Hard Facing

The production of a hard wear-resistant surface layer on metals by welding is known as hard facing. This method is relatively easy to apply, requiring only the hard facing alloys in the form of welding rods and an oxyacetylene flame or electric arc. The advantages of hard facing are that    

1.    it might be applied to localised areas subjected to wear.
2.    Hard-wear-resistant compounds are available, and
3.    This provides effective use of expensive alloys and protection in depth.

The hard facing material is provided in the form of an electrode or welding rod. The rods are generally used bare for oxyacetylene-gas welding and are flux-coated for electric arc welding. The flux coating contains materials for arc stability, oxidation protection of the molten weld, fluxing of impurities, thermal and electrical insulation and control of metal transfer.

Oxyacetylene-gas welding produces smoother deposits that can be positioned more precisely, whereas the heating & cooling rates are slower. Electric arc welding is  less expensive, might be faster, & lends itself better to automatic procedure. The hard facing material is melted and spread over the base metal and bonds with it to form a new surface ranging from 10.5 to 6 mm thick based on the application. Only the surface of the base metal is brought to melting temperature. This prevents mixing of the alloy along with too much of the base metal and thus changing the properties of both the coating and the base metal.

Hard facings can be applied to most ferrous metals but with a few exceptions it is not advisable to face non-ferrous alloys having melting points below 1090oC. Carbon steels are relatively easy to hard face, particularly below 0.35% carbon. Welding becomes more difficult with increasing carbon content, and the high carbon and alloy steels might be heated before & after hard facing.

Hard facing is most extensively used where systematic lubrication against abrasion is impossible as on mining tools, refinery and engine valves. Hard facing also extends the life of lubricated parts. Usually, hard faced surfaces are more wear-heat and corrosion resistant. Hard facing serves to increase operating efficiency by extending the life of equipment, by decreasing replacement cost and loss of production time, and by permitting the use of a low-cost base metal for parts that wear or corrode.

Tungsten carbide hard facings have the highest hardness and best wear resistance. Although it is relatively costly this is important because the material has a long life and can be applied in the form of inserts only where needed as in the case of rock drill bits.

The cobalt base alloys contain from 45 to 63 percent cobalt, 24 to 29 percent chromium, 5.50 to 13.50 percent tungsten and 1.10 to 3.20 % carbon. These alloys are utilized where wear and abrasion resistance combined with resistance to heat and oxidation is required. They have been used as hard facing materials for ladle linings and pouring spouts to resist hot gases and liquids. Some of distinctive applications are for hard facing of piercing mandrels, hot heading dies and shear blades exposed to hot solid metals.