The Changes In Mechanical Properties Of Quenched Steel During Tempering

• Mechanical properties of mild steel after tempering:When tempered below 200°C, the strength and hardness will not decrease much, and the plasticity and toughness will basically remain unchanged. This is due to the segregation of carbon atoms without precipitation at this temperature. The solid solution strengthening is maintained.When tempering at a temperature higher than 300°C, the hardness is greatly reduced and the plasticity is increased. This is due to the disappearance of solid solution strengthening, the accumulation and growth of carbides, and the recovery and recrystallization of the α phase. The overall performance obtained is not better than that after low-carbon martensite tempering at low temperature.
• High carbon steel generally adopts incomplete quenching, so that the carbon content in austenite is about 0.5%. After quenching, it is tempered at low temperature to obtain high hardness, and a large number of dispersed carbides are generated to improve wear resistance and refine austenite grains.When the temperature is higher than 300℃, the hardness and strength will decrease obviously, the plasticity will increase, and the impact toughness will decrease to the lowest. This is because the flaky θ carbide precipitates between the martensite bars and fully grows, thereby reducing the impact toughness, while the α matrix increases the plasticity and reduces the strength due to the combined effect of recovery and recrystallization.When tempering below 200℃, the hardness will increase slightly, which is due to the precipitation of dispersed ε(η) carbides, which causes age hardening.
• Mechanical properties of medium carbon steel after tempering:When tempering is lower than 200℃, a small amount of carbides will precipitate, the hardening effect is not great, and the hardness can be maintained without decreasing. When tempering is higher than 300℃, with the increase of tempering temperature, the plasticity increases, and the fracture toughness KIC increases sharply. Although the strength has declined, it is still much higher than that of low carbon steel.
• Tempering brittleness:When some steels are tempered, as the tempering temperature increases, the impact toughness decreases instead. The brittleness caused by tempering is called temper brittleness.
• When tempered at 300°C, the hardness decreases slowly. On the one hand, the further precipitation of carbon will reduce the hardness; on the other hand, the transformation of more retained austenite to martensite in high carbon steel will cause hardening. This causes the hardness to drop gently, and may even rise. It is still brittle after tempering.

The occurrence at 200~350℃ is called the first type of temper brittleness; the occurrence at 450~650℃ is called the second type of temper brittleness.

1. The first type of temper brittleness is irreversible temper brittleness.

When the first type of temper brittleness appears, it can be eliminated by heating to a higher temperature for tempering; if tempering in this temperature range, such brittleness will not appear. Therefore, it is called irreversible temper brittleness. In many steels, the first type of temper brittleness exists. When Mo, W, Ti, Al are present in the steel, the Type I temper brittleness can be weakened or suppressed.

At present, there are many opinions on the cause of the first type of temper brittleness, and there is no conclusive conclusion. It seems that it is likely to be a comprehensive result of multiple reasons, and for different steel materials, it is also likely to be caused by different reasons.

Initially, based on the temperature range of the first type of temper brittleness that happened to coincide with the second transformation of carbon steel when tempering, that is, the temperature range of the retained austenite transformation, the first type of temper brittleness was considered to be retained austenite. Caused by the transformation, the plastic phase austenite will disappear as a result of the transformation. This point of view can well explain the phenomenon that elements such as Cr and Si push the first type of temper brittleness to high temperature and the increase in the amount of retained austenite can enter the first type of temper brittleness. But for some steels, the first type of temper brittleness does not completely correspond to the transformation of retained austenite. Therefore, the retained austenite transformation theory cannot explain the first type of temper brittleness of various steels.

After that, the theory of retained austenite transformation was once again replaced by the carbide thin shell theory. It is confirmed by electron microscope that when the first type of temper brittleness occurs, a thin carbide shell is formed along the grain boundary. Based on this, it is believed that the first type of temper brittleness is caused by the thin carbide shell. It is recognized that the formation of brittle phases along grain boundaries can cause brittle intergranular fractures. The question is how the thin carbide shells observed are formed.

As mentioned earlier, after quenching low and medium carbon steels, lath martensite and thin shell-like retained austenite with high carbon content distributed along the lath boundaries are obtained. When tempering at low temperature, only carbon segregation occurs in the lath martensite with a carbon content of less than 0.2% without precipitation of carbides, while the martensite with a carbon content of more than 0.2% may be uniform in the martensite Disperse and precipitate metastable transition carbides.

When the tempering temperature exceeds 200°C, fine needle-like carbides may also precipitate in the low-carbon martensite. At the same time, the θ-carbide nuclei will be formed at the lath martensite bar boundary and grow into strips of θ-carbide. The formation of this θ-carbide not only relies on the decomposition of retained austenite, but also relies on the dissolution of the dispersed metastable transitional carbides and fine needle-like θ-carbides that have been precipitated in the martensite. This strip-shaped θ-carbide is the thin shell-shaped carbide observed under the electron microscope. It can be seen that for steels with more high-carbon retained austenite in the lath boundary, the retained austenite transformation theory is consistent with the carbide thin shell theory.

When high-carbon martensite is tempered below 200℃, metastable transition carbides are dispersed and precipitated in the flaky martensite, and when the tempering temperature is higher than 200℃, strips will precipitate at the carbon-rich twin interface. Shape χ and θ-carbides. At the same time, the θ-carbides that have been precipitated will re-dissolve. The strips of χ and θ- carbides distributed on the same twin interface will be connected into carbide sheets, so fracture is likely to occur along such a surface, which increases the brittleness of the steel. When the tempering temperature is further increased, the flaky carbides break, aggregate, and grow to become granular carbides, so the brittleness decreases and the impact toughness increases.

The third theory is the theory of grain boundary segregation. That is, impurity elements P, Sn, Sb, As, etc. will be concentrated in the grain boundary during austenitization. The segregation of impurity elements causes the weakening of the grain boundaries and leads to brittle fracture. The segregation of impurity elements in the austenite grain boundary has been confirmed by Auger electron spectrometer and ion probe [43,44]. Mn, Si, Cr, Ni, V can promote the segregation of impurity elements in the austenite grain boundary, so it can promote the development of the first type of temper brittleness. Mo, W, Ti, Al can prevent the segregation of impurity elements in the austenite grain boundary, so it can suppress the development of the first type of temper brittleness.

2. The second type of temper brittleness is reversible temper brittleness.

That is, after embrittlement, if reheated to above 650℃, and then quickly cooled to room temperature, embrittlement can be eliminated. After the embrittlement is eliminated, embrittlement can occur again, so it is called reversible temper brittleness. The chemical composition is a factor that affects the second type of temper brittleness. According to different functions, it is divided into three categories:

• (1) Impurity factors P, Sn, Sb, As, B, S;
• (2) Ni, Cr, Mn, Si, C, which promote the second type of temper brittleness;
• (3) Mo, W, V, Ti and rare earth elements La, Nb, Pr that inhibit the second type of temper brittleness;

Impurity elements must coexist with elements that promote the second type of temper brittleness to cause temper brittleness.

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