Abstract: Repair welding and surfacing are both considered in the field of maintenance welding and are covered together since they are both done by the same welders. Often it is extremely difficult to separate what is considered repair welding from maintenance welding, and surfacing can be included in both situations. The same basic factors apply to both weld repair and surfacing. Repaired parts may be more serviceable than the original part, since they can be reinforced and the weaknesses of the original part corrected. It is often more economical to weld repair since the delay in obtaining the replacement part could be excessive and the cost of the new part would normally exceed the cost of repairing the damaged part The need for weld repair and surfacing
There are probably more welders employed doing maintenance and repair welding than there are in any other industry grouping. The work done in the primary metal industry is primarily maintenance and repair. This is true also of the utility services category and by combining these with repair services you find that approximately 18% of the welders are engaged in this type of work. In addition, it has prime importance to welding since the earliest use of welding was for repair work. The most famous incident happened at the outbreak of World War I when German ships were interned in New York harbor. Their crews, hoping to make the ships inoperable, sabotaged the engines and machinery. However, by means of welding, repairs were quickly made and the ships were placed in transatlantic service to deliver material from the U.S. to Europe. Repair welding and surfacing are both considered in the field of maintenance welding and are covered together since they are both done by the same welders. Often it is extremely difficult to separate what is considered repair welding from maintenance welding, and surfacing can be included in both situations. The same basic factors apply to both weld repair and surfacing. Parts break and wear out continually. It may be impossible to obtain another part exactly like the one that broke or wore out. This is particularly true of older industrial machinery, construction machinery, agricultural machinery, machine tool parts, and even automobiles. Repaired parts may be more serviceable than the original part, since they can be reinforced and the weaknesses of the original part corrected. It is often more economical to weld repair since the delay in obtaining the replacement part could be excessive and the cost of the new part would normally exceed the cost of repairing the damaged part. Weld repair is commonly used to improve, update, and rework parts so that they equal or exceed the usefulness of the original part. This is normally attained, with the possible exception of weld-repaired cast iron parts that are subjected to heating and cooling. Weld repairs on cast iron parts subjected to repetitive heating and cooling may or may not provide adequate service life. The problem is that cast iron parts subjected to high-temperature heating and cooling, such as machinery brakes, furnace sections, etc., fail originally from this type of service and due to metallurgical changes the weld may fail again without providing adequate service life. Except for emergency situations, it is not wise to repair cast iron parts of this type. The metal that the part to be repaired is made of has a great influence on the service life of the repaired parts. Parts made of low-carbon and low-alloy steels can be repaired without adversely affecting the service life of the part. On the other hand, high-carbon steels may be weld repaired but must be properly heat treated if they are to provide adequate service life. It is absolutely essential that we know the type, specification, or composition of the metal that we are planning to weld. As mentioned above, it may be unwise to weld repair certain metals. But we should not weld on any metal unless we know its composition. The economics of weld repairing are usually very favorable and this applies to the smallest or the largest weld repair job. Some weld repair jobs may take only a few minutes and others may require weeks for proper preparation and welding. Even so, the money involved in a repair job may be less than the cost of a new part. A part made of any metal that can be welded can be repair welded or surfaced. In fact, some of the metals that are not normally welded can be given special surfacing coatings by one process or another. All the arc welding processes are used for repair and maintenance work. In addition the brazing processes, the oxy-fuel gas welding processes, soldering, thermit welding, electro slag welding, electron beam welding, and laser beam welding are also used. The thermal spraying processes are all widely used for surfacing applications. In addition, the various thermal cutting processes are used for preparing parts for repair welding. The selection of the appropriate preparation process and welding process depends on the same factors that are considered in selecting a welding or cutting process for the original manufacturing operation. In the case of repair welding, there are usually limitations, such as the availability of equipment for a one-time job and the necessity of obtaining equipment quickly for emergency repair work. This limits the selection and it is for this reason that the shielded metal arc welding process, the gas metal arc welding process, the gas tungsten arc welding process, and oxyacetylene welding and torch brazing are most commonly used. However, for many routine and continuous types of repair work some of the other welding processes may be the most economical. For example, submerged arc welding is widely used for building up the surface of worn parts. The electro slag process has been used to repair and resurface parts for hammer mills, for construction equipment, and for rebuilding rolls for steel mills. Thus there is a difference in the selection of the welding process for the routine, continuing types of repair and surfacing work versus the one-of-a-type or breakdown emergency repair job.
Analyze and develop rework procedure
The success of a repair or surfacing job depends on the thought and preparation prior to doing any actual work on the project. Many factors must be considered in making a thorough analysis. A thorough analysis as outlined may not be required in many situations. This is due to experience gained by welders and others in analyzing jobs, making repairs, and then checking on the service life of the repaired part. As experience is gained many short cuts can be taken, but it is the intent to provide a detailed method of analyzing jobs so that the repair will be as successful as possible. One of the reasons for such an investigation is to establish the cause of the failure in the case of a broken part or the cause of wear or erosion in the case of a part to be surfaced. The four points outlined are:
- Make a detailed study of the actual parts that failed.
- Learn the background information concerning the specifications and design.
- Make an investigation of the materials used.
- Make a listing of all of the facts so that at the conclusion the reason of failure will be as accurate as possible.
There are certain situations and certain types of equipment for which repair welding may not be done or may be done only with prior approvals. Certain types of containers and transportation equipment must not be weld repaired or may be welded only with special permission and approval. These include railroad locomotive and car wheels, high-alloy high-strength truck frames, and compressed gas cylinders. Most pieces of power-generating machinery, including turbines, generators, and large engines, are covered by casualty insurance. Weld repair on such machinery can be done only with the prior approval of the welding procedure by the insurance underwriters. In some cases, approval may not be granted. An example of this can be cast iron crankshafts in large stationary diesel engines. Certain weld repairs may be made but it is necessary to develop a written procedure which must be approved in writing by the underwriting company’s representative. Repairs by welding to boilers and pressure vessels require special attention. Pressure vessels that carry an ASME stamp or are under the jurisdiction of any state or province or government agency must be repaired in accordance with the National regulations issued by responsible authorities. Repairs by welding are limited to steels having known weldable quality. It provides a maximum carbon content of 0.35% for carbon steels and a carbon content of 0.25% for low-alloy steels. For welding high-alloy materials and nonferrous materials the work must be done in accordance with the ASME code. Welders making such repairs must be qualified based on the thickness of the material and the type of material being welded. Full-penetration welds are required with welding recommended from both sides. Permissible welded repairs are defined as cracks, corroded surfaces, and seal welding, patches, and the replacement of stays. A repair is the work necessary to return a boiler or pressure vessel to a safe and satisfactory operating condition. Alterations are also permitted and this is a change in a boiler or pressure vessel that substantially alters the original design and in this case work can be done only by a manufacturer possessing a valid certificate authorization from ASME. All alterations must comply with the section of code to which the original boiler or pressure vessel was constructed. A written repair procedure is required for doing either repair work or alterations. In the case of an alteration a record must be made and all alteration work must be approved. These records must be filed with the inspection agency or the jurisdictional agency, the National Board of Boiler and Pressure Vessel Inspectors, and all work must be inspected. Alterations on bridges, large steel frame buildings, and ships may be done only with special authorization. The alteration work must be designed and approved. The welders must be qualified according to the code used and the work must be inspected. Written welding procedures are required. Once the decision has been made to make a weld repair it is then necessary to establish why the part failed or wore out. This relates to the type of repair job since it also determines whether reinforcing may be required. Reasons for the part to fail or wear out can be among the following:
If the part failed because of an accident or an overload, it may be returned to service with the weld repair made to bring it back to its original strength. The same consideration applies if the part has been abused or misapplied. It may be necessary to reinforce the part so that it will stand temporary overloads, misapplication, or abuse. This decision should be made prior to the weld repair. In the case of poor workmanship, poor design, or incorrect material the weld repair should eliminate the poor workmanship that was responsible for the failure. In this case, the part would be returned to its original design. If failure is due to poor design, design changes may be required and reinforcement may be added. In a case of wrong material it will be assumed that the material was of a lower strength level which contributed to the failure. In this case reinforcing would be required. If the repair or alteration job is to modify the part, it is necessary that the modification be designed by competent designers who have the knowledge of the design conditions of the original part. This may require reinforcing to make sure the modification or alteration is satisfactory. Another important factor that must be considered is what results are expected of the repaired or reworked part. Should it be reinforced or should it be redesigned and altered to provide necessary service life? Finally, in the case of surfacing, what better surface could be provided to withstand the service that caused the premature wear or failure?
A written repair procedure is required for all but the most simple jobs. It is absolutely necessary that the type of material being welded is known. This can be found in several ways. If possible, refer to the drawing of the part and the specifications that are shown for the part or parts to be welded. If this is not possible, particularly in the field or at the maintenance shop, look for clues as to the type of metal involved. Analyze the application of the metal, for clues. For example, an automobile engine block is normally cast iron except for some which might be cast aluminum. Aluminum and iron are easily distinguishable. The spring of an automobile or truck would normally be high-carbon steel. The body structure of a car or truck would be mild steel. The appearance often helps provide clues. As a final resort it may be necessary to obtain a laboratory analysis of the metal. Filings or a piece of the metal must be sent to a laboratory capable of making such determinations. The normal method of selecting the welding process will be followed once the material to be welded has been identified. This involves the type of metal, the thickness of the metal, the position of welding, etc. This also leads into the question of filler metal to be used. After this, the normal method of filler metal selection is followed. This involves matching base metal composition, matching the base metal properties, particularly strength, and providing weld metal that will withstand the service involved. In surfacing, the surface characteristics desired for the finished job depend entirely on the service to which the surface will be exposed. This is based on knowledge and experience and on the fact that the surface has deteriorated to the point that it needs to be reworked or resurfaced. When wear is involved, surfaces can be rebuilt many times without reducing the strength of the part and the service life will be greatly extended. The repair procedure should be very similar to a procedure developed for welding a critical part. It should include the process and filler metal and the technique to be used in making welds.