Investigation the effect of precipitating characteristics on the creep behavior of HR3C austenitic steel at 650 ℃

Abstract

The microstructure evolution in HR3C (25Cr-20Ni-Nb-N steel) austenitic heat-resistant steel after creep rupture at 650 ℃ are carefully characterized in this paper. The long-term creep property of HR3C heat-resistant austenitic steel mass-produced has been conducted at 650 ℃, The threshold strength at 650 ℃ was determined by extrapolation method to be σ105650=105.0MPa. Corresponding microstructure investigation indicates that several kinds of carbides/nitrides precipitated during creep exposure, and the main precipitates are M23C6 carbides and NbCrN nitride or Z-phase. Dense Z-phase particles precipitate dispersively in the matrix along dislocation lines as stripes and keep well coherent relation with the matrix, which have distinct effect of precipitation strengthening and show highly stable against coarsening during long-term creep exposure. The Z-phase has a crystallgraphic orientation relationship with the matrix as [001]γ//[101]Z, [001]γ//[021]Z and (001) γ//(111) Z. M23C6 carbides mainly precipitated on the grain boundaries, which grow and coarsen distinctly and widen the grain boundary obviously under the function of high stress at elevated temperature. Some fine M23C6 carbides also precipitated in the matrix and are dense close to the grain boundaries with no Z-phase around. So the fine Z-phase has a trendy to transfer into stable M23C6 carbides under long-term creep exposure. Creep cavities formed on the grain boundaries are connected to the M23C6 particles at the triple junctions between grain boundaries and the sites where acicular shape particles growing into the matrix, which indicate that the coarse M23C6 carbides at triple junctions on grain boundaries serve as high stress concentration and creep cavities nucleation sites. So coarse M23C6 carbides along grain boundaries no only have a negative effect to grain boundary strength and induce intergranular brittle tendency, they also play a major role in the creep performance and fracture mechanism. The Metallographic observations corroborated that coarse M23C6 carbides residing on the grain boundaries lead to initergranular fracture eventually.