Mechanism of Mg treatment on pinning particles for shipbuilding steel
The capacity of shipbuilding steel for large heat input welding could be significantly improved by Mg treat-ment.On the one hand,the addition of Mg in steel could refine the inclusions and improve the probability of IAF inducing.On the other hand,Mg could enhance the high-temperature stability of pinning particles,and effectively restrain the coarsening of original austenite grains in HAZ.The high-temperature stability of second phase particles in steel is the key to the effect of Mg oxidation metallurgical,but further in-depth research is still insufficient,espe-cially the influence mechanism of Mg on pinning particles.Therefore,this study starts from the welding simulation experiments of different welding heat input,systematically analyzes the influence of heat input on the constitution of pinning particles in steel,and analyzes and discusses based on the thermodynamics of particle precipitation,thermody-namics of MgO generation,crystallography relationship between particles,and diffusion behavior of elements in steel.The purpose is to ascertain the influence mechanism of Mg on pinning particles in steel.The results show that,for the industrial shipbuilding steel with Mg treatment,when the welding heat input exceeds 200 kJ/cm,some pin-ning particles will lose their effectiveness.With the increase of welding heat input,the size of pinning particles in HAZ first decreases and then increases,and the more large-sized TiN particles with higher welding heat input.At this time,most pinning particles are composite particles,with TiN-MgO-Al2O3 as the matrix,coated with NbC par-ticles on the surface.The results of particle precipitation thermodynamics analysis,MgO generation thermodynamics analysis,particle precipitation mismatch relationship and diffusion coefficient analysis of elements in steel show that,due to the close temperature range of MgO generation and TiN precipitation,and their co-precipitation behavior,MgO-TiN composite precipitates will generate with long-term high temperature,and greatly enhance the high-temperature stability of second phase particles in steel.The results of welding simulation experiment and theoretical analysis show that,with Mg treatment,composite particles with high temperature stability such as MgO-TiN-Al2O3 may be generated in steel during continuous casting and large heat input welding processes.However,for the Mg treated steel,the amount of MgO in steel is far lower than the precipitation amount of TiN,which would affect the pinning effect of second phase particles in steel.Therefore,improving the Mg content in steel is the key to improving the capacity of Mg treated shipbuilding steel for large heat input welding.
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