Phase Transformation Behaviour in P91 During Post Weld Heat Treatment: A Gleeble Study

Grade P91 is a creep strength enhanced ferritic steel used widely at high temperature applications in thermal power plants. P91 weldments are subjected to post weld heat treatment (PWHT) at a typical temperature of 760 °C for stress relieving purpose and to obtain optimal mechanical properties. In general PWHT temperature is about 30–50 °C lower than the lower critical temperature (Ac1). Depending on the chemical composition (particularly Ni + Mn), heating rate and prior austenite grain size, Ac1 temperature can vary from 780 to 860 °C. In this investigation an attempt was made to understand the effect of surpassing the typical PWHT temperature of 760° and approaching Ac1 of the material. The investigation was based on physical simulation conducted using 3500 Gleeble thermo mechanical simulator. The study was performed on cylindrical specimens which were extracted from weld metal and parent metal. Specimens were subjected to normalizing treatment at 1050 °C for achieving austenitisation and to determine the Ac1 temperature and then to different PWHT temperatures in the range of 790–850 °C at suitable intervals. American Society for Testing and Materials (ASTM) A1033-10 based procedure was used to find Ac1 temperature for the parent metal and weld metal at slow heating rate condition to compare with practical heat treating condition. Dilatometry plots, microstructure and hardness tests performed on the specimen were used to analyse the phase transformation. Results indicated that alpha ferrite phase and fresh untempered martensite could be formed in P91 steel when PWHT temperature was about 12 °C less than the Ac1 temperature. It was also seen that the heating rate had strong influence on the Ac1 temperature. At the heating rate of 28°/hr, Ac1 was about 792 °C for weld metal, while Ac1 was about 812 °C for the heating rate of 220 °C/hr.