锻压技术

NOS425钢铝合金压铸模早期开裂失效分析

英文标题：Failure analysis on early cracking of diecasting mold for NOS425 steelaluminum alloy

作者：郭联金金林奎孟鑫沛李清彬

分类号：TG147

出版年,卷(期):页码：2019,44(8):129-136

摘要：

NOS425钢铝合金压铸模在使用过程中出现早期开裂。采用直读光谱仪、数显洛氏硬度计、光学显微镜、扫描电镜及能谱仪，对失效模具的化学成分、断口形貌以及显微组织进行检测和分析。检测结果显示：模具从裂纹起始区至扩展终断区的整个断面以解理断口特征形貌为主，存在粗大放射条纹和二次裂纹；基体中存在大量白亮色淬火马氏体及残余奥氏体组织、较多晶间裂纹和封闭网状裂纹，表明模具在热处理过程中回火不充分，使得基体中存在较大的组织内应力和热应力，产生应力开裂的晶间裂纹，显著降低材料强度，增加材料脆性；同时，材料中的亚稳定组织在模具服役时产生再回火相变和二次硬化，模具在交变热应力的作用下发生热疲劳开裂；此外，机加工过程使得应力驰豫，模具表层应力得到释放，在晶界露头处形成应力释放的锲形凹坑，加速了模具开裂失效的进程。通过改进模具的热处理工艺和使用时的操作，显著提升了模具的质量和寿命，避免了类似失效的发生。

Early cracking occurred during the use of NOS425 steel-aluminum alloy diecasting mold, and the chemical composition, fracture morphology and microstructure of the failed mold were detected and analyzed by direct reading spectrometer, digital Rockwell hardness tester, optical microscope, scanning electron microscope and energy spectrometer. The results show that the entire section of the mold from the crack initiation to the extended termination is mainly characterized by cleavage fracture features, and there are gross radiation fringes and the secondary cracks. However, lots of white bright quenched martensite, retained austenite structure，intergranular cracks and closed network cracks appear in the matrix to indicate the insufficient tempering of the mold during the heat treatment, and large tissue stress and thermal stress are produced in the matrix to form the intergranular cracks. Thus, the material strength reduces and the material brittleness increases. Meanwhile, the metastable structure in material generates the retempering phase transformation and the secondary hardening when the mold is in service, and the thermal fatigue crack is yielded under the alternating thermal stress action. In addition, the stress releases on the surface of mold in machining process, and the squatted pits are formed at the outcrop of the grain boundary to accelerate the process of mold cracking failure. By improving the heat treatment process and the applied operation, the quality and life of the mold are significantly improved, and the similar failures are avoided.

基金项目：

2018年广东省大学生科技创新培育专项（pdjhb0902）；东莞职业技术学院政校行企项目（201813）

作者简介：郭联金(1981-)，女，硕士，副教授 E-mail：echo-guo@163.com

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