Grain Boundary Martensite Formation And Tempering Brittleness At Low Temperature Of Steels

The martensite nucleation and tempering brittleness of industrial steel have been studied in the article, such as: 65Mn, CrWMn, 50GrMn, 5GrMnMo, 42GrMo. It indicated that the martensite nucleation and growing at boundary of the patent phase at first, and the shape of the martensite is lamellar. These steel have large temper brittleness. The plate martensite at the boundary of the patent phase is less evident in the steels including Mo. In our opinion, the plate martensite at the boundary of the patent phase is the main reason that lead to the occurrence of tempering brittleness. We also researches the tempering brittleness of CrWMn, 65Mn and 50CrMn after different heat treatment by impact experiment. The result is that the tempering brittleness is smaller after tempering when the lower bainite instead of the plate martensite at the grain-boundary of the patent in the process of quenching. Observation of morphology of the plate martensite were performed. Analysis of the plate martensite at the grain-boundary effect on the microstructure in the tempering process. The martensite at the grain-boundary promotes the diffusion of carbon and P segregation in the tempering process. It promotes tempering brittleness of steels. Martensite substructure at the patent phase grain-boundary have twin crystal and dislocation. It is found that the parent phase transforms around the defects forming a sessile metastable martensitic embryo, and the dislocation loop or group of loops provide dislocation source for martensite transformation.