements, and X-ray diffraction can be used for determining stress magnitude in the specimen [30].

Figure 2.11 Experimental set-up for the stressed O-ring test [30].

2.8.10 Bar-Bending Test For this test method, the specimens are attached to a fixture and bending moment is applied to the specimen’s free end with the use of a bolt. Since the load is in the form of a moment, relatively low loads are needed compared to a conventional tensile test. Other advantages of this method are that it is simple to conduct and that is uses simple specimen geometries. The load is maintained until failure and time to failure analysis is conducted for hydrogen embrittlement evaluation [31].

Figure 2.12 Experimental set-up for the bar bending test [31].

2.8.11 Stepwise Increased Load If the load is step-wise increased the test time can be significantly shortened compared to constant load and constant displacement rate tests. The test is an accelerated method by the means that the load is stepwise increased until crack initiation occurs. Its mechanism is principally the same as for the constant load tests since the load is held constant for a predetermined time. If fracture has not incurred during the predetermined time, the load is increased and this procedure is repeated until fracture. Evaluation of hydrogen embrittlement is most commonly done by time to failure analysis. Normally, this type of test method lasts less than 24 hours with holding times of about one hour at each stress level. If the holding time is too short, the obtained result might be incorrect because of insufficient time for the hydrogen diffusion process to take place [18].

Figure 2.13 Stepwise increased load [18].

2.9 Chemical Test Methods Chemical test methods are principally conducted by measuring the steel’s hydrogen contents, which is a well known technique. However, it is problematic to find a good correlation between hydrogen contents and hydrogen embrittlement. One difficulty is that hydrogen exists at two different types of states in the steel, diffusible and trapped, where the diffusible hydrogen is considered to be of greatest importance for hydrogen embrittlement [14].

2.9.1 Hydrogen Extraction Hydrogen is extracted from the steel by heat treatment in an inert gas or vacuum environment. The diffusible hydrogen is extracted when heating the steel up to about 400 °C, and the trapped hydrogen is also extracted if the steel is melted at a temperature of 2000 °C. Evaluation of hydrogen embrittlement is made by measuring the amount of hydrogen in the material, where the measurement can be done by following methods:

• Separating the hydrogen from the inert gas. • Using a heated Palladium filter. • Measure the volume of the hydrogen gas. • Examining the conductivity of the exerted gas. • Mass spectronometry. • Gas chromatography [14].

2.9.2 Electrochemical Method Hydrogen content is determined by measuring the current created from two specimens put into an alkali solution. One of the specimens contains hydrogen and the other is hydrogen free. In the solution, the hydrogen containing specimen acts as cathode and the hydrogen free specimen acts as anode, together they create an electric current [14].

2.9.3 Electrochemical Hydrogen Probe In situ measurements of mobile hydrogen in steel are conducted by an electrochemical cell which is in contact with the sample. The cell contains NaOH solution together with a Ni/NiO electrode. The steel is kept at an electric potential which is 150 mV higher than the electrode’s potential. Hydrogen atoms oxides when diffusion out from the steel and the oxidation current is measured. NaOH solution keeps the steel passive to prevent effects of iron oxidation currents [14].

2.10 Aggressive Environment By exposing the specimens to an aggressive environment such as acid solutions or hydrogen gas, the test times can be significantly reduced. Generally, steel exposed to acidic solutions leads to a higher amount of absorbed hydrogen compared to hydrogen gas exposure except if the gas pressure is very high. Sulphuric acid (H2SO4), hydrochloric acid (HCl), and phosphoric acid (H3PO4) are examples of acids which can be used for these kinds of tests. Louthan et al found that among the mentioned acids, the highest hydrogen absorption for steel was in sulphuric acid, as illustrated in figure 2.14 [3].

Figure 2.14 Hydrogen absorption of low carbon steels exposed to three different acidic solutions at 38°C and 90°C [3].

In a test employed by McIntyre et al, it was found that hydride sulphide gas environment resulted in the highest crack growth rate among the tested conditions which also included hydrogen gas and seawater [7].

Figure 2.15 Environmental influences on crack growth rate for a high-strength steel [7].

When conducting hydrogen embrittlement evaluation tests in acidic solutions, the results are very sensitive to the test conditions. The following four test conditions exemplify the effects of test conditions:

1. Tensile testing a commercial low-alloy steel in 0,1 M sulphuric acid solution at room temperature. For this condition, two interrelated electrochemical reactions take place at the steel surface; cathodic reaction (2.4) and anodic reaction (2.5).

+ 2 3 + 2 2 + 2 2 (2.4) − 𝐻𝐻 𝑂𝑂 𝑒𝑒 → 𝐻𝐻 𝐻𝐻 𝑂𝑂 2+ + 2 (2.5) − 𝑀𝑀𝑀𝑀 → 𝑀𝑀𝑒𝑒 𝑒𝑒 Consequently, the steel assumes corrosion potential. In addition, the steel surface is chemically attacked and evolves sulphide:

+ 2 4 4 + 2 (2.6)

𝑀𝑀𝑀𝑀𝑀𝑀 𝐻𝐻 𝑆𝑆𝑂𝑂 → 𝑀𝑀𝑀𝑀𝑀𝑀𝑂𝑂 𝐻𝐻 𝑆𝑆 Under these conditions cracking initiates at the interface of metal and electrolyte, namely at sites where sulphides are attacked and the hydrogen absorption into the steel

is enhanced by H2S evolution.

2. Same conditions as for 1) but with polarization of the steel specimen. The polarization is achieved with electric current to a value slightly more cathodic than the open-circuit corrosion potential. For this condition, the anodic reaction (2.5) does not occur at the steel surface, but reactions (2.4) and (2.6) do occur. The amount of absorbed hydrogen is much higher than for condition 1). Crack initiation is expected in the specimen’s interior.

3. For this condition, the same steel, stress and temperature are used as for 1) and 2) but the steel is exposed to the neutral electrolyte 0,1 M sodium chloride solution of pH 7. The sample is polarized to a potential more cathodic than the open-circuit corrosion potential of the solution. A cathodic reaction occurs on the steel surface:

2 20 + 2 2 + 2 (2.7) − − 𝐻𝐻 𝑒𝑒 → 𝐻𝐻 𝑂𝑂𝐻𝐻 The amount of absorbed hydrogen is much lower than in both cases 1) and 2). If cracking occurs it is expected inside the specimen.

4. The same steel, stress and temperature as in cases 1-3 are used and with the same solution as in condition 3), but now with a polarized potential more anodic than the open-circuit corrosion potential of the solution. Because of the presence of chlorides in the solution, the passivating oxide film is ruptured. At these locations, pits are formed due to the anodic reaction (2.5). The hydrogen absorption can be very high within the pits. Thus, cracks initiate from the pits [32]. 17

Castellote et al performed slow strain rate tests on pearlitic steels under cathodic potential in aggressive environment. The environment consisted of a naturally aerated 0,05 M sodium bicarbonate (NaHCO3) aqueous solution with pH = 8,5. The test was carried out in two different conditions:

• Sodium bicarbonates solution with cathodic potential of -1200 mV, which promotes embrittlement by producing proton discharge that diffuses into the metal. • Under inert environment.

Specimens tested in the aggressive environment showed more brittle behaviour compared to the specimen tested in inert conditions. Hydrogen contents were measured by nuclear resonance reaction analysis. It was found that the inert sample had less hydrogen content than the sample tested in the sodium bicarbonate solution. Neither pits nor corrosion were developed for the specimen at cathodic potential, instead the hydrogen was believed to be absorbed and diffused to internal regions with triaxial stresses [33].

Figure 2.16 Stress-strain curves from specimens in a slow strain rate test [33].

Chapter 3 METHODS

In this chapter, the basic concepts behind the developed test methods that evaluate hydrogen embrittlement are explained. The material used in the testing is described and also the technique which was used to charge the specimens with different levels of hydrogen contents.

3.1 Material For the different hydrogen embrittlement evaluation test methods, material from the products Audi Q5 and Jaguar X250 are used. Both products are side impact beams produced by USIBOR 1500P steel which is boron steel coated with a 25 μm thick layer consisting of aluminium and silicon. The material’s yield strength is approximately 1050 MPa and its tensile strength is about 1550 MPa. The Q5 pillars have a total thickness of 1,75 mm while the corresponding value is 1,60 mm for the X250 pillars. From the plane surfaces of the pillars, specimens are cut out with dimensions suited for the different test methods.

3.1.1 Hydrogen Contents The conditions in the press-hardening process are altered to achieve various hydrogen contents in the pillars. Three different atmospheres are used when the pillars are press- hardened which charge the steel with three levels of hydrogen contents. The materials from the different process conditions are termed PC1, PC2 and PC3:

• Process condition 1 (PC1) is the ordinary process condition for USIBOR 1500P steel. The press-hardening atmosphere consists of nitrogen and has a low dew-point. This process atmosphere leads to relatively low levels of hydrogen in the steel. • Process condition 2 (PC2) is an atmosphere consisting of both nitrogen and methanol. Initially, there is only nitrogen gas in the press-hardening device. The products are press-hardened 5-10 minutes after a methanol gas flow has been introduced to the atmosphere. Moderate levels of hydrogen contents in the steel are achieved by this condition. • Process condition 3 (PC3) corresponds to the state when the methanol flow has endured for 1 hour which is the ordinary process condition for non-coated boron steel at the Gestamp HardTech production facility. This atmosphere has a relatively high dew-point which results in high hydrogen contents in the steel products.

The pillars’ hydrogen contents are measured by the Swerea KIMAB research institute in Stockholm. Before sending the specimens to Stockholm, sample preparation is performed which includes removing the coating by grinding and also to cut out small samples with a mass within the interval of 0,7-1,1 g. Until the hydrogen contents measurement is conducted, the samples are kept in a canister filled with liquid nitrogen which prevents the hydrogen from

19 diffusing out from the steel. Hydrogen contents measurement is executed by extracting the hydrogen from the specimens by heating in an argon atmosphere. The hydrogen contents are determined by observing the changes in the argon gas’ heat conductivity as the temperature rises. Diffusible hydrogen is measured for temperatures up to 420 °C, and the trapped hydrogen is measured for temperatures up to 1800 °C.

3.1.2 Micro-Structure The steel’s micro-structure is determined by optical examination. Sample preparations include inserting the steel specimens into a thermosetting polymer mould, grind and polish the sample surfaces, and also etching the specimens to clarify the micro-structure. Martensitic micro- structure is expected for the press-hardened components produced by Gestamp HardTech.

3.1.3 Hardness Vickers’ hardness test is used to investigate if there are any differences in hardness for PC1, PC2 and PC3 materials. For these tests, a load of 2,0 kg is used.

3.2 Test Methods A number of new test methods have been developed and evaluated with the aim of detecting the phenomena hydrogen embrittlement in high-strength steel products. Three conditions must be fulfilled for the occurrence of hydrogen embrittlement; a material that is susceptible to hydrogen embrittlement, the presence of hydrogen in the material or the environment, and tensile stresses which can be either residual or externally applied. The USIBOR 1500P steel is known to be susceptible for hydrogen embrittlement and different hydrogen contents are achieved by altering the press-hardening atmosphere. Common for the evaluated test methods are that stresses are applied to the specimens but with different means. Specimens with different hydrogen contents are tested, and the results are compared with each other to determine hydrogen’s role in the results.

3.2.1 Stamped Hole By stamping circular holes in the specimens, residual stresses are created in the material. The stresses are present in the area closest to the hole and the stress magnitude decreases rapidly with increasing distance from the hole. Earlier simulations and tests made by Gestamp HardTech have shown that the stress magnitude can be more than 700 MPa. To evaluate if any degradation due to hydrogen embrittlement has occurred, the edges around the hole are examined with microscope immediately after stamping and also 24 and 72 hours after the stamping. From the microscope pictures it can be determined if any crack nucleation and propagation has occurred within the test period.

Figure 3.1 Principal image of the steel specimens used in the stamped hole test method. The red colour illustrates the stress concentration within the specimen.

Additional testing is conducted in an acid environment for 24 hours. The purpose of exposing the specimens to an aggressive environment is to enhance the effects from the hydrogen embrittlement process. Rectangular specimens with several stamped holes in each steel piece are used, where each hole is considered as an individual specimen. Hydrogen is already present in the steel prior to the testing and additional small amounts of hydrogen may be absorbed into the steel from the acid during the test. Three different acid conditions are utilized:

• 0,10 M sulphuric acid (H2SO4) solution.

• 1,0 M sulphuric acid (H2SO4) solution. • 0,10 M hydrochloric acid (HCl) solution.

To determine whether the steel has been hydrogen embrittled, the edges of the stamped holes are examined with microscope after the acid bath. From the pictures taken with the microscope, it can be seen if any crack nucleation and propagation have taken place during the test. The results are compared between specimens with low hydrogen content to those with high hydrogen content.

Figure 3.2 Schematic image of specimens exposed to an acid environment.

3.2.2 The Curved Coin Test In the curved coin test method, stresses are applied to the material by bending. Small, circular specimens (Ø20 mm) are bent approximately 45 degrees by hammering which causes stresses in the material. To evaluate if the specimens have been hydrogen embrittled, the specimens’ surfaces are examined with microscope immediately after the bending, 24 hours and 72 hours after the bending. If any differences can be seen in crack density at the specimens at different occasions, then some sort of time-dependent embrittling process might have taken place.

Figure 3.3 A circular specimen used in the curved coin test method.

3.2.3 The Triangular Wedge Test Tensile stresses are applied to the specimens by hammering a wedge into a hole in the specimens. The wedge has a triangular shape and is made of tooling steel with very high hardness which ensures that the specimen, and not the wedge, is deformed during the test. For this test method, plane specimens with a stamped hole (Ø20 mm) in the centre are used. The geometry of the wedge is shown in appendix 1. The wedge is hammered into the position where the part of the wedge which is in contact with the specimen has the width of 21,0 mm. Thus, the size of the hole is increased which results in stresses around the hole. If failure does not occur earlier, the wedge is maintained in its position for one week. Evaluation of hydrogen embrittlement is done by examining if cracks are present at the specimens and the results are compared between specimens with different hydrogen contents.

Figure 3.4 The set-up of the triangular wedge test method seen from the side and above.

3.2.4 The Conical Wedge Test In the conical wedge test method, stresses are created in the specimens by inserting a conical wedge into a hole in the specimens. Plane specimens with a circular hole (Ø20 mm) in the centre are used. The wedge geometry is presented in appendix 1. With the use of a hydraulic press device, the conical wedge is pressed into the specimen’s circular hole. As the wedge is pressed into the hole, the hole increases in size because of the pressure that the wedge is exerting. The wedge is made of tooling steel with very high hardness which ensures that the specimen, and not the wedge, is deformed during the test. Radial stresses and tensile ring stresses around the hole are created as a result of the increased hole diameter.

Figure 3.5 The hydraulic press device used in the conical wedge test method.

Thanks to the wedge’s conical geometry the entire hole is in contact with the wedge which makes it possible to establish a theoretical model for the stress distribution within the specimen. The theoretical model is presented in appendix 2. For the particular wedge used in this evaluation, the theoretical tensile ring stress magnitude at the hole’s edge is described as follows:

= 3,54 107 (0,010+0,022 )2 [Pa], (3.1) 𝑦𝑦 𝜎𝜎 𝜑𝜑 ∙ ∙ ∙𝑦𝑦 where y is the depth in meters that the wedge is inserted measured from the position where the wedge diameter is 20,0 mm. For small insertion depths, equation (3.1) has a linear appearance as shown in figure 3.6.

1600 1400 1200 1000 800 600

Ring stress (MPa) 400 200 0 0 1 2 3 4 Wedge insertion (mm)

Figure 3.6 Ring stress magnitude at the hole’s edge for different wedge insertion depths.

In the conical wedge test method, it was decided to use a load of 875 MPa. This stress magnitude is present at the hole’s edge, and decreases rapidly with increasing distance from the hole as shown in figure 3.7. To achieve this stress level, the wedge is inserted 2,5 mm which increases the hole’s diameter from 20,0 mm to 20,1 mm.

1000

800

600

400 Ring stress (MPa) 200

0 0 1 2 3 4 5 6 7 8 9 10 Radial distance from the hole (mm)

Figure 3.7 The ring stress magnitude’s decline for increasing distance from the hole’s edge for the wedge insertion of y = 2,5 mm.

Evaluation of hydrogen embrittlement is done by studying the fraction of specimens that fail and compare the results from specimens with different hydrogen contents. The wedge is maintained in its position for 96 hours if fracture has not occurred before.

3.2.5 Reference Test Methods The strip test and three point bending test are two methods known to be detecting hydrogen embrittlement successfully in Gestamp HardTech’s products. Thus, both methods are used to achieve reference results for comparison with the results mainly from the conical wedge test.

In the strip test method, rectangular specimens with dimensions 230×16 mm are tested. By bending the strips, stresses are created in the specimens. The strip’s ends are fixed while the strip’s centre can be raised which bends the specimen. The bending is accomplished by a screw which position can be adjusted resulting in different stress levels in the specimens. A fixture which can hold 20 specimens is used in the testing. To evaluate hydrogen embrittlement, the time is measured until cracks are present at the specimens’ surface. The test period is two weeks.

Figure 3.8 Part of the fixture used for bending strips.

In the three point bending test, whole pillars are used in the testing. The bending is accomplished with a device at the Gestamp HardTech facility similar to the one in figure 2.6. In these tests, the pillars’ centre is bent vertically 150 mm which induces plastic deformation and thus stresses of high magnitude in the pillars. Hydrogen embrittlement evaluation is conducted by studying the time to failure for the pillars within the test period of three weeks.

Figure 3.9 HardTech’s three point bending equipment.

3.3 Fracture Mode Characterisation Some of the specimens that have failed in the different test methods are analysed to characterise the fracture mode. The examination is made by studying the fracture surface with the use of a scanning electron microscope (SEM) at Luleå University of Technology. Before the examination, samples are prepared by cutting out small samples of the specimens that contains a fracture surface. The fracture surface is treated with acetone and methanol before the samples are inserted in the SEM equipment. From the pictures taken with the microscope, it can be determined whether the fracture mechanism has been intergranular, transgranular, or a mixture of both.

Chapter 4 RESULTS

The results from the evaluated hydrogen embrittlement test methods are presented in this chapter. Furthermore, some general material properties from the specimens used in the tests are described.

4.1 Material The side impact beams Audi Q5 and Jaguar X250 were press-hardened in three different process atmospheres to achieve different levels of hydrogen contents in the steel. Some general properties were examined to determine if there are any differences between materials from pillars produced at the three different process conditions.

4.1.1 Hydrogen Contents The diffusible hydrogen contents were measured by the Swerea KIMAB laboratory. Totally twelve samples were analysed with four samples from each process condition PC1, PC2 and PC3. From the results it can be noted that the products processed at PC1 conditions have the lowest hydrogen contents with values in the range of 0,24 – 0,42 ppm, while the PC2 specimens lies in the interval of 0,48 – 0,63 ppm. The PC3 samples have the highest amount of diffusible hydrogen as the results from the four samples are within 0,67 – 1,63 ppm. More detailed information about hydrogen contents is presented in appendix 3.

1,80 1,60 1,40 1,20 1,00

0,80 Audi Q5 0,60 Jaguar X250 0,40

Diffusible hydrogen content (ppm) content hydrogen Diffusible 0,20 0,00 PC1 PC2 PC3 Process condition

Figure 4.1 Diffusible hydrogen contents (ppm) in samples processed in PC1, PC2 and PC3.

4.1.2 Micro-Structure Material from the products Jaguar X250 and Audi Q5, processed in the three press-hardening atmospheres PC1, PC2 and PC3, were studied to determine the micro-structure. From the images taken with microscope at 1000 times magnification it can be seen that all examined specimens have martensitic micro-structure. Thus, there are no structural differences between PC1, PC2 and PC3 material for neither the Q5 pillars nor the X250 pillars.

Figure 4.2 Micro-structure for PC1, PC2 and PC3 material from Audi Q5 pillars.

Figure 4.3 Micro-structure for PC1, PC2 and PC3 material from Jaguar X250 pillars.

4.1.3 Hardness By using Vickers hardness test, material produced in the three process conditions PC1, PC2 and PC3 could have its hardness measured. Three measurements were conducted on each type of material and also type of product. The average results from the measurements are presented in table 4.1.

Material Audi Q5 Jaguar X250 PC1 512,3 483,7 PC2 506,7 507,0 PC3 530,7 507,0 Table 4.1 Hardness values (HV) for PC1, PC2, and PC3 specimens from Q5 and X250.

It turned out that the average hardness is slightly higher than 500 HV which is a typical hardness value for USIBOR 1500P steel. Two of the values stand out; the PC3 hardness for Audi Q5 with a hardness of 530,7 HV and the PC1 hardness for Jaguar X250 with a hardness of 483,7 HV. However, these values are also in the range of acceptable hardness values for the particular press-hardened steel.

4.2 Test Methods A number of new test methods were developed and evaluated with the aim of detecting hydrogen embrittlement in high-strength steel products.

4.2.1 Stamped Hole In the stamped hole test method, specimens from the product Audi Q5 processed at PC1 and PC3 were evaluated. 24 specimens from each process condition were examined with microscope immediately after the stamping, and also 24 hours and one week after the stamping. Images from the examination are shown in appendix 4. From the microscope pictures, the crack density was appreciated for which a reference scale was innovated. The scale consists of four different levels of crack densities which are all given a numerical value.

Crack density Numerical value No cracks 0 Low 1 Medium 2 High 3 Table 4.2 The scale used for appreciating crack densities.

Every microscope picture was categorised into one of the scale’s four levels of crack densities. Average numerical values of the crack density were calculated for each examination and the results are presented in table 4.3.

Process condition t = 0 t = 24 h t = 1 week PC1 0,0 0,0 0,0 PC3 0,6 0,5 0,6 Table 4.3 Average crack densities for specimens evaluated in the stamped hole test method.

The results show that no cracks were present at the specimens produced in PC1 atmosphere as the crack density appreciations are 0,0 for all of the three examinations. For the PC3 specimens, about half of the specimens have low crack density while the other half has no cracks resulting in an average value of about 0,5 in crack density. No further crack initiation can be observed since the crack density value remains constant during the test period of one week.

The specimens which were tested in different acids were examined with microscope after the 24 hours exposure to aggressive environment. From the pictures taken at 45 times magnification at the hole edge, crack density appreciations were made. It could be seen that both the PC1 and PC3 specimens had similar amount of cracks in each of the three acid conditions. Specimens in the hydrochloric acid and the 1,0M sulphuric acid had experienced the most cracking with crack density values slightly higher than two. For the 0,10 M sulphuric acid, the crack density was 1,2 for PC1 specimens and 1,5 for PC3 specimens. The crack density appreciation is shown in table 4.4.

Process condition 0,10 M HCl 0,10 M H2SO4 1,0 M H2SO4 PC1 2,1 1,2 2,2 PC3 2,0 1,5 2,3 Table 4.4 Crack density appreciation for specimens exposed to three different acid conditions.

It could also be seen that corrosion had taken place along the cracks at the specimens in the hydrochloric acid. The specimens in the 1,0 M sulphuric acid had undergone severe corrosion. Only few signs of corrosion were observed at the specimens who had been exposed to 0,10 M sulphuric acid.

4.2.2 Curved Coin Totally 40 specimens from the product Audi Q5 were evaluated; 20 processed at PC1 conditions and 20 processed at PC3 conditions. The specimens were examined with microscope immediately after the bending and also 24 and 72 hours after the bending. The crack density appreciation scale from table 4.2 was used.

Process condition t = 0 t = 24 h t = 72 h PC1 1,1 1,0 0,9 PC3 1,1 1,2 1,1 Table 4.5 Crack density appreciation for specimens evaluated in the curved coin test method.

From the images it could be seen that both PC1 and PC3 specimens have similar amount of cracks with crack density values of around one. No further crack nucleation or propagation occurs within the test period of 72 hours as the crack density values remains constant.

Figure 4.4 A specimen evaluated in the curved coin test method at three different occasions.

4.2.3 Triangular Wedge In the triangular wedge test method, six PC1 specimens and six PC3 specimens from Audi Q5 pillars were evaluated. During the test period of one week, none of the PC1 specimens failed while all of the six PC3 specimens failed. Thus, a difference in behaviour between specimens with low and high hydrogen contents could be observed since the PC3 specimens experienced delayed cracking. The failed specimens’ cracks are macroscopic and can be observed without optical equipment. The initiation of the cracks took place at the hole edge in the area in

30 between the two wedge indents. From the hole, the cracks propagated in a straight line in a direction perpendicular to the wedge as seen in figure 4.5. The wedge indents are caused by the pressure exerted from the wedge which has induced plastic deformation in the areas where the wedge and the specimen were in contact.

Figure 4.5 A PC3 specimen that failed in the triangular wedge test method.

4.2.4 Conical Wedge Specimens from both Audi Q5 and Jaguar X250 pillars were evaluated in the conical wedge test method. The test period lasted for four days and totally 32 specimens were tested; 18 from Audi Q5 (four PC1, seven PC2 and seven PC3 specimens) and 14 from Jaguar X250 (four PC1, five PC2 and five PC3 specimens). Neither of the totally eight evaluated PC1 specimens failed within the test period. However, nine of the twelve PC2 specimens and eleven of the twelve PC3 specimens failed. Thus, delayed failure caused by hydrogen embrittlement occurred in the specimens with enhanced hydrogen contents. In this test method, the crack arose at apparently random locations around the hole and propagated in a straight line towards the outer edge of the specimen as shown in figure 4.6.

Figure 4.6 A PC3 specimen that failed in the conical wedge test method.

Figure 4.7 The crack in figure 4.6 seen from the hole edge. 31

For those specimens that failed, the average time to failure was approximately 74 hours and no significant differences between the time to failure for PC2 and the PC3 specimens could be observed. More detailed information about the specimens’ time to failure is presented in appendix 5.

4.2.5 Reference Tests In the bent strip test method, 17 PC3 specimens from Audi Q5 pillars were evaluated. Three different loads were used by positioning the screws to three different levels. Six specimens were evaluated at the load corresponding to the screw position of six mm, six specimens were evaluated at the load of eight mm, and five specimens were evaluated at the load of ten mm. One of the strips exposed to the highest load failed within 8-14 days, while the other 16 strips remained intact during the test.

Jaguar X250 pillars were tested in the three point bending test method. Totally 48 pillars were evaluated with 16 pillars from each of the three process conditions PC1, PC2 and PC3. Neither of the pillars with the lowest hydrogen content failed during the test period of three weeks. For the specimens with medium and high hydrogen contents, several pillars were cracked. PC3 specimens failed to the highest extent as 12 of the 16 pillars cracked within 3 weeks, and the corresponding number is 9 pillars for the PC2 material.

Process Condition t = 24 hours t = 72 hours t = 3 weeks No failure PC1 0 0 0 16 PC2 6 0 3 7 PC3 5 3 4 4 Table 4.6 The number of pillars that had failed at the three examination occasions.

Figure 4.8 An undamaged PC1 pillar.

Figure 4.9 A cracked PC2 pillar.

Figure 4.10 Severe delayed failure in a PC3 pillar.

4.3 Fracture Mechanism Characterisation Specimens that had failed in the triangular wedge test method and the conical wedge test method were examined with scanning electron microscope to characterise the fracture mode. The fracture surfaces of specimens with high hydrogen contents, previously used in the triangular wedge test method, were studied initially. From the pictures taken with the microscope it can be seen that the fracture mode has been a mixture of both intergranular and transgranular fracture. In figure 4.11 both plane surfaces, which are typical for intergranular fracture, as well as more uneven parts which are characteristic for transgranular fracture, can be observed.

Figure 4.11 A PC3 specimen from the triangular wedge test method at 1000X and 2500X.

From the conical wedge test method, both specimens with moderate and high hydrogen contents were examined. The fracture mode seems to be a combination of both intergranular and transgranular fracture for the two types of specimens. However, for the PC2 specimens there seems to be a larger amount of intergranular fracture than for the PC3 specimens. This is because a larger fraction of the fracture surface is plane compared to the rougher fracture surface of the PC3 specimen.

Figure 4.12 A PC2 specimen from the conical wedge test method at a) 1000X and b) 2500X.

Figure 4.13 A PC3 specimen from the conical wedge test method at a) 1000X and b) 2500X.

DISCUSSION

The aim of this thesis has been to develop two methods that detect degradation in mechanical properties in high-strength steel products due to the phenomena hydrogen embrittlement. Of the two methods, one was supposed to be quick and make a coarse detection within 24 hours, and the other endures longer but gives more precise detection. Both methods should be easy to conduct, do not require advanced equipment, and being able to evaluate specimens of complex geometries. During the project, it has proven to be problematic to develop a method which detects hydrogen embrittlement within 24 hours. With the available equipment it seems like it is not possible to attain satisfying results within that short period of time for constant load test methods. Instead, some sort of accelerated method might be the answer to how to develop a quick test method [18]. Some initial trials were made to conduct the conical wedge test method with stepwise accelerated load. However, satisfying results could not be obtained. The specimens did not fail as expected even though the applied stress was very high. Another problem was that the wedge tended to slip out from its position when the load reached the highest evaluated levels. Perhaps it is advantageous to use a wedge with less inclination or some kind of device which fixes the wedge’s position in order to avoid the problems with the slippery wedge.

Typically for constant load test methods, the testing endures for at least 200 hours [15]. Two of the evaluated constant load tests methods have been detecting hydrogen embrittlement successfully within a week; the triangular wedge test method and the conical wedge test method. In the first-mentioned method, the wedge is hammered into its position. Hammering is not an accurate method since it is problematic to repeat the same wedge insertion position. Another disadvantage with the method is that the stress distribution within the specimen is complex and thus difficult to estimate by theoretical models. Furthermore, the plastic deformation present at the wedge indents may complicate the stress distribution even more. The conical wedge test method has been given the most attention thanks to its better repeatability and the theoretical model which could be established for explaining the stress distribution within the specimens. The method is a further development of the triangular wedge test method which in turn was developed from the principal idea behind the o-ring test [30]. Within a time interval of four days, the conical wedge test method effectively detects enhanced levels of hydrogen as specimens with high hydrogen content fail. However, due to the many influencing factors behind the phenomena hydrogen embrittlement the time to failure results for similar specimens are relatively scattered. Furthermore, 25 % of the specimens with medium hydrogen contents and 8 % of the specimen with the highest hydrogen contents did not fail during the test. These results prove that it is essential to evaluate large numbers of specimens when evaluating hydrogen embrittlement.

Three point bending is a test method known to detect hydrogen embrittlement in high-strength steel [18]. At the Gestamp HardTech facility in Luleå, it has been recognized that pillars with 35 enhanced hydrogen contents have cracked within a couple of days after being bent. Hence, this test method was conducted to achieve reference results to compare with the results from the conical wedge test method. It turned out that the results from the two test methods correlates with each other rather well. None of the evaluated specimens with low hydrogen contents failed in either method. For specimens with medium hydrogen contents, 56,3 % of the tested pillars failed in the bending test while 64,3 % failed in the conical wedge test method. The corresponding numbers for specimens with high hydrogen contents are 75,0 % for the three point bending test and 92,9 % for the conical wedge test method. Accordingly, the conical wedge test method is a more effective test method than the three-point bending test method.

The Audi Q5 and Jaguar X250 pillars were charged with different amounts of hydrogen by altering the process atmosphere in the press-hardening process. In total three different atmospheres were utilized with the aim of achieving three different levels of hydrogen contents in the steel; low, medium and high. The hydrogen contents measurement show that the use of three different process conditions actually resulted in three different levels of hydrogen contents in the steel. However, the hydrogen contents results were somewhat scattered. The largest deviation was found for specimens with high hydrogen contents as the lowest measured value was 0,67 ppm and the highest 1,63 ppm. Differences of this magnitude aggravate the interpretation of the results from the test methods as the hydrogen content is a critical factor when evaluating hydrogen embrittlement. It is desirable to achieve a more precise distribution of hydrogen contents when charging the steel. The fact that the Audi Q5 and Jaguar X250 pillars were processed in different production lines is a likely explanation for the differences in hydrogen contents. Even though the same atmospheres were utilized in the two production lines, there are several other factors that could have influenced the process conditions in each production line. The material with low, medium and high hydrogen contents were further examined to determine if there are any other differences between the materials except for the hydrogen contents. As expected, the material types have the same micro-structure and hardness. However, the Audi Q5 steel with high hydrogen contents had a hardness of 530,7 HV while the other values were around 500 HV. One possible explanation might be that the material with the highest hardness had enhanced levels of carbon contents. When the hydrogen contents are associated with the results from the conical wedge test, a hydrogen contents threshold limit can be seen below which delayed fracture does not occur. This limit is somewhere around 0,40 ppm of diffusible hydrogen. Earlier experience and recommendations at Gestamp HardTech have shown that this limit lies around 0,30 ppm for the USIBOR 1500P steel. Unfortunately, the two limit values of 0,40 and 0,30 ppm cannot be compared with each other in this context because they have been measured with different methods.

It is problematic to develop hydrogen embrittlement detection test methods for industrial use. The main issue is that it is almost impossible to standardize specimen geometry since the produced pillars are of various designs. This problem was apparent especially for the Jaguar X250 pillar which has few and small plane surfaces. Thus, specimens with the optimal geometry could not be obtained. In most cases it is desired to utilize plane specimens with

36 circular or quadratic dimensions. Circular specimens provide isotropic geometry but are in many cases complicated to cut out. It is likely that the specimen geometry influences the results in the test methods, but no investigations were made within this field. In the different test methods, the time to failure should be shorter for small specimens which are not entirely flat than for the standardised specimen. This is because the standardised specimens have more material which can absorb energy from the applied load.

Fracture surfaces from specimens evaluated in the triangular wedge test and the conical wedge test were studied with SEM. It could be seen that the fracture mode is a mixture of transgranular and intergranular cracking. According to the theory, the fracture mode can be both transgranular and intergranular for hydrogen embrittlement but is typically the latter for delayed failure. The SEM pictures indicate that the specimens with medium hydrogen contents have experienced intergranular fracture as the dominant fracture mode. The specimens with high hydrogen contents seem to have a larger fraction of transgranular fracture mode. Thus the hydrogen contents might play a crucial role for the fracture mode, where increasing hydrogen contents cause transgranular fracture to a larger extent.

Stamped holes and curved coin test methods were not further studied since their results were not promising. In the stamped hole method, differences could be observed between specimens with low and high hydrogen contents. However, as the cracks arose immediately after the stamping and no further crack initiation occurred afterwards, it was concluded that it was not an appropriate method for analysing the time-dependent phenomena hydrogen embrittlement. A possible explanation to why the cracks did not propagate is that the residual stresses are sufficient to create small cracks, but since the stresses are only present locally at the hole’s edge the crack cannot propagate further. Another disadvantage with this method is that it is rather complicated to interpret the results because the cracks are microscopic. The results were similar for the curved coin test method; cracks were present immediately after the bending but no further crack propagation occurred. The method was thus excluded from further examination because delayed failure was not observed at the specimens.

CONCLUSIONS

From this master’s thesis a number of conclusions can be drawn:

• The triangular wedge test method and the conical wedge test method detect delayed failure in press-hardened martensitic steel within one week. • With the available equipment it is not possible to achieve satisfying detection of hydrogen embrittlement within 24 hours by the use of constant load test methods. • It is essential to test large numbers of specimen when evaluating hydrogen embrittlement since a relatively large fraction of the specimens with enhanced hydrogen contents did not fail in the test methods. • The fracture mode of the specimens evaluated in the wedge tests is a mixture of intergranular and transgranular fracture, but with the first-mentioned as the dominating mechanism. • The presence of methanol in the press-hardening process atmosphere has a great influence on the products hydrogen contents. • There is a threshold limit of approximately 0,40 ppm of diffusible hydrogen. Below the limit, delayed fracture does not seem to occur in the evaluated test methods.

FUTURE WORK

To develop test methods which detect hydrogen embrittlement in high-strength steel, some areas are of great interest for further studies: l

• To further evaluate the conical wedge test method with a large number of specimens. • Try to execute the conical wedge test with higher applied loads which might shorten the evaluation time. • Develop a more precise method for charging the specimens with hydrogen. • Test to perform the conical wedge test with step-wise accelerated load. • Examine the possibilities to prevent corrosion in the stamped holes method performed in acid environment. The equipment potentiostat is frequently mentioned in the literature in this context.

REFERENCES

[1] Dini J.W., Electrodeposition: The Materials Science of Coatings and Substrates, William Andrew Publishing (1993) ISBN: 0815513208

[2] Shreir L.L., Jarman R.A., Burstein G.T., Corrosion Volumes 1-2 Third Edition, Elsevier (1994) ISBN: 978-0-7506-1077-3

[3] Louthan M.R. Jr., Hydrogen Embrittlement of Metals: A Primer for the Failure Analyst, (2008)

[4] Nagumo M., Nakamura M., Takai K., Hydrogen Thermal Desorption Relevant to Delayed-Fracture Susceptibility of High-Strength Steels (2001)

[5] Kim, C.D., Hydrogen Embrittlement. Part of: ASM Handbook Volume 11 Failure Analysis and Prevention, ASM International (2002) ISBN: 0-87170-704-7

[6] Katz Y., Tymiak N., Gerberich W.W., Evaluation of Environmentally Assisted Crack Growth. Part of: ASM Handbook Volume 8 Mechanical Testing and Evaluation, ASM International (2000) ISBN: 0-87170-389-0

[7] McIntyre P., Hydrogen Effects in High Strength Steels. Part of: Oriani R.A, Hirth J.P, Smialowski M., Hydrogen Degradation of Ferrous Alloys, Noyes Publications, New Jersey (1985) ISBN: 0-8155-1027-6

[8] McCall J.L., French P.M., Metallography in Failure Analysis, Plenum Press, New York (1977) ISBN 0-306-40012-X

[9] Tiwari, G.P. et al, A study of internal hydrogen embrittlement of steels (2000)

[10] Gray, H.R., Opening Remarks. Part of: Raymond, L., Hydrogen Embrittlement Testing, American Society for Testing and Materials, Baltimore (1974) ISBN: 978-0-8031-5579-4

[11] Nagumo M., Function of Hydrogen in Embrittlement of High-strength Steels (2000)

[12] Hertzberg R.W., Deformation and Fracture Mechanics of Engineering Materials, John Wiley & Sons (1996) ISBN: 0-471-01214-9

[13] Chatterjee U.K., Bose S.K., Roy S.K, Environmental Degradation of Metals, Marcel Dekker, New York (2001) ISBN: 0-8247-9920-8

[14] Skogsmo J., Väteförsprödning – Mekanismer, Orsaker och Åtgärder, (1997)

[15] Interrante C.G., Raymond L., Hydrogen Damage. Part of: Corrosion Tests and Standards: Application and Interpretation, ASTM International (2005) ISBN: 978-0-8031- 2098-3

[16] Nordling C., Österman J., Physics Handbook for Science and Engineering Studentlitteratur, Lund (2004) ISBN: 91-44-03152-1

[17] Kim J.S., Microstructural Influences on Hydrogen Delayed Fracture of High Strength Steels (2008)

[18] Phull B. Evaluating Hydrogen Embrittlement. Part of: ASM Handbook Volume 13A Corrosion: Fundamentals, Testing, and Protection ASM International (2003) ISBN: 0-87170- 705-5

[19] Colangelo V., Metallographic analysis of corrosion failures. Part of: Metallography in Failure Analysis, J.L. McCall Plenum, New York (1977) ISBN: 0-306-40012-X

[20] Russo M., Analysis of fractures utilizing the SEM. Part of: Metallography in Failure Analysis, J.L. McCall Plenum, New York (1977) ISBN: 0-306-40012-X

[21] Callister W.D., Materials Science and Engineering an Introduction, John Wiley & Sons, Utah (2003) ISBN: 0-471-22471-5

[22] Bernd I., Test Methods for Assessing the Susceptibility of Prestressing steel to Hydrogen Induced SCC, Maney Publishing (2004) ISBN: 978-1-904350-24-8

[23] Komazaki S., Maruyama R., Misawa T., Effect of Applied Cathodic Potential on Susceptibility to Hydrogen Embrittlement in High Strength Low Alloy Steel (2002)

[24] Lukito H., Szklarska-Smialowska Z., Susceptibility of Medium-Strength Steels to Hydrogen-Induced Cracking (1997)

[25] Tsay L.W. et al, Hydrogen Embrittlement Susceptibility and Permeability of Two Ultra- high Strength Steels (2005)

[26] Shapiro E., Bend Testing. Part of ASM Handbook Volume 8, Mechanical Testing and Evaluation, ASM International (2000) ISBN 0-87170-389-0

[27] Fidelle J.P., Broudeur R., et al, Disk Pressure Technique. Part of: Raymond L., Hydrogen Embrittlement Testing, American Society for Testing and Materials, Baltimore (1974) ISBN: 978-0-8031-5579-4

[28] Jankowsky E.J., Notched C-Ring Test. Part of: Raymond L., Hydrogen Embrittlement Testing, American Society for Testing and Materials, Baltimore (1974) ISBN: 978-0-8031- 5579-4

[29] Jones W.C., Notched Test Strips. Part of: Raymond L., Hydrogen Embrittlement Testing, American Society for Testing and Materials, Baltimore (1974) ISBN: 978-0-8031-5579-4

[30] Hyter W.H., Stressed O-Ring Test. Part of: Raymond L., Hydrogen Embrittlement Testing, American Society for Testing and Materials, Baltimore (1974) ISBN: 978-0-8031- 5579-4

[31] Movich R.C., Notched Bar-Bending Test. Part of: Raymond L., Hydrogen Embrittlement Testing, American Society for Testing and Materials, Baltimore (1974) ISBN: 978-0-8031- 5579-4

[32] Smialowski M., Initiation of Hydrogen-Induced Cracking in Iron and Iron Alloys. Part of: Oriani R.A, Hirth J.P, Smialowski M., Hydrogen Degradation of Ferrous Alloys, Noyes Publications, New Jersey (1985) ISBN: 0-8155-1027-6

[33] Castellote M. et al. Hydrogen Embrittlement of High-Strength Steel Submitted to Slow Strain Rate Testing Studied by Nuclear Resonance Reaction Analysis and Neutron Diffraction, (2007)

[34] Lundh, H. Grundläggande hållfasthetslära, Fingraf AB, Södertälje (2000) ISBN: 91- 972860-2-8

Appendix 1 WEDGE GEOMETRY

Dimensions for the wedges used in the triangular wedge test method and the conical wedge test method are declared in figure A.1.

Figure A.1 The dimensions of the triangular wedge and the conical wedge.

Appendix 2 THEORETICAL MODEL FOR THE CONICAL WEDGE TEST

A theoretical model which describes the stress distribution within the specimens used in the conical wedge test method was made. To be able to make the theoretical estimation, some assumptions and simplifications must be done:

• The specimens’ geometry is considered as a thick-walled cylinder even though the geometry is quadratic with a hole in the centre as illustrated in figure A.2. For the thick-walled cylinder, a is the inner diameter and b is the outer diameter.

Figure A.2 Quadratic and thick-walled cylinder specimens.

• If there are any stresses in the specimen’s thickness direction, they are considered to not affect the stress distribution in radial and ring direction. • The wedge is much harder than the specimen, meaning that the wedge is not deformed plastically during the test. • No stresses are present at the area close to the specimen’s outer radius. The specimen’s dimensions are considered to be sufficient large that the stress magnitude has declined to a value which is negligible at the outer diameter. • No regard is taken for the effects from the friction between the specimen and the wedge.

From the differential equation (A.1), the radial stress (A.2) and the ring stress (A.3) can be derived:

d  1 d   (ur ⋅ r) = 0 dr  r dr  (A.1) E  1  σ = ⋅ ⋅ +ν − ⋅ −ν ⋅ r 2  A1 (1 ) A2 (1 ) 2  1−ν  r  (A.2)

E  1  σ = ⋅ ⋅ +ν + ⋅ −ν ⋅ ϕ 2  A1 (1 ) A2 (1 ) 2  1−ν  r  (A.3)

with E (Young’s modulus) and ν (Poisson’s constant) as material constants. A1 and A2 are integration constants. To simplify the coming calculations, the two new constants A and B are defined as:

A ⋅ E ⋅ (1+ν ) = 1 A 2 1−ν (A.4) A ⋅ E ⋅ (1−ν ) = 2 B 2 1−ν (A.5)

By inserting the new constants A and B into the equations for the radial stress (A.2) and the ring stress (A.3), those equations are reduced to:

B σ = − r A 2 r (A.6) B σ = + ϕ A 2 r (A.7)

[34]

The constants A and B are determined through the boundary conditions for:

• The inner diameter (r = a) where the displacement is known. The displacement corresponds to the increase of the specimen’s hole’s radius caused by the insertion of the conical wedge. Consequently, the displacement depends on how deep the wedge is inserted. The displacement is described as follows:

1  B  u (r) = ⋅ A(1−ν )⋅ r + (1+ν )  (A.8) r E  r 

• The outer diameter (r = b) where the radial stress’ magnitude is known to be zero through one of the made assumptions.

The constant A is determined by equation (A.6) with the radial stress as zero at the outer diameter (r=b):

B B σ = = = − ⇔ = r (r b) 0 A 2 A 2 b b (A.9)

B is determined by equation (A.8) with the displacement ur known at the inner radius, and also with A=B/b2 from equation (A.9):

1  B B  u (a) ⋅ E u (r = a) = ⋅ ⋅ (1−ν ) ⋅ a + (1+ν ) ⋅  ⇔ B = r (A.10) r E  b 2 a   (1−ν ) ⋅ a (1+ν )   +   b 2 a 

The outer diameter is assumed to be very large compared to the inner diameter. Hence the term which contains b2 can be neglected from the denominator of equation (A.10) resulting in a simplified expression for B:

u (a) ⋅ E ⋅ a B = r (A.11) (1+ν )

The ring stress from equation (A.7) can, as the constants A and B now are known and the outer radius b is estimated to be very large, be expressed as:

B B B B u (a) ⋅ a ⋅ E σ = + = + = = r ϕ (r) A 2 2 2 2 2 (A.12) r b r r (1+ν )⋅ r

The specimens are made of USIBOR steel which has following material constants:

• Young’s modulus, E = 207 GPa. • Poisson’s ratio, ν = 0,30. • Inner radius, a = 0,010 m.

With these constants known, the ring stress can be expressed as:

9 207 ⋅10 ⋅ 0,010 ⋅ur (a) 9 ur (a) σ ϕ (r) = = 1,5923⋅10 ⋅ (A.13) 1,30 ⋅ r 2 r 2

Now the ring stress’ magnitude at the hole edge can be found by inserting different values of the displacement ur(a) into equation (A.13). In order to find the stress magnitude at the hole edge, the radius is defined as r = 0,010 + ur since the hole’s original radius is 0,010 m and it is increased by the displacement caused by the wedge. Further calculations have to be made to find the correlation between the wedge’s insertion and the stress field in the material. To accomplish that, the displacement must be expressed as a function that depends on the wedge’s shape:

= = ⋅ α ur x y tan (A.14)

Where x is the wedge’s radius that exceeds 0,010 m, y is the wedge’s height and α is the angle of the wedge; all declared in figure A.3.

Figure A.3 Variable declaration.

With the displacement expressed as a function of the wedge’s geometry, the ring stress can now be expressed as:

y ⋅ tanα σ = ⋅ 9 ⋅ ϕ (r) 1,5923 10 2 (0,010 + y ⋅ tanα) (A.15)

For the particular wedge used in the tests, the wedge’s angle is α = 1,27°. Now the ring stress at the hole edge can be expressed as equation (A.16) where the stress only depends on the depth of the wedge insertion.

= 3,54 107 (0,010+0,022 )2 (A.16) 𝑦𝑦 𝜎𝜎 𝜑𝜑 ∙ ∙ ∙𝑦𝑦

Appendix 3 HYDROGEN CONTENTS

Specimen Product Process condition Hydrogen content 1 Q5 PC1 0,24 2 Q5 PC1 0,30 3 X250 PC1 0,25 4 X250 PC1 0,42 5 Q5 PC2 0,48 6 Q5 PC2 0,60 7 X250 PC2 0,59 8 X250 PC2 0,63 9 Q5 PC3 0,67 10 Q5 PC3 0,98 11 X250 PC3 1,54 12 X250 PC3 1,63 Table A.1 Diffusible hydrogen contents (ppm).

Appendix 4 EXAMPLE IMAGES FROM THE STAMPED HOLE TEST METHOD

Figure A.4 The hole edge of a PC1 specimen after 24 hours in air.

Figure A.5 The hole edge of a PC3 specimen after 24 hours in air.

Figure A.6 A PC1 specimen tested in HCl.

Figure A.7 A PC3 specimen tested in HCl.

Figure A.8 A PC1 specimen tested in 0,10 M H2SO4.

Figure A.9 A PC3 specimen tested in 0,10 M H2SO4.

Figure A.10 A PC1 specimen tested in 1,0 M H2SO4.

Figure A.11 A PC3 specimen tested in 1,0 M H2SO4.

Appendix 5 RESULTS FROM THE CONICAL WEDGE TEST METHOD

Specimen Product Time to failure (h) 1 Q5 - 2 Q5 - 3 Q5 - 4 Q5 - 5 X250 - 6 X250 - 7 X250 - 8 X250 - Table A.2 Time to failure for PC1 specimens.

Specimen Product Time to failure (h) 9 Q5 63 10 Q5 68 11 Q5 71 12 Q5 84 13 Q5 90 14 Q5 - 15 Q5 - 16 Q5 - 17 X250 66 18 X250 71 19 X250 74 20 X250 82 21 X250 - 22 X250 - Table A.3 Time to failure for PC2 specimens.

Specimen Product Time to failure (h) 23 Q5 58 24 Q5 61 25 Q5 64 26 Q5 70 27 Q5 77 28 Q5 83 29 Q5 89 30 Q5 - 31 X250 58 32 X250 64 33 X250 70 34 X250 73 35 X250 75 36 X250 94 Table A.4 Time to failure for PC3 specimens.

Figure A.12 Fracture surface from a specimen used in the conical wedge test method.