关键词: 20CrMnTi；脆性断裂；Cr、Mn 元素晶界偏析；相变应力；断口微裂纹

Abstract: The transmission output shaft broke during the comprehensive durability test of a certain vehicle. Via chemical composition detection, microstructure observation, fracture morphology and energy spectrum analysis, the fracture causes of transmission output shaft made of 20CrMnTi are analyzed. The results show that during the casting and solidification process of raw materials, chromium (Cr) and manganese (Mn) elements converge in the dendrites at a slow diffusion rate, and the solute elements in the liquid phase region cause element segregation at the grain boundaries, thereby precipiting Cr and Mn compounds and weakening the grain boundaries. Under the comprehensive action of internal stress and phase transition stress generated by subsequent heat treatment process, micro-cracks occur on the weakened grain boundaries. When subjected to alternating load, the microcracks gradually spread inward along the original austenite grain boundary, and finally lead to the fracture of the parts. Users can learn from the direction of failure analysis introduced in this paper to control the occurrence of such failure problems at the user end.

Key words: 20CrMnTi; brittle fracture; grain boundary segregation of Cr and Mn elements; phase change stress; fracture microcrack