Common types of repair for boilers with welded construction are replacing sections of boiler tubes, replacing tubes and stays, window weld, weld build-up of wasted area, welding gage holes, and many others. Under all conditions, only electric arc welding process such as shielded metal arc welding (SMAW) and gas tungsten arc welding (GTAW) is accepted. GTAW is normally used for root pass or on thin plate. Welding process and procedures must be taken seriously since any welding defects can lead to serious problems. Here are some fundamental guidelines for boiler repair by welding. I cannot cover all aspects of repair in a short article but I will try my best to help you learn a little bit more about this topic in simple technical terms.
First of all, you must know that the owner shall obtain approval from the Authorized Inspector responsible for the jurisdictional inspection prior to making any repair or replacement that affects the pressure retaining capacity of a boiler. Only Authorized Inspectors licensed by the Department of Occupational Safety and Health (Malaysia) or the National Board may authorize and document the repair forms.
The welding procedure should be carried out by a firm approved for Class 1 welding under the direct supervision of an experienced foreman and to the satisfaction of the third party surveyor and the authorized inspector. Where Class 1 permission is not available, appropriate performance tests should be carried out by selected operators to the surveyor’s satisfaction. Recognized codes such as ASME Codes or BS codes must be followed whenever applicable. The material should comply with the original specifications or be equal to them. The specifications can be obtained from the boiler blueprints. Acceptable welded attachments, weld form, and welding preparation outlined in the blueprint should be adhered to. Only welders who are experienced in Class 1 welded pressure vessel construction shall be employed on boiler repair works. Very careful supervision should be given at all stages of the repair work. Substandard workmanship, should never under any circumstances, be compromised. Low-hydrogen electrode must be used to prevent hydrogen-induced cracking. Low hydrogen electrode used must have the tensile properties similar to that of parent metal.
In welding cracks, carry out dye penetrant test or magnetic particle inspection first to locate the cracks or discontinuities. If defective areas need to be cropped by flame cutting, a suitable final weld preparation of double V, U, or J form should be made by careful chipping or grinding. ASME Code Section IX AF-613 specifies that preparation of plate edges, welding bevels, and chamfers and similar operations involving the removal of metal shall be by machining, chipping, or grinding, by gas cutting or gauging. Where welding is to be carried out in both the down hand and overhand position, the larger V or U preparation should be arranged for the down hand position. With this approach, the flow and penetration of weld material into the parent metal is the optimum. Welding strength depends on the penetration capabilities of the electrode onto the parent metal and the flow of the electrode material into the parent metal.
Where flame cutting or overheating has occurred on boiler shell, hardness plot should be made to determine the corresponding tensile strength by referring to the published table. By doing so, the affected area can be separated from ground material or area. This also gives a clear indication as to whether or not the material has been overheated to some extent that the microstructure of the material has been affected. Under no circumstances may welded repairs be undertaken when the microstructure of the parent material is suspected to have undergone any change from its original form. In any case, MPI should be used to ensure that there are no cracks present before the welding is commenced. The design of the repair should be such that there are no sharp corners and the new insert plate should be of similar quality and specification to that of parent material. This method is called window weld (or window patching).
Areas to be welded should be preheated to a minimum of 93oC for material thickness above ½” and up to 1½”; thicker plate requires higher preheat temperatures. Maintain at that temperature throughout welding process. Care should be taken to eliminate any cold draught from impinging upon the area under repair. Insulation mattress could be made available to protect the welded area from rapid cooling. After welding, grind flush the weld, and carry out DPI or MPI again.
Changes can occur in the metallurgical structure of the base metal adjacent to the weld (heat affected zone or HAZ). The changes can cause embrittlement or degrade the material properties. In addition, upon cooling, the weld metal shrinks to a greater extent that the base metal in contact with the weld (HAZ) exerts a drawing action. Therefore, post weld heat treatment (PWHT) is necessary to restore the ductility of the material and thus prevent stresses, shrinkage, and cracking. Finally, because welding usually affects the pressure-retaining parts of the boiler, the boiler shall be subjected to a hydrostatic test of 1½ times the MAWP for at least 15 minutes.