Effect of Preferential Heat Treatment on Microstructure of New Martensitic Heat Resistant Steel G115

Abstract

Intermediate heat treatment and thermo-mechanical treatment are the two possible ways of preferential heat treatment. Microstructure of G115 martensitic heat resistant steel after traditional heat treatment and preferential heat treatment was tested by field emission scanning electron microscope (FESEM) and electron back-scattered diffraction (EBSD). The results show that intermediate heat treatment can hardly change the size of precipitates while the precipitates can be obviously refined after thermo-mechanical treatment. Additionally, there is no obvious difference in precipitate size after deforming at different thermo-mechanical treatment temperatures. The reason why intermediate heat treatment failed in G115 steel can be attributed to that the content of Nb is low and most of V is in solution. The reason for the success of thermo-mechanical treatment is that after thermo-mechanical treatment but without tempering, the amount of large angle boundaries has a three-fold increase compared with that after traditional heat treatment. Since M₂₃C₆ particles are mainly precipitated along large angle boundaries, the increase of large angle boundaries can induce the increase of the precipitation site of M₂₃C₆ particles, resulting in the refinement of M₂₃C₆ particles. After deforming at different temperatures, the amount of large angle boundaries is close, so the size of M₂₃C₆ is nearly the same.