Study On Solidification Microsegregation Of Steels

The solidification segregation decreases the alloy properties especially toughness and corrosion resistance,and it degrades the product performance as well.With the increase of alloying degree,segregation becomes more serious,which generally deteriorates the mechanical properties of materials.Therefore,studying the solute repartition and segregation behavior of alloy elements during solidification has guiding significance to produce homogenized alloys.In this study,310S austenitic stainless steel,316 austenitic stainless steel and 10Cr17 ferritic stainless steel were chosen to investigate the solidification characteristics.Isothermal solidification quenching(ISQ)and solidification microstructure observation by optical microscope(OM)were used,combining with Thermo-Calc thermodynamic calculation,quantitative metallography,scanning electron microscope(SEM),energy dispersive spectrometer(EDS)analyses,the solidification process and the segregation behavior of alloy elements of three different solidification modes of stainless steels were studied.The main research work and results are as follows:(1)Study on solidification segregation behavior of 310S austenitic stainless steelThe phase composition of 310S austenitic stainless steel was investigated with Thermo-Calc thermodynamic software.The hquidus and solidus temperatures of 310S austenitic stainless steel are 1404 C and 1322 C,respectively.The solidification temperature range is 82 C.A series of samples were prepared by ISQ,and the distribution of alloy elements in 310S austenitic stainless steel during solidification has been analyzed by energy spectrum.The results show that the segregation coefficients of Ni and Mn elements in 310S austenitic stainless steel are larger than 1,tending to be concentrated in the solid dendrite arms,which are negative segregation elements.The segregation coefficient of Nb element is less than 1,tending to be enriched in interdendritic regions and being a positive segregation element.The segregation coefficient of Nb element increases obviously at 1340 C,which is related to the decrease of the diffusion rate of Nb in solid phase and the precipitation of NbC in the y phase in the later stage of solidification.(2)Study on solidification segregation behavior of 3 16 austenitic stainless steelThe liquidus and solidus temperatures of 316 austenitic stainless steel are 1430 C and 1397 ?,respectively.The solidification temperature range is 33 ?.ISQ and solidification microstructures observation show that the segregation coefficient of Mn element in 316 austenitic stainless steel is less than 1,tending to be concentrated in the interdendritic liquid phase,which is a positive segregation element.The segregation coefficients of Si and Mo elements are larger than 1 at 1410 C and higher,tending to be enriched in the solid dendrite arm,which are negative segregation elements.At 1410 C and lower,due to the occurrence of L+??? peritectic reaction,the segregation coefficients of Si and Mo elements are smaller than 1,turning to be enriched in the interdendritic liquid,and becoming positive segregation elements.(3)Study on solidification segregation behavior of 10Cr17 ferritic stainless steelThe results show that the liquidus and solidus temperatures of 10Crl7 ferritic stainless steel are 1508 C and 1475 C,respectively.The solidification temperature range is 33 C.ISQ and solidification microstructures observation show that the segregation coefficient of Si element in 10Crl7 ferritic stainless steel changes from smaller than 1 to larger than 1,tending to be concentrated in the solid dendrite arm,which is a negative segregation element.The segregation coefficient of Mn element is smaller than 1,and tending to be enriched in the interdendritic region and being a positive segregation element.