Study On The Characterization Of Surface Ferrite Transformation In Carbon Steel Using Thermoelectrical Power In-Situ Measurement

The preparation of surface ferrite layer on the surface of carbon steel by fine control heat treatment has significantly improved the material properties,so it is important to further control the surface ferrite phase transition.Thermoelectric potential(TEP)is very sensitive to the evolution of the microstructure of materials,so this paper proposes an in-situ thermoelectric potential measurement method,which successfully obtains the in-situ thermoelectric potentialtemperature curve and the transition of thermoelectric potential near the phase transition temperature.It provides a new detection method for the quantitative analysis of phase transition information and control of phase transition in the heat treatment process of materials.In the process of online measurement at high temperature,the self-built thermoelectric potential measuring device was used to first carry out the online measurement of high temperature in-situ thermoelectric potential for carbon steel without phase change,and then the in-situ thermoelectric potential of carbon steel containing γ-α phase change at high temperature was measured,and the in-situ thermoelectric potential-temperature curve was obtained,the surface ferrite phase transition was studied,and the phase change temperature and process were analyzed,thereby optimizing the preparation process of the surface ferrite layer and realizing further control of the surface ferrite phase transition;Based on this,the mechanical properties of the materials before and after the optimization of the surface ferrite layer preparation process,and the corrosion properties at high temperature and room temperature were compared and analyzed,which provided a basis for the design and development of gradient functional materials with good combination of strength and bending properties and corrosion resistance.On the other hand,through a series of verification experiments,the reliability of thermoelectric potential accurately characterizing the phase transition of surface ferrite is shown.In situ thermoelectric potential measurement experiments at different cold end solder joint positions show that the transition of the thermoelectric potential curve is indeed caused by the phase transition of ferrite on the surface,which will move accordingly with the migration of the phase interface.The quenching experiment at the phase transition temperature(765°C)confirmed the existence of two phases from the cold end to the hot end,and gave a microscopic explanation of the transition of the thermoelectric potential near the phase transition temperature.In addition,the quenching experiment at the phase transition temperature(765°C)identified the formation mechanism of the ferrite layer on the surface from the perspective of the phase transition process.The result of this discrimination is that the surface layer is caused by a ferrite phase transition.The cold junction surface is first generated,and the phase transition pushes the carbon atoms to diffuse in a directional direction to the hot junction surface and core,so the cold junction forms a thicker surface ferrite layer.EPMA measured the carbon distribution at the point where the growth of ferrite grains was interrupted,which verified the previous surface ferrite layer formation mechanism.The study in this paper confirms that in situ thermoelectric potential measurement technology has practical application value in surface ferrite phase transition characterization,and provides a new detection method for quantitative analysis of phase transition information and control phase transition in material heat treatment process.On this basis,the preparation process of the surface ferrite layer is further optimized,so that the material can obtain a good combination of strength and toughness and better corrosion resistance.In addition,from the perspective of phase transition process,this paper identifies the formation mechanism of the above-mentioned surface ferrite layer,which provides experimental support for revealing the formation mechanism of the surface ferrite layer.