Understanding crack growth within the γ′ Fe4N layer in a nitrided low carbon steel during monotonic and cyclic tensile testing

Nitriding is a cost-effective method to realize simultaneous improvements in tensile and fatigue properties and resistance to abrasion and corrosion. Previous studies reported that nitriding pure Fe enhances tensile strength by ~ 70% and fatigue limit by ~ 200%. It is due to the increase in surface hardness caused by the formation of γ′(Fe 4 N) and ε(Fe 2-3 N) nitrogen-containing intermetallic compound phases. However, the intermetallic compound layer is prone to brittle-like cracking. To better design nitrided steels, it is crucial to identify the crack growth mechanisms via analysis of the microstructural crack growth paths within the ~ 4–6 µm thick nitride layer. In the current work, we statistically evaluate the crack propagation behavior in the γ′ Fe 4 N layer during monotonic and cyclic tensile deformation in nitrided low-carbon steel (0.1 wt% C). Since nitriding typically results in the formation of columnar grains, the effect of morphology needs to be clarified. To this end, the steel was shot-peened and subsequently nitrided to promote equiaxed nitride grains morphology (~ 16% increase). Crack growth paths were comparatively evaluated for multiple cracks, and no significant effect of nitride morphology was observed. {100} γ′ is the predominant transgranular crack path in the monotonic tensile tested specimen, followed by {111} γ′ . It is despite the elastic modulus of {111} γ′ < {100} γ′ . This contrary behavior is explained by {100} γ′ plane having the lowest surface energy (density functional theory calculations). In the cyclic tensile loaded specimen, experiments revealed that transgranular cracking along {100} γ′ (cracking via symmetric dislocation emission) or {111} γ′ (slip plane cracking) is equally likely.