轴坯校直断裂分析

材料为3Cr13钢的轴坯在进行机械校直时，有30%的轴坯发生了横向断裂。经分析，轴坯的化学成分符合技术条件要求。经宏、微观检验，发现轴坯的外表面有数条裂纹；其显微组织为位向明显的回火索氏体，裂纹表面有轻微的氧化脱碳；轴坯断口为萘状断口，其微观断裂途径为沿晶断裂。通过对断裂轴坯表面裂纹的分析，认为轴坯表面裂纹分为两类：一类裂纹是由于轴坯原材料表面有较深的纵向划伤缺陷，热处理时沿缺陷处进一步扩展形成的；另一类裂纹是轴坯在淬火加热过程中由于温度偏高而形成的过热裂纹。因此，淬火加热时的过热导致轴坯在淬火冷却时形成表面裂纹，这样在轴坯机械校直时表面裂纹又诱发轴坯发生断裂。

RUPTURE ANALYSIS OF AXLE STOCK DURING THE STRAIGHTENING

Some factory produces 3Cr13 roughcast shafts. When doing mechanical alignment, 30% of the roughcast shafts have been fractured transversely. By means of the chemical composition analysis, it accords with the technical requirement. There are many crack on the surface of these cracking roughcast shafts. These cracks can mostly be divided into two kinds. The first kind of the cracks is longitudinal distributed, comparatively wide, very straight, and throughout the whole roughcast shaft. The second kind is irregularly reticular crack, interweaved transversely and lengthways, and very fine. The break of these roughcast shafts is transverse split. The metallographic test shows that the microstructure of these roughcast shafts is tempered sorbite that is exposed apparently. Oxidization of the face of these cracks is rather lighter. From macroscopic view, the break of these roughcast shafts is naphthalene-way split. Its microscopic split is intergranular fracture. Microscopic examination of the surface crack of these cracking roughcast shafts shows that the formation of the first kind of cracks is because there are much deeper lengthways cut shortcomings on the surface of the raw material of these roughcast shafts which much expended during heat treatment. The second kind of cracks are over heated splits that were formed by too high temperature when these roughcast shafts were quenched. So the cracks of these roughcast shafts is because that over heat appear when these roughcast shafts are heated during quench leads to formation of surface cracks when these roughcast shafts are cooled during quench. These surface cracks place a premium on that the break of these roughcast shafts occur during mechanical alignment.