Stress-relief cracking of a new ferritic steel

The mechanism of stress-relief cracking in the coarse-grained heat-affected zone (CGHAZ) of low-alloy ferritic steels was studied through a tempering study, stress-relaxation testing, and detailed microstructural characterization. A new ferritic alloy steel, HCM2S, was used as the model system. Common 2.25Cr-1 Mo steel, which is susceptible to stress-relief cracking, was used for comparison to HCM2S.; The CGHAZ was simulated using Gleeble techniques. A dense distribution of small tungsten-rich carbides within the prior austenite grains induced secondary hardening in the CGHAZ of HCM2S. The CGHAZ of 2.25Cr-1 Mo steel exhibited secondary hardening due to the intragranular precipitation of many Fe-rich M₃C carbides. The hardness of HCM2S was more stable at longer times and high temperatures than 2.25Cr-1 Mo steel due to the intragranular precipitation of small W and V-rich carbides.; The CGHAZs of HCM2S and 2.25Cr-1 Mo steel were susceptible to stress-relief cracking between 575 and 725°C. HCM2S exhibited C-curve behavior with respect to the time to failure as a function of post-weld heat treatment (PWHT) temperature. No segregation of tramp elements to prior austenite grain boundaries was detected in HCM2S. Both intergranular and intragranular carbide precipitation controlled the stress-relief cracking behavior. The amount of intergranular failure increased with test temperature due to the increasing amounts of Fe-rich M3C carbides at the prior austenite grain boundaries. These carbides acted as cavity nucleation sites. The cavities coalesced to form microcracks along prior austenite grain boundaries. Eventually, the remaining uncracked areas could not support the load and failed by ductile rupture.; The balance of intergranular and intragranular carbide precipitation resulted in the C-curve behavior. The nose of the C-curve occurred at 675°C. The intragranular regions were strong because of a dense distribution of W/Fe-rich carbides, but the prior austenite grain boundaries were weak due to a large amount of intergranular M₃C carbides.; A mechanism for stress-relief cracking in the CGHAZ of HCM2S has been proposed. The results of this study form a basis for heat treating and welding processing variables for HCM2S and 2.25Cr-1 Mo steel to avoid stress-relief cracking. In addition, these results can be applied to other materials to avoid microstructures susceptible to stress-relief cracking.