Knowledge

Heat treatment is an important method to improve the performance of metal materials. Cold treatment, as an extension and supplement to heat treatment, plays a significant role in enhancing the hardness, tensile strength and dimensional stability of workpieces. This article mainly introduces the basic principles, process flow, applicable materials and application scope of cold treatment.

 

The basic principle of cold treatment

Cold treatment (also known as deep cryogenic treatment or ultra-low temperature treatment) refers to the process of rapidly cooling the quenched workpiece to a low temperature of -60°C to -80°C and maintaining it for a certain period of time, allowing the residual austenite to further transform into martensite.

 

During the quenching process of steel, due to limitations in cooling speed or material composition, some austenite may not completely transform into martensite. These residual austenites are in a metastable state at room temperature and can affect the hardness, wear resistance, and dimensional stability of the workpiece.

 

Cold treatment can significantly reduce the content of residual austenite. This is because at low temperatures, the stability of austenite decreases, and the transformation of martensite can continue. This transformation not only increases the hardness and strength of the material but also enhances the dimensional stability of the workpiece.

 

The process flow of cold treatment

Cold treatment is not an independent process but a subsequent procedure after quenching. The specific steps are as follows:

1) After quenching, the workpiece should first be cooled to room temperature to avoid direct entry into the cold treatment stage, which may cause excessive internal stress or cracking.

2) Cold treatment should be carried out as soon as possible after quenching. Prolonged stay at room temperature may lead to the stabilization of austenite, thereby reducing the effectiveness of cold treatment.

3) Place the workpiece in a cold treatment device and cool it to the low-temperature range of -60°C to -80°C. The cooling process should be uniform to prevent deformation or stress concentration caused by temperature gradients.

 

4) Once the temperature of the workpiece's cross-section is uniform, maintain it for a certain period of time to ensure the complete transformation of residual austenite.

5) After the cold treatment is completed, the workpiece should be removed and allowed to naturally return to room temperature in the air. If necessary, low-temperature tempering can be performed to eliminate internal stress.

 

During the entire cold treatment process, precise temperature control and the duration of heat preservation are key parameters that affect the effectiveness of cold treatment and should be optimized based on the specific material and workpiece size.

 

Applicable Materials and Scope of Application

Cold treatment is mainly applicable to high alloy steel, tool steel, die steel and certain special alloy materials, especially those with a high content of retained austenite and prone to martensitic transformation at low temperatures.

1) Precision cutting tools: such as reamers, milling cutters, drills, etc. Cold treatment can enhance hardness and wear resistance, thereby extending service life.

2) Measuring tools and measuring elements: such as gauge blocks, gauges, templates, etc. Cold treatment can significantly improve dimensional stability and ensure measurement accuracy.

3) High-precision gears and transmission components: mainly used in the automotive, aerospace and precision machinery industries, it can enhance fatigue strength and resistance to deformation.

4) Magnetic materials: such as magnetic steel. Cold treatment helps stabilize the magnetic properties of magnetic materials and improve the consistency of magnetic performance.

In addition, some non-ferrous metals and composite materials can also improve performance through cold treatment, but the application is relatively rare.

 

Equipment and Key Technologies for Cold Treatment

Cold treatment requires specialized low-temperature equipment, such as mechanical refrigeration devices and liquid nitrogen cooling systems. When choosing equipment, factors such as cooling rate, temperature uniformity, and temperature control accuracy should be considered. Excessively fast cooling rates may cause internal cracks in the workpiece, while overly slow rates can reduce processing efficiency. Therefore, key technologies include:

1) Ensuring uniform temperature reduction across all parts of the workpiece to avoid local overcooling or overheating.

2) Developing reasonable cold treatment temperatures, holding times, and heating rates based on material properties.

3) Internal stresses generated during cold treatment can be eliminated through subsequent tempering processes to prevent workpiece deformation or cracking.

 

The Practical Benefits of Cold Treatment

In actual production, the cold treatment process can bring about numerous benefits.

1) The hardness, strength, and wear resistance of the workpiece are significantly enhanced, especially in high-load and high-wear working environments.

2) For precision parts and measuring tools, cold treatment can reduce dimensional changes during use and extend their service life.

3) By reducing residual austenite, the material structure becomes more uniform, and performance fluctuations are greatly reduced.

4) The cold treatment process does not require complex chemical media, is relatively simple to operate, and does not produce harmful substances, aligning with the modern trend of green manufacturing.

 

As an important subsequent process of heat treatment, cold treatment plays an irreplaceable and significant role in improving workpiece performance, stabilizing workpiece dimensions, and enhancing durability. Through the scientific and standardized application of cold treatment, manufacturing enterprises can not only enhance product quality but also reduce production costs and strengthen their market competitiveness.

 

Vigor team has more than 20 years experience in casting, forging, cold forming processes and the post treatment, as well as a robust surface treatment supply chain. If anything we can help or any parts you want to develop, please contact us at info@castings-forging.om