Dean - welding cast always breaks ??? I beg to differ there. Some cast is crap to begin with but weldable cast is no problem when done right. I know of impliments still being used that I welded 30 years ago PLUS a cast planitary gear in a tractor rear end that I MIG welded (wrong weld) but it's going for 11 years now.
Lonny - What Jim said is pretty close. I don't always preheat unless it's a big piece. But the secret is the cool down. To fast and it'll crack. I've welded parts and put them in my secret cool down stuff and let them sit over night sometimes. I torch welded an exhaust manifold and 4 hours later it was still to warm to hold. Slow cool is the way to do it. I use nickel rods and sometimes just the old reliable 7018. My 30 year ago weld was a 7018 on a turning plow. Used every year in rocky clay and still turning sod
Lonny - What Jim said is pretty close. I don't always preheat unless it's a big piece. But the secret is the cool down. To fast and it'll crack. I've welded parts and put them in my secret cool down stuff and let them sit over night sometimes. I torch welded an exhaust manifold and 4 hours later it was still to warm to hold. Slow cool is the way to do it. I use nickel rods and sometimes just the old reliable 7018. My 30 year ago weld was a 7018 on a turning plow. Used every year in rocky clay and still turning sod