3 W - 3 Blade Co Is A New Type Ferrite Steel Reinforcement Mechanism Research

The new type ferritic blade steel3W-3Co has been studied in this paper. To develop ferritic blade steel for26-35MPa/620-650"C advanced USC power plants is the goal of this project. The best heat treatment has been confirmed by using OM, SEM, TEM, X-ray diffraction and mechanical properties tests. The types of precipitates and the effect of microstructures evolution on mechanical properties have been researched in this paper. The effect of precipitates’growth on the creep strengthen during creep process at high temperature and the precipitate strengthening mechanism have also been discussed.In this research there were four heat treatments for this ferritic steel. According to the results of microstructures and mechanical properties, this steel has the better room temperature mechanical properties after720℃and750℃tempering. This steel has the better620℃short-time mechanical properties after720℃tempering. Thus, the best heat treatment is1100℃×1h(OQ)+720℃×2h tempering. The main precipitates is M23C6phase, Nb(C,N) phase and M3B2phase(formed during smelting) after heat treatment. The Laves phase precipitates during long-term aging. The hardness increases at the early stage and then decreases after long-term aging at620℃/0-2000h. The strength decreases at initial stage and increases during aging. The ductility decreases obviously at the early stage and keeps in a stable level during long-term aging.The creep stress rupture curve of this steel at620℃keeps linear relation. There is an inflection point at300MPa/1265h in this curve. The creep stress decreases quickly after that inflection point. According to the results, the coarsening rate of M23C6phase at grain boundaries is the highest one and then is Laves phase. M23C6phase at the sub-boundaries grows slowly and MX phase is a kind of stable strengthening phase. Thus, one of the reasons for creep strength degeneration is that the growth of M23C6phase at grain boundaries. The martensite laths restored and the density of dislocation decreased. The characteristic of some of the martensitic lathwas not appreciable and the recrystallization grains formed during long-term creep process. According to this results as mentioned, the other reason for creep strength degeneration is that the weakening of grain boundaries and the dislocation strengthening.The creep stress rupture curve of this steel at620℃/0～7508h, Laves phase grew up the slowly. Laves phase can effectively slow down the growth rate of the M23C6phase.The main of Laves phase raise persistent strength. MX size is smaller, stable in high temperature creep process,plays an enhanced role for lasting strength. Distribution of fine precipitates along the lath boundary and lath can be effective pinning dislocation slip and improve the rupture strength.