Tempering is a heat treatment process in which the workpiece is quenched and heated to a temperature below Ac1 (the starting temperature for pearlite to austenite transformation during heating), held for a certain period of time, and then cooled to room temperature.
Tempering is generally followed by quenching, with the aim of:
(a) Eliminate residual stress generated during workpiece quenching to prevent deformation and cracking;
(b) Adjust the hardness, strength, plasticity, and toughness of the workpiece to meet the performance requirements for use;
(c) Stable organization and size, ensuring accuracy;
(d) Improve and enhance processing performance. Therefore, tempering is the last important process for obtaining the required performance of the workpiece. By combining quenching and tempering, the required mechanical properties can be obtained. [2]
According to the tempering temperature range, tempering can be divided into low temperature tempering, medium temperature tempering, and high temperature tempering.
Tempering classification
Low temperature tempering
Tempering of the workpiece at 150-250°
The purpose is to maintain high hardness and wear resistance of quenched workpieces, reduce residual stress and brittleness during quenching
Tempered martensite obtained after tempering refers to the microstructure obtained during low-temperature tempering of quenched martensite. Mechanical properties: 58-64HRC, high hardness and wear resistance.
Application scope: Mainly used in various types of high carbon steel tools, cutting tools, measuring tools, molds, rolling bearings, carburized and surface quenched parts, etc. [1]
Medium temperature tempering
Tempering of the workpiece between 350 and 500 ℃.
The purpose is to achieve high elasticity and yield point, with appropriate toughness. After tempering, tempered troostite is obtained, which refers to the duplex structure of ferrite matrix formed during martensite tempering, where extremely small spherical carbides (or cementite’s) are distributed within the matrix.
Mechanical properties: 35-50HRC, high elastic limit, yield point, and certain toughness.
Application scope: mainly used for springs, springs, forging dies, impact tools, etc. [1]
High temperature tempering
Tempering of workpieces 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 duplex structure of ferrite matrix formed during martensite tempering, where small spherical carbides (including cementite) are distributed within the matrix.
Mechanical properties: 25-35HRC, with good comprehensive mechanical properties.
Application scope: Widely used for various important load-bearing structural components, such as connecting rods, bolts, gears, and shaft parts.
The composite heat treatment process of workpiece quenching and high-temperature tempering is called quenching and tempering. Quenching and tempering can not only be used for final heat treatment, but also for pre heat treatment of some precision parts or induction quenched parts。
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Grace Ma
Post time: Nov-03-2023
