Hard facing is a technique by which a wear-resistant overlay is welded on a softer and usually tougher base metal. The method is versatile and has a number of advantages:
1. Wear resistance can be added exactly where it is needed on the surface.
2. Hard compounds and special alloys are easy to apply.
3. Hard facings can be applied in the field as well as in the plant.
4. Expensive alloying elements can be economically used.
5. Protection can be provided in depth.
6. A unique and useful structure is provided by the hard-surfaced, tough-core composite.
Many of the merits of hard facing stem from the hardness of the special materials used. For example, ordinary weld deposits range in hardness up to about 200 Brinell, hardened steels have a hardness up to 700 to 800 Vickers, and special carbides have hardness up to about 3000 Vickers. However, it is important to note that the hardness of the materials does not always correlate with wear resistance. Thus, special tests should be performed to determine the resistance of the material to impact, gouging abrasion, grinding (high-stress abrasion), erosion (low-stress scratching abrasion), seizing or galling, and hot wear.
Another important point is that durable overlays are not necessarily hard. Most surfacing is used to protect base metals against abrasion, friction, and impact. However, many "hard facings" such as the stainless steels, related nickel-base alloys, and copper alloys are used for corrosion-resistant applications where hardness may not be a factor. Also, the relatively soft leaded bronzes may be used for bearing surfaces. Other facings are also used for heat- and oxidation-resistant applications.
Methods of Application Hard facings can be applied by:
1. Manual, semiautomatic, and automatic methods using bare or flux coated electrodes
2. Submerged-arc welding
3. Inert-gas shielded arc welding (both consumable and tungsten electrode types)
4. Oxyacetylene and oxyhydrogen gas welding
5. Metal spraying
6. Welded or brazed on inserts
Gas welding and spraying usually provides higher quality and precise placement of surfaces; arc welding is less expensive. Automatic or semiautomatic methods are preferred where large areas are to be covered, or where repetitive operations favor automation.
Surfacing filler metals are available in the form of drawn wire, cast rods, powders, and steel tubes filled with ferroalloys or hard compounds (e.g., tungsten carbide). The electrodes may take the form of filled tubes or alloyed wires, stick types, or coils specially designed for automated operations. The stick-type electrodes may have a simple steel core and a thick coating containing the special alloys. In submerged-arc welding the alloys may be introduced through a special flux blanket. In spray coating, the materials are used in the form of powders or bonded wire.
Sprayed facings are advantageous in producing thin layers and in following surface contours. With this method it is usually necessary to fuse the sprayed layer in place after deposition to obtain good abrasion resistance. However, under boundary lubrication conditions the as-sprayed porosity of the facings may aid against frictional wear.
Hard facings are used in thicknesses from 0.031 to 25.4 cm or more. The thinnest layers are usually deposited by gas welding, usually with low melting alloys that solidify with many free carbide or other hard compound crystals. The thick deposits are usually made from air-hardening or austenitic steels.
Hard overlays are usually strong in compression but weak in tension. Thus, they perform better in pockets, grooves, or low ridges. Edges and corners must be treated cautiously unless the deposit is tough. Brittle overlays should be deposited over a base of sufficient strength to prevent subsurface flow under excessive compression.
Gas welding is a useful method for depositing small, precisely located surfacing in applications where the base metal can withstand the welding temperatures (e.g., steam valve trim and exhaust valve facings). On the other hand, heavy layers and large areas may be impossible to surface without cracks with the harder, more wear-resistant alloys because of the severe thermal stresses that are encountered (e.g., usually in arc welding). Thus, the opposing factors of wear resistance and freedom from cracking frequently require a compromise in process and material selection.
Materials
Basically, hard-facing materials are alloys that lend themselves to weld fusion and provide hardness or other properties without special heat treatment. Thus, for hard surfacing, the steels and the matrices of high-carbon irons may contain enough alloys to cause the hardening transformation during weld cooling, rather than after a quenching treatment.
The properties of the iron-, nickel-, and cobalt-base alloys are strongly affected by carbon content and somewhat by the welding technique used. For example, gas welding usually provides superior abrasion resistance, although carbon pickup may lower corrosion resistance. Arc welding tends to burn out carbon and alloys, thereby lowering abrasion resistance but increasing toughness; high thermal stresses from arc welding may also accentuate cracking tendencies.
The martensitic irons, martensitic steels, and austenitic manganese steels are suited for light, medium, and heavy impact applications, respectively. Gouging abrasion applications usually require an austenitic manganese steel because of the associated heavy impact. Grinding abrasion is well resisted by the martensitic irons and steels. Erosion is most effectively resisted by a good volume of the very hard compounds (e.g., high-chromium irons). Tungsten carbide composites have outstanding resistance to abrasion where heavy impact is not present, but deposits may develop a rough surface.
Selection of materials for hot-wear applications is complicated by oxidation, tempering, softening, and creep factors. Oxidation resistance is provided by using a minimum of 25% chromium. Tempering resistance (up to 593°C) is provided by chromium, molybdenum, tungsten, etc. Creep resistance is provided by the austenitic structure in nickel- or cobalt-bearing alloys. The chromium-cobalt-tungsten grade of materials usually provides a good combination of properties above 649°C.
