Effect Of Ni,Mo And Heat Treatments On Microstructure And Properties Of Low Alloy Cast Steel

Automobile industry is the symbol of modern industrial civilization,which plays an important role in production and life.Heavy truck is widely used because of its large transportation volume,convenient loading and unloading,economy and practicality.The traction base is the key component in the heavy truck that connects the tractor and semi-trailer,which directly affects the safety performance of the vehicle system.Therefore,the steel used for the traction base is required to have high strength,high toughness and good welding performance.There are few researches on the cast steel for the traction seat in China.The research on the low alloy cast steel for the traction seat can improve the technical level of the key auto parts and promote the development of heavy truck manufacturing technology in China.In this paper,the chemical composition of low alloy steel was designed as following: C 0.22% ? 0.28%,Si 0.4% ? 0.8%,P ? 0.03%,S ? 0.03%,Mn 0.5% ? 1.1%,Cr 0.5% ? 0.7%,Ni 1.0% ? 2.0%,Mo 0.3% ? 0.7%,Nb 0.04% ? 0.05%.By optimizing alloy composition and designing three heat treatment processes,the effects of Ni?Mo and heat treatments on the structure and properties of low alloy cast steel were studied.The results of the research showed that:The microstructure of as-cast low-alloy steel with different Ni content and different Mo content is composed of pearlite and granular ferrite.With the increase of Ni content and Mo content,the microstructure of low alloy steel becomes more refined and uniform.The microstructure of low alloy cast steel with different Ni content after normalizing and annealing is sorbite and ferrite.The microstructure is tempered sorbite after quenching and annealing.After normalizing and tempering,with the increase of Ni content,the microstructure becomes finer and more uniform,the hardness and tensile strength increase gradually,and the elongation and impact energy decrease gradually.When the Ni content is 1.0%,the hardness,tensile strength,elongation and impact energy is 191 HB,627.5 MPa,21.5% and 51.7 J,respectively.After quenching and tempering,with the increase of Ni content,strip-shaped ferrite appears in the matrix structure,the hardness and tensile strength gradually increase,the elongation decreases rapidly and then slowly,and the impact energy gradually decreases.When the Ni content is 1.0%,the hardness,tensile strength,elongation and impact energy is 217 HB,720 MPa,20.5% and 80.04 J,respectively.The microstructure of low alloy cast steel with different Mo content after normalizing and tempering is sorbite.The microstructure is tempered sorbite after quenching and tempering.Under the two heat treatment conditions,with the increase of Mo content,the microstructure is more uniform and finer,the hardness and strength increase,and the elongation and impact toughness decrease.When the Mo content is 0.5%,the comprehensive properties of low alloy cast steel are better.After normalizing and tempering,the hardness,tensile strength,elongation and impact energy is 252 HB,820 MPa,15.5% and 13.68 J,respectively.After quenching and tempering,the hardness,tensile strength,elongation and impact energy is 278 HB,890 MPa,13.5% and 18.62 J,respectively.The microstructures of low alloy cast steel treated by different Q&P processes are both lath martensite and block martensite without corrosion.With the increase of initial quenching temperature,the morphology of the martensite lath becomes clearer,the lath becomes longer,the tensile strength decreases,elongation and impact energy first increase and then decrease.With the increase of carbon distribution time,a small amount of massive M/A island structure and white granular carbide appear in the matrix.The strength decreases,elongation and impact energy first increase and then decrease.When the initial quenching temperature is 260 oC and the carbon distribution time is 10 min,the comprehensive properties of low alloy cast steel are better.The tensile strength,elongation and impact energy is 982 MPa,14.4% and 21.90 J,respectively.Based on the influence of Ni and Mo on the structure and properties of low alloy cast steel,it is suggested that the composition of low alloy cast steel for traction seat is C 0.26% ? 0.28%,Si 0.5% ? 0.7%,P ? 0.03%,S ? 0.03%,Mn 0.9% ? 1.1%,Cr 0.5% ? 0.7%,Ni 0.8% ? 1.2%,Mo 0.4% ? 0.6%,Nb 0.04% ? 0.05%.Different heat treatment processes should be selected according to different requirements of traction seat.