Minor phase precipitation in 9 wt.% Cr power plant steels

A 9 wt.% Cr creep resistant power plant steel, P91, was subjected to transmission electron microscopy, energy dispersive x-ray and optical analysis in order to identify a method that may be used to assess the thermal history of the steel and to identify material which may have been subjected to incorrect pre-service heat treatment. As a first step, the relatively simple technique of optical image analysis was used. Photographs of etched ex-plant P91 specimens were analysed using an automated optical image analyser in order to identify any apparent trends that may be used as an indicator to either the service history or the pre-service heat treatment of the steel. The main parameter used was the amount of black space on each image, itself a function of both precipitation within the material and etching procedure. This technique provided inconsistent results due to the inherent problems in creating reproducible images and sensitivity. [Continues.]