Temper Brittleness Of Steel
1. Temper brittleness refers to the phenomenon that the toughness of the quenched steel decreases after tempering. When the quenched steel is tempered, the hardness decreases and the toughness increases as the tempering temperature increases. However, in the relationship between tempering temperature and impact toughness of many steels, two valleys appear, one between 200 and 400 °C, which is low temperature temper brittleness; the other is between 450 and 650 °C, which is high temperature tempering. brittleness. In the tempering brittle temperature range, as the tempering temperature increases, the impact toughness decreases.
Alloy steel quenched to obtain martensite structure, tempered in the temperature range of 450 ~ 600 ° C; or tempered at 650 ° C after a slow cooling rate of 350 ~ 600 ° C; or tempered at 650 ° C, 350 ~ 650 ° C Long-term heating in the temperature range causes embrittlement of the steel. If the already brittle steel is reheated to 650 ° C and then cooled quickly, the toughness can be restored, so it is also called “reversible temper brittleness”.
2. Causes of temper brittleness: It is due to the segregation of impurity elements at the grain boundaries, which reduces the fracture strength of the grain boundaries and produces temper brittleness.
3. Mechanism of high temperature temper brittleness: 1 When temper brittleness occurs, Ni, Cr, Sb, Sn, P, etc. are all segregated to the original A (austenite) grain boundary (the main type of temper brittleness is produced) The reason) is concentrated on the grain boundary of 2~3 atomic thickness, and the temper brittleness increases with the increase of impurity elements. Ni and Cr not only segregate themselves but also promote segregation of impurity elements. 2 If quenching is not tempered or tempered without embrittlement, no segregation of alloying elements and impurity elements is observed. 3 The alloying element Mo can suppress the segregation of the impurity element to the A grain boundary, and it is not segregated by itself. Carbon also plays a catalytic role. Generally, carbon steel is not sensitive to high temperature temper brittleness, and binary or multi-alloy steels containing chromium, manganese, nickel and silicon are sensitive, and the brittleness sensitivity varies depending on the type and content of the alloy elements.
4. Methods to prevent temper brittleness:
(1)Improve the purity of steel and minimize impurities;
(2)Adding an appropriate amount of beneficial alloying elements such as Mo and W; when the molybdenum content in the steel is increased to 0.7%, the tendency of high-temperature temper embrittlement is greatly reduced, and a special carbide rich in molybdenum is formed in the steel beyond this limit. The molybdenum content in the matrix is reduced, and the embrittlement tendency of the steel is increased. For the long-term work in the high-temperature temper embrittlement zone, it is difficult to prevent embrittlement by adding molybdenum alone. Only the content of impurity elements in the steel is lowered, and the purity of the steel is improved.
(3)For parts with small size and simple shape, use the method of quick cooling after tempering;
(4)Sub-temperature quenching (lower than A1~A3 temperature line): Refine the grains and reduce the segregation of impurities. After heating, it is A+F (F is ferrite, thin strip shape), impurities are concentrated in F, and F has a large ability to dissolve impurity elements, and can suppress segregation of impurity elements to A grain boundaries.
(5)Using high temperature deformation heat treatment, the crystal grains are ultra-fine, the grain boundary area is increased, and the concentration of impurity element segregation is reduced.
(6) After high-temperature tempering, it is cooled by oil cooling or water to suppress the segregation of impurity elements at the grain boundary.
5. Tempering or stress relief annealing of die-casting molds should be avoided as much as possible within the high temperature brittleness range.