Heat treatment is the soul that imparts intrinsic quality to machinery. Currently, the situation is as follows: the technical personnel in state-owned enterprises are aging and experiencing significant attrition, while there are very few heat treatment technicians in existing private enterprises. Newly established private companies require a large number of technical personnel. As a result, domestic heat treatment technology still lags behind that of foreign countries. However, the demand for the development of heat treatment in China is also substantial. Let the machinery rise to self-improvement!

Heat treatment of metals and other materials is one of the key processes in mechanical manufacturing. Unlike other processing techniques, heat treatment generally does not change the shape and overall chemical composition of the workpiece. Instead, it modifies the internal microstructure or alters the surface chemical composition of the workpiece to impart or enhance its performance. Its characteristic lies in improving the intrinsic quality of the workpiece, which is often not visible to the naked eye. Without good heat treatment, even the most visually appealing materials are merely superficial. Everyone hopes that the tools they use will not be brittle but instead will perform exceptionally well! But how can we achieve this? Please take a seat and read carefully!

What is Heat Treatment?

In simple terms, heat treatment involves heating materials to a specified temperature, maintaining that temperature for a certain period, and then cooling them at a controlled rate to room temperature or lower. This process improves the material's microstructure to obtain superior performance. Generally, we refer to the treatment of metal materials.

The Importance of Heat Treatment

Heat treatment is crucial for improving the quality and performance of materials from within. It enhances mechanical properties, eliminates residual stresses, and improves the machinability of metals. These benefits are often not visible to the naked eye.

Historical Insights into Heat Treatment

When did humanity gain insight into heat treatment? The understanding of its importance gradually emerged during the transition from the Stone Age to the Copper Age and then to the Iron Age. The invention of the "annealing process" marked the beginning of human engagement with metal heat treatment.

By the sixth century BC, iron weapons were increasingly adopted. To enhance the hardness of steel, the "quenching process" rapidly developed in China.

Common Heat Treatment Methods

Annealing and Normalizing

The purposes of annealing and normalizing are to homogenize the composition of steel, refine the grain structure, improve the microstructure, eliminate processing stresses, reduce hardness, and enhance machinability. These processes serve as preparatory heat treatments for subsequent cold or hot working or other heat treatment steps.

For steel parts with lower performance requirements, normalizing can be used as the final heat treatment process.

Quenching and Tempering

Quenching is the most critical step in heat treatment for strengthening materials, aimed at achieving high strength and hardness in steel.

During the tempering process, the hardness and strength of quenched steel gradually decrease, while plasticity and toughness improve. This process also reduces and eliminates residual stresses, preventing cracking.

In other words, quenching followed by tempering results in excellent overall mechanical properties and maintains dimensional stability during use.

Quenching can be combined with various tempering processes, with the combination of quenching and high-temperature tempering referred to as "quenching and tempering treatment."

Surface Hardening

Surface hardening allows the surface layer of a workpiece to achieve high hardness, wear resistance, and fatigue strength, while the core retains good toughness and plasticity.

Three Essential Heat Treatment Techniques

Heat treatment distortion in workpieces can be categorized based on the timing of its occurrence: distortion during quenching (quenching distortion) and distortion that occurs during the period after heat treatment (time effect distortion). It can also be classified by form: shape distortion (geometric warping, twisting, bending) and volume distortion (expansion or contraction). In practice, these two forms of distortion rarely exist in isolation; they often occur simultaneously due to factors such as the composition of steel, the shape of the workpiece, and the operational processes.

1. Shape Distortion

Shape distortion in heat-treated workpieces can arise from various causes. The release of residual stresses during heating, thermal stresses and organizational stresses during quenching, and the self-weight of the workpiece can lead to uneven plastic deformation, resulting in shape distortion.

For slender workpieces, if the furnace floor is uneven or if the workpiece is placed in a bridging state, it can experience creep distortion due to its own weight during the holding period at austenitizing temperatures. This type of distortion is unrelated to heat treatment stresses. Additionally, workpieces may have internal stresses due to factors like straightening, excessive feed during machining, or improper pre-heat treatment operations, which contribute to residual stresses. During heating, as the yield strength of steel decreases with rising temperatures, any residual stresses in certain areas of the workpiece may reach their yield point, causing uneven plastic deformation and relaxation of residual stresses, leading to shape distortion.

Thermal stresses generated during heating are significantly influenced by the chemical composition of the steel, the heating rate, and the size and shape of the workpiece. High-alloy steels with poor thermal conductivity, rapid heating rates, large sizes, complex shapes, and uneven thickness can lead to significant thermal stresses due to differential thermal expansion, resulting in uneven plastic deformation and shape distortion.

Compared to heating, thermal and organizational stresses during cooling have a more substantial impact on the deformation of the workpiece. Deformations caused by thermal stresses primarily occur in the early cooling phase, as the workpiece remains at high temperatures and is still plastic. Under initial thermal stresses, the core may yield under multi-directional compression, causing plastic deformation. As cooling progresses and the yield strength increases, it becomes increasingly difficult for further plastic deformation to occur, leading to the retention of initial non-uniform plastic deformations as the workpiece cools to room temperature.

2. Volume Distortion

After heat treatment, the microstructure of the workpiece changes, resulting in proportional expansion or contraction due to the differences in specific volumes of various phases. Volume changes do not affect the original shape of the workpiece. For example, a gear shaft may experience axial elongation or contraction. Such volume distortions are generally small and difficult to detect visually.

Volume distortion is related to the composition and combined stresses during phase transformations, rather than the magnitude of heat treatment stresses. The extent of volume change is influenced by several factors:

1. The greater the difference in specific volumes before and after quenching, the larger the volume distortion.
2. Higher quenching temperatures increase the alloy content in austenite, resulting in a larger specific volume for martensite and an increase in retained austenite.
3. Workpieces that are fully hardened exhibit maximum volume distortion.

3. Micro Distortion

Micro distortion occurs due to unstable microstructures (such as martensite and retained austenite after quenching) and unstable stress states (whether compressive or tensile). Over extended periods at room temperature or sub-zero temperatures, these structures gradually transform and stabilize, leading to the appearance of distortion. For instance, the shape changes in the teeth of gears after carburizing or induction hardening (such as variations in the effective profile length and tooth thickness) can be one cause of noise during gear operation.