Effect of Simulated Heat Affected Zone Thermal Cycle on the Creep Deformation and Damage Response of Grade 91 Steel Including Heat-to-Heat Variation

Abstract

Grade 91 and other creep strength enhanced ferritic steels are widely used in fossil power plants as the structural materials of high temperature piping and components. As the operating temperature of these plants is as high as 600°C, creep deformation and accompanying damage cannot be neglected in evaluating their integrity. In particular, creep damage tends to concentrate in the softened heat-affected zone in welded joints and often caused so-called type-IV cracking, in advance of failure in base metal or weld metal. In evaluating the likelihood of such a failure, it is important to understand the mechanical property of such region in comparison with base metal and weld metal. Previous studies have shown that considerable degradation of creep strength brought by thermal cycles simulating the temperature history experienced by such a zone. In order to strengthen the understanding, thermal cycle was given to six Grade 91 heats encompassing a wide range of creep properties and their creep behavior was evaluated in this study. As a result, it was found that resistance for creep deformation and rupture life of six heats showed different trend than what was observed in the original metal but the heat-dependency of the ductility seems to be inherited.