Tempering of steel
Tempering is a heat treatment process in which the workpiece is hardened, heated to a temperature below AC1 (the starting temperature of 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 for the purpose of:
(a) Eliminate the residual stress generated during quenching of the workpiece to prevent deformation and cracking;
(b) Adjust the hardness, strength, plasticity and toughness of the workpiece to meet the performance requirements;
(c) Stable structure and size to ensure accuracy;
(d) Improve and improve processing performance. Therefore, tempering is the last important process for the workpiece to obtain the required 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.
Tempering classification
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 is obtained after tempering, 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 high carbon steel tools, cutting tools, measuring tools, molds, rolling bearings, carburized and surface hardened parts, etc.
Medium temperature tempering
Tempering of workpiece at 350 ~ 500 ℃.
The purpose is to obtain high elasticity, yield point and appropriate toughness. Tempered troostite is obtained after tempering, which means that the ferrite matrix formed during tempering of martensite is distributed with extremely fine spherical carbides (or cementites).
Mechanical properties: 35 ~ 50HRC, high elastic limit, yield point and certain toughness.
Scope of application: mainly used for springs, springs, forging dies, impact tools, etc.
High temperature tempering
Tempering of workpiece above 500~650 ℃.
The purpose is to obtain comprehensive mechanical properties with good strength, plasticity and toughness.
Tempered sorbite is obtained after tempering, which means that the ferrite matrix formed by martensite tempering is distributed with fine spherical carbides (including cementite).
Mechanical properties: 25 ~ 35hrc, good comprehensive mechanical properties.
Scope of application: it is widely used in various important stressed structural parts, such as connecting rod, bolt, gear and shaft parts.
The compound heat treatment process of workpiece quenching and high temperature tempering is called quenching and tempering. Quenching and tempering can be used not only for final heat treatment, but also for pre heat treatment of some precision parts or induction quenched parts.
When the steel is tempered at about 300 ℃ after quenching, it is easy to produce irreversible tempering brittleness. In order to avoid it, it is generally not tempered at 250 ~ 350 ℃.
The alloy steel containing chromium, nickel, manganese and other elements is tempered at 500 ~ 650 ℃ after quenching, and the slow cooling is easy to produce reversible tempering brittleness. In order to prevent it, small parts can be rapidly cooled during tempering; Large parts can be made of alloy steel containing tungsten or molybdenum.
Tempering refers to the metal heat treatment process in which the quenched workpiece is reheated to an appropriate temperature lower than the lower critical temperature AC1 (the starting temperature of pearlite to austenite transformation during heating), and cooled in air, water, oil and other media after holding for a period of time (or the quenched alloy workpiece is heated to an appropriate temperature, held for a certain period of time, and then cooled slowly or rapidly).