(1) Quenching. The quenching heating temperature of hypoeutectoid steel (low and medium carbon steel) with carbon content less than 0.8% is 30 ~ 50 ℃ above A3 line. At this temperature, it is maintained for a certain time to change all the steel structure into austenite, and then it is cooled rapidly in water or oil, so that austenite can not be decomposed into pearlite and ferrite, but form martensite. This process is called quenching. However, ordinary low carbon steel with carbon content less than 0.25% is not easy to quench into martensite because of its low carbon content.
Martensite is a solid solution of carbon dissolved in body centered lattice iron. Under the microscope, martensite is white acicular structure. It has high hardness, brittleness and strength, but low plasticity and toughness.
When welding medium carbon steel and some low alloy structural steels, quenching may occur in the near seam area to produce high hardness martensitic belt. In order to avoid martensite structure and prevent cold cracks during welding, the following measures are often taken when welding steel with quenching tendency:
① Preheat to slow down the cooling rate of weld and heat affected zone, which is conducive to avoid quenching structure.
② Larger welding current and lower welding speed can also slow down the cooling rate of the joint, which is helpful to avoid quenching structure and cold cracks.
(2) Tempering. Tempering after quenching can restore the toughness of steel to a certain extent. The tempering temperature is below line a (723 ℃). According to different tempering temperatures, it can be divided into high temperature tempering (400 ~ 650 ℃), medium temperature tempering (250 ~ 400) and low temperature tempering (150 ~ 250 ℃). High temperature tempering can completely eliminate the internal stress in the steel, reduce the strength and hardness of the steel, and improve the plasticity and toughness; After low temperature tempering, the hardness of the steel does not decrease much, or even does not decrease, but the toughness is improved. The purpose of medium temperature tempering is to eliminate internal stress and make the steel have high elastic limit.
If the weldment is heated at 650 ℃ only to eliminate the welding residual stress, it can also be called stress relief tempering.
(3) Conditioning. Some alloy steels and their welded structures are tempered at high temperature immediately after quenching. This continuous heat treatment operation is called quenching and tempering treatment. Through quenching and tempering treatment, the steel can maintain high impact toughness and obtain high strength at the same time. This is beyond the reach of other treatment methods.
(4) Annealing. Heat the steel to above 30 ~ 50 ℃, keep it at this temperature for a period of time, then slowly and evenly cool it to room temperature or lower than a certain temperature, stay for a certain time, and then cool it in the air. This process is called annealing. Annealing can reduce hardness, facilitate cutting, refine grain size and eliminate internal stress. The stress relief annealing of welded structure belongs to low-temperature annealing, and its heating temperature is similar to that of high-temperature tempering, so it is also called stress relief tempering. The temperature is generally 600 ℃ ~ 650 ℃, and the holding time is calculated as 4 ~ 5 minutes per mm thickness (but not less than one hour), and then cooled in air or furnace.
(5) Normalize. The steel is heated to above 30 ~ 50 ℃ and cooled in air after heat preservation. This process is called normalizing. Due to the faster cooling rate in air, the microstructure of steel after normalizing is finer than that after annealing. Therefore, the strength and hardness of the same steel after normalizing are higher than those after annealing.