Work Hardening Response of Case Hardened M50nil Steel Induced by Rolling Contact Fatigue

MISSING IMAGE

Material Information

Title:
Work Hardening Response of Case Hardened M50nil Steel Induced by Rolling Contact Fatigue
Physical Description:
1 online resource (75 p.)
Language:
english
Creator:
Lee, Myong Hwa
Publisher:
University of Florida
Place of Publication:
Gainesville, Fla.
Publication Date:

Thesis/Dissertation Information

Degree:
Master's ( M.S.)
Degree Grantor:
University of Florida
Degree Disciplines:
Mechanical Engineering, Mechanical and Aerospace Engineering
Committee Chair:
Arakere, Nagaraj K
Committee Co-Chair:
Subhash, Ghatu
Committee Members:
Ifju, Peter

Subjects

Subjects / Keywords:
case-hardened-steel -- m50nil -- rolling-contact-fatigue -- work-hardening
Mechanical and Aerospace Engineering -- Dissertations, Academic -- UF
Genre:
Mechanical Engineering thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract:
This study focused on investigation of the fundamental characteristics of M50NiL and its responses to Rolling Contact Fatigue (RCF). The carbon gradient caused by carburizing leads a microstructural and hardness variation as a function of depth in the case. In addition, M50NiL tool steels contain a fine dispersion of MxCy, carbides in tempered martensite structure. Under a laboratory-based experimental system, the material's response to RCF was investigated in the shake down stage, namely cyclic work hardening stage. The hardness increased up to 1GPa near the surface with the maximum increment, and gradually decreased as a function of depth, finally merging with the virgin hardness plot at the depth of about 300microns. The plastically deformed area was determined by nital etching. The increased hardness is a consequence of cyclic hardening and cyclic strain accumulation in the RCF affected region. This study will provide insight into cyclic hardening of M50NiL due to RCF.
General Note:
In the series University of Florida Digital Collections.
General Note:
Includes vita.
Bibliography:
Includes bibliographical references.
Source of Description:
Description based on online resource; title from PDF title page.
Source of Description:
This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility:
by Myong Hwa Lee.
Thesis:
Thesis (M.S.)--University of Florida, 2012.
Local:
Adviser: Arakere, Nagaraj K.
Local:
Co-adviser: Subhash, Ghatu.
Electronic Access:
RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2013-05-31

Record Information

Source Institution:
UFRGP
Rights Management:
Applicable rights reserved.
Classification:
lcc - LD1780 2012
System ID:
UFE0044150:00001


This item is only available as the following downloads:


Full Text

PAGE 1

1 WORK HARDENING RESPONSE OF CASE HARDENED M50NIL STEEL INDUCED BY ROLLING CONTACT FATIGUE By MYONG HWA LEE A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2012

PAGE 2

2 20 1 2 Myong Hwa Lee

PAGE 3

3 To my family and friends

PAGE 4

4 ACKNOWLEDGMENTS First of all, I would like to greatly appreciate my adviser, Dr. Nagaraj K. Ar a kere a nd co adviser, Dr. Ghatu s ub h ash for all of their strong support and guidance. Particularly, I appreciate their patient and belief while I was exploring and challenging th e several different concepts and skills for my research. I also would like to thank m y committee member, Dr. Peter G. I f j u for his permission and guidance. In addition, I would like to acknowledge Dr. Laurie Gower for her generosity and help growing me up as a research beginner during my previous work. I also would like to thank my family and my fianc Jun Aoyagi, for their best support and endless love. They have always been on my side and encourage me to finish this work. I should mention the great hel p and motivation from my mentor, Chuljun Park. Much of this work has been performed by lively discussion with him. Also, thank you for all the helpful idea s and discussion with my lab mates Nathan Branch, Sh aw n English, and Anu p Pankar. To my emotional anc hor, all friends of mine and my roommate, Young Mee Yoon, I greatly appreciate. This work would not have been possible without the Major Analytical Instrumentation Center (MAIC) facilitating me to use their various equipments. I would like to acknowledge the help of Dr. Kerry Siebein and Kyeong Won Kim as teacher s Without their help this would not have been completed.

PAGE 5

5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLE S ................................ ................................ ................................ ............ 7 LIST OF FIGURES ................................ ................................ ................................ .......... 8 LIST OF ABBREVIATIONS ................................ ................................ ........................... 10 ABSTRACT ................................ ................................ ................................ ................... 11 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 12 2 CHARACTERISTICS OF CASE HARDENED BEARING STEEL ........................... 15 Materials and Methods ................................ ................................ ............................ 18 Sample preparation ................................ ................................ .......................... 18 X Ray Diffraction (XRD) Analysis ................................ ................................ ..... 18 Etching Characte ristics ................................ ................................ .................... 19 Vickers Hardness Test ................................ ................................ .................... 19 Scanning Electron Microscopy (SEM) Analysis ................................ ................ 19 Results and Discussion ................................ ................................ ........................... 20 Effect of the Carbon Gradient of Case Hardened M50NiL on Microstructure .. 20 Effect of the Carbon Gradient of Case Hardened M50NiL on Mechanical Property (H ardness) ................................ ................................ ..................... 24 Summary ................................ ................................ ................................ ................ 25 3 WORK HARDENING RESPONSE OF CASE HARDENED M50NIL STEEL INDUCED BY ROLLING CONTACT FATIGUE ................................ ..................... 41 Materials and Methods ................................ ................................ ............................ 43 Sample preparation ................................ ................................ .......................... 43 X Ray Diffraction (XRD) Analysis ................................ ................................ ..... 44 Etching Characteristics ................................ ................................ .................... 44 Vickers Hardness Test ................................ ................................ .................... 44 Scanning Electron Microscopy (SEM) Analysis ................................ ................ 44 Tra nsmission Electron Microscopy (T EM) Analysis ................................ .......... 45 Results and Discussion ................................ ................................ ........................... 45 Analysis of the Initial Cyclic Work Hardening Response of M50NiL ................. 45 Analysis of Cyclic Work Hardening Response of M50NiL on Shake Down Stage ................................ ................................ ................................ ............ 47 Summary ................................ ................................ ................................ ................ 53

PAGE 6

6 4 CONCLUSION ................................ ................................ ................................ ........ 69 LIST OF REFERENCES ................................ ................................ ............................... 7 2 BIOGRAPHICAL SKETCH ................................ ................................ ............................ 7 5

PAGE 7

7 LIST OF TABLES Table page 2 1 Chemical composition of M50 and M50NiL steels. ................................ ............. 28 3 1 RCF testing time and the corresponding number of revolutions in accordance with ASTM standard (ASTM STP 771) for elastic shake down .......................... 56 3 2 RCF testing time and the corresponding number of revolutions in accordance with ASTM s tandard (ASTM STP 771) for plastic shake down. .......................... 57

PAGE 8

8 LIST OF FIGURES Figure page 2 1 Characteristics of M50 steel in terms of undissolved carbides and hardness during tempering ................................ ................................ ................................ 29 2 2 Characteristics of the case of M50Ni L plate as a function of depth: hardness, residual stress, half value breadth, and retained austenite ................ 30 2 3 Experimental preparation method for M50NiL specimen ................................ .... 31 2 4 XRD analysis of the radial cross section of M50NiL performed by Cu radiation ................................ ................................ ................................ .............. 32 2 5 Light optical micrographs o f the etched M50NiL surfaces by nital etching ......... 33 2 6 The SEM micrographs of the surface of the radial c ross section of M50NiL taken at ................................ ........................ 34 2 7 The SEM micrographs of the morphology of martensite : lath martensite of the as quenched ASTM A514 steel and plate martensite of AISI/SAE 1095 steel. .. 35 2 8 The SEM micrographs of Cronidur 30 showing the transformation from retained austenite to martensite at the depth of the maximum equivalent st ress showing lamellar shape of etching response ................................ ........... 36 2 9 The SEM micrographs of the surface in the case of M50NiL lightly re polished after etching ................................ ................................ ......................... 37 2 10 The SEM micrograph of M50NiL at a depth of 100 microns from the surface taken b y BSE mode ................................ ................................ ............................ 38 2 11 The SEM micrographs of the radial cross section of M50NiL taken by the interaction with backscattered electrons and the corresponding result of EDS analysis. ................................ ................................ ................................ ............. 39 2 12 Micro hardness test on the radial cross section of M50NiL plotted as a function of depth ................................ ................................ ................................ 40 3 1 Schematic illustration of the stages of a material s evolution by RCF cycles: shake down, steady state response, and instability ................................ ........... 58 3 2 Images of surface initiated spall and subsurface initiated spall .......................... 59 3 3 Experimental preparation method for RCF test specimens ................................ 60 3 4 A micro hardness test of M50NiL after exposure to a small numver of RCF cycles ................................ ................................ ................................ ................. 61

PAGE 9

9 3 5 Light optical micrographs of the etched surfaces of RCF exposed M50NiL ....... 62 3 6 A micro hardness test on the radial cross section of M50NiL exposed to millions of cycles of RCF to develop cyclic strain hardening in the shake down stage ................................ ................................ ................................ ......... 63 3 7 A micro hardness test on the longitudinal cross section of virgin and RCF ex posed M50NiL specimens to determine cyclic strain hardening in the shake down stage after 246 million cycles ................................ ......................... 64 3 8 Comparison of SEM micrographs between virgin and a RCF exposed specimen with 246 million revolutions at three dif ferent depths of 100 microns, 250 microns, and 500 microns ................................ ................................ ............ 65 3 9 TEM micrographs of RCF exposed specimen taken at a depth of 100 microns ................................ ................................ ................................ .............. 66 3 10 TEM micrographs of RCF exposed specimen taken at a depth of 100 microns ................................ ................................ ................................ ............... 67 3 1 1 The SEM micrographs of the comparison of the carbide distribution and population between the virgin and RCF exposed specimen with 246 million revolutions at a depth of 100 microns and the EDS analysis on the indic ated areas on RCF exposed specimen with 246million revolutions ........................... 6 8

PAGE 10

10 LIST OF ABBREVIATION S ASTM American Society of Testing Materials BCC Body Centered Cubic BF Bright Field BSE Back Scattered E lectron EDS Electron Diffraction Spectroscopy EHD Elastohydrodynamic EHL Elastohydrodynamic Lubrication FIB Focused Ion Beam HCP Hexagonal Close Packed L OM Light Optical Microscopy RCF Rolling Contact Fatigue SAED S elected Area Electron Diffraction SEM Scanning Electron Microscopy TEM Transmission Electron Microscopy VIMVAR Vacuum Induction Melting Vacuum Arc Remelting Method XRD X ray diffraction

PAGE 11

11 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science WORK HARDENING RESPONSE OF CASE HARDENED M50NIL STEEL INDUCED BY ROLLING CONTACT FATIGUE By Myong Hwa Lee May 2012 Chair: Nagaraj K. Ar a kere Cochair: Ghatu S u b h ash Major: Mechanical Engineering This study focused on investigation of the fundamental characteristics of M50NiL and its responses to Rolling Contact Fatigue (RCF). The carbon g radient caused by carburizing leads a microstructural and hardness variation as a function of depth in the case. In addition, M50NiL tool steels contain a fine dispersion of M x C y carbides in tempered martensite structure. U nder a laboratory based experimental system the material s response to RCF was investigated in the shake down stage, namely cyclic work hardening stage The hardness increased up to 1GPa near the surface with the maximum increment, and gradually decreased as a function of depth, finally merging with the virgin hardness plot at the depth of about 300microns. The plastically deformed area was determined by nital etch ing. The increase d hardness is a consequence of cyclic hardening and cyclic strain accumul ation in the RCF affected r egion This study will provide insight into cyclic hardening of M50NiL due to RCF

PAGE 12

12 CHAPTER 1 INTRODUCTION A r olling bearing use s rolling elements between two moving parts, which allow a rotational motion while supporting a load [1] They are composed of an inner ring, separator rolling elements, and outer ring [2] According to the rolling bearing industry statics [3] the industry of rolling bearings is a 20 billion U.S dol lar global business, and 500 bearings are approximately manufactured per second. As important machinery parts, 37 different bearing steels were specified by ASTM international in 2002. The bearing technology has been more complicated, and the ability of t he bearing is emphasized to tolerate extreme service environments over recent years. Most research in the bearing industry has been focused on performance and reliability. However, it is difficult to precisely predict the specific fatigue life of an individual bearing since the life of rolling element bearings is various, depending on quality of materials and surface finishing, rolling speed, applied load, lubricant, temperature, contaminants, and design concept [4, 5] This study focused on investigation of the fundamen tal characteristics of M50NiL and its responses to RCF in order to predict a more precise estimation of the life and prevent a sudden unexpected fracture. Chapter 2 contains fundamentals of M50NiL steel as a utilized material that may potentially be appli ed to aerospace bearings. Case hardened M50NiL has an inhomogeneous microstructure as a function of depth from the surface. The carbon gradient caused by carburizing also leads a hardness variation in the case. It shows enhanced bearing life due to strong resistance to the surface wear and tough core with a high degree of ductility. In addition, M50NiL tool steels contain a fine dispersion of

PAGE 13

13 molybdenum rich M 2 C, vanadium rich MC and possibly chromium rich M 6 C carbides; in addition, the amount of retained austenite is minimized by transformation to martensite [6, 7] These characteristics allow M50N iL to perform at a high temperature without structural instability. Therefore, the work presented in chapter 2 investigates the influence of the carbon gradient on the properties of M50NiL, including etching and hardness responses At a higher magnificatio n the details of microstructure are also studied via X ray diffraction analysis, and Scanning Electron Microscopy (SEM). This investigation will be used to determine the association between the properties of the material and its plastic responses to RCF. A ball bearing undergoes significant microstructural changes during its operation, eventually leading to failure. Failure of ball bearing is mainly caused by fatigue when the bearing is ideally loaded, installed, and lubricated [1] The process of material degradation in rolling b earings is comprised of three stages: shake down or running in wear, steady state operation, and instability with accelerated rolling contact fatigue. In the initial shake down stage, the material experiences work hardening by cyclic stress. In addition, t ransformation of retained austenite into martensite occurs with development of residual stress and plastic strain [8] This stage continues until the load exposed volume is fully modified by micr o straining allowing the next stage bears elastic cyclic loading. The amplitude of the resulting cyclic strain decreases as a function of the depth from the running surface [9] In t h e second incubation stage, a large elastic response continues until its threshold limit. Finally, in the last stage, the material is softened, acce lerating RCF [10] In this chapter, investigation of the RCF response of M50NiL was begun at the shake down stage: cyclic hardening and the

PAGE 14

14 saturation l imit of the plastic deformation. Work hardened M50NiL was characterized by its etching response, high magnification micrographs taken by SEM and TEM, and a micro hardness test. The hardness test performed in this chapter is expected to show unique plastic flow induced by the RCF of cyclic loadings and the strain induced microstructural changes based on our previous study utilizing the new reverse analysis of case hardened M50NiL bearing steels [11] The characterization of the RCF response of M50NiL is detailed in Chapter 3

PAGE 15

15 C HAPTER 2 CHARACTERISTICS OF C ASE HARDENED BEARING STEEL Over recent years, most machines have been growing more complex while decreasing in size. In addition, they require the ability to tolerate extreme service environments [3] Due to the importance of r olling b earings in machinery, methods to enhance their performance and reliability have been investigated A s mentioned in the previous chapte r bearing technology is highly complicated due to the various factors influencing service life (e.g., quality of materials and surface finishing, rolling speed, applied load, lubricant, temperature, contaminant s and design concept ) [4] Therefore, a major research focus of bearing technology is how these variables affect bearing life and how they can enhance performance and reliability H ow ever, before evaluating the effectiveness and efficiency of the se factors it is also important to investigate the fundamental characteristics of th e applicable materials and the ir response s to rolling contact fatigue (RCF) from the initial to failure. If the relationship between a specific material s characteristics and its distinctive reactions through the fatigue life can be defined it may allow for a more precise estimation of the life and prevent a sudden unexpected fracture. In order to de fine this relationship, th e present study has utilized materials that may potentially be applied to aerospace bearings In e arly aerospace applications which operated at moderate speed and temperature, AISI 52100 was the predominant material used even tho ugh periodic replace ment was required to prevent a catastrophic failure [12] However, as the technology has progressed, the bearings have been ex posed to more demanding operating conditions. Therefore, the materials are required to bear high er temperature s and speed s as well as have a higher resistance to

PAGE 16

16 indentation and corrosion. AISI 52100 is not suitable for use under those extreme conditions due to its structural instability at high temperature s, which is caused by fine scale meta stable carbides and the decomp osition of large amount s of retained austenite to martensite. Due to theses requirement s molybdenum based through hardened M50 steel s were introduced, and are called secondary hardening steel s Secondary h ardening refers to a multi step tempering process which allow s M50 tool steels to contain a fine dispersion of molybdenum rich M 2 C, vanadium rich MC and possibly chromium rich M 6 C carbides ; in addition, the amount of retained austenite is minimized by transform ation to martensite [6, 7] as shown in Figure 2 1 An excess of retained austenite in a structure lowers its strength, causing reduced hardness, wear resistance, and fatigue resistance Also, transformation from martensite to austenite after quenching reduces the compressive residual surface stress Finally, the decomposition of retained austenite causes structural instability and can be induced in the service period [13] Therefore, the amount of retained austenite should be minimized Due to secondary hardening, M50 can maintain its hardness and strength at high temperatures even after a repeated tempering process. However, M50 has encountered the se vere problem of sudden ring fracture result ing from high tensile hoop stress due to tight shaft fit and centrifugal forces [14] This problem is mainly caused by the characteristics of through hardened steels: in addition to limited ductility and fracture toughness the presence of relatively large size of solute serve as a potential site for chemical segregation [15] Ther efore, there was need for a material with similar surface fatigue characteristics a s M50 but with greater ductil ity and a tougher Since th en,

PAGE 17

17 case hardened M50NiL has been developed for better performance and reliability [16, 17] As seen in Table 2 1, the chemical composition s of M50 and M50NiL are similar. The suffix NiL in M50NiL represents a greater nickel content and reduced carbon inclusion [18] The added nickel enhances cleavage resistance while the lower carbon composition limits the formation of primary carbides, which leads to improved core fracture toughness and ductility Similar to M50 steel the hot hardness of M5 0NiL is guaranteed by second ary hardening along with the presence of M x C y carbides (where M is Mo, V, Cr, or Fe ) However, unlike the slightly tensile residual stress present at the surface in M50, M50NiL has a residual compressive stress induced by case hardening R esidual compressive stress is required for bet ter rolling contact fatigue life as previously explained. The general characteristics of a case layer of M50NiL are shown in Figure 2 2 in terms of residual stress, Vickers hardness, half value breadth, and the amount of retained austenite. The hardness and half value breadth is greatest at the surface and gradually decreases with depth The compressive residual stress is a maximum at the subsurface; however, the trend is similar. The slope of r etained austenite is little changed due to the minimized amounts. Based on the resultant characteristics, it can be assumed that the grain size and compressi ve residual stress induce the hardness increase and the carbon or carbide gradient plays an import ant role i n determining the characteristics of M 50NiL Therefore, th e work presented in th is chapter focuses on how the carbon gradient affects the prop erties of M50NiL including etching and hardness responses which will be later used to determine the association between the properties of the raw material and its plastic responses to RCF.

PAGE 18

18 Even though this study emphasizes the role of M50NiL, there are many useful materials currently employed in aerospace bearing applications. The phys ical and mechanical properties of both contemporary and new alloys for aerospace applications have been thoroughly reviewed in terms of hot hardness, recovery hardness, fracture toughness, corrosion resistance, abrasive wear resistance, and structural fati gue strength [19] Material s and Method s Sample Preparation A commercial grade case hardened M50 NiL rod (Diameter: 9.525mm) was employed with a chemical composition defined in Table 2 1. To ensure consistency, an identical case hardened rod was also prepared to study the plastic deformation of M50NiL induced by RCF, which will be discussed in the next chapter. The rod was sectioned into small cyli nder s and was further cut in half to reveal the radial and longitudinal cross sections ( Figure 2 3 ) E ach cross section was then mechanically polished following a standard metallographic polishing procedure until a smooth surface wa s obtained X Ray Diffraction (XRD) A nalysis The crystal structure of M50NiL was determined by X ray analysis (Philips APD 3720). The radial cross section of the sample was placed on a glass slide and fixed with double sided scotch tape. Cu K ( wave length : 1.54 ) was used at 40KV and 20mA in the 2 theta ranging from 35 to 120. Step size was 0.02 and time per step was 0.2 seconds.

PAGE 19

19 Etching Characteristics The carbide gradient and surface characteristics were established by natal etching. A c ommercially available n ital etchant ( 3% nitric acid in methanol ) was used The surface of the specimen was swabbed with the etchant for 45 seconds, then washed with excess water T he specimen was then treated with ethanol and air dried. The resulting etched surface was probed by Light Optical Microscopy ( L OM) and Scanning Electron Microscop y (SEM) Vickers Hardness Test The carbide distribution and gradient of M50NiL was measured perpendicular ly from surface to the core via Vickers micro hardness tes ting system (Wilson Instruments, Tukon 2100B). A standard Vickers indenter with a 100g load was applied to the polished surface and fixed for 11 seconds. The spacing between indents was maintained at 75m to minimize the interaction between neighbor ing indents followed by ASTM E 384 T he measured hardness was plotted with respect to the d epth from the contact surface. Scanning Electron Microscopy (SEM) Analysis The surface etched by nital etchant was placed on a carbon stub for SEM (JEOL 6335F FEG SEM) to investigate the detailed surface topography as well as compositional difference s at a higher magnification. The SEM system uses a Field Emission Gun as the electron source. During operation, voltage for SEM was maintained at 15kV. The composition of carbides was also probed by Electron Diffraction Spectroscopy (EDS)

PAGE 20

20 Results and Discussion M50NiL is a case hardened steel developed to alleviate the limitations of M50 steel. I t is compo sed of a tempered martensite matrix, with finely dispersed carbides near the surface and minimal retained austenite. X ray analysis confirms the body centered cubic structure of tempered martensite ( a mixture of ferrite and carbides) and the characteristic peaks are matched with JCPDF # 35 1375 as shown in Figure 2 4. Th ese data were compared to the preliminary resul s t of our x ray analysis of M50 steel We found that M50 steel has the same crystal structure and characteristic peaks with spectrum shifting present due to misaligned sample loading. Therefore, it can be assumed that the two materials have similar base crystal structure s regardless of their manufacturing process ; however, the effects of the carbides should not be overlooked. Effect of the Carbon Gradient of Case Hardened M50NiL on Microstructure C ase hardened steel has an inhomogeneous micro structure w ith variation observed as a function of depth from the surface which directly depend ent on the carbon gradient through the c ase Since the carbon content is greatest at the surface and decreases with depth, the microstructure also gradually shifts from high carbon martensite to low carbon martensite [13] As sho wn in Figure 2 5, the etched surfaces of both radial and longitudinal cross sections clearly depict the characteristic gradation of the etching response. Since nital etchant is an effective means to observe both martensite structure and carbides, it is oft en used to investigate the detailed microstructure of steels. The increased microstructure boundaries and carbide population ma inly caused by the higher carbon content generate more reaction sites which are a darker color as they approach the surface. B o t h radial and longitudinal sections show a similar trend.

PAGE 21

21 D etails are shown at a higher magnification via SEM in Figure 2 6. In order to investigate topographical features, micrograph s w ere taken at a depth of 100 micron s and 200 250 micron s. As mentioned above, carburized steel has a gradational microstructural transition from high carbon martensite to low carbon martensite. According to the literature [20] lath martensite is formed at low to medium carbon content under 0.6%C while plate martensite is developed at carbon concentration s greater than 1.0%C ( Figure 2 7 ) The structure of plate martensite consists of a zig zag array of black colored needle like structure s enclosed by retained austenite of a lighter color M ixed martensite appears at concen tra tion s between 0.6 and 1.0%C. C ommercially available M50NiL has a 0.8%C content at the surface case layer, so lath and plate martensite may coexist. However, due to tempering, those martensite structures undergo partial decomposition to ferritic matrix and carbides as indicated by X ray analysis. D istinct features listed in Figure 2 6, may not be apparent. H owever, due to the transformation of retained austenite to untempered martensite, short scaled needlelike morphologies may be observed at some locations. The retained austenite directly depends on the carbon content so greater transformation occurs near the surfa ce. Figure 2 8 provides a description of the concurrent topographical changes that occur during the the martensitic transformation of Cronidur 30 when induced by applied loading at the depth of maximum stress [21] When compari ng the micrographs taken from different locations at 100 and 250 micron s from the surface the most distinctive difference is the length of the martensite. The longer needle or the packet (lath) of martensite is observed at the greater depth M artensite is transformed from austenite via a dif f usionless pr ocess, so the size of the marten site is determined by the former

PAGE 22

22 grain size of austenite which is depend ent on the depth from the surface. The austenite at the s urface has a much finer structure than at the core; therefore, the size of the martensite becomes coarse r as a function of depth [13, 22] Finally, the carbides are m ainly precipitated at the grain or phase boundaries. The overall size of the carbides is very fine ( less than 1 m ) and smaller carbides are located nearer to the surface. In addition, the micrograph of the specimen which is lightly polished after the nital etching shows carbide s observed at the boundaries or in the martensite plate ( Figu r e 2 9 ) The topographical and compositional images are both provided for comparison The carbides formed from austenitic origin are precipitated at the grain boundary while supersaturated carbon stored with in the plate are precipitated as a fine dispersion during tempering [23] As shown in the micrograph taken by B ack Scattered E lectron (BSE) interaction, M50NiL has a various kind of carbides which are distinguished by contrast It can be assumed that the carbides are composed of M 2 C and MC carbides as well as small primary carbide s. According to the literature [24] the mostl y found carbides are either VC or V 7 C 8 carbide and the next frequently observed one s are either Mo 2 C or MoC based on the x ray diffraction analysis on the extracted carbides from M50NiL. In order to confirm the details of the composition and morphology the surface was in vestigated by using Back Scattere d Electron (BSE) in a FE SEM system BSE mode shows a compositional difference which is depend ent on atomic number. Based on this knowledge, it can be assumed that the s mall shiny carbide is caused by strong interaction of m olybdenum with electrons with its higher atomic number while the darker one is composed of vanadium due to its lower atomic number However, the needle like martensite obscures the clear observation of the details ( Figure 2 10 )

PAGE 23

23 In order to reveal the carbide morphology and distribution without disturbance, the specimen was re polished to remove any martensite structure s ( Figure 2 11 ) A micrograph was taken at a depth of 150 micron s from the surface as shown in the first picture, where the small square is the area exposed by the electron beam. Based on the micrograph on the left, it is presumed that the black area is a vanadium based carbide due to its lower atomic number, while white area is a molybdenu m based carbide due to its strong interaction caused by a higher atomic mass. In order to confirm the composition of the carbides, an EDS study was performed on the four different representative spots. The first probe of the tiny black one (A) show ed no di stinction between the peaks of alloying elements with the exception of a 3 fold greater intensity of the iron peak. It may be assumed that the spot may be formed by either carbide detachment s from the surface, caused by the mec hanical polishing, or may be primary carbide s However, this elemental analysis is limited to confirming the primary carbide As previously mentioned the shiny particle (B) was determined to be a molybdenum rich carbide based on the exceptional ly high intensity of the Mo peak on the spectrum The third spot at (C) is the base structure of the M50NiL and shows a strong iron peak. The black large particle (D) show ed the intense vanadium peak as well as evidence of molybdenum and chromium Since th e elements are mutually inter soluble, the particle can be considered V rich carbide. Our previous EDS analysis study o n M50 steel also supported that most carbides w hich are similar to, but much larger than, that spot are mainly composed of vanadium. T herefore, it can be accept ed that the two peaks c a me from neighbor ing par ticles or the base structure The result attained can be explained by the size of the electron beam Moreover, due to this effect, dim white spots located just

PAGE 24

24 below the surface appear to be on t he surface. Interestingly, it has been reported that MC carbide is a multi component system partitioned by V, Nb, and Mo in V and Nb bearing Cr Mo steels [25] However, in th at case, the presence of niobium hin dered the formation of M 2 C carbide and promoted MC carbide. To summarize the results, very fine size d carbides are dispersed sporadically in the tempered martensite matrix The carbides are mainly composed of alloying elements such as V, Mo, and Cr forming metallic carbide s Based on the electron diffraction analysis, the carbides were confirmed to be Mo and V rich ones in accordan ce with the literature [24] However, the possible presence of the primary carbides cannot be ignored Effect of the Carbon Gradient o n a Mechanical Property (Hardness) C arburizing is a manufacturing process which dissolves the carbon in the surface layer s of low carbon steel through heat treatment and includ es austenizing, quenching, and tempering [13] Since the surface carburized layers are rendered by carbon diffusion, the depth of the carbon grad ation created below the surface is depend ent on temperature, exposure time, carbon supply, and alloying elements. D ue to t his carbon gradient, the hardness also varie s from a strong and hard surface to a soft and ductile core Since a hardness test is a method of measuring the resistance of a material to localized deformation, it is sensitive to the presence and population of carbides in the matrix. T he numerical hardness value decreases as a function of depth and merges with the core hardness at the boundary between the carbon diffused and unaffected area Therefore, the carburizing depth and gradient which plays a critical role on bearing performance can be easily detected via a hardness test. The results of the

PAGE 25

25 Vicker s hardness test on two specimens ar e plotted in Figure 2 12 Vicker s hardness test also known as a microhardness test is based on the indentation produced by a diamond indenter and converted to hardness. The test was used because i t requires a small specimen size and accurately recognizes case hardening depth and its variation through a hardness profile obtained by a ser ies of indent ations One specimen designated with a square was exposed to longer carburizing time than the second Both specimens show ed a similar hardness value at the surface (7.3 GPa) and at the diffusion boundary (4.5 GPa) regardless of the differen ce in carburizing time Also, both cases showed a gradual decrease in hardness as the distance from the surface increase d However, the depth where hardness reached the constant core value was different between specimens The specimen exposed for a longer time show ed a deeper carburized layer with a 750 micron difference as well as a shallow slope decrease Due to the nature of the diffusion process, as exposure time increases a greater amount of carbons can travel further from the surface However, the depth of the carburized case did not increase linearly with exposure time showing that complex relationships among the variables determin ing the depth of the case ( Figure 2 12 ) Despite a longer exposure time, a similar hardness value wa s observed at the surface of both specimens This may be explained by the solubility limit of the steel. Since the amount of austenite depends on the carbon concentration the relatively soft retained austenite may be increased reducing surface hardness Summary T h is chapter i s intended to investigate the f undamentals of M50NiL steel which will be useful in interpret ing it s RCF response s. Through the process of case hardening (carburizing), M50NiL develops its characteristic case layer near the surface. Due to t h e

PAGE 26

26 carbon gradient this creates the microstructure varies as a function of depth from the surface causing an inhomogeneous structure. This phenomena lead s a hardness variation it the case. LOM confirmed the carbide gradient as a color gradation represented by a nital etching response. The o verall crystal structu re of the steel wa s confirmed by X ray analysis to be tempered martensite which is the result of partial decomposition from martensite to ferritic matrix and carbides. T h e characteristic features of martensite as well as the structural decomposition were observed on the SEM micrographs. The size of the crystal structure also depend ed on the depth in the case. C arbides smaller than 1 micron were mainly formed at the grain and phase boundaries : namel y the electrochemically unstable region. Also, c arbon saturated in the plate martensite is precipitated to fine carbide within the plate. The composition and distribution of the carbides w as determined by BSE micrographs and EDS analysis. A fine dispersion of the carbides was observed at the surface. Carbides are composed of alloying elements (e.g., Mo, V, and Cr ) Based on t he spectra of the spotted EDS analysis it confirmed the molybdenum rich shiny carbide and the V rich black carbides. Finall y, the case depth was confirmed through a hardness test, with a gradual change in the numerical hardness value from 7.3GPa to 4.5GPa. T h e carbon gradient creates variation in the microstructure that correspond s to the gradational change in hardness. A hard, strong surface with compressive residual stress is obtained with a tougher and ductile core. T h e se factors are expected to contribute to better wear resistance at the surface. R etained austenite is minimized for structural stability and a fine dispe rsion of primary and intermetalic carbides are used to

PAGE 27

27 enhance hot hardness. The presence of small carbides is also advantageous to avoid subsurface initiated failure. In the next chapter, the contribution of these factors will be investigated by the RCF r esponse of M50NiL

PAGE 28

28 Table 2 1. Chemical composition of M50 and M50NiL steels. Alloys Melt Method Elemental Composition (%) C Cr Mo V Ni Fe M50 VIMVAR 0.83 4.20 4.25 1.00 Remaining M50NiL VIMVAR 0. 1 3 4.10 4.40 1.15 3.40 Remaining VIMVAR* indicates Vacuum Induction Melting Vacuum Arc Remelting Method.

PAGE 29

29 Fi gure 2 1. Characteristics of M50 steel (a) Concentration of undissolved carbides at a n elevated heat treatment for 7 minutes, (b) change in hardness and amount of retained austenite during tempering [26]

PAGE 30

30 Fi gure 2 2 Characteri stics of the c ase of M50NiL plate as a function of depth : ( order from top to bottom ) Vicker s h ardness, residual stress, half value breadth, and retained austenite [14]

PAGE 31

31 Fi gure 2 3 Experimental method s of sample prepar ation: two types of surfaces of M50NiL longitudinal (A) and radial cross section (B) through the sectioning of the rod in to small cylinder s and further cutting in half.

PAGE 32

32 Fi gure 2 4 XRD analysis of the radial cross section of M50NiL performed by Cu K radiation (a wave length of 1.54 ) at 40KV and 20mA the 2 theta range between 35 and 120 and step size of 0.02.

PAGE 33

33 Fi gure 2 5 Light optical micrographs of the etched surfaces of M50NiL: (a) the radial cross section and (b) the longitudinal cross section characterized by color gradation caused by n ital etch ing (scale bar: 200 m).

PAGE 34

34 Fi gure 2 6. The SEM micrographs of the surface of the radial cross section of M50NiL taken at (a) a depth of 100 m and (b) 200 250 m to determine the morphology of martensite and carbides.

PAGE 35

35 Fi gure 2 7 The SEM micrographs of the morphology of martensite (a) lath martensite of the as quenched ASTM A514 steel ( b) plate martensite of AISI/SAE 1095 steel [20]

PAGE 36

36 Fi gure 2 8. The SEM micrographs of Cronidur 30 showing the transformation from retained austenite to martensite (a) unloaded area with clear grain boundar ies of the former austenite and undamaged surface, (b) surface at the depth of the maximum equivalen t stress showing lamellar shape of etching response [21]

PAGE 37

37 Fi gure 2 9. The SEM micrograph s of the surface in the case of M50NiL which is lightly re polished after etching : m any carbides (sphere like) are founded along the boundaries (black line) (a) c ompositional difference s with BSE mode (b) the t opography of the grain structure taken by SE interaction s

PAGE 38

38 Fi gure 2 10 The SEM micrograph of M50NiL at a depth of 100 micron s from the surface taken by BSE mode : different phase of the carbides is distinguished by the atomic number

PAGE 39

39 Fi gure 2 11 The SEM micrographs of the radial cross section of M50NiL taken by the interaction with backscattered electrons: the investigated area pointed by arrow is 150 micron s away from the surface (left), detail s at a higher magnification reve a l the morphology and dispersion of carbides showing selected four different spots (A to D) for determining the composition (right), and the corresponding result of EDS analysis (below).

PAGE 40

40 Fi gure 2 12 Micro hardness t est on the radial cross section of M50NiL: series of h ardness is plotted as a function of depth as indicated by the indentation mapping. Hardness profile is matched with the carbon gradient shown in LOM image. The t wo specimens having the different depth of the case show the different slope of the hardness variation.

PAGE 41

4 1 CHAPTER 3 WORK HARDENING RESPONSE OF M50NIL STEEL INDUCED BY ROLLING CONTACT FATIGUE A b all bearing experience s significant microstructural changes under complex loading condition s during its operation, eventually leading to failure. Ball bearing failure is mainly caused by material fa tigue assuming the bearing is ideally loaded, installed, and lubricated in the absence of contaminants [1] T h e fatigue induced decay of the microstructure results in a re set of both the new and existing residual stress es changes in hardness and etching response and development of pref erred crystallographic orientation With respect to the observed changes, t h e material s response to RCF is described by three stepwise process es : shake down, steady state operation, and instability ( Figure 3 1 ) The initial shake down stage is represented by plastic deformation caused by cyclic work hardening and residual stress build up result ing from martensitic transformation of retained austenite. This stage continue s until the load exposed volume is fully modified by mi cr o straining allowing the next stage bears elastic cyclic loading. The amplitude of the result ing cyclic strain decreases as a function of the depth from the running surface [9] In t h e second incubation stage, a large elastic response continues until severe plastic deformation s develop Finally, in the last stage, cracks are nucleated and propagated producing wear particles The types of cracks are grouped by the origin of the nucleat ed site into subsurface or surface initiated ( Figure 3 2 ) S ubsurface initiated cracks are considered the dominant failure mechanism of bearing s exposed to ideal lubrication ; however, in practic e about 70% of bearing failure resu lts from surface distress with another 10% due to corrosion [27 29] Therefore, it

PAGE 42

42 is assumed that bearing life depends g reatly on the operating conditions a nd the quality of the materials The process of microstructural degradation of M50NiL is presented in this chapter The M50NiL sample was subjected to RCF and was expected to undergo work hardening after a certain nu mber of cycles during the shake down stage. The conditioned plastic deformation at the load exposed area wa s then allowed to experience an elastic steady state process eventually developing wear par t icles To date, the majority of research has focused on analyzing failure in terms of bearing life or the mechanism of wear particle formation induced by either spalling or pitting. T hough it is very important to investigate how the failure is initiated and how the microstructure is altered at its final stage it is also crucial to understand the material s distinctive alteration s accompanied by RCF throughout its service life Also, understanding the characteristics of the material is necessary because the y are the primary determinants of the degradation proces s T h e fundamentals of M50NiL were well de tailed in the previous chapter in order to better interpret the evolution of the material s response under cyclic stress In this chapter, investigation of the RCF response of M50NiL was begun at the shake down stage: cyclic hardening and the saturation limit of the plastic deformation This study when combined with current studies on crack initiation and propagation, will provide valuable in sight on the mechanism of RCF ov er the material s service life. W ork hardened M50NiL was characterized by its etching response, high magnification micrographs taken by SEM and TEM and a micro hardness test.

PAGE 43

43 Hardness is not considered a typical physical pro perty of a material; however, it is an easy method to estimate and compare a material s resistance to permanent plastic deformation. It c an be used to determine the stress strain response of the material via the so called reverse analysis method [30 33] S train hardened material increas es in hardness due to accumulated plastic strain which is possibly related to increase d flow stress [34] O ur previous study utilizing the new reverse analysis of case hardened M50NiL bearing steels was successfu l in showing the gradient of the plastic flow coincident with the experimental Vickers hardness test [11] T h erefore, the h ardness test performed in this chapter is expected to show unique plastic flow induced by the RCF of cyclic loadings and the strain induced m icrostructural changes Materials and Methods Sample Preparation A commercial grade case hardened M50 NiL rod (Diameter: 9.525mm) was em ployed with a chemical composition defined in Table 2 1. The M50NiL rod was subjected to RCF tests Ball STP 771 Briefly, M50NiL rod was surrounded by three silicon nitride balls (Diameter: 12.7mm) constrained by steel 4340 cage in M50 race at 177 C The result ant contact stress was 5.5 GPa A serial study of material s cyclic hardening was performed with respect to the different number of cycles After the test, t he rod was sectioned in to small cylinder s and was further cut in half to reveal the radial and longitudinal cross sections ( Figure 3 3 ) E ach cross section was then mechanically polished following a standard metallographic polishing procedure until a smooth surface wa s obtained

PAGE 44

44 X Ray Diffraction (XRD) A nalysis The crystal structure of work hardened M50NiL was determined by X ray analysis (Philips APD 3720). The radial cross section of the sample was placed on a glass slide and fixed with double sided scotch tape. Cu K ( wave length : 1.54 ) was used at 40KV and 20mA in the 2 theta ranging from 35 to 120. Step size was 0.02 and time per step was 0.2 seconds. Etching Characteristics Changes in t he carbide gradient and surface characteristics were established by natal etching. A c ommercially available n ital etchant ( 3% nitric acid in methanol ) was used The surface of the specimen was swabbed with the etchant for 45 seconds, then wa shed with excess water. T he specimen was then treated with ethanol and air dried. The resulting etched surface was probed by Light Optical Microscopy ( L OM) and Scanning Electron Microscopy (SEM) Vickers Hardness Test P lastic flow and surface hardening induced by RCF was measured perpendicular ly from surface to the core via Vickers micro hardness testing system (Wilson Instruments, Tukon 2100B). A standard Vickers indenter with a 100g load was applied to the polished surface and fixed for 11 seconds. The spacing between indent ation s was maintained at 75m to minimize the interaction between two neighbor ing indents followed by ASTM E 384 T he measured hardness was plotted with respect to the d e pth from the contact surface. Scanning Electron Microscopy (SEM) Analysis The surface etched by nital etchant was placed on a carbon stub for SEM (JEOL 6335F FEG SEM) to investigate the detailed surface topography as well as

PAGE 45

45 compositional difference s at a higher magnification. The SEM uses a Field Emission Gun a s the electron so urce During operati o n voltage for SEM was maintained at 15kV. The composition of carbides was also probed by Electron Diffraction Spectroscopy (EDS) Transmission Electron Microscopy ( T EM) Analysis N ano structure of the plastically deformed area was proved by TEM (JEOL 200CX) working at 200kV. Bright Field (BF) image s and S elected Area Electron Diffraction (SAED) pattern s were obtained in order to determine crystal structure and grain size. For TEM analysis, sample wa s prepared by Focused Ion Beam (FIB FIB Dual Beam Strata DB235 ) technique. The sample was taken from a depth of 100 micron s away from the surface by milling the neighbor of the interested area via FIB until thickness of the remained plate wa s less than 200 micron. The thin plate was under cut and was lifted out from the bulk specimen. Finally, it was transported to a copper TE M grid. Results and Discussion Analysis of the Initial Cyclic Work Hardening Response of M50NiL Voskamp concluded that the initial shake down stage end s at near 1000 revolutions after he investigated the rolling contact surface of a 6309 type deep groove bearing inner ring. Based on this result, it can be assumed that work hardening saturation of the load exposed subsurface is reached at the very beginning of operation Si nce RCF greatly depends on the material and applied load, single cr iteria cannot be universally applied to every material and condition However, a thorough investigation of the early stages of revolution is needed. For this reason a series of RCF test s wa s performed on M50NiL for variable time periods from 30 second s to 3 minutes. The

PAGE 46

46 corresponding number of revolution s are provided in Table 3 1 showing calculated revolutions ranging from over 4 thousand to over 25 thousand Based on this, t he specimen wa s expected to undergo significant material shake down with an increase d hardness. However, a s shown in Figure 3 4 in the difference in hardness among the f ive specimens is minor Using a curve fit method, the hardness of the tested samples seem ed greater than the virgin sample; however, the maximum increase is around 0.2GPa. In addition, the hardness data were heavily scattered. The hardness increase may not only come from hardening, but also may result from a neighbor ing fine dispersion of carbides. Therefore, caution should be used in r ehard to concluding that t he specimens were work hardened An e tched surface test of the specimens showed no variation. Hence it is concluded that t he microstructure has started to respond to the contact tress and cyclic loading, and that further alteration is expected to be proportional to the loading time M50NiL has a hard carburized case on its surface with favorable compressive residual stress and tough core with a high level of ductility Therefore, it shows good wear resistance to an applie d loading condition. Also, the amount of the retained austenite is minimized to less than 4% through the multi stepped tempering process resulting in the limited martensitic transformation of the retained austenite Therefore, this may inhibit cyclic work hardening in M50NiL steel. However, further appli cation of contact stress can lead to plastic deformation continuing until the elastic steady state stage is reached

PAGE 47

47 Analysis of the Cyclic Work Hardening Response of M50NiL in the Shake Down Stage A RCF test on M50NiL was performed for over one million cycles in order to investigate work hardening and residual stress showing the calculated number s of revolutions in Table 3 2. The plastically deformed area with a resultant contact stress of 5.5GPa from rolling, was visualized by nital etchant under an LOM system ( Figure 3 5 ) The damaged ar ea can be distinguished by a lighter color etching framed by a dark er color band than the unaffected area. A distinctive pale colored layer is apparent on the rad ial cross section along the circumferential direction, while a light ly etched semicircular feature i s observed on the longitudinal cross section. Such contrast is due to the difference in the material s resistance to the nital etchant. M ore etching occurs when there is a greater difference in electrochemical potential between two neighbor ing phases or grains. A lighter color indicates that the area is less etched with a greater resistance to the chemical attack caused by the etchant and vice versa Etching resistance can normally be enhanced by removing phase or grain boundar ies In the case of tempered martensite, the easiest way to improve the resistance may be the removal or dissolution of the carbides or coarseness of grain size reducing the attackable site. However, more boundaries result in increased strength and hardness since the boundary plays an important role i n blocking dislocation motion. Therefore, t he lower hardness value may be accompanied by a reduced boundar y area However, t he main mechanism of action in the shake down stage is cyclic strain hardening leading to an increased hardness The micro hardness test performed on

PAGE 48

48 radial cross section s of specimens that underwent different periods of rolling contact fatigu e confirmed an increase d hardness in the plastically deformed area ( Figure 3 6 ) As shown in the previous chapter, t he virgin specimen exhibit ed the ha rdness gradient from the surface hardness of 7.24GPa to the core hardness of 4.5GPa in the case hardened layer Compared to the control the t wo specimen s subjected to the RCF test show increased numerical hardness value s near the surface down to a depth of 3 00 micron s, which merges into the best fitted virgin curve The increase in hardness i s localized. Also, the depth of the plastic ally hardened area on the plot i s coincident to the lightly etched area on the LOM image ( Figure 3 5 ) The maximum increase of 1GPa in hardness is observed near the surface on both specimens which is due to th e maximum cyclic strain amplitude induced by RCF. The strain amplitude also decrease s as a function of depth reaching near zero at the boundary between the deformed zone and unaffected area Even though those two specimen s showed a hardness increase of 1GPa compared to t he virgin one there was not a noticeable discrepancy on the hardness among them in spite of the huge gap in the cycles they were subjected to (i.e., from 1 3.5 and 246 million ). In a detailed view with a best curve fitting in the damaged area (b), it show ed that t he longer the specimen wa s exposed to RCF, the higher the hardness value that is obtained Additional strain hardening and plastic deformation was accumulated at the longer expos ure time ; however, the difference wa s about 0. 1 GPa and may not be cle arly noticeable without the curve fitting method. Based upon this result it can be assumed that in 1 3. 5 million cycles the load=exposed volume almost reache s its maximum capacity of work hardening that can be attained by the constant amplitude of a given cyclic stress. T h erefore, increasing t he number of revolutions

PAGE 49

49 cannot further increase the hardness, since the full volume of the load exposed area had already experience d sufficient plastic work harden ing Moreover a three dimensional surface plot of th e hardness variation revealed the plastic strain flow on the longitudinal section ( Figure 3 7 ) The virgin specimen show ed moderate surface variation of the hardness with the exception of a few distinctive error peaks. The difference between the maximum and minimum hardness is less than 0.5GPa. As shown in the ra dial cross section in Figure 3 6 the hardness valu e near the surface is steady near 7.2GPa until a distance of 300 micron s and then decrease s gradually along the carbon gradation. T he RCF specimen that underwent 246 million RCF cycles successfully show s the localized plastic deformation t hrough the surface variation The deformed area i s semicircular with the greatest increase in hardness having occurred nearest to the origin (the lo ading center) T he variation is lessened as the distance increase s either laterally, through changes in depth, or both. Therefore, it can be concluded that the cyclic strain hardening of RCF i s governed by the contact stress mechanism. The great er amount o f flow stress induced near the loading center the greater the accu mulated plastic strain produced resulting in the material s strengthening In order to further detail t he induced plastic deformation that is responsible for the strong etching resistance to the etching solution and 1GPa hardness increase, the microstructu re of the RCF tested M50NiL was investigated by SEM and TEM. Based on the optical micrograph and sample geometry, the depth of the hardened area was calculated. The t wo specimens showed a depth of about 220 micron s below the surface Theref ore, SEM micrographs were taken at depth s of 100, 250, and 500 micron s The

PAGE 50

50 image of a depth of 100 micron s is representative to the heavily deformed but lightly etched area while the one taken at 250 m icron s is on the severely etched area. The overall characteristics of the final micrograph are expected to be similar The microstructural characteristic s of M50NiL were introduced in the previous chapter. The crystal structure of the virgin specimen was confirmed as tempered martensite partial decompos ition into ferrite and cementite. A needle l ike plate martensite structure wa s also observed and its size increased as a function of depth from the surface In Figure 3 8 the micrograph taken at a depth of 100 microns is heavily populated by t he shorter length of ra ndomly disorganized martensite Also, larger numbers of carbide s are observed i ncluding white colored Mo ba sed, and V based black carbides. As the depth increase s the length of the structure increase s while the population decrease s Compared to the control condition, the surface of the RCF exposed specimen i s smoothe r, with an absence of the needle structures at a depth of 100 micron s As s hown in the next micrographs, in the test specimen, th e shorter length of martensite i s heavily developed due to structural breakage and the topogra phy i s similar in characteristic to the virgin specimen obtained at 100 mic ron s However, at 250 microns deep, the test sample shows a decayed structure confirming partial decomposition. To summarize the results, fi rst ly grain size i s decreased leading to the shorter length of the needle structure s Secondly, the transformation from retained austenite to martensite occurs, even though the maximum con tent of the retained austenite wa s reduced to less than 4% at the tempering stage As shown in Figure 2 8, th e transformation induce s numerous scratch es on the grain surface due to the nature of the martensite formation. Finally, the broken and newly formed structure s

PAGE 51

51 are decayed by RCF, following the gradient of the strain amplitude Carbon saturated martensite is de composed into ferrite and carbide ; thu s, the c yclic stress induces m ore highly tempered martensite. Therefore, the image at the border of the deformed area at 250 micron s deep, shows severe etching characteristics than the surrounding area. T he population of distinguishable whi te Mo based carbides appear s similar ly in both specimens The black carbides s such as the V based one and cementite seem s larger, but it is hard to discriminate them due to limitation s in contrast. As confirmed i n Figure 3 8 (a), the surface i s much smoothe r, with sh allow pockmarks rather than deep groove s of random needle structures. This surface smoothness i s the direct result of the less active etching behavior It can be assumed that this is because t he decayed structure s shown in (c) subjected to further structural break down into very fine grained ferrite matrix and carbide under RCF Generally, finer grain result s in more boundaries leading to heavier etching damage. However, if the size is too small (e.g., nano size ) the etchant cannot effectively damage the all boundaries due to the oversheling amount of them The relevance of the grain size is discussed below. TEM micrographs show a more detailed microstructure at the nano scale level ( Figure 3 9 ) The first BF image (a) show e s martensite grain s at a width of approximately 100nm ; the boundar ies between each grain were filled with needle like cementite unknown hard material and/or dislocation accumulation. They may serve as an obstacle to dislocation movement induced by strain hardening In addition, sphere carbides are randomly spread throughout the surface T he layered grain structure s, showing a few misfit of the orientation, are observed at (b), and the detailed orientations are depicted in (c). Such a grain structure is als o well known as a dislocation obstacle;

PAGE 52

52 therefore, it can also be assumed that it contributed to the material s hardening. In addition, the clusters of very fine carbide grains are observed indicate d by arrows. However, the nano structure of the virgin specimen should be taken into account. The corresponding TEM SAED patterns are given in Figure 3 10 with the solution. Previously, it was expected that the grain size was severely reduced to a very fine structure because of t he strong etching resistance However, the result ing pattern d oes not show the perfect ring pattern that is one of the main characteristic s of nano scaled material. Instead of the ring type pattern the diffraction is composed of a multitude of spots which almost form ed a ring Therefore, consideration should be given that the grain i s subjected to substantial size reduction. Based on the calcula tion, each spot wa s indexed and the beam direction wa s [111]. From the first pattern, it wa s confirmed that the ferrite matrix has a BCC structure and the alloying VC carbide has a cubic structure The composition of the carbides i s V 8 C 7 which corresponds to the {002} and {012} planes. Through another diffraction under the same [111] dir ection(c), the ferrite BCC structure of the {110} plane wa s also confirmed as well as the grain size reduction. However, the result was also not a perfectly connected circle. The s tructural mismatch i s also observed at the guided mapping on (d). O ne structure is tilted to a very minor degree against another. This may be a result of the neighbor ing grain which has a small orientation incongruity as depicted in Figure 3 9 T he molybdenum carbide wa s confirmed to be a H exagonal C lose P acked (HCP) st ructure namely Mo 2 C As indicated on the solution o n (e), the hexagonal structure i s a little stretched out along the y axis (up and down). The crystal structure and lattice constant have been well studied in the open literature [24] A further EDS analysis o f the radi al cross section at a depth of 150

PAGE 53

53 micron s confirmed the morphology and elemental composition of the carbides ( Figure 3 11 ) As seen previously in the virgin specimen, there are three distinguishable carbide structures: black chunk carbide, white chuck car bide, and a very fine acicular type In addition, there i s only a small difference in the population and distribution of the carbides between the two samples. However, due to the contrast li mitation s it is difficult to determine the contribution of the carbides on the strengthening by using these two pictures. Also, simila r to the previous results, it was confirmed that the black chunk carbide is V based ( namely V 8 C 7 ) e ven though the adjacent carbide showed Mo peak. The white one is Mo 2 C carbide, and the matrix is a tempered martensite. Summary The main topic of this chapter is the study of RCF behavior of M50NiL in an experimental system. Of the three stages of the M50NiL respons e s to cyclic loading, the focus was on the initial shake down stage. The shake down is re ferred to as cyclic work hardening or the strain hardening stage where the bearing steel work hardened The M50NiL rod was subjected to the ball and rod RCF testing, and the plastic response s were studied by etching response, hard ness variation, and microstructural investigation. In order to determine the time period in which cyclic hardening is induced the material was subjected to short testing times in accordance with the literature. Howeve r, there was no discernable change in a numerical hardness value or etching response. S ince a m icro hardness test is proved as an effective way to verify plastic strain flow in a material ; based on the results of the scattered hardness and curve fitting me thod, it was concluded that the specimens were not plastically deformed but had nearly become strain hardened based

PAGE 54

54 I ncreas ing the number of revolutions in to the millions successfully yielded work hardened material. In order to chronicle material s response the experiment was performed for two different time periods ( 13.5 and 246 million revolutions, respectively ) Firstly, compared to the control sample, the hardness of the test specimens both increased a maximum of 1GPa at the surface T he increme nt gradually decreased with depth and merged with the virgin hardness plot at the boundary between the deformed and unaffected area near a depth of 300microns Between the two tested specimens, there was no significant discrepancy of the resulting hardness in the plastically deformed region without curve fitting method which showed an increment of about 0.1GPa This infers that the fatigued specimen with 13.5 million cycles was almost fully strain hardened to its limit of plastic deformation and t he other test specimen might have run at the second steady static response stage Hence, a further increase in cycle number could not result in additional hardening of the material The load exposed volume bec ame strengthened due to the hardening process; therefore, the amount s of the deformation under a fixed amplitude of stress decrease s as the increased number of revolution s, finally reaching zero and entering the next stage Moreover, the hardness increase was the maximum at the center of loading, and decreased with increased lateral distance away from the center. The shape of the plastically deformed area was similar to what was theoretical ly speculat ed, and depth also corresponded to the depth of hardness increased zone. The damaged area showed strong resistance to nital etchant and was enclosed by a heavil y etched band at the boundary between the deformed and unaffected area s The greater etch ing area resulted from structural break down caused by tempered martensite decomposition

PAGE 55

55 and martensi tic transformation of retained austenite The needle like structures disappeared and the short ly broken structure s w ere subjected to further structural decay. The surface became roughened providing more etchant attackable sites. The area with less etch ing underwent further structural break down, decay and decomposition than the band with more etch ing Th e initial structure had originally smaller grain and rougher surface due to the carbon gradient. Moreover, i n the less etched area, the damage gradient wa s also induced by the resultant strain amplitude from RCF as shown in the hardness plot I t lost the characteristics of martensite and experienced grain size reduction. The TEM analysis showed the ring like diffraction pattern s, confirm ing the grain size decrease In accordance with the SEM results, the solution of the electron diffraction pattern also confirmed the cubic structure of V 8 C 7 carbide, and HCP structure of Mo 2 carbide on a base of BCC ferrite matrix. In conclusion, c yclic work hardening of M50NiL induced by RCF occurs through grain size decrease, structural decay of tempered martensite and minor effect from retained austenite transformation Further study on the subsequent stage will be helpful to understand the comp lete life of M50NiL under RCF and predict the service life more precisely

PAGE 56

56 Table 3 1. RCF testing time and the corresponding number of revolutions in accordance with ASTM standard (ASTM STP 771) for elastic shake down Time (min.) T otal cycles 1 0.5 4300.2 2 1 8600.4 3 2 17200.8 4 3 25801.2

PAGE 57

57 Table 3 2 RCF testing time and the corresponding number of revolutions in accordance with ASTM standard (ASTM STP 771) for plastic shake down. Time (min.) T otal cycles 1 26.19 13514668.56 2 476.64 245957679.4

PAGE 58

58 Fi gure 3 1 Schematic illustration of the stages of a material s evolution by RCF cycles: shake down, steady state response, and instability.

PAGE 59

59 Fi gure 3 2 Images of surface initiated spall (top) and subsurface initiated spall (bottom) [35]

PAGE 60

60 Fi gure 3 3 Experimental preparation method for RCF test specimens. The M50NiL rod was RCF tested following the ASTM STP 771 standard over different time periods. The individually sectioned specimen was further cut in half to create (A) longitudinal and (B) radial cross sections.

PAGE 61

61 (a) (b) Fi gure 3 4 A m icro hardness test of M50NiL after exposure to a small numver of RCF cycles : (a) h ardness plot of the e ntire carburizing layer and (b) t he enlarged area of the deformation observed at a depth of 1000 micron s Compared to the dashed fitting curve of the virgin specimen, the hardness of the tested sample seems slightly increased

PAGE 62

62 Fi gure 3 5 Light optical micrographs of the etched surfaces of RCF exposed M50NiL. Durin g the RCF test, the M50NiL rod wa s pressu rized by silicon nitride ball s and the cyclic loadings, causing the localized load exposed volume to be p lastically deformed This appears as a white etching area on the LOM images: ( top right ) the radial cross section and (b ottom left ) the longitudinal cross section (scale bar: 200 m).

PAGE 63

63 Fi gure 3 6 A m icro hardness test on the radial cross section of M50NiL exposed to millions of cycles of RCF to develop cyclic strain hardening in the shake down stage (a) S chematic diagram of micro indentation mapping as a function of the depth away from the surface (b) a detailed hardness plot along with the curve fitting and (c) a plot of m icro hardness versus depth of the virgin and test specimens ( 13.5M cycles, and 246M cycles )

PAGE 64

64 Fi gure 3 7 A m icro hardness test on the longitudinal cross section of virgin and RCF exposed M50NiL s pecimen s to determine cyclic strain hardening i n the shake down stage after 246 million cycles : (a) micro indentation mapping, and (b) a three dimensional surface plot of the hardness

PAGE 65

65 Fi gure 3 8 Comparison of SEM micrographs between virgin (right column : b, d, and f ) and a RCF exposed specimen with 246 million revolutions (left column : a, c, and e ) at three different depths : (first row : a b ) 100 micron s (second row : c d ) 250 micron s and (third row : e f ) 500 micron s

PAGE 66

66 Fi gure 3 9 TEM micrographs of RCF exposed specimen taken at a depth of 100 micron s : (a) BF image of the overall grain structure, (b) BF image of the mis orientat ion of neighboring grains, and (c) the solution of the orientation.

PAGE 67

67 Fi gure 3 10 TEM micrographs of the electron diffraction patterns and solution: (a) a ring like pattern and its solution (b) the confirmation of tempered martensite and VC carbides, (c) an other type of ring like pattern, (d) a misfit of the diffraction patterns, (c) the evidence of Mo 2 C carbide.

PAGE 68

68 Fi gure 3 11 The SEM micrographs of the comparison of the carbide distribution and population between the virgin and RCF exposed specimen with 246 million revolutions at a depth of 100 microns. The EDS analysis on the indicated area on RCF exposed specimen wi th 246 million revolutions : the evidence of V rich carbide (A), ferrite matrix (B), Mo rich carbid e (C).

PAGE 69

69 CHAPTER 4 CONCLUSION In order to predict a better service life and prevent an unexpected sudden fracture of bearings, t h is study is concentrated on investigation of the fundamental characteristics of M50NiL, and its responses to RCF. Firstly, M50NiL develops its characteristic case layer near the surface and the microstructure varies as a function of depth from the surface causing a hardness variation it the case. LOM confirmed the carbide gradient as a color gradation represented by a nital etching response. The crystal structure was confirmed by X ray analysis to be tempered martensite, resulting from partial decomposition from martensite to ferritic matrix and carbides. T h e characteristic features of the M50NIL steel w as observed on the SEM micrographs. The size of the crystal structure also depended on the depth and c arbides were smaller than 1 micron The composition and distribution of the carbides was determined by BSE micrographs and EDS analysis. A fine dispersi on of the carbides was observed, and it confirmed the molybdenum rich shiny carbide and the V rich black carbides through the spotted EDS analysis A gradual change in the numerical hardness value also caused by the carbon gradient was also confirmed from 7.3GPa to 4.5GPa. The study of RCF behavior of M50NiL under a laboratory based experimental system was performed based on the fundamentals of M50NiL previously investigated. T h e material s response to RCF is described by three stepwise process es : shake down, steady state opera tion, and instability The first shake down stage was begun to be investigated. The shake down is represented by cyclic work hardening or strain hardening stage, allo w ing the bearing steel work hardened. The experiment was

PAGE 70

70 performed on two different time intervals, resulting in the corresponding number of revolutions is 13.5 and 246 million respectively. The hardness of the both tested specimens increased up to 1GPa near the surface with the maximum increment, and gradually decreased as a function of depth finally merging with the virgin hardnes s plot at the depth of about 300microns. T here was no discrepancy of the hardness observed between the two tested specimens since the fatigued specimen with 13.5 million cycles was fully strain hardened to its maxim um capacity of plastic deformation T h erefore, the more hardening cannot be achieved by the further increased number of cycles In addition, t he hardness increase was the maximum at the center of loading, and decreased with increased lateral distance from the center. The damaged area showed strong resistance to nital etchant and it was framed by heavily etched area. The more actively etched area was resulted from structural break down, caused by tempered martensite decomposition, and martensitic transformat ion of retained austenite. The needle like character disappeared, and the shortly broken structures were subjected to further structural decay. The surface became roughened, providing more etchant attackable sites. However, t he less etched area experienced further structural break down, decay, and decomposition than the heavily etched band previously discussed In this area, the damage gradient was also induced by the resultant strain amplitude from RCF It also showed grain size reduction. The TEM result confirm ed the grain size decrease. In accordance with SEM results, the solution of the electron diffraction pattern also confirmed cubic structure of V 8 C 7 carbide, and HCP structure of Mo 2 carbide in the BCC ferrite matrix. These carbides are responsible t o hot hardness. In order to conclude the cyclic work hardening of M50NiL induced by RCF grain size decrease,

PAGE 71

71 structural decay of tempered martensite, and minor effect from retained austenite transformation must be taken in account in terms of the gradient of carbon and strain amplitude These alterations can accommodate blocking of the dislocation movement This study, when combined with current studies on crack initiation and propagation, will provide valuable insight on the mechanism of RCF over the mate rial s service life.

PAGE 72

72 LIST OF REFERENCES 1. Farshid Sadeghi, B.J., Trevor S. Slack, Nihar Raje, and Nagaraj K. Arakere, A Review of Rolling Contact Fatigue. Journal of Tribology, 2009. 131 : p. 041403 1 15. 2. Howard, I., A review of Rolling Element Bearing Vibration "Detection, Diagnosis, and Prognosis" in DSTO Aeronautical and Maritime Research Laboratory 1994: Melbourne. 3. Masamichi Shibata, M.G., Atsuhiko ohta, and Kazutoshi Toda, Bearing Steel Technolog y in Bearing Steel Technology, ASTM 1419 J.M. Beswick, Editor. 2002, American Society for Testing and Mateials International: West Conshohocken. 4. Daniel Girodin, F.V., Roger Guers, and Gilles Dudragne, Rolling contact Fatigue Tests to Investigate Surface Initiated Damage and Tolerance to Surface Dents in Bearing Steel Technology, ASTM STP 1419 J.M. Beswick, Editor. 2002, American Society for Testing and Materials International: West Conshohocken. 5. R. Fougeres, G.L., A. Vincent, D. Nelias, G. D udragne, D. Girodin, G. Baudry, and P. Daguier, A New Physically Based Model for Predicting the Fatigue Life Distribution of Rolling Bearings in Bearing Steel Technology, ASTM STP 1419 J.M. Beswick, Editor. 2002, American Society Testing and Materials In ternational: West Conshohocken, PA. p. 197 212. 6. Bridge J.E., M.G.N., and Philip T.V., Carbides in M50 high speed steel. Metallurgical Transactions, 1971. 2 : p. 2209 2214. 7. M.S., I.A.a.B., Heat treatment response of triple and quinuple tempered M50 h igh speed steel. European Journal of Scientific Research, 1997. 17 : p. 150 159. 8. Voskamp, A., Material response to rolling contact loading. Journal of Tribology, 1985. 107 (359 364). 9. Hahn, G.T., Bhargava, V., and Chen, Q., The Cyclic Stress Strain Pr operties, Hysteresis Loop Shape, and Kinematic Hardening of Two High Strength Bearing Steels. Metallurgical Transactions A Physical Metallurgy and Materials Science, 1990. 21 : p. 653 665. 10. J Halme, P.A., Rolling contact fatigue and wear fundamentals fo r rolling bearing diagnostics state of the art. Proceedings of The Institution of Mechanical Engineers Part J: Journal of Engineering Tribology, 2010. 224 (4): p. 377 393.

PAGE 73

73 11. Nathan A. Branch, G.S., Nagaraj K. Arakere, Michael A. Klecka, A new reverse analysis to determine the constitutive response of plastically graded case hardened bearing steels. International Journal of Solids and Structures, 2011. 48 : p. 584 591. 12. Franz Josef, E., An Overview of Performance Characteristics, Experiences and Tre nds of Aerospace Engine Bearings Technologies. Chinese Journal of Aeronautic, 2007. 20 : p. 378 384. 13. Davis, J.R., Surface Hardening of Steels: Understanding the Basics 2002, Materials Park, OH: ASM International. 14. J.C.Hoo, J., ed. Creative Use of Bearing Steels, ASTM STP 1195 1993, American SOciety for Testing and Materials: Philadelpohia. 33 48. 15. J.E., I.A.a.K., The role of primary carbides in fatigue crack propagation in aeroengine bearing steels. International Journal of Fatigue, 1990. 12 : p. 234 244. 16. Bamberger, E.N., Material for Rolling Element Bearings in BBearing Design Historical Aspects, Present Technology, and Future Problems W. Anderson, Editor. 1980, American Society of Mechanical Engineers: San Francisco. 17. Clark, J.C., F racture Tough Bearings for High Stress Applications. American Institute of Aeronautics and Astronautics, 1985: p. 1 7. 18. Boehmer H. J., E.F.J., Trojahn W., M50NiL bearing material heat treatment, material properties and performance in comparison with M5 0 and RBD. STLE Publication, 1991. 91 : p. AM.3G 2. 19. Ragen, M.A., Anthony, D.L., and Spitzer, R.F, A comparison of the mechanical and Physical Properties of Contemporary and New alloys for Aerospace Bearing Application in Bearing Steel Technology, ASTM STP 1419 J.M. Beswick, Editor. 2002, American Society for Testing and Materials International: West Conshohocken. 20. B. L. Bramfitt, a.A.O.B., Metallographer's guide: practices and procedures for irons and steels 2002, Materials park, OH: ASTM interna tional. 21. Bohmer, H.J., Hersch, Th., Streit, E., Rolling Contact Fatigue Behavior of Heat Resistant Bearing Steels at High Operational Temperature in Bearing steels: into the 21st century a.W.B.G. J. J. C. Hoo, Editor. 1996, American Society for Testi ng and Materials: New Orleans. 22. William D. Callister, J., Materials Science and Engineering: An Introduction 7th ed, ed. J. Hayton. 2007, New York, NY: John Willey & Sons, Inc.

PAGE 74

74 23. Bhadeshia, H.K.D.H., Martensite and Bainite in Steels: Transformation Mechanism & Mechanical Properties. Journal de Physique IV, 1997. 07 (C5): p. 367 376. 24. Dennis W. Hetznera, W.V.G., Crystallography and metallography of carbides in high alloy steels. Materials Characterization, 2008. 59 (7): p. 825 841. 25. Kaori Miyata T.K., Tomohiro Omura and Yuichi Komizo, Coarsening Kinetics of Multicomponent MC Type Carbides in High Strength Low Alloy Steels. Metallurgical and Materials Transactions A, 2003. 34 (8): p. 1565 1573. 26. Bridge JE, M.G., Philip TV, Carbides in M50 high speed steel. Metallurgical Transactions, 1971. 2 : p. 2204 2214. 27. Bhadeshia, H.K.D.H., Steels for bearings. Materials Science, 2012. 57 : p. 268 435. 28. Bamberger, E. Status of understanding for bearing materials in Tribology in the 80's 1984. Cleveland, OH: NASA Lewis Research Center. 29. Bamberger, E. Materials for rolling element bearings in Bearing design historical aspects, present technology and future problems 1980. San Antonio, TX: ASME. 30. Dao, M., Chollacoop, N., Van Vliet, K.J., Venkatesh, T.A., Suresh, S., Computational modeling of the forward and reverse problems in instrumented sharp indentation. Acta Materialia, 2001. 49 : p. 3899 3918. 31. Gu, Y., Nakamura, T., Prchlik, L., Sampath, S ., Wallace, J., Micro indentation and inverse analysis to characterize elastic plastic graded materials. Materials Science and Engineering A, 2003. 345 (1 2): p. 223 233. 32. Tho, K.K., Swaddiwudhipong, S., Liu, Z.S., Zeng, K., Hua, J., Uniqueness of rever se analysis from conical indentation tests. Materials Research Society, 2004. 19 (8): p. 2498 2502. 33. Antunes, J.M., Fernandes, J.V., Menezes, L.F., Chaparro, B.M., A new approach for reverse analyses in depth sensing indentation using numerical simulati on. Acta Materialia, 2007. 55 : p. 69 81. 34. Tabor, D., The hardness of solids. Review of Physics in Technology. Surface Physics 1970, Cambridge: Cavendish Laboratory. 35. Stachowiak, G., Batchelor, A W, Engineering Tribology 3rd ed. 2000, Boston: Else vier Butterworth Heinemann.

PAGE 75

75 BIOGRAPHICAL SKETCH Myong Hwa Lee was born in Suwon, South Korea in 1983. She grew up in the same city with her parents, Janghan Lee and Kisoon Hwang, and her older sister, Myong Jin Lee. After she graduated from Bulkok High School in Sungnam in 2002, she was admitted to the D epartment of Materials Science and Engineering in Kookmin University in Seoul, South Korea. While Kookmin Unive rsity, she was a leader of the L iterary Society. She attended the program of English as a Se condary Language at the University Oregon in 2004. Also, she obtained the teac i n secondary school education from the Ministry of Education in Seoul, South Korea in 2007. With her ree, she was accepted into the D epartment of Materials Science and Engineering at the University of Florida in 2007. She had worked for Dr. Laurie Gower on the biomimetics, specifically study on biomineralization of bone for 2 years. After field and was accepted into the department of Mechanical and Aerospace Engineering at the University of Florida in 2010. She has worked for Dr. Nagaraj Arekere in the fi el d of failure analysis, specifically investigation of microstructural changes of the b all bearings for aerospace application. Under his generosity and support, she successfully defended thesis and graduated from the University of Florida in the spring of 201 2