Effect of surface treatment:
1. Improve the corrosion resistance and wear resistance of the surface, and slow down, eliminate and repair the change and damage of the material surface;
2. Make ordinary materials obtain surfaces with special functions;
3. Save energy, reduce costs and improve the environment.
Classification of metal surface treatment processes
Classification description of surface treatment process
The surface modification technology changes the surface morphology, phase composition, microstructure, defect state and stress state of materials through physical and chemical methods to obtain the surface treatment process with required performance. The chemical composition of the material surface remains unchanged.
The surface alloying technology enables the added materials to enter the matrix through physical methods to form an alloying layer to obtain the surface treatment process with required properties.
The surface conversion film technology is a surface treatment process that chemically reacts the added materials with the matrix to form a conversion film to obtain the required performance.
The surface replica technology is a surface treatment process that enables the added materials to form plating and coating on the substrate surface through physical and chemical methods to obtain the required performance. The matrix 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.
1、 Surface modification technology
1. Surface hardening
Surface quenching refers to the heat treatment method of strengthening the surface of parts after austenitizing the surface layer with rapid heating without changing the chemical composition and central structure of steel.
The main methods of surface quenching include flame quenching and induction heating, and the common heat sources include flame such as oxyacetylene or oxypropane.
2. Laser surface strengthening
Laser surface strengthening is to use a focused laser beam to shoot at the workpiece surface, heat the extremely thin material on the workpiece surface to the temperature above the phase change temperature or melting point in a very short time, and then cool it in a very short time to harden the workpiece surface.
Laser surface strengthening can be divided into laser transformation strengthening treatment, laser surface alloying treatment and laser cladding treatment.
Laser surface hardening has small heat affected zone, small deformation and convenient operation. It is mainly used for locally strengthened parts, such as blanking die, crankshaft, cam, camshaft, spline shaft, precision instrument guide rail, high-speed steel cutter, gear and cylinder liner of internal combustion engine.
3. Shot peening
Shot peening is a technology to spray a large number of high-speed moving projectiles onto the surface of parts, just like countless small hammers hitting the metal surface, so that the surface and sub surface of parts will have certain plastic deformation to achieve strengthening.
Shot peening can improve the mechanical strength, wear resistance, fatigue resistance and corrosion resistance of parts; Commonly used for surface matting and descaling; Eliminate the residual stress of castings, forgings and weldments.
4. Rolling
Rolling is a surface treatment process in which hard rollers or rollers are used to press the surface of a rotating workpiece at room temperature and move along the generatrix direction to plastically deform and harden the workpiece surface to obtain an accurate, smooth and strengthened surface or a specific pattern.
It is often used for simple parts such as cylinder, cone and plane.
5. Wire drawing
Wire drawing refers to the surface treatment method that makes the metal pass through the die forcibly under the action of external force, the metal cross section area is compressed, and the required shape and size of cross section area are obtained, which is called metal wire drawing process.
Drawing can be made into straight lines, random lines, ripples and spiral lines according to the decorative needs.
6. Polishing
Polishing is a finishing method to modify the surface of parts. Generally, only smooth surfaces can be obtained, and the original machining accuracy cannot be improved or even maintained. With different pre machining conditions, the Ra value after polishing can reach 1.6~0.008 μ m。
It is generally divided into mechanical polishing and chemical polishing.
2、 Surface alloying technology
1. Chemical surface heat treatment
The typical process of surface alloying technology is chemical surface heat treatment, which is a heat treatment process that places the workpiece in a specific medium for heating and insulation, so that the active atoms in the medium penetrate into the workpiece surface to change the chemical composition and structure of the workpiece surface, 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 different elements infiltrated, chemical heat treatment can be divided into carburization, ammoniation, multi-element penetration, penetration of other elements, etc. The chemical heat treatment process includes three basic processes: decomposition, absorption and diffusion.
The two main methods of chemical surface heat treatment are carburizing and nitriding.
Contrast carburizing and nitriding
Objective To improve the surface hardness, wear resistance and fatigue strength of the workpiece, while maintaining good toughness of the heart. Improve the surface hardness, wear resistance, fatigue strength and corrosion resistance of the workpiece.
The material contains 0.1-0.25% C low carbon steel. The higher the carbon, the lower the core. It is medium carbon steel containing Cr, Mo, Al, Ti and V.
Common methods: gas carburizing, solid carburizing, vacuum carburizing, gas nitriding and ion nitriding
Temperature 900~950 ℃ 500~570 ℃
The surface thickness is generally 0.5~2mm, not more than 0.6~0.7mr
It is widely used in mechanical parts such as gears, shafts, camshafts, etc. of aircraft, automobiles and tractors. It is used for parts with high wear resistance and precision requirements, as well as heat resistance, wear resistance and corrosion resistance parts. Such as small shaft of instrument, light load gear and important crankshaft.
3、 Surface conversion film technology
1. Blackening and phosphating
Blackening: The process of heating steel or steel parts to an appropriate temperature in air water vapor or chemicals to form a blue or black oxide film on their surface. It also becomes bluish.
Phosphating: the process of workpiece (steel or aluminum or zinc parts) immersed in phosphating solution (some acid phosphate based solutions) to deposit a layer of water-insoluble crystalline phosphate conversion film on the surface, which is called phosphating.
2. Anodizing
It mainly refers to the anodizing of aluminum and aluminum alloys. Anodizing refers to the process of immersing aluminum or aluminum alloy parts in acid electrolyte, acting as anode under the action of external current, and forming an anti-corrosion oxidation film firmly combined with the substrate on the surface of the parts. This oxide film has special characteristics such as protection, decoration, insulation and wear resistance.
Before anodizing, polishing, degreasing, cleaning and other pre-treatments shall be carried out, followed by washing, coloring and sealing.
Application: It is commonly used for the protective treatment of some special parts of automobiles and airplanes, as well as the decorative treatment of handicrafts and daily hardware products.
4、 Surface coating technology
1. Thermal spraying
Thermal spraying is to heat and melt metal or non-metallic materials, and continuously blow and spray them onto the surface of the workpiece by compressed gas to form a coating firmly bonded with the substrate, so as to obtain the required physical and chemical properties from the surface of the workpiece.
Thermal spraying technology can improve the wear resistance, corrosion resistance, heat resistance and insulation of materials. It has applications in almost all fields, including aerospace, atomic energy, electronics and other cutting-edge technologies.
2. Vacuum plating
Vacuum plating is a surface treatment process for depositing various metal and non-metallic films on metal surfaces by evaporation or sputtering under vacuum conditions.
By vacuum plating, a very thin surface coating can be obtained, which has the advantages of fast speed, good adhesion and less pollutants.
Principle of Vacuum Sputtering Plating
According to different processes, vacuum plating can be divided into vacuum evaporation plating, vacuum sputtering plating and vacuum ion plating.
3. Electroplating
Electroplating is an electrochemical and redox process. Take nickel plating as an example: Dip the metal parts in the solution of metal salt (NiSO4) as the cathode, and use the metal nickel plate as the anode. After the DC power is turned on, the metal nickel coating will be deposited on the parts.
Electroplating methods are divided into ordinary electroplating and special electroplating.
4. Vapor deposition
Vapor deposition technology is a new type of coating technology, in which a vapor phase substance containing deposition elements is deposited on the material surface by physical or chemical methods to form a thin film.
According to the different principles of the deposition process, the vapor deposition technology can be divided into physical vapor deposition (PVD) and chemical vapor deposition (CVD).
Physical Vapor Deposition (PVD)
Physical vapor deposition (PVD) refers to the technology of vaporizing materials into atoms, molecules or ions by physical methods under vacuum conditions, and depositing a thin film on the surface of materials through a vapor process.
Physical deposition technology mainly includes vacuum evaporation, sputtering and ion plating.
Physical vapor deposition has a wide range of suitable matrix materials and film materials; Simple process, material saving and pollution-free; The obtained film has the advantages of strong adhesion between film and substrate, uniform film thickness, compactness, less pinholes, etc.
It is widely used in the fields of machinery, aerospace, electronics, optics and light industry to prepare wear-resistant, corrosion resistant, heat-resistant, conductive, insulating, optical, magnetic, piezoelectric, lubricating, superconducting and other films.
Chemical Vapor Deposition (CVD)
Chemical vapor deposition (CVD) is a method of forming metal or compound films on the substrate surface by the interaction of mixed gases and the substrate surface at a certain temperature.
Because of its good wear resistance, corrosion resistance, heat resistance, electrical and optical properties, CVD films have been widely used in mechanical manufacturing, aerospace, transportation, coal chemical industry and other industrial fields.
