75钢拉拔开裂的表面白亮层分析

Analysis of White Layer of Steel 75 Drawing Crack

在现代的金属工艺中,为节约材料和提高材料性能,对线材或棒材等长材经常采用拉拔工艺进行加工。在这种加工艺中,会出现一种表面因拉拔而产生的白亮层,这一白亮层很薄,但硬度很高,常常引起拉拔件表面开裂,有时甚至会引起拉拔件断裂。在以往的文献记载中,常根据其硬度接近马氏体,将其归结为拉拔过程中表面异常升温而产生的马氏体。作者有幸接触到了一起因表面白亮层而导致的表面开裂,但分析结果是白亮层为极其破碎的渗碳体与铁素体的机械混合物,只是腐蚀剂由于表面张力无法进入,而使得其在金相显微镜下观察呈白色。

In modern metal process, in order to save material and improve its property, wires or rods are often processed by drawing. In the process, a white layer would arise from drawing. The white layer, which is very thin and hard, often leads to surface crack and even fracture of drawing units. In the previous literatures, because of its hardness approaching to that of martensite, the layer was considered martensite generated from abnormal surface temperature in the drawing process. It was fortunate that the author came across a surface crack caused by white layer . The analysis shows that the layer was a mixture of easily broken cementite and ferrite. The mixture looks like white under metallographic microscopy because the corrosive agent could not enter due to the surface tension.