2009:084 MASTER'S THESIS

Introduction to an impact fatigue testing method in order to assess the properties of different steel grades with various applied surface treatments

Kamrooz Riyahimalayeri

Luleå University of Technology Master Thesis, Continuation Courses Minerals and Metallurgical Engineering Department of Chemical Engineering and Geosciences Division of Process Metallurgy

2009:084 - ISSN: 1653-0187 - ISRN: LTU-PB-EX--09/084--SE Abstract

During this project, several different steel grades that were made by Ovako steel company were chosen, then they were treated differently (Annealed, through hardened, carburized, nitridized, induction hardened, and DLC coated), finally by the means of an in house Vibrophore machine, and a costume made cyclic Impactor a test setup were introduced.

Samples were tested in two different force conditions of 1900N and 2400N, and in four different impact cycles as per 103, 104, 105, 5x105, then surfaces of these specimens were analyzed both macroscopically and microscopically and achieved results were documented.

Finally a comparison among the size of the biggest appeared surface ring cracks, number of appeared surface ring cracks that were above 40µm, indentation radius and penetration depth of the impacted surface areas after applying cyclic impact force were made, and results were shown.

As the final result of the introduced impact fatigue test it is concluded that samples designated as I and H (Ovako steel grade 804Q_DLC coated, 528Q_DLC respectively) had the same and best impact fatigue resistance among the tested samples, following by DLC coated samples designated as G (803J), these samples were followed from the best to worst as per samples E (528Q_induction hardened), samples B (256G_carburized), samples J (804Q_through hardened), samples D (277Q_nitrided), samples C (225A_nitrided), and worst achieved results were from samples designated as A (804Q_annealed).

Key Words: Introducing an impact fatigue Test - Surface treated Steels - Surface crack evaluation

Acknowledgments

This master thesis is written in order to document the accomplished project in Ovako steel company as the final report for achievement of the master degree in Minerals and metallurgical engineering from Luleå University of Technology.

I would like to express my sincere gratitude to all people who have assisted me regarding this project; first and foremost Patrik Ölund, who was my industrial supervisor and without his assistance and support it would not be possible to finish this project, Professor Bo Björkman who has been my accademical advisor, and the person from whom I learned a lot during my studies at Luleå University of Technology, and all other personnel of the Ovako R&D department who have contributed to this work as per Jan-Erik Andersson, Stefan Akterhag, Garry Wicks, Johan Borg, Christopher Fallqvist, and Christer Malmquist.

My final gratitude always goes to my family for all their supports and understandings.

Kamrooz Hofors, Sweden June 2009

I II Table of contents List of figures ...... IV List of tables ...... VII Presentation of Ovako ...... 1 Introduction ...... 5 Theoretical Background ...... 6 Fatigue and Fracture Mechanics Introduction ...... 6 Cyclic Deformation Prior to Fatigue Crack Initiation ...... 6 Initiation of Microcracks ...... 7 Propagation and Coalescence of Microcracks ...... 9 Growth of Macrocracks ...... 9 Impact Fatigue ...... 13 Hertzian Contacts ...... 15 Area Calculation of the Spherical Segments ...... 16 Surface Modifications ...... 17 Induction Hardening ...... 17 Carburizing ...... 18 Nitriding ...... 20 Diamond-like Carbon (DLC) ...... 22 Experimental ...... 25 Test Procedure ...... 25 Material ...... 27 Machine ...... 28 Impactor ...... 29 Specimen ...... 29 Results ...... 30 Discussion ...... 40 Conclusion ...... 47 Future work ...... 50 References ...... I Index 1: Experimental Data, Test Conditions and Observed Results ...... III Index 2: Experimental Data, Hardness, and Calculated Pressure ...... V Index 3: Stereo Macroscopic Pictures of the Impacted Area at 20X ...... VI Index 4: Optical Microscopic Pictures of the Biggest Observed Cracks ...... VIII Index 5: Result of 2D Surface Topography of the Impacted Area ...... XII Index 6: Material Properties of the Tested Steels ...... XVIII

III List of figures

Figure 1: Group structure and net sales by division charts of Ovako from [19] ...... 1 Figure 2: Some of the examples of various applications of Ovako’s products from [19] ...... 2 Figure 3: Production plan: Billet manufacturing in Hofors from [19] ...... 2 Figure 4: Production plan: Hot rolled tubes from [19] ...... 3 Figure 5: Production plan: Cold rolled tubes from [19] ...... 3 Figure 6: Production plan: Hot rolled rings from [19] ...... 4 Figure 7: Intrusion and extrusion formation according to Cottrell-Hull model from [3] ...... 7 Figure 8: Paired dislocation pile ups from [3] ...... 8 Figure 9: Slip band in circle that is believed to be already cracked, originating from a grain boundary from [3] ...... 8 Figure 10: Rate of fatigue crack propagation, dA/dN, versus range of stress intensity, ΔK from [4] ...... 10 Figure 11: The sequence of procedures throughout metal fatigue from [5] ...... 10 Figure 12: Elastic contact between ball and plane [11] ...... 15 Figure 13: Schematic of a spherical segment from [27] ...... 16 Figure 14: Relation between the surface hardness based on the carbon content of the steel from [14] ...... 18 Figure 15: Hardness range of carburized microstructures versus distance from the surface from [15] ...... 18 Figure 16: Plate martensite and retained austenite in the case (14NiCr18, 0.7% C) from [15]19 Figure 17: Plate martensite, SEM micrograph (16MnCr5) from [15] ...... 19 Figure 18: Plot of nucleation of nitrides on iron from [15] ...... 20 Figure 19: Hardness range of nitrided microstructures versus distance from the surface from [15] ...... 21 Figure 20: Pure iron nitrided, etched with Nital from [15]...... 21 Figure 21: Pure iron nitrided, SEM micrograph from [15] ...... 21 Figure 22: DLC’s atomic structure and composition from [17] ...... 23 Figure 23: Nanostructure of pure DLC showing the nodules from [18] ...... 23 Figure 24: TEM image of cross-section of the DLC from [18] ...... 24 Figure 25: Pin and disk wear rate of three different DLC coated substrates in dry and 50%RH. Applied force was 5N and velocity of sliding 0.004 ms-1 , taken from [30] ...... 24 Figure 26: Mounted specimen under dynamic load ...... 25 Figure 27: Perthometer PGK used for 2D surface topography ...... 26 Figure 28: Pictures of the applied machine (left picture is the impact part and right picture is the control panel) ...... 28 Figure 29: Sketch of the Impactor ...... 29 Figure 30: Sketch of the specimens ...... 29 Figure 31: Picture taken by stereo macroscope from the area of impact from steel 804Q, Annealed, after 105 cycles, at 2400N dynamic force ...... 30 Figure 32: Picture taken by optical microscope showing the biggest observed surface ring crack on steel 804Q, Annealed, after 5x105 cycles, at 2400N dynamic force ...... 30 Figure 33: Result of the linear surface topography of steel 804Q, Annealed, after 5x105 cycles at 2400N dynamic force ...... 31 Figure 34: Picture taken by stereo macroscope from the area of impact from steel 804Q, through hardened, after 5x105 cycles, at 2400N dynamic force ...... 31 Figure 35: Picture taken by optical microscope showing the biggest observed surface ring crack on steel 804Q, through hardened, after 5x105 cycles, at 2400N dynamic force ...... 32

IV Figure 36: Result of the linear surface topography of steel 804Q, through hardened, after 5x105 cycles at 2400N dynamic force ...... 32 Figure 37: Picture taken by stereo macroscope from the area of impact from steel 256G, carburized, after 5x105 cycles, at 2400N dynamic force ...... 33 Figure 38: Picture taken by optical microscope showing the biggest observed surface ring crack on steel 256G, carburized, after 5x105 cycles, at 2400N dynamic force ...... 33 Figure 39: Result of the linear surface topography of steel 256G, carburized, after 5x105 cycles at 2400N dynamic force ...... 33 Figure 40: Picture taken by stereo macroscope from the area of impact from steel 225A, nitride, after 5x105 cycles, at 1900N dynamic force ...... 34 Figure 41: Picture taken by optical microscope showing the biggest observed surface ring crack on steel 225A, nitrided, after 5x105 cycles, at 1900N dynamic force ...... 34 Figure 42: Result of the linear surface topography of steel 225A, nitrided, after 5x105 cycles at 1900N dynamic force ...... 34 Figure 43: Picture taken by stereo macroscope from the area of impact from steel 277Q, nitride, after 5x105 cycles, at 2400N dynamic force ...... 35 Figure 44: Picture taken by optical microscope showing the biggest observed surface ring crack on steel 277Q, nitrided, after 5x105 cycles, at 2400N dynamic force ...... 35 Figure 45: Result of the linear surface topography of steel 277Q, nitrided, after 5x105 cycles at 2400N dynamic force ...... 35 Figure 46: Picture taken by stereo macroscope from the area of impact from steel 803J, DLC coated, after 5x105 cycles, at 1900N dynamic force ...... 36 Figure 47: Result of the linear surface topography of steel 803J, DLC coated, after 5x105 cycles at 1900N dynamic force ...... 36 Figure 48: Picture taken by stereo macroscope from the area of impact from steel 528Q, DLC coated, after 5x105 cycles, at 1900N dynamic force ...... 37 Figure 49: Result of the linear surface topography of steel 528Q, DLC coated, after 5x105 cycles at 1900N dynamic force ...... 37 Figure 50: Picture taken by stereo macroscope from the area of impact from steel 804Q, DLC coated, after 5x105 cycles, at 2400N dynamic force ...... 38 Figure 51: Result of the linear surface topography of steel 804Q, DLC coated, after 5x105 cycles at 2400N dynamic force ...... 38 Figure 52: Picture taken by stereo macroscope from the area of impact from steel 528Q, induction hardened, after 5x105 cycles, at 2400N dynamic force ...... 39 Figure 53: Picture taken by optical microscope showing the biggest observed surface crack on steel 528Q, induction hardened, after 5x105 cycles, at 2400N dynamic force ...... 39 Figure 54: Result of the linear surface topography of steel 528Q, induction hardened, after 5x105 cycles at 2400N dynamic force ...... 39 Figure 55: The longest observed surface ring crack at four different chosen impact cycles and two different test forces ...... 41 Figure 56: Number of the observed surface ring cracks longer than 40µm at four different chosen impact cycles and two different test forces ...... 42 Figure 57: the observed indentation radius of the impacted area at four different chosen impact cycles and two different test forces ...... 43 Figure 58: The measured penetration depth of the Impactor on the impacted area at four different chosen impact cycles and two different test forces ...... 44 Figure 59: Approximation of the applied pressure calculated in Hertzian contact pressure at the area of the impact at four different chosen impact cycles and two different test forces .... 45 Figure 60: Approximation of the applied pressure calculated in mean pressure at the area of the impact at four different chosen impact cycles and two different test forces ...... 46

V Figure 61: Tested samples are compared by four different observed test criteria and according to their comparative relations to each other (1 shows the worst and 9 shows the best position in their relative positions) ...... 48 Figure 62: Comparative position of the tested samples against their measured hardness number ...... 49

VI List of tables

Table 1: Important variables that affect the fatigue resistance from [2] ...... 12 Table 2: Material properties of DLC materials and other carbon allotropies from [16] ...... 22 Table 3: Test material matrix ...... 27 Table 4: Chemical composition of the steel matrix of the Impactor from [20] ...... 29

VII Presentation of Ovako

Ovako is an European company in production of long special steels. Ovako’s production range covers low alloy steels and carbon steels in the form of bars, wire rod, tubes and rings. Ovako’s total net sales are around EUR 1.7 billion, and number of employees reaches around 4,000 people at 15 production sites and several sales companies in Europe and the USA. Their total production is 2 million tonnes of steel annually that is supplied by the following respective group divisions.

Figure 1: Group structure and net sales by division charts of Ovako from [19]

And following figures show some of the examples of various applications of Ovako’s products, and main production schemes of the Ovako Hofors AB based in Hofors Sweden.

1

Figure 2: Some of the examples of various applications of Ovako’s products from [19]

Figure 3: Production plan: Billet manufacturing in Hofors from [19]

2

Figure 4: Production plan: Hot rolled tubes from [19]

Figure 5: Production plan: Cold rolled tubes from [19]

3 And following figure shows three different ways for production of rings in Ovako Hofors AB. Generally as could be seen from bellow figure, for line 1, after having bars with the demanded temperature, rings are cut, then forged and finally rolled, but in lines 2 and 3, small bars that have already been cut to the requested length are heated up, then forged and finally they are either rolled or forged to get the final shape.

Figure 6: Production plan: Hot rolled rings from [19]

4 Introduction

The most common test used for evaluation of metal's fatigue properties is called rotating bending test, this type of fatigue evaluation method is very well known, well explained and standardized with several internationally respected organizations; as a mater of fact there are plenty of sources that cold be referred in order to have a fairly detailed knowledge about these tests.

It is believed that there are numerous applications and situations that could lead to metal fracture and failure as a result of fatigue and could not be perfectly explained by the rotating bending fatigue tests.

Without a doubt several engineering parts are under dynamic impact forces, to have a general view about this claim it is possible to number various examples, for instance a drilling head that is used in mining applications (for demolishing rocks) , or even various metallic parts that are used in different parts of crushers that are utilized in mineral processing applications, and there is no well specified and/or standardized method for having a grasp about resistance of the products or components in such conditions.

Consequently at this time no reliable comparison among different metals could be made for such applications.

This thesis work is an attempt to introduce a new testing method for evaluation of the metal's fatigue properties; this project is accomplished due to the market demand of Ovako Company for the assessment of the different steel's resistance under cyclic impact pressures.

During this project, several different steel grades that were made by Ovako were chosen, then they were treated differently (Annealed, through hardened, carburized, nitridized, induction hardened, and DLC coated), finally by the means of an in house Vibrophore machine, a costume made cyclic Impactor and a test setup were introduced.

Following pages are more detailed descriptions of the accomplished project.

5 Theoretical Background

Fatigue and Fracture Mechanics Introduction

Metal fatigue could be defined as the progressive, confined to a small area, and continuing structural damage that takes place when a material is under repeated strains at insignificant stresses that are less than (and usually much less than) the actual yield strength of the material [1].

According to reference 1, the whole process of metal failure caused by fatigue could be metallurgicaly divided into following five steps:

1. Repeated plastic deformation proceeding to the initiation of fatigue crack. 2. Initiation of microcracks (that could be one or several). 3. Propagation or merging of microcracks in order to form one or several microcracks. 4. Appearance and propagation of one or several bigger cracks (macrocracks). 5. Fatigue failure of the metal.

In general, three following concurrent states of affairs are necessary for the incidence of fatigue damage (in case there is lack of any of these following conditions, it could be expected that no crack initiate and propagate): 1. Cyclic stress that could be defined as a stress that exists over time in a repetitive mode. 2. Tensile stress that is the stress at which a material permanently deforms or breaks. 3. Plastic strain that is a strain in that the deformed material does not return to its original size and shape after the deforming force is removed.

As a matter of fact, cyclic stress causes the plastic strain, and this occurred plastic strain is the reason of crack initiation, and subsequently the tensile stress (that could be localized tensile stresses) progressively propagates the crack [1].

Cyclic Deformation Prior to Fatigue Crack Initiation

In order to study the whole process that would lead to metal fatigue it is required to have a detail understanding about fatigue steps. It is known that microcracks could initiate as a result of an adequately large number of alternating plastic strain amplitude. As soon as a dislocation appears at the surface, a slip step could occur. When several slip steps accumulate in a specific area, surface roughening happens.

Following figure shows the appearance of extrusions and intrusions according to the shown steps, in another word the process of happening of two intersecting slip systems is believed to be according to the depicted stages [3].

According to this figure indentation is produced in the slip system (c), this indentation or notch is followed by the appearance of protrusion and finally throughout the second half-cycle, both first and second slip systems are assumed to function again, resulting to appearance of an intrusion and extrusion pair [3].

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Figure 7: Intrusion and extrusion formation according to Cottrell-Hull model from [3]

Nonetheless, areas of severe surface roughness are produced as a result of slip bands plastic deformation, and this is rather common for different metals, however slip bands that are coarse or localized plastic strains happen more likely as a consequence of cyclic deformation rather than throughout monotonic deformation [3].

Initiation of Microcracks

According to reference 4 fatigue cracks initiation are likely to take place alongside slip bands, in inclusion, in boundary of grains, in particles that are counted as second-phase, and matrix phase interfaces with the second-phase.

The approach that could lead to the initiation of fatigue crack depends on any of above mentioned items that are likely to happen most easily; however it is claimed that grain boundaries are principally more vulnerable to fatigue crack initiation [3].

Throughout recurring plastic deformation, dislocations either appear at the metal surface or stack up adjacent to obstacles. If the dislocations always appear at the surface more than stack up next to obstacles, then slip bands that finally turn into cracks emerge in the central sections of the grains, where the stress is smaller. As a matter of fact, inclusions, grain boundaries, oxide layers, and field boundaries are different types of obstacles that could lead to pile up of dislocations during stress recurrence [3].

7 Above mentioned dislocation pile-ups raise elastic strain energy. As soon as the strain energy density goes beyond double the surface free energy, a state of instability happens that energetically is in favour of the microcracks initiation [3].

Following figure shows how paired dislocation pile-ups could cause an intrusion or extrusion by an avalanche [3]. In this figure an oxide inclusion on the surface is supposed to be the barrier, however the same conclusion could be made for other types of obstacles.

Figure 8: Paired dislocation pile ups from [3]

Following figure depicts a slip band in a HSLA steel that is known to be cracked according to the above mentioned procedure by originating from a grain boundary. This picture is taken by scanning electron micrograph of polished notched steel following 300,000 cycles at 850 MN/M2, slip band that is shown in circle is believed to be already cracked.

Figure 9: Slip band in circle that is believed to be already cracked, originating from a grain boundary from [3]

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Propagation and Coalescence of Microcracks

Microcracks that are already emerged may not develop further for several known reasons. For instance, a fatigue crack may originate inside an inclusion, but might be hindered or stopped by the interface between the inclusion and metal matrix because the needed stress to enlarge the crack toward the matrix is bigger than that demanded to expand the crack within the inclusion [4].

Generally the numbers of microcracks that appear throughout fatigue are dependence on stress or plastic strain amplitude, at high amplitudes, numerous cracks appear, and merging alongside grain boundaries is the prevailing sort of microcrack development; however at low stresses, development of individual microcracks is the matter of most importance [4].

Growth of Macrocracks

When a macrocrack with a length of several millimetres is appeared, the range of stress- intensity of linear fracture mechanics appear to be the rate-dominating factor with the condition that just low scale yielding happens at the crack tip [4].

Here ΔK (stress intensity) could be calculated as Δσ , where Δσ is equal to the maximum nominal stress minus the minimum nominal stress; respective (a) is the length of the crack; and f(g) is a function of sample geometry, conditions of loading, and the respective ratio of crack length to sample’s width [4].

As a final explanation regarding the theory behind the fatigue procedure it could be said that fatigue crack growth manners is categorized by three regimes that could contain the area in which the extent of yielding at the tip of the crack is not essentially low.

By knowing that fatigue crack extension amounts are normally plotted as graphs of da/dN versus ΔK above mentioned three regions could be described as per below[4]:

• Area I: Threshold and near threshold area that in which da/dN declines quickly by reduction in ΔK to a threshold value. • Area II: Middle area in which relation of Paris is supposed to be dominant, or da/dN=C(ΔK)N (C and N are both constants). • Area III: High rate area in which the Max stress intensity, goes toward the critical stress intensity.

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Figure 10: Rate of fatigue crack propagation, dA/dN, versus range of stress intensity, ΔK from [4]

Following figure depicts the single procedures that ultimately cause fatigue failure based on the sequential order of their incidence.

Figure 11: The sequence of procedures throughout metal fatigue from [5]

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The fatigue resistance property of a metal is determined by carrying out several tests over a range of values of strain range or stress amplitude.

Finally the counted number of cycles that lead to metal failure is plotted versus the strain range or the stress amplitude to gain a curve that is called fatigue curve. It could be mentioned that because of the different factors affecting the fatigue resistance of metals, several tests might be required to assertively draw a fatigue curve.

Fatigue analysis are conducted on fairly small, and assumed to be primarily crack-free, samples of a metal that is meant to be representative of the metallurgical and mechanical properties of the tested metal.

Prepared samples are generally smooth and have evenly polished surfaces within the section that is to be tested. The aim of this polishing is to reach a reproducible surface finish, in another word this polishing is required in order to have control on the fatigue properties of different samples based on their surface qualities and/or surface treatments (The whole idea is to eradicate surface finish as an uncontrolled variable) [2]. However, this could be mentioned that in case the fatigue test is performed in order to evaluate the effects of surface treatments (surface modifications) on the fatigue limit of the samples, samples should not be polished.

Usually, the fatigue failure life of a test sample is assumed as the required number of repeated cycles that lead to breakage of the specimen into two pieces.

According to reference 2 microcracks are likely to initiate rather early in cyclic life (roughly saying in the first 1–10%) in the high strain, plasticity-controlled, low-period life stage. In this area, periodic plasticity is prevalent throughout the test sample; consequently the plastic strain range is utilized as a measure of the severity of fatigue loading; however macrocracks start to initiate rather lately in cyclic life (roughly saying 90–99%) in the very low-strain, elastically controlled, high-cycle life area.

Even though the results of fatigue tests that are achieved from representative test samples do not exactly depict the fatigue life of an actual final part, such results do provide practical data on the fatigue crack initiation manner of a metal. Consequently, such information can be used to build up engineering design principles to avoid initiation of fatigue cracks in real engineering components.

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Following table depicts the main important variables that affect the fatigue resistance of different metals.

Table 1: Important variables that affect the fatigue resistance from [2]

12 Impact Fatigue

Impact fatigue could be defined as fracture of components or specimens that are repeatedly subjected to impacts. By accomplishing a quiet extensive literature survey it is claimed that the knowledge on the properties of surface modified component under cyclic contact loads is rather small [6].

There have been extensive researches on the matter of tribomechanical properties of the surface treated components, however most of these characterizations are based on the scratch, indentation test, or well known evaluating techniques like abrasive wheel or pin on disc test.

By looking at the results of these researches it could be concluded that cyclic impacts plastically deforms the substrate, this cause the appearance of cracks and surface damages, finally it would lead to degradation or failure of the coating layer or interface. However, most of these tests either concentrate on the cyclic behavior of the coating layer without considering the possible effects of the substrate properties on the results or just look at fatigue life of samples under a specific load [6].

One of the main differences between ordinary fatigue tests and impact fatigue test is the amount of involved strain rate that for impact fatigue evaluations is known to be around 103s-1 that is much higher than conventional fatigue tests [7].

According to [7] during last century different testing methods are applied in order to measure the impact fatigue properties of different ferrous alloys, however because of the lack of a standard method these testing procedures are rather different from each other, and as a consequence achieved results are not comparable.

In reference 7 it is expressed that different testing methods which have been used could be divided into following groups (based on the applied dynamic impact load):

• Impact load by using falling tubs. • Falling cylinder that dynamically impacted the notched specimens. • Falling hammer with improved acceleration that was achieved by a coil spring. • Use of cyclic tensile impacts instead of compressive loads. • Use of Hopkinson split bar method • Use of falling grinding balls

It is said that several published studies are mainly dedicated to quenched-tempered steels [7]; bellow some achieved results of these tests are briefly written:

• According to [21] best tempering temperature in order to maximize the impact fatigue life for Ni-Cr low alloy steels is around 250ºC.

• According to [22] for tool steels containing 11.5%Cr and 1.48% C, at high stress range impact fatigue properties are improved by presence of retained austenite, but at low stress range contrary is believed to be true.

13 • According to [23] for maraging steels retained austenite that is achieved by thermal cycling will lead to formation of a dispersed austenite that enhances the impact fatigue properties at low cycle impact ranges.

• According to [24] carburizing surface modification has very low or no effect on the impact fatigue properties for low hardenability steels however enhances the impact fatigue properties for high hardenability steels.

• According to [25] impact fatigue properties are improved by increasing core hardness.

• According to [21] for Ni-Cr steels that were tested between 25 ºC and –120ºC, by reducing temperature, impact fatigue properties are enhanced.

• According to [26] for plain carbon steels that were tested in 25ºC and –30ºC, these steels have lower impact fatigue properties at 25ºC in comparison with –30ºC.

According to [8], regarding the surface coated metals it is possible to obtain rough estimation of the load in the contact area using the Hertz theory, however, it should be mentioned that due to the fact that the treated surfaces are not necessarily completely elastic and fully homogeneous, the actual stress is not totally same as calculated.

Based on the Hertz theory in case of uniform material, a hydrostatic compressive stress situation appears in the centre of the impact surface, and for an elastic situation, the highest shear stress appear at a depth of 0.47-0.5a (a is the diameter of contact zone which is elastically flattened under the load) [8].

So it could be said that for thin layers on the surfaces (coatings), the major fraction of the applied impact pressure is passed throughout the coating layer to the substrate, that is the place in which most of the stress arises in the form of shear. However, in coating systems with very high hardness the stress distribution is totally different [8].

According to studies done by reference [9] regarding the impact fatigue properties of the surface treated metals circular cracks that appear on the coating surface are more suitable criteria than subsurface or interfacial cracks for evaluating significant degradation beneath spherical indenter. This declaration originates from the fact that these surface cracks not only appear as the initial failure aspect observed by rising load or cycle's numbers but also cause appearance of cracks in substrate with no in-between breakdown at the interface layer [9].

For loads more than endurance limit and after proper cycles (104-106 cycles) two following kinds of surface cracks are expected to appear [9]:

• Ring-Cone cracks - Ring shaped cracks that emerge outside the circular contact area. After cutting the samples and looking at the cut view it was observed that these cracks twist outward making a cone. By increasing the total amount of applied load in the experiment these cracks gets the tendency to appear in more distances from the contact circle. • Sub-surface tangential cracks - These types of cracks were observed under the contact area after applying around ten times more cycles required for appearance of Ring- Cone cracks. These cracks are shallow U-shape and they appear at a specific distance from the surface independent of the applied load.

14 Hertzian Contacts

In numerous applications, engineering parts surfaces are regularly exposed to contact pressures, in which huge loads are put on a limited localized area. These kinds of pressure are called contact pressure and are categorized as Hertzian contact loadings. Fatigue wear is resulted from contacting surfaces under Hertzian contact stresses with very high magnitude.

To be more precise according to [10], Hertzian contact pressure is valid and could be calculated when:

• Loading over the surfaces is continuous. • The contact area is limited, and is considerably smaller than the members' dimensions and their respective surface curvature radius. • The nature of the contact is just elastic and neither of the surfaces have plastic strain. • friction is supposed to be missing or neglected

According to Hertzian theory from [11] the actual contact area between two bodies is dependent on the applied load, and hardness of the softer contact surfaces. A practical approximation for the actual contact area (in mm2) could be given by:

W

10H

Where W is the applied load in Newtons and H is the hardness of the softer body (Vickers or Brinell), and in case there is a contact involving a plane and a ball (see the below figure), it could be said that [11]:

Figure 12: Elastic contact between ball and plane [11]

Radius of the contact is:

1 3 ⎛WR ⎞ 3 a = ⎜ ⎟ 4 ⎝ E ⎠

Hertz pressure or maximum contact pressure is:

15 1 ⎛WE 2 ⎞ 3 PH ≅ 0.6⎜ ⎟ ⎜ 2 ⎟ ⎝ R ⎠

Where E and R could be achieved from followings:

1 1− v 2 1− v 2 = 1 + 2 E E1 E2

E1 and E2 are Young's moduli and V1 and V2 Poisson's ratios for the two contact members.

1 1 1 = + R R1 R2

And R1 and R2 are the respective curvature radius of the contact members.

Area Calculation of the Spherical Segments

According to [28] a spherical segment is the solid area that is achieved by cutting a sphere with two parallel planes. It could also be thought as a spherical cap with a flat top (see below figure).

Figure 13: Schematic of a spherical segment from [27]

In case the radius of the above sphere is called R and the segment height (the distance from the top of the segment to the bottom) is assumed h and radii of the upper and lower planes are called b and a respectively, following relations could be used in order to calculate the surface area of the spherical segment that is called S excluding area of top and bottom planes [28].

[(a − b) 2 + h 2 ][(a + b) 2 + h 2 ] R = and S = 2πRh 4h 2

The calculated surface area that is achieved by the above mentioned formula plus the surface area of the top plane could be used in order to calculate the respective applied mean force in MPa or N/mm2 on the specimen, and the respective above mentioned formula could also be used in order to calculate R that is needed for calculating Hertzian pressure.

16 Surface Modifications

Case hardening treatments for example carburising and nitriding have successfully been used in order to enhance fatigue properties of steels for several decades. The cause of this enhancement in fatigue properties is explained by considering both tensile and compressive residual stress of the area that is treated after the heat treatment [12].

It is known that occurred compressive residual stress in the modified area of the surface is neutralized by the tensile stress that is occurred in the metal core; the extent and allocation of residual stress in the area as well as generated strength in the region could be used for making a conclusion about the location of fatigue crack origin [12].

Predominantly in even components, residual stresses have the chief function on fatigue properties; However the enhancement in fatigue properties of different metals that are assumed to be due to surface modifications might be credited to two factors as per the greatest compressive residual stress and the distribution of that [12].

Induction Hardening

Induction hardening is a widely utilized process for hardening the surface of steel parts. During this process steel components are heated by using an alternating magnetic field to a temperature that is about or higher than the transformation temperature followed by quick quenching. As the result the core or center of the component stays untouched by the heat treatment and its mechanical properties are still those of the original part; however the achieved surface hardness (called case hardness) could be varied in the range of 37 to 58 HRC [13]. A high frequency source of electricity is utilized to generate an alternative current throughout a coil. The current channel through this coil cause a strong and rapidly altering magnetic field in the opening within the coil. The metal part that is supposed to be heat treated is positioned within this magnetic field, consequently as a result of generated eddy currents the metal part is heated [13]. Metallurgically it could be said that induction hardening causes a core that is tough and a surface layer that is hard. According to reference [13] wear resistance performance of parts that are induction hardened depends on hardened depth and the extent and distribution of remaining stress in the surface layer. Following figure shows the relation between the possible archived surface hardness based on the carbon content of the steel after the induction hardening treatment.

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Figure 14: Relation between the surface hardness based on the carbon content of the steel from [14]

Carburizing

Carburizing could be defined as the process in which the surface of the steel part with a carbon content of typically 0.15 to 0.25% is exposed to a carbon carrying medium at about 870-980ºC, as the result of carbon diffusion, the steel surface is carburized and a carbon contents in the range of 0.7–to 1.0 % C is achieved, finally by the means of quenching the structure of this carburized surface is transformed to martensite [15].So it could be said that carburizing leads to a hard and in comparison with parts dimensions usually thin surface on comparatively originally soft metals.

Following figure taken from reference [15] shows the changes of the surface hardness of three different carburized steel grades based on the measured distance from their respective surfaces.

Figure 15: Hardness range of carburized microstructures versus distance from the surface from [15]

18 Following figures depict the microstructure of the carburized surfaces of two different steel grades.

Figure 16: Plate martensite and retained austenite in the case (14NiCr18, 0.7% C) from [15]

Figure 17: Plate martensite, SEM micrograph (16MnCr5) from [15]

19 Nitriding

Nitriding also known as nitridization modifies the surface composition of steels in a way pretty similar to carburization. Nitriding steel is achieved by the diffusion of nitrogen at 500 to 550 °C to the surface of the components. The supreme benefit of nitriding over carburizing is that it does not require further heat treatments in order to harden the steel surface; as a result nitriding leads to a better final dimensional control [15]. In nitridization, surface of a steel part is heated and exposed to nitrogen in a bath of organic compounds containing nitrogen or nitrogenous gas flow (often ammonia ). Final achieved surface hardness due to nitriding could be as high as 70 HRC.

Following figure shows the nucleation scheme of nitrogen atoms on the surface of the treated steel.

Figure 18: Plot of nucleation of nitrides on iron from [15]

Following figure taken from reference [15] shows the changes of the surface hardness of four different nitrided steel grades based on the measured distance from their respective surfaces.

20

Figure 19: Hardness range of nitrided microstructures versus distance from the surface from [15]

Following figures depict the microstructure of the nitrided surfaces of some parts made of pure iron.

Figure 20: Pure iron nitrided, etched with Nital from [15].

Figure 21: Pure iron nitrided, SEM micrograph from [15]

21 Diamond-like Carbon (DLC)

According to reference 16 two types of DLC materials could be categorized as amorphous carbon and hydrogenated amorphous carbon that are respectively shown as (a-C) and (a-C:H); these unique materials play a significant roles in managing wear resistance and friction properties between two or several sliding surfaces of various moving components.

These materials depict the following valuable properties: • Resistance to wear as the cause of a very high hardness • Low friction coefficient • Chemical inertness and electrical insulation • Optical transparency and smoothness • Biological compatibility

Following table shows the general material properties of DLC materials as a comparison with other types of carbon allotropies

Table 2: Material properties of DLC materials and other carbon allotropies from [16] Crystal Structure Cubic Amorphous Amorphous Cubic Hexagonal 2 3 3 2 ao=3.561Å Mixed sp and sp bonds sp /sp ao=3.567Å a=2.47 Form Faceted crystals Smooth or rough Smooth Faceted crystals Hardness (Hv) 3000-12000 1200-3000 900-3000 7000-10000 3 Density (g/cm ) 2.8-3.5 1.6-2.2 1.2-2.6 3.51 2.26 Refractive Index - 1.5-3.1 1.6-3.1 2.42 2.15 Electrical Resistivity (Ω/cm) >1013 >1010 106-1014 >1016 0.4 Thermal Conductivity (W/m.K) 1100 - - 2000 3500 Chemical Stability Inert Inert Inert Inert Inert Hydrogen Content (H/C) - - 0.25-1 - - Growth Rate (µm/hr) ~1 2 5 1000 (synthetic) -

Following methods are some of the best known processes for covering surfaces of the components by DLC material [16].

• IBD (ion beam deposition) • PVD (physical vapour deposition) • PACVD (plasma assisted chemical vapour deposition)

Following ternary phase diagram could be used in order to find out the possible ratio among two carbon allotropies and hydrogen in order to achieve the desired coating material.

22

Figure 22: DLC’s atomic structure and composition from [17]

Following figures show the microstructure of the pure DLC material and is taken by electron microscopy.

Figure 23: Nanostructure of pure DLC showing the nodules from [18]

23

Figure 24: TEM image of cross-section of the DLC from [18]

It is worthy to mention that it is claimed that DLC coatings show different properties in different environments, for example following figure that is taken from reference [30] shows that DLC coating that contained hydrogen and tested by pin and disk method failed catastrophically in presence of water, however DLC coating that was hydrogen free had no wear at all.

Figure 25: Pin and disk wear rate of three different DLC coated substrates in dry and 50%RH. Applied force was 5N and velocity of sliding 0.004 ms-1 , taken from [30]

24 Experimental

Test Procedure

In order to accomplish the Impact fatigue test a material matrix based on the applications and costumer demands was chosen, then chosen materials were machined in the demanded shape and their surfaces were treated accordingly (based on the material matrix), then an Impactor was designed and made, and finally test started.

Following lines state the applied stages of the Impact fatigue test:

1. A static load of 78N (chosen based on several experiments) was applied between the Impactor and the specimen, this load was chosen constant for all specimens. This static force was necessary in order to make sure that the Impactor starts from the exact same position in all tests.

2. Applied static load was removed.

3. An active air gap between Impactor and specimen was applied; this air gap was constant for all tests and all specimens. This should be mentioned that active air gap makes sure that no matter what happens between Impactor's carbide head and the area of the contact, distance between them remains constant during the whole test (this set up helps to have the situation very similar to reality).

4. A dynamic force was applied for a demanded cycle of impacts (samples were tested in two different dynamic loads of 1900N and 2400N at four different cycles of 103, 104, 105, and 5x105). Following figure shows the mounted specimen under dynamic force.

Figure 26: Mounted specimen under dynamic load

25 5. Specimen was un-mounted and it's surface was observed for the following items:

• Number of cracks with the length more than 40µm. • Length of the longest observed Circular Surface Ring crack. • Position of the cracks. • Radius and deepness of the indentation area. • Linear surface topography. • Surface hardness.

6. Based on the measured radius and deepness of the contact areas mean applied pressure in MPa and Hertzian contact pressure were calculated.

Following machine that is called Perthometer PGK was used in order to achieve linear surface topography of the impacted specimens.

Figure 27: Perthometer PGK used for 2D surface topography

26 Material

Following table depicts the material matrix that was used for the impact fatigue test, as it could be seen nine different steel grades with various surface modifications were chosen for this matrix.

Table 3: Test material matrix

Sample Designation Ovako Grade Surface Modification Hardness HRC A 804Q Annealed 36 B 256G Carburized 57 C 225A Nitrided 39 D 277Q Nitrided 44 E 528Q Induction hardened 59 G 803J Hardened & DLC coated 62 H 528Q Hardened & DLC coated 58 I 804Q Through Hardened & DLC coated 59 J 804Q Through hardened 59

As it could be seen first column shows the designated letter for each grade with its respective surface modification; this sample designation was used during all tests and material handling in order to ease tracing samples and documenting results.

Surface modifications and heat treatments were done according to respective datasheets of each steel grade; it could be mentioned that the thickness of the nitriding layer was between 0.1 to 0.2mm, the thickness of the induction hardened layer was about 2mm, and carburized layer was about 0.6 to 1mm.

Respective hardness number of every sample was measured by hardness tester machine type LECO RT-240. It must be considered that in order to measure the hardness of the DLC coatings, due to the very thin thickness of the DLC coated layer, special hardness tester machines are required.

27 Machine

Utilized testing machine was a High Frequency Vibrophore type Amsler HFP 5000 with the maximum force 150 KN (picture is shown is following figure). This machine is a type of resonance machine that runs on the electromagnetic drive, its upper part works as the pure dynamic unit and its lower part is solely acting as a static unit.

Figure 28: Pictures of the applied machine (left picture is the impact part and right picture is the control panel)

28 Impactor

Impactor that was used in the test set up was made from carburized steel grade Ovako 253G (SAE 1020) with the following chemical composition, and then W-Co carbide insert made by Atlas Copco Secoroc AB was placed in that.

Table 4: Chemical composition of the steel matrix of the Impactor from [20] C Si Mn P S Cr Mo Ni Cu V Al Ti Min 0.17 0.2 0.3 0 0.015 1.2 0.2 2.5 0 0 0.02 0 Max 0.21 0.3 0.5 0.02 0.03 1.4 0.26 2.9 0.25 0.1 0.04 50

And following figure depict the geometrical features of the Impactor.

Figure 29: Sketch of the Impactor

Specimen

Following figures depict the sketch of the utilized specimens, it was highly desired to have the same machining background for surfaces of all specimens; consequently before respective surface treatments all specimens were fine machined.

Figure 30: Sketch of the specimens

29 Results

Achieved results of impact fatigue test on steel grades could be numbered as below:

1- Material designated A with the test numbers 1 to 9: this tested steel grade was Ovako steel grade 804Q, annealed with the hardness of 36HRC. As could be seen from following figures, as the results of the impact fatigue test, this grade depicted several very long surface ring cracks with the maximum length of 223.7µm, these cracks were scattered from the surface of the impact circle and continued for about 0.3 mm outside the outer radios. The biggest measured indentation radios of the impacted area was 2.15mm and the deepest penetration depth was 170µm (more figures are available in the index part of this report).

Figure 31: Picture taken by stereo macroscope from the area of impact from steel 804Q, Annealed, after 105 cycles, at 2400N dynamic force

Figure 32: Picture taken by optical microscope showing the biggest observed surface ring crack on steel 804Q, Annealed, after 5x105 cycles, at 2400N dynamic force

30

Figure 33: Result of the linear surface topography of steel 804Q, Annealed, after 5x105 cycles at 2400N dynamic force

2- Material designated J with the test numbers 10 to 17: this tested steel grade was Ovako steel grade 804Q, through hardened with the hardness of 59HRC. As could be seen from following figures, as the results of the impact fatigue test, this grade depicted several long surface ring cracks with the maximum length of 86µm, these cracks were scattered from the surface of the impact circle and continued for about 0.1 mm outside the outer radios. The biggest measured indentation radius of the impacted area was 1.34mm and the deepest penetration depth was 62µm (more figures are available in the index part of this report).

Figure 34: Picture taken by stereo macroscope from the area of impact from steel 804Q, through hardened, after 5x105 cycles, at 2400N dynamic force

31

Figure 35: Picture taken by optical microscope showing the biggest observed surface ring crack on steel 804Q, through hardened, after 5x105 cycles, at 2400N dynamic force

Figure 36: Result of the linear surface topography of steel 804Q, through hardened, after 5x105 cycles at 2400N dynamic force

3- Material designated B with the test numbers 18 to 25: this tested steel grade was Ovako steel grade 256G, carburized with the hardness of 57HRC. As could be seen from following figures, as the results of the impact fatigue test, this grade depicted several very short surface ring cracks with the maximum length of 67.46µm, these cracks were scattered from the surface of the impact circle and continued for about 0.1 mm outside the outer radios. The biggest measured indentation radios of the impacted area was 1.71mm and the deepest penetration depth was 50µm (more figures are available in the index part of this report).

32

Figure 37: Picture taken by stereo macroscope from the area of impact from steel 256G, carburized, after 5x105 cycles, at 2400N dynamic force

Figure 38: Picture taken by optical microscope showing the biggest observed surface ring crack on steel 256G, carburized, after 5x105 cycles, at 2400N dynamic force

Figure 39: Result of the linear surface topography of steel 256G, carburized, after 5x105 cycles at 2400N dynamic force

33 4- Material designated C with the test numbers 26 to 33: this tested steel grade was Ovako steel grade 225A, nitrided with the hardness of 39HRC. As could be seen from following figures, as the results of the impact fatigue test, this grade depicted several extremely long surface ring cracks with the maximum length of 818.1µm, these cracks were scattered from the surface of the impact circle and continued for about 0.1 mm outside the outer radios. The biggest measured indentation radios of the impacted area was 1.89mm and the deepest penetration depth was 80µm (more figures are available in the index part of this report).

Figure 40: Picture taken by stereo macroscope from the area of impact from steel 225A, nitride, after 5x105 cycles, at 1900N dynamic force

Figure 41: Picture taken by optical microscope showing the biggest observed surface ring crack on steel 225A, nitrided, after 5x105 cycles, at 1900N dynamic force

Figure 42: Result of the linear surface topography of steel 225A, nitrided, after 5x105 cycles at 1900N dynamic force

34 5- Material designated D with the test numbers 34 to 41: this tested steel grade was Ovako steel grade 277Q, nitrided with the hardness of 44HRC. As could be seen from following figures, as the results of the impact fatigue test, this grade depicted several very long surface ring cracks with the maximum length of 112.9µm, these cracks were very scattered outside the outer radios. The biggest measured indentation radios of the impacted area was 1.81mm and the deepest penetration depth was 60µm (more figures are available in the index part of this report).

Figure 43: Picture taken by stereo macroscope from the area of impact from steel 277Q, nitride, after 5x105 cycles, at 2400N dynamic force

Figure 44: Picture taken by optical microscope showing the biggest observed surface ring crack on steel 277Q, nitrided, after 5x105 cycles, at 2400N dynamic force

Figure 45: Result of the linear surface topography of steel 277Q, nitrided, after 5x105 cycles at 2400N dynamic force

35 6- Material designated G with the test numbers 42 to 49: this tested steel grade was Ovako steel grade 803J, DLC coated with the hardness of 62HRC. As the results of the impact fatigue test, this grade depicted no crack. The biggest measured indentation radios of the impacted area was 1.3mm and the deepest penetration depth was 40µm (more figures are available in the index part of this report).

Figure 46: Picture taken by stereo macroscope from the area of impact from steel 803J, DLC coated, after 5x105 cycles, at 1900N dynamic force

Figure 47: Result of the linear surface topography of steel 803J, DLC coated, after 5x105 cycles at 1900N dynamic force

36 7- Material designated H with the test numbers 50 to 57: this tested steel grade was Ovako steel grade 528Q, DLC coated with the hardness of 58HRC. As the results of the impact fatigue test, this grade depicted no crack. The biggest measured indentation radios of the impacted area was 0.88mm and the deepest penetration depth was 30µm (more figures are available in the index part of this report).

Figure 48: Picture taken by stereo macroscope from the area of impact from steel 528Q, DLC coated, after 5x105 cycles, at 1900N dynamic force

Figure 49: Result of the linear surface topography of steel 528Q, DLC coated, after 5x105 cycles at 1900N dynamic force

37 8- Material designated I with the test numbers 58 to 65: this tested steel grade was Ovako steel grade 804Q, DLC coated with the hardness of 59HRC. As the results of the impact fatigue test, this grade depicted no crack. The biggest measured indentation radios of the impacted area was 1.12mm and the deepest penetration depth was 40µm (more figures are available in the index part of this report).

Figure 50: Picture taken by stereo macroscope from the area of impact from steel 804Q, DLC coated, after 5x105 cycles, at 2400N dynamic force

Figure 51: Result of the linear surface topography of steel 804Q, DLC coated, after 5x105 cycles at 2400N dynamic force

38 9- Material designated E with the test numbers 66 to 73: this tested steel grade was Ovako steel grade 528Q, induction hardened with the hardness of 59HRC. As could be seen from following figures, as the results of the impact fatigue test, this grade depicted some surface ring cracks with the maximum length of 65.4µm, these cracks were scattered from the surface of the impact circle and continued for about 0.1 mm outside the outer radios. The biggest measured indentation radios of the impacted area was 1,44mm and the deepest penetration depth was 35µm (more figures are available in the index part of this report).

Figure 52: Picture taken by stereo macroscope from the area of impact from steel 528Q, induction hardened, after 5x105 cycles, at 2400N dynamic force

Figure 53: Picture taken by optical microscope showing the biggest observed surface crack on steel 528Q, induction hardened, after 5x105 cycles, at 2400N dynamic force

Figure 54: Result of the linear surface topography of steel 528Q, induction hardened, after 5x105 cycles at 2400N dynamic force

39 Discussion

By considering the achieved results of the impact fatigue tests it would be a better idea to summarize these results into respective graphs, details of the test specifications that lead to these following graphs could be found at the index parts of this report.

It was decided to conclude the final result of the impact fatigue test based on the following four criteria:

1. Longest observed surface ring crack (due to the difficulties of crack recognition by the optical microscope, cracks more than 40µm were considered)

2. Number of observed surface ring cracks that were longer than 40µm (cracks were counted until 100, and any number more than 100 is shown as 100)

3. Indentation radius of the impacted area

4. Penetration depth of the Impactor or the deepness of the impacted area.

This should be mentioned that due to the lack of any standard achieved results of the introduced impact fatigue test is just for comparing specific samples with their respective shown modification conditions, in another word various treatments are highly dependent on their respective suppliers, for example if according to the result of the accomplished impact fatigue test samples designated as B is much better than samples designated as C it doesn't mean that a fact could be concluded that carburized surfaces are always better than nitrided surfaces, the result could be different based on different treatment procedures and different suppliers or workshops.

One of the criteria that is important for making a comparison among different samples is the length of the longest observed surface ring crack after accomplishing impact fatigue experiment.

Following graph shows the longest observed surface ring crack at four different chosen impact cycles and two different test forces. It could be mentioned that due to the fact that longest observed surface ring crack on the surface of the steel grade 804Q_annealed that was designated as A was about 225µm, and because these samples were considered as the reference samples, any surface ring crack that was longer than 225µm on the surface of the other samples is shown as 225µm, however the actual length of these cracks are shown in the respective table at the index parts of this report.

By looking at this graph it could be said that DLC coated samples with respective designations of G, H, and I (803J, 528Q, and 804Q respectively) showed no surface crack, the shortest seen surface ring cracks among all test samples (that was the longest surface ring crack seen on surface of the respective specimen) were observed on the surfaces of samples designated as B and E (256G_carburized and 528Q_induction hardened respectively), after these grades comes samples designated as J (804Q_through hardened), then samples D (277Q_nitrided), then samples A (804Q_annealed), and finally samples designated as C (225A_nitrided) that had the longest observed surface ring crack.

40

250

225 225 225223,7

200

180,7

150 A_1900N_804Q_Annealed A_2400N_804Q_Annealed J_1900N_804Q_TH J_2400N_804Q_TH B_1900N_256G_Carburized B_2400N_256G_Carburized C_1900N_225A_Nitrided C_2400N_225A_Nitrided Crack size µm size Crack 112,9 D_1900N_277Q_Nitrided D_2400N_277Q_Nitrided E_1900N_528Q_IH 101,8 102,7 100 E_2400N_528Q_IH

91,86 86

77,83 70,11 67,46 65,53 65,4 63,22 62,3

50 44,43 43,05 40,16 40,8440,99 40,3840,08

0 000000 1000 10000 100000 500000 Number of Cycles

Figure 55: The longest observed surface ring crack at four different chosen impact cycles and two different test forces

41 Second criteria that is important for making a comparison among different samples is the number of the observed surface ring crack that were longer than 40µm after accomplishing impact fatigue experiment. Following graph shows the number of the observed surface ring cracks that were longer than 40µm at four different chosen impact cycles and two different test forces. It could be mentioned that cracks were counted maximum until 100.

By looking at this graph it could be said that DLC coated samples with respective designations of G, H, and I (803J, 528Q, and 804Q respectively) showed no surface ring crack, the lowest number of observed cracks were on the surfaces of samples designated as B and E (256G_carburized and 528Q_induction hardened respectively), after these grades comes samples designated as C (225A_nitrided), then samples J (804Q_through hardened), then samples A (804Q_annealed), and finally samples designated as D (277Q_nitrided) that had the highest number of observed cracks longer than 40µm.

120

100

A_1900N_804Q_Annealed 80 A_2400N_804Q_Annealed

J_1900N_804Q_TH J_2400N_804Q_TH B_1900N_256G_Carburized B_2400N_256G_Carburized 60 C_1900N_225A_Nitrided C_2400N_225A_Nitrided D_1900N_277Q_Nitrided

Number of Cracks>40µm D_2400N_277Q_Nitrided E_1900N_528Q_IH 40 E_2400N_528Q_IH

20

0 1000 10000 100000 500000 Number of Cycles

Figure 56: Number of the observed surface ring cracks longer than 40µm at four different chosen impact cycles and two different test forces

42 Third criteria that is important for making a comparison among different samples is the indentation radius of the impacted area. Following graph shows the measured indentation radius of the impacted area at four different chosen impact cycles and two different test forces.

By looking at this graph it could be said that DLC coated samples with respective designation of H (528Q) showed the smallest indentation radius, following by samples I (804Q_DLC), and samples G (803J_DLC), then come samples designated as E (528Q_induction hardened), after these grades comes samples J (804Q_through hardened), then samples B (256G_carburized), then samples designated as D (277Q_nitrided) following by samples designated as C (225A_nitrided), and finally samples A (804Q_annealed) showed the biggest indentation radius.

2,5

2 A_1900N_804Q_Annealed A_2400N_804Q_Annealed J_1900N_804Q_TH J_2400N_804Q_TH B_1900N_256G_Carburized 1,5 B_2400N_256G_Carburized C_1900N_225A_Nitrided C_2400N_225A_Nitrided D_1900N_277Q_Nitrided D_2400N_277Q_Nitrided G_1900N_803J_DLC G_2400N_803J_DLC

Indentation Radiusmm 1 H_1900N_528Q_DLC H_2400N_528Q_DLC I_1900N_804Q_DLC I_2400N_804Q_DLC E_1900N_528Q_IH E_2400N_528Q_IH

0,5

0 1000 10000 100000 500000 Number of Cycles

Figure 57: the observed indentation radius of the impacted area at four different chosen impact cycles and two different test forces

43 Fourth criteria that is important for making a comparison among different samples is the penetration depth of the Impactor or the deepness of the impacted area. Following graph shows the measured penetration depth of the Impactor on the impacted area at four different chosen impact cycles and two different test forces.

By looking at this table it could be said that samples designated as E (528Q_induction hardened) had the shortest deepness at the impacted area, following by DLC coated samples with respective designation of I (804Q) that had higher penetration depth, then come samples designated as H, G, and B (528Q_DLC, 803J_DLC, 256G_carburized), following by samples J (804Q_through hardened), then come samples designated as D (277Q_nitrided) following by samples designated as C (225A_nitrided), and finally samples A (804Q_annealed) had the biggest penetration depth or highest deepness at the impacted area.

180

160

140 A_1900N_804Q_Annealed A_2400N_804Q_Annealed J_1900N_804Q_TH 120 J_2400N_804Q_TH B_1900N_256G_Carburized B_2400N_256G_Carburized C_1900N_225A_Nitrided 100 C_2400N_225A_Nitrided D_1900N_277Q_Nitrided D_2400N_277Q_Nitrided G_1900N_803J_DLC 80 G_2400N_803J_DLC µm Depth Penetration H_1900N_528Q_DLC H_2400N_528Q_DLC 60 I_1900N_804Q_DLC I_2400N_804Q_DLC E_1900N_528Q_IH E_2400N_528Q_IH 40

20

0 1000 10000 100000 500000 Number of Cycles Figure 58: The measured penetration depth of the Impactor on the impacted area at four different chosen impact cycles and two different test forces

44 Apart from the formerly mentioned results it would be useful to have an idea about the approximate applied pressures at the contact area. Following figures depict an approximation of the applied pressure calculated in both mean pressure and Hertzian contact pressure at the area of the impact at four different chosen impact cycles and two different test forces.

It should be mentioned that due to the plastic nature of these contacts Hertzian contact pressure could just be used in order to make a comparison among different coatings and to have a rough estimation about these pressures, however mean contact pressure seems more reliable to be considered.

60000

50000

A_1900N_804Q_Annealed A_2400N_804Q_Annealed J_1900N_804Q_TH 40000 J_2400N_804Q_TH B_1900N_256G_Carburized B_2400N_256G_Carburized C_1900N_225A_Nitrided C_2400N_225A_Nitrided D_1900N_277Q_Nitrided 30000 D_2400N_277Q_Nitrided G_1900N_803J_DLC G_2400N_803J_DLC H_1900N_528Q_DLC Hertzian Contact Pressure MPa H_2400N_528Q_DLC 20000 I_1900N_804Q_DLC I_2400N_804Q_DLC E_1900N_528Q_IH E_2400N_528Q_IH

10000

0 1000 10000 100000 500000 Number of Cycles

Figure 59: Approximation of the applied pressure calculated in Hertzian contact pressure at the area of the impact at four different chosen impact cycles and two different test forces

45

2000

1800

1600 A_1900N_804Q_Annealed A_2400N_804Q_Annealed J_1900N_804Q_TH 1400 J_2400N_804Q_TH B_1900N_256G_Carburized B_2400N_256G_Carburized 1200 C_1900N_225A_Nitrided C_2400N_225A_Nitrided D_1900N_277Q_Nitrided 1000 D_2400N_277Q_Nitrided G_1900N_803J_DLC G_2400N_803J_DLC

Mpa Mean Pressure 800 H_1900N_528Q_DLC H_2400N_528Q_DLC 600 I_1900N_804Q_DLC I_2400N_804Q_DLC E_1900N_528Q_IH 400 E_2400N_528Q_IH

200

0 1000 10000 100000 500000 Number of Cycles Figure 60: Approximation of the applied pressure calculated in mean pressure at the area of the impact at four different chosen impact cycles and two different test forces

By looking at these calculated contact pressures it could be seen that the bigger the contact area the lower the applied pressure, and by considering the fact that applied impact forces were kept constant it is acceptable, at the same time it could be seen that initial achieved pressure is very high, but becomes smaller and smaller until about 10000 cycles that reaches a constant slope until 500000, it is also acceptable at contact pressure must be calculated after a specific number of cycles in order to find out the correct approximate contact pressure.

After 10000 till 500000 cycles apart from DLC coated samples all samples shows a contact area that by considering the applied force in Newton, impact pressure could be calculated as roughly 200MPa until 400MPa.

46 Conclusion

As the conclusion of the introduced impact fatigue test following statements could be made:

1. It was seen that the introduced impact fatigue test method could be utilized as a tool in order to compare the impact fatigue properties of different steel samples with their different respective heat treatments and/or surface modifications, it could obviously be seen that the achieved results are following the expected experimented results. However it is not claimed that the suggested test setup is the only possible way neither is the best one, on the contrary it is believed and named as introduction to an impact fatigue test that could surely be completed or even revised by reviewing and spending more time on the suggested procedure.

2. Achieved results are highly dependent on the material preparation history, result achieved from different material suppliers or different surface treatment companies could be different, consequently before making any absolute judgments it is desired to do the same impact fatigue tests on the same treatment or surface materials of different suppliers and/or coating companies.

3. Calculated contact mean pressure of the samples (apart from DLC coated samples) during the impact fatigue testing after 10000 cycles follows almost the same constant slope that could be approximated between 200 and 400MPa based on various samples.

4. Due to the lack of an international or even a known standard it is very difficult or almost impossible to make a comparison between achieved test results and other probable results that are achieved from any other experiment in another places with other material matrix.

5. After accomplishing impact fatigue test results as per experimental data of the respective test number, pictures taken at 20X by the stereo macroscope, pictures of the longest observed surface ring cracks taken by optical microscope, result of the 2D linear surface topography of the impacted area are observed, measured, calculated, and finally documented on this report. All of the results are shown on the index part of this report.

6. The final test result regarding comparison of the materials tested could be concluded into the following figure. As could be seen on the below graph all tested samples are compared by four different observed test criteria and according to their comparative relative positions to each other they were given from 1 to 9 points (1 shows the worst and 9 shows the best position in the relative chart), it means that the higher the given point, the better the impact fatigue resistance of the tested samples (in below figure IH means induction hardened, TH means through hardened).

47

4 5 3 TH 4

8 8 9 TH & DLC 9

7 9 9 9 TH & DLC

7 7 9 TH & DLC 9

9 6 6 IH 6

3 3 1 Nitrided 3

2 2 4 Nitrided 1

7 4 6 6 Carburized

1 1 2 2 ABCDEGH I J Annealed

804Q 256G 225A 277Q 528Q 803J 528Q 804Q 804Q

0 5 10 15 20 25 30 35 40 Given Points

Crack Size Number of cracks Indentation radius Penetration depth

Figure 61: Tested samples are compared by four different observed test criteria and according to their comparative relations to each other (1 shows the worst and 9 shows the best position in their relative positions)

7. By looking at the above graph that depicts the final result of impact fatigue test on nine different sample designations it could be concluded that these materials may be numbered as per bellow (from top to bottom come best to worst): • Samples designated as I and H (804Q and 528Q respectively, both were through hardened and DLC coated) • Samples designated as G (803J_ through hardened and DLC coated) • Samples designated as E (528Q_induction hardened) • Samples designated as B (256G_carburized) • Samples designated as J (804Q_through hardened) • Samples designated as D (277Q_nitrided) • Samples designated as C (225A_nitrided) • Samples designated as A (804Q_annealed)

48 8. It could be worthy to look at the relation between these samples' impact fatigue resistance and their measured hardness. As could be seen on the below graph that plots comparative positions of tested samples (based on the above graph) against their measured hardness number, there is a trend between specimens impact fatigue resistance and their respective hardness, or in another word it could approximately be concluded that the higher the materials hardness the more resistance they were against dynamic impacts.

70 60 50

40 30

Hardness HRC 20 10

0 IH

TH

Nitrided Nitrided Annealed TH & DLC TH TH & DLC TH & DLC Carburized 528Q 804Q 803J 528Q 256G 804Q 277Q 225A 804Q HIGEBJDCA

Comparative Position

Best Worst

Figure 62: Comparative position of the tested samples against their measured hardness number

49 Future work

Based on the experience of the project regarding introduction of an impact fatigue testing method, following items could be recommended for any possible future work.

1. Due to the importance of the impact fatigue conditions, an internationally recognized standardized method seems necessary, it will be a great attempt to set a standard for the test set up, sample size, test conditions, etc.

2. Applied forces could be more varied from those used in this project; a higher range of applied forces will lead to better understanding regarding materials resistance in different applications.

3. Test cycles instead of hundreds of thousands cycles could be extended to millions of cycles.

4. During the test applied force was not stable all the time, so it could be useful to write some proper software in order to trace the real applied force and other test properties during the experiment.

5. It could be very useful to do the impact fatigue test with the test environment like the real conditions of different applications; an attempt to make a cover around the sample and Impactor gives the opportunity to do the test in oil, water, mud, etc.

6. A proper three dimensional surface topography could be very useful; with 3D topography it could be possible to have a better understanding about the impact fatigue properties of different samples.

7. It would be a good idea to evaluate the surface residual stress of the samples before doing the impact test; this could be very helpful in case results of the test are not as expected due to the different machining histories.

8. It could be easier to find the biggest surface ring crack and number of cracks very accurate in case some suitable software for automatic crack rating could be prepared.

9. Regarding coatings, different suppliers have various qualities; it could be a good idea to test different suppliers of one specific coating before issuing an idea about the impact fatigue properties of the samples.

50 References

[1] Fine, M.E. And Chung, Y.-W. Fatigue and Fracture, ASM Handbook, ASM International, Vol. 19, 1996, pp. 63–72

[2] ASM International Handbook Committee, Fatigue Failure in Metals, ASM Handbook, Vol. 8, 2000, pp 1571–1650

[3] Cottrell, A.H. And Hull, D. Proc. R. Soc. (London) A, Vol. A242, 1957, P. 211

[4] ASM International Handbook Committee, Fatigue and Fracture, ASM Handbook, Vol. 19, 1996.

[5]. H. Mughrabi, DISLOCATIONS AND PROPERTIES OF REAL MATERIALS, THE INSTITUTE OF METALS, LONDON, Vol. 323, 1985, pp. 244-261

[6] Tarres, E. Ramirez, G. Gaillard, Y. Jimenez-Pique, E. Lianes, L. Contact fatigue behavior of PVD-coated hardmetals, Departament de Ciencia dels Materials i Enginyeria Metal_lurgica, ETSEIB, Universitat Politecnica de Catalunya, www.sciencedirect.com, 2009 (accessed 2 February 2009)

[7] Alan, J. Randall, J. Storeyb, A. The impact fatigue properties of iron and steel, Metals Research Inc and University of Louisville, www.sciencedirect.com, 2007 (accessed 2 February 2009)

[8] Bantle, R. Matthews, A. Investigation into the impact wear behavior of ceramic coatings, Research Centre in Surface Engineering, University of Hull, www.sciencedirect.com, 1995 (accessed 2 February 2009)

[9] Alfredsson, B. Olsson, M. Applying multiaxial fatigue criteria to standing contact fatigue, Department of Solid Mechanics, Royal Institute of Technology, www.sciencedirect.com, 2001(accessed 5 February 2009)

[10] Buschow, K.H. Jürgen, Cahn. Robert, W. Flemings, Merton C. Ilschner, B; Kramer, Edward, J. Mahajan, S. Encyclopedia of Materials - Science and Technology, Elsevier, Vol.1-11, 2001

[11] Neale, M.J. Estimate of contact area, THE TRIBOLOGY HANDBOOK, Butterworth-Heinemann, issue 2, 2001

[12] Genel, K. Estimation method for the fatigue limit of case hardened steels, Mechanical Engineering Department, Sakarya University of Turkey, www.sciencedirect.com, 2004 (accessed 3 February 2009)

[13] Davies, J. Simpson, P. Induction Heating Handbook, McGraw-Hill, 1979

[14] http://www.matter.org.uk/steelmatter/manufacturing/surface_hardness/7_2_2.html, 2008 (accessed 2 February 2009)

[15] ASM International Handbook Committee, Heat Treating, ASM Handbook, Vol. 4, 1996.

[16] Monaghan, D.P. Laing, K.C. Logan, P.A. Teer P. Teer, D.G. Diamond-Like Carbon, Coatings Materials World, Vol. 1, 1993, pp. 347-49.

[17] James, C. Sung, A. Ming-Chi Kan, A. Michael Sung, B. Fluorinated DLC for tribological applications, Kinik Company and Advanced Diamond Solutions, Inc.2002.

[18] Hieke, A. Bewilogua, K. Taube, K. Bialuch, I. Weige, K. Efficient Deposition Technique for Diamond- Like Carbon Coatings, Fraunhofer Institute for Surface Engineering and Thin Films (IST), 2008

[19] Ovako external presentation 2008, www.ovako.com, (accessed 3 February 2009)

[20] Ovako steel applications www.ovako.com/applications/Ovako_Search/index.htm (accessed 5 February 2009)

I [21] Yarema, S. Ya. Kharish, E.L. Crack nucleation and growth periods in 30CrMnSiNiA steel under repeated- impact tensile loads, Materials Sciences, Vol. 5, 1972, pp. 432–434.

[22] Kozyrev, G.V. Toporov, G.V. Effect of retained austenite on the impact fatigue strength of steel, Metallovedenie i Termicheskaya Obrabotka Metallov , Vol. 12, 1973, pp. 45–48.

[23] Li, W. Tu, X.-h. Su, J.-i. Zhou, Q.-d. Microstructure impact fatigue resistance and impact wear resistant low Cr–Si cast iron, Journal of Iron and Steel Research International, Vol. 8, 2001, pp. 17–20.

[24] Diesburg, D.E. Bella, W. Fairhurst, W. Resistance of carburized steels to crack initiation in impact fatigue, Proceedings of the International Heat Treating Conference, International Federation for the Heat Treatment of Materials, Spain, 1978.

[25] Horimoto, M. Matsimoto, H. Makino, T. Murai, N. Onita, K. Arimi, Y. Fujikawa, S. Nishino, T. Effect of core hardness and case depth on low-cycle-impact-fatigue property in carburized steel, Journal of the Society for Materials Science Japan, Vol. 52, issue 11, 2003, pp. 1318–1324.

[26] Matsumure, T. Inoue, Y. Takagi, T. Impact fatigue fracture of axle steel and tire steel, Tetsudo Gijitsu Kenkyogo, Quarterly Report, Vol. 24, issue2, 1983, pp. 94–95.

[27] Wolfram Mth World, http://mathworld.wolfram.com/SphericalSegment.html (accessed 7 April 2009)

[28] Beyer, W. H. CRC Standard Mathematical Tables, Boca Raton, FL: CRC Press, issue 28, 1987, pp. 130

[30] Ronkainen, H. Varjus, S. Holmberg, K. Friction and wear properties in dry, water- and oil-lubricated DLC against alumina and DLC against steel contacts, VTT Technical Research Centre of Finland, www.sciencedirect.com, 1998 (accessed 29 May 2009)

II Index 1: Experimental Data, Test Conditions and Observed Results

Designation Test Conditions Observed results Test A- Number I 1,2… Freq Dyn Load No of Cycles No of cracks >40µm Crack length Crack position a b h 1 A 8 NA 1,9 1000 0 0 0 0,76 0,07 0,25 2 A 8 205 1,9 100000 34 112,9 0,3 1,38 0,54 1 3 A 8 205 1,9 10000 0 0 0 1,13 0,7 0,65 4 A 8 218 1,9 500000 60 102,7 0,3 2,035 0,3 1,3 5 A 1 115 1 100000 0 0 0 0,63 0,14 0,15 6 A 1 222 2,4 100000 85 180,7 0,33 1,95 1,12 1,3 7 A 1 NA 2,4 1000 0 0 0 0,81 0,06 0,27 8 A 1 230 2,4 10000 2 40,16 0,3 1,39 0,79 0,75 9 A 1 227 2,4 500000 >100 223,7 0,3 2,15 0,54 1,7 10 J 1 228 2,4 100000 23 70,11 0,1 1,23 0,45 0,42 11 J 1 231 2,4 10000 0 0 0 1,12 0,4 0,23 12 J 1 NA 2,4 1000 0 0 0 0,8 0,22 0,15 13 J 1 236 2,4 500000 >100 86 0,1 1,34 0,7 0,62 14 J 3 234 1,9 500000 20 62,3 0,1 1,34 0,3 0,57 15 J 3 223 1,9 100000 4 44,43 0,1 1,07 0,31 0,25 16 J 3 230 1,9 10000 0 0 0 0,73 0,36 0,12 17 J 3 NA 1,9 1000 0 0 0 0,57 0,33 0,05 18 B 2 245 1,9 100000 0 0 0 1,57 1,03 0,25 19 B 2 245 1,9 10000 0 0 0 1,47 0,86 0,2 20 B 2 NA 1,9 1000 0 0 0 0,8 0,1 0,1 21 B 6 228 1,9 500000 2 40,38 0,1 1,64 1,15 0,3 22 B 6 250 2,4 100000 2 40,84 0,1 0,97 0,1 0,1 23 B 6 250 2,4 10000 0 0 0 0,87 0,1 0,07 24 B 1 NA 2,4 1000 0 0 0 0,73 0,33 0,05 25 B 2 250 2,4 500000 6 67,46 0,1 1,71 1,3 0,5 26 C 3 212 1,9 100000 6 607,3 0,1 1,86 1,45 0,8 27 C 3 244 1,9 10000 2 706,6 0,1 1,36 0,93 0,4 28 C 3 NA 1,9 1000 0 0 0 0,95 0,1 0,2 29 C 1 230 1,9 500000 2 818 0,1 1,89 1,37 0,8 30 C 1 250 2,4 100000 5 702,3 0,1 1,76 1,14 0,7 31 C 1 236 2,4 10000 ∞ ∞ 0 1,34 1,03 0,45 32 C 4 NA 2,4 1000 0 0 0 0,95 0,1 0,2 33 C 4 230 2,4 500000 ∞ ∞ 0 1,63 1,1 0,6 34 D 3 225 1,9 100000 3 43,05 Distributed 1,65 1,1 0,6 35 D 3 230 1,9 10000 0 0 0 1,5 1 0,45 36 D 3 NA 1,9 1000 0 0 0 0,94 0,1 0,15 37 D 1 230 1,9 500000 >100 77,83 Distributed 1,81 1,2 0,6 38 D 1 240 2,4 100000 >100 101,8 Distributed 1,53 0,83 0,5 39 D 1 230 2,4 10000 >100 65,53 Distributed 1,4 0,74 0,25 40 D 2 NA 2,4 1000 >100 63,22 Distributed 1,04 0,1 0,2 41 D 2 250 2,4 500000 >100 112,9 Distributed 1,7 0,87 0,6 42 G 1 212 1,9 100000 0 0 0 1,02 0,8 0,2 43 G 1 235 1,9 10000 0 0 0 0,55 0,1 0,15 44 G 1 NA 1,9 1000 0 0 0 0,33 0,1 0,05 45 G 4 250 1,9 5E+105 0 0 0 1,3 0,5 0,4 46 G 4 222 2,4 100000 0 0 0 0,85 0,5 0,15 47 G 3 230 2,4 10000 0 0 0 0,37 0,1 0,2 48 G 3 NA 2,4 1000 0 0 0 0,35 0,1 0,15 49 G 3 230 2,4 500000 0 0 0 1,29 0,64 0,4 50 H 6 217 1,9 100000 0 0 0 0,54 0,1 0,3 51 H 6 228 1,9 10000 0 0 0 0,37 0,1 0,15 52 H 6 NA 1,9 1000 0 0 0 0,23 0,1 0,1 53 H 8 240 1,9 500000 0 0 0 0,7 0,32 0,3

III 54 H 8 220 2,4 100000 0 0 0 0,66 0,31 0,2 55 H 8 220 2,4 10000 0 0 0 0,3 0,1 0,15 56 H 3 NA 2,4 1000 0 0 0 0,28 0,1 0,1 57 H 3 223 2,4 500000 0 0 0 0,88 0,4 0,3 58 I 5 230 1,9 100000 0 0 0 1 0,1 0,4 59 I 5 250 1,9 10000 0 0 0 0,9 0,1 0,15 60 I 5 NA 1,9 1000 0 0 0 0,6 0,1 0,05 61 I 6 230 1,9 500000 0 0 0 0,93 0,1 0,25 62 I 6 230 2,4 100000 0 0 0 0,51 0,1 0,1 63 I 6 230 2,4 10000 0 0 0 0,4 0,1 0,1 64 I 4 NA 2,4 1000 0 0 0 0,34 0,1 0,1 65 I 4 227 2,4 500000 0 0 0 1,12 0,1 0,4 66 E 3 225 1,9 100000 0 0 0 0,98 0,5 0,04 67 E 3 220 1,9 10000 0 0 0 0,75 0,35 0,02 68 E 2 NA 1,9 1000 0 0 0 0,74 0,34 0,05 69 E 3 230 1,9 500000 3 40,08 0,1 1,2 0,4 0,15 70 E 1 215 2,4 100000 8 40,99 0,1 1,06 0,7 0,1 71 E 1 215 2,4 10000 0 0 0 1,12 0,7 0,3 72 E 2 NA 2,4 1000 0 0 0 0,83 0,4 0,1 73 E 1 215 2,4 500000 10 65,4 0,1 1,44 0,9 0,35

IV Index 2: Experimental Data, Hardness, and Calculated Pressure

Designation Hardness Calculated pressure Designation Hardness Calculated pressure Test A- Test A- Number I 1,2… HRC MPa Hertzian Number I 1,2… HRC MPa Hertzian 1 A 8 36 943,89 19341,41 37 D 1 44 148,81 13473,06 2 A 8 36 194,01 18030,25 38 D 1 44 276,52 15089,15 3 A 8 36 302,02 20520,61 39 D 1 44 369,56 11695,25 4 A 8 36 102,84 13311,25 40 D 2 44 681,03 12414,43 5 A 1 36 755,39 15085,72 41 D 2 44 220,92 14287,11 6 A 1 36 119,49 15580,98 42 G 1 62 510,90 18492,31 7 A 1 36 1047,43 20150,95 43 G 1 62 1853,70 21917,31 8 A 1 36 269,29 18935,65 44 G 1 62 5407,86 22428,19 9 A 1 36 98,72 14551,11 45 G 4 62 318,60 14020,31 10 J 1 59 440,13 16430,74 46 G 4 62 996,22 17073,89 11 J 1 59 578,18 13235,81 47 G 3 62 4208,26 43154,27 12 J 1 59 1147,58 15169,29 48 G 3 62 5154,19 41138,98 13 J 1 59 319,82 18471,59 49 G 3 62 399,36 16114,17 14 J 3 59 281,56 15312,24 50 H 6 58 1565,93 30989,31 15 J 3 59 496,73 13249,23 51 H 6 58 3726,36 35673,45 16 J 3 59 1088,02 15478,19 52 H 6 58 9091,73 52714,39 17 J 3 59 1834,30 13386,12 53 H 8 58 980,47 25153,10 18 B 2 57 231,38 10662,13 54 H 8 58 1543,03 24580,30 19 B 2 57 270,03 9403,58 55 H 8 58 6543,93 48873,45 20 B 2 57 930,47 10454,73 56 H 3 58 8407,48 44880,26 21 B 6 57 206,19 11750,04 57 H 3 58 845,35 21266,45 22 B 6 57 803,62 8742,43 58 I 5 59 520,39 17801,22 23 B 6 57 1003,15 8004,61 59 I 5 59 726,36 11588,26 24 B 1 57 1424,20 9316,00 60 I 5 59 1668,58 9802,38 25 B 2 57 208,67 15606,12 61 I 6 59 651,51 15137,89 26 C 3 39 117,18 14735,84 62 I 6 59 2821,79 20564,25 27 C 3 39 271,33 15902,85 63 I 6 59 4463,37 28340,14 28 C 3 39 641,43 12909,08 64 I 4 58 6003,35 35033,62 29 C 1 38 119,12 14298,13 65 I 4 58 539,52 16851,44 30 C 1 38 186,91 15503,33 66 E 3 59 628,26 5282,48 31 C 1 38 321,27 19034,68 67 E 3 59 1074,54 4587,22 32 C 4 38 810,22 13954,51 68 E 2 59 1097,32 8511,58 33 C 4 38 221,69 16204,16 69 E 3 59 412,19 8509,28 34 D 3 44 172,84 14756,79 70 E 1 59 665,42 11090,63 35 D 3 44 223,39 14811,53 71 E 1 59 530,03 18045,44 36 D 3 44 667,43 10945,48 72 E 2 59 1084,46 12586,32 73 E 1 59 327,68 14623,68

V Index 3: Stereo Macroscopic Pictures of the Impacted Area at 20X

Number of Cycles 103 104 105 5x105

A_1900N_804Q_Annealed

A_2400N_804Q_Annealed

J_1900N_804Q_TH

J_2400N_804Q_TH

B_1900N_256G_Carburized

B_2400N_256G_Carburized

C_1900N_225A_Nitrided

C_2400N_225A_Nitrided

D_1900N_277Q_Nitrided

D_2400N_277Q_Nitrided

VI Number of Cycles 103 104 105 5x105

E_1900N_528Q_IH

E_2400N_528Q_IH

G_1900N_803J_DLC

G_2400N_803J_DLC

H_1900N_528Q_DLC

H_2400N_528Q_DLC

I_1900N_804Q_DLC

I_2400N_804Q_DLC

VII Index 4: Optical Microscopic Pictures of the Biggest Observed Cracks

VIII

IX

X

XI Index 5: Result of 2D Surface Topography of the Impacted Area

XII

XIII

XIV

XV

XVI

XVII Index 6: Material Properties of the Tested Steels

Following datasheets are taken from reference [20].

XVIII

XIX

XX

XXI