Heat treatment crack of casting and forged parts

There are many kinds of defects in heat treatment, and the causes are various. The causes of defects in heat treatment may include improper process, improper operation, improper equipment and environmental conditions, etc. Therefore, in order to prevent the occurrence of heat treatment defects, we should solve it from many aspects related to heat treatment, and control all relevant links before, during and after heat treatment, which is to carry out comprehensive quality control over the whole process of heat treatment. In addition, we should strengthen the research on heat treatment defects, deeply study the mechanism of various heat treatment defects and new heat treatment process defects, and explore new measures and remedies to reduce and avoid heat treatment defects.

1. Tempering crack

Tempering crack is a crack caused by improper process control in the tempering process. Tempering cracks may occur when large tools and dies are made of steel with good hardenability.

2. Cold treatment crack

For some measuring tools and precision mechanical parts, in order to ensure high dimensional stability, residual austenite needs to be reduced as much as possible. Cold treatment at minus 800C is usually adopted. The service life of tools and dies can be significantly improved after ultra-low temperature treatment. However, improper cold treatment will cause workpiece cracking. The main reasons are as follows: 1) after quenching, the temperature of the workpiece is high. At this time, if the cooling speed is accelerated, part of the untransformed austenite will be further transformed into martensite, the tensile stress will increase, and the brittle fracture resistance of the material will be reduced at low temperature. When the stress exceeds the brittle fracture resistance of the material, it will lead to cracks. 2) Because the workpiece size is too large, the structure is complex, the cold treatment temperature is too low, the medium used for cold treatment cools quickly, or increases the original internal stress, these may form cold treatment cracks.

3. Aging crack

In some alloys, all carbides dissolve into the matrix during solution treatment, so as to embrittle the alloy. The weldments of this material will produce strain aging cracks during standard heat treatment.

4. Grinding cracks

Hardened tool steel parts, or parts carburized, nitrocarburized and quenched, sometimes have a large number of grinding cracks during subsequent grinding.

5. Plating cracks

The surface layer with internal stress will be produced during electroplating or pickling of quenched parts before electroplating. The hydrogen generated by the interaction between the workpiece and the solution will penetrate into the steel. The higher the temperature, the greater the amount of hydrogen adsorbed. The stronger the dissociation degree of the solution, the more hydrogen content adsorbed and infiltrated by the workpiece. If the quenched parts stay in acid, alkali and other chemically active media for a long time, the internal stress in the parts can cause stress corrosion. In short, the causes of heat treatment defects and cracks are various. Generally speaking, they can be divided into three aspects: before heat treatment, during heat treatment and after heat treatment. Before heat treatment, cracks may occur during heat treatment due to poor design, raw material or blank defects, such as improper material selection, improper technical requirements for heat treatment, unreasonable design such as sharp change of section, acute angle transition and stress concentration at marking, resulting in heat treatment defects. Various defects of raw materials and various processing process defects before heat treatment can also lead to heat treatment defects during heat treatment. After heat treatment, defects related to heat treatment may also occur due to improper subsequent processing procedures or improper use.

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