Annealing

heating the workpiece to a certain temperature, holding it for a sufficient time, and then cooling at an appropriate rate (usually slow cooling, sometimes controlled cooling). One of the most important process parameters of annealing is the highest heating temperature (annealing temperature), the annealing temperature of various steels, depending on the specific annealing purpose, and a certain temperature above Ac3, Ac1 or below the corresponding steel. The annealing temperature of various non-ferrous alloys is below the solidus temperature of the corresponding alloy, above or below the solid solubility temperature. Its purpose is to reduce hardness, improve machinability, reduce residual stress, stabilize size, reduce deformation and crack tendency, refine grain, adjust organization, eliminate organization defects, uniform material organization and composition, improve material performance or prepare for future heat treatment. In actual production, the annealing process is widely used. According to the workpiece requirements for different purposes of annealing, annealing process specifications also have a variety of commonly used full annealing, spheroidizing annealing, and stress relief annealing.

Normalizing

After heating the steel or workpiece austenitizing (Ac3 above 30 to 50 degrees C), keep it for a certain period of time, and then cool it in still air or protective atmosphere to become a heat treatment process of pearlite. The main feature is that the cooling rate is faster than annealing and lower than quenching. The purpose is to refine the grains and homogenize the carbide distribution to improve the strength, hardness and toughness of the steel.

Quenching

the steel is heated to a temperature above the critical temperature Ac3 (hypoeutectoid steel) or Ac1 (hypereutectoid steel), held for a period of time to make it fully or partially austenitized, and then rapidly cooled to below Ms (or near Ms) at a cooling rate greater than the critical cooling rate for martensite (or bainite) transformation heat treatment process. The solid solution treatment of aluminum alloy, copper alloy, titanium alloy, tempered glass and other materials or the heat treatment process with rapid cooling process is usually called quenching. The purpose of quenching is to transform the undercooled austenite into martensite or bainite to obtain martensite or bainite, and then to cooperate with tempering at different temperatures to greatly improve the rigidity, hardness, wear resistance, fatigue strength and toughness of the steel, so as to meet the different requirements of various mechanical parts and tools. It can also meet the special physical and chemical properties such as ferromagnetism and corrosion resistance of some special steels by quenching. According to the different cooling medium and cooling method, the quenching process can be divided into single medium quenching (such as gas quenching, oil quenching, water quenching, salt quenching, etc.), double medium quenching, graded quenching, isothermal quenching, etc.

Tempering

heat treatment process in which the workpiece is hardened and heated to a temperature below Ac1, held for a certain period of time, and then cooled to room temperature. Generally used to reduce or eliminate the internal stress in quenched steel, or reduce its hardness and strength, in order to improve its ductility or toughness. The quenched workpiece should be tempered in time to prevent deformation and cracking, stabilize the organization and size, and ensure accuracy. According to the tempering temperature range, tempering can be divided into low-temperature tempering (generally 150~250 ℃, tempered martensite is obtained after tempering, in order to maintain high hardness and wear resistance of quenched workpieces, reduce quenching residual stress and brittleness, and is mainly applied to various high-carbon steel tools, cutting tools, measuring tools, molds, rolling bearings, carburized and surface quenched parts, etc.), medium-temperature tempering (generally 350~500 ℃, tempered troostite is obtained after tempering in order to obtain higher elasticity, yield point and appropriate toughness. It is mainly used for springs, springs, forging dies, impact tools, etc.) and high temperature tempering (generally 500~650 ℃, and tempered soostite is obtained after tempering in order to obtain better comprehensive mechanical properties of strength, plasticity and toughness. It is widely used in various more important stressed structural parts, such as connecting rods, bolts, gears and shaft parts, quenching and high temperature tempering composite heat treatment process is also called quenching and tempering).