Investigatiom of deposition and wear problems with hardfaced coatings for high temperature liquid metal service

Abstract

We have examined hardfaced coatings on two valves that were designed for use in high temperature liquid sodium environments. The valve bodies were type 304 stainless steel and were hardfaced with AWSRCoCr-A (cobalt-base material) using the gas tungsten-arc process. One valve was a cold leg trap valve that was removed from the Sodium Component Test Installation at the Liquid Metal Engineering Center. Owing to system redesign this valve was removed from sodium service after 40 000 h at temperatures up to 427°C (800°F). It had experienced some sticking but this was not of significant consequence. The microstructure of RCoCr-A consists of an austenite matrix and a carbide and eutectic carbide interdendritic structure from which the material gains its wear resistance. Evaluation revealed evidence of erosion/corrosion of the hard interdendritic material, thus leaving a wear surface of softer matrix material. As a result, some galling (self-wedding) of the wear surfaces apparently occured. The other valve was a hot leg isolation valve intended for use in the Fast Flux Test Facility. This valve was rejected owing to hot cracking during deposition of the hardfacing material (coating) and was not put into operation. The hardfaced surface was inspected for liquid penetration and was sectioned for evaluation by metallographic examination, scanning electron microscopy and microprobe analysis. It was concluded that hot cracking occured in the base metal, due to liquation of grain boundaries in the weld heat-affected zone, and was followed by crack propagation through the hardfaced deposit.