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An overview of austenitic irons in foundries.
The austenitic irons are covered by a number of national and international specifications and specifically, in the United Kingdom by BS 3468:1986.Most of these alloys were developed or refined in the laboratories of INCO and promoted under the trade name, now generic, of "Ni-Resist."
The first Ni-Resist alloy developed was Type 1, a flake graphite grade of cast iron containing approximately 15% nickel, 6% copper, and 2% chromium to provide the austenitic structure.
Although this alloy had limited strength, it exhibited good resistance to corrosion and useful physical properties. In particular, Ni-Resist Type 1 had a thermal expansivity matching the aluminum alloys used for many cast pistons fitted in high-speed diesel engines, allowing a top ring carrier of Ni-Resist to be cast-in.
In the early 1950s, following the development of the magnesium process for the production of spheroidal graphite iron, Ductile Ni-Resist was introduced, exhibiting superior mechanical properties, retaining the good resistance to corrosion, and providing improved stability to long-term exposure at temperatures in excess of 750deg C.
It is not the intention of this article to discuss the merits of the austenitic irons in terms of their applications but to deal with the practical aspects of their production in the foundry. The article will normally refer to the flake graphite austenitic irons as Ni-Resist and the spheroidal graphite austenitic irons as Ductile Ni-Resist using the type designations contained in BS 3468:1986.
Solidification of Ni-Resist and Ductile Ni-Resist
In his book, "The Solidification of Castings," R.W. Ruddle describes the theoretical and practical aspects of solidification for many metals and alloys, and the reader quickly becomes aware of the fundamental difference in the way by which metals and alloys solidify.
Extensive work has been carried out on the mechanism of solidification of gray cast iron and ductile iron. The work seems to indicate that while both start to solidify in a similar way, gray cast iron follows the path more akin to a "skin former," and it is also affected by the precipitation of the graphite flakes from the liquid. On the other hand, the solidification of ductile iron is more complicated, and it is more akin to solidification in the "pasty" mode, but it is also...