Cracking of a Cobalt-Based Hardfacing of a Gate Valve Disk in a Desalination Power Plant

Cobalt-based alloys of the Stellite family are used as hardfacing for sealing surfaces of valves operating in desalination and power plants because of their excellent low friction and anti-galling properties in high-load sliding contact under the prevailing conditions. However, insufficient control of pressure and temperature during operation can degrade the integrity of the hardfaced material thus leading to its premature failure. This article presents a failure investigation carried out on the disk of a main stop gate valve that was used in a desalination plant. The disk was manufactured from X20 as a substrate material and a cobalt-based alloy for hardfacing. The cobalt-based hardfacing suffered from many surface and subsurface cracks that degraded its integrity. It was concluded that high-pressure steam flowing against the disk had tilted it and, thus, disturbed the alignment between the surfaces of the disk and the seat, leading to wear and large frictional heat that resulted in the degradation of the microstructure of the hardfacing layer and formation of the observed cracks.