Thermal Stability And Microstructures Homogeneity In Quenching And Partitioning Treated 60Si2Mn Steel

Quenching and partitioning process is a heat treatment for steels proposed by J G Speer et al in 2003. In the present paper, commercial 60Si2Mn steel, based on the requirement on the steels for the quenching and partitioning process, is adopted as experimental material, and a facilities family of air resistance furnace-oil bath-air resistance furnace is used, to study the application of Q-P process in this kind of alloy structural steels, rather than TRIP steels. Cryogenic treatments at different temperatures are performed to assess the thermal stability of retained austenite. Samples of bigger size (Φ10×20)are designed, and microstructures and its homogeneity are analyzed using transmission electron microscope(TEM), scanning electron microscope(SEM), X-ray diffraction(XRD), and optical microscope combined with hardness, aiming the application in structural members.The results show that the Q-P process could be realized using a range of facilities of air electric furnace-oil bath-air electric furnace, rather than salt baths. Optimum process parameters, austenitizing at 880℃for 900s followed by quenching at 220℃for 10s and partitioning at 400℃for 30s, were selected based on repeated experiments. More than 13%(volume fraction) retained austenite combined were got in steels.Multiphase microstructures of lath martensite, with thin films of residual austenite between martenite laths and block shape of retained austenite were achieved in the Q-P treated 60Si2Mn steel. There is no obvious change in martensite morphology while a decreasing in martensite size from the center to the edge in the microstructures of different locations of the 1/2 height cross section of the sample under FE-SEM. The homogeneity of microstructures is uniform, namely.Some specimens were cryogenic treated at -196℃～30℃for 60-300s. Mixtures of liquid nitrogen and ethyl alcohol were used for cryogenic treatment. The volume fraction of retained austenite in samples subjected only Q-P process and cryogenically treated under -30℃for 60s, in -30℃for 300s and in -50℃for 60s followed with Q-P process are determined as 13.0%,10.9%,9.2% and 8.3%, respectively. After cryogenic treatment under -80℃and -196℃, the volume fraction of austenite in samples are too low to be determined. However, diffraction peaks of (111)γand (200)γcan be still observed. The result reveals that retained austenite in 60Si2Mn grade steel after Q-P heat treatment can be stabilized to -80℃, indicating the possibility he possibility of application of Q-P processed steels in low-temperature environment.