Annealing
Fully annealed
Process: Heat the steel component to the critical temperature (different steels have different critical temperatures), usually
Ac3 30-50 ℃, the critical temperature of some alloy steels may vary, and then slowly cooled with the furnace or buried in sand for cooling.
Purpose: Refine grain size, achieve uniform structure, reduce hardness, and fully eliminate internal stress. Complete annealing is suitable for carbon steel with a carbon content (mass fraction) below 0.8%. This helps to improve the plasticity and toughness of steel pipes, enabling them to obtain good comprehensive mechanical properties, and stabilize the size of steel pipes, so that the structure of steel pipes does not change during use.
Spheroidization annealing
Process: Heat the steel to a higher temperature, then cool it to the spheroidization temperature, hold for a period of time, and finally cool it to room temperature.
Purpose: To reduce hardness, improve cutting performance, and prepare for future quenching to reduce deformation and cracking after quenching. Spheroidal annealing is suitable for carbon steel and alloy tool steel with a carbon content of 0.7-1.0%
Stress relief annealing
Process: Generally, furnace cooling is used.
Purpose: To eliminate the internal stress generated during welding and cold straightening of steel parts, and to eliminate the internal stress generated during precision cutting of parts, in order to prevent deformation during future processing and use. Stress relief annealing is suitable for various castings, forgings, welded parts, and cold extruded parts, including seamless carbon steel pipes, which can effectively prevent deformation and cracking of steel pipes after processing.
Normalization
Process
Heat the steel component above the critical temperature (usually 30-50 ℃ above Ac3 or Acm), hold it for a certain period of time, and then cool it in air.
Objective
Improve organizational structure and cutting performance; For parts with low mechanical performance requirements, normalizing is commonly used as the final heat treatment; Eliminate internal stress. Normalization can also refine the grain size of steel, homogenize the internal structure, change the residual stress state, improve the comprehensive performance of steel pipes, and reduce or eliminate the banded structure and mixed grains formed during the deformation process of steel pipes; Eliminating network carbides in eutectoid steel to facilitate spheroidization annealing; Used for pre-treatment of medium carbon steel and alloy structural steel before quenching, to refine grain size and make the structure uniform, reducing steel pipe defects caused by quenching process. Low carbon steel and low-alloy steel can be used as a substitute for annealing to improve the cutting performance of steel pipes, and can be used as the final heat treatment for ordinary structural steel pipes with low requirements.
Quench
Process
Heat the steel parts above the critical temperature (30-50 ℃ above Ac3), keep them warm for a period of time, and then rapidly cool them in water, salt water, or oil (some materials are in air).
Objective
Enable steel components to achieve certain special properties, such as higher strength, elasticity, and toughness, after tempering. However, the quenched structure generally has high hardness and brittleness, and is a product of rapid cooling of austenite. Subsequent tempering treatment is usually required to improve performance.
Quenching treatment is usually divided into different methods such as water quenching, oil quenching, and gas quenching. Water quenching is the process of rapidly immersing steel in water, resulting in the fastest cooling rate and the best quenching effect; Oil quenching is the rapid immersion of steel into oil, with a fast cooling rate, suitable for some special carbon steels; Gas quenching is the process of exposing steel to a gas environment, with a slower cooling rate, and is suitable for some carbon steels that do not require high cooling rates.
Tempering
Process
Heat the quenched steel to a certain temperature and then slowly cool it down.
Pbjective
Eliminate internal stress generated by quenching and improve the toughness and tensile strength of steel (including seamless carbon steel pipes). Tempering treatment is usually divided into several different temperature ranges, such as low-temperature tempering (150-250 ℃), medium temperature tempering (350-500 ℃), and high-temperature tempering (500-650 ℃). Tempering at different temperatures can result in different mechanical properties.
