Hard chrome process
The workpiece to be treated must have the smallest possible roughness depth of Rz 2 – 3 μm, as the hard chrome layer does not cover any scratches, pores or structural defects. For this reason, the workpiece is ground or belt polished in preparation. It should also be noted that the material must be metallically bright before treatment. This is the only way to ensure adequate protection.
In hard chrome plating, the workpiece is electrolytically coated with the chrome layer in an electroplating bath. The process temperatures here are usually between 50 and 60° C. A rectifier is used to reduce the chromium ions to metallic chromium. This supplies the electrons needed for the process. In this way, chromium is only deposited at the points opposite an electrode (anode). As is usual in electric baths, edges build up due to the field line distribution at the points favoured by the current. However, this can be reduced with the help of a special rack.
The required thickness of the chrome layer is decisive for the subsequent chemical and mechanical stress. The following layer thicknesses are applied:
- A layer thickness of 5 to 15 μm is applied in case of light wear. In addition, such a layer thickness serves to reduce friction.
- Hard chrome coatings of 15 to 30 μm protect against moderate wear.
- For adhesive wear resistance, a layer of 30 to 60 μm is applied to the workpiece.
- For heavy wear resistance, a chromium layer of 60 to 150 μm is used.
- To protect against abrasion and erosion as well as heavy wear, layer thicknesses of 150 to 350 μm are used.
- Chrome layers of over 250 μm are used for repair parts.
The properties of the hard chrome surface after treatment
The main focus of the procedure is to improve the wear resistance of the workpiece. In addition, the hard chrome layer gives the workpiece a hardness of 900 – 1200 HV without distortion. The hard chrome plating makes the surface water and dirt repellent, which further improves the corrosion resistance of the workpiece. Also, the machined workpieces have low adhesion and a minimal coefficient of friction.
In summary, the workpiece has the following advantages after treatment:
- no material distortion, despite high hardness
- high wear resistance
- low coefficient of friction and low adhesion tendency
- improved protection against chemical stress
- higher corrosion protection
- improvement of tribological properties
Hard chromium plating is suitable for a wide range of metals. These include all steels, aluminium, non-ferrous metals and their alloys. Due to its excellent chemical, technical and mechanical characteristics, the process is important for a wide range of industries:
- Cylinders in the printing industry: To ensure the highest print quality, it is of great importance that the cylinders have a flawless surface. Dimensional accuracy in the micrometre scale also plays a crucial role. Furthermore, the cylinders must have strong corrosion protection, as the inks and cleaning agents used are often very aggressive. For these reasons, optimized hard chrome coatings are often used here. Other possible applications for hard chrome coatings in the printing industry are duct rollers and metering rollers.
- Screen baskets for waste material processing in the paper industry: The screen baskets used must have a high level of wear protection as well as good repairability. Hard chrome plating is the best choice here. Other possible applications of hard chrome plating in the paper industry are creping cylinders, applicator and ink metering rollers.
- Pistons in mechanical engineering: Hard chrome plating is used for piston rings and piston ring grooves, as high wear protection is required here. Hard chrome plating is also used in mechanical engineering for various tools, shafts and measuring rollers.
The process is also used in the foil, automotive and food industries.