疲劳损伤及中间退火修复的40Cr试件的微观结构分析

MICROSTRUCTURE ANALYSIS OF STEEL 40Cr SPECIMENS FATIGUE DAMAGED AND RECOVERED BY INTERMEDIATE ANNEALING

利用透射电子显微镜对原始态、疲劳损伤态以及疲劳损伤后经中间退火修复的 40Cr钢的微观结构进行研究。结果表明 ,经高温回火的 40Cr钢的原始试样由于合金元素铬的存在使其回火稳定性较高 ,组织基本上保留了原板条马氏体的形貌。沿板条界或在板条中分布着粒状碳化物M7C3和M2 3C6 ,另外在较宽的板条中还有细长的杆状M3C碳化物存在。疲劳损伤态的试样的组织中位错组态与原始态的不同 ,部分区域位错缠结成位错墙结构 ,还有一些区域观察到位错胞结构。而且损伤后M2 3C6 碳化物的形貌变大 ,杆状M3C碳化物的数量增加 ,杆的长度减小。经过中间退火修复后 ,碳化物周围的位错缠结减少 ,而且位错胞的壁厚减小 ,位错密度降低。在整个组织中只发现了大块圆球状的M2 3C6碳化物均匀分布。从微观亚结构分析这三种状态的试样的组织存在一定的差异 ,因而表现为其力学性能和疲劳总寿命都有所不同。

The transmission electron microscopy was used to study the microstructure of steel 40Cr for virgin, fatigue damaged, and recovered after fatigue damageing by intermediate annealing. The results indicate that the stability of temper is raised due to the existence of chromium in high temperature tempering. The original martensite lathes is remained. There are M_7C_3 and M_ 23 C_6 carbides particle along the boundaries of lathes or inside the lathes. In addition, thin rod-like M_3C carbides are distributed inside some broader lathes. Configuration of dislocation in the specimens endured fatigue damage is different from that of virgin specimens. Some dislocations tangle and become dislocation walls in specific zones, and dislocation cells are observed in some regions. After the fatigue damage, the size of M_ 23 C_6 carbides particle becomes larger, the quantity of rod-like M_3C carbides increases and the length of rod shrinks. Through the recovery by intermediate annealing, there are fewer dislocation tangles exists around carbides and the wall thickness of dislocation cells thinned. So it is considered that the density of dislocation is reduced. Only big clump M_ 23 C_6 carbides, which is spherical and evenly distributed, can be observed in all region of the recovered specimens. There are some differences in microstructures among above three kind of specimens, so their mechanical properties and total fatigue life times are different.