Main Principles and Facilities of Cryogenic Treatment and Heat Treatment

Heat treatment and cryogenic treatment are two core processes for regulating the properties of metallic materials. The former alters the material’s internal structure through a phase transformation process of heating, holding, and cooling. At the same time, the latter eliminates residual stress and improves service stability through atomic diffusion and structural transformation in a cryogenic environment. Both are widely used in high-end manufacturing fields such as aerospace, precision machinery, and new energy equipment. This article systematically reviews the working principles and core supporting facilities of these two types of processes, providing a reference for process selection and production adaptation.

I. Principles and Core Facilities of Heat Treatment Process

The essence of heat treatment is to control the heating temperature, holding time, and cooling rate of the material, using phase transformation laws in the solid state to achieve structural reconstruction and adjust the material’s key properties, such as strength, hardness, toughness, and wear resistance, as needed. According to the purpose of the process, heat treatment can be divided into four main categories: annealing, normalizing, quenching, and tempering. Annealing involves heating the material above its critical phase transformation point, holding it at that temperature, and then slowly cooling it. The purpose is to eliminate residual forging stress, homogenize the microstructure, and reduce hardness for easier subsequent machining. Normalizing involves cooling in air, which is faster than annealing, resulting in a finer grain structure and improved overall mechanical properties of low- and medium-carbon steel. Quenching, by rapidly cooling to form a metastable martensitic structure, is a core process for improving the strength and hardness of steel parts. Tempering involves heating the quenched steel part below its critical phase transformation point and holding it at that temperature to eliminate quenching stress, reduce brittleness, and achieve a balance between strength and toughness.

The core facilities for heat treatment are built around the requirements of “thermal control,” with the heating furnace as the primary piece of equipment. Furnaces can be categorized into three types based on the heating medium: air furnaces, protective atmosphere furnaces, and vacuum furnaces. Ordinary air furnaces are the least expensive and suitable for rough-machined components with low surface oxidation requirements. Protective atmosphere furnaces introduce argon, nitrogen, or controlled-carbon atmospheres to prevent surface oxidation and decarburization during heating, making them the mainstream equipment for heat treatment of automotive and construction machinery parts. Vacuum furnaces heat in a near-zero-oxygen environment, resulting in smooth, deformation-free workpiece surfaces suitable for the high-end heat treatment needs of aerospace precision components and mold steel. The temperature control accuracy of the furnace body is a core performance indicator. For medium- and low-temperature furnaces (below 1000℃), the temperature control accuracy needs to be ±5℃; for high-temperature furnaces (above 1200℃), it needs to be ±10℃ to avoid uneven microstructural transformation caused by temperature fluctuations.

Cooling facilities are crucial for controlling the performance of heat treatment processes. They are categorized into three types based on different cooling rate requirements: Quenching oil baths have a cooling rate of approximately 10~30℃/s, suitable for alloy steel structural parts with good hardenability and minimal quenching deformation; water baths can reach cooling rates of 50~80℃/s, suitable for medium carbon steel components with lower hardenability, but with a higher risk of deformation and cracking; polymer quenching media baths, by adjusting the concentration of polyalkylene glycol, can control the cooling rate within a wide range of 20~60℃/s, combining the hardenability advantages of water quenching with the low deformation characteristics of oil quenching, and are currently the mainstream cooling system for high-end heat treatment. In addition, tempering furnaces, hardness testers, and metallographic microscopes are essential auxiliary facilities in the heat treatment process, used for tempering and performance verification.

II. Cryogenic Treatment Process Principles and Core Facilities

Cryogenic treatment generally refers to placing materials in a low-temperature environment below -80℃ for a period of time. Based on the temperature range, it can be divided into ordinary cryogenic treatment (-80℃ to -120℃) and deep cryogenic treatment (-120℃ to -196℃). Its core principle is based on the transformation of retained austenite and the release of internal stress. Taking quenched high-carbon steel and high-alloy steel as examples, approximately 5% to 15% of the austenite fails to transform into martensite during quenching. This metastable retained austenite undergoes phase transformation expansion during service, leading to a decrease in the dimensional accuracy of the components. Cryogenic treatment can promote the decomposition of retained austenite into a stable martensite structure while simultaneously precipitating a large number of nanoscale carbides in the martensite matrix, thereby refining the grain size. This not only improves the material’s hardness and wear resistance but also increases the dimensional stability of the components by more than 90%. Furthermore, the shrinkage of interatomic spacing at low temperatures can relieve microscopic internal stress generated during quenching, reducing the risk of deformation and cracking during component use.

The core facility for cryogenic treatment is the cryogenic treatment chamber. Currently, liquid nitrogen refrigeration technology is the mainstream approach, achieving a minimum temperature of -196℃ with a temperature control accuracy of ±2℃, enabling programmed increases and decreases in temperature. The main body of the equipment consists of three parts: an insulated chamber, a liquid nitrogen delivery system, and a temperature control system. The insulated chamber uses a double-layer structure of polyurethane foam and a vacuum insulation layer, with a thermal conductivity of less than 0.02 W/(m·K), ensuring the stability of the cryogenic environment. The liquid nitrogen delivery system controls the injection volume through a solenoid valve, working in conjunction with an internal circulating fan to achieve uniform temperature distribution. The internal temperature difference in large cryogenic chambers can be controlled to ±3℃, preventing uneven treatment of components at different locations. The temperature control system can set the heating, holding, and cooling rates to meet process requirements, preventing thermal-stress cracking of components caused by sudden temperature changes. A typical process involves cooling to -180℃ at 5℃/min within 1 hour after quenching, holding at that temperature for 2-4 hours, and then allowing the furnace to return to room temperature to achieve optimal treatment results.

For different industrial scenarios, cryogenic treatment facilities can be divided into two categories: batch and continuous. Batch cryogenic chambers are suitable for processing multiple varieties of small batches of precision parts, and process parameters can be flexibly adjusted. Continuous cryogenic treatment lines are equipped with automated loading and unloading mechanisms that connect to heat-treatment and quenching production lines, increasing processing efficiency by more than 3 times. They are suitable for large-scale production of standard parts such as bearings and cutting tools. Supporting residual stress detectors and metallographic analyzers are used for post-treatment performance verification, ensuring that the residual austenite content is controlled within 2% and that the residual stress elimination rate exceeds 80%.

III. Synergistic Application Logic of the Two Processes

Heat treatment and cryogenic treatment are not independent processes; they are often used together in high-end manufacturing scenarios: 42CrMo wind turbine bolts, after tempering heat treatment, undergoing a -180℃ cryogenic treatment process, can improve impact toughness by 15% and extend fatigue life by 40%; high-speed steel cutting tools, after quenching + tempering + cryogenic treatment, have wear resistance improved by more than 30% and service life doubled. During the production process, process parameters need to be matched to material characteristics and performance requirements, while also ensuring facility compatibility: the low-temperature treatment process should be set after quenching and before tempering to maximize the effect of residual austenite transformation.