Effects of external stresses on the carbide precipitation in bainite transformation

TEM observation of carbide precipitates in bainite has been carried out for steel 40CMD8. Specimens transformed at 350°C and 450°C without external stress contain respectively the lower and the upper bainite. No fundamental change in carbide precipitation is observed in specimen transformed at 350°C under stress of 128 MPa. The very fine cementite is found precipitated within the upper bainite plates in specimen transformed at 450°C under stress of 144 MPa, which can be explained as the result of the difficulty of carbon diffusing out of the massive ferrite plate within a much shortened transformation time due to the external stress. For all the specimens examined, Bagaryatskii orientation relationship is found existing between the carbides and ferrite matrix.