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.