This specification covers centrifugally cast cylinders with an outer layer of white cast iron and the remainder of the material of gray cast iron. These castings are suitable for pressure containing parts of the design strength of which is based on the gray iron portion of the cylinder. The white iron portion of the cylinder shall be made to a minimum hardness and the casting process shall be controlled to produce a metallurgical bond between the two metal layers. All surfaces shall be machined prior to the cylinders being placed into service. The tensile strength of the cast irons shall be measured by tension testing while the thickness of the white cast iron shall be determined by ultrasonic testing.