3DAP INVESTIGATION OF PRECIPITATION BEHAVIOR OF Cu-RICH PHASE IN SUPER304H HEAT RESISTANT STEEL

Super304H austenitic heat resistant steel is based on 18/8 Cr-Ni stainless steel alloyed mainly with 3%Cu and a small amount of Nb combined with N, which is used as superheater/reheater tubes in ultra-super critical (USC) power plants all over the world, due to its good combination of elevated temperature strength with hot corrosion resistance. The excellent high temperature strengths of this steel are mainly contributed by the precipitation strengthening effect of fine Cu-rich phase. Comprehensive study of the characteristic of Cu-rich phase is very important to understand strengthening effect on this steel. However, the Cu-rich phase is very fine and difficult to be detected at the beginning of precipitation. In this paper, three dimensional atom probe (3DAP) was used to study the early stage of precipitation behavior and the composition change in Cu-rich phase of Super304H aged at 650 degrees C for different times after solution treatment at high temperature. Cu-rich phase is formed from Cu-rich segregated rigion by the concentration of Cu atoms in it at very beginning stage of aging. Homogeneously distributed Cu-rich phase precipitates with about 1 nm radius are obviously detected after aging at 650 degrees C for 5 h. With increasing aging time, Cu-rich phase is growing slowly while other elements in the Cu-rich phase decreased obviously. The copper element has almost concentrated to 90% in the center part of Cu-rich phase after 500 h aging. The homogenous distribution of fine Cu-rich phase in austenitic matrix effects excellent hardening with increasing aging time. The stability of fine Cu-rich phase is one of the most important reasons for keeping good strength of Super304H heat resistant steel at high temperature.