Precipitation of Iron Carbides in Tempered Martensite

The precipitation of carbides in 0.7 wt%-carbon steel as a function of tempering temperature has been studied through the Mössbauer effect, magnetic measurements, and x-ray observations. The magnetic behavior of the extracted precipitates is discussed. The tiny clusters obtained by extraction after the early stages of tempering (up to 250°C) show the relation of these clusters to martensitic bct cells (``Johnson clusters''). The results obtained by further stages of tempering (250–500°C) show a decrease in the relative amount of the Johnson clusters, followed by a precipitation of ferromagnetic carbides. The behavior of the internal field, the TC and θD, as a function of the tempering temperature suggests a decrease of the relative carbon content with the increasing tempering temperatures. The Fe3C is formed toward 550°C.