What is Tempering and How Does it Work？

Tempering is a heat treatment process for ensuring the performance of various steel components. The tempering process is applied to components in automotive engines and structural parts in large-scale construction machinery. For instance, automotive transmission gears can accurately transmit power, guaranteeing the smooth operation of the gearbox after quenching and tempering. This article will discuss the basics of tempering.

Overview of Tempering

Tempering is a heat treatment process that involves heating a hardened workpiece to a specific temperature below the Ac1 point, holding it for a certain period, and then cooling it to room temperature. Tempering allows the workpiece to attain the required service properties.

Steel is rarely used immediately after quenching because the resulting microstructure consists of martensite and retained austenite, accompanied by significant internal stresses. Although martensite has high strength and hardness, it possesses poor plasticity and high brittleness, making the component susceptible to deformation and cracking under internal stresses. Furthermore, the quenched structure is unstable and can slowly decompose at room temperature, causing volume changes and workpiece distortion. Therefore, quenched parts must be tempered before being put into service.

Function of Tempering

The purposes of tempering are:

Internal Stress Reduction

Quenched metallic materials contain substantial internal stresses that can lead to part deformation or even cracking. Tempering provides an effective means to release and balance these internal stresses.

For instance, in large quenched dies and molds, if internal stresses are not promptly relieved after quenching, the mold may undergo severe deformation during use, compromising product precision and quality. Tempering ensures a more uniform stress distribution within the mold, thereby guaranteeing its structural stability.

Hardness Adjustment

While quenched metallic materials possess high hardness, they are often excessively brittle, making them unsuitable for many practical applications. Tempering allows the hardness to be precisely adjusted to an appropriate range based on application requirements.

For mechanical components subjected to impact loading, such as hammerheads or excavator buckets, tempering can moderately reduce the hardness while simultaneously enhancing toughness. This improved toughness allows the part to better withstand impact forces during operation without fracturing easily.

Microstructure Stabilization

The microstructure of quenched metallic materials is in a metastable state. Tempering enables the microstructure to transform into a more stable form.

Taking steel as an example, the as-quenched martensite structure decomposes during tempering to form more stable structures like tempered martensite, tempered troostite, or tempered sorbite. These structures exhibit excellent comprehensive mechanical properties, ensuring the long-term stability of the material’s performance during extended service. For example, in the manufacturing of automotive engine crankshafts, the stable structure achieved through tempering guarantees the crankshaft’s long-term, stable operation under high-temperature, high-speed, and high-load conditions, preventing performance degradation or failure due to microstructural instability.

Toughness Improvement

The toughness of a metallic material is a measure of its resistance to fracture when subjected to impact or severe plastic deformation. Tempering significantly enhances the toughness of metallic materials.

For thin-walled parts or components with complex geometries, insufficient toughness after quenching can easily lead to failure during use. Tempering improves the toughness of these parts, enabling them to better adapt to various complex working conditions. In critical aerospace components, for instance, extremely high toughness is required to ensure flight safety, making the tempering process an indispensable step in their manufacturing.

Types of Tempering

According to the tempering temperature, it can be divided into 3 types:

Low-Temperature Tempering (Below 250°C)

Low-temperature tempering produces a tempered martensite microstructure. The objective is to reduce quenching stresses and brittleness. The resulting tempered martensite exhibits high hardness (typically 58–64 HRC), high strength, and excellent wear resistance. Therefore, low-temperature tempering is particularly suitable for components requiring high hardness and wear resistance, such as cutting tools, measuring instruments, rolling bearings, carburized parts, and induction-hardened surfaces.

Medium-Temperature Tempering (250°C – 500°C)

Medium-temperature tempering produces a tempered troostite microstructure. This gives the steel a high elastic limit, relatively high strength and hardness (typically 35–50 HRC), and good plasticity and toughness. Medium-temperature tempering is primarily used for various elastic components and hot work dies.

High-Temperature Tempering (500°C – 650°C)

High-temperature tempering produces a tempered sorbite microstructure. The combined heat treatment process of quenching and high-temperature tempering is known as Quenching and Tempering, or “Conditioning.” After conditioning, the steel possesses excellent overall mechanical properties (typically a hardness of 220–230 HBS). High-temperature tempering is mainly applied to critical machine components made of medium-carbon or low-alloy structural steel, such as crankshafts, connecting rods, bolts, automotive axles, machine tool spindles, and gears.

Temper Brittleness

Generally, when quenched steel is tempered, its strength and hardness decrease, while its plasticity and toughness increase as the tempering temperature rises. However, when tempering is performed within certain temperature ranges, the steel’s impact toughness can significantly decrease. This embrittlement phenomenon, where quenched steel is tempered within specific temperature ranges or cooled slowly through these ranges from the tempering temperature, is called temper brittleness. Temper brittleness is divided into Type 1 and Type 2.

Type 1 Temper Brittleness

This type is an irreversible temper brittleness. The embrittlement occurs when quenched steel is tempered around 300°C and is known as Type 1 or irreversible temper brittleness. Almost all steels exhibit this type of brittleness. It is attributed to the precipitation of discontinuous, thin-shell-like carbides along the boundaries of martensite laths or plates during tempering, which substantially reduces the fracture strength of the grain boundaries. Consequently, workpieces are generally not tempered in the 250°C to 350°C range.

Type 2 Temper Brittleness

This type is a reversible temper brittleness. The brittleness affects alloy steels containing elements like Cr, Mn, and Ni. It occurs when these steels are tempered in the embrittlement temperature range of 400°C–550°C, or when they are slowly cooled through this range after tempering at a higher temperature. This type is called reversible temper brittleness because it can be eliminated by re-tempering above the embrittlement temperature and rapidly cooling. If the steel is subsequently re-tempered in the embrittlement range or slowly cooled through it, the brittleness reappears.

The cause is generally the segregation of impurity elements such as Sb, Sn, and P at the prior austenite grain boundaries. Alloying elements like Ni and Cr promote this impurity segregation and also segregate to the grain boundaries themselves, thereby increasing the susceptibility to temper brittleness.

Prevention methods include:

• Minimizing the content of impurity elements in the steel.
• Adding elements like Molybdenum (Mo), which can inhibit grain boundary segregation.
• For small to medium-sized workpieces, this type of temper brittleness can be suppressed by rapid cooling after tempering.