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Effect of alloying elements on vacuum tempering of quenched steel

Alloying elements and strengthening and toughening of steel

The main purpose of adding alloy elements to steel is to make the steel have better properties. For structural materials, the first is to improve

To improve its mechanical properties, it is necessary to have both high strength and sufficient toughness. However, the strength and toughness of materials are often.

It is often a contradiction. Increasing strength often requires sacrificing part of the plasticity and toughness of steel, and vice versa. Therefore, seeking materials with high strength and high toughness is an important part of metal material research.

Effect of alloying elements on the strength of steel

The strength of metal generally refers to the resistance of a metal material to plastic deformation. The higher the stress required for plastic deformation, the higher the strength. Since the actual strength of steel materials is closely related to a large number of dislocations, its mechanical essence is plastic deformation resistance. In order to improve the strength of steel materials, we must focus on improving plastic deformation resistance and preventing the movement of dislocations. Starting from this basic point, the strengthening effects of alloying elements in steel mainly include the following four ways: solid solution strengthening, fine grain strengthening, second phase strengthening and dislocation strengthening. By using these four methods individually or in combination, the strength of steel can be effectively improved.

Effect of alloying elements on vacuum tempering of quenched steel

(1) Improve vacuum tempering stability

During the vacuum tempering process, alloying elements can delay the decomposition of martensite and the transformation of retained austenite, increase the recrystallization temperature of ferrite, and make it difficult for carbides to aggregate and grow, thus improving the resistance of steel to vacuum tempering softening. , that is, the vacuum tempering stability of steel is improved.

Elements that have a strong effect on improving the stability of vacuum tempering include V, Si, Mo, W, Mi, Co, etc. Therefore, when alloy steel and carbon steel with the same carbon content reach the same hardness, the vacuum tempering temperature of alloy steel should be higher than that of carbon steel, and the vacuum tempering time should also be longer, which is beneficial to stress relief, so the alloy steel The plasticity and toughness are better than carbon steel; and when vacuum tempered at the same temperature, the strength and hardness of alloy steel are higher than carbon steel.

(2) Secondary hardening and secondary quenching occur

When high-alloy steel with a high content of strong carbide-forming elements such as W, Mo, and V is vacuum tempered, the hardness does not decrease monotonically with the increase in vacuum tempering temperature. Instead, after a certain vacuum tempering temperature, the hardness decreases. increases and reaches a peak value at a certain temperature (usually around 550°C). This phenomenon of peak hardness at a certain vacuum tempering temperature is called secondary hardening. The cause of secondary hardening is, on the one hand, the highly dispersed alloy carbide particles (such as W2C, Mo2C, VC) precipitated from martensite during high-temperature vacuum tempering. This type of carbide particles is very stable at high temperatures, is not easy to aggregate and grow, and maintains a coherent relationship with the α phase, so that the steel has good high-temperature strength; on the other hand, it is due to the residual aerosol in the vacuum tempering and cooling process. It is caused by the secondary quenching that transforms martensite into martensite. For steels that work at high temperatures, especially for high-speed cutting tools and hot-working mold steels, the secondary hardening phenomenon is extremely important for tool and mold steels that require higher red hardness.

(3) Increase the tendency of vacuum tempering brittleness

When quenched alloy steel is vacuum tempered within a certain temperature range, it shows obvious embrittlement phenomenon. This phenomenon is vacuum temper brittleness. The first type of vacuum temper brittleness between 250 and 400°C is determined by the phase change mechanism itself. It is an irreversible vacuum temper brittleness that cannot be eliminated by heat treatment and alloying and can only be avoided. However, elements such as Mo, W, V, and Al can slightly weaken this type of vacuum temper brittleness; while Mn and Cr promote the development of this type of vacuum temper brittleness. Adding Si, Cr, etc. can shift the temperature of this type of vacuum temper brittleness toward high temperatures. The second type of vacuum temper brittleness that occurs between 450 and 600°C is mainly related to the severe segregation of certain impurity elements and alloy elements themselves on the original austenite grain boundaries. This type of vacuum temper brittleness is common in all types of alloy steels. It does happen, but to varying degrees. This is a reversible vacuum temper brittleness.

According to the effect of alloying elements on the second type of vacuum temper brittleness, alloying elements can be divided into three categories: ① Elements that increase the sensitivity of vacuum tempering brittleness are: Mn, Cr, Ni (when added together with other elements), P , V, etc.; ② Elements that have no obvious effect are: Ti, Zr, Si, Ni (when a single element acts); ③ Elements that reduce the sensitivity of vacuum tempering brittleness are: Mo, W.

In order to prevent the second type of vacuum tempering brittleness in alloy steel, the following methods have been summarized in long-term production practice: ① Quick cooling after vacuum tempering, generally small parts are cooled with oil, and larger parts are cooled with water. However, when the size of the workpiece is too large, it is difficult to prevent brittleness even if it is water-cooled; ② Add alloying elements Mo and W to suppress the second type of vacuum temper brittleness; ③ Improve the metallurgical quality and reduce the content of harmful elements in steel as much as possible.