Welding cast iron to steel is a process that really doesn't take much time to get the hang of, even if you've never welded before. The reason for this is that steel is slightly harder than cast iron, and therefore combines fairly easily. The most difficult aspect of welding is when the cast iron is relatively small or thin compared to the steel; because cast iron is softer and has a lower melting point, it is easy to burn through if you aren't careful.
Things You'll Need
Set up your welding workspace, beginning with an overview of the basic safety precautions necessary while welding. Ensure that there are no puddles of standing water in which you might stand, and that there are no flammable materials nearby. For personal protection, use a welding helmet. It is also more comfortable to weld if you wear a long-sleeved shirt and welding gloves.
Turn on the MIG welder and set it to the necessary power you'll need to weld the metals together. A reference chart is usually located just inside the cover of the welder that tells you what power level and wire speed to use for the thickness of steel that you are welding. Measure your metal and set the welder's power and wire speed accordingly.
Position the cast iron and steel in the position that you want them to be welded into, and then extend the welding electrode approximately 1/4 inch from the welding nozzle by pulling the trigger on the welding gun. Clamp the welder's ground clamp onto the workpiece.
Touch the electrode to the steel, not the cast iron, and then pull the trigger. Through your welding helmet, you will see a small pool of molten steel form where the wire touches the steel. While you are pulling the trigger, move the electrode slowly toward the cast iron until the pool of molten metal forms there.
Move the electrode back and forth between the steel and the cast iron in a stitching motion. The molten pool of metal should remain approximately 1/4 inch in diameter. If it grows smaller, slow down how quickly you pass the electrode between the steel and cast iron. If it grows larger, speed up how quickly you move.
Finish the weld and release the welder's trigger. When completed, there will be a crust of slag covering the weld area. Clean away this slag with the wire brush and allow the weld to cool before attempting to handle the metal.
Always wear a welding helmet when you are using any kind of welder. Not only can you damage your eyesight permanently by not wearing a helmet, but you can also weld the metal together incorrectly, destroying the workpiece.