RELATIONS BETWEEN FAILURE BEHAVIOUR AND MICROSTRUCTURE IN TEMPERING STRUCTURE STEELS
YU Xuejie (Shanghai Iron and Steel Research Institute) (Manuseript(?)eceived 16 August; 1982; revised manuscript 14 March; 1983)
Cite this article:
YU Xuejie (Shanghai Iron and Steel Research Institute) (Manuseript(?)eceived 16 August; 1982; revised manuscript 14 March; 1983). RELATIONS BETWEEN FAILURE BEHAVIOUR AND MICROSTRUCTURE IN TEMPERING STRUCTURE STEELS. Acta Metall Sin, 1984, 20(1): 41-146.
Abstract The relations between the microstructure and failure behaviour have been investigated in two structure steels by means of TEM etc. The results show that the appearance and disappearance of qusicleavage fracture after tempering are related to the precipitation and the distribution of Fe_3C. When tempering at 250℃, the carbide precipitated is ε-carbide. At the same time the fracture is dimple only. When tempering temperature is raised beyond 350℃, the Fe_3C precipitates and the qusi-cleavage of fracture also appears. The proportion of qusi-cleavage fracture reaches the maximum after tempering at 450℃. If the tempering temperature is raised again, the particles of Fe_3C coalesce and the fracture become dimple alone in 60Si2Mn steel after tempering at 550℃. However, the coalescence tendency of Fe_3C particles is relatively less in the 37SiMnCrNiMoV steel, so the qusi-cleavage fracture still exists after tempering at 550℃. Observation of microcracks retained in failure samples reveals that the microcracks always propagate along the twin boundaries of twinned martensites and along the lath boundaries of lath martensites. So it is suggested that these boundaries that were already made brittle may be the cleavage planes of qusi-cleavage fracture. The reason why they become brittle is due to precipitation and continuous distribution of Fe_3C along these boundaries.
