The Effect Of Alloying Elements On Microstructure Transformation In The Deposited Mental Of Low Transformation Temperature Weld Filler

In this study,nine low transformation temperature(LTT)flux cored wires were designed by orthogonal design.The nine kinds of LTT cored wires were welded with MAG overlay on the clean surface of Q345 steel plate.The martensite transformation(M_s)temperature of the LTT deposited metal was tested using a Gleeble 3500 thermal simulator.Based on the XRD pattern of LTT deposited metal,the content of retained austenite in deposited metal was calculated by multiphase full-spectrum fitting Rietveld method.As the Cr and Ni contents increasing,the transformation temperature of austenite to martensite decreased,and the residual austenite content increased.The microstructure of LTT deposited metal was analyzed by optical microscope(OM),electron back-scattered diffraction(EBSD),scanning electron microscope(SEM)and transmission electron microscopy(TEM).When the Ni content was relatively high,the primary phase of LTT deposited metal was a regular cellular structure,and its initial phase maybe austenite.A large amount of?-ferrite in skeleton shape was found in two LTT deposited metals with relatively high content of Cr,and the initial solidification phase maybe?-ferrite.The retained austenite distributed along the growth direction of the primary grains in the form of stripe/block or between the martensite single crystals in the film-like.The martensite morphology of the LTT deposited metal was lath-like or polygonal.The width of martensite single crystal was different and distributed in the range of 50nm~600nm.The orientation differences between martensite lathes or blocks were the large angle,which were mostly distributed in the range of 45°~65°.With the residual austenite increasing,the hardness of the deposited metal decreased.When the residual austenite content was the same,LTT deposited metal hardness increased with the Cr alloying element increasing.The coarsening of the martensite can reduce the hardness of the LTT deposited metal.The residual austenite was able to improve the low temperature impact toughness of the weld metal.The dense distribution of the skeleton shape residual?-ferrite would reduce the low temperature impact toughness of the LTT deposited metal.Cr and Ni can improve the impact toughness of LTT weld metal,and the effect of Ni element was more significant.In a certain range,Mo has little effect on the impact toughness.But more than a certain amount,Mo element would reduce the impact toughness.With the toughness increasing,the fracture morphology of the impacted specimen was changed from brittle cleavage morphology to dimples.