Microstructure And Properties Of Laser Additive Carbide-free Bainitic Steel

Carbide-free bainite steel is high strength material with good toughness,widely used in the manufacturing of parts that require superb mechanical properties in aerospace,shipping and other fields.It would be valuable if achieve additive manufacturing(LAM)of such components and remanufacturing of damaged parts.However,the additive metal parts is non-equilibrium solidified microstructure,with usual defects such as porosity,segregation,and crack of the microstructure,and it is difficult to maintain toughness while obtaining high strength.In this paper,aiming at the control problem of the mechanical properties of LAM carbide-free bainite steel.Firstly,four carbide-free bainite steel powders for LAM were optimized by thermodynamic calculation method from the aspects of processability and mechanical properties.Then,the microstructure and properties of the deposited materials were studied,and the effects of forming process and chemical composition were discussed.Finally,the influence of post-treatment such as heat treatment and hot rolling on the microstructure and properties of the material was systematically discussed,and the high strength and toughness carbide-free bainitic steel by LAM was realized.The work was as follows:Firstly,carbide-free bainite steel powders for laser additive manufacturing were optimized.Based on the analysis of the strength-toughness mechanism of carbide-free bainite steel and the formation mechanism of defects in LAM,the chemical composition of the powder was optimized by thermodynamic calculation based on the microstructure and properties of the material and the process of LAM.Finally,four carbide-free bainite steel powders for LAM were designed for medium-low carbon and high carbon.The LAM of defect-free and carbide-free bainite steel was realized,and the microstructure and properties of the deposited samples were characterized and analyzed.There were no defects such as pores,cracks and serious segregation in the deposited samples.For medium-low carbon alloys,the microstructure was granular pearlite + retained austenite,and the carbides were nanoscale.The diffusive transformation was slightly.The tensile strength,yield strength and elongation of the L2 component sample reached 1051 ± 65 MPa,662 ± 71 MPa and 13.03 ± 1.33 %,respectively,and the hardening ability was high.For the high carbon alloy,the microstructure was martensite + bainite + retained austenite.The microstructure was fine and uniform,and there was no obvious segregation.The tensile strength,yield strength and elongation of the H1 component sample steel were 1498 ± 41 MPa,836 ± 38 MPa and 13.86 ± 1.63 %,respectively.The comprehensive mechanical properties were excellent.The effect of typical alloying elements on microstructure and properties of LAM carbide-free bainite steel was investigated.Al and Ni accelerated the bainite transformation rate of high carbon alloy,increased the bainite content,refined the reduce the content of martensite and retained austenite,increased the strength,and reduced the plasticity and hardness.But the effects of Al and Ni on medium and low-carbon alloy was not significant.The increase of C content significantly reduced the Ms temperature,extended the bainite transformation time significantly,and refined the lath-like bainite.The retained austenite content decreased but was more stable,and the strength and hardness of the material increased,but it had a negative effect on plasticity.The influence of austempering and hot rolling + austempering on the microstructure and properties of LAM carbide-free bainite steel were revealed.The typical conclusions were as follows: Austempering could obtain uniform and fine bainite structure,and the strength of the material was significantly improved to 1300 MPa,but the retained austenite content was reduced,the hardening ability was weak,and the elongation was not significantly improved.Austempering after hot rolling could obtain bainite + retained austenite.The microstructure was fine and uniform,the retained austenite content increased,and the distribution was uniform.The comprehensive mechanical properties were significantly improved.The tensile strength,yield strength and elongation of L2 component sample steel were 1398 ± 57 MPa,1177 ± 44 MPa and 19.80 ± 0.79 %,respectively.