【Mechanical Equipment Technology Exchange Edition】Mechanical Equipment 【Daily Question】20190726

In pearlitic steel, other properties such as strength and ductility generally do not change simultaneously with the occurrence of thermal brittleness； In contrast, austenitic steels experience changes in properties such as strength and plasticity alongside thermal embrittlement. Thermal embrittlement is primarily related to factors such as chemical composition, service time, plastic deformation caused by creep, new phases, as well as the organizational characteristics and stability of the steel.

In pearlitic steel, other properties such as strength and ductility generally do not change simultaneously with the occurrence of thermal brittleness； In contrast, austenitic steels experience changes in properties such as strength and plasticity alongside thermal embrittlement. Thermal embrittlement is primarily related to factors such as chemical composition, service time, plastic deformation caused by creep, new phases, as well as the organizational characteristics and stability of the steel.

In pearlitic steel, other properties such as strength and ductility generally do not change simultaneously with the occurrence of thermal brittleness； In contrast, austenitic steels experience changes in properties such as strength and plasticity alongside thermal embrittlement. Thermal embrittlement is primarily related to factors such as chemical composition, service time, plastic deformation caused by creep, new phases, as well as the organizational characteristics and stability of the steel.

For pearlitic steel, when the impact value decreases due to the occurrence of thermal brittleness, its plasticity and strength remain unchanged. Only in individual cases do both the elongation rate and the reduction in cross-sectional area decrease. For austenitic steels, when the impact value decreases due to the occurrence of thermal brittleness, the plasticity also tends to decline simultaneously.

For pearlitic steel, when the impact value decreases due to the occurrence of thermal brittleness, its plasticity and strength remain unchanged. Only in individual cases do both the elongation rate and the reduction in cross-sectional area decrease. For austenitic steels, when the impact value decreases due to the occurrence of thermal brittleness, the plasticity also tends to decline simultaneously. Steel used in power plants operates under high temperature and stress conditions; carbides, nitrides, and intermetallic compounds in the solid solution precipitate more rapidly in steels susceptible to thermal embrittlement, thereby accelerating the development of this phenomenon. Therefore, some steels retain quite high impact values even after aging treatment, yet the time at which thermal brittleness occurs during operation **comes earlier**, and this is because stress and plastic deformation accelerate the development of thermal brittleness. The temperature range at which pearlitic steel develops thermal brittleness is 400–500°C; carbon steel only exhibits thermal brittleness when plastic strain is present, while Mn and Cr contribute to the development of this phenomenon ； Cu≤0.5% has no significant effect, while Cu>0.5% accelerates the development of thermal embrittlement ； W, V, etc. are elements that help to slow down the development of thermal embrittlement. The thermal brittleness of annealed steel develops rapidly ； The development of thermal brittleness in quenched and high-temperature tempered steel is slow. 2. Thermal brittleness of austenitic steels: By holding 18–8 stainless steel at temperatures between 500 and 850°C and then testing it at room temperature, the development of its brittleness can be observed. As the carbon content in steel increases, its brittleness also increases. When the tempering temperature is around 900°C, brittleness becomes more severe. Increasing the holding time during tempering will cause Cr-containing carbides to precipitate along the grain boundaries, which likewise leads to embrittlement. In the microstructure of the embrittled steel, a reticular martensite structure has appeared. The formation of this type of structure is due to the precipitation of Cr carbides, which locally depletes the Cr in the solid solution state, thereby resulting in the formation of martensitic structure. In steels containing Ti and Nb, brittleness occurs after tempering at 700°C and 900°C. The development of temper brittleness at 700°C is due to the precipitation of Cr carbides. After tempering at 900℃, carbides of Ti and Nb precipitate, and the increase in brittleness is slow. Steels containing less than 3% Mo will experience an increase in brittleness after tempering at 800–900°C.

In pearlitic steel, other properties such as strength and ductility generally do not change simultaneously with the occurrence of thermal brittleness； In contrast, austenitic steels experience changes in properties such as strength and plasticity alongside thermal embrittlement. Thermal embrittlement is primarily related to factors such as chemical composition, service time, plastic deformation caused by creep, new phases, as well as the organizational characteristics and stability of the steel.

1. For pearlitic steels, when the impact value decreases due to the occurrence of thermal brittleness, their plasticity and strength remain unchanged. Only in individual cases do both the elongation rate and the reduction in cross-sectional area decrease. For austenitic steels, when the impact value decreases due to the occurrence of thermal brittleness, the plasticity also tends to decline simultaneously. Steel used in power plants operates under high temperature and stress conditions; carbides, nitrides, and intermetallic compounds in the solid solution precipitate more rapidly in steels susceptible to thermal embrittlement, thereby accelerating the development of this phenomenon. Therefore, some steels retain quite high impact values even after aging treatment, yet the time at which thermal brittleness occurs during operation **comes earlier**, and this is because stress and plastic deformation accelerate the development of thermal brittleness. The temperature range at which pearlitic steel develops thermal brittleness is 400–500°C; carbon steel only exhibits thermal brittleness when plastic strain is present, while Mn and Cr contribute to the development of this phenomenon ； Cu≤0.5% has no significant effect, while Cu>0.5% accelerates the development of thermal embrittlement ； W, V, etc. are elements that help to slow down the development of thermal embrittlement. The thermal brittleness of annealed steel develops rapidly ； The development of thermal brittleness in quenched and high-temperature tempered steel is slow. 2. Thermal brittleness of austenitic steels: By holding 18–8 stainless steel at temperatures between 500 and 850°C and then testing it at room temperature, the development of its brittleness can be observed. As the carbon content in steel increases, its brittleness also increases. When the tempering temperature is around 900°C, brittleness becomes more severe. Increasing the holding time during tempering will cause Cr-containing carbides to precipitate along the grain boundaries, which likewise leads to embrittlement. In the microstructure of the embrittled steel, a reticular martensite structure has appeared. The formation of this type of structure is due to the precipitation of Cr carbides, which locally depletes the Cr in the solid solution state, thereby resulting in the formation of martensitic structure. In steels containing Ti and Nb, brittleness occurs after tempering at 700°C and 900°C. The development of temper brittleness at 700°C is due to the precipitation of Cr carbides. After tempering at 900℃, carbides of Ti and Nb precipitate, and the increase in brittleness is slow. Steels containing less than 3% Mo will experience an increase in brittleness after tempering at 800–900°C.

In pearlitic steel, other properties such as strength and ductility generally do not change simultaneously with the occurrence of thermal brittleness； In contrast, austenitic steels experience changes in properties such as strength and plasticity alongside thermal embrittlement. Thermal embrittlement is primarily related to factors such as chemical composition, service time, plastic deformation caused by creep, new phases, as well as the organizational characteristics and stability of the steel.

1. For pearlitic steels, when the impact value decreases due to the occurrence of thermal brittleness, their plasticity and strength remain unchanged. Only in individual cases do both the elongation rate and the reduction in cross-sectional area decrease. For austenitic steels, when the impact value decreases due to the occurrence of thermal brittleness, the plasticity also tends to decline simultaneously. Steel used in power plants operates under high temperature and stress conditions; carbides, nitrides, and intermetallic compounds in the solid solution precipitate more rapidly in steels susceptible to thermal embrittlement, thereby accelerating the development of this phenomenon. Therefore, some steels retain quite high impact values even after aging treatment, yet the time at which thermal brittleness occurs during operation **comes earlier**, and this is because stress and plastic deformation accelerate the development of thermal brittleness. The temperature range at which pearlitic steel develops thermal brittleness is 400–500°C; carbon steel only exhibits thermal brittleness when plastic strain is present, while Mn and Cr contribute to the development of this phenomenon ； Cu≤0.5% has no significant effect, while Cu>0.5% accelerates the development of thermal embrittlement ； W, V, etc. are elements that help to slow down the development of thermal embrittlement. The thermal brittleness of annealed steel develops rapidly ； The development of thermal brittleness in quenched and high-temperature tempered steel is slow. 2. Thermal brittleness of austenitic steels: By holding 18–8 stainless steel at temperatures between 500 and 850°C and then testing it at room temperature, the development of its brittleness can be observed. As the carbon content in steel increases, its brittleness also increases. When the tempering temperature is around 900°C, brittleness becomes more severe. Increasing the holding time during tempering will cause Cr-containing carbides to precipitate along the grain boundaries, which likewise leads to embrittlement. In the microstructure of the embrittled steel, a reticular martensite structure has appeared. The formation of this type of structure is due to the precipitation of Cr carbides, which locally depletes the Cr in the solid solution state, thereby resulting in the formation of martensitic structure. In steels containing Ti and Nb, brittleness occurs after tempering at 700°C and 900°C. The development of temper brittleness at 700°C is due to the precipitation of Cr carbides. After tempering at 900℃, carbides of Ti and Nb precipitate, and the increase in brittleness is slow. Steels containing less than 3% Mo will experience an increase in brittleness after tempering at 800–900°C.