The outstanding sulfidation-resistant alloy available in the art has been alloy 6B invented by E. Haynes (U.S. Pat. No. 1,057,423) and marketed under the registered trademark STELLITE. STELLITE.RTM. alloy 6B is cobalt base and contains about 30% chromium, 4% tungsten, 1.1% carbon and is essentially free of iron and nickel.
The high cost and strategic limitations of cobalt prevent the full marketing of the alloy for wide spread use in combating sulfidation damage. The production costs of alloy 6B are especially high because of the difficulty in forging and hot and cold rolling this alloy. Furthermore, it is difficult to fabricate the alloy into components such as heat exchangers for applications.
U.S. Pat. Nos. 4,195,987 and 4,272,289 disclose alloys containing iron, nickel, cobalt, chromium and selected metals including lanthanum to increase resistance to high temperature oxidation. A commercial alloy, marketed under the registered trademark HAYNES.RTM. alloy 556, is a typical example of this prior art. The alloy normally contains essentially about 18% cobalt, 22% chromium, 3% molybdenum, 2.5% tungsten, 20% nickel, 0.6% tantalum, 0.02% lanthanum and the balance iron with minor contents of nitrogen, manganese, aluminum, carbon and zirconium.
U.S. Pat. No. 3,418,111 discloses Haynes alloy 188, well-known in the art for its resistance to high temperature oxidation. The alloy normally contains about 22% nickel, about 22% chromium, about 14% tungsten, 0.10% carbon, 0.03% lanthanum, and the balance essentially cobalt (about 40%).
Known in the art is UMCo-50 alloy or HAYNES alloy 150. The alloy contains normally about 28% chromium, about 50% cobalt and the balance iron with minor contents of carbon, manganese, and silicon. The alloy has good high temperature properties including stress-rupture and sulfidation resistance.
Many prior art alloys, including those mentioned above, are used as components in industrial installations where resistance to chemical reactions such as oxidation and sulfidation is required. Equally the weldability and thermal stability characteristics must be acceptable.
Each of the prior art alloys provides one or more of the desired characteristics but may be deficient in one or more of the other required characteristics. In some cases an alloy may provide nearly all the desired characteristics but its use may be limited because of the cost of raw materials and processing. Thus, the art is in need of an alloy that provides all of the desired characteristics at a lower cost.