The selection of the proper metal to be used for a job is the most important step in doing a job successfully. Each metal has different characteristics of strength, shrinkage, hardness etc so that there is a metal for almost every purpose. When a metallic wire is sprayed, it forms an alloy different from the original one, which is why when choosing a metal it is necessary to always consider the properties of metal deposition and not just the properties of the original metallic wires. All sprayed metal is porous and brittle as compared with the wire from which it was sprayed as a result of “spraying”. When metal is sprayed it tends to shrink slightly and hence set up internal stresses. The real strength of any sprayed coating is the difference between its original inherent strength and these internal stresses. Since the stresses are produced by shrinkage, those metals which shrink least will have the least internal stresses. Sprayed metals which have high tensile strengths and low shrinkage factors will obviously give the strongest coatings which are least likely to crack or lift. These points are very important, and explain why wires for spraying should be chosen on the basis of the properties of the metallic coatings and not on the basis of imitation and consideration of the parent material.
In general, the maintenance welder will be required to weld according to absolute specification, in which case the filler metal and welding procedures are spelled for him. In the absence of a specification, if the welder can identify the parent metal, he may match the filler metal to the parent metal as closely as possible. If he cannot identify the metal, other than that it is a high nickel alloy (for example), the best practice is to stay with the higher nickel content and choose the other alloying elements according to his knowledge of what those elements might contribute to the weld deposit under the welding in-service environment. When depositing comparatively thin coating on known parent metal, any metal can successfully be sprayed, since the metal is chosen with respect to its hardness and wear resistance characteristics or machinability or its corrosion resistance properties. This type of coating is usually carried out on the majority of parts for thin coatings (up to 1.5mm).
Surface roughening is generally necessary for coatings over 1.5mm thickness wherever there is an edge, such as at a key way. In this case, high shrinkage metals, such as SPRASTEEL 10 or METCOLOY 1 should be used. They should also be used for coatings over 3.0mm thickness wherever there are edges and for coatings that do not have edges (such as continuous coatings on cylindrical surfaces, where the conditions of the service are extremely severe, where there is the danger of layer separations in the coating or where the coatings are heavy and high shrink metals so that danger of cracking exists). In the cases where coating metals are chosen for wear resistance and hardness, steels are not machinable on lathes, and where grinding equipment is available, it is recommended to use METCOLOY 2, SPRASTEEL 80, Nickel-Base Alloys and Self-Fluxing Alloys for heavy coatings with the highest shrinkage.
At this juncture, it is worthy to note that, grinding heavy coatings of hard alloy materials do produce tensile stresses, which will definitely reduce the hardness and wear resistance of components. In order to obtain maximum hardness and wear resistance of components at minimum cost, there is therefore the need to apply technological inheritance technique.
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