Effects of PWHT Temperature on Mechanical Properties of High-Cr Ferritic Heat-Resistant Steel Weld Metals

Welded joints of ASME Gr. 91 steel are subject to post-weld heat treatment (PWHT) to obtain sufficient mechanical properties. However, when the PWHT temperature is higher than the lower critical transformation temperature (AC1) of both the base metal and the weld metal, the mechanical properties of the weld may be degraded. To avoid this, it is important to know the AC1 of the weld. The authors have investigated several methods of measuring the AC1 of Gr. 91 steel weld metals to select the most credible method and examined the effects of the PWHT temperature on the mechanical properties of the weld metal. In addition, the proper PWHT temperature was discussed in consideration of the measurements of AC1, test results of creep-rupture performance, and observation results of precipitates. In the case of a low Mn+Ni weld metal, the upper PWHT temperature limit should be lower than AC1. On the other hand, in the case of the high Mn+Ni weld metal, remarkable negative effects on the mechanical properties of the weld metal were not found, even though the PWHT temperature exceeds AC1 by about 30 °C.