Once a part has been quenched, it must be tempered. This accomplishes two things: First, it relieves the thermal and transformational stresses from quenching. Second, it transforms the hard, brittle martensite to the tougher tempered martensite. For high-alloy steels, it may also convert any residual retained austenite to tempered martensite or bainite.
Tempering is usually conducted in recirculating air furnaces. Depending on the temperature, nitrogen may be used to prevent surface oxidation of the parts. To prevent quench cracking, parts are usually placed in the temper furnace within approximately 60 to 90 minutes after quenching. In fact, in some specifications, the time between quenching and tempering is mandated to be less than 90 minutes.  In general, the higher the carbon content, the more critical it is to get the parts into temper immediately. If the carbon equivalent is greater than 0.52 percent (discussed previously), it is imperative that the parts are tempered as soon as possible. If it is not possible to temper the parts within 90 minutes, it is recommended that the parts be snap-tempered at 121° C (250°F) for two hours per inch of thickness.
The furnaces used for tempering are generally very simple and are typically a box furnace with a recirculating fan. The uniformity is good, because of the recirculation of the air and is typically around ±10 to 15°F around the setpoint. A typical tempering furnace used for tempering steel parts is shown in Figure 1.
Toughness increases as the part is tempered above 150°C. In general, as the toughness increases, the hardness decreases. For high hardness applications, the tempering temperature is kept low, usually between 150 and 200°C. The martensite partially decomposes and forms very fine carbide precipitates . The precipitates that form are transition carbides of epsilon-carbide (ε-carbide) and eta-carbide (η-carbide) . They are not cementite. There is a small increase in toughness, but the matrix remains hard.
When steels are tempered between 200 and 400°C, the martensite precipitates cementite (χ-carbide) and any retained austenite transforms to ferrite and cementite. These carbides are coarse and occur within the plates or laths of martensite. The retained austenite begins to transform above 200°C . There is a slight decrease in toughness associated with tempering in the range of 250 to 400°C, called tempered martensite embrittlement.
Tempering above 400°C results in coarsening of cementite. The martensitic structure is also changed. The laths are now nearly completely ferrite because all the carbon has precipitated out as carbides (Figure 2). Hardness and strength fall rapidly as the tempering temperature is increased above 400°C. Toughness improves significantly. In alloy steels, fine alloy carbides may form. The formation of these fine dispersions of alloy carbides can overcome the softening effect of coarsening cementite carbides and result in an increase in hardness. This increase in hardness is called secondary hardening.
As indicated above, increasing the tempering temperature above 400°C can greatly increase toughness. However, if tin, antimony, or arsenic are present as impurities in the steel, then the steel may become brittle. This is called temper embrittlement. It is caused by the impurities segregating to the grain boundaries .
The tempering reactions in steel are summarized in Table 1.
As the tempering temperature is increased, the hardness of the part will decrease. Generally, parts are tempered for one hour per 25 mm of thickness. Some highly alloyed steels require a double temper to increase toughness and ductility. The change in hardness for plain carbon steels as a function of tempering temperature is shown in Figure 3.
Certain alloying elements retard the rate of softening during tempering. The most effective elements are those that are strong carbide formers, such as chromium, molybdenum, and vanadium (Figure 4). Without these elements, Fe-C alloys will soften rapidly with increasing tempering temperature as shown in Figure 3. This softening is due to the coarsening of cementite with increasing tempering temperature. If the alloying elements are present in sufficient quantity, the carbide-forming elements slow the coarsening of cementite by reducing the diffusion of carbon. These carbide formers also form very fine carbides in the matrix that produce an increase of hardness at higher tempering temperatures. This hardness increase is called secondary hardening.
The presence of these alloying elements slows the process of tempering by retarding the diffusion of carbon. Therefore, it is often necessary to temper for a longer time. Tempering for two hours per inch of thickness is not uncommon.
Figure 4 shows the differences in retarding hardness for different alloying elements. The strong carbide formers do not show much effect until higher temperatures are reached. Nickel has a small effect on tempered hardness since it is not a carbide-forming element. Manganese has a small effect at lower temperatures, but at higher temperatures has a much stronger effect, due to being incorporated into carbides at elevated temperatures.
If the steel contains a significant amount of austenite stabilizers, such as nickel, austenite often remains at the tempering temperature. This can occur during martempering, or even normal quench and temper if the temperature after quenching is still above the martensite finish temperature. When this occurs, the retained austenite must be converted to martensite, or to bainite. This is accomplished by performing a double tempering operation. Often the second temperature is about 10°C or so below the first tempering temperature. This also allows the part to hit the desired hardness, by controlling the final tempering temperature, and adjusting it up or down, depending where the hardness was at the end of the first tempering cycle.
In this article, we have discussed the type of equipment used for tempering, and the various reactions that occur during tempering, as a function of temperature.
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