Fatigue crack growth of AISI 304 stainless steel welds in air and hydrogen

Fatigue crack growth behavior of AISI 304 stainless steel plate and its plasma weld was evaluated in laboratory air and gaseous hydrogen. Observed variation in crack growth characteristics are explained based on the martensitic transformations near the crack tip due to mechanical loading and environmental action. The microstructures of fusion zone consisted of lacy ferrites in the columnar grain and subgrain boundaries within the austenite matrix, versus equiaxial austenite grains with some twins inside the parent metal. The threshold Δ K of the as-welded (AW) specimen required to initiate crack growth was clearly higher than that of the other specimens, regardless of testing environments. The considerable retardation of crack growth vanished for welds subjected to stress-relief treatment. In addition, all specimens tested in gaseous hydrogen exhibited enhanced crack growth. The fatigue-fractured surface of welds was found to be very rough suggesting a zig-zag crack path, unlike the straight path in the base metal. The lack of twin boundary failures in addition to the irregular solidified microstructure could be the causes for the improved resistance to fatigue crack growth in case of the stress-relieved weld. Moreover, the thin martensite layer formed on the cracked surface was correlated with the hydrogen-enhanced crack growth, resulting in inducing extensive quasi-cleavage (QC) fracture for the specimens tested in hydrogen.