Effect of creep constitutive equation on simulated stress mitigation behavior of alloy steel pipe during post-weld heat treatment

Abstract

Post-weld heat treatment (PWHT) is usually conducted to reduce the residual stress and to improve the mechanical properties of the welded region. The thermal elasto-plastic creep analysis is used to estimate the mitigation of the residual stress in the welded region analytically. The stress mitigation is caused by effect of the creep relaxation behavior during PWHT. Thus, creep constitutive equation is used to estimate the residual stress mitigation. The material properties of Norton's law and the Norton-Bailey law for alloy steel pipe JIS STPA23 (equivalent to ASME SA335P11 seamless steel tubes) were experimentally investigated. Creep tests were conducted at 400, 500, 600, and 700 °C. Specimens were subjected to certain stresses, and creep strain was measured. The material properties of Norton's law and the Norton-Bailey law were calculated from the measured creep strain data. Five bead-on curved plate specimens were fabricated in order to verify the thermal elasto-plastic creep analysis done with these creep constitutive equations. The curved plate was made from a pipe cut in the hoop direction. The specimens were welded without filler metal by gas tungsten arc weld (GTAW) on both surfaces in the center of the curved plate. The residual stress in one specimen was measured without PWHT. The other specimens were measured under different PWHT conditions. The results of analysis with the two creep constitutive equations agreed well with the experimentally measured results. In addition, no differences in the creep constitutive equations were observed at high PWHT temperature. Accordingly, these analyses conducted with both types of creep constitutive equation were verified to be effective. The effect of creep constitutive equation on simulated stress mitigation behavior during PWHT has been clarified for analytically estimating residual stress in the welded region.