内裂纹高温修复组织演化过程分析

Microstructure evolution of internal crack healing in a low-carbon steel at elevated temperatures

针对大型锻件在生产过程中常因裂纹等孔隙性缺陷探伤超标导致改锻甚至判废的问题,采用辐射加热方式研究了低碳钢内部裂纹高温修复及组织演变过程。结果表明:温度为900℃时,裂纹附近富集析出铁素体,裂纹修复以原子的扩散迁移为主;1 000℃时,裂纹附近发生再结晶,裂纹间隙被大量细小的等轴铁素体晶粒所填充,1 200℃时,等轴的铁素体晶粒长大为链状,并有晶粒跨过原裂纹区长大,形成共有晶粒;温度高于1 000℃时,裂纹修复以再结晶和晶粒长大为主,再结晶的发生实现了裂纹的快速愈合。裂纹修复过程可分为4个阶段:裂纹面局部凸起的形成与接触,凸起的长大及裂纹的分段离散化,分段裂纹的球形化及孔洞的形成,孔洞的缩小及裂纹区的消失。该研究结果对揭示高温裂纹修复机理和演化过程具有重要意义,为大型锻件缺陷探伤标准的制定及质量控制提供参考依据。

Heavy forgings are often reforged and even disposed of as waste because of cavity-type defects such as cracks seen by ultrasonic flaw detectors. The microstructure evolution of internal crack healing in low-carbon steel is investigated with radiation heating.The results show that the crack healing is not just controlled by a single mechanism.At 900℃,ferrite grains enrich around the crack and the crack healing mechanism is mainly atomic diffusion.At 1000℃,large amounts of fine equiaxed ferrite grains fill the crack gaps.At 1 200℃,equiaxed grains become chain-like ferrite,with some grains growing across the crack gaps to generate mutual grains.At temperatures above 1 000℃,the crack healing is mainly controlled by the recrystallization and grain growth.Recrystallization leads to rapid crack healing.The internal crack healing can be divided into four stages：formation and contact of bulges on local crack surfaces, bulge growth and crack segmentation,spheroidization of intersecting cracks and formation of spherical voids,and diminishing of spherical voids and disappearance of the crack healing zone.This understanding of the crack healing mechanisms and crack evolution will facilitate crack healing control in practice.