The main purpose of surface treatment of powder metallurgy parts:
- Improve wear resistance
- Improve corrosion resistance
- Improve fatigue strength
The surface treatment methods applied to powder metallurgy parts can basically be divided into the following five categories:
- Coating: Cover the surface of the processed part with a layer of other materials without any chemical reaction
- Surface chemical treatment: the chemical reaction between the surface of the processed part and the external reactant
- Chemical heat treatment: other elements such as C and N diffuse to the surface of the processed part
- Surface heat treatment: the phase change is generated by the cyclic change of temperature, which changes the microstructure of the surface of the processed part
- Mechanical deformation method: to produce mechanical deformation on the surface of the processed part, mainly to produce compressive residual stress, while also increasing the surface density
Ⅰ. Coating
Electroplating can be applied to powder metallurgy parts, but it can only be carried out after the powder metallurgy parts are pretreated (such as dipping copper or dipping wax to seal holes) to prevent the penetration of electrolyte. After electroplating treatment, the corrosion resistance of the parts can usually be improved. Common examples are galvanizing (reusing chromate for passivation after galvanizing to obtain a black or army green shiny surface) and nickel plating
Electroless nickel plating is superior to electrolytic nickel plating in some aspects, such as controlling the thickness of the coating and plating efficiency.
The “dry” zinc coating method does not need to be carried out and does not need to be sealed. It is divided into powder galvanizing and mechanical galvanizing.
When anti-rust, anti-corrosion, beautiful appearance and electrical insulation are required, painting can be used. The methods can be further divided into: plastic coating, glazing, and metal spraying.
Ⅱ.Surface chemical treatment
Steam treatment is the most common of all surface treatment processes for powder metallurgy parts. Steam treatment is to heat the parts to 530-550°C in a steam atmosphere to produce a magnetic (Fe3O4) surface layer. Through the oxidation of the surface of the iron matrix, the wear resistance and friction properties are improved, and the parts are resistant Rust performance (further strengthened by oil immersion) The oxide layer is about 0.001-0.005mm thick, covering the entire outer surface, and can diffuse to the center of the part through interconnected pores. The filling of this pore increases the apparent hardness, thereby Improve the wear resistance and make it have a moderate degree of compaction.
Cold phosphate treatment is a chemical reaction in a salt bath to form complex phosphate on the surface of the workpiece. Zinc phosphate is used for pretreatment of coatings and plastic coatings, and manganese phosphate is used for friction applications.
The bluing is done by placing the workpiece in a potassium chlorate bath at 150°C by chemical corrosion. The surface of the workpiece has a dark blue color. The thickness of the bluing layer is about 0.001mm. After bluing, the surface of the parts is beautiful and has anti-rust function.
Nitriding coloring uses wet nitrogen as the oxidant. During the cooling process of the workpiece after sintering, an oxide layer is formed in the temperature range of 200-550°C. The color of the formed oxide layer changes with the processing temperature.
Anodized anti-corrosion treatment is used for aluminum-based parts to improve its appearance and anti-corrosion performance.
Passivation treatment is applied to stainless steel parts, mainly to form a surface oxide protective layer. These oxides can be formed by heating or by chemical methods, that is, soaking with nitric acid or sodium chlorate solution. In order to prevent the solution from immersing, chemical The method requires pre-sealing wax treatment.
