12.9级高强螺栓冷镦开裂的原因分析

Cause Analysis of Cracking during Cold Heading of Class 12. 9 High-strength Bolt

对合金冷镦钢SCM440加工12.9级内六角螺栓时发生冷镦开裂的现象进行了分析。开裂原因主要是退火过程中保护气氛不当,导致材料表面出现深20~30μm的完全脱碳层,铁素体晶界的抗氧化性降低,同时合金元素在晶界富集,在冷却时产生沿晶裂纹。同时用户的退火工艺不当,盘条组织未完全球化,也导致冷镦性能不佳。

The causes of cracking during cold heading of Class 12.9 hex socket bolt of SCM440 al oyed cold heading steel were analyzed. The main cause was improper protective atmosphere in the annealing process,which led to 20-30 um thickness of complete decarburization layer. The anti-oxidation property in the boundary area of ferrite phase deteriorated,and al oys concentrated at grain boundaries,resulting in intergranular cracking during cooling. Improper annealing process by customers and incomplete pearlite spheroidization are also harmful to cold heading property.