Effect of tungsten carbides properties of overlay welded WC/NiSiB composite coatings

Sammanfattning: To get a long life of a product which is subjected to heavy mechanical wear, materials with particularly good properties such as high hardness and the ability to resist high temperature is required. Tungsten carbide is a material which has these properties, but to manufacture large products for mining industry or agriculture entirely of this material is neither practical nor quite beneficial when the disadvantages of the materials such as brittleness, high weight and high price will have a major affect. To exploit material properties in a beneficial way it is more convenient to coat the surface on ordinary steel with a composite coating consisting of tungsten carbides in a nickel base matrix. By this way it is possible to utilize the steels ductility, lower weight and lower price along with the tungsten carbides superior abrasion resistance. The method for coating the steel surfaces of this composite material is usually some welding process. Laser or plasma welding with filler material in powder form is often used as a beneficial method for this. Höganäs AB is a manufacturer of metal powder and conducts development of metal powder mixtures for surface coating purposes. This report is a study of various types of tungsten carbide powder used for the surface coating process “Laser Cladding”. As tungsten carbide powders are available in various shapes and compositions, the goal is to find out what the different tungsten carbide types characteristics, advantages and disadvantages are when used in surface coatings, and also how they behave in the welding process. For this, the different types of powders were analyzed with respect to their properties in the unwelded condition. Thereafter samples of surface coatings were welded with the different types of tungsten carbides and their coating properties were analyzed. This has been done by measuring the dilution with X-ray diffraction, metallographic studies in both the light optical and scanning electron microscopy and hardness measurements. Finally, the coatings are subjected to abrasion test according to ASTM G65-4 for abrasive wear. The different types of tungsten carbides covered in this report are: 1. Spherical Cast Carbide2. Fused and crushed tungsten carbide, FTC3. Mono Cristalline tungsten carbide4. PA2, WC-Co carbide5. Agglomerated WC-Co carbide It appeared early in welding attempts that the carbide type PA2 (WC-Co) could not be weld with appropriate results, so further experiments of this type were canceled. After analysis of the welded coatings it can be concluded that Spherical Cast Carbide has the best wear resistance closely followed by Fused and crushed tungsten carbide and Mono Cristalline tungsten carbide. Agglomerated WC-Co carbide showed poor abrasion resistance in overlap areas, but in between these the result has appropriate properties were also a low surface roughness is achieved compared to the other materials. This could provide opportunities for new applications for this type of carbide where good surface finish is required, if it is possible to eliminate the problems in overlap areas.