<i>In Situ</i> Microstructure Observation of Steel Grades in the Semi-Solid State for Thixoforging Process by Using Confoncal Laser Scanning Microscopy

Abstract

It is necessary to well understand the microstructure evolution during high speed heating and forming for steel thixoforging, since it determines the thixotropic flow behavior of materials in the semi-solid state. A new in situ technique - high temperature Confocal Laser Scanning Microscopy (CLSM) - was developed and used for studying the microstructure evolution directly at high temperature where the microstructure in the semi-solid state could not be preserved by quenching experiments for conventional 2D characterization. Several steel grades (C38LTT, 100Cr6 and M2) were investigated during heating from the as-received state to the semi-solid state and finally cooled to the solid state).It has been found that there is a significant difference in diffusion rate of alloying elements between these grades during heating and cooling. In M2, thanks to the high content of alloying elements and their low diffusion rate, the semi-solid temperature range is greater and its microstructure in the semi-solid state could be preserved by quenching or even at a low cooling rate, which means the microstructure of M2 in the semi-solid state can be characterized in room temperature on quenched M2 samples. On the contrary, the microstructure of other steel grades 100Cr6 and C38LTT in semi-solid state can only be revealed by CLSM at high temperature because of the lower volume fraction of alloying elements and their high diffusion rate. It is very interesting to use high temperature CLSM to in situ investigate the microstructure evolution in the semi-solid state, especially at low liquid fraction.