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Cast iron is one of the more difficult metals to weld. And given the brittle nature of the material and that it’s used everywhere from kitchen to workshop, broken cast iron parts are not unusual. The process of cast iron welding has a few potential pitfalls. It requires a lot of precision and high heat. However, if done correctly, it’s not that difficult. In this article, we’ll cover the basics of how to weld cast iron. Keep in mind, though, that this is only an informational article to help you understand the basics. You shouldn’t attempt to weld cast iron only on the strength of a brief internet article.
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Challenges Of Welding Cast Iron – Can Cast Iron Be Welded?
The main challenge of welding cast iron is its high carbon content. Typically, it has about 2-4% of carbon and that is roughly 10 times more than most steels. The advantage of this is that it gives cast iron its characteristic hardness, but that comes at the expense of ductility. Cast iron is not a very malleable material. When it’s heated or stressed, it doesn’t stretch or deform. Instead, if not done correctly, it will crack.
Cast Iron Alloys
- Grey cast iron. This is the most common form of cast iron. It’s more ductile in comparison to white cast iron. However, the welder still needs to make sure that the graphite flakes within grey cast iron don’t enter the weld pool because it can cause metal embrittlement.
- White cast iron. White cast iron doesn’t precipitate the carbon out, as graphite. Instead, it retains it as iron carbide. This makes the microstructure of the material hard and unweldable.
The Process Of Welding Cast Iron
Clean the Casting
No matter what alloy you’re welding, the casting should always be properly cleaned before you start welding. Remove any materials from the surface of the alloy, clean away any paint, grease, dirt or any other foreign material. Also, when applying heat to the weld area, do it carefully and slowly. This will help to remove the entrapped gas in the weld zone.
If the part will also be machined after you finish welding, then choose a nickel-type electrode.
Preheating Cast Iron
As we’ve mentioned above, cast iron is vulnerable to cracking when it’s under stress. To avoid these cracks and other defects it’s important to have proper heat control of the weld. It means that it requires proper preheating before you start welding and slow cooling after the weld is finished.
Typical preheat temperatures are 500-1200 Fahrenheit. Preheating should be done slowly and uniformly. Also, make sure that you don’t overheat the part. Anything over 1400 degrees will put the material into a critical temperature range.
Welding To Repair Cast Iron
The most important thing to remember when welding cast iron is to move low and slow. Use only low current, which will minimize residual stress and admixture. Also, move slowly in a stitch-like manner. Work on one inch at a time and allow about 45 seconds for the segment to cool down before moving on to the next one. This will help to prevent overheating and buildup of residual stress.
Once the weld is completed, inspect it for any slag holes or cracks. Due to the nature of cast iron, it’s likely that the cracks will still appear, even if you’re properly following all the procedures. If you do find cracks, you can eliminate them by using a sealing compound. They can also rust shut with time after being returned to service.
If the weld is all good, brush away any excess slag using a slag brush to make the weld nice and smooth.
After completing these steps, it’s also important to allow gradual cooling of the part. You can wrap the part in an insulated blanket or bury it in dry sand.
- Stick welding. Stick welding involves working with a consumable electrode that is covered with flux. There are different electrodes available that can be chosen depending on the application at hand. For cast iron welding, it’s recommended to use nickel alloy electrodes. The mix of nickel and iron has a lower coefficient of thermal expansion, which lowers welding stresses and makes it less prone to cracking. As a result, it produces a stronger and more durable weld.
- Oxy-acetylene welding. In oxy acetylene welding you also use an electrode. However, the arc is generator not by the current, but by the energy provided by the oxy-acetylene torch. You can choose either cast iron or copper zinc electrodes for welding cast iron using this technique.
- Braze welding. A common method that is used for joining cast iron parts together is brazing. Its advantage is that it has minimum impact on the base metal. The filler adheres to the cast iron surface and since it has a lower melting point than the base metal it doesn’t dilute with it.
To guarantee a quality weld, it’s best to contract cast iron to a professional. However, by choosing the appropriate weld technique and properly preparing the material, it’s also possible to complete in-house weld repair. If you do decide to do it yourself, don’t forget about safety. Wear proper protective clothing and get an auto-darkening helmet for protection against the sparks and infrared light of the arc.