empering is a metal heat treatment process that reheats the quenched workpiece to an appropriate temperature lower than the lower critical temperature Ac1 (the starting temperature of pearlite to austenite transformation during heating), and cools it in air, water, oil and other media after holding for a period of time. Or heat the quenched alloy workpiece to an appropriate temperature, hold it for a certain time, and then slowly or rapidly cool it. It is generally used to reduce or eliminate the internal stress in hardened steel parts, or reduce their hardness and strength to improve their ductility or toughness. The quenched workpiece shall be tempered in time, and the required mechanical properties can only be obtained through the coordination of quenching and tempering.

Tempering purpose

Generally, steel is rarely used directly after quenching, because the quenched structure is martensite and residual austenite, and internal stress is generated. Although martensite has high strength and hardness, it has poor plasticity and high brittleness, and is prone to deformation and cracking under the action of internal stress; In addition, the structure after quenching is unstable, which can be slowly decomposed at room temperature, resulting in volume change and workpiece deformation. Therefore, quenched parts must be tempered before use.

The purpose of tempering is: (1) to reduce or eliminate the quenching internal stress; (2) Stable tissue and size; (3) Reduce brittleness and obtain required mechanical properties.

Change of structure and properties during tempering

The microstructure transformation of quenched steel can be divided into four stages: decomposition of martensite (below 200 ℃) → decomposition of retained austenite (200~300 ℃) → formation of cementite (250~400 ℃) → cementite aggregation and growth (above 400 ℃). With the increase of tempering temperature, the internal stress in quenching decreases or is eliminated, the hardness decreases gradually, and the plasticity and toughness increase gradually.

Common tempering methods

Low temperature tempering: tempering of workpiece at 150~250 ℃. The purpose is to maintain the high hardness and wear resistance of the quenched workpiece, and reduce the quenching residual stress and brittleness. Tempered martensite obtained after tempering refers to the structure obtained when quenched martensite is tempered at low temperature. Mechanical property: 58~64HRC, high hardness and wear resistance.

Scope of application: mainly used in various high carbon steel tools, cutting tools, measuring tools, moulds, rolling bearings, carburized and surface hardened parts, etc.

Medium temperature tempering: tempering of workpiece at 350~500 ℃. The aim is to obtain higher elasticity, yield point and proper toughness. Tempered troostite is obtained after tempering, which refers to the duplex structure with extremely fine spherical carbide (or cementite) distributed in the ferrite matrix formed during martensite tempering. Mechanical properties: 35~50HRC, high elastic limit, yield point and certain toughness.

Scope of application: mainly used for springs, clockwork, forging dies, impact tools, etc.

High temperature tempering: tempering of workpieces above 500~650 ℃. The aim is to obtain comprehensive mechanical properties with good strength, plasticity and toughness. Tempered sorbite is obtained after tempering, which refers to the multiphase structure with fine spherical carbide (including cementite) distributed in ferrite matrix formed during tempering of martensite. Mechanical property: 25~35HRC, good comprehensive mechanical property.

Scope of application: widely used for various important stressed structural parts, such as connecting rod, bolt, gear and shaft parts.

Precautions for tempering

① After high temperature tempering, if slow cooling materials are used, temper brittleness will occur, so water and oil are often used for rapid cooling.

② Due to the low hardenability of carbon steel, the cost of normalizing for large size parts is lower than that of quenching and tempering (both are sorbite), and the effect is similar, normalizing is often used to replace quenching and tempering.

③ Large size parts shall be hardened and tempered after rough turning and forming, otherwise, the quenching and tempering effect will not be achieved if the quenching and tempering layer is shallow and then turned off.

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