In-situ Observation of Microstructural Characterization of Lath Martensite by Cryogenic Treatment

The lath martensite microstructure of 20 steel before and after cryogenic treatment was observed by OM and AFM. The samples were quenched at 1100 ℃ for 4 min after heating, and then were cryogenic treated at -196 ℃ for different time. By XRD and microhardness tester, the lattice constants and the hardness of the martensite were measured, and their changes were studied. The results show that the microstructure of lath martensite is disintegrated in cryogenic treatment and that the precipitation of minute carbide in lath martensite is relate to the times and the restore temperature of cryogenic treatment. The results also show that the c/a of tetragonal martensite tends to become 1 and the microhardness of lath martensite is increased a little about 1%~3% in cryogenic treatment.