Microstructural evolutions of IN738LC during multiple reheat treatment and aging

Abstract

IN738LC is a polycrystalline superalloy which is still widely used for gas turbine blading in many industrial applications. It has been the centre of many research programmes during the last four decades in areas such as alloy design, processing, various degradation regimes and even after heat treatment during repair. The subject of this study has been the microstructural evolution during multiple reheat treatments combined with long term aging. Material has been exposed at two different aging temperatures (875 and 950°C) for a period of 4000 h. This has been followed by a reheat treatment and then further aging, up to a total of 16 000 h. The microstructural evolution of the alloy, γ′ coarsening and carbide transformations, have been investigated in detail after the heat treatments and further aging periods. The results highlighted that although the γ′ structure has largely been recovered by the heat treatments, that of the carbides did not follow a similar trend. This partial recovery of the alloy microstructure and its potential impact on the long term integrity of gas turbine blading which has been heat treated periodically will be discussed in this paper.

Keywords:

• IN738
• Microstructure
• Property recovery
• Refurbishment
• Heat treatment

Some of this work was funded by Siemens AG, Siemens Westinghouse, Siemens Industrial Turbines, PX Ltd, Laborelec, Elsam, Vattenfall and EDF. This paper is based on a contribution to the 8th International Parsons Turbine Conference, organised by the Institute of Materials, Minerals and Mining at Portsmouth, UK on 5–8 September 2011.