Transactions of Materials and Heat Treatment

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

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ClassificationCode：TG147

year,vol(issue):pagenumber：2019,44(8):129-136

Abstract：

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.

Funds：

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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