1Cr18Ni9Ti焊缝疲劳短裂纹的形态演化

Evolutionary Morphology of Short Fatigue Cracks of 1Cr18Ni9Ti Weld Metal

试验研究了1Cr18Ni9Ti焊缝光滑试样的单轴疲劳短裂纹形态演化规律。基于“有效短裂纹准则”,把注意力放在“主导有效短裂纹(DESFC)”萌生及裂尖前沿区域。观测发现,ESFCs萌生于焊缝局部delta铁素体与奥氏体基质界面,取向在与加载轴垂直截面小于45o随机分布。之后,受微观结构约束,表面倾向于沿垂直于轴向不规则扩展;深度方向,裂纹初始与加载方向垂直,之后倾向于与柱状晶生长方向垂直。当若干取向基本一致,裂尖前沿相邻的ESFCs发生合并即形成真实DESFC,由微观结构短裂纹(MSC)阶段进入物理短裂纹(PSC)阶段。PSC阶段,表面DESFC近似与轴向垂直,在微观结构干扰下不规则扩展;深度方向,逐步演变与柱状晶生长方向垂直,后期及长裂纹阶段倾向于与加载轴成45o。最后,试样沿与加载轴垂直方向瞬时断裂。这说明了疲劳短裂纹与长裂纹形态变化具有明显差异。

Evolutionary morphologies of the short crack propagation of 1Cr18Ni9Ti weld metal are investigated on specimen surface and in depth direction. To be agreement with the previously effective short fatigue crack （ESFC） criterion, attention is paid to the formation zone of the dominant ESFC （DESFC）, which results in the specimen failure, in the micro-structural short crack （MSC） regime and then the tip zone（s） ahead of the DESFC in the physical short crack （PSC） regime. Results show that the ESFCs imitated from the bounds between the ferrites and the austenite matrix. The ferrites are distributed randomly in an angle range less than 45 degree to the cross-section perpendicular to the loading axle. The initiated ESFCs on surface propagate to be inclined to the direction perpendicular to the loading axle due to the stronger microstructural constraints. In the depth direction, the initiated ESFCs grew perpendicular to the loading axle first and then to be inclined to the direction perpendicular to the material columnar grain structure formation. When the sizes of some longer ESFCs, with closer tips ahead the growth paths and similar directions, get the material maximum barrier size, coalescence may occur to form at a true DESFC. In the PSC regime the formed DESFC grows to be more and more inclined to the direction perpendicular to the loading axle on the surface. But in the depth direction, it grows to the direction perpendicular to the formation direction of the material columnar grain structure furst and then to be inclined to the direction with 45 degree angle to loading axle. Finally, the specimens are fractured mostly in direction perpendicular to the loading axle. It revealed that there is significant difference between the evolutionary morphologies of long and short fatigue cracks.