Kent Henry
Former EASA Technical Support Specialist
In the power transmission industry, a fair amount of cast iron is used. Whether it’s for motors, pumps, or gear reducers, many use cast iron for the bulk of their structure. This variety of usage results in service opportunities involving the repair of cast iron components.
Cast iron has a very high carbon content, so much so that the concentrations of carbon form graphite flakes that result in a high resistance to wear. The drawback of cast iron is that the high carbon content also makes castings brittle. Examples of brittle castings are terminal boxes and fan covers. If a forklift operator rounded a corner a little wider than normal and bumped into the terminal box and fan cover of a Totally Enclosed Fan Cooled (TEFC) motor made from steel, the impact would bend the steel components. Steel is a fairly ductile material. The repair of these parts may Figure 1. Example of crack prepared for welding. and fully weld this side of the be limited to hammering out dents in the terminal box and fan cover. If the same collision happened with cast iron components, the damage would be quite different. They would likely be cracked or even break into pieces due to the brittleness.
Repair in the service center
The damage to cast iron components can range from minor cracks to major reconstruction of components that have suffered multiple fractures.
To assess the extent of damage to a cast iron part, we need to clean the surface and remove coatings such as paint that may mask damage. First, apply aerosol gasket or paint remover, then scrub the area with a wire brush and then pressure wash the casting to remove any paint. Once cleaned, the best practice is to perform a dye penetrant test.
If you don’t have a dye penetrant test kit, you can take a low-tech approach. Use a rag or brush with kerosene on it to scrub the damaged areas. Then immediately follow up by rubbing the areas with chalkboard chalk. Any cracks will readily appear after a few minutes as the kerosene bleeds through the chalk. Just past the end of each crack, drill through the casting using a 1/8” (3 mm) drill bit to stop the crack from continuing. Cracks need to be prepared for welding. From one side of the crack, grind and bevel the sides approximately 45°, leaving a total of approximately 90° combined bevel. See Figure 1. Leave approximately 1/8” (3mm) of parent metal from the crack bottom. Preheat the casting Cast iron has a very high carbon content, so much so that the concentrations of carbon form graphite flakes that result in a high resistance to wear. The drawback of cast iron is that the high carbon content also makes castings brittle. casting. Working from the backside of the crack, grind and bevel as on the first side to a depth of half the casting thickness. Fully weld this side of the casting. In some circumstances the backside of the casting is inaccessible and welding one side must suffice.
A stator or pump frame with a foot that is fractured into several pieces may not arrive at the service center with all the broken pieces. Even with all the pieces, you may find that they have been deformed and no longer fit together properly to reassemble the foot. In these circumstances, it is better to start with a piece of cast iron large enough to create a new foot.
Design a replacement foot and remove as much of the original as necessary to allow a weld of minimum length. Design the replacement part so that it can be properly welded. In some instances the remaining section of foot may need to be removed to arrive at the best repair solution. Always bevel the areas to be welded to allow proper penetration and maximum strength.
Use creativity
Severe cases may require reconstructing the casting using pieces just as in solving a jigsaw puzzle. These situations require some creativity and finesse.
A critical task is planning how to hold the pieces in position for welding. If we have a heavy cross-sectional casting, we can pin the pieces together by drilling and tapping the pieces in proper relationship using mild steel bolts or screws. See Figure 2. Once screwed in place the heads of the bolts or screws are cut off with a die grinder. The edges of the pieces can now be beveled and prepared for welding as previously described. Do not grind into the bolts holding the casting together. Preheat the casting and weld the beveled sections. Now grind through the bolts as needed to obtain the proper bevel as previously described and fully weld the repair.
If the material has a smaller cross-sectional thickness that makes use of screws impracticable, get pieces of steel plate, bent and formed to position the parts. These supports or backers can be fastened to the casting pieces using tack welds to hold the assembly for preparation and during welding.
Preheat prior to welding
The biggest risk in cast iron repair is cracking from expansion or contraction during welding due to heating and cooling. Pre-Hole Drilled heating reduces Just Beyond The Crack the chance of cracking. Therefore, you should preheat cast iron prior to welding. However, there are some limitations. For instance, if welding in the field, preheating the part to over 100° F (38° C) can be the difference between success and failure.
When we have a motor stator with a good winding and a broken foot that needs repair, we can’t preheat to a temperature that would compromise the winding. In this case, a safe preheat temperature will be dependent upon the motor’s insulation system. If in doubt as to the insulation system design, assume the worst case and do not exceeded a Class “A” insulation system rating of 225° F (105° C). The best solution for preheating motors with good windings is a bake oven.
If the part to be welded can be separated from any lower temperature components, it should be preheated to a minimum of between 500°F(260°C) and 750°F(400°C). In a service center,
a great source of preheating to these higher temperature ranges is the burnout oven. Since the typical operating temperature for burn out ovens is about 680°F (360°C),it will not require any temperature adjustment for preheating of cast iron parts. When handling and working around preheated parts, use proper safety precautions to avoid contact burns.
The welding process
Increased carbon content, porosity, sand and brittleness present challenges to the welder. The use of a nickel welding electrode is typically best for the repair of cast iron using arc welding methods. A stainless electrode works well but in some instances may lead to increased stress and cracking. If brazing with an acetylene torch, it is important to obtain a quality “cast iron brazing flux” for use with the brazing rod. The best practice is to consult your welding supplier for their recommendations.
Preheating allows the technician to use less weld heat or flame; this translates to less temperature variation and less potential for cracking. When welding with either arc or acetylene torch, it is best to weld in 1.0” (25 mm) sections, alternating areas to limit the heat induced. See Figure 3.
This also enables heat to spread more evenly throughout the part. Be sure to weld in the same direction, right to left or left to right as the materials will expand and contract more consistently, reducing weld stress. When welding cast iron, several weld passes with low to moderate heat are better than a single pass with high heat. Following each weld or braze, use a small ball peen hammer or a pneumatic needle scaler at reduced air pressure to peen the bead. This peening removes weld scale and stress relieves the weld. Monitor the part temperature so that no
area reaches or exceeds1,400° F(760°C) or unexpected cracking may occur.
When backing plates of mild steel were utilized to hold the casting together for a repair, they may be removed or left in place. If left in place, they should be welded or brazed to the repair. If they are removed the underlying cracks or voids need to be welded or brazed to maximize the strength of the repair.
Post weld
The welded part must be allowed to cool slowly to avoid cracking. It is important to plan ahead and have a method that promotes a slow rate of cooling.
Suggestions to slow the rate of cooling:
- Wrap high-temperature insulation around the welded part to allow it to cool slowly. Be sure the temperature range of the insulation is capable of the part temperature or
- it could catch on fire.
- Preheat sand or vermiculite in a curing or burn oven while the part is being welded. When the welding is completed bury the part in the warm sand or vermiculite allowing it to cool slowly.
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