Mechanism of White Band (WB) Formation due to Rolling Contact Fatigue in Carburized SAE4320 Steel

Abstract

We investigated the microstructural changes in carburized steel due to high stress rolling contact fatigue (RCF) in this study. The changes consisted of the formation of white bands (WBs), including the low angle bands (LABs) and high angle bands (HABs), following the formation of a dark etching area (DEA) in the bearing steels below the contact surface. Although several studies have analyzed the characteristics of WBs, their formation mechanism has not been sufficiently elucidated. We analyzed the orientation of the crystal constituting the WBs and investigated their relationship with the direction of the shear stress generated by the rolling contact. The morphology of the WBs as a function of depth from the surface was studied using an optical microscope, and the crystal orientation was analyzed using scanning electron microscopyelectron backscattering diffraction. It was found that the LABs and HABs respectively formed at a depth where the principal and orthogonal shear stresses were at their maximum. The results of the crystal orientation analysis revealed that the crystal were rotated under the principal shear stress at a specific depth, resulting in the formation of textures such as {111}©211ª and {122}©411ª in LABs and HABs, respectively. Thus, the WBs were revealed to be a type of shear band. The WB formation behaviors of the specimens with varying amounts of initially retained austenite (£ R ) were compared to elucidate the sub-surface initiated spalling life improvement mechanism of the £ R . However, the WB formation behavior showed no difference regardless of the amount of initial £ R . This suggested that the WB formation was not directly related to the sub-surface initiated spalling life.

Deformation-Induced Martensitic Transformation Behavior of Retained Austenite during Rolling Contact in Carburized SAE4320 Steel

“…2(b), although there is no change in the cementite. This result is the same as previously reported; 49) however, the fine, elongated structure corresponding to white bands (WB) as confirmed in the previous report were not observed because different sections of the material were observed. Moreover, the reduction in retained austenite due to RCF was similar to the that observed by X-ray diffraction in previous research.…”

“…During rolling contact, principal shear stress is the highest at a particular depth from the rolling contact surface. 49) Using the Herzian contact theory, the principal shear stress at depth z 0 , at which we conducted the microstructural analyses, is determined to be approximately 1 807 MPa. Additionally, since no known mean critical resolved shear stress (τ CRSS ) is available for austenite in SAE4320 steel or equivalent steels, we used yield stress (σ Y ) of austenite for Fe-Si-Mn-C alloy as reported by Jacques et al, 53) who showed that the σ Y of austenite increases as carbon content increases.…”

“…Since the microstructure contains retained austenite along with tempered martensite and cementite, numerous studies have been conducted into the impact of this retained austenite on the rolling contact fatigue (RCF) life. [37][38][39][40][41][42][43][44][45][46][47][48][49][50] These studies report that retained austenite effectively improves RCF life, and may be related to the deformation-induced martensitic transformation of retained austenite as the underlying principle. Other than the suppression of crack propagation like that found in TRIP steel, 37) the possibilities of suppressing martensite decomposition 37,38) and introducing compressive residual stress 39,40) owing to the generation of new martensite have also been reported.…”

Inclusion orientation dependent flaking process in rolling contact fatigue observed by laminography using ultrabright synchrotron radiation X‐ray

“…[9][10][11] Microstructural alterations of the matrix beneath the surface have been considered as important causes of RCF without friction under oil lubrication. Such alterations may be a dark etching area (DEA), 12,13 dark etching constituent (DEC), 12 dark etching region (DER), 12,14 and martensite transformation from retained austenite, 13 which occur in high-stress regions under the ball track. As the number of fatigue cycles increases, characteristic microstructures appear, such as the white band (WB), 12,13 which is type a of shear band, the white etching band (WEB), 12,14 white etching area (WEA), 12,[15][16][17][18][19] white etching cracks (WEC), 14,16 brown etching layer (BEL), 15 and butterfly cracks.…”

“…Such alterations may be a dark etching area (DEA), 12,13 dark etching constituent (DEC), 12 dark etching region (DER), 12,14 and martensite transformation from retained austenite, 13 which occur in high-stress regions under the ball track. As the number of fatigue cycles increases, characteristic microstructures appear, such as the white band (WB), 12,13 which is type a of shear band, the white etching band (WEB), 12,14 white etching area (WEA), 12,[15][16][17][18][19] white etching cracks (WEC), 14,16 brown etching layer (BEL), 15 and butterfly cracks. 17 The WEA was found to be composed of ultrafine nanocrystalline ferrite grains, voids, and spherical carbides, as observed by transmission electron microscopy.…”