Stress-Relief Cracking in Simulated-Coarse-Grained Heat Affected Zone of a Creep-Resistant Steel

Abstract

Cracking has been reported in newly constructed water wall panels of fossil power plants during startup testing. Both high hardness (exceeding 350 HV) and high level of welding residual stress have been reported in welds of waterwall panels made of T23 and T24 steels. Stress-relief cracking (SRC) is being considered as a possible failure mechanism during high temperature exposure such as PWHT. High temperature exposure of non PWHT-ed welds of Grade T23 and T24 steels leads to hardening in the weld and coarse-grained heat-affected zone (CGHAZ). It has been suggested that such a hardening mechanism can lead to stress-relief cracking (SRC). The objective of this study is to evaluate the susceptibility to SRC in the coarse grained heat affected zone (CGHAZ) of Grade T24 steel utilizing a Gleeble-based SRC test developed at The Ohio State University. The strain-age cracking test developed at The Ohio State University was modified in order to better replicate the conditions of PWHT in highly restrained welds and quantify the stress-relief cracking susceptibility in creep resistant steels. In addition to reduction in area and time to failure, the modified test allows quantification of the stress and strain that cause failure during SRC testing. This test utilizes the Gleeble^® 3800 thermo-mechanical simulator. SRC testing of simulated-CGHAZ in Grade T24 Steel has revealed ductile failure for the sample tested at 600 °C, predominantly intergranular with ductile features for the sample tested at 650 °C, and brittle intergranular failures for the samples tested at temperatures of 675 °C and above. For PWHT above 600 °C at residual stress levels close to the yield stress, the CGHAZ in Grade T24 steel welds may be susceptible to SRC.