Investigation On Strength-Toughness Mechanism Of Low Nickel, Cobalt Free Maraging Steel

Based on the calculated results of Thermo-Calc software,physicochemical phase analysis,SEM morphologies of steel, the solution treatment of 00Ni14Cr3Mo3Ti maraging steel has been optimized. Effecting solution temperature on austenite grain size of 00Ni14Cr3Mo3Ti,00Ni13Cr9MoTi maraging steel were studied.The solutionized and aging microstructure of steel were observed by optical microscope(OM) and scanning electronic microscope(SEM). Effecting solution temperature on the aging impact toughness of 00Ni14Cr3Mo3Ti,00Ni13Cr9MoTi maraging steel were studied. Impact fracture morphology of steel at different heat treatment were observed by scanning electronic microscope(SEM). The tensile properties of aging steel was researched.The test results show that: the equilibrium precipitated phases of Co-free 00Ni14Cr3Mo3Ti maraging steel are calculated by Thermo-Calc software. The undissolved Laves-Fe2(Mo,Ti)phase is discoveried at lower solution temperature, the Laves phase has been dissolved at 805℃. The undissolved phase at 750℃,800℃solution is Laves by phase analysis, it isn't detected at 850℃.But a little Laves phase in matrix is detected at 850℃solution by SEM microstructure, the Laves phase has been dissolved at 900℃.The best range of solution temperature of 00Ni14Cr3Mo3Ti maraging steel is 850-900℃, so the undissolved phase is dissolved and grain isn't coarsening.00Ni14Cr3Mo3Ti maraging steel were treated during 750-1050℃. The austenite grain size is finer below 900℃, it is almost 22μm,but the grain growth is faster over 900℃.The morphology of martensite lath isn't changed with increasing solution temperature.The undissolved Laves phase dissolves with increasing solution temperature, so the matrix structure is homogeneous, impact toughness gradually increases and hardness reduces in the solutionized condition. After aging at 510℃, the strength isn't almost change, impact toughness and plasticity gradually increases during 750-900℃, the maximal impact absorption power is 61J at 900℃solution temperature. Because the austenite grain size is growing, the strength,impact toughness and plasticity drop above 900℃.Compared with 00Ni14Cr3Mo3Ti maraging steel, the Cr content of 00Ni13Cr9MoTi maraging steel increases from 3.19% to 8.92% so as to reduce Ms point to 60℃.So water quenching is needed before cold treatment 2h at -73℃.The aim is that austenite transforms martensite as much as possible duringγ→αtransformation and the steel has higher strength.The austenite grain size of 00Ni13Cr9MoTi maraging steel grows with increasing solution temperature. Grain growth is faster over 900℃. Ti2S,AlN,TiN phases precipitate in grain boundary at 930℃solution treatment. The matrix structure is lath martensite and a little retained austensite. 00Ni13Cr9MoTi maraging steel also has undissolved Laves-Fe2(Mo,Ti) phase below 850℃solution temperature by means of thermodynamic calculation and analyzing SEM structure. Because the Mo content decreases from 3.06% to 1.46%, the Laves is very fine.Because the austenite grain doesn't begin to grow, the undissolved Laves is very fine and there is retained austensite below 850℃solution temperature, the impact toughness of aging steel is very high. Because of grain growth and decreasing retained austentite over 850℃, the impact toughness gradual drops. Precipitated phases in grain boundary make impact toughness sharply drops at 930℃.The yield strength and tensile strength are invariable below 900℃solution temperature, but they have a little drop over 900℃with grain growing. The plasticity reaches maximum value at 850℃.00Ni14Cr3Mo3Ti maraging steel is treated at 900℃before aging at 510℃, Rm=1750MPa,Rp0.2=1670 MPa, A=10.5%, Z=55%, AKU2=61J, 00Ni13Cr9MoTi maraging steel is treated at 850℃before cold treatment2h at -73℃and aging at 510℃, Rm=1700MPa, Rp0.2=1625 MPa,A=11.5%,Z=64%,AKU2=70J.the strength,toughness and plasticity of steel are the best combination.