The effect of simulated post weld heat treatment temperature overshoot on microstructural evolution in P91 and P92 power plant steels

Creep strength enhanced ferritic (CSEF) steels, in particular modified 9Cr steels Grade 91 and 92, are becoming more widely used in the electrical power generation industry for the construction of header and steam piping in advanced coal-fired power plants. They typically enter service having received a standard high temperature normalizing treatment following by a lower temperature tempering treatment designed to produce an optimum microstructural condition. However, situations may arise in practice, particularly during welding operations for example, whereby the component may receive an additional heat treatment which briefly exceeds the Ac, and possibly the Ac , temperature before stabilizing at the tempering temperature. In this research, simulated post weld heat treatments (PWHT) have been applied to Grade 91 and 92 materials using carefully controlled heating and cooling rates within a dilatometer. Peak temperatures applied were below Ac, between Ac and Ac, and above Ac, prior to a subsequent heat treatment at 750°C for 2 hours. Hardness measurements demonstrated a significant reduction once the Ac temperature was exceeded. Advanced electron microscopy has been carried out to investigate the effect of the PWHT excursions on subsequent microstructural evolution. Electron back scatter diffraction has been used to quantify the nature of the martensite laths and grain structure changes as a function of temperature. The detailed size distribution of carbides within the microstructure has also been determined using both scanning and transmission electron microscopy. These results are discussed in respect of the likely consequences of such a PWHT overshoot on subsequent mechanical properties during high temperature service. Copyright © 2011 Electric Power Research Institute Distributed by ASM International®. All rights reserved.