ML20MnTiB-1合金冷镦钢螺栓镦头开裂原因分析

Analysis of Heading Head Cracking of Cold Heading Alloy Steel Bolt ML20MnTiB-1

材质为ML20MnTiB-1的合金冷镦钢盘条在经过拉拔、球化退火处理后冷镦加工成螺栓的过程中,部分螺栓头部出现开裂现象。对头部开裂的螺栓试样进行了宏观检测、非金属夹杂物检测、金相检测、扫描电镜及能谱检测分析,检测结果表明,螺栓头部开裂对应的螺杆表面的近表面区域存在多条大尺寸黑色夹渣。材料近表面区域存在大尺寸夹渣会严重破坏材料基体的连续性,降低材料的韧塑性。在金属基体相对于夹渣发生流动时,夹渣与金属基体脱离并划伤基体而出现微裂纹及空隙,金属冷拔、冷镦变形会进一步加剧这些微裂纹及空隙的扩展。由此判断,材料近边缘大尺寸夹渣是导致材料镦头开裂的主要原因。

When the alloy cold heading steel wire rod ML20MnTiB-1 is processed into bolts after drawing and spheroidizing annealing,part of the bolt heads crack.The macroscopic inspection,non-metallic inclusion detection,metallographic examination,scanning electron microscope and energy spectrum analysis were carried out on the bolt samples with head cracking.There are many large-size black slag inclusions in the near surface area of the screw surface corresponding to the bolt head cracking.The existence of large size slag inclusion near the surface of the material will seriously destroy the continuity of the material matrix and reduce the toughness and plasticity of the material.When the metal matrix flows relative to the slag inclusion,the slag inclusion detaches from the metal matrix and scratches the matrix,resulting in microcracks and voids.Metal cold drawing and cold heading deformation will further aggravate the propagation of these microcracks and voids.Therefor,it can be concluded that the large size of slag inclusion near the edge of the material is the main reason for the cracking of the upsetting head of the material.