Mechanism of Crack Initiation / Propagation for Ultra-high Strength Stainless Steel under Corrosion Environment - Fatigue Load Coupling

In this work, to clarify the mechanism of corrosion fatigue crack initiation / propagation for 10Cr13Co13Mo5Ni3W1VE ultra-high strength stainless steel (UHSS) under corrosion environment - fatigue load coupling, fatigue, pre-corrosion fatigue, and corrosion fatigue behaviors have been investigated. Under uncoupling conditions (fatigue), cracks mainly initiated from internal defects, with quasi-cleavage fracture. Under coupling conditions (salt-spray pre-corrosion fatigue and corrosion fatigue), cracks mainly initiated from surface pits, with quasi-cleavage + cleavage fracture. During salt-spray pre-corrosion, the percentage of pits at prior austenite grain boundaries (PAGBs) increased (from 20% to 42%), leading to greater influence on crack initiation. Cleavage fracture for pre-corrosion fatigue was only observed at crack origin, while subsequent crack propagation was quasi-cleavage mode. Surface cracks of corrosion fatigue mainly initiated from pits at PAGBs and block boundaries (BBs) which were controlled by slip-dissolution mechanism. Pits at BBs accounted for 70%, which played a dominant role in crack initiation. Comparing S-N curves for fatigue and corrosion fatigue, the coupling effect accelerated crack propagation and resulted in fractured at lower lives. Transgranular corrosion fatigue cracks with block schmid factor (SF) > 0.463 at deflection sites mainly propagated across blocks, suggested that transgranular cracks were primarily controlled by slip-dissolution mechanism. Intergranular cracks mainly propagated along PAGBs where the misorientation angles (MAs) ranged from 40° to 55°, which were influenced by hydrogen-induced cracking. The corrosion fatigue crack propagation in UHSS was governed by the synergistic interaction of slip-dissolution and hydrogen-induced cracking mechanisms. This study provides guidance for the corrosion protection design of UHSS.