Alloy steel tempering heat treatment process

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Tempering is a heat treatment process by which steel (and other metals) are imparted with more beneficial properties, usually involving strengthening of the material. When steel is subjected to other heat treatment processes, the associated heating and rapid cooling (quenching) can cause the material to become brittle. Tempering solves this problem by reintroducing some strength into the assembly to match the increase in hardness due to previous heat treatments such as carbonitriding. The most common steels are alloys of iron and carbon, but there are other alloyed steels, each containing additional alloying elements and having their own unique properties. These alloying elements include chromium (used in stainless steel), manganese, nickel, silicon, aluminum, cobalt, molybdenum and vanadium. Depending on the exact steel alloy in question, tempering is done to achieve different properties and use slightly different methods.

When the quenched steel is tempered, its mechanical properties also change correspondingly due to the change in the structure.

(1) Hardness

Variation of hardness of quenched steel during tempering. The general trend is that the hardness of the steel decreases continuously as the tempering temperature increases. However, when the high carbon steel with more carbon content is tempered at about 100 ℃, the hardness increases slightly, which is caused by the segregation of carbon atoms in the martensite and the precipitation of ε-carbide caused by dispersion hardening. When tempering at 200~300℃, the hardness decreases gently. This is due to the decomposition of martensite on the one hand, which reduces the hardness, and on the other hand, the transformation of retained austenite into lower bainite or tempered martensite, which increases the hardness, which is the result of the combined effect of the two. After the tempering temperature exceeds 300 °C, the coherence relationship is destroyed due to the transformation of ε-carbides into cementite, and the cementite aggregates and grows, so that the hardness of the steel decreases linearly.

Alloying elements in steel can reduce the tendency of hardness decrease during tempering to different degrees and improve tempering stability. Strong carbide forming elements can also precipitate special carbides that are dispersed during high temperature tempering, which significantly increases the hardness of the steel and causes secondary hardening.

(2) Strength and toughness

With the increase of tempering temperature, in general, the strength index yield point (σ s ) and tensile strength (σ b ) of steel continue to decrease, while the plastic index elongation (δ) and area shrinkage (ψ) continue to decrease. rise. When tempering at around 350℃, the elastic limit of steel reaches a maximum value, and when tempering above 400℃, the elongation (δ) and area shrinkage (ψ) of steel increase most significantly. The strength of 45 steel after quenching is not high, and the plasticity is very poor. For example, tempered martensite is obtained by tempering at 200~300 °C, and its strength reaches a maximum value due to the elimination of internal stress; tempering at 350~500 °C, the structure is tempered troostite, and the elastic pole is the highest. Toughness is also good! Tempered at 450~600℃, the obtained structure is tempered sorbite, which has good comprehensive mechanical properties, that is, high strength is matched with good plasticity and toughness.

In general, as the tempering temperature increases, the general trend is that the strength and hardness of the steel decrease, while the ductility and toughness increase. However, in many steels (mainly structural steels), it is found that when the tempering temperature increases, the impact toughness of the steel does not increase continuously, but when tempered in certain temperature ranges, the impact toughness decreases significantly. This embrittlement The phenomenon is called temper brittleness of steel.

(1) The first type of temper brittleness

When the quenched steel is tempered in the range of 250 to 400 °C, the impact toughness is significantly reduced, which is called the first type of temper brittleness, also known as low temperature temper brittleness. Almost all industrial steels have this type of temper brittleness to some extent, and the appearance of brittleness is independent of the rate of cooling during tempering.

The reason for low-temperature tempering brittleness is not very clear, but it is generally believed to be related to the initial nucleation of cementite during the decomposition of martensite, and it is believed to be due to the presence of film-like carbides with a certain critical size at the martensite grain boundaries and result on the formation of subgrain boundaries. It is also believed that the appearance of brittleness is related to the segregation of trace elements such as S, P, Sb, and As at grain boundaries, phase boundaries or subgrain boundaries. In addition, when the retained austenite decomposes, brittle carbides precipitate along grain boundaries, subgrain boundaries or other interfaces, and the disappearance of ductile retained austenite is also an important cause of brittleness. This kind of temper brittleness cannot be eliminated after it occurs, so it is also called irreversible temper brittleness.

In order to avoid the brittleness of low temperature tempering, tempering should generally not be performed in the range of embrittlement temperature (especially the temperature corresponding to the lowest toughness value), or use isothermal quenching process, or add alloying elements such as Mo and W to reduce the first type of tempering brittleness.

(2) The second type of temper brittleness

After tempering in the range of 450~650℃, the impact toughness of quenched steel is significantly reduced after slow cooling, which is called the second type of temper brittleness, also known as high temperature temper brittleness. If this kind of steel that has been tempered and brittle is reheated to a temperature above 650 ℃, and then rapidly cooled, the brittleness disappears. If it is tempered again in the brittle temperature range, and then slowly cooled, the brittleness will reappear, so it is also called It is reversible temper brittleness. The occurrence of such brittleness is closely related to the chemical composition of the steel, tempering temperature, tempering time and cooling rate after tempering. The second type of temper brittleness mainly occurs in alloy structural steel, and carbon steel generally does not appear this kind of temper brittleness.

The generation mechanism of the second type of temper brittleness has not been fully understood yet. Recent studies have pointed out that it is due to the segregation of trace impurity elements such as Sb, Sn, As, P and other trace impurity elements on the prior austenite grain boundaries or in the form of compounds during tempering. Due to precipitation, alloying elements such as Cr, Mn, and Ni in steel can not only promote the segregation of the above impurity elements to the grain boundary, but also segregate to the grain boundary, further reducing the strength of the grain boundary and increasing the brittleness tendency.

Temper stability

The ability of a quenched steel to resist a decrease in hardness during tempering is called tempering stability. Because the alloy element hinders or delays the microstructure transformation of quenched steel during tempering, it can delay the decomposition of martensite and the transformation of retained austenite, increase the recrystallization temperature of ferrite, and make carbide difficult to aggregate. grow, while maintaining a greater degree of dispersion. Therefore, the tempering stability of alloy steel is better than that of carbon steel. For steel with higher tempering stability, higher tempering temperature can be used, the quenching stress can be eliminated more thoroughly, and the comprehensive mechanical properties after tempering can be better.

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