Thread Content
Annealing of steel 1. Concept: A heat treatment process in which steel is heated to an appropriate temperature, held there for a certain period of time, and then cooled slowly in a furnace. 2. Purpose: 1) Reduce hardness, increase plasticity, and improve workability ； 2) Refine grains and eliminate structural defects ； 3) Eliminate internal stress. 3. Classification: Depending on the composition of the steel and the purpose of the treatment, it can be divided into full annealing, spheroidizing annealing, and stress-relief annealing. 1. Complete annealing: The steel part or blank is heated to 20°C–30°C above Ac3, held at that temperature for a certain period of time to allow the structure within the steel to fully transform into austenite. After that, it is cooled slowly (usually by cooling it in the furnace) to below 500°C–600°C, after which it is cooled in air. By “complete,” it means achieving a fully austenitic microstructure upon heating. 1) Purpose of full annealing Improve the coarse and uneven microstructure resulting from hot processing ； Medium-carbon and higher carbon steels as well as alloy steels have their hardness reduced in order to improve their machinability; generally, it is easy to machine workpieces with a hardness between 170HB and 230HB, while machinability becomes difficult when the hardness is above or below this range ； Eliminate the internal stresses in castings, forgings, and welded parts. 2) Scope of application Fully annealing is mainly applicable to carbon steels, alloy steels, as well as engineering castings, forgings, and hot-rolled profiles with a carbon content of 0.25%–0.77%, which are hypoeutectoid in composition. Hyper-eutectoid steel should not be subjected to complete annealing, because when such steel is heated above Accm and cooled slowly, secondary carbides will precipitate in a network pattern along the austenite grain boundaries, resulting in a significant reduction in the steel’s strength, ductility, and impact toughness. 2. Isothermal annealing: The steel part or blank is heated to a temperature 20°C–30°C above Ac3 (or Ac1), held at that temperature for a certain period of time, then cooled rapidly to a temperature near the \"peak\" of the isothermal transformation curve for supercooled austenite, and held there again (in the pearlite transformation zone) to allow the austenite to transform into pearlite. After that, it is cooled slowly. This type of heat treatment is known as isothermal annealing. The purpose of isothermal annealing is the same as that of full annealing, but the transformation during isothermal annealing is easier to control, allowing for the attainment of a uniform desired microstructure. It is suitable for large components and alloy steel parts, and can shorten the annealing cycle. Fully annealed structure of 45 steel (F+P), 200×; Spheroidized annealed structure of T12 steel, 500×3. Spheroidized annealing: This is a process in which the steel part or blank is heated to a temperature slightly above Ac1, held at that temperature for an extended period of time to allow the secondary cementite in the steel to transform into granular (or spherical) cementite, and then cooled slowly to room temperature. (1) Purpose of spheroidizing annealing: to reduce hardness, homogenize the microstructure, improve machinability, and prepare for quenching. (2) Scope of application for spheroidizing annealing Spheroidizing annealing is mainly applicable to eutectoid steels and hypereutectoid steels such as carbon tool steel, alloy spring steel, rolling bearing steel, and alloy tool steel (with a carbon content greater than 0.77%). 4. Diffusion annealing: To reduce the unevenness in the chemical composition and microstructure of steel ingots and castings, they are heated to a temperature slightly below the solidus line (100°C to 200°C below the melting point of steel), held at that temperature for an extended period of time, and then cooled slowly, thereby homogenizing their chemical composition and microstructure. Due to the high heating temperature in diffusion annealing, the grains become coarse after annealing; complete annealing or normalizing can be used to refine these grains. Stress-relief annealing and recrystallization annealing: Stress-relief annealing is also known as low-temperature annealing. It is a processing method in which steel is heated to 400°C–500°C (below the Ac1 temperature), held at that temperature for a period of time, and then slowly cooled to room temperature. Its purpose is to eliminate the internal stresses resulting from processing such as casting, forging, welding, and cold deformation. It is widely used because of its low stress-relief annealing temperature and the fact that it does not alter the original structure of the workpiece. 6. Recrystallization annealing Recrystallization annealing is primarily used to eliminate the distorted microstructure resulting from cold deformation processes such as cold rolling, cold drawing, and cold stamping, as well as to reduce work hardening through low-temperature annealing. The heating temperature is 150°C to 250°C above the recrystallization temperature (the temperature at which deformed grains recrystallize into undeformed grains). Recrystallization annealing allows the grains that have been elongated after cold deformation to re-nucleate and grow into uniform equiaxed grains, thereby eliminating the effect of work hardening.