Secondary Precipitation Behavior of Nitride in the Electro-slag Remelted Structure of Alloy 690

The precipitation and evolution of secondary nitrides （S-nitrides） in the Alloy 690 electro-slag remelted （ESR） structure were investigated. Experimental results indicate that S-nitrides precipitated in the interdendritic region of the ESR structure at temperatures higher than 1100 ℃. S-nitrides could spread throughout the entire interdendritic area after sufficient exposure, and they were more numerous and much finer than primary nitrides. Furthermore, after studying the evolution of S-nitride particles at 1100, 1200 and 1300℃, it was determined that the precipitation of S-nitrides was controlled by the diffusion of nitrogen. In addition, by investigating the elemental segregation of ESR structure and calculating critical Ti concentrations, S-nitride precipitation was found to be thermodynamically inevitable in the Alloy 690 ESR structure.

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 ofηphase during long-term aging treatment.In K4750-Co alloy,the morphology of MC carbide at the grain boundary(GB)remained dispersed blocky shape and noηphase was observed after aging at 750℃ for 3000 h.However,in K4750 alloy,almost all the MC carbides at GBs broke down into granular M23C6 carbide and needle-likeηphase.The addition of cobalt could delay the decomposition of MC carbides,which accordingly restricted the elemental supply for the formation ofηphase.The stress rupture tests were conducted on two alloys at 750℃/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 inγ’phase in K4750-Co alloy,which further enhanced the strengthening effect ofγ’phase.After aging at 750℃ for 3000 h,substitution of Co for Fe can also cause the stress rupture life at 750℃/430 MPa to increase from 48.72 to 208.18 h.The reason was mainly because MC carbide degradation andηphase precipitation in K4750 alloy,which promoted the initiation and propagation of micro-crack during stress rupture testing.

Effect of heat treatments on Charpy impact properties of 15Cr12MoVWN ferritic/martensitic steel

The Charpy impact properties of 15Cr12MoVWN ferritic/martensitic steel for sodium-cooled reactors with variation in heat treatment factors and parameters are reported.The results show that the ductile-to-brittle transition temperature(DBTT)increased and the upper shelf energy(USE)decreased with increase in normalizing temperature.However,the variation tendency of DBTT and USE was the opposite with increase in tempering temperature.The tempering temperature showed a greater influence on USE than the normalizing temperature,and normalizing and tempering temperatures had the equally significant effects on DBTT,but the cooling method was not a significant factor for DBTT and USE.The prior austenite grain and M_(23)C_(6) size were the main influences on DBTT,and the dislocation density was the main factor affecting the variation of USE.The heat-treatment regime recommended for 15Cr12MoVWN steel was composed of normalizing at 1000-1050℃ for 0.5 h followed by water quenching or air cooling and tempering at 760℃ for 1.5 h.