材料期刊网

用透射电镜研究了两种结构钢的断裂行为与显微组织的关系。结果表明,回火后准解理断口的出现和消失与Fe_3C的析出和聚集密切相关:经250℃回火时析出ε碳化物,呈现单一韧窝断口,回火温度超过350℃时,有Fe_3C析出,这时出现准解理断口,而于450℃回火后达最大比例。在550℃回火后,60Si2Mn钢因Fe_3C聚集,准解理断口消失,又变成单一韧窝断口,37SiMnCrNiMoV钢Fe_3C聚集的倾向小,经550℃回火后仍有少量准解理断口保留下来。观察试样上由负荷产生的残留显微裂纹发现,它们常沿孪晶马氏体的内孪晶界及板条马氏体的条界发展,因此认为这两种脆化了的界面是准解理的解理面,而Fe_3C沿这类界面析出并呈连续分布是导致界面脆化的原因。

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.