Strengthening-toughening Mechanisms Of Deposited Metals Producing By Low Transformation Temperature Metal Powder Cored Wires

Low transformation temperature(LTT)welding material has remarkable effect on improving fatigue strength and reducing tensile residual stress of high strength steel welded joints.However,the poor toughness of the LTT deposited metal limits its application.In order to obtain the better combination of strength and toughness,in present study,the strengthening and toughening mechanisms of LTT deposited metal are investigated The effective methods to improve the strength and toughness of LTT deposited metal are explores,meanwhile,the LTT welding wires can still reduce tensile residual stress and improve fatigue strength of welded joint.High density of tangled dislocation and a small amount of twin martensite are the main reasons for embrittlement of traditional martensite LTT deposited metals.In this paper,the martensite-retained austenite dual-phase microstructure of LTT deposited metals are designed,and the content of retained austenite is optimized.The toughening mechanisms of dual-phase LTT deposited metal are attributed to the deformation compatibility and TRIP effect of retained austenite,the retained austenite on crack steering and crack arrest,as well as the high angle martensite/retained austenite phase boundary on hindering crack propagationThe welding&partitioning process(200/300°C)is developed based on the microstructure characteristics of dual-phase LTT deposited metal.The carbon atoms will diffuse from martensite to retained austenite during partitioning,then the stability of carbon-enriched retained austenite increases.With the increase of the stability of retained austenite,the stress-induced martensitic transformation of retained austenite at low stress is inhibited,the deformation coordination effect of retained austenite is enhanced,the sustained TRIP effect and high strain hardening rate are obtained.Therefore,the yield strength,plasticity and toughness of dual-phase LTT deposited metal are significantly improved.In addition,the fatigue test results indicate that the residual compressive stress and fatigue strength of LTT welded joint are not been weaken after welding&partitioning process(200°C×1h).In order to meet the application requirements of ultra-high strength steel,the reversed transformation tempering process for martensite LTT deposited metal is developed and optimized.The mixed microstructure of tempered martensite,reversed austenite and fine carbide is obtained.The reversed austenite distributes in the boundaries of martensite laths and blocks.Then,the hierarchical structure with soft reversed austenite and strong martensite forms and it is beneficial to improve toughness.The softening caused by tempered martensite and the strengthening of fine carbides make the martensite LTT deposited metal maintain high strength.In addition,the residual compressive stress and fatigue strength of LTT welded joints are weakened after reversed transformation tempering with short-time,but the fatigue strength of LTT welded joints is still significantly increased(increased by 139%)compared with that of conventional welded joints.Due to the influence of the dilution of base metal,the M_s temperature of LTT weld metal increase and the transformation expansion decrease,which weaken the effect of LTT welding wire on improving the fatigue life of welded joint.A method to predict the reasonable range of M_s temperatures of LTT welding material is proposed,and the prediction formulas is related to the chemical composition of base metal and dilution rate.The fatigue test results indicate that the fatigue strength of the welded joints using dual-phase LTT wire with M_s temperatures of 97°C is higher,which is consistent well with the predicted result.