The rustless chromium steels were first used for table cutlery and aero-engine exhaust valves. When the valves were required to offer great resistance to scaling, silicon was added with good effect. When valves were required to be as hard at red heat as ordinary structural steel is at ordinary temperatures, 12 to 30% of nickel was added. To resist other specific conditions, molyb denum, tungsten, manganese and copper have been added.
The development of rustless steel is hampered by two difficul ties. The first relates to its adaptability to existing manufacturing processes. This difficulty starts with the actual making of the steel and is met in every hot-working and cold-working process. The second difficulty relates to the smelting of the ore. The ferro chromium alloy can be produced cheaply by smelting chrome-iron ore in the blast furnace but it is useless for making rustless steel because it contains too much carbon. For soft varieties of rustless steel the steel maker requires ferro-chromium alloys containing practically no carbon. It is still not known how to adapt cheap smelting operations to chromium ore without contaminating the product with carbon, though much progress in this respect has been made during the past few years.
With introduction of rustless steels special causes of corrosion have been more sharply separated and defined. The most notable causes of corrosion, apart from direct solubility, are due to dis tortion and contact. A piece of rustless steel used for stainless knives and containing, say, 13% chromium, will remain bright if exposed in a country garden providing it has been suitably heat treated. If, however, a piece of the same material be machined
in a lathe the machined surface and the turnings themselves will rust, for they are distorted and to some extent their adjacent parts are distorted unequally. If the surface be smoothly ground and polished it will not rust. The polished surface does not offer easy lodgment for casual dirt but otherwise polishing in itself does not appear to exert a great restraint upon rusting. A piece of the same 13% chromium steel, for example, in the form of cold drawn wire which has simply been highly polished after cold drawing rusts with surprising readiness (see also CORROSION ).
BIBLIOGRAPHY.—Sir R. A. Hadfield, "On the Alloys of Iron and Chromium," Jour. Iron and Steel Inst., vol. 2 (1892), with a good bibliography of early work ; F. G. Bell, "Stainless Steel," Iron and Coal Trades Review (Aug. Io, 1923) ; J. H. G. Monypenny, "Stain less Steel, with particular Reference to the Milder Varieties," Trans. Amer. Inst. Mining and Met. Engineers (Feb. 1924) ; W. H. Hatfield, "Stainless and Similar Corrosion Resisting Steels in the Chemical and Allied Industries," The industrial Chemist (March, 1925) ; "Chromium Nickel Rustless Steels," The Metallurgist, supplement to The Engineer (Oct. 3o, 1925) ; J. H. G. Monypenny, Stainless Iron and Steel (1926).
(H. B.)