Temper brittleness, as a common phenomenon during metal heat treatment, has a significant impact on the mechanical properties of materials, especially impact toughness. This article will start from the basic concept of temper brittleness, discuss its formation causes in detail, and propose corresponding prevention methods.
1. Basic concept of temper brittleness
Temper brittleness, as the name suggests, refers to the embrittlement phenomenon that occurs when metal is tempered after quenching. According to the tempering temperature range that produces brittleness, temper brittleness can be divided into low-temperature brittleness and high-temperature brittleness. Low temperature temper brittleness, also known as the first type of temper brittleness, mainly occurs in the temperature range of 250 to 400°C; while high temperature temper brittleness, also known as the second type of temper brittleness, mainly occurs in the temperature range of 450 to 650°C.
2. Causes of temper brittleness
a. Low temperature temper brittleness (type 1 temper brittleness)
The reasons for the formation of low-temperature temper brittleness are relatively complex. Currently, there are mainly the following theories:
• Retained austenite transformation theory: It is believed that low temperature temper brittleness is caused when retained austenite transforms into tempered martensite or bainite during the tempering process.
• Carbide precipitation theory: It is believed that low temperature temper brittleness is due to the precipitation of flake carbides from martensite. These carbides precipitate along the interface of martensite strips or sheets, reducing the fracture strength of the grain boundaries and turning them into cracks. Expanded path.
• Impurity segregation theory: It is believed that low-temperature temper brittleness is due to the segregation of impurity elements in steel such as phosphorus, sulfur, arsenic, tin, antimony, etc. at the original austenite grain boundaries, resulting in the weakening of the grain boundaries.
b. High temperature temper brittleness (second type of temper brittleness)
The cause of high-temperature temper brittleness is relatively clear, mainly due to the segregation of trace impurity elements (such as phosphorus, tin, antimony, arsenic, etc.) or alloying elements in the steel to the original austenite grain boundaries during the tempering process. The segregation of these elements at the grain boundaries reduces the fracture strength of the grain boundaries, leading to brittle fracture. In addition, alloying elements such as nickel, chromium, manganese, etc. will also promote the segregation of impurity elements, further aggravating the embrittlement phenomenon.
It can be seen from Figure 1 that during the tempering process of 30Ni-Cr steel, as the tempering temperature increases, the impact toughness reaches minimum values at 300°C and 550°C. The first type of temper brittleness appears near 300℃. After this type of temper brittleness occurs, temper it at a temperature ≥300℃ to avoid the embrittlement temperature range and restore its toughness; if it is again in the range of 250~400℃ Internal tempering will not reduce the toughness of the material, so it can be seen that the first type of temper brittleness is irreversible. As can be seen from Figure 1, the second type of temper brittleness appears near 500°C; after tempering at a temperature ≥650°C, cooling slowly and staying at 500~650°C for a long time will also cause embrittlement, while rapid cooling will not Embrittlement will occur, as shown in Figure 1. After the brittleness of the material that produces the second type of temper brittleness is eliminated, if it is heated to the brittle temperature range again, or if it is slowly cooled in the brittle range for a long time, the brittleness will reappear. It can be seen that the second type of temper brittleness is reversible.
3. Methods to prevent temper brittleness
a. Methods to prevent low temperature temper brittleness
• Reduce the content of impurity elements in steel: Reducing the content of phosphorus, sulfur, arsenic, tin, antimony and other impurity elements in steel through refining and other processes can fundamentally reduce the risk of low-temperature brittleness.
• Adding elements that refine austenite grains: such as niobium (Nb), vanadium (V), titanium (Ti), etc., can refine austenite grains and increase the grain boundary area, thereby reducing impurity elements per unit area The amount of segregation.
• Adjust the tempering process: Using isothermal quenching instead of quenching and high-temperature tempering process can effectively avoid the occurrence of low-temperature brittleness.
• Alloying: Adding appropriate amounts of molybdenum (Mo), tungsten (W) and other alloying elements can reduce low temperature temper brittleness. At the same time, elements such as silicon (Si) and chromium (Cr) can also adjust the temperature range where low-temperature brittleness occurs to avoid the required tempering temperature.
Contact Us
For more information, please contact us at metal@welongpost.com.
