Nondestructive Evaluation of Temper Embrittlement in Turbine Casing Steels

Abstract

In order to develop a nondestructive method of evaluating embrittlement for turbine casing steels, a study was conducted using lab-charged P-doped steels. P-doped steels showed evident embrittlement due to segregation of P at grain boundaries. The chemical etching test used was found to be a very good measure of embrittlement for casing steels. There was a good correlation between the width W of the etched grain boundary measured by the penetration of replicas and FATT. The correlation between W and FATT was also recognized for the samples taken from long-term-serviced casings. Multiple regression analysis was conducted in order to express FATT using W and other variables which are known or nondestructively measurable. The regression equation obtained gave an estimate of the actual FATT with the scatter of ±20°C. On the basis of this result a nondestructive evaluation method of temper embrittlement for turbine casing is proposed.