Search

The formation mechanisms for two carbides with co-existence and increasing stacking faults nearby the carbides during surfacing welding were investigated in this study. The results indicated that the surfacing layer had a two-phase structure of a matrix phase and a second phase, the density of the second phase was relatively small and the second phase was discontinuous, by contrast with the as-received welding wire. Relatively uniform microstructure of surfacing layer and the welding interface without melting of base metal were obtained, resulting in an inapparent change in the microhardness. A few carbides with blocky shape were identified in the surfacing layer. The mechanisms for (Cr,Co)23C6 of face-centered cubic crystal structure co-existing with (Cr,Co)7C3 of hexagonal close-packed crystal structure and (Cr,Co)23C6 subsequently forming close to the pre-formed (Cr,Co)7C3 during the cooling process were discussed in alloying element, calculated equilibrium phase diagram, Gibbs energy, etc. The relatively higher density of the stacking fault present around the carbides was understood from forming energy of stacking faults.

The microstructure evolution, elements diffusion and fracture behavior of the Stellite 6 weld overlay, deposited on 10Cr9Mo1VNbN (F91) steel by the tungsten inert gas (TIG) cladding process, were investigated after long-time service. Obvious diffusion of Fe occurred from the steel and fusion zone to the Stellite overlay, resulting in the microstructure evolution and hardness increase in the coating, where hard Co&ndash, Fe phases, &sigma, phases (Fe&ndash, Cr metallic compounds) and Cr-rich carbides (Cr18.93Fe4.07C6) were formed. Besides, the width of the light zone, combined with the fusion zone and diffusion zone, increased significantly to a maximum value of 2.5 mm. The fracture of the Stellite coating samples mainly occurred in the light zone, which was caused by the formation and growth of circumferential crack and radial crack under high temperature and pressure conditions. Moreover, the micro-hardness values in the light zone increased to the maximum (470&ndash, 680 HV) due to the formation and growth of brittle Co&ndash, Fe phases. The formation of these cracks might be caused by formed brittle phases and changes of micro-hardness during service.