RUSTLESS STEEL. The simplest and cheapest rustless steels are ordinary mild steels containing chromium. Resistance of the steel to rusting depends on the amount of chromium it con tains. The addition of to% of chromium to pure iron suppresses its solubility in water; the resulting pure chromium-iron might be regarded as a satisfactory rust-resisting metal. But such pure chromium irons cannot be produced commercially. Commercial rustless steels contain carbon, even when little, causing rustings. Between 1912-14, a series of steels containing between 9 and 16% of chromium was investigated by Brearley (see U.S.A. patent 1197256/16), who concluded that with the stipulated amounts of chromium and not more than .7o% of carbon, rustless steel articles could be produced providing they were submitted to suitable hardening operations. From this series of rustless steels developed by Brearley the familiar "stainless" cutlery is made.
The resistance of chromium steels to rusting is increased by adding chromium and decreased by adding carbon. Small amounts of carbon are always unavoidably present but definite amounts are also added with the object of producing required degrees of hardness, resistance to wear or ability to cut. The useful extent to which chromium alone can be added to iron is limited because such comparatively pure chromium irons are brittle.
Steels containing more than g% chromium were made as early as 1892. Their rust-resisting properties were not realized because at that time the resistance of steel to general corrosion was esti mated by its measured resistance to solubility in dilute sulphuric acid. Since the solubility of high chromium steels in dilute sul phuric acid is greater than that of ordinary steel, the quest was directed away from and not towards chromium.
Up to the year 1912 the least corrodible steels then in general use were nickel steels. The more nickel they contained the less they were supposed to corrode. Steels containing 25% nickel were held in high regard, as they were notably resistant to dilute sulphuric acid, an active corrosive agent in industrial towns or under special conditions as in chemical works or mines.
The addition of nickel to steels which already contain chromium produces useful results : conferring a certain degree of resistance to the corroding effect of cold sulphuric acids, whether in the liquid or vapour form; (2) making those higher chromium alloys tough which, without the nickel, would be brittle, and (3) making homogeneity comparatively easy without submitting the alloy to special forms of heat treatment. Materials of this class were investigated by Strauss in Germany 1912-14 (English pat ents, 13414/13 and 13415/13). Strauss appears to regard the pres ence of nickel as an indispensable constituent of rustless steels, and Krupp, with whom he was associated, has devoted most attention to a steel containing about 20% chromium and 6% nickel. Steels of this composition cannot be hardened by the usual heat treatment processes. They are hardened by cold-working, as are all metals and alloys (see METALLURGY).
If scaling at high temperatures be regarded as a form of cor rosion, then the work of Marsh must be noticed. Looking for some substitute for platinum wire or strip to wind resistance coils, Marsh found what he required in an alloy of nickel and chro mium. This material, alloyed with more or less iron, is familiar as the coiled filament of heated metal in office and bedroom radiators. Its value for that purpose, apart from its electrical properties, is that it does not scale when it becomes red hot, but neither Marsh nor anyone else appears to have connected its resis tance to scaling at red heat with its possible value as a rustless alloy. Haynes, who is reputed to have made the first motor-car in America, worked with cobalt-chromium alloys. From his ob servations and experiments arose the well-known cutting tool material called "stellite" and a similar material in a malleable form from which Haynes made pocket-knife blades.