C-Mn钢及焊缝金属的缺口和裂纹试样的微观断裂行为

Microscopic Fracture Behavior of C-Mn Steel and Weld Metals in Notched and Precracked Specimens

本文通过起裂源粒子的特征和尺寸分布、裂源处显微组织和残留裂纹的尺寸分布情况,研究了C-Mn钢和两种焊缝金属的Charpy V,COD和预制裂纹冲击试样的微观断裂行为。发现焊缝中的解理裂纹形核于夹杂物或第二相粒子,母材形核于珠光体团;对于同一材料用缺口试样和裂纹试样测得的微观解理断裂应力的不同值是由于断裂过程中临界事件发生了变化。在-45～-65℃范围内,Charpy V 缺口试样中的临界事件是铁素体晶粒尺寸的微裂纹扩展进入铁素体基体;而在-110℃的裂纹试样中,是第二相粒子尺寸的微裂纹扩展进入临近的铁素体晶粒。临界事件的变化认为与缺口根部和裂纹前端不同的有效剪应力有关。建立Charpy V 和COD实验结果之间关系的条件是有关材料铁素体晶粒和第二相粒子的尺寸应当是类似的。

The microscopic fracture behavior of a C-Mn steel plate and of two types of weld metal in charpy V,COD,and precracked impact specimens have been studied by observations of the size distribution of the cleavage-initiating particles,the microstructure in the region initiating the cleavage crack, and the size distribution of microcracks remaining in specimens. It has been found that the cleavage fracture in the weld metals is initiated by inclusions or second phase particles and in the base metal is initiated by pearlite colony, and the different values of the microscopic cleavage fracture stress, σf measured in notched and precracked specimens of the same metal is caused by the change in the critical event in the fracture processes. The critical event in the notched charpy V specimens at -45--65℃ is the propagation of a ferrite grain-sized microcrack into the ferrite matrix. For the precracked COD specimens at - 110℃ ,it is the propagation of a second phase particle-sized microcrack into the neighboring ferrite grain. The variation of the critical event is considered to be related to the difference of effective shear stress ahead of a notch root or a crack tip. The prerequisite condition for establishing a correlation between charpy V and COD test is that the ferrite grain and second phase particle sizes in the related metals should be similar.