Method of centrifugally casting a metal tube

A method of centrifugally casting a metal tube comprising:

• pouring an easily oxidized metal onto the inside diameter of a rotating centrifugal mold and allowing the easily oxidized metal to solidify in the mold so that it has a thick section at one end of the mold, tapering to a thinner section;
• pouring a denser metal initially onto the thick section of the solidified metal while rotating the mold, the denser metal having a higher melting point than the easily oxidized metal thereby gradually remelting the easily oxidized metal and progressing down the length thereof as a protective blanket for preventing oxidation of the easily oxidized metal at the pouring temperature of the denser metal;
• continuing rotation of the centrifugal mold until the easily oxidized metal is entirely covered by the blanket and also until the easily oxidized metal has moved radially inward through the denser metal to reach the inside surface of the tube, whereby the easily oxidized metal has been prevented from undergoing objectionable oxidation;
• and
• allowing the metals to solidify to complete a centrifugally cast tube of both metals characterized by a centrifugal casting having at and near the outside surface a first zone having a high concentration of the denser metal alloyed with a low concentration of the easily oxidized metal;
• a second zone having a higher concentration of the easily oxidized metal, higher than in the first zone, alloyed with a lower concentration of the denser metal, lower than in the first zone, located at and near the inside surface;
• and a transition zone between the other two zones where the concentration of the easily oxidized metal is continuously increasing radially in the direction of the inside surface.