Effects of Strength and Microstructure on Hydrogen-Assisted Crack Propagation in 22Cr-13Ni-5Mn Stainless Steel Forgings

The objective of this study was to evaluate the effects of hydrogen on the fracture toughness and fracture mechanisms for the nitrogen-strengthened, austenitic stainless steel 22Cr-13Ni-5Mn, an alloy with potential value in high-pressure hydrogen containment components. The fracture initiation toughness and crack-growth resistance were measured before and after thermal precharging with hydrogen and as a function of crack-growth orientation and material strength. The effects of crack-growth orientation and material strength dominated over the effect of hydrogen exposure. The former two variables caused changes in fracture initiation toughness of up to 400 pct, while dissolved hydrogen resulted in only modest decreases in fracture initiation toughness of 20 to 40 pct. Coarse Z- phase (CrNbN) particles aligned in bands governed the measured fracture toughness and observed fracture mode. Fracture progressed by void nucleation and growth in the Z- phase bands, forming microcracks that ultimately linked through the remaining austenite matrix. Crack-growth orientation, material strength, and hydrogen exposure affected the nucleation and growth of voids in the Z- phase bands and the subsequent linking of microcracks. Control or elimination of the coarse, banded Z phase would likely enhance the fracture resistance of this alloy.