Rod Flame Spray – The Metal Spray Process

Metal spray is the process of applying molten metal to the surface of the rod to form a hard, wear-resistant coating. The coating material is melted in a flame and its minute particles are sprayed at relative low velocities onto a prepared surface by a stream of air. The molten particles impinging on the rod are flattened and interlocked to provide a mechanical bond. Alloying with the base metal does not occur.

A subsequent diffusion or sintering heat treatment may be required to obtain acceptable bonding conditions. The metal spray process requires roughing of the base metal (sandblasting, rough turning, etc.) prior to coating. Both pure metal and alloy materials in powder and wire form can be applied. The term “metallizing” is often used to describe the type of metal spray process which uses metal in wire form. The term “thermospray” is used to describe the process of using metals in powder form. Oxyacetylene torches or electrodes are common methods of melting coating materials. To seal the resulting porous coating, several types of sealers are employed. Phenolic sealers and silicone-alloyed resins are two common examples. Powders and application equipment are available from suppliers such as Metco, Wall Colmonoy, Stellite Division (Cabot), and others.

Advantages include:
• Low base material temperatures are maintained during application.
• Minimal distortion or warping (if diffusion heat treatment is not required).
• Applicable to a wide variety of rod base materials.
• Good lubricant retention characteristics.
• Relatively low cost.
• Can be applied to thicknesses up to approximately 40 mils.

Disadvantages include:
• Bond strength is low. Coatings are mechanically bonded to the base metal.
• Fracture/peeling will occur unless the coating is continuously bonded to itself.
• Coatings are very porous. Must be impregnated with suitable sealers to minimize porosity (and avoid base metal corrosion).
• Coatings have relatively low hardness (Rc 30-40).
• Surface preparation prior to coating is critical to adequacy of bond.
• Coating quality is likely to vary widely from shop to shop.
• Relatively slow powder/wire heating results in greater oxidation and some change in coating chemical composition.
• Fair surface finishing characteristics.
• Finish machining is required.

An extension of the basic metal spray process is the post-application fusing of coatings. Coatings are applied in the manner described above. Then one additional step is taken. The deposited metal spray coating is fused with the base metal by use of an oxyacetylene torch or controlled furnace atmosphere. The resulting bond is molecular in nature and is claimed to be much stronger. Coatings up to 0.065 inch can be applied. Hardness ranges from approximately Rc 55 to 63. Corrosion resistance is excellent.

In order to utilize the metal spray and fusion process, the base metal must have a melting point higher than 1950°F. High temperatures required to achieve fusing of the coating may result in rod distortion. In addition, when the carbon content of steel rods exceeds 0.25%, special precautions must be taken to avoid an annealed metal. Any previous heat treatment applied to the rod to achieve improvement in physical properties is lost. Annealed rods must be derated to maintain safe operating stress levels. Fusing followed by air cooling could result in the formation of brittle martensite, depending on the hardenability of the base metal alloy.

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