Improvement of Surface Cracking Risks by Controlling Dilution of Cobalt-Based Hardfacings

Abstract

Cobalt-based hardfacings have been widely applied to the main valve seats of steam turbines for the purpose of improving durability. With the aim of decreasing the surface cracking risks of the hardfacings, the dilution of the hardfacing on their surface was controlled and the properties of the dilution-controlled materials were investigated in this study. Mixtures of cobalt-based hardfacings and base materials were cast in order to simulate the dilution of base material to hardfacing, which occurs in the process such as PTA welding. It was confirmed that the grain size matched that produced by PTA welding by controlling the cooling rate. To investigate the effect of aging on the microstructures and mechanical properties of the cast samples, the samples were aged in electric furnaces where the temperature was controlled to the steam temperature of steam turbines and above. The test results show that the phase transitioned from face-centered cubic to hexagonal closed-packed in low-dilution samples, while carbides precipitated along the grain boundaries in high-dilution samples after aging. Both samples showed an increase in hardness and reduction in ductility and fracture toughness. In addition, the variation in microstructures and strength properties was suppressed for a 20 % dilution sample. To validate the influence of dilution as investigated with the cast samples, test specimens were machined from the surface and bottom layers of multi-layered hardfacing that had different dilutions by PTA welding. It was confirmed that the influence of dilution on the strength properties of hardfacing layers had a similar tendency to the cast samples. The results above lead to the possibility of reducing the surface cracking risks by controlling the dilution of hardfacings and suppressing embrittlement after aging. We have applied dilution-controlled hardfacings to steam valves and successfully reduced surface cracking.