Cast iron is a very popular and widely used metal, but it’s also one of the most confusing materials to work with due to the many different varieties in use. From a welding standpoint, cast iron can be a very difficult metal to weld, and any attempt at doing so often ends in failure or completely unacceptable results. In order to minimize the chance of failure when welding cast iron pieces, one of the most important things you need to know is which type of cast iron you’re working with. Quite often a weld or repair is attempted on a type of cast iron that is NOT weldable. This only adds to the bad reputation of cast iron. Certain types of cast iron are readily weldable while others are not. Lets take a quick look the first of the cast iron types – Grey Cast Iron
The first type of cast iron is grey cast iron (ASTM A48). It is the most popular type of cast iron, and it’s alleged that it accounts for up to 80+% of all cast iron in use. It’s normally made from pig iron, scrap steel, scrap cast iron, coke, limestone, and small amounts of other materials.
The three main components in grey cast iron are iron (fe), carbon (c), and silicon (si). The iron provides strength, and the carbon is converted to graphite which provides wear resistance and lubrication; an important characteristic when performing any machining operations. The silicon in the mixture is used to cause the carbon to come out of solution as graphite. Combined with the cooling rate of the casting, the amount of graphite in the metal can be precisely controlled.
Grey cast iron is strong, but it’s also somewhat brittle. And while the graphite lends itself to the wear resistance and lubrication properties of the metal, it also causes stress concentrations and makes it easier for cracks to form due to the sharp edges of the graphite flakes. Also, due to the high carbon content, the elongation properties are very low. This alone causes the most problems when trying to weld cast iron. Grey cast iron is also not a metal that can be bent, or easily forged because of the graphite within it.
It gets it’s name from the grey appearance caused by graphite within it. You may not notice the color on the outside of the casting, but if you fracture a piece, you should see it. Some typical uses for grey cast iron include machine castings, engine blocks, cast iron cookware (frying pans for instance), and brake rotors to name a few. So if you’d like to practice your welding skills on grey cast iron, get some old brake rotors. Engine blocks would work well too, but due to the slightly porous nature of cast iron, there may be a lot of oil in it and it’s normally much harder to weld if it’s not completely clean. Brake rotors are pretty clean and due to the nature of their usage, typically have little if any oil or grease on them. And most importantly, they’re readily available and cheap.