Exotic Alloy Finds Niche
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Nickel aluminide, an intermetallic alloy, cuts the cost of heat-treating. By Gerald
Crawford
Nickel magazine, April 8, 2003 -- An intermetallic material, nickel aluminide
(Ni3Al), is enabling manufacturers to slash the energy, product recycle, and capital equipment
costs associated with austenitizing, carburizing and other heat-treating furnace applications.
Containing 80% nickel, this advanced alloy is beginning to challenge more traditional materials in some
applications. For example, a steel plate austenitizing furnace at Bethlehem Steel's Burns Harbour
plant, in Chesterton, Indiana, U.S.A., has more than 130 tonnes of the alloy in the form of transfer rolls,
while a carburizing furnace at the Delphi Saginaw Steering Systems' automotive parts plant in Saginaw,
Michigan, U.S.A., has more than 110 tonnes in various fixtures.
Demand could double in two years as new applications, such as radiant burner tubes and seal rolls
(now under trial), come into use.
Developed jointly by the U.S. Department of Energy’s Oak Ridge National Laboratory (ORNL) in Tennessee,
U.S.A., and Delphi Automotive Systems Corp. in Troy, Michigan, U.S.A., the alloy becomes stronger
at around 900°C. That's because of its highly ordered structure and creep resistance, the properties
underlying its superior performance.
The alloy, known as IC-221M, contains nickel combined with aluminum, chromium, molybdenum, zirconium and
boron. The addition of boron is critical in increasing the ductility of the alloy and it was this discovery
that allowed the alloy to become commercially producible. Melted and cast by ORNL’s Exo-MeltTM
process, the alloy is supplied by half a dozen licensees in the U.S.
IC-221M is superior to conventional alloys. For example, traditional heat-resistant steel alloy HP (N08705, containing 25% chromium, 35% nickel) transfer rolls blister under use at 950°C, with several costly consequences:
- the furnace is shut down weekly to grind blisters off the roll surfaces, wasting heat and cutting production;
- as much as 40% of the steel plate product is down-graded by scratches caused by the blisters, resulting in decreased revenues, and
- the rolls must be replaced frequently, some annually, with corresponding capital replacement costs.
However, the nickel aluminide rolls do not blister or need grinding, enabling manufacturers to save on energy costs and enhance production revenue. In addition, they last three to five times longer than steel alloy rolls, so life-cycle costs are lower.
Conventional alloy steel fixtures in carburizing furnaces cause other problems: catastrophic failure of a fixture can jam the furnace, which must be shut down, wasting energy and possibly compromising the quality of the product being treated. Nickel aluminide fixtures, on the other hand, have three times the life of steel fixtures, decreasing both the number of fixtures needed and also the number of furnaces required for a given production rate. Energy, product replacement and capital costs are therefore all lower with the nickel aluminide alternative.
Few engineering materials are developed without technical challenges, and IC-221M alloy is no exception. The very features that underpin the remarkable performance of nickel aluminide – its highly ordered, stable structure and great creep resistance – also make it difficult to weld. In a roll application, for example, welding is essential for connecting the trunnion to the roll body. This was a problem in early trials, but the welding technology has developed so that automated GMAW (MIG) procedures now produce consistent, reproducible welds.
Not surprisingly, the initial cost of nickel aluminide can be up to twice that of conventional heat-resistant steel alloys, but because of its performance, the life-cycle cost of the nickel aluminide alternative is considerably lower. Consequently, we are likely to see more Ni3Al in rolls, fixtures, burner tubes and other applications.
IC-221M alloy is covered by ASTM A1002-99, ‘Standard Specification for Castings, Nickel-Aluminum Ordered Alloy’.
Gerald Crawford, Ph.D., P.Eng, is a Toronto-based consultant to the Nickel Development
Institute.
Photos: ORNL
 Dr. Vinod Sikka |
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