Effect Of Restraint Stress On Martensite Transformation Behavior And Microstructure Of Low Phase Transformation Weld Metal

With the rapid development of world economy and the continuous improvement of manufacturing level,low alloy high strength steel is widely used in various fields of industrial manufacturing.In the welding structure of low alloy high strength steel,the problem of fatigue failure of welding joint is more and more prominent.With the appearance of low phase transformation(LTT)welding materials,the fatigue resistance of low alloy high-strength steel welding structure has been significantly improved.But so far,LTT welding materials have not been widely used and promoted,mainly because the comprehensive mechanical properties of the welded joints welded with LTT welding materials are not good enough.It is the basis of studying LTT welding materials to clarify the effect of restraint stress on the martensite transformation behavior and microstructure of LTT weld metal.It can make us better understand the transformation law of LTT weld metal in the actual welding process,and provide theoretical guidance and basis for the design and application of LTT welding materials.In this paper,the thermal simulation method is used to quantitatively simulate the transformation process of LTT weld metal under different restraint stress conditions by applying displacement control and stress control to the specimen.The results show that the restraint stress can provide additional driving force for the martensite transformation of LTT weld metal,increase the starting temperature of martensite transformation(M_s),and increase the M_stemperature of LTT weld metal by about85K compared with the unrestrained state.When the axial restraint tensile stress of50MPa,100MPa and 150MPa is applied to the weld metal,the M_stemperature of the weld metal increases 23K,43K and 65K respectively.In addition,due to the orientation of confinement stress,the transformation rate of martensite in the whole transformation process decreases and the transformation range expands.Fe-X-C ternary solid solution elastic energy model can effectively predict the effect of restraint stress and external stress on M_stemperature of LTT weld metal,which can be used for quantitative calculation.The effect of restraint stress on martensite transformation of LTT weld metal can be divided into three stages:stress-induced nucleation stage with martensite transformation fraction range(f)of 0?0.2,transformation stage(f:0.2?0.63)and without effect stage(f:0.63?1.0).Due to the effect of restraint stress,the martensite nucleation site and transformation temperature of LTT weld metal increase,resulting in the decrease of martensite size and dislocation density with the increase of restraint stress.However,with the increase of restraint stress,the hardness and martensite orientation of LTT weld metal have no obvious change.