Mechanisms of fatigue crack propagation in a Q&P-processed steel

Abstract

The fatigue crack propagation (FCP) behavior of a quenched and partitioned (Q&P), low-alloyed CSiCrMn steel containing 12% of retained austenite is investigated in ΔK-controlled tests at different R-ratios and compared with its quenched and tempered (QT) counterpart, containing 4% of retained austenite. Characteristic features of the crack wake and FCP surfaces are examined by scanning electron microscopy and related to the presence or absence of martensitic transformation. Comparing the FCP behavior of the Q&P condition with its QT counterpart, the fatigue threshold ΔKth is higher, and fatigue crack growth rates in the Paris-regime are lower, for Q&P. With an increase of ΔK, Paris-lines intersect and, at high ΔK, the fatigue crack growth resistance of the Q&P condition becomes inferior. Such FCG behavior is attributed to the amount and stability of retained austenite: at low ΔK, contributions of the martensitic transformation are negligible, and thus a higher fraction of retained austenite accounts for an increased FCG resistance. With increasing ΔK, an increasing percentage of retained austenite in the cyclic plastic zone at the crack tip is transformed, providing brittle pathways for fast crack propagation and thus lowering FCG resistance. Analysing the crack surface, cleavage-like intergranular crack propagation can be directly related to martensitic transformation: in the threshold region, FCP occurs predominantly in a transgranular mode and the percentage of cleavage planes increases as ΔK increases. This study is the first one that examines the FCP behavior in a Q&P-processed steel and one of very few which deal with the near-threshold FCP behavior in TRIP steels.