Tempering reheats the quenched workpiece to an appropriate temperature below the lower critical temperature Ac1 (the starting temperature of pearlite to austenite transformation during heating), and the metal cooled in air or water, oil and other media after holding for a period of time heat treatment process. Or heat the quenched alloy workpiece to an appropriate temperature, hold it for a period of time, and then cool it slowly or rapidly. Generally used to reduce or eliminate internal stress in quenched steel, or reduce its hardness and strength to improve its ductility or toughness. The quenched workpiece should be tempered in time, and the required mechanical properties can be obtained through the combination of quenching and tempering.
Tempering is generally followed by quenching, the purpose of which is to:
(a) Eliminate the residual stress generated when the workpiece is quenched to prevent deformation and cracking;
(b) Adjust the hardness, strength, plasticity and toughness of the workpiece to meet the performance requirements;
(c) Stable organization and size to ensure accuracy;
(d) Improve and enhance processability.
Therefore, tempering is the last important process for the workpiece to obtain the desired properties. Through the combination of quenching and tempering, the required mechanical properties can be obtained.
According to the tempering temperature range, tempering can be divided into low temperature tempering, medium temperature tempering and high temperature tempering.
1. 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
After tempering, tempered martensite is obtained, which refers to the structure obtained when quenched martensite is tempered at low temperature. Mechanical properties: 58～64HRC, high hardness and wear resistance.
Scope of application: Mainly used in various types of high carbon steel tools, cutting tools, measuring tools, molds, rolling bearings, carburized and surface-hardened parts, etc.
2. Medium temperature tempering
Tempering of workpieces between 350 and 500 °C.
The purpose is to get higher elasticity and yield point, and proper toughness. After tempering, tempered troostite is obtained, which refers to the complex phase structure in which extremely fine spherical carbides (or cementite) are distributed in the ferrite matrix formed during martensite tempering.
Mechanical properties: 35～50HRC, higher elastic limit, yield point and certain toughness.
Application: Mainly used for springs, forging dies, impact tools, etc.
3. High temperature tempering
Tempering of workpiece above 500~650℃.
The purpose is to obtain comprehensive mechanical properties with good strength, plasticity and toughness.
After tempering, tempered sorbite is obtained, which refers to the complex phase structure in which fine spherical carbides (including cementite) are distributed in the ferrite matrix formed during martensite tempering.
Mechanical properties: 25～35HRC, good comprehensive mechanical properties.
Scope of application: Widely used in various important stress-bearing structural parts, such as connecting rods, bolts, gears and shaft parts.
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