| 21CoCrNi steel is a new type of secondary hardening ultra-high strength steel.The mass fraction of Cr reaches 10%,so it has good corrosion resistance and has been sucessfully applied to variety of aviation devices.21CoCrNi steel is heat treated by double tempering process,which is very complex,and there are few reports on the optimization of its smelting process and heat treatment process.The effect of heat treatment process on the microstructure and toughness of the 21CoCrNi steel is also unclear.In this paper,the effect of different kinds of rare earth and Al additions on the cleanliness of 21CoCrNi steel was compared,the effects of different kinds of rare earth and Al addition on cleanliness were compared,and the effects of traditional double tempering process and low cost single tempering process on its microstructure and properties were studied.The purpose of the study was to optimize the preparation process and improve the strength and toughness of 21CoCrNi steel.In this study,seven different 21CoCrNi-RE/Al steels were smelted with vacuum induction furnace and vacuum arc remelting furnace.Rare earth La,rare earth Ce and Al were added in the smelting process.Metallographic microscopy and scanning electron microscopy were used to analyze the effects of different rare earth and Al on the types and distribution of inclusions in steel.The heat treatment experiments of 21CoCrNi steel at different solution temperature,tempering temperature and aging time were carried out.The change of austenite and secondary phase in 21CoCrNi steel during different heat treatment process were analyzed by metallographic microscope,scanning electron microscopy,transmission electron microscopy,X-ray diffractometer and deformation dilatometer.The room temperature tensile and impact properties of steels under different heat treatment processes were tested.The relationship between the microstructure of 21CoCrNi steel and its mechanical properties was studied.And the effects of rare earth or Al addition on the mechanical properties were also studied.According to the research above,some important conclusions are as follows:(1)The inclusions in 21CoCrNi steel that treated with rare earth are mainly rare earth oxides and rare earth oxysulfide.The average size of inclusions in steel with Ce content of 0.03%is the smallest,which is 1.458 μm.While the average size of inclusions in steel with La content of 0.2%is the largest,which is 3.959 μm and the inclusion content is also the largest.The inclusion distribution and mechanical properties of Ce treated steel are better than those of La treated steel.The tensile strength of Ce treated 21CoCrNi-Ce steel can reach 1914 MPa after double tempering,the total elongation of it can reach 14%,and the Charpy impact energy is 7.5 J.(2)The main secondary phases in the annealed 21CoCrNi-Ce steel after are M23C6,and the segregation of secondary phases is serious.Some of the secondary phases are larger than 1 μm.,while most of them are about 100 nm in size.The main components of the secondary phases are C,Cr,Mo and W.These secondary phases can be completely dissolved after solid solution treatment at 1085℃ for 1 hour.(3)There are three kinds of twins in 21CoCrNi-Ce steel before aging.During aging process,the twin interface is the preferred nucleation position of M2C carbide.The orientation relationship between M2C carbide and martensite matrix is as follows:[211]carbide//[135]α,(111)carbide//(121)α.There are blocky retained austenite produced by quenching and thin-film reversed austenite produced by tempering in the aged samples.The width of reversed austenite is about 5 nm,and the reversed austenite is uniformly distributed at the martensite lath interface.(4)The secondary quenching of retained austenite occurs when the tempering temperature above 440℃,and the volume fraction of austenite is the lowest when the temperature is between 500-540℃,which is about 3.2%.The α→γ phase transformation occurs when the steel is tempered above 580℃,and large amount of austenite containng Ni and Cr will formed.There are two kinds of carbides,M7C3 and M2C in 21CoCrNi-Ce steel when it is tempered at 482℃,and the sizes of them are both less than 10 nm.When the steel is tempered at 630℃,there are a large number of M2C and M23C6 carbides with sizes above 50 and 500 nm,respectively.(5)During single tempering heat treatment,the decomposition of retained austenite leads to phase transformation strengthening and the secondary phase leads to precipitation strengthening,which together result in the strength peak of steel at 482℃.The 21CoCrNi-Ce steel obtained good properties with tensile strength of 1945 MPa,yield strength of 1543 MPa,elongation of 11.9%and impact energy of 18 J when tempered at at 482℃ for 6 hours.Compared with the double tempering process,the single tempering process not only keeps the strength of steel,but also improves the impact toughness of steel at room temperature.(6)The effect of Al on the mechanical properties of 21CoCrNi-Ce steel is mainly due to its effect on the retained austenite.Al decreases the volume fraction of austenite after tempering at high temperature and increases the peak strength value of steel.The tensile strength of 21CoCrNi-Ce steel containing 1.66%Al can reach 2117 MPa after the steel tempered at 540℃.Al also increases the formation temperature of reverted austenite,which raises the tempering temperature corresponding to the strength peak of 21CoCrNi-Ce steel.The tempering temperature corresponding to the highest strength of steel with 3.2%Al is 580℃. |
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