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Performance description of steel in tempering furnace after tempering

　Published by：2023-4-7　　Categories:Industry news　Viewed by: 3884 times

The properties of quenched steel after tempering depend on its internal microstructure; The microstructure of steel varies depending on its chemical composition, quenching process, and tempering process. Carbon steel can achieve good mechanical properties after tempering between 100 and 250 ℃. The typical changes in mechanical properties of alloy structural steel after tempering in a tempering furnace between 200 and 700 ℃ are shown in Figure 5. From Figure 5, it can be seen that as the tempering temperature increases, the tensile strength of the steel σ B monotonically decreases; yield strength σ 0.3 first slightly increases and then decreases; Reduction of area ψ And elongation δ Continuously improving; The overall trend of toughness (using fracture toughness K1c as an indicator) is upward, but there are two minimum values between 300-400 ℃ and 500-550 ℃, which are correspondingly referred to as low-temperature tempering brittleness and high-temperature tempering brittleness. Therefore, in order to obtain good comprehensive mechanical properties, alloy structural steel is often tempered in three different temperature ranges: ultra-high strength steel is about 200-300 ℃; Spring steel at around 460 ℃; Tempering of quenched and tempered steel at 550-650 ℃. Carbon and alloy tool steels require high hardness and strength, and the tempering temperature generally does not exceed 200 ℃. Alloy structural steel, mold steel, and high-speed steel with secondary hardening during tempering are all tempered within the range of 500 to 650 ℃.

Tempering brittleness is a problem that must be paid attention to during tempering in tempering furnace: low temperature tempering brittleness brittleness occurs in many alloy steels after quenching to martensite at 250~400 ℃. The embrittlement that has already occurred cannot be eliminated by reheating, hence it is also known as irreversible tempering embrittlement. The causes of low-temperature tempering brittleness have been extensively studied. It is generally believed that when quenched steel is tempered at 250~400 ℃, cementite precipitates at the original austenite grain boundary or at the martensite interface, forming a thin shell, which is the main reason for low temperature tempering brittleness. Adding a certain amount of silicon to the steel can delay the formation of cementite during tempering and increase the temperature at which low-temperature tempering brittleness occurs. Therefore, ultra-high strength steel containing silicon can be tempered at 300-320 ℃ without embrittlement, which is beneficial for improving comprehensive mechanical properties.

High temperature tempering embrittlement refers to the phenomenon of embrittlement that occurs when many alloy steels are tempered between 500 and 550 ℃ after quenching, or when they pass through the 500 to 550 ℃ range at a slow cooling rate after tempering above 600 ℃. If reheated to a temperature above 600 ℃ and rapidly cooled, toughness can be restored, hence it is also known as reversible tempering brittleness. It has been proven that impurity elements such as P, Sn, Sb, As in steel tend to migrate towards the original austenite grain boundaries at temperatures ranging from 500 to 550 ℃, leading to high-temperature tempering brittleness; Elements such as Ni and Mn can undergo grain boundary co segregation with impurities such as P and Sb, while Cr promotes this co segregation. Therefore, these elements all exacerbate the high-temperature tempering brittleness of steel. On the contrary, the interaction between molybdenum and phosphorus hinders the segregation of phosphorus at grain boundaries, which can reduce high-temperature tempering brittleness. Rare earth elements also have a similar effect. Rapid cooling of steel after tempering in a furnace above 600 ℃ can suppress phosphorus segregation, and is often used in heat treatment operations to avoid high-temperature tempering

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