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

Abstract

The deformation-induced martensitic transformation is a phenomenon that significantly improves the mechanical properties of steels, and is well known to be beneficial for the rolling contact fatigue (RCF) of bearings. In the present study, the characteristics of the deformation-induced martensitic transformation in the RCF of carburized, quenched and tempered SAE4320 steel were investigated in detail using scanning electron microscopy with electron backscattering diffraction and transmission electron microscopy with automated crystal orientation mapping. These analyses clarified that different variants of the extremely fine deformation-induced martensites as small as several tens of nm were formed within an austenite grain with RCF, and the martensites were speculated to have the Kurdumov-Sachs or the Nishiyama-Wasserman relationship with the retained austenite. Furthermore, the deformation-induced martensites were preferentially formed within the retained austenite grains rather than at the interface between the tempered martensite and retained austenite. This suggests that the deformation-induced martensites were formed from some localized regions that were plastically introduced within the retained austenite grains.

Carbon Migration Behavior during Rolling Contact in Tempered Martensite and Retained Austenite of Carburized SAE4320 Steel

“…Analysis using automated crystal orientation mapping by TEM (ACOM-TEM), which has a higher spatial resolution than SEM-EBSD, confirmed that the smallest deformation-induced martensite was a few nanometers in size. 29) From 3.7 × 10 6 cycles onwards, at least up to 44 × 10 6 cycles, there was no significant progression of deformation-induced martensitic transformation, as can be also inferred from the results in Fig. 2.…”

“…4) showed that deformation-induced martensite was formed in the retained-austenite grains via rolling contact. In our previous paper using ACOM-TEM, 29) we investigated the morphology of the deformation-induced martensite, which had not been previously elucidated, and confirmed that fine martensite, with a minimum size of several nanometers, was dispersed in the retained austenite. Therefore, the state of the carbon in the deformation-induced martensite was investigated using APT together with the results of ACOM-TEM in the previous study.…”

“…An appropriate amount of retained austenite may improve the RCF life by transforming it into deformation-induced martensite during rolling contact. 3,[22][23][24][25][26][27][28][29] Various possibilities have been proposed regarding the role of deformationinduced martensite on RCF, such as the suppression of crack propagation via deformation-induced martensitic transformation at the stress-concentration part in the tip of the internal crack, 23) delayed fatigue of tempered martensite via formation of new martensite, 3,23) or introduction of compressive residual stress. 24,26) However, the mechanism remains largely unclear.…”

“…Therefore, the state of the carbon in the deformation-induced martensite was investigated using APT together with the results of ACOM-TEM in the previous study. 29) First, the average interparticle spacing of the deformation-induced martensite in the retained-austenite grains was evaluated for the specimen after 3.7 × 10 6 cycles based on the results of ACOM-TEM. 29) As schematically shown in Fig.…”

“…29) First, the average interparticle spacing of the deformation-induced martensite in the retained-austenite grains was evaluated for the specimen after 3.7 × 10 6 cycles based on the results of ACOM-TEM. 29) As schematically shown in Fig. 11, circular deformation-induced martensite grains of equal sizes were assumed to be aligned at regular intervals within an initially (pre-RCF) square retained-austenite grain.…”