The mechanism of stress-relief cracking in a ferritic alloy steel

Stress-relief cracking is a major cause of weld failures in creep-resistant, precipitation-strengthened materials such as ferritic alloy steels, stainless steels, and Ni-based superalloys. Stress-relief cracking occurs primarily in the coarse-grained heat-affected zone of weldments. Although the general causes of stress-relief cracking are known, the underlying mechanisms are very much a topic of debate. The mechanism of stress-relief cracking in the coarse-grained heat-affected zone (CGHAZ) of a new ferritic alloy steel (HCM2S) was investigated through stress-relaxation testing and detailed microstructural characterization. The CGHAZ simulation and stress-relaxation testing was performed using Gleeble techniques. The time to failure exhibited C-curve behavior as a function of temperature. A balance of intergranular and intragranular carbide precipitation controlled the stress-relief cracking susceptibility. Cracking initiated at prior austenite grain boundaries by cavity nucleation on incoherent, Fe-rich M3C carbides. The grain interiors were resistant to plastic deformation due to precipitation strengthening by small (5-40 nm) alloy carbides. Elemental segregation played no detectable role in the stress-relief cracking failures. Much of the microstructural characterization was performed using a VG603 FEG STEM having a probe size of about 1.5 mn. The small probe size allowed nano-sized precipitates to be individually analyzed by using EDS and elemental EDS traces taken across prior austenite grain boundaries. In addition, SE STEM imaging with the VG603 FEG STEM was able to resolve small precipitates that were previously unobservable using conventional TEM and STEM techniques. The results of this study form a basis for heat treatment and welding process variables for HCM2S to avoid stress-relief cracking. In addition, these procedures and analytical results can be applied to other materials to avoid microstructures that are susceptible to stress-relief cracking.