General heat treatment knowledge of steel

Most parts need to undergo heat treatment during manufacturing. Heat-treated steel parts can enhance hardness and strength, improve wear resistance, fully exploit the potential mechanical properties of steel, and also reduce hardness, increase plasticity and toughness, improve the machinability of steel and the internal structure of steel parts, eliminate internal stress caused by processing forces, and enhance the wear resistance, heat resistance, corrosion resistance, fatigue resistance and impact resistance of the steel surface.

Heat treatment can be classified into different types based on its various requirements and purposes, including annealing, normalizing, quenching, tempering and chemical heat treatment (such as carburizing, nitriding and cyaniding).

The following are the main types of heat treatment for steel:

Annealing: Heat the steel parts to a temperature above the critical point, hold for a period of time, and then cool slowly in the furnace or in an insulating material. Annealing can reduce the hardness of steel, improve its processing performance, increase plasticity and toughness, eliminate internal stress, and improve the internal structure.

Normalizing: Heat the workpiece to a temperature above the critical point, hold it for a period of time, and then cool it in the air. After normalizing treatment, steel can obtain a fine microstructure, increase its hardness, strength and toughness, reduce internal stress, and improve processing performance.

Quenching: Heat the workpiece to a temperature above the critical point, hold it for a period of time, and then rapidly cool it in water, brine or oil. After quenching treatment, steel can increase its hardness and strength and enhance its wear resistance.

Tempering: Heat the hardened steel to a temperature below the critical point, hold it for a period of time, and then cool it in air or oil. Tempered steel can eliminate the brittleness and internal stress after quenching, and improve its plasticity and impact toughness.

Quenching and tempering: high-temperature tempering after quenching. It can achieve high toughness and strength, eliminate internal stress, and reduce the deformation of steel.

Chemical heat treatment: Chemical heat treatment includes carburizing, nitriding, and cyaniding. Carburizing saturates the surface of the workpiece with carbon elements, nitriding saturates the surface of the workpiece with nitrogen elements, and cyaniding saturates the surface of the workpiece with both carbon and nitrogen elements, that is, carbo-nitriding.

Common Heat Treatment Methods and Their Applications

Annealing

Full annealing: Heat to 30-50°C above Ac₃ and then cool slowly. It is suitable for various steels with a carbon content of less than 0.8%. After full annealing, the steel not only reduces hardness and eliminates stress, but also refines the grain structure and homogenizes the microstructure.
Partial annealing: Heat between Ac₁ and Ac₃ (Ac₃m) and then cool slowly. It is suitable for all types of steel and is mainly used for high-carbon steel. It has the effect of reducing hardness and eliminating stress.
Spheroidizing annealing: Heat to 10-30°C above Ac₁ or near Ac₁ and perform cyclic changes, that is, cycle several times above Ac₁ (730-740°C) and below Ac₁ (680°C), and finally cool. It is suitable for high-carbon steel and alloy steel as a post-forging annealing process for manufacturing precision parts and tools. Spheroidizing annealing can spheroidize cementite, making it easier to machine.
Low-temperature annealing: Heat to 650-720°C and then cool slowly or rapidly. It is suitable for processed parts and welded structures. It can eliminate internal stress generated during welding and machining, eliminate cold work hardening, improve plasticity and eliminate internal stress.

Quenching

Single quenching: Heat above Ac₃ or Ac₁ by 30 to 50℃, then cool down to room temperature in one go in water, oil or other media. It is suitable for carbon steel and alloy steel.
Double liquid quenching: The heating temperature is the same as above. First, quickly cool in water, then slowly cool in oil or air. It is mainly used for medium carbon steel and high carbon steel. This method can achieve high hardness and prevent excessive cooling speed, which may cause cracks in parts.
Step quenching: The heating temperature is the same as above. Quench into molten salt at a temperature higher than the MH point (the starting point of martensite) and hold for a period of time to ensure uniform temperature across the cross-section of the part without decomposing the austenite. This quenching method is mainly used for carbon steel, alloy steel, and high-speed steel.
Isothermal quenching: The heating temperature is the same as above. Hold in molten salt at a temperature higher than the MH point (200 to 400℃). It is suitable for carbon steel and alloy steel.

Surface hardening

Flame surface hardening: Heat the steel to above Ac₃ with a high-temperature flame, and then cool it by spraying water or emulsion onto the surface of the part or by immersing it in oil. It is suitable for large parts made of carbon steel or alloy steel with a carbon content greater than 0.35%.
High-frequency surface hardening: Heat by current induction to 100-150℃, and then cool it by spraying water or immersing it in oil water. It is mostly used for medium carbon steel.

Tempering (tempering treatment is generally required after quenching)

Low-temperature tempering: By heating to 150-250℃ and then slowly or rapidly cooling, tempered martensite structure is obtained. It is suitable for various steels (such as tools and carburized parts that require high hardness and wear resistance), which can reduce the stress during quenching, eliminate brittleness, and not reduce the hardness of the parts.
Medium-temperature tempering: By heating to 270-450℃ and then slowly or rapidly cooling, troostite structure is obtained. Medium-temperature tempering is also suitable for various steels. Such as various springs, forging dies and parts subjected to impact force, it has high elasticity and sufficient strength and hardness under the condition of maintaining certain toughness.

High-temperature tempering: The temperature is higher than 450℃ but lower than Ac₁, and spheroidite structure is obtained after slow or rapid cooling. It is suitable for various steels, such as shafts, gears and general structural parts. High-temperature tempering can obtain a good comprehensive mechanical property with good plasticity, toughness and strength.

Quenching and tempering involves high-temperature tempering after quenching. It is suitable for the final heat treatment of general medium carbon structural steels before precision machining. It is also used as a pre-heat treatment for important parts made of alloy tool steel before quenching. This method can eliminate internal stress more easily than annealing and can achieve better comprehensive mechanical properties. Moreover, it can reduce deformation during quenching and enable the steel to achieve a higher surface finish after precision machining.