Tempering is also called flame matching. A process of metal heat treatment. Reheat the quenched workpiece to a proper temperature lower than the lower critical temperature, and heat treat the metal cooled in air, water, oil and other media after holding for a period of time. Or heat the quenched alloy workpiece to a proper temperature, keep it warm for some time, and then cool it slowly or rapidly. Generally used to reduce or eliminate the internal stress in the quenched steel, or reduce its hardness and strength to improve its ductility or toughness. According to different requirements, low temperature tempering, medium temperature tempering or high temperature tempering can be used. Generally, with the increase of tempering temperature, hardness and strength decrease, ductility or toughness gradually increase.
After quenching, the workpiece has the following characteristics: ① the unbalanced (i.e. unstable) structures such as martensite, bainite and retained austenite are obtained. ② There is a large internal stress. ③ The mechanical properties can not meet the requirements. As a result, steel parts are usually tempered after quenching.
The function of tempering is: ① to improve the stability of the structure, so that the structure of the workpiece will not change in the use process, so that the geometric size and performance of the workpiece remain stable. ② Eliminate the internal stress, so as to improve the performance of the workpiece and stabilize the geometric dimension of the workpiece. ③ Adjust the mechanical properties of steel to meet the use requirements.
The reason why tempering has these effects is that when the temperature increases, the ability of atom activity increases, and the atoms of iron, carbon and other alloy elements in steel can diffuse rapidly, so as to realize the rearrangement and combination of atoms, so as to gradually transform the unstable unbalanced structure into the stable balanced structure. The elimination of internal stress is also related to the decrease of metal strength with the increase of temperature. In general, the hardness and strength of steel decreased and the plasticity increased when tempered. The higher the tempering temperature, the greater the change of these mechanical properties. Some alloy steels with high content of alloying elements will precipitate some fine metal compounds during tempering in a certain temperature range, which will increase the strength and hardness. This phenomenon is called secondary hardening.
It is required that workpieces with different applications shall be tempered at different temperatures to meet the requirements in use. ① Tools, bearings, carburized and quenched parts and surface quenched parts are usually tempered at low temperature below 250 ℃. After low temperature tempering, hardness changes little, internal stress decreases and toughness slightly increases. ② The spring can obtain high elasticity and necessary toughness by tempering at 350 ~ 500 ℃. ③ The parts made of medium carbon structural steel are usually tempered at 500-600 ℃ to obtain a good fit of strength and toughness. The heat treatment process of quenching and high temperature tempering is generally called tempering.
Tempering brittleness: during tempering, the hardness and toughness of quenched steel decrease and increase with the increase of tempering temperature, but there are two troughs in the relation curve between tempering temperature and impact toughness of many steels, one is between 200-350 ℃, the other is between 450-650 ℃. With the increase of tempering temperature, the impact toughness decreases. Tempering brittleness can be divided into the first type and the second type.
The first kind of temper brittleness is also called irreversible temper brittleness. Low temperature temper brittleness mainly occurs when the temper temperature is 250-400 ℃, characterized by:
(1) It has irreversibility; (2) it has nothing to do with the cooling rate after tempering; (3) the fracture is intergranular brittle fracture. There are three reasons: (1) the theory of residual a transformation; 2) the theory of carbide precipitation; 3) the theory of impurity segregation.
Prevention methods
It can’t be eliminated that there is no alloy element which can effectively restrain the temper brittleness.
(1) Reducing the content of impurity elements in steel;
(2) Grain a was refined by Al deoxidization or adding Nb, V, Ti and other alloy elements;
(3) Mo, W, etc. can be added to reduce;
(4) Adding CR and Si to adjust the temperature range (pushing to high temperature);
(5) Isothermal quenching is used instead of quenching and tempering.
The second kind of temper brittleness is also called reversible temper brittleness and high temperature temper brittleness. The temperature is 400-650 ℃,
Features:
(1) Reversible;
(2) It is related to the cooling speed after tempering; after tempering and heat preservation, slow cooling appears, but fast cooling does not appear. After embrittlement, it can be reheated and fast cooling can be eliminated.
(3) It has nothing to do with the state of organization, but M tends to be brittle;
(4) After tempering, the embrittlement is independent of the cooling rate;
(5) The fracture is intergranular brittle fracture.
Factors affecting the second kind of temper embrittlement
(1) Chemical composition (2) a grain size (3) hardness after heat treatment
Mechanism:
(1) When temper embrittlement occurs, Ni, Cr, Sb, Sn, P and so on all tend to converge to the original a grain boundary, and all concentrate on the grain boundary with the thickness of 2-3 atoms. The temper embrittlement increases with the increase of impurity elements. Ni and Cr not only segregate themselves, but also promote the segregation of impurity elements. (2) No segregation of alloy elements and impurity elements was found in the case of quenching without tempering or tempering without embrittlement treatment. (3) Mo can inhibit the segregation of impurity elements to a grain boundary, and it does not.
It is shown that the segregation of sb, Sn, P and other impurity elements to the original a grain boundary is the main reason for the second kind of tempering brittleness, while Ni and Cr not only promote the segregation of impurity elements, but also promote the segregation of themselves, so as to reduce the fracture strength of grain boundary and produce tempering brittleness.
Prevention method
(1) Improve the purity of steel and minimize impurities;
(2) Add some Mo, W and other beneficial alloy elements;
(3) For small size and simple shape parts, the method of quick cooling after tempering is adopted;
(4) The sub temperature quenching (A1 ~ A3) was used to refine the grains and reduce the segregation. After heating, it is a + F (F is fine strip), impurities will be enriched in F, and f has a greater ability to dissolve impurities, which can inhibit the segregation of impurities to a grain boundary.
(5) High temperature thermomechanical treatment can make the grain super fine, increase the area of grain boundary, and reduce the concentration of impurity elements segregation.