Study On Microstructure And Mechanical Properties Of 27.5Cr-3.5Mo-2Ni Super Ferritic Stainless Steel After Hot Rolled

Super ferritic stainless steel has good mechanical properties and excellent corrosion resistance.It is widely used in the fields of condenser,chemical processing and construction in power plant.However,the brittleness of this kind of stainless steel has become the technical obstacle that hinders its large-scale production and wide application.In this paper,27.5Cr-3.5Mo-2Ni super ferritic stainless-steel is taken as the research object.Scanning electron microscopy(SEM)and transmission electron microscopy(TEM)were used to observe the microstructure.The micro component analysis was carried out by the energy dispersive spectrometer(EDS),and the mechanical properties were tested by electronic universal testing machine and microhardness tester.The effects of hot rolling at 1100? on the microstructure and mechanical properties of 27.5Cr-3.5Mo-2Ni were investigated.The effects of hot rolling at 1100? on the aging behavior of 27.5Cr-3.5Mo-2Ni at different aging time were studied.After hot rolled at 1100?,a new heat treatment with different parameters was carried out.The effects of heat treatment with different parameters on microstructure,aging precipitation behavior and mechanical properties of 27.5Cr-3.5Mo-2Ni after hot rolled at 1100? were studied.The results of the study show that:27.5Cr-3.5Mo-2Ni mainly occurs dynamic recovery during hot rolling at 1100?,and a large number of dislocations and vacancies produced by rolling exist in the matrix resulting in the work hardening of 27.5Cr-3.5Mo-2Ni.Nb and Mo atom segregate at dislocations,promote the nucleation and precipitation of sigma phase and Laves phase which are rich in Nb and Mo elements and result in the second-phase strengthening of 27.5Cr-3.5Mo-2Ni.The hot rolling at 1100? has no obvious effect on the precipitation behavior of Ti N and Nb C.Due to the work hardening and second-phase strengthening,the tensile strength of 27.5Cr-3.5Mo-2Ni increased from 610 MPa to 730 MPa,the hardness increased from 218.76 HV to 287.15 HV,and the elongation decreased from 22.4% to 14.4%.During aging at 800?,the hot rolling at 1100? obviously promotes the precipitation of sigma phase in 27.5Cr-3.5Mo-2Ni.The microstructures of 27.5Cr-3.5Mo-2Ni after heat treated at 980? for 5min,15 min and 30 min are multilateral structure.The work hardening and second phase strengthening have not been eliminated,because a large number of dislocations still exist in matrix after heat treated at 980?.After heat treated at 1150? for 20 s,hot-rolled strips are observed in matrix and no recrystallization occurs.A large number of dislocations still exist in matrix and no precipitation of second phase was found.After heat treated at 1150? for 40 s,the hot-rolled strips transform into equiaxed grains and recrystallization occurs,the dislocation density also decrease sharply.After heat treated at 1150? for 80 s,the equiaxed grains obviously coarsening.The heat treatment at 1150? eliminates the work hardening and second phase strengthening of 27.5Cr-3.5Mo-2Ni after hot rolled at 1100?.During aging at 800?,the heat treatment at 1150? obviously decreases the precipitation of sigma phase in hot-rolled 27.5Cr-3.5Mo-2Ni.After heat treated at 1150? for 20 s,the volume fraction of the sigma phase decreases sharply comparing with the hot-rolled 27.5Cr-3.5Mo-2Ni during aging at 800?.After heat treated at 1150? for 40 s and 80 s,the volume fraction of sigma phase decreases by more than one order of magnitude.Because the volume fraction of the sigma phase is still very large after heat treated at 1150? for 20 s,the heat preservation should be above 20 s.