A More Efficient Way to Harden Steel
High nickel alloy enables auto-parts-makers to harden components at higher temperatures
By Dr. Gerry Crawford
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THE MAGAZINE DEVOTED TO NICKEL AND ITS APPLICATIONS |
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BASKETS THAT HOLD automotive parts in vacuum carburizing furnaces must make many trips to the furnace.
Therefore, they need to be made of an alloy that does not crack or distort at high temperature in a
carburizing environment. |
Nickel Magazine, July 2005 -- High-nickel alloy N06025 may some day challenge N08330 as the workhorse material for baskets and fixtures in the carburizing of steel automotive gears and other components.
To enhance wear resistance of automotive gears, they are typically exposed to a hydrocarbon gas, such as
acetylene.This is done at high temperatures where carbon can diffuse into the metal, forming an austenitic
surface layer with about 0.8 to 1.0% carbon. Upon quenching, the high carbon austenite transforms to
martensite, producing a hardened layer at the surface of the gears.
Carburizing automotive gears has traditionally been carried out, and to a large extent still is, at
temperatures of about 900°C. However, there is a growing trend to even higher temperatures in the range of
940 to 1,040°C, to decrease carburizing times and losses due to distortion, thereby increasing throughput,
production, and revenue.
"N08330 has been for some time, and presently remains, the workhorse material for baskets and fixtures that hold the gears during the carburizing process," says Jason Wilson, technical marketing manager, Rolled Alloys Inc. However, one of the operating problems is that the baskets are themselves subject to carburization and distortion over time, to the point they eventually have to be replaced.
The performance of N08330, with only 35% nickel and 19% chromium, may suffer compared to alloy N06025, which contains 62% nickel and 25% chromium, because N06025 has greater resistance to carburization under vacuum and greater high-temperature strength than N08330.
Baskets of N08330 suffer from carburization even at 900°C, and eventually crack and distort beyond further use, commonly after eight months or so. N06025, on the other hand, has shown no distortion in trial runs to date, even after 12 months of service.
The cost of N06025 is presently five times the cost of N08330, however, so it should ideally provide several times the service life to be competitive with N08330 on a life-cycle cost basis. This enhanced performance has yet to be demonstrated.
Robert Peters, Director of Operations, ALD Thermal Treatment Inc., in Blythewood, South Carolina, U.S.A., is confident. "N06025 will outperform N08330, particularly as carburizing temperatures continue to rise above 900°C into the 940 to 1,040°C range and possibly higher," he says.
While N06025 has been in widespread use for about ten years in many high-temperature and other challenging applications, its testing for carburizing baskets and fixtures is still a ‘work in progress’. It is high-cost today because it is not yet available in commercial quantities in the rod forms and dimensions suitable for carburizing baskets and fixtures.
When its superior performance at higher temperatures is demonstrated and recognized in the industry, however, production of N06025 for carburizing baskets should increase and its price should decrease to the point where it will challenge N08330 as the nickel alloy of choice for vacuum carburizing automotive gears.
Dr. Gerry Crawford is a Toronto-based consultant to the Nickel Institute.
PHOTOS: Rolled Alloys Inc.
Jason Wilson Robert Peters |
