These Ag cutting discs are the new Infinium steel. They are 55 Rockwell through and through. They are the better answered to the French Forges de Niaux 200. ImThese are 18 1/2" inch (actual size is about 18.675"). These are for John Deere Pro Series, 95, 90, 60, and 50 series drills and air seeders. I also have Standard Ingersoll discs at 18 inch made from their common steel.