First, it helps to understand how cast iron differs from other materials. Most of what we call cast iron is more accurately called gray iron. Gray iron is an iron alloy that is about 95% iron, 4% graphite and 1% silicon. In gray iron the graphite takes the form of flakes. This makes it durable but easily machinable since the graphite acts as a lubricant. Although it has almost no ductility, it has relatively good tensile strength, and great compressive strength. It’s used in things such as large pipes, engine blocks, and fire hydrants. Then there’s white iron. White iron is gray iron that’s been quenched. This changes the form that the graphite takes from flakes to pearlite or cementite. It’s very hard, brittle, non-machinable, and used where abrasion and wear resistance are necessary. It’s also used in making malleable iron. Malleable iron is white iron that’s been held at 1700 degrees F for 100 to 120 hours. This process makes it incredibly tough, resistant to shock, and gives it a high ductility. There are more types of cast irons but the only one that can be successfully welded without changing the physical properties of it is gray iron.
Welding gray iron can be easy if the weld area is prepared properly. In most cases the easiest way to weld gray iron will be brazing. If it’s done right, it can be just as strong as the original material. Whether you are brazing or fusion welding, proper preheating is essential. With brazing, the torch will be enough. Preheat the metal until it's a dull red. Then tin it with your rod. Tinning is where you allow a thin layer of the brazing material to flow out over the entire weld joint. If you've done it correctly, it will flow almost like water and adhere to the metal. Then you're free to start filling the joint. If you happen to be trying fusion welding with a torch, you'll just have to get the piece hotter before you start. If you’re using arc welding, it’s best to preheat the entire piece. And like with any other weld project, make sure you prepare the site properly and select the correct rod.
Written by Dustin Saunders
