Weld-cladding,Proper Answer to Suitable Jobs.SOLUTIONS with Effective, Practical Advice
Weld-cladding is generally intended as the application of thick layers of corrosion resistant alloys upon common steel base metal. Cladding materials are usually stainless steels or nickel alloys. Other materials combinations are also possible but will not be treated here. The application of hard, wear resistant layers on new or worn out items is called Hardfacing. Although the materials used are different, processes and methods are much the same as those used for Weld-cladding. Applications here are containers or pressure vessels and reactors for chemical processing where an economic advantage is sought by modifying the surface in contact with the corrodent, through application of a different material with improved corrosion resistant properties. A composite clad material can be purchased ready to be used for fabricating the reactors, containers etc., if it is available in time and if the economics of this option are more favorable. The manufacturing process for realizing such a composite is described elsewhere (see Explosion Welding). In that case special welding procedures must be adopted for Weld-cladding the edges of the material on itself. In this page however Weld-cladding is intended for the cases where special composite material is either not available or not economic. One method consists in preparing strips of the clad material and welding them at the edges on the surface to be protected. Sufficient distance must be left between the strips so that the first passes that include the maximum dilution will be covered completely by further passes, to guarantee top welded passes of correct composition. Usually the finished fabricated and clad part will be heat treated for stress relieving after welding. In this case holes must be drilled in the strips to let the heated air at the interface escape freely. The holes will be plugged later by welding. Otherwise Weld-cladding is performed by any suitable and economic process of adequate deposition rate, by mechanized means under constant control of an operator. Flux cored arc welding (FCAW), Plasma arc hot wire process or any available version of submerged arc welding (SAW), with wire or strip electrodes possibly with more than one welding head, with or without oscillation to cover a larger area. Also hot or cold wire GTA Welding and Laser Beam overlaying with fused powders are used in certain cases. The main limitation of SAW is that it can be performed only in almost flat position, so that suitable accessories like turning rolls (welding rotators) and side carriages mounted on horizontal beams placed at the proper height for each job must be available. As briefly hinted at above, dilution of the base steel metal in the weld is a major concern. Dilution is likely to increase with heat input, so that a proper balance must be struck between optimum deposition rate and dilution, generally preferred between 10 and 15%. Dilution is calculated as the quotient of the amount of base metal melted divided by the sum of the melted base and filler metal added, expressed as a percentage. Control of dilution is achieved by adjusting welding parameters including amperage, polarity, electrode size, electrode extension, travel speed, oscillation, weld pool position relative to the arc, arc shielding and additional filler metal. A suitable evaluation of the microstructure obtained at any dilution can be obtained by using the Schaeffler diagram or any of the more modern versions thereof. The first Weld-cladding layer should avoid any presence of martensite. Further weld bead layers should aim to be mostly austenitic, but contain between 3 and 15% of ferrite to avoid that the deposit be sensitive to fissuring. Summing up, Weld-cladding as a method of providing corrosion resistance by welding metal overlays onto less resistant substrates is a proven and technically mature industrial development capable of supplying adequate solutions to different projects. Experience, as usual, is the most important ingredient for success.
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