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Tig Weld Cast Iron With A Tig Welding Rod In A Way You Never Considered Before Welding Cast iron is always a crap shoot. There are so many different grades that things are made of and it is not always apparent which one you are about to weld. And there are ways of telling, like spark testing but I have yet to see anyone who could tell one type of iron casting from another by spark testing. So why is cast iron so hard to weld? Because it has a metric ton of carbon in it. Consider this for a minute. Tool steels contain around 1 percent carbon and are considered difficult to weld because of excessive hardening and even cracking problems. Cast iron contains over 2 percent carbon and sometimes up to 4 percent. When that much carbon is present in iron, it hardens easily. But all that carbon serves a purpose... it turns into graphite and that actually serves a purpose as a lubricant on cylinder walls of a combustion engine. But for welding, all that carbon and graphite is horrible. Carbon in those percentages cause hardening of the weld metal and surrounding heat affected area. To make matters worse, cast iron is very brittle. It does not bend, it breaks. It may be good for engine blocks, but it is unpredictable under load. Back to tig welding cast. What is the best rod to overcome the excessive amount of carbon present in the cast iron base metal? The most popular choice in arc welding rods seems to be nickel rods also called ni rod. But tests have shown that for tig welding cast iron, aluminum Bronze is a very good choice. Aluminum Bronze. yep, you heard me, these tig welding rods work great for tig welding iron castings. Lets look at why... 1. The weld deposit will not harden from high carbon content. 2. The solidifying weld metal does not have high shrinkage stresses to cause the metal to crack next to welded areas. 3. aluminum bronze tig welding rods are almost as strong as steel 4. when welded on A/C, these Al bronze tig welding rods flow and wet easily and achieve a minimum amount of dilution from the cast iron base metal. that is always a good thing. 5. aluminum bronze tig welding rods are much cheaper than nickel rods. A good tip for using this rod on welding iron castings is to set the gtaw machine on A/C just like you would weld aluminum. That will let the bronze filler metal flow without mixing too much of the iron and carbon into the weld.