Tempering is a heat treatment process in which a quenched workpiece is reheated to an appropriate temperature below the critical temperature Ac₁, held at that temperature for a certain period, and then cooled at a specific rate to enhance the material’s toughness. It is a critical process that determines the microstructure and properties of steel in its service state.
Although quenched steel (e.g., steel with martensitic structure) exhibits extremely high hardness and wear resistance, it has two key issues: first, a large amount of residual quenching internal stress remains inside, which easily causes the steel part to crack or deform; second, it has extremely poor toughness and high brittleness, making it unable to withstand impact or complex loads. By adjusting the heating temperature and holding time, tempering specifically addresses these problems, with the following specific objectives:
- Eliminate internal stress: Heating provides steel atoms with a certain degree of mobility, relieving the internal stress generated by the rapid microstructural transformation during quenching (e.g., rapid cooling of austenite to martensite) and reducing the risk of cracking during subsequent processing or service of the steel part.
- Adjust mechanical properties: "Customize" the properties of steel according to requirements. For applications requiring high hardness (e.g., cutting tools, dies), low-temperature tempering can be selected to retain most of the quenched hardness; for applications requiring high toughness (e.g., shafts, gears), high-temperature tempering can be used to significantly improve toughness while reducing some hardness.
- Stabilize microstructure and dimensions: Transform the unstable quenched microstructure (e.g., martensite, retained austenite) into a more stable one (e.g., tempered martensite, sorbite), preventing dimensional fluctuations caused by continued microstructural changes during long-term service and ensuring precision.
- Improve machinability: Reduce the hardness of quenched steel to facilitate subsequent mechanical processing such as cutting and grinding (e.g., tool steel becomes easier to grind after tempering).
- Steel parts cannot be used directly after quenching and must be tempered promptly.
- Adjust the hardness, strength, plasticity, and toughness of the workpiece to meet service performance requirements. Quenched steel generally has high hardness and brittleness, and tempering can adjust its hardness and toughness.
- Eliminate or reduce residual internal stress generated during quenching to prevent deformation or cracking.
- Stabilize microstructure and dimensions to ensure precision. The microstructure of quenched steel consists of quenched martensite and retained austenite, both of which are metastable and tend to transform into stable structures (i.e., spontaneously transform into ferrite and cementite). This transformation can cause changes in the workpiece’s size and shape. Tempering converts quenched martensite and retained austenite into more stable microstructures, ensuring no changes in size or shape during service.
The tempering process involves four types of reactions: decomposition of martensite; precipitation, transformation, aggregation, and growth of carbides; recovery and recrystallization of ferrite; and decomposition of retained austenite.
When quenched steel is heated below 200°C, fine ε-carbides (FeₓC) begin to precipitate from martensite, reducing the supersaturation of martensite and partially eliminating internal stress. The microstructure composed of these ultra-fine ε-carbides and low-saturation α-solid solution is called tempered martensite (Mₜₑₘₚ).
After quenched martensite transforms into tempered martensite, its volume shrinks, reducing the pressure on retained austenite. At this point, retained austenite decomposes to form a bainitic structure.
This transformation is relatively obvious in medium-carbon and high-carbon steels. For carbon steels and low-alloy steels with a carbon content below 0.4%, the amount of retained austenite is so small that this transformation is essentially negligible. Therefore, the tempered microstructure remains tempered martensite at 200–300°C.
When the temperature exceeds 250°C, martensite decomposition is complete, and its tetragonality disappears. ε-carbides transform into fine, stable cementite (Fe₃C), and most of the quenching internal stress is eliminated. Tempering between 300–500°C produces a mixed microstructure consisting of a ferrite matrix and a large number of finely dispersed granular cementite, known as tempered troostite (Tₜₑₘₚ).
When the tempering temperature reaches above 400°C, lamellar ferrite undergoes polygonization, while fine granular Fe₃C spheroidizes and aggregates to grow. Quenching internal stress is completely eliminated. Tempering between 500–650°C results in a microstructure composed of a polygonal ferrite matrix with relatively large granular cementite distributed throughout, called tempered sorbite (Sₜₑₘₚ).
Tempering is classified into low-temperature tempering, medium-temperature tempering, and high-temperature tempering based on the temperature range.
- Tempering temperature: 150–250°C
- Microstructure: Tempered martensite (Mₜₑₘₚ) — a mixture of ultra-fine ε-carbides and low-saturation α-solid solution. Under an optical microscope, tempered martensite appears black, while retained austenite appears white.
- Purpose: Maintain high hardness (typically 58–64 HRC) and wear resistance of the quenched workpiece, while reducing residual quenching stress and brittleness.
- Applications: Mainly used for heat treatment of high-carbon steel tools, cutting tools, measuring tools, dies, rolling bearings, carburized parts, and surface-quenched parts.
- Tempering temperature: 350–500°C
- Microstructure: Tempered troostite (Tₜₑₘₚ) — a microstructure of fine granular cementite distributed on a ferrite matrix that retains the martensitic morphology.
- Purpose: Achieve high elastic limit, yield strength, and a certain degree of toughness; most internal stress is eliminated, with a hardness of 35–45 HRC.
- Applications: Primarily used for heat treatment of springs, clockwork springs, forging dies, and impact tools.
- Tempering temperature: 500–650°C
- Microstructure: Tempered sorbite (Sₜₑₘₚ) — a microstructure of granular Fe₃C distributed on a polygonal ferrite matrix.
- Purpose: Obtain excellent comprehensive mechanical properties (balanced strength, plasticity, and toughness); internal stress is completely eliminated, with a hardness of 25–35 HRC. Since tempered sorbite has good comprehensive mechanical properties, the combination of "quenching + high-temperature tempering" is commonly referred to as quenching and tempering (QT), or simply "tempering" in a broad sense.
- Applications: Quenching and tempering are mainly used as the final heat treatment for various important structural parts (e.g., connecting rods, shafts, and gears subject to alternating loads and high fatigue resistance requirements). It is also often used as the preheat treatment for surface-quenched parts, nitrided parts, precision cutting tools, measuring tools, and dies.
It must be noted that the above temperature ranges for tempering processes apply to carbon steels and low-alloy steels, and are not applicable to medium and high-alloy steels with high alloy element content.
The impact toughness of quenched steel changes with tempering temperature. As the tempering temperature increases, the hardness of the steel tends to decrease, while toughness generally increases. However, two minimum values appear in the temperature ranges of 250–400°C and 450–650°C. This phenomenon is called temper embrittlement and is divided into low-temperature temper embrittlement and high-temperature temper embrittlement.
This refers to the brittleness exhibited by quenched steel when tempered at 250–400°C. Since the embrittlement once formed cannot be eliminated by reheating, it is also called irreversible temper embrittlement. The main cause is that during tempering in this temperature range, martensite decomposes, and cementite precipitates at grain boundaries, reducing the grain boundary fracture strength and destroying the continuity of the matrix. Almost all steels exhibit this type of temper embrittlement, and there is currently no effective method to completely eliminate it. Therefore, quenched steel is generally not tempered in the range of 250–350°C.
This refers to the brittleness exhibited by quenched steel when slowly cooled after tempering in the range of 450–650°C. Toughness can be restored if the steel is reheated to above 600°C and rapidly cooled, so it is also called reversible temper embrittlement.
This type of embrittlement mainly occurs in structural steels containing alloying elements such as Cr, Ni, Si, and Mn. A key characteristic is that rapid cooling (oil cooling) after tempering does not cause brittleness, while slow cooling (air cooling) does. When these steels must be tempered at high temperatures, they are usually heated to above 600°C and rapidly cooled. Of course, rapid cooling from this temperature does not cause hardening, as austenitization does not occur.
In general, to obtain good comprehensive mechanical properties, alloy structural steels are often tempered in three different temperature ranges: ultra-high-strength steels at approximately 200–300°C; spring steels around 460°C; and quenched-and-tempered steels at 550–650°C. Carbon and alloy tool steels, which require high hardness and strength, are generally tempered at temperatures not exceeding 200°C. Alloy structural steels, die steels, and high-speed steels are all tempered in the range of 500–650°C.
- Tempering is meaningless for untreated quenched steel; therefore, it is used as a final heat treatment process in conjunction with quenching.
- To prevent deformation or cracking of quenched parts during storage, steel parts must be tempered promptly after quenching.
- Insufficient tempering can be compensated by an additional appropriate tempering process; however, if over-tempering occurs, all previous efforts will be wasted, and the part must be quenched again.
- Tempering is not a hardening method; on the contrary, it involves reheating heat-treated hardened steel to relieve stress, soften the material, and improve plasticity.
- The microstructural changes and property modifications caused by tempering depend on the temperature to which the steel is reheated. The higher the temperature, the more significant the effects. Therefore, the choice of temperature usually depends on the extent to which hardness and strength are sacrificed to gain plasticity and toughness.
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