Other basic processes for heat treatment of steel

1) Normalizing

Normalizing is a heat treatment process that heats the workpiece to a certain temperature, holds it for a certain time, and takes it out of the furnace to cool in the air.

Main differences between normalizing and annealing:

① The normalizing temperature is higher, and the annealing temperature is lower.

The cooling rate of normalizing is faster than that of annealing, so the strength and hardness of normalizing steel are generally higher than that of annealed steel.

③ The use effect is different, after carburizing treatment, normalizing can eliminate the network cementite, annealing can not. For carbon content below 0.25%, normalizing can improve the hardness, improve the cutting performance, annealing can not be done.

Reinforcement of steel structure

The normalizing period is short, the operation is convenient, the annealing period is long, and the operation is more troublesome (referring to the need to control a certain cooling speed).

2) Quenching

Quenching is a heat treatment process in which the steel is heated above the critical point and cooled at a rate greater than the critical cooling rate to obtain martensite or lower bainite structure. Its purpose is to increase strength and hardness, and to improve certain physicochemical properties. However, due to rapid cooling, internal stress will occur, and its plasticity and toughness are also reduced, so it is usually necessary to be tempered after quenching.

3) Tempering

Tempering is a heat treatment process in which quenched steel is heated to a certain temperature and held for a certain time, and then cooled to room temperature in an appropriate manner. It is the next heat treatment process followed by quenching, and at the same time determines the organization and performance of the steel in the use state, which is related to the service life of the workpiece, so it is a key process.

The purpose of tempering is to reduce or eliminate residual stress, adjust strength and hardness, reduce brittleness, increase plasticity and toughness, and form a more stable organization. The quenched steel is heated to 500~650°C and cooled in the air after holding heat, which is called high temperature tempering. After tempering at high temperature, the steel has good comprehensive effect of strength, plasticity and initial property. The process of quenching and tempering is usually called tempering treatment. Steels with higher strength, such as grade C, D and E steels in Q420 and steels with high strength bolts, are subject to tempering treatment.