Analysis of the creep behavior and microstructure of PWHT steam piping exposed to service

A comparative study was made of the material of a steam piping system (1.25Cr0.5Mo steel operating at 480 °C for over 20 years) and the areas affected by post-weld heat treatment (PWHT) applied during a simulated weld repair of this component in terms of its creep and microstructural behavior. This study was based on the results of a qualitative microstructural characterization by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and on constant load creep rupture tests. The PWHT caused precipitation of rod-shaped Fe2MoC carbide in the ferrite matrix, which, during service, had already undergone precipitation and coarsening of the M2C carbide, coupled with impoverishment of the matrix’s alloying element. From the standpoint of the stress and temperature values close to those specified for 1.25Cr0.5Mo steel, the creep strength of the material affected by PWHT decreased and creep rupture occurred approximately 30% sooner than in the material exposed to service and unaffected by the weld repair process.