How To Solve The Problem Of Heat Treatment Deformation Of Large Forgings

Large forgings are forgings and forgings on major equipment such as metallurgy, electric power, vehicles, ships, and chemical industries. For these major equipment, the large-scale silverware industry is the basic and equipment sector, which is absolutely different from the general processing industry.

Large forgings refer to forgings produced with presses of 1,000 or more tonnage.

With the increase in size and quality of large forgings, the effective thickness during heat treatment also changes. Due to the increase in cross section, component segregation is inevitable. Metallurgical defects such as non-metallic inclusions and microscopic voids plus the influence of the latent heat of phase change. The stress generated in the heating and cooling process is large, which can easily cause the darkening and cracking of the workpiece. In the early 1950s, due to the equipment, materials and process technology of the time, the heat treatment of large forgings was mostly normalized and tempered. Later, with the continuous in-depth study of quenching and tempering of large forgings and a large amount of practice showed that quenching and tempering can significantly improve the performance of forgings. The author discusses the heat treatment deformation of large forgings through the analysis of typical quality accidents.

The mechanism of heat treatment deformation

After the large forgings undergo quenching and tempering heat treatment, the austenite structure must be obtained after heating, and then a reasonable quenching medium is selected for cooling according to the specific technical requirements of the forgings. Obtain the required organization and obtain the ideal mechanical properties. Therefore, forgings are subject to the dual effects of heating and cooling during the heat treatment process. When the structure changes during the cooling process, the forging is subjected to the combined effect of thermal stress and structure stress, which will inevitably cause the shape and volume of the forging to change. Among them, the deformation is a problem that often occurs in the heat treatment process, and it cannot be eliminated. Only necessary measures and reasonable methods can be taken. Reduce the deformation of the forgings during the heat treatment process, and control the deformation barrier as much as possible to the required range, minimize the deformation of the forgings and prevent cracking. In the actual heat treatment of forgings, the internal and external factors affecting deformation must be fully considered. After repeated process verification, the best heat treatment process parameters can be determined.

There are usually two forms of deformation of satin parts after heat treatment, namely volume deformation and shape change of forgings. They are introduced separately below,

1) Volume changes: the metallographic structure of the forgings has changed after heat treatment. The difference in the specific volume of various structures will cause the forgings to expand and shrink proportionally, and the volume changes will not affect the shape of the original clamps of the forgings. The volume change is selected by the following two reasons

① The volume change caused by thermal expansion and contraction occurs during the heating and cooling process of the forging, and the temperature of each part inside and outside is different, so the amount of change caused by thermal expansion and contraction is different. Since the temperature of the forgings before and after heat treatment is the same, the effect of specific heat expansion and contraction has no significant influence on the volume change after heat treatment.

② Volume changes caused by tissue transformation. In the heat treatment process of steel forgings, the transformation of the pseudo-structure is bound to occur. Due to the asynchronous transformation of the internal and external organization of the forging, the obtained organization is different. There are differences in the specific volume of various tissues, and the stress generated at this time is called tissue stress. The cooling rate of forgings is fast during quenching, so the difference in specific volume before and after the structure transformation is large. The product structure and character constant of various organizations are different. Especially the volume change of steel in the quenching process is the most obvious. After quenching, the specific volume of martensite increases, while the retained austenite makes the specific volume smaller, so the quenching volume change is directly related to the amount of retained austenite and the amount of undissolved infiltrate