What is the surface treatment process?
The surface treatment process is a procedure that uses modern technologies of physics, chemistry, metallography, heat treatment and other disciplines to modify the surface condition and properties of a component and optimize its combination with the core material to meet performance predetermined. requirements.
Surface treatment function:
- Improve surface corrosion and wear resistance, slow down, eliminate and repair material surface changes and damage;
- Allow common materials to obtain surfaces with special functions;
- Save energy, reduce costs and improve the environment.
2. Classification of Metal Surface Treatment Process
It can be divided into four categories: surface modification technology, surface alloy technology, surface conversion film technology and surface coating technology.
1. Surface modification technology
1. Surface quenching
Surface hardening refers to the heat treatment method of austenitizing the surface layer by rapid heating and quenching to strengthen the surface of parts without changing the chemical composition and structure of the steel core.
The main surface hardening methods are flame quenching and induction heating.
Common heat sources are oxyacetylene or oxypropane flames.
2. Laser surface strengthening
Laser surface strengthening is the process of using a focused laser beam to irradiate the surface of a workpiece, heating the extremely thin surface material to a temperature above the phase transformation temperature or melting point in a time Very short. The material is then rapidly cooled to harden and strengthen the surface of the part.
Laser surface strengthening can be divided into laser phase transformation strengthening treatment, laser surface alloying treatment and laser cladding treatment.
The laser surface strengthening has a small heat-affected zone, small deformation and convenient operation. It is mainly used to locally strengthen parts such as blind dies, crankshafts, cams, camshafts, splined shafts, guide rails of precision instruments, high-speed steel cutters, gears and cylinder liners of internal combustion engines.
3. Shot peening
Shot peening is a strengthening technology that involves spraying a large number of high-speed moving particles onto the surface of parts. This process creates numerous small impacts on the metal surface, inducing plastic deformations in the surface and underground regions of the parts to obtain reinforcement.
Function :
- Improve the mechanical strength, wear resistance, fatigue resistance and corrosion resistance of parts;
- Used for extinguishing surfaces and descaling;
- Eliminate residual stress from castings, forgings and welds.
4. Rolling
Rolling is a surface treatment process in which hard rolls or rollers are pressed onto the rotating surface of the part at room temperature and move along the busbar direction to plastically deform and harden the surface of the part so as to obtain a precise, smooth and reinforced surface or specific pattern.
Application: cylindrical surface, conical surface, flat and other parts with relatively simple shape.
5. Drawing
Wire drawing is a surface treatment method that involves passing metal through a die under external force, compressing the cross-sectional area of the metal, and obtaining the shape and size of the required cross-sectional area, which is called of metal wire drawing process.
Drawing can be done in straight, random, wavy and spiral lines according to the decoration needs.
6. Polishing
Polishing is a finishing method used to modify the surface of parts. Generally, it can only obtain a smooth surface and cannot improve or even maintain the original machining accuracy. With different pre-processing conditions, the Ra value (a measure of surface roughness) after polishing can reach 1.6 to 0.008 μm.
It is generally divided into mechanical polishing and chemical polishing.
2. Surface alloy technology
Chemical surface heat treatment
The typical process of surface alloy technology is surface chemical heat treatment.
It is a heat treatment process that involves placing the part in a specific medium for heating and insulation. This allows active atoms from the medium to penetrate the surface of the part and change the chemical composition and organization. This process changes the surface of the part, which in turn changes its performance.
Compared to surface quenching, surface chemical heat treatment not only changes the surface structure of steel, but also changes its chemical composition.
Carburizing and nitriding can be categorized into various elements and other elements depending on the different elements used.
The thermal chemical treatment process comprises three fundamental steps: decomposition, absorption and diffusion.
The two main methods of surface chemical heat treatment are carburizing and nitriding.
| Comparison | Carburization | Nitriding |
| goal | Improve the surface hardness, wear resistance and fatigue resistance of the workpiece while maintaining good heart toughness. | Improve the surface hardness, wear resistance and fatigue resistance of the workpiece, and improve corrosion resistance. |
| Material | Low carbon steel containing 0.1 ~ 0.25% C. High carbon reduces the toughness of the core. | It is medium carbon steel containing Cr, Mo, Al, Ti and V. |
| Common method | Gas carburizing, solid carburizing and vacuum carburizing | Gas nitriding method, ion nitriding method |
| temperature | 900~950℃ | 500~570°C |
| Surface thickness | Generally 0.5~2mm | No more than 0.6 ~ 0.7mm |
| Purpose | It is widely used in mechanical parts such as aircraft, automobiles and tractors; Such as gear shaft, camshaft, etc. | It is used for parts with high wear resistance and precision requirements and parts resistant to heat, wear and corrosion. Such as small instrument shaft, light load gear and important crankshaft, etc. |
3. Surface Conversion Film Technology
1. Darkening and phosphating
Darkening:
The process of heating steel or steel parts to a suitable temperature in air, steam or chemicals to form a blue or black oxide film on its surface is known as “blueing”. Steel or steel parts may also turn blue during the process.
Phosphating:
Phosphating is a process in which a part made of steel, aluminum or zinc is immersed in a phosphating solution (usually an acidic phosphate-based solution) and a water-insoluble crystalline phosphate conversion film is deposited on the surface.
2. Anodic oxidation
It mainly refers to the process of creating an oxide coating on the surface of aluminum and its alloys.
During anodizing, aluminum or aluminum alloy parts are immersed in an acidic electrolyte and act as an anode under the influence of external current. This results in the formation of an anti-corrosion and oxidizing film that firmly combines with the substrate on the surface of the parts.
This oxide film has unique properties such as protection, decoration, insulation and wear resistance.
Before anodizing, polishing, degreasing, cleaning and other pre-treatments must be carried out, followed by washing, coloring and sealing.
Application: It is commonly used in the protective treatment of some special parts of automobiles and aircraft, as well as the decorative treatment of handicrafts and hardware in daily use.
4. Surface coating technology
1. Thermal spraying
Thermal spraying involves heating and melting metals or non-metallic materials and continuously blowing them onto the surface of the workpiece with compressed gas. This creates a coating that blends tightly with the substrate, allowing the surface of the part to acquire the necessary physical and chemical properties.
Thermal spray technology can improve the wear resistance, corrosion resistance, heat resistance and insulation resistance of materials.
Applications: aerospace, atomic energy, electronics and other cutting-edge technologies in almost all fields.
2. Vacuum coating
Vacuum coating is a surface treatment process that involves the deposition of various metallic and non-metallic films on a metallic surface through distillation or sputtering under vacuum conditions.
Vacuum coating can produce very thin surface coatings. Furthermore, it offers advantages such as high speed, good grip and less pollution.
Vacuum sputter coating principle
According to different processes, vacuum plating can be divided into vacuum evaporation, vacuum sputtering and vacuum ion plating.
3. Electroplating
Electroplating is an electrochemical and redox process.
To illustrate nickel plating: the metal parts are immersed in a metal salt solution (NiSO4) as the cathode, while the nickel metal plate serves as the anode. When connecting the DC power supply, a nickel metallic coating will be deposited on the parts.
Electroplating methods are classified as ordinary electroplating or special electroplating.
4. Vapor deposition
Vapor deposition technology is a new coating technology that uses physical or chemical methods to deposit vapor phase substances containing deposition elements on the surface of materials. This creates thin films.
Vapor deposition technology is categorized into physical vapor deposition (PVD) and chemical vapor deposition (CVD) based on the different principles of the deposition process.
Physical vapor deposition (PVD)
Physical vapor deposition is a technology that involves vaporizing materials into atoms, molecules, or ions by physical methods under vacuum conditions. A thin film is subsequently deposited on the surface of the materials through the steam process.
Physical deposition technology comprises three fundamental methods: vacuum evaporation, sputtering and ion plating.
Chemical vapor deposition (CVD)
Chemical vapor deposition is a process that involves the formation of metallic or composite films on the surface of a substrate through the interaction between mixed gas and the substrate surface at a specific temperature.
Chemical vapor deposition films are widely used in various industries such as mechanical manufacturing, aerospace, transportation, and coal chemical industries. These films have unique properties including good wear resistance, corrosion resistance, heat resistance, and electrical and optical properties.
