The effect of duration of stress relief heat treatments on microstructural evolution and mechanical properties in grade 91 and 92 power plant steels

A detailed examination has been carried out of the microstructural evolution and mechanical properties of samples of T91 and T92 steels which have been subjected to both a 'normal' pre-service heat treatment and an extended stress relief heat treatment at 765°C for up to 16 hours. The samples have subsequently been creep tested to failure at different stresses ranging from 66 to 112 MPa. In each case, a reduction in rupture time was observed of 20-30% in the samples which had experienced the additional stress relief heat treatment compared to those which had not. It is shown that these data, when compared with the mean values expected from European Creep Collaborative Committee (ECCC) Datasheets, result in a reduction in stress of approximately 10% of the mean value predicted from the ECCC data, which is within the allowable scatter band. In order to investigate the reasons behind this reduction in life, extensive analyses have been carried out using a range of advanced electron microscopy techniques. In particular, imaging using both ion and electron beams in combination with image analysis methods have been used to provide a rapid and accurate technique for quantifying the particle size distributions of the M C carbides and also the Laves phase, which was found only in the T92 samples. Electron backscatter diffraction techniques have also been used to quantify the changes occurring in the martensitic matrix with respect to the nature and length of the grain boundaries and the grain size distribution. The effect of the additional heat treatment on microstructural evolution is to coarsen the secondary phase particle distribution and enhance matrix recovery before the steel enters service, to the detriment of the hardness and creep fracture life. It is also demonstrated that there is a significant effect of applied stress on the particle size distributions and matrix recovery during creep testing. Copyright © 2011 Electric Power Research Institute Distributed by ASM International®. All rights reserved.