Welding-cobaltsafely for demanding applications.SOLUTIONS with Effective, Practical Advice
Other applications of welding cobalt alloys involve hardfacing valve seats of internal combustion engines. Cobalt alloys are used in Dental applications and for Prosthetic Bone Implants. (Sponsored Links) A previous page dedicated to heat resisting alloys includes also a discussion of some Cobalt alloys that have suitable properties for certain applications. See Heat Resisting Alloys Welding. The microstructure of cobalt alloys at room temperature is called austenitic, face centered cubic structure, and is similar to that of austenitic stainless steels. Cobalt and cobalt alloys in annealed wrought form are quite ductile and readily formed to the required shapes. Their Welding-cobalt characteristics are similar to those of wrought nickel alloys. Development and qualification of Welding Procedure Specifications is required as the best way to assure quality production. Heat input for Welding-cobalt should be kept in the low to moderate level for better results. Copper contamination must be avoided because it will readily cause cracking while Welding-cobalt. Liquid Metal Embrittlement (LME) is initiated by molten copper in the heat affected zone. Care must be taken therefore and any contact with copper from whatever sources must be prevented. Stainless steel wire brushes are recommended, if needed, for interpass cleaning. Cast cobalt alloys may need to be welded for fabrication or repair. Welding-cobalt is done with arc welding, plasma arc welding, resistance spot welding and with high energy processes. Oxyacetylene welding is not recommended. Submerged arc welding if used with high heat input could cause solidification cracking of the weld metal. Electron Beam Weldability is reported good for HS-21 in unrestrained joints, but poor in restrained joints. Cast alloy H-31 (X-40) is reported good to fair. Alloy S-816 is reported fair. Good Welding-cobalt practices require adequate joint preparation and thorough cleaning before welding to assure sound joints. All thermal or mechanical cutting methods are acceptable for preparation, provided that all the surfaces are then ground or polished before Welding-cobalt, to clean and bright metal condition including a 25 mm Cleanliness is very important. Contamination by greases, oils, cutting oils, crayon marks, corrosion products, lead, sulfur, and other low melting point elements, paint, scale, dye penetrant solutions, and other foreign matter should be completely removed because theycan cause severe cracking problems. Before Welding-cobalt, the joint surfaces and their surroundings must be thoroughly degreased with proper clean solvents. Shop compressed air, if used, should be free from traces of oil. Preheating of cobalt base corrosion and heat-resistant alloys is not required. Metal frozen in cold weather should be allowed to reach room temperature in the heated shop before welding, to avoid condensing moisture on the surfaces. Interpass temperature should be below 93 0C (200 0F). To control the interpass temperature, auxiliary cooling methods like water quenching may be used. Stainless steel wire brushing with clean brushes is normally adequate for interpass cleaning of GTA and GMA weldments. The grinding of start and stop craters is recommended for all fusion welding processes. The following cobalt base alloys are strengthened by the presence of tungsten and molybdenum in solid solution: Haynes 25 (L-605), Haynes 188, S-816, Stellite 6B and UMCo-50. The composition includes chromium for corrosion resistance, precise content of carbon and of other elements that provide carbide formation. Among the Precipitation Hardenable alloys, these are found: AR-213, MP-35N, MP-159. Welding results in solutioning of age-hardening constituents with consequent reduction of mechanical properties that can partly be restored by suitable post weld heat treatment. Fusion Welding-cobalt is performed usually with matching composition of filler metal to that of base metal.Where dissimilar metal welds are involved, selection of the welding filler metal depends upon the specific circumstances. The base metals and filler metal should be metallurgically compatible, avoiding crack susceptible microstructure or dangerous phases which might promote hot cracking, embrittlement, or other metallurgical problems. Furthermore the as-welded joint should have mechanical strength and toughness at all the temperatures developing during the intended service. If low ductility interferes with further forming of wrought cobalt alloy weldments, a solution anneal at 1121 0C (2050 0F) with water quenching is recommended. Any questions or comments or feedback? Write them down and send them to us by e-mail. Click on the Contact Us button in the NavBar at top left of every page.
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