铝合金用ASSAB 8407钢压铸模早期断裂失效分析

Failure analysis of early fracture of ASSAB 8407 steel die casting mold for aluminum alloy

对铝合金用ASSAB 8407钢压铸模断口进行宏观观察,初步推断断口的特征形貌,确定裂纹源的位置以及裂纹扩展的状态特征。采用金相显微镜及扫描电子显微镜,对失效件断口及组织进一步检测,分析断口裂纹形成和扩展机理。检测结果表明,由于基体中存在大量沿晶裂纹,材料强度大幅度降低,脆性显著增大,模具在服役过程中发生早期断裂失效。沿晶裂纹的产生是由于模具锻造过热后慢冷析出片状碳化物,材料强度显著降低。在热处理过程中,热应力及组织应力的影响,使材料基体组织产生沿晶的应力开裂裂纹。同时锻造过热使得材料强度大幅度降低,进一步促使模具表面裂纹的萌生和扩展。

The fracture of ASSAB 8407 steel die casting mold for aluminum alloy was observed macroscopically,the characteristics of fracture surface were preliminarily inferred,the location of crack source and the state characteristics of crack propagation were determined. Using the metallographic microscope and scanning electron microscope,the fracture and the microstructure of the failed part were further tested,and the formation and propagation mechanism of fracture were analyzed. The test results show that due to the large number of intergranular cracks existing in the alloy,the strength of the material is significantly reduced and the brittleness is significantly increased,which results in the early failure of the mold during service. The inter-crystal cracks are initiated along the flake carbides which are precipitated during slow cooling after forging of the mold due to overheating,and therefore the strength of the mold is remarkably decreased. In the heat treatment process,thermal stress and phase transformation stress make the material to produce cracking. At the same time,the overheating during forging further promotes crack to initiate and propagate on the mold surface.