Failure Analysis of Early Cracking of Die Casting Die

1. Overview

The aluminum alloy die-casting mold material is 4Cr5MoSiV1 (H13) steel, and the hardness requirement is 42-48HRC. The manufacturing process route is: forging→annealing→roughing→heat treatment→finishing. The mold is heated in a vacuum furnace at a temperature of (1030 ± 10) ° C, held for 90 min, quenched with argon gas, tempered at 560 to 580 ° C for 6 h and cooled in air. The hardness after the treatment was 51 to 52.5 HRC.

2. Failure mold inspection

The mold is cracked after a short period of use, and the chemical composition analysis meets the national standard of 4Cr5MoSiV1 steel. There are obvious bead lines in the fracture observation. The source and extension areas of the crack show different oxidation colors due to high temperature oxidation, which makes the fatigue cracks clearer. The crack source is located at the corner of the mold profile. The other pair of molds have cracks before use and become a source of cracks in use. There are black oxides at the cracks on the fractures, and there are no obvious bead lines in the extended area, but there are long fast tear strips.

The metallographic structure is tested as tempered martensite + tempered tortite (black network). The black network was analyzed by SEM spectroscopy, where there were chromium, vanadium carbides, and segregation of impurity elemental sulfur. Metallographic examination also found that due to the uneven distribution of the chemical composition of the steel, the carbide segregation zone (carbon and alloy element enrichment zone) has massive and stacked carbides, and is very small and dense. Dot carbide. The area of ​​carbon-poor alloy-poor elements on both sides, although it has been tempered at high temperature, the microstructure has changed, but the bainite morphology (black needle-like) formed during the original quenching is still visible.

3. Failure analysis and preventive measures

Component segregation of large modules is inevitable. Forging makes it difficult to eliminate component and tissue unevenness, and bulk-stacked and chain-network eutectic carbides are not easily broken and homogenized. Therefore, the large module can be firstly subjected to solid solution-high temperature tempering pretreatment at 1100~1150 °C to improve the tissue non-uniformity, eliminate the secondary carbide net in the raw material, and increase the toughness of the mold. It is tempered at a high temperature of about 680 ° C, and air-cooled after heat preservation.

The quenching temperature is selected from 1000 to 1050 ° C, and a fine needle-like martensite structure with fine crystal grains and good properties and a metastable eutectic carbide can be obtained. If the quantity is small and the distribution is uniform, the shape is rounded, which is beneficial to the improvement of steel performance. 4Cr5MoSiV1 steel may obtain martensite structure under air-cooling conditions, but it is slowly cooled in the temperature range of 800~860 °C, and secondary carbides are precipitated along the grain boundary. If oil quenching (oil temperature 55~150 °C) or classification is adopted Quenching can inhibit the precipitation of secondary carbides along the grain boundaries and avoid brittleness.

4Cr5MoSiV1 steel has temper brittleness, and the larger size mold should be oil-cooled after high temperature tempering to avoid temper brittleness, improve toughness and prolong mold life.

4. Conclusion

(1) The segregation of chemical components of the steel raw material is serious, resulting in uneven organization and additional stress, which is an important cause of early cracking.

(2) The argon quenching pressure is not enough, and the cooling rate is slow, which causes black torsite to appear in the quenched structure, and there may also be reticulated carbide precipitation, which reduces the strength and impact toughness of the material, and is also the cause of early cracking. One.

(3) Due to the large size of the mold, the air cooling rate is insufficient after high temperature tempering, causing temper brittleness.

(4) The R angle at the change of the section of the mold is too small, causing the heat treatment stress to concentrate and causing micro cracks, and the micro cracks expand during operation, leading to early failure of the mold.

About the author: Su Yiren, male, (1965-), School of Mechanical and Electrical Engineering, Wuhan Vocational and Technical College, Bachelor of Engineering, Engineer, Senior Lecturer.

Chen Dongli, Jingzhou Huanyu Auto Parts Co., Ltd.