Classification and Applications of Surface Heat Treatment of Steel

I. Introduction

In industrial production, many components not only require good overall performance but also have special requirements for surface properties. Surface heat treatment, as an important material surface-strengthening technology, can significantly improve the surface hardness, wear resistance, fatigue strength, and other properties of parts without altering their core microstructure or properties, thereby extending service life and improving reliability and safety. Therefore, surface heat treatment technology has been widely used in modern industry.

II. Classification of Surface Heat Treatment

Surface heat treatment is mainly divided into two categories: surface hardening and chemical heat treatment, which will be described in detail below.

2.1 Surface Hardening

Surface hardening is a process that rapidly heats the surface of a workpiece to its phase transformation temperature, then rapidly cools it to achieve surface martensitization (or bainitization). At the same time, the core retains its original microstructure. It is a “physical” surface strengthening method that does not require altering the material’s chemical composition. It offers advantages such as a short processing cycle and minimal workpiece deformation, and is suitable for materials such as medium-carbon steel and medium-carbon alloy steel. Surface hardening can be further classified according to the heating method as follows:

• High-frequency induction heating surface hardening: This method utilizes a high-frequency alternating current (100-500kHz) to generate an alternating magnetic field through an induction coil. When the workpiece is placed in the magnetic field, eddy currents are generated on the surface. Using the eddy-current heating effect, the surface is heated to 850-950℃ (above Ac3) within seconds to tens of seconds. Immediately afterward, water is sprayed for cooling, resulting in a hardened martensitic microstructure on the surface. The gap between the induction coil and the workpiece must be uniform (usually 2-5mm). The heating time should be adjusted according to the thickness of the hardened surface layer (0.5-3mm). The cooling water pressure must be stable (0.15-0.3MPa). This process is suitable for shaft parts (such as crankshaft main journals and motor shafts), gear teeth, machine tool guideways, and other parts that require wear-resistant surfaces and impact-resistant cores.

 Classification and Applications of Surface Heat Treatment of Steel (1)

 

• Flame-heated surface hardening: This method uses an acetylene-oxygen flame (approximately 3000℃) or a propane-oxygen flame (approximately 2800℃) to directly irradiate the workpiece surface, rapidly heating it to the hardening temperature. Cooling is then achieved using compressed air or water mist. The hardened layer is relatively thick (3-5mm), and the surface hardness is slightly lower than that of high-frequency induction hardening (HRC48-55). However, it offers lower equipment cost and higher flexibility, making it suitable for large or irregularly shaped parts, such as large rolls, excavator bucket teeth, and flywheel gear rings.

 

• Laser-heated surface hardening: This method uses a high-energy-density laser beam (power density 10⁴-10⁶ W/cm²) to irradiate the workpiece surface, heating it to the phase transition temperature within microseconds to milliseconds. After irradiation stops, rapid cooling via the workpiece’s own thermal conductivity completes the hardening process. Its hardened layer is extremely thin (0.1-0.5 mm) and extremely hard (e.g., HRC58-62 after quenching of 40Cr steel), resulting in minimal workpiece deformation (nearly no deformation), refined surface microstructure, and significantly improved fatigue strength. This process is suitable for surface strengthening of precision parts (e.g., engine camshafts and precision gears) and thin-walled parts (e.g., light alloy components in the aerospace industry).

Classification and Applications of Surface Heat Treatment of Steel (2)

• Electron beam-heated surface hardening: This method is performed in a vacuum. When an electron beam strikes a metal surface, the impact point rapidly heats. The depth of electron beam penetration depends on the accelerating voltage and material density. The heating rate is fast, with an austenitization time of only a fraction of a second or even less. Therefore, the workpiece surface has very fine grains, higher hardness than that of conventional heat-treated materials, and excellent mechanical properties.

Classification and Applications of Surface Heat Treatment of Steel (3)

2.2 Chemical Heat Treatment

Chemical heat treatment is a process in which a workpiece is placed in a specific medium at a certain temperature and time, allowing one or more elements to diffuse into the surface layer through atomic diffusion, thereby altering the surface chemical composition, microstructure, and properties. Its core advantage is the ability to customize surface properties (such as corrosion resistance, red hardness, and friction reduction) to meet specific requirements, and it applies to a wide range of metallic materials. Chemical heat treatment can be divided into the following types based on the different diffusion elements:

• Carburizing: Low-carbon steel (carbon content 0.10%–0.25%) or low-carbon alloy steel workpieces are placed in a carburizing medium (such as solid carburizing agent or gaseous carburizing agent) and held at 900–950℃ for 2–10 hours, allowing carbon atoms to diffuse into the surface layer (surface carbon content rises to 0.8%–1.2%). This is followed by quenching and low-temperature tempering to achieve a “high-hardness, wear-resistant surface layer and low-carbon, tough core.” This process is suitable for parts that withstand impact and have a wear-resistant surface, such as automotive gearbox gears, clutch shafts, and tool holders.

Classification and Applications of Surface Heat Treatment of Steel (4)

• Nitriding: The workpiece (usually steel containing alloying elements such as Cr, Mo, and Al, such as 38CrMoAlA) is placed in an ammonia decomposition atmosphere or nitrogen-based medium and held at 500-560℃ for 10-50 hours, allowing nitrogen atoms to penetrate the surface layer, forming nitrides (such as AlN and CrN). A high-hardness surface layer can be obtained without subsequent quenching. Its surface hardness ranges from HV800 to HV1200 (equivalent to HRC65-70), with wear resistance far superior to that of carburizing and quenching, and it also possesses good corrosion resistance (dense surface nitrides) and red hardness (hardness remains essentially unchanged below 600℃). Workpiece deformation is minimal (low-temperature process, no phase transformation stress). This process is suitable for high-precision, high-wear-resistant, and high-temperature-resistant parts, such as machine tool spindles, precision lead screws, internal combustion engine valves, and mold cavities.

Classification and Applications of Surface Heat Treatment of Steel (5)

• Boronizing: The workpiece (carbon steel, alloy steel, or cast iron) is placed in a boride medium (such as ferroboron + fluoride infiltration agent) and held at 850-950℃ for 2-6 hours, allowing boron atoms to penetrate the surface layer, forming a double-layer compound layer of FeB and Fe2B. This surface layer has extremely high hardness (HV1200-2000), wear resistance 3-5 times that of carburizing and quenching, and good corrosion resistance (especially acid and wear resistance). However, the surface layer is relatively brittle (the FeB layer is more brittle than the Fe2B layer), requiring control of the compound layer thickness (typically 50-150 μm). This process is suitable for parts subjected to severe wear but low-impact loads, such as die cutting edges, rolling mill rolls, and conveyor chain pins.

• Sulfurizing: The workpiece is placed in a sulfur-containing medium (such as sulfide gas or solid sulfurizing agent) and kept at 150-600℃ for 1-3 hours, allowing sulfur atoms to penetrate the surface layer and form a sulfide (such as FeS, FeS₂) lubricating layer. This surface layer has an extremely low coefficient of friction (0.05-0.15), exhibiting excellent friction reduction and anti-galling properties. Still, the hardness increase is not significant (the surface hardness is slightly higher than the substrate), and its temperature resistance is poor (the lubricating layer is prone to failure above 200℃). This process is suitable for sliding friction parts that require reduced friction and wear, such as sliding bearings, piston rings, and gear-meshing surfaces (especially under low-speed, heavy-load conditions).

III. Application Areas of Surface Heat Treatment

Surface heat treatment technology is widely used in the processing of key components in the automotive, machine tool, aerospace, and rail transportation industries. The following sections will introduce its applications in these fields.

3.1 Automotive Industry

In automotive manufacturing, surface heat treatment technology is crucial for improving the performance and reliability of components. For example, automotive gearbox gears need to withstand significant impact loads and friction. Carburizing and quenching treatments can impart the gear surface with high hardness and wear resistance while maintaining good toughness in the core, thereby improving the gear’s service life and transmission efficiency. Furthermore, automotive engine components, such as valves and piston rings, are often nitrided to improve their high-temperature, corrosion, and wear resistance.

3.2 Machine Tool Field

Machine tools are fundamental equipment in manufacturing, and their precision and reliability directly affect the quality of the products they process. Surface heat treatment technology plays a crucial role in the manufacturing of machine tool components. For example, machine tool spindles require high precision, high rigidity, and good wear resistance. High-frequency induction hardening or nitriding treatments can give the spindle surface high hardness and wear resistance while maintaining the core’s toughness, thereby improving the spindle’s rotational accuracy and service life. In addition, machine tool guideways, gears, and other components are frequently strengthened using surface heat treatment.

3.3 Aerospace Industry

The aerospace industry demands extremely high performance from its components, requiring high strength, high hardness, high wear resistance, and excellent high-temperature and corrosion resistance. Surface heat treatment technology is widely used in the manufacturing of aerospace components. For example, turbine blades for aero-engines need to withstand high temperatures, high pressures, and high-speed airflow. Laser surface alloying or electron beam cladding can form a high-temperature and corrosion-resistant alloy layer on the blade surface, thereby improving the blade’s service life and reliability. Furthermore, structural and transmission components in the aerospace industry are frequently strengthened through surface heat treatment.

3.4 Rail Transit Industry

Rail transit equipment requires high reliability, high safety, and long service life. Surface heat treatment technology plays a crucial role in the manufacturing of rail transit components. For example, high-speed rail track fastening systems need to meet stringent standards of a hardened layer depth of 4-6 mm and a surface hardness of 58-62 HRC. Dual-frequency quenching technology successfully achieves this requirement, reducing wear rate by 70%. In addition, surface heat treatment is commonly used to strengthen shafts, gears, and other components in rail transit vehicles, thereby improving their wear resistance and fatigue strength.

IV. Conclusion

Surface heat treatment technology, as an important material surface strengthening technique, has been widely used in industrial production. Through processes such as surface quenching and chemical heat treatment, the surface hardness, wear resistance, and fatigue strength of parts can be significantly improved, while maintaining the toughness and plasticity of the core. Surface heat treatment technology plays a crucial role in the processing of key components in the automotive, machine tool, aerospace, and rail transit industries. It is of great significance for improving product performance and reliability, extending service life, and reducing production costs. In the future, as materials science and heat treatment technology continue to develop, surface heat treatment technology will continue to innovate and improve, providing higher-quality, more efficient technical support for industrial production.