Resistance of Austenitic Stainless Steels to Ductility-Dip Cracking: Mechanisms

Abstract

In Ni-based alloys, precipitates that form along grain boundaries (GBs) during terminal solidification have been shown to pin GBs and resist GB sliding, which can cause ductility-dip cracking (DDC). As a result, it is often suggested that the stainless steel skeletal/lacy δ in a γ matrix resists DDC because it pins GBs. In the present study, austenitic stainless steels 304, 316, 310, and 321 were quenched with liquid Wood’s metal (75̊C) during welding. Quenching captured the elevated-temperature microstructure and simultaneously induced cracking, thus revealing the mechanisms of the resistance to DDC. In addition, DDC was much higher in 310 than 304, 316, and 321, which is consistent with results of conventional tests. Both 304 and 316 solidified as columnar δ grains, with continuous γ formed along GBs soon after solidification to resist DDC along the GBs. 321 solidified as equiaxed grains of δ instead of columnar, and the tortuous GBs associated with equiaxed grains resisted DDC. 310, however, solidified as coarse, straight γ grains with little δ along the GBs, and solidification GBs migrated to become locally straight. The resulting GBs were long, straight, and naked, which is ideal for DDC. In 304, 316, or 321, skeletal/lacy δ in a γ matrix did not exist in the fusion zone near the mushy zone, where DDC occurs. This proved skeletal/lacy δ cannot resist DDC as often suggested. Instead, the present study identified two new mechanisms of resistance to DDC: 1) formation of continuous or nearly continuous γ along boundaries of columnar δ grains and 2) solidification as equiaxed δ grains.