Effect of Co on Microstructure and Stress Rupture Properties of K4750 Alloy

The effects of substituting Co for Fe on the microstructure and stress rupture properties of K4750 alloy were studied. The microstructure of the alloy without Co (K4750 alloy) and the alloy containing Co (K4750-Co alloy) were analyzed. Substitution of Co for Fe inhibited the decomposition of MC carbide and the precipitation of eta phase during long-term aging treatment. In K4750-Co alloy, the morphology of MC carbide at the grain boundary (GB) remained dispersed blocky shape and no eta phase was observed after aging at 750 degrees C for 3000 h. However, in K4750 alloy, almost all the MC carbides at GBs broke down into granular M23C6 carbide and needle-like eta phase. The addition of cobalt could delay the decomposition of MC carbides, which accordingly restricted the elemental supply for the formation of eta phase. The stress rupture tests were conducted on two alloys at 750 degrees C/430 MPa. When Co is substituted for Fe in K4750 alloy, the stress rupture life increased from 164.10 to 264.67 h after standard heat treatment. This was mainly attributed to increased concentration of Al, Ti and Nb in gamma ' phase in K4750-Co alloy, which further enhanced the strengthening effect of gamma ' phase. After aging at 750 degrees C for 3000 h, substitution of Co for Fe can also cause the stress rupture life at 750 degrees C/430 MPa to increase from 48.72 to 208.18 h. The reason was mainly because MC carbide degradation and eta phase precipitation in K4750 alloy, which promoted the initiation and propagation of micro-crack during stress rupture testing.