Steel-finishing,The Final Necessary Touch.SOLUTIONS with Effective, Practical Advice
Whatever treatment is selected, it must be preceded by thorough degreasing and cleaning to eliminate from the surfaces all traces of contaminants likely to hinder the successful application. The information covering this kind of preparation can be the subject of another page. Steel-finishing is needed because usual mild or low alloy steels are readily rusting when exposed to the atmosphere, especially in presence of moisture. For getting an idea of the measure and the success of protective processes one has only to think of the millions of cars traveling for tens of years around the world without rusting. Important AnnouncementWhen you need Welding-metallurgy knowledge (Sponsored Links) Steel moving parts are usually dipped in oil or covered with grease that, besides lubricating, prevents the formation of corrosion or rust. During fabrication, storage or shipping, steel surfaces may be protected with rust-preventive removable compounds made of various materials with additives and inhibitors and with variable degree of protection. The application of paints is generally performed on steel surfaces chemically treated. Among the most used nonmetallic coating processes are phosphate coatings, where the surface is converted, by reacting with phosphoric acid, to an integral mildly protective layer of insoluble crystalline phosphate. Similarly chromate conversion coatings (from reaction with chromic acid) provide some protection when applied on steel either bare or cadmium or zinc plated. Painting for Steel-finishing means applying a thin organic coating on the surface for protection or decoration on suitably prepared surfaces, in a wide range of pigments and media with a large variation of characteristics and properties. Enamels and lacquers are different classes of protective coatings for specific purposes. Electroplating is a special type of Steel-finishing where electro deposits of various metals are applied on the surface for a variety of purposes. Dissolved metal ions in an electrolyte solution are moved by an electric field and are reduced at the interface such that they attach onto the cathode. Cadmium and zinc are used for protecting from atmospheric corrosion. In recent years rules for environment protection imposed severe restrictions on the use and disposition of chemical solutions for Steel-finishing, considered a threat to the environment. Steel parts submitted to Cadmium electroplating (and possibly to other metals too) should be thoroughly stress relieved before plating. For hardened steels with hardness equal to Rockwell C 35 or higher, special procedures must be adopted including a long baking period between 3 and 24 hours at a temperature between 175 to 200 0C (350 to 400 0F) after plating because of the danger of Hydrogen Embrittlement. Copper plating is used as first layer for other metal plating, as a stop-off in thermal diffusion treatments (carburizing), or for decorative purposes if overcoated with a clear lacquer. Nickel plating, with or without underlying copper strike, is one of the oldest decorative Steel-finishing treatments. The distribution of the nickel layer upon the substrate is strictly dependent on the density of current flowing from the cathodic surface, which is higher at protruding areas, with consequent accumulation of excess metal unless special provisions are put in place. To avoid the problem of non uniform metal deposition, special non electrolytic nickel plating methods were developed, called also electroless nickel plating, that use an auto-catalytic reaction. It provides a deposit of uniform thickness regardless of part geometry. Two types of Steel-finishing chromium plating are in use: the decorative type, and the hard chromium type. Because the chrome plating chemicals are very toxic, the disposal of chemical effluents is regulated almost everywhere. Severe limitative requirements are now in place in the use of compounds (mainly hexavalent chromium) from the process, for environmental protection. Much work was dedicated to develop suitable alternatives. Other metals like tin, lead, silver and gold can be electroplated for Steel-finishing, for decorative and other purposes. Other coating processes include hot dipping of aluminum, pack diffusion, slurry process, metal spray and more. Hot dip galvanizing of steel parts in molten zinc and in baths containing also other elements. Tin is also coated by immersion or by wipe tinning. Lead was applied in the past but is no longer approved for current use because of poisoning concerns. Porcelain enameling and ceramic coating are additional types of Steel-finishing used in the past for a variety of household objects, although they are nowadays largely supplanted by plastic materials. Steel-finishing instructions are an essential part of engineering drawings, and if special requirements must be observed during fabrication, they must be spelled out clearly and adequately in manufacturing documents. SAE AMS2400W SAE AMS2403L SAE AMS2404F SAE AMS2406L SAE AMS2410K SAE AMS2411G SAE AMS2412J SAE AMS2418G SAE AMS2424F SAE AMS2460 SAE AMS2485K
To reach a Guide to the collection of the most important Articles from Past Issues of Practical Welding Letter, click on Welding Topics. Hardfacing
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