20.5 Welding Cast Iron and Cast Steel
David Colameco, M.Ed.
Basics of welding cast iron and cast steel
Cast Iron and Cast Steel are like many other casts such as Cast Aluminum and even Cast Stainless Steel in that they tend to be porous and are brittle. As a student you start by welding low carbon steel, such as A36, because it is easy to weld on and is used throughout industry. Once you have the techniques down to weld low carbon steel, it is much easier to learn how to weld on other alloys that are much more difficult to weld such as stainless steel, chrome-moly, and casts for example.
In the previous sections we discussed how cleanliness and shielding gas were important for welding stainless steel above and beyond the requirements for mild steel. For cast iron and cast steel, cleanliness is also important because many casts are repairs of equipment that were used in service and could be covered in or have soaked up oil, greases, fuels, and other hydrocarbons used to lubricate the machine parts or fuel the parts if it is an engine block for example. You may not be able to remove all of the impurities but you must do the best that you can.
If you are repairing a casting, it is very important to identify the size of the repair. If you are repairing a crack in a casting you must determine how long the crack is. First clean the surface of the casting in the area of the crack by removing any paint but also being careful not to smear metal over the crack through mechanical removal of the paint. You can use dye penetrant tests if doing a repair at home and follow the instructions for timing of the application. The crack is likely longer than you can see with the naked eye, and the dye penetrant might show you a crack that is a few inches longer than first thought.
- The next step is to end drill the cracks to prevent them from spreading any further. Drilling the ends of the cracks removes the sharp point of the crack and replaces it with a round hole. Structurally round holes do not allow a stress concentration to form and the stress flows around the hole. When you end drill the hole make sure to countersink it so you can inspect the metal below the surface to see if the crack is extending below the surface.
Preheating cast iron and cast steel parts is very important due to the brittle nature of casts. The thermal conductivity, the ability of heat to travel through a material, is lower than other materials. Due to this inability of heat to move through the material quickly, the heated metal will naturally expand while nearby metal that is not heated has not expanded as much. This creates thermal stresses and can cause casts to fail by cracking.
It is equally important to have all of your welding equipment and materials ready to go before preheating begins. That way once preheating is completed you are ready to start welding and are not off spending 30 minutes looking for something while the part cools off. Preheating of casts costs money, and any addi