焊缝金属解理断裂微观机理

Micromechanism of Cleavage Fracture of Weld Metals

本文为低合金高强钢的解理断裂提出了一个新的理论框架:解理断裂不仅决定下平台的冲击韧性,而且对过渡温度区的韧性也起决定性的作用,因为在这个温度区间韧性取决于先前产生的塑性裂纹扩展的长度,而解理断裂终止了塑性裂纹的扩展,从而决定其长度。解理断裂包含3个不间断的阶段:(1)裂纹在第二相颗粒起裂成核;(2)第二相裂纹穿过第二相颗粒和晶粒边界扩展;(3)晶粒裂纹穿过晶粒边界扩展进入相邻晶粒。本理论框架对整个过程进行了诠释:解理断裂的临界事件是在裂纹形成过程中提供最大困难的阶段,它控制了解理断裂过程,并确认了最薄弱的微观组分及其临界尺寸,这个尺寸决定了微观解理断裂应力sf。在断裂过程研究中最为重要的就是揭示各种条件下断裂的临界事件。提出引发解理断裂的3个准则:(1)裂纹起裂成核准则;(2)防止裂纹核钝化准则;(3)裂纹扩展准则。并由这3个准则形成了一个活性区,在活性区中应力和应变的综合作用可以引发解理断裂,这个活性区被用来建立断裂的统计模型。本研究利用这个新的理论框架进行了8%Ni钢焊缝的实例研究,以说明实验研究的结果。

Cleavage fracture is the most dangerous form of fracture. Cleavage fracture usually hap- pens well before general yielding at low nominal fracture stress and strain. Cleavage fracture is often spurred by low temperature and determines the toughness in the lower shelf temperature region. This pa- per describes a new framework for the micromechanism of cleavage fracture of high strength low alloy （HSLA） steel weld metals. Cleavage fracture not only determines the impact toughness in the lower shelf but also plays a decisive role on the impact toughness in the transition temperature region. The tough- ness is determined by the extending length of a preceding fibrous crack which is terminated by cleavage fracture. Three non-stop successive stages, i.e. crack nucleation, propagation of a second phase particle- sized crack across the particle/grain boundary, propagation of a grain-sized crack across the grain/grain boundary are explained. The ＂critical event＂ of cleavage fracture is emphasized which offers the greatest difficulty during crack formation and controls the cleavage process. The critical event indicates the weak- est microstructural component and its critical size which specifies the local cleavage fracture stress σf for cleavage fracture. In toughness-study it is paramount important to reveal the critical events for various test specimens. Three criteria for crack nucleation, for preventing crack nucleus from blunting and for crack propagation are testified. An active region specified by these criteria is suggested where the com- bined stress and strain are sufficient to trigger the cleavage fracture. It can be used in statistical analy- ses. A case study, using the new framework of micromechanism for analyzing toughness of 8% Ni steel welding metals is presented to analyze the experimental results.