For the successful hard surfacing or overlaying operation a welding procedure should be established. The procedure should be related to the particular part being surfaced and the composition or analysis of the part. It should specify the welding process to be used, the method of application, the prewelding operations such as cleaning, undercutting, etc.
The welding procedure should also give the preheat and interpass temperature and any special techniques that should be employed, such as the pattern of hardsurfacing, the method of welding whether beading or weaving, the interface between adjacent beads, and finally, any postwelding operations such as peening and the method of cooling. When a properly developed procedure is followed the service life of the job will be predictable.
The selection of the surfacing alloy was discussed. In many cases two separate materials may be required: the buildup alloy, which is used when the part is to be reclaimed or is excessively worn, and the hardfacing alloy. In general, over three layers of hardfacing alloys are not deposited. The hardfacing alloys are considerably more expensive than buildup alloys. The hard-surfacing should be replaced when the hardfacing alloy is worn away.
When deposit exceeds three layers other problems may be encountered such as cracking, etc., which will influence the service life of the deposit. The other factor to be considered is dilution. This is the diluting of the hardfacing alloy with base metal. Excessive dilution will reduce the effectiveness of the hard-facing material. Excessive penetration and poor tie-in of adjacent beads should be avoided.
In common with all welding operations, fabrication, repair, or surfacing, the base metal to be welded must be clean. Shot blasting, grinding, machining, and brushing are all methods for cleaning the base metal prior to welding. A major consideration is the location of finished surface with respect to the worn surface. In many cases, the first layer of surfacing may have sufficient dilution of base metal so that it is unsuitable for the desired service.
In this case, the worn surface should be further removed so that there is sufficient room for two layers of surfacing metal. This will provide a better service life. There are other situations in which the part is to be re-machined after surfacing and it is not recommended that the machining surface be at the interface between weld surfacing metal and the base metal. Here again, pre-machining may be required. This is particularly important when the base metal is of hardenable material.
Preheating, interpass temperature, and cooling of the part being surfaced are as important in surfacing as they are in repair welding or in original fabrication. The factors that apply to the welding of the base metal in normal fabrication should be followed when overlaying with surfacing weld metal. Preheating is used to minimize distortion, to avoid thermal shock, and to prevent surfacing cracking. The temperature of preheat depends on at least two factors, the carbon and the alloy content of the base metal and the mass of the part being surfaced.
A soak type preheat should be used and sufficient time must be allowed for the preheat temperature to stabilize throughout the part. If it is extremely complex in shape, preheat should be increased. If the ambient temperature is low, preheat should also be increased. Any part that is preheated should be maintained at that temperature throughout the entire welding operation and should then be allowed to slow cool.
The base metal composition must be known in order to provide proper preheat temperatures. Certain materials such as austenitic manganese steel should be treated in accordance with the requirements of the steel. In this case preheat should not exceed 260°C (500 °F). Cast iron should be given sufficient preheat. Cast iron is crack sensitive and normally it is not hardsurfaced because it is relatively inexpensive compared to the cost of surfacing metal and it may be more economical to replace the part than to surface it.
The thickness of the surfacing deposit is extremely important. If the deposit is too heavy, problems can be encountered. Hardfacing alloys should be restricted to two layers. The first will include dilution from the base metal, but the second layer should provide the properties expected. Some types of alloys can be used in three layers.
Surfacing for corrosion resistance and others are recommended for use of one layer only. When edges are being built up with surfacing material make sure that sufficient material is removed so that the edge has at least two layers of surfacing metal prior to re-machining or grinding. Consult the manufacturer’s data for the particular product involved.
A weaving technique is recommended instead of stringer bead welding. In addition, the pass thickness or layer thickness should not exceed 5 mm. The adjacent beads must fair into the previous bead to provide as smooth a surface as possible.
There is considerable controversy concerning the exact pattern of welds that should be made when applying the surfacing deposits. In general, the direction of welding should not be transverse to the load on the part. This can create stress concentrations and may affect service life of the part. Diagonally-shaped welds have an advantage in this regard. In certain types of metal, peening is recommended but this is based on the metal. The manufacturer’s instructions should be followed.
Hardfacing by welding is an excellent method of reclaiming parts and will save considerable time and money. It will often reduce downtime of equipment and may keep equipment going without as much downtime. It is considerably cheaper than replacing original parts and should be used whenever possible. It is now becoming popular for original equipment manufacturers to actually hardface wear parts on new equipment to provide better service life of the equipment.
The corrosion of metals is one of the less known but more expensive of the factors that cause premature failures of many things. These range from automobile bodies to chemical plant equipment to ship hulls. The cost of corrosion is difficult to measure but should include the loss of efficiency of operating equipment such as pumps, mixers, valves, etc., as well as total failures. Fortunately, corrosion can be prevented or at least substantially reduced so that metal parts will have a longer life cycle.
One of the best ways to reduce corrosion is to protect the metal with an overlay or surface of a material less susceptible to corrosion in a specific environment. Coated metals such as galvanized steel and clad metals with nonferrous facings have long been used to reduce the effect of corrosion. In more and more applications, surfaces that are sprayed or welded are contributing to longer service life of parts exposed to corrosive atmospheres.
It was mentioned previously that the deterioration of metal surfaces is caused by the combination of factors, such as corrosion and oxidation, corrosion and erosion, or cavitation.
In repairing corroded or deteriorated surfaces it is necessary to analyze the reason for the deterioration. These factors should be considered in designing new surfaces for specific types of service. That is, consideration should be made in selecting a material for overlays to prevent corrosion.
There is considerable confusion in this field concerning the proper terms to use for the weld surface that is applied. The general term surfacing is sometimes used but more often the term cladding is used. The terms weld buildup and buttering have no official status; however, the term corrosion-resistant weld-overlay cladding does provide an understanding of what is being covered in this section.
Cladding of this type is applied for many reasons:
- to produce a corrosion-resistant surface as on the inside of a nuclear pressurized vessel,
- to produce a corrosion-resistant material to replace a higher-priced or unavailable high-alloy material,
- to produce a metallurgical structural composition that is more weldable,
- to deposit weld metal which would later be used as a filler metal such as in a tube-to-tube sheet weld, or
- to produce a wear- or erosion-resistant surface.
Various methods or techniques can be used to provide these surfaces, such as explosive clad metal, roll bond clad, and loose cladding liners, plug and seam welded to the inside of a vessel or tank. Many of the welding processes can be used for applying liners. When attaching liner plates or sheets to carbon mild steel the problems of welding dissimilar metals must be considered. This involves the metallurgical requirements of the clad material and the compatibility of the two materials.
When the solid solubility, that is, ability of one element to be dissolved in another, is exceeded, cracking may occur. In addition, the effects of elements such as sulfur and phosphorous from dilution can be a source of trouble. The welding technique and procedure involving the selection of filler metals, coatings, fluxes, etc., must be considered as with any dissimilar welding operations. These same factors apply whether the material is being applied as a weld surfacing or as separate sheets or plates welded to the carbon steel structure.
There are a number of alloys that are used for overlays or clads for corrosion and oxidation resistance. These are usually standardized compositions commonly used by themselves for the same requirements. These are summarized as follows.
The copper-based alloys are used for certain corrosion requirements. The copper silicon alloys and the copper tin alloys are used for certain corrosion-resistance requirements.
The austenitic stainless steels, which include the standard alloy types 308, 309, 310, 316, and 347, are all used for corrosion-resistant surfaces. These alloys exhibit moderate resistance to high-stress abrasion and have excellent oxidation-resistance and impact properties.
The nickel-base alloys are also used for this purpose. This includes 100% nickel, the Monel (67 Ni-30 Cu) and Inconel (72 Ni-7 Fe-16 Cr). These alloys are frequently used as overlays on carbon and low-alloy steels for cladding of tanks and vessels.
The high-cobalt chromium alloys are used for specific overlays when corrosion is a major problem. These are used quite often in refineries where high pressures, high temperatures, and corrosive materials are pumped and stored. These alloys can be applied in several ways; as a powder applied by the plasma process, as a cold wire, or by covered electrodes with the shielded metal arc welding process. The selection of the overlay is based entirely on the requirements of the materials to which the product is exposed. The selection must be based on normal metallurgical factors.
A unique but rather typical application of weld overlay is used in the repairing of digesters used in pulp and paper mills. Digesters are tanks or pressure vessels ranging in height from 25-50 ft and in diameter from 8-12 ft. They are used for the first chemical processing step of converting wood chips into pulp for paper manufacturing, primarily in the sulphate or kraft paper process.
The wood chips are placed in the digester and are cooked in a highly corrosive alkaline solution. The mixture of wood chips and alkaline liquor is under pressure and operates at a relatively high temperature. The digester is made of carbon steel of from 1 to 2 inches thick. The internal surfaces of digesters corrode at a high rate at the surface of the liquor due to the corrosive action of the alkaline solution. The steel walls of the vessel will gradually deteriorate until they become so thin that pressures and temperatures must be reduced for safety. Unless the metal is replaced by welding the digester will eventually become unsafe and will have to be abandoned.
Welding has been employed to repair the pitted or corroded areas and to rebuild wall thickness to original dimension. Originally, carbon steel weld metal was used. It was found, however, that stainless steel electrodes provide a surface that is less subject to the corrosive action. Tests revealed that stainless overlay outlasts the original carbon steel many times.
Recently the gas metal arc welding (GMAW) process has been used for this overlaying operation. Automatic methods have been used to make the overlay welds more rapidly than with manual application. The automatic application will deposit weld metal in horizontal beads on the vertical inside circumference of the tank. The automatic welding heads are mounted on a boom that rotates about the centerline of the tank and deposits metal as it revolves inside the tank. It is possible to utilize two or even three automatic heads that automatically travel around the inside circumference of the tank. This work is done starting at the lower portion to be welded and moves upwards as it revolves. The most popular procedure uses either 316 or 310 stainless alloy in the 0.035-in. diameter electrode wire with argon for shielding.
In normal applications the inside diameter of the digester is prepared for welding by grit blasting the entire surface to be welded. This may be followed by an acid wash and water rinse. The welding operation, once it is begun, is usually continuous to eliminate any voids in the surface. Each pass must fair smoothly into the previous one and the depth of the surface should be from 1/8 to 3/16 of an inch thick.
This technique is used occasionally for new digesters to reduce the rate of corrosion and the length of time between maintenance repair work. Penetration must be closely controlled so that dilution will not appreciably lower the alloy content of the deposit.
Other procedures for accomplishing an overlay on the inside diameter of smaller tanks are done by rotating the tank and doing the welding in the flat position. In this case, the welding is done by the submerged arc process using one or more electrode wires. There are some situations in which the strip overlay method is used. The submerged arc welding process increases the speed of making the overlay. In some cases the gas tungsten arc welding (GTAW) or plasma hot wire process is used. The process should be selected which is most appropriate for the position and the job to be done.
Normally single layers are used; however, for certain applications such as pump linings and wear areas a second layer of surfacing is applied. The second layer can be made with an electrode of lower alloy content since the dilution factor is drastically reduced.