Core microstructure-dependent bending fatigue behavior and crack growth of a case-hardened steel

Abstract

Abstract Carburizing is a thermo-chemical surface treatment through which a very hard martensitic layer develops in the external surface (case) of steel components resulting in substantial improvement in the fatigue life. Nevertheless, the overall fatigue properties of carburized steel components are yet severely dependent on the microstructure which develops in the interior region (core). This paper deals with the effects of core microstructure on bending fatigue behavior and fatigue crack growth of carburized steel parts. V-notched steel specimens were fabricated and subjected to two case hardening cycles where, respectively, bainitic-martensitic and ferritic-bainitic-martensitic microstructures developed in the core regions supported by similar fully martensitic microstructures in the case. 4-point plane bending fatigue tests were conducted to study the fatigue behavior of the heat-treated specimens. Furthermore, the effects of the core microstructures on fatigue crack growth resistance were also investigated. Hardness measurements revealed that both batches of specimens have similar hardness properties on the exterior surfaces, in the case-hardened layers and also in the cores. Moreover, the results showed that the specimens with the bainitic-martensitic core microstructure provide a marginally better fatigue performance in the finite life regime as compared to the ferrite-containing counterparts. More noticeable difference was, however, observed in the corresponding endurance limits where the former demonstrated a higher magnitude than the latter. Besides, the bainitic-martensitic core microstructure resisted the fatigue crack propagation more effectively than the ferrite-containing specimens.