Research On Precipitation Behavior And Linear Friction Welding Process Of Austenitic Heat-resistant Steel

S31042 steel is one of typical 25Cr-20Ni type austenitic heat-resistant steels with excellent resistance to oxidation and corrosion,and the creep rupture strength is improved by additional amounts of Nb and N.This austenitic steel is widely used as super-heaters and re-heaters in ultra-super critical plants,and its performance at elevated temperature is related to the formation and evolution of Z,MX and M₂₃C₆phase.Few studies have addressed the precipitation behavior of Z phase in austenitic steel so far,and the reinforcing mechanism of different M₂₃C₆phase is confusing.In order to clarify the precipitation mechanism of Z phase and summarize the reinforcing mechanism of different M₂₃C₆phase,aging experiments and creep tests of the S31042steel have been carried out at 700°C.The evolution behavior,thermal stability and strengthening mechanism of precipitates under the condition of 700°C were investigated and the relationship between precipitate evolution and high temperature performances was clarified by optical microscopy(OM),scanning electronic microscopy(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD),microhardness test and tensile test.Moreover,the methods of controlling microstructure to improve the high temperature resistance were explored by deformation and heat treatments,which can lay a solid fundamental basis for the improvement and development of the new-type high temperature materials.S31042steel and GH4169 alloy were jointed by linear friction welding to explore the process parameter and technical reliability.The main precipitates in aged S31042 steel include the Z,MX,M₆C and M₂₃C₆phase,of which Z phase is the dominated precipitate due to the smallest size and the best thermal stability.The Z phase precipitated preferentially along the(002)?and was coherent with the austenite during aging treatment.Besides,the Z phase grew into each other,which led to the formation of a net-like microstructure.Furthermore,the Z phase nucleated on the pre-precipitated M₆C or M₂₃C₆phase,and then grew by the consumption of M₆C and M₂₃C₆phase.The stable Z phase is important to promote the strength of S31042 steel by precipitation strengthening.At the beginning of aging,M₂₃C₆phase precipitated swiftly along grain boundaries,then the connection of M₂₃C₆phase resulted in a chain-like structure.The thickness of M₂₃C₆phase increased with aging time.Finally,the chain-like M₂₃C₆phase partly turned to be granular.M₂₃C₆phase has an important effect on mechanical properties of S31042 steel.The chain-like M₂₃C₆phase can enhance the strength of31042 steel,while the plastic descend sharply.Large amount of deformation twins were formed after cold deformation,cold deformation promoted the precipitation of Z phase,and led to the fragmentation of rod-like MX phase.The sample with 80%reduction exhibits excellent strength and ductility at 700°C,which is related to the work hardening effect of cold deformation and the precipitation strengthening of M₂₃C₆phase.During the aging at 700°C,M₂₃C₆phase precipitated along grain boundaries and twin boundaries.The amount of M₂₃C₆phase increases rapidly with the increase of the cold rolling deformation degree.The?phase nucleated on the pre-precipitated M₂₃C₆phase and then grew with the consumption of M₂₃C₆phase.The amount of?phase is increased with the increase of deformation strain.The supersaturation degree of alloying elements in solution-treated S31042 steel decreased after aging at 1050°C,and the driving force for the formation of M₂₃C₆phase is weak,leading to a discontinuous distribution of rod-like M₂₃C₆phase along austenite grain boundaries upon creep tests.At high stress levels,the discontinuous distribution of the M₂₃C₆phase along grain boundaries can increase the resistance to boundary sliding,without changing the ductility.Thus the creep rupture life of steel is improved.Nevertheless,under low stress levels,strengthening effects of M₂₃C₆phase discontinuously distributed along boundaries in aged samples were not strong as those in solution-treated steels.S31042 steel joint free of cracks and pores can be obtained by using linear friction welding.The process of linear friction welding is feasible,and the welding parameters of 25 Hz/2 mm/80 MPa/120 MPa are appropriate.The joint between dissimilar materials(S31042/GH4169)was studied by linear friction welding,and good metallurgic bonding was obtained under the optimized welding process parameters of 25 Hz/2 mm/100 MPa/150 MPa.Dynamic recrystallization occurred and the secondary phase particles precipitated within the weld zone.The microhardness and tensile property of the welded joint were higher than those of the base metal,which was attributed to the fine grain strengthening and the dispersion strengthening.