Ductile Iron

is what we use for metal water mains, and it's some incredibly tough stuff- I've seen guys run across a piece of 6" pipe with a D8 CAT and not break it, and I pulled about 800 ft of 6" out of the ground by getting an excavator bucket under one end, and pulling it straight up out of the ground through 3' of cover, and not one piece broke- it would deflect a little pulling it up through the dirt, but snap right back straight when it pulled free