| Austenitic stainless steel has a stable austenitic microstructure,including the widely used 304 stainless steel(18Cr-8Ni)and the highly developed nickel-chromium stainless steel by increasing the alloying elements such as Cr,Ni,Mo,Cu,Si,Nb,and Ti.Austenitic stainless steel exhibits non-magnetic,high toughness,excellent corrosion resistance but low strength.Austenite,as a metastable phase,may undergo martensitic transformation under the conditions of plastic deformation and heat treatment,which provides favorable conditions for grain refinement and improving the strength of austenitic stainless steel.For metastable austenitic stainless steel,refinement can be achieved through a combination of cold deformation and heat treatment.This method can apparently improve the strength of the steel without losing the plasticity of the steel.Fe-Cr-Ni-Mo-Co-Al series new high-performance austenitic stainless steel(11Cr-8Ni-5Mo-8Co-Al)was investigated in this project by conducting the research on phase transformation and mechanical property.Furthermore,strengthening mechanism was explored through the combination of cold rolling deformation and tempering heat treatment,and it was compared with that of duplex stainless steel(15Cr-2Al-2Ni-10Mn).The main research contents and conclusions are as follows:(1)Analyze the phase transformation mechanism and determine the appropriate heat treatment system.The qualitative analysis and quantitative calculation of the mircostructure of the new type A steel were performed by X-ray diffraction(XRD),transmission electron microscope(TEM)and optical microscopy(OM).The results show that it mainly contains austenite phase(γ)and a small amount of ferrite phase(α);After the heat treatment,the relative contents of the γ phase and the α phase in the new type A steel(was)changed,and the maximum relative content of the γ phase after heat treatment at 600℃-1h was up to 89.40%.It is generally believed that the γ-phase content in austenitic stainless steel should be not less than 70%. Therefore,the suitable heat treatment regime of the new A steel is in the range of 400℃ to650℃.(2)Test the mechanical properties and corrosion resistance.The mechanical properties of the new A steel were tested by tensile and microhardness tests.The results showed that the new A steel exhibits ductile fracture and the TRIP effect during the tensile process.Among them,the tensile strength(Rm)obtained by heat treatment at 600℃-1h is the largest,which is 798.41 and about 16.29% higher than that of non-heat-treated steel(Norm).At the same time,the micro Vickers hardness of both the γ phase and the α phase was improved.The γ phase was202.65 hv,which was about 8.79% higher than that of Norm;the α phase was 235.45 hv,which was about 9.86% higher than that of Norm.The immersion corrosion and electrochemical corrosion tests on the new A steel show that the new A steel exhibits excellent corrosion resistance with a very low corrosion rate.After heat treatment,the corrosion resistance is slightly improved.(3)Research and analysis of strengthening mechanism.The new A steel was strengthened through cold rolling deformation and tempering heat treatment.The results show that for the new A steel,cold rolling causes partly martensite transformation,and martensite phase was transformed back to γ phase under tempering heat treatment.It achieves the purpose of refining the grains and improving the strength.The Rm of the new type A steel after cold rolling at 50% and heat treatment at 600℃-0.5h can reach 1492.29 MPa,which is about117.50% higher than that of Norm.For duplex stainless steel,the cold rolling deformation refines the grains without the TRIP effect.The strength of new A steel is higher than that of duplex stainless steel,and the strengthening effect is obviously different.The former is phase-transition fine-grain strengthening,while the latter is cold-deformation fine-grain strengthening. |
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