Evolution Of TiN Inclusion During Heating And Its Effect On Microstructure And Mechanical Properties In Fe-Ni Alloys

TiN is a very important type of non-metallic inclusions in the field of oxide metallurgy.The pinning effect of TiN inclusions on the austenite grain boundary is a current research hotspot and has been widely studied in steels with various compositions.The Fe-Ni alloy with austenite microstructure maintained from high temperature to room temperature is very suitable for the study of pinning effect because it avoids the interference of phase transition on the study of pinning effect.Moreover,the evolution of TiN inclusions in Fe-Ni matrix is rarely reported,and there are many unknowns.Therefore,Fe-Ni alloys are used as the research object to study the evolution of TiN inclusions during heating and pinning effect.In order to study the pinning effect of TiN inclusions on the austenite grain boundaries in the Fe-Ni alloys during heating,the Fe-Ni alloys with TiN addition were cast using a vacuum induction furnace by air cooling.In order to avoid the influence of phase transformation on the pinning effect,the Fe-Ni alloys retained as austenite from high temperature to room temperature were selected for heating experiment.Using a resistance furnace,the austenitic Alloy 1 containing TiN and the austenitic Alloy 2 without TiN were kept at 1473 K for 0.5 h,1 h,2 h,and 24 h,respectively,and then air-cooled.Optical microscope(OM),scanning electron microscope(SEM),energy dispersive spectrometer(EDS),confocal scanning laser microscopy(CSLM)were used to study the evolution of TiN inclusions in Fe-Ni alloys and the changes in microstructure and mechanical properties at 1473 K.In addition,the melting and solidification process of Fe-Ni alloys were in-situ observed by CSLM to study the casting process of Fe-Ni alloy with inclusions.The results show that the evolution of TiN inclusions can be divided into three stages according to the quantity distribution of different sizes:Casting stage,0.5?2 h stage,24 h stage.In the casting stage,the size distribution of TiN inclusions is relatively average and the quantity is relatively small.During the 0.5h?2 h stage,the peak value of TiN inclusions is concentrated in the range of 0.5?1.5?m.Compared with the casting stage,the number density of TiN inclusions increases significantly and the average size decreases.The number density and the average size of TiN inclusions change little in this stage.During the 0.5 h?2 h stage,the peak value of TiN inclusions is concentrated in the range of<1?m.The density of TiN inclusions increases further,and the average size of inclusion decreases significantly.The reason for the evolution of TiN inclusions in the alloy kept within 2 hours is that Oswald ripening occurs.The number density of TiN inclusions increases significantly during the heating process because the larger TiN inclusions decompose into smaller TiN inclusions.After a long time of holding for 24 hours,the grain size of Alloy 1 containing TiN is obviously smaller than that of Alloy 2.The reason for the slow growth of grain size of Alloy 1 is that TiN inclusions are located at austenite grain boundary and the triple junction,which hinders the migration of grain boundary and TiN inclusions leads to the formation of fine new grains when heated.The grain refinement of Alloy 2 occurres at 1 h,which may be due to the effect of annealing twins on grain refinement during heating.The hardness of Fe-Ni alloys can be improved by adding TiN inclusions,and the hardness of the alloy can be improved by dispersing TiN inclusions.The melting and solidification process of Fe-Ni alloys were observed in-situ by CSLM.Before melting,with the increase of temperature,only the grain boundary and precipitated inclusions appeare in alloys,without the change of phase transformation microstructure,while the surface microstructure of non-austenite alloys changed.The melting of Fe-Ni alloys begin at the grain boundaries with more inclusions.During solidification,the metal liquid near the inclusions first solidifies due to heterogeneous nucleation,and the first solidified part meets to form grain boundaries,then the metal liquid between the grain boundaries solidifies.