I think it might be cast steel , your welder could tell by the sparks when he starts grinding the vee on the crack , cast iron will throw off dull red sparks while steel will throw off bright orange/red sparks , I would highly recomend you have it Tig weded as that is the purest type of weld and I would probably use stainless welding rod as it has a slight flexability to it , my second choice would be a high nickel content rod , go to a reputable weld shop in your area , it should be ok for casual riding