Microstructure Of Weld Metal And Solidification Characteristics Of Molten Pool In High Manganese Austenitic Steel

With the rapid development of clean energy liquefied natural gas(LNG),the high production and application costs of traditional cryogenic steel materials,such as 9%Ni steel,will continue.Therefore,the development and promotion of new high manganese austenitic steel for cryogenic has a very high application value.This paper adopts the submerged arc welding process to prepare welded joints of high manganese austenitic steel,and analyzes the solidification behavior and characteristics of the weld pool of high manganese austenitic steel by studying and analyzing its microstructure and elemental segregation behavior,combined with the classical solidification theory.Microstructure analysis methods such as optical microscope and scanning electron microscope were used to analyze the microstructure of high manganese austenitic steel base metal,welded joint fusion zone and weld metal.The structure of base metal is composed of equiaxed austenite grains,and there are hot-rolled segregation bands widely distributed along the rolling direction.High manganese austenitic steel joints fusion zone can be divided into inhomogeneous mixed zone and partially melted zone(PMZ).The hot-rolled strip in base metal is melted by the effects of welding heat,forming the partially melted zone.The heterogeneous mixture zone is formed by the mechanical mixing of the liquid metal of the base metal and the liquid filler metal at the bottom of the molten pool,which is the area where the chemical composition transitions from the base metal to the weld zone.By using the Electron Probe Micro-analyzer to analyze the alloying elements distribution in the fusion zone of the joint and the weld metal,and using the thermodynamic calculation software Thermo-calc to calculate the binary and ternary equilibrium phase diagram of high manganese austenitic steel and its solidification path,combined with the solidification theory,the elemental segregation behavior of high manganese austenitic steel and its melt pool solidification characteristics are proposed.As the alloying elements are more easily distributed in the liquid phase,after the solidification of the melt pool,the alloying elements segregate into the intercellular and inter-dendritic crystals,with the highest concentration of solute elements occurring in the inter-cellular region of the weld metal.The decrease of the equilibrium solid-liquid phase line temperature due to the elemental segregation on the hot rolled strip in the base metal is the main reason for the formation of the partially melted zone,which leads to the formation of a solid-phase peninsula protruding from the melt pool in the fusion zone,and because of the solute segregation at the concave valley of the solid-liquid interface.Since the tip of the solid phase peninsula is more conducive to solute expulsion,the new phase is more conducive to the growth of associative nucleation at the tip of the peninsula and grows into the melt pool in the form of cellular crystallization,and the initial cellular crystallization width has a certain correlation with the segregation strip spacing,and the initial cellular crystallization transforms into cellular dendrite growth during the solidification process.