Tempering is a heat treatment process where a quenched workpiece is reheated to a temperature below A1, held for a specific time, and then cooled to room temperature. Quenched steel should not be used directly; it must undergo tempering, which determines the microstructure and properties of the steel and is a crucial heat treatment step.
3.1 Purpose of Tempering
To Achieve Desired Mechanical Properties
After quenching, the workpiece has high hardness but low ductility and toughness. To meet different performance requirements for various parts, tempering is used to modify the quenched microstructure, adjust hardness, and reduce brittleness, resulting in the desired mechanical properties of the workpiece.
To Stabilize Workpiece Dimensions
The martensite and retained austenite formed during quenching are unstable structures that may decompose over time, causing dimensional and shape changes. Tempering transforms the quenched microstructure into a stable one, ensuring that the workpiece maintains its dimensions and shape during use.
To Reduce or Eliminate Internal Stresses from Quenching
Quenching induces significant internal stress. If not promptly relieved through tempering, these stresses may cause the workpiece to deform or even crack.
3.2 Transformations During Tempering of Quenched Steel
Quenched martensite and retained austenite are metastable phases that decompose into ferrite and carbides when tempered from room temperature to below A1. The specific transformations depend on the tempering temperature:
Decomposition of Martensite (≤200°C)
When tempered below 80°C, no significant microstructural change occurs, except for the clustering of carbon atoms in martensite. Between 80°C and 200°C, martensite begins to decompose, with carbon atoms precipitating as ε-carbides (Fe2.4C), reducing the carbon supersaturation in martensite and decreasing tetragonality. As the tempering temperature is low, only part of the excess carbon precipitates, leaving the martensite as a supersaturated solid solution of carbon in α-Fe. The fine ε-carbides are dispersed along the interfaces of the supersaturated α-solid solution, maintaining a coherent relationship (where the atoms at the phase boundaries are shared by the two crystal lattices). This microstructure, consisting of a less supersaturated α-solid solution and ε-carbides, is called tempered martensite. Due to the fine and highly dispersed nature of the ε-carbides, the hardness of the steel does not decrease significantly when tempered below 200°C. However, the precipitation of ε-carbides reduces lattice distortion, lowering quenching stress and slightly increasing the plasticity and toughness of the steel.
Decomposition of Retained Austenite (200°C–300°C)
Retained austenite is similar to undercooled austenite, so its tempering transformation products are the same as those of undercooled austenite under similar temperature conditions, forming martensite, bainite, or pearlite depending on the temperature.
When steel is tempered between 200°C and 300°C, martensite continues to decompose, and retained austenite begins to transform into lower bainite (200°C–300°C is the lower bainite transformation range). At this temperature range, quenching stress further decreases, but hardness does not significantly drop.
Transformation of Carbides (250°C–450°C)
When tempered above 250°C, the increased diffusion ability of carbon atoms causes ε-carbides to gradually transform into stable cementite. By 450°C, all ε-carbides convert into highly dispersed cementite. The continuous precipitation of carbon lowers the carbon content in the α-solid solution to its equilibrium level, turning it into ferrite, though it remains needle-like in shape. This structure, composed of needle-like ferrite and highly dispersed cementite, is called tempered troostite. The tempered troostite structure of 45 steel is shown in the figure below. At this point, the hardness of the steel decreases, and its toughness and plasticity increase further, with quenching stress nearly eliminated.
Aggregation and Growth of Cementite and Recrystallization of Ferrite (450°C–700°C)
Above 450°C, the highly dispersed cementite gradually spheroidizes into fine particles, and as the temperature rises, these particles grow. Simultaneously, ferrite begins to recrystallize between 500°C and 600°C, transforming from lath or needle-like shapes into polygonal grains.
This structure, consisting of granular cementite distributed on a polygonal ferrite matrix, is called tempered sorbite. The tempered sorbite structure of 45 steel is shown in the figure below. If the temperature is further increased to 650°C–A1, the granular cementite coarsens, forming a microstructure of polygonal ferrite and larger granular cementite, known as tempered pearlite.
The transformation of quenched steel during tempering occurs over different temperature ranges. Even at the same tempering temperature, multiple types of transformations may occur. The properties of tempered steel depend on these microstructural changes, which, in turn, influence its mechanical performance. Generally, as the tempering temperature increases, strength and hardness decrease while ductility and toughness improve, with these changes becoming more pronounced at higher temperatures.
3.3 Types and Applications of Tempering
The primary factor determining steel's microstructure and properties is the tempering temperature. Tempering is categorized into three types based on temperature and resulting microstructure:
Low-Temperature Tempering (150°C–250°C)
Low-temperature tempering produces tempered martensite. The aim is to retain the high hardness and wear resistance of quenched steel while reducing internal stress and brittleness, and improving ductility and toughness. This method is mainly used for high-carbon and alloy steels in cutting tools, measuring tools, cold stamping dies, rolling bearings, carburized parts, and surface-quenched parts. The hardness after tempering is typically between 58–64 HRC.
Medium-Temperature Tempering (350°C–500°C)
This method yields tempered troostite. Its purpose is to achieve high yield strength, elastic limit, and significant toughness. Medium-temperature tempering is primarily used for various elastic components and hot-working dies. The hardness after tempering generally ranges from 35–50 HRC.
High-Temperature Tempering (500°C–650°C)
This method produces tempered sorbite. The goal is to achieve a balance of strength, hardness, ductility, and toughness. When quenching and high-temperature tempering are combined, the process is commonly referred to as "quenching and tempering." It is widely used for critical structural components in the production of automobiles, tractors, and machine tools (such as connecting rods, studs, gears, and transmission shafts). The hardness after tempering generally ranges from 200–330 HBW.
Although the hardness values of steel after normalizing and quenching-tempering are quite similar, critical structural components in production usually undergo quenching-tempering rather than normalizing. This is because the microstructure of tempered sorbite has granular cementite, whereas sorbite obtained from normalizing has lamellar cementite. Therefore, quenched and tempered steel not only exhibits higher strength but also has better ductility and toughness compared to the normalized state.
Quenching and tempering can serve as the final heat treatment process or as a preliminary treatment before surface hardening and chemical heat treatment. Since the hardness of tempered steel is not high, it allows for easy machining and low surface roughness values.
In addition to these three common tempering methods, some high-alloy steels undergo high-temperature softening tempering at 20°C–40°C below A1 to obtain tempered pearlite as an alternative to spheroidizing annealing.
To ensure thorough microstructural transformation during tempering, the workpiece must be held at the tempering temperature for a sufficient time, usually between 1 and 3 hours, depending on material, temperature, thickness, load, and heating method. The cooling method after tempering has little effect on the performance of carbon steel, but to avoid inducing new stresses, workpieces are generally slowly cooled in air after tempering.