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Large forgings refer to parts with larger volume or heavier weight, which are usually forged directly from steel ingots. In the process of machining, the forgings need to go through several steps such as blank grinding, preliminary heat treatment, rough machining, final heat treatment, and finishing. To reduce its internal stress and improve its structure and properties, we will strictly follow the process requirements in terms of heating temperature and operation methods, and prepare for the final heat treatment in advance. The equipment for heat treatment is usually medium frequency induction heating equipment, and the final heat treatment process of forgings includes quenching, normalizing, high temperature tempering, etc.
The purpose of tempering treatment for large forgings is to eliminate or reduce the internal stress of the forgings after quenching or normalizing treatment, so that a stable tempered structure can be obtained to meet the technical requirements of forgings in heat treatment. The temperature at which the medium frequency induction heating equipment tempers large forgings should be determined according to its performance, organizational requirements, and shape.
Large-scale forgings will undergo post-forging heat treatment immediately after forging. On the one hand, austenite is fully transformed into ferrite-carbide structure, and dehydrogenation annealing is performed to reduce the residual stress of forgings; on the other hand, through crystallization Annealing treatment, refines grains, improves the structure of forgings, and improves its performance. The temperature for quenching or normalizing large forgings should be determined according to its chemical composition and size. In order to avoid large thermal stress, an appropriate heating rate will be selected. After many experiments, it was found that water cooling is a more common and effective method for cooling large forgings, because the strength, hardness and toughness of the water-cooled forgings have been greatly improved after being tempered by medium frequency induction heating equipment.
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