Thermal brittleness of steel

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The thermal brittleness of steel refers to the phenomenon where, when steel is exposed to a specific temperature range for an extended period of time (usually between 400 and 500°C), its impact toughness decreases significantly upon cooling back to room temperature, resulting in brittle fracture. This phenomenon is not apparent at high temperatures; it becomes evident only after the steel is cooled to room temperature, through room-temperature impact tests, with the impact value possibly dropping by 50%-60% or more compared to the normal value. The causes of thermal brittleness are closely related to the chemical composition of the steel, the holding time, and the heat treatment processes. Specifically, thermal brittleness is mainly caused by the following factors: Chemical composition: Sulfur in steel is the main factor that promotes thermal brittleness. The solubility of sulfur in solid iron is extremely low; it can form a low-melting-point FeS with iron, and the eutectic of FeS and Fe has an even lower melting point. When this low-melting-point eutectic melts at the grain boundaries, it can cause cracks in the steel during heat treatment or use. In addition, elements such as Mn and Cr also contribute to the development of thermal brittleness, whereas elements like W and V help to mitigate it. Insulation time: The longer the steel stays at high temperatures, the greater the likelihood of heat embrittlement developing. Different steel grades require different holding times to develop thermal brittleness; for steels such as low-alloy chromium-nickel steel, manganese steel, and chromium-manganese steel, their impact toughness at room temperature decreases significantly after being held at a constant temperature for 100–200 hours. Heat treatment process: The heat treatment process of steel also affects its thermal brittleness. For example, by holding austenitic steel at a temperature between 500 and 850°C and then testing it at room temperature, the development of its brittleness can be observed. Furthermore, the choice of tempering temperature and time also affects the thermal brittleness of the steel. Measures to prevent thermal brittleness: To prevent thermal brittleness in steel, the following measures can be taken: Control of chemical composition: By adjusting the chemical composition of steel, the content of harmful elements such as sulfur can be reduced, while increasing the content of elements such as W and V that help to mitigate thermal brittleness. Optimize heat treatment processes: Properly control heat treatment parameters such as the heating temperature, holding time, and cooling rate of the steel, in order to avoid keeping the steel at high temperatures for extended periods. Addition of alloying elements: By adding appropriate amounts of alloying elements such as Mn, compounds with high melting points are formed with sulfur, thereby preventing the formation of low-melting-point eutectics. Improving the microstructure of steel: Enhancing the toughness and resistance to thermal embrittlement of steel through methods such as grain refinement and microstructural homogenization. In summary, the thermal brittleness of steel is a complex phenomenon, and both its occurrence and prevention involve multiple factors. In actual production and application, appropriate measures need to be taken based on specific circumstances to prevent the occurrence of thermal brittleness.