Influence Of Al/Ni On Microstructure And Properties Of Cr-Ni-Mo-V-Al Maraging Steel

The influence of Al and Ni elements on the microstructure and mechanical properties of Cr-Ni-Mo-V-Al maraging steel were investigated.The effects of Al and Ni contents and heat treatment parameters on the grain size,microstructure,residual austenite,precipitated phase,tensile properties,impact toughness and hardness of the test steel were systematically studied,which provided a theoretical basis for the optimization of alloy composition and heat treatment parameters.The influence of Al content on microstructure and mechanical properties of Fe-6Ni-5Cr-Al-Mo-V maraging steel was studied.The results show that of the addition of 2%Al in the steel results the increases of the tensile strength and yield strength by 700 MPa and559 MPa at room temperature,respectively.However,the further increase of Al content has no obvious effect on the strength.With the increase of Al content,the toughness of the material decreases and the hardness increases.Moreover,the addition of Al significantly improves the mechanical properties of the steel at high temperature.After addition of 2%Al,the tensile strength and yield strength of the test steel are increased by 53.6%and 40.7%,respectively,after solution at 1020?for 30 min and aging at 550?for 3 h.The further increasing of Al content show no obvious effect on the high temperature strength.The influence of Ni content on the microstructure and mechanical properties of Fe-Ni-5Cr-2Al-Mo-Vmaraging steel was studied.The variation of Ni content has little effect on the strength of the test steel.With the increase of Ni content,the tensile strength remains about 2200 MPa and the yield strength is about 1800 MPa;while the impact toughness of the steel improves.The addition of Ni affects the M_Spoint and A_C1point of the steel.The M_Spoint decreases from 350?to 300?with the increase of Ni content to 7%.With the increase of Ni content,the grain size increases slightly,which is attributed to the decrease of the onset austenitizing temperature due to the addition of Ni.When the Ni content increases from 4%to 7%,the A_C1temperature decreases from 752?to 701?.The increasing of Al and Ni results in the decrease of the?phase in the test steel.Al increases the area of?phase,and?-ferrite is stable at high temperature.The precipitation of?-ferrite in the solidification structure reduces the?-ferrite content.Since the?-ferrite decompositions into?phase,so the?phase also decreases.The increase of Ni leads to the decreases of the ferrite phase area and ferrite content,and therefore,the amount of?phase decreases.Finally,the influence of the different heat treatment temperatures on the microstructure and mechanical properties of Fe-6Ni-5Cr-2Al-Mo-V maraging steels were studied.The results show that the increase of solution treatment temperature has little effect on the strength,impact toughness and hardness of the test steel.However,the martensitic laths and grains become coarse,and the amount of?phase increases,which shows a negative effect on the toughness of the steel.Therefore,the solution treatment temperature should not exceed1020?.The hardness of the test steel increases first and then decreases with the increase of aging temperature,and it reaches a maximum value of 57.6 HRC at 550?,indicating that the test steel has good tempering stability.The reason why the test steel with the addition of Al and Ni show good tempering stability and high temperature mechanical properties is that the steel is considerably strengthened by Ni Al,Ni₃Al and M₆C carbides precipitated during aging.Ni Al phase is abundant and it precipitates in the steel with the addition of Al and Ni.The amount of Ni₃Al phase is relatively small and it is only found in test steels with the Ni/Al ratio of no lower than 3:1.