The latest research achievements and future development directions of surface heat treatment

The Definition and Basic Principles of Surface Heat Treatment

Surface heat treatment refers to the process of heating the surface of a metal workpiece to a certain temperature and then cooling it in different ways to make the surface harden. The aim is to enhance the physical and chemical properties of the workpiece’s surface, such as hardness, wear resistance, and corrosion resistance, without altering the internal structure of the workpiece.

The basic principle of surface heat treatment is to control the phase transformation of the surface and interior of the workpiece by using different temperatures, times and cooling rates, thereby changing its microstructure. Common surface heat treatment methods include quenching, annealing, carburizing, nitriding, etc.

The core principle of surface heat treatment: Atomic-level transformation and performance enhancement

The reason why surface heat treatment can significantly alter the properties of metal materials lies in the fact that through a series of processes, it causes changes in the atomic structure and chemical composition of the metal surface layer. To deeply understand surface heat treatment technology, one must start from its inherent principle and explore how it achieves the regulation of metal properties at the atomic level.

The relationship between the microstructure of metals and their properties

The properties of metallic materials are closely related to their internal microstructure. Metals are polycrystalline materials composed of numerous grains. The size, shape, distribution of grains, as well as the crystal structure all have significant impacts on the properties of metals such as hardness, strength, toughness, and wear resistance.

For instance, a fine grain structure usually enables metals to have higher strength and hardness because grain boundaries can hinder the movement of dislocations, thereby enhancing the metal’s ability to resist deformation. Face-centered cubic crystal structures in metals (such as austenitic stainless steel) typically have better toughness and plasticity, while body-centered cubic crystal structures in metals (such as ferritic steel) tend to become brittle at low temperatures.

Phase transformation and diffusion during the heating process

Heating is a crucial step in the surface heat treatment process. Through heating, the metal surface undergoes phase transformation and atomic diffusion, thereby changing its microstructure and chemical composition.

In the surface quenching process, when the metal surface is heated to the austenitization temperature, the pearlite and other structures will transform into austenite. Austenite has a face-centered cubic crystal structure and good plasticity and toughness, and can dissolve more alloy elements such as carbon. As the heating temperature increases and the holding time prolongs, the austenite grains will gradually grow larger. Therefore, it is necessary to precisely control the heating parameters to obtain fine and uniform austenite grains.

In the chemical heat treatment process, the role of heating is mainly to provide energy for atomic diffusion. When the metal workpiece is heated in a specific medium, the active atoms (such as carbon, nitrogen, boron, etc.) in the medium will obtain sufficient energy to overcome the potential barriers on the metal surface and diffuse towards the metal surface. The diffusion process follows Fick’s law, and the diffusion rate is related to factors such as temperature, concentration gradient, and diffusion coefficient. The higher the temperature, the more intense the thermal motion of atoms, and the faster the diffusion rate; the greater the concentration gradient, the greater the driving force for atomic diffusion, and the easier the diffusion process is.

The organizational transformation during the cooling process

The cooling process is a crucial step in determining the microstructure and properties of the metal surface layer after surface heat treatment. Different cooling rates will cause different microstructural transformations in the metal surface layer, thereby resulting in different properties.

In surface quenching, when the austenitized metal surface layer is rapidly cooled (such as water quenching or oil quenching), since the cooling rate exceeds the critical cooling rate, the austenite cannot transform into balanced microstructures such as pearlite, but instead transforms into martensite. Martensite is an oversaturated solid solution with a body-centered tetragonal crystal structure, which has extremely high hardness but poor toughness. By controlling the cooling rate, different quantities and forms of martensite can be obtained, thereby regulating the hardness and toughness of the metal surface layer.

If the cooling rate is slower, the austenite may transform into tissues such as pearlite, bainite, etc. Pearlite is a layered structure composed of ferrite and carbonitride, with certain strength and toughness; bainite is a tissue between pearlite and martensite, and its properties are also between the two.

The performance synergy between the surface layer and the matrix

One of the major advantages of surface heat treatment is the ability to achieve a synergistic performance between the metal surface layer and the internal matrix. By treating the surface layer, it can acquire properties such as high hardness and high wear resistance, while the internal matrix retains good toughness and plasticity, enabling the entire part to both withstand external wear and impact and avoid overall brittle fracture.

This performance synergy is achieved through the transition zone between the surface layer and the matrix. The microstructure and properties gradually transition from the surface layer to the matrix, avoiding abrupt changes in performance, thereby reducing stress concentration and improving the overall strength and reliability of the part. For example, after surface quenching of automotive gears, a high-hardness martensite layer forms on the tooth surface, while the core maintains a ductile pearlite structure. When subjected to loads, the tooth surface can resist wear, and the core can absorb impact energy. The combined effect significantly extends the service life of the gears.

Main methods of surface heat treatment

Quenching

Quenching is a heat treatment method where metal workpieces are heated to a certain temperature and then rapidly cooled. The surface of the metal after quenching has high hardness but poor toughness. It is often used for parts that need to increase hardness, such as tools, gears, bearings, etc.

Tempering

Tempering is a heat treatment method where metal workpieces are heated to a certain temperature and then slowly cooled. The main purpose of tempering is to eliminate the internal stress in the workpiece and improve its mechanical properties. It is often used for processing cast iron, steel, etc.

Carburizing

Carburizing is a heat treatment method in which the surface of the workpiece is placed in an atmosphere containing carbon elements, allowing the surface to absorb carbon and form a hardened layer. Carburizing is often used for parts that require high hardness and wear resistance, such as gears and bearings in automotive engines.

Nitriding

Nitriding is a heat treatment method in which metal workpieces are heated in nitrogen or ammonia gas to form a nitrided layer on their surfaces. The nitrided layer has excellent wear resistance, corrosion resistance and high-temperature strength, and is often used in high-load parts such as engines and hydraulic systems.

Surface laser treatment

Laser surface treatment is a technology that uses the high energy density of the laser beam to heat the metal surface, causing it to rapidly melt and solidify. By adjusting parameters such as laser power and scanning speed, various effects such as surface hardening and oxide layer removal can be achieved.

Advantages of surface heat treatment technology

Enhance hardness and wear resistance

Surface heat treatment technology can significantly increase the hardness of the metal surface, making it have better wear resistance and fatigue resistance, and prolonging the service life of the parts.

Improve surface performance

Through surface heat treatment, the surface of the metal can form a hardened layer, which has higher corrosion resistance, high-temperature resistance and other characteristics, and can effectively protect the parts from being eroded by the external environment.

Improve mechanical properties

Surface heat treatment not only can increase hardness, but also can improve the mechanical properties of components, such as strength and toughness, thus making them more suitable for complex working environments.

Economic benefits

Through surface heat treatment technology, the durability of components can be greatly improved, reducing the cost of frequent replacement and maintenance, thereby improving production efficiency and economic benefits.

Applications of surface heat treatment

Surface heat treatment is widely used in various industrial fields, especially in those industries where improving the surface performance of components is necessary.

Automotive industry

Many components in the automotive industry, such as engine valves, gears, bearings, etc., require surface heat treatment to enhance wear resistance, corrosion resistance and service life. Especially for components that operate under high loads and high speeds, surface heat treatment technology is the key to ensuring their long-term stable operation.

Aerospace industry

In the aerospace field, the working environment of components is usually very demanding, requiring them to have excellent strength, hardness, and high-temperature resistance. Surface heat treatment can effectively enhance the performance of components such as aircraft engines and turbine blades, ensuring their reliability under extreme conditions.

Mechanical Manufacturing

Many components in mechanical manufacturing have high working requirements, especially under high-speed rotation and heavy loads. Surface heat treatment can effectively increase the service life of components and reduce maintenance costs. For example, components such as gears, bearings, and guides in the transmission system all need to undergo surface heat treatment.

Electronic and Electrical Industry

In the electronic and electrical industry, surface heat treatment technology is widely used in the manufacturing of electrical components, contact points, and electronic devices. Through surface treatment, their conductivity, corrosion resistance, and reliability can be improved.

Future Development Trends

Surface heat treatment technology is still evolving. The future trends focus on:

Digitalization and Intelligence: By leveraging sensors, the Internet of Things (IoT), and big data technologies, all process parameters such as furnace temperature, atmosphere, and flow rate can be monitored in real time. Through AI algorithms, adaptive control and process optimization can be carried out to achieve “zero defect” production and predictive maintenance.

Low energy consumption and environmental friendliness: Develop technologies such as vacuum heat treatment and plasma heat treatment that do not require quenching media, consume less air, and have no pollution. Develop new processes to replace cyanide salts and reduce CO₂ emissions.

Composite surface technology: Combining two or more technologies to achieve an “1+1>2” effect. For example, “nitriding + PVD coating”, the nitriding layer provides strong support for the coating, preventing it from collapsing; “laser quenching + laser cladding”, etc.

For additive manufacturing (3D printing): Provide customized surface performance improvement solutions for 3D printed parts, addressing issues such as poor surface quality and anisotropic performance of additive manufacturing parts.

New material adaptation: Develop new surface treatment technologies suitable for lightweight alloys (such as aluminum alloys, magnesium alloys, titanium alloys), high-entropy alloys, and composite materials.

Conclusion

Surface heat treatment, this ancient yet vibrant technique, has long transcended the boundaries of traditional heat treatment and has evolved into a comprehensive cutting-edge technology that integrates materials science, physics, chemistry, mechanical engineering, and intelligent control. With its ingenious “two ounces moving a thousand pounds” approach, it perfectly resolves the core contradiction between material performance and cost, and is an indispensable cornerstone of modern industry.

From the cars speeding on the ground to the planes soaring in the sky, from the noisy machines in factories to the precise artificial joints inside the body, surface heat treatment technology invisibly safeguards their strength, durability and reliability. With the advancement of technology, it will become more precise, efficient and green, continuing to empower humanity to create more outstanding products and opening up endless possibilities.

CNC machining on demand

Start your project today. Get free DFM from professional engineer at ProtoTi.

Get instant quote