Heat treatment of alloy structural steel forgings. The carbon content of alloy quenched and tempered steel is in the middle carbon range. For steel with less alloy content, the structure after normalizing is mostly pearlite (called pearlitic steel alloy); for steel with more content, it is martensitic structure. (called martensitic steel). In order to facilitate cutting and improve the coarse-grained structure and banded structure caused by improper thermal processing in steel, preparatory heat treatment is required first.
For this reason, for pearlitic steel, it can be heated above Ac3 for normalizing or annealing; for martensitic steel, it should be heated above Ac3 for normalizing, and then tempered at a high temperature below Ac3 to transform its structure into Temper sorbite to reduce hardness. After the forgings are processed into parts, the final heat treatment is carried out. First, the steel is heated to the above quenching to obtain the martensitic structure, and then tempered to obtain the required properties of the forgings. The tempering temperature of quenched and tempered steel is 500-650°C (high temperature tempering).
The heat treatment specification for alloy carburizing steel is to perform quenching + low temperature tempering after carburizing. According to the composition of the steel, the heat treatment method is different (direct quenching, one quenching or two quenching, and then low temperature tempering). The structure of the carburized layer is cryptocicular tempered martensite and uniformly distributed fine-grained carbides. The hardness of the surface is HRC58-63, and the hardness of the core varies with the steel type, and there are structures such as low-carbon tempered martensite, troostite and sorbite respectively, and the hardness is between HRC20-45.
There are two main heat treatment methods for spring steel in forging factories. One is medium temperature tempering after quenching; the other is low temperature stress relief treatment. Which heat treatment method to take depends on the supply status of the steel before the spring is wound.