Repair-weldingonly when feasible.SOLUTIONS with Effective, Practical Advice
Repair-welding is one of the most used welding applications because of a long record of successful realizations. An experienced welder with substantial knowledge in the type of equipment to be repaired can be expected to get away easily with it. In other cases the results can be ruinous. It all comes back to basic principles and to a good dose of common sense, an important ingredient even in high tech implementations. One should be cautious not to attempt any welding to highly stressed parts like gears and shafts, or to items of lifting equipment likely to endanger people in case of collapse. Special cases where Repair-welding may be required are those affecting pressure vessels or equipment designed to operate at elevated service temperatures. These are regulated by appointed authorities prescribing definite procedures that must be followed. On this subject see the note on the "R" Stamp, published (11) in Issue 54 of Practical Welding Letter for February 2008. The preparation and the repair process should be allowed to be carried out in a calm and encouraging atmosphere. What often happens however is that the failure of an essential element of complex machinery occurs at the most damaging moment, when a large manufacturing effort is under way to meet pressing delivery schedules. Therefore the production manager may have little patience for the doubts and needs of the repair team. What is needed is the most complete knowledge on the material and condition of the broken parts. This information could be included in the manufacturing drawings of the equipment, but if not, it should be achieved by indirect methods. The additional missing link is a clear understanding of the causes determining the failure, in order to avoid its recurring, after the equipment is being returned to operation. Failure analysis is a complex discipline that is seldom pursued in sufficient depth except in cases where human lives were endangered or could have been. It should be understood however, that if sufficient effort and thought are not devoted to learn what happened, improper Repair-welding could result in greater harm. Even if a failure investigation is not attempted, people involved in disassembling the equipment should leave the appearance of the fractured surfaces as is. One should not try to match the broken surfaces together, because important clues referring to fracture origins might be canceled in the process, in case someone is asked to investigate that later. Management should be fair to the maintenance team attempting Repair-welding. As they may be urged by management to proceed with the repair while complete information is lacking, they cannot be made accountable for errors. Not all repairs should include welding, especially if doubts are raised as to the weldability of the materials involved. Mechanical fastening and even adhesive bonding might be suitable methods, especially if intended as a temporary solution until spare parts are obtained. To decide on the material type involved, x-ray fluorescence (XRF) is the suggested qualitative method because it is immediate and non destructive. It must be carried out by knowledgeable people that understand the makeup of the parts, in case these are not homogeneous. Plating and possible surface treatments should be accounted for. See our page on Material Identification. The material condition, assuming it was not affected by failure, can be determined by having hardness testing performed by knowledgeable people. One should remember that Repair-welding will usually include the application of heat. Its influence must be accounted for, because it may affect the parts to the point that their original strength and hardness are reduced remarkably. In that case the repaired parts may no longer be able to sustain service stresses. If it is decided that Repair-welding is an acceptable solution one has still to select the process, the filler metal and the procedure most suitable to the application. Depending on the base metal, special precautions may be needed like preheating and postheating. Finally one has still to inspect the Repair-welding according to acceptable best practice. Will the production manager be satisfied? He/she will, only if the broken equipment goes quickly back to normal operation. In any case don't be disappointed if exceptional thanks are not profuse. Our article on Repair-welding of service failures of welded parts is available by clicking on Weld Repair.
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