SA-387Gr.91钢球形封头淬火裂纹分析

为提高表面硬度,使用淬火+回火对 SA-387Gr.91钢球形封头进行热处理,热处理后在封头内表面发现大量裂纹。对裂纹及附近母材的化学成分、显微组织、硬度、断口形貌进行分析。结果表明,裂纹为淬火裂纹。淬火裂纹与原材料化学成分及冶金质量无关。封头母材的原始奥氏体晶粒粗大,显微硬度超过标准值。基于试验分析认为裂纹的产生主要与热冲压温度偏高造成的晶粒粗大及冷却介质选取不当造成的冷速过快有关。在采取了降低冲压温度,淬火冷却方式由水冷改为强制风冷,降低回火温度等改进措施后,封头未出现淬火裂纹,表面硬度达到了预期的要求。

Analysis on quenching cracks of SA-387Gr.91 steel spherical head

A large number of cracks were found on the inner surface of SA-387Gr.91 steel spherical head after quenching and tempering processes which were used for improving the hardness. The chemical composition, microstructure, hardness and fracture appearance of the cracks and the nearby base metal were detected and analyzed. The results show that the cracks are quenching ones, and they are not related to the chemical composition and the metallurgical quality of the base metal. The prior austenite grain size of the base metal is too large and the microhardness is above standard value. It is revealed the by the experimental results that the cracks are mainly caused by coarse grains resulted from too high hot stamping temperature and the high cooling rate resulted from inappropriate quench medium. By lowering the hot stamping temperature, changing water cooling to forced air cooling and lowering the tempering temperature, no quenching cracks ever appear in the spherical head, and the surface hardness meet the expected requirements.