Microstructure and Mechanical Properties of SP2215 Austenitic Heat-Resistant Steel Tube After Aging at 650 ℃
The microstructure,mechanical properties and fracture mechanism of SP2215 austenitic heat-resistant steel tube after aging at 650 ℃ for different time were studied by OM,SEM,TEM,and tests of microhardness,room temperature impact and high temperature tension.The results show that the microstructure of solid solution treated SP2215 steel is composed of austenite,a small amount of twins and undissolved NbN and Z phases.Cr23C6 preferentially precipitates at austenite grain boundaries during aging at 650 ℃,and gradually increases in amount,coarsens and forms a continuous network with the extension of aging time.The spherical Cu-rich phase is precipitated in the austenite grains;when the aging time reaching 2012 h,its size is about 15 nm.The room temperature microhardness in grains of SP2215 steel reaches the maximum value after aging for 50 h,and then tends to be stable,which is related to the precipitation strengthening effect of stable Cu-rich phase.SP2215 steel has an obvious tendency of high temperature aging embrittlement.The room temperature impact absorption energy of sample aged for 2012 h is about 78.5％lower than that of solid solution treated sample.The impact fracture mechanism at room temperature changes from ductile fracture to intergranular brittle fracture with the increase in aging time,which is caused by the precipitation,aggregation and coarsening of Cr23C6 at austenite grain boundary.Portevin-Le Chatelier(PLC)appears when SP2215 steel was stretched with the strain rate of 2.5 × 10-4 s-1 at 650 ℃,and the serrated type is Type(A+B).With the increase in aging time,the PLC gradually"weakens",but the serrated type remains unchanged.With increasing the aging time,the high temperature tensile yield strength of SP2215 steel remains basically stable,the tensile strength and reduction of area gradually decrease,and the high temperature tensile fracture mechanism changes from ductile fracture to quasi-cleavage brittle fracture.
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