Using modern physics, chemistry, metal science and heat treatment and other disciplines technology to change the condition and properties of the surface of parts, so that it and the core material for optimization combination, in order to achieve predetermined performance requirements of the process method, called the surface treatment process.
The role of surface treatment:
1. Improve surface corrosion resistance and wear resistance, slow down, eliminate and repair material surface changes and damage;
2. Make ordinary materials have special functional surface;
3. Save energy, reduce costs and improve the environment.
Classification of metal surface treatment processes
| Description of surface treatment process | Classification |
| Surface modification technology |
Surface modification technology Through physical and chemical methods, the surface morphology, phase composition, microstructure, defect state and stress state of the material surface are changed to obtain the surface treatment technology of demand performance. The surface chemical composition of the material remains unchanged. |
| Surface alloying technology | A surface treatment process in which the added material is physically transferred into the matrix to form an alloying layer to obtain desired properties. |
| Surface conversion membrane technology | By chemical method, the additive material reacts with the matrix to form the transformation film, so as to obtain the surface treatment technology of the required performance. |
| Surface remolding technology | By means of physical and chemical methods, the additive material is plated and coated on the surface of the substrate to obtain the required properties of the surface treatment process. The substrate does not participate in the formation of the coating |
It can be divided into four categories: surface modification technology, surface alloying technology, surface conversion film technology and surface coating technology.
First, surface modification technology
1. Surface hardening
Surface quenching refers to the heat treatment method of hardening the surface of parts by quenching the surface austenitizing with rapid heating without changing the chemical composition and core structure of steel.
The main surface quenching methods are flame quenching and induction heating, commonly used heat sources such as aerobic acetylene or oxypropane flame.
2. Laser surface enhancement
Laser surface strengthening is to use focused laser beam to the surface of the workpiece, in a very short time to heat the surface of the workpiece thin material to the temperature above the phase change temperature or melting point, and in a very short time to cool, so that the workpiece surface hardening and strengthening.
Laser surface strengthening can be divided into laser phase transformation strengthening treatment, laser surface alloying treatment and laser cladding treatment.
Laser surface strengthening has small heat-affected zone, small deformation and easy operation. It is mainly used for local strengthening parts, such as punching die, crankshaft, CAM, camshaft, spline shaft, precision instrument guide rail, high speed steel tool, gear and internal combustion engine cylinder liner.
3. Shot peening
Shot peening is a technology that ejects a large number of high-speed projectiles onto the surface of parts, just like countless small hammers hitting the metal surface, so that the surface and subsurface surface of parts have certain plastic deformation and realize strengthening.
Shot peening can improve the mechanical strength and wear resistance, fatigue resistance and corrosion resistance of parts. Often used for surface extinction, oxidation skin; Eliminate the residual stress of casting, forging and welding parts.
4. The roller
Rolling is at room temperature with hard roller or roller pressure on the rotating surface of the workpiece, and move along the bus direction, so that the workpiece surface plastic deformation, hardening, to obtain accurate, smooth and enhanced surface or specific pattern surface treatment process.
Often used in cylinder, cone, plane and other simple shape parts.
5. Wire drawing
Wire drawing refers to the metal forced through the die under the action of external force, metal cross-sectional area is compressed, and obtain the required cross-sectional area shape and size of the surface treatment method called metal wire drawing process.
Drawing can be made according to decorative needs, straight lines, disorderly lines, corrugations and spiral lines, etc.
6. Polishing
Polishing is a kind of finishing method to modify the surface of parts. Generally, it can only get a smooth surface, but can not improve or even maintain the original machining accuracy. The Ra value after polishing can reach 1.6~0.008μm depending on the pre-machining condition.
Generally divided into mechanical polishing and chemical polishing.
Surface alloying technology
1. Chemical surface heat treatment
The typical process of surface alloying technology is chemical surface heat treatment. It is a heat treatment process in which the workpiece is heated and kept warm in a specific medium, so that the active atoms in the medium penetrate into the surface of the workpiece to change the chemical composition and organization of the surface of the workpiece, and then change its performance.
Compared with surface quenching, chemical surface heat treatment not only changes the surface structure of steel, but also changes its chemical composition. According to the infiltration of different elements, chemical heat treatment can be divided into carburizing,
ammoniation, multiple infiltration, infiltration of other elements. The process of chemical heat treatment includes three basic processes: decomposition, absorption and diffusion.
The two main methods of chemical surface heat treatment are carburizing and nitriding.
| Contrast | Carburizing | Nitride |
| Purpose | Improve the surface hardness, wear resistance and fatigue strength of the workpiece, while maintaining good toughness of the core. | Improve workpiece surface hardness, wear resistance and fatigue strength, improve corrosion resistance. |
| Timber | Mild steel containing 0.1-0.25%C. When carbon is high, heart primacy decreases. | Medium carbon steel containing Cr, Mo, Al, Ti and V. |
| Commonly used method | Gas carburizing, solid carburizing, vacuum carburizing | Gas nitriding, ion nitriding |
| Temperature | 900 ~ 950 ℃ | 500~ 570℃ |
| The thickness of the surface | 0.5 ~ 2 mm | No more than 0.6 ~ 0.7 Mr |
| Use | Widely used in aircraft, automobiles and tractors and other mechanical parts such as gear, shaft, camshaft and so on. | Used for wear resistance, high precision parts and heat, wear and corrosion resistant parts. Such as instrument small shaft, light load gear and important crankshaft. . |
Three, surface conversion membrane technology
1. Blackening and phosphating
Blackening: Process in which steel or steel parta are heated to the right temperature in air,water vapor or chemicalss to form a blue or black oxide film on their surface,and become blue.
Phosphating: workpiece (steel or aluminum, zinc parts) immersed in phosphating solution (some acid phosphate based solution), on the surface deposition to form a layer of insoluble crystalline phosphate conversion film process, called phosphating.
2. Anodizing
It mainly refers to the anodic oxidation of aluminum and aluminum alloy. Anodic oxidation is the aluminum or aluminum alloy parts immersed in acidic electrolyte, under the action of external current as an anode, on the surface of the parts to form an anti-corrosion oxide film layer firmly combined with the matrix. This oxide film has the special characteristics of protection, decoration, insulation and wear resistance.
Before anodizing, it should be pretreated with polishing, oil removal and cleaning, followed by washing, coloring and sealing.
Application: It is often used in the protective treatment of some special parts of automobiles and airplanes, as well as decorative treatment of handicrafts and daily hardware products.
Four, surface coating technology
1. The thermal spraying
Thermal spraying is to heat and melt metal or non-metallic materials, by continuous blowing of compressed gas to the surface of the workpiece, forming a coating firmly combined with the matrix, and obtain the required physical and chemical properties from the surface of the workpiece.
The wear resistance, corrosion resistance, heat resistance and insulation of materials can be improved by thermal spraying technology. It is used in almost all fields including advanced technologies such as aerospace, atomic energy and electronics.
2. The vacuum plating
Vacuum plating is a surface treatment process of depositing various metallic and non-metallic films on the metal surface by means of evaporation or sputtering under vacuum conditions.
Principle of vacuum sputtering plating
According to different processes, vacuum plating can be divided into vacuum evaporation, vacuum sputtering and vacuum ion plating.
3. The plating
Electroplating is an electrochemical and REDOX process. Take nickel plating as an example: the metal parts immersed in the solution of metal salt (NiSO4) as the cathode, the metal nickel plate as the anode, after switching on the DC power supply will be deposited on the metal nickel plating layer.
Electroplating method is divided into ordinary electroplating and special electroplating.
4. Vapor deposition
Vapor deposition technology refers to a new coating technology in which vapor material containing deposited elements is deposited on the surface of the material by physical or chemical methods to form a thin film.
Physical vapor Deposition (PVD)
Physical vapor deposition refers to the technology of vaporizing materials into atoms, molecules or ionizing them into ions by physical methods under vacuum conditions, and depositing a thin film on the surface of materials through the vapor phase process.
Physical deposition technology mainly includes three basic methods: vacuum evaporation, sputtering and ion plating.
Physical vapor deposition has a wide range of applicable matrix materials and film materials; Simple process, material saving, pollution-free; The advantages of strong adhesion, uniform thickness, density and less pinhole were obtained.
Widely used in machinery, aerospace, electronics, optics and light industry and other fields to prepare wear-resistant, corrosion-resistant, heat-resistant, conductive, insulating, optical, magnetic, piezoelectric, smooth, superconducting films.
Chemical vapor Deposition (CVD)
Chemical vapor deposition (CVD) is a method by which mixed gases interact with the substrate surface to form metal or compound films on the substrate surface at a certain temperature.
Because of its good wear resistance, corrosion resistance, heat resistance and electrical, optical and other special properties, chemical vapor deposition film has been widely used in mechanical manufacturing, aerospace, transportation, coal chemical industry and other industrial fields.
