Comparison of Stress Relaxation Cracking Susceptibility of Austenitic Stainless Steels

Coal-fired power plants often have welded joints made up of 347H stainless steel. However, this alloy is known to fail because of stress relaxation cracking. Thus, quantitative evaluation methods are needed as screening measures. In this study, a Gleeble® thermomechanical simulator was implemented in 347H and Super 304H alloy heat- affected zone (HAZ) simulation and stress relaxation testing. In the case of 347H, carbide dissolution in the HAZ reduced the hardness value and promoted grain growth. Alternatively, the respective extent of precipitate dissolution and hardness reduction in the nitrogen-containing Super 304H was relatively small. The stress relaxation tests were performed at a temperature of 700°C (1292°F), which was maintained for up to 70 h. Consequently, all 347H specimens fractured within 32 h. Furthermore, the time to rupture substantially decreased as the strain was increased from 5 to 10% and then to 15%. Additionally, the hardness near the fractured surface increased, and the plastic deformation primarily occurred near the grain boundaries. Conversely, the Super 304H specimens did not fracture during the 70-h testing period, at which time their hardness distribution was observed to still be relatively uniform. These results demonstrate that the susceptibility of stress relaxation cracking can be quantitatively determined according to the material and strain.