Gray Iron was the original "cast iron". It is relatively easy and inexpensive to make. Compared to the more modern engineered irons Gray Iron has a lower tensile strength and lower ductility. In other words it will fail more easily, and its mode of failure will be sudden fracture (it will not bend). Used for housings where tensile strength is non critical. Engine blocks, pump housings, valve bodies, electrical boxes, decorative castings. Cast iron usually refers to grey cast iron, but can mean any of a group of iron-based alloys containing more than 2% carbon (alloys with less carbon are carbon steel by definition). ... The tensile strength of a material is the maximum amount of tensile stress that it can be subjected to before it breaks. ...