| As one of the revolutionary cutting-edge technologies in the field of advanced manufacturing,laser melting deposition(LMD)has been widely concerned and rapidly developed at home and abroad.This technology includes that the raw materials(metal powder or wire usually involved)fed synchronously are melted in real time by high-power focused laser beam.Then,the molten pool will be formed on the substrate and cool down rapidly with metallurgical bonding generated.The technology uses the laser as an energy source.It has high heat input efficiency,realizing rapid and accurate one-time near net forming of the complex large-size structural component.At present,a lot of scientific research has been carried out in the manufacturing fields of LMD forming titanium alloy,nickel-based superalloy,aluminum alloy and stainless steel globally,and many breakthroughs have been realized in the industrial application.However,few researches on LMD preparing alloy steel can be accessible.During the LMD process,the effective control of the microstructure of alloy steel is challenged.The main reason is that the LMD forming process is a rapid non-equilibrium solidification process,where phase transformation of alloy steel itself is complex.and There is no guidance of non-equilibrium solidification theory to control the deposited microstructure and its evolution effectively.These fact results in poor comprehensive mechanical properties and limit the popularization and application of LMD alloy steel components.Because of this research status,post heat treatment is necessary to improve the structural stability and comprehensive mechanical properties of LMD alloy steel components.In this paper,the LMD preparing 12CrNi2 alloy steel is mainly studied.The microstructure of as-deposited 12CrNi2 alloy steel is consisted of ferrite-austenite dual phases,which show good plasticity and toughness,but poor strength.This type of steel is widely used in the camshaft of nuclear power diesel engines.The service environment is harsh,and the components are easily affected by wear and alternating load.It requires high surface strength and core toughness.Therefore,it is necessary to ensure high toughness(? 11%)based on improving the tensile strength(? 1000 MPa).This paper mainly studies the post heat treatment process optimization LMD morphology 12CrNi2 alloy steel and reveals the internal relationship among post-heat treatment process,microstructure and mechanical properties.The main research contents and conclusions are summarized as follows:(1)Optimization of plastic deformation ability:Control of martensite-ferrite dual-phase structure and optimization mechanism of plastic deformationThe ferrite-austenite dual-phase structure of LMD alloy steel was transformed into three types of martensite-ferrite one by direct quenching,critical quenching and step quenching,in which the volume fraction of martensite phase is the same(62%)and the prior austenite grain size is different.The result shows that the critical quenching can refine the grain size of prior austenite to less than 4 ?m,which causes the internal lath substructure to change from high-density dislocations to {112}<111>type nanotwins.Thus,significantly improving the uniform deformation ability of the dual-phase structure and uniform elongation was obtained.(2)Optimization of strength-toughness balance:Control of martensite-ferrite dual phase structure and optimization mechanism of strength-toughness balanceTo further improve the comprehensive mechanical properties of the two-phase structure,i.e.strength-ductility balance,two types of martensite-ferrite dual-phase microstructure were obtained in the as-deposited structure through direct quenching and multi-step quenching,in which the volume fraction of martensite is the same,and the morphology is different.The result shows that the strength-toughness balance of 12CrNi2 alloy steel is significantly improved by multi-step quenching that caused the martensite in the dual-phase structure to transform into fine fibrous one.The reasons are summarized as follows:1)Hard martensite improves the strength of the alloy steel;2)The alloy steel treated by multi-step quenching has higher elongation.On the one hand,the composite treatment eliminates the preferred orientation of ferrite grains which are not conducive to plastic deformation in the deposited alloy steel;on the other hand,the alloy steel has a higher density of {112}<111>nanotwins interface in the dual-phase structure after the treatment,which can promote the continuous plastic deformation of the dual-phase structure.(3)Simultaneous optimization of strength and toughness:Regulation of lath substructure of single-phase martensite and its mechanism of simultaneous strengthening and tougheningThe deposited microstructure of LMD was transformed into single phase martensite by direct quenching and cyclic quenching.The results show that the prior austenite grains of single-phase martensite are refined to less than 4 ?m by cyclic quenching,which leads to the transformation of lath substructure from high-density dislocation to {112}<111>type nanotwins in martensite,which realizes the simultaneous optimization of strengthening and toughening of alloy steel.In addition,by considering the orientation distribution characteristics of lath substructures in coarse and fine grain martensite,the formation and evolution models of different substructures in lath martensite are established from an atomic scale.It opens up a new way to obtain numerous {112}<111>type nanotwins in low-carbon lath martensite and provides a new idea for the research of martensitic transformation mechanism.(4)Evolution mechanism of the substructure of single-phase martensite after tempering and its mechanical propertiesThe substructure of fine martensite was controlled by tempering at 200,300,400 and 500 ?.The results show that the nanotwins structure within the lath degenerates after tempering treatment;After tempering at 200? and 300? the Fe(C)precipitates grow in short rod shape along the three planes of ?-Fe {112} family.Compared with tempered martensite,the quenched fine martensite has better strong toughness matching due to the existence of high density {112}<111>type nanotwins.In addition,the transformation mechanism model of nanotwins substructure in the fine-grained lath martensite during tempering at different temperatures is proposed,which is in line with the new ideas of martensite transformation mechanism in the last part. |
PDF Full Text Request