Stress relaxation study of HAZ reheat cracking in type 347 stainless steel

Four different compositions of Type 347 austenitic stainless steel were stu died using Gleeble(TM)-based stress relaxation tests to evaluate susceptibi lity to weld HAZ reheat cracking. Coupons extracted from plate were thermal ly cycled to reproduce the coarse-grained region of the HAZ known to be mos t prone to such cracking. These samples were then reheated to various PWHT temperatures, and a strain comparable to 70% of the strain to cause yieldin g at the temperature was applied. Force in the specimens was recorded for u p to 3 h while this total strain was kept constant. Force vs. time curves f or all samples exhibited an increase attributed to volumetric shrinkage upo n precipitation of Nb-rich particles as confirmed by constant-stress tests. Formation of Nb-rich precipitates strengthens the alloy and stress buildup due to volumetric shrinkage in the age-hardening matrix leads to cracking along grain boundaries when stress cannot relax fast enough by yielding or creep.