Early Martensitic Transformation in a 0.74C–1.15Mn–1.08Cr High Carbon Steel

Abstract

The martensitic transformation in a high carbon steel was studied by a new experimental approach focusing on the nucleation and growth as well as the variant pairing of the early-formed martensite. A mixed microstructure with tempered early-formed martensite and fresh later-formed martensite was achieved by a heat treatment with an isothermal hold below the martensite start temperature. In-situ high-energy X-ray diffraction showed no further transformation of austenite to ferrite/martensite during the isothermal hold. The tempered early-formed martensite was characterized with a combination of light optical microscopy and local tetragonality determination by electron backscatter diffraction. The characterization allowed qualitative as well as quantitative analysis of the tempered early-formed martensite with regard to the prior austenite grain boundaries (PAGB) and variant pairing. The early-formed martensite was shown to grow predominantly along the PAGBs and clustering was observed indicating an autocatalytic nucleation mechanism. The variant pairing of the early-formed martensite had a stronger plate character compared to the later-formed martensite.