Alloy Steels

High speed transmission gears in automobiles must operate with out much noise, and this requires a degree of accuracy of shape on the order of 0.0005 inch. Warpage increases with the depth of carburization, so shallow cases were tried to give quieter gears. This was successful, but the hard case now tended to flake off, due to fatigue failure of the weaker low carbon metal at junction of case and core. To strengthen this metal it has been necessary to increase carbon content, and the result is that in 1940 American automobiles the great majority of transmission gears are of steels that used to be called "oil quenching gear steels," high enough in carbon and alloy to quench to a file hard surface without carburization. An upper carbon limit is set by the demands of reasonable machinability. Whereas the old oil quenched gears were heated in molten lead for quenching (in or der to avoid any trace of decarburized soft skin), the more modern practice is to hold them in a cyanide bath at 815° C. for about 25min. which is enough to induce a 0.002-in. case of carbides and nitrides, exceedingly hard and wear resistant, or to heat them in a controlled gas of carburizing nature. A representa tive steel would contain 0.45% carbon, 1.25% nickel, o•6o% chro mium. After the above quenching from cyanide and a 2-hr. draw at 235° C. the core would have a tensile strength of 275,000lb. per sq.in., and a correspondingly high fatigue strength. After such treatment the involute errors seldom would amount to 0.0004 inch. It is seen that the result is a gear with high surface hardness to resist wear, high core strength just under the case to resist fatigue failure (flaking or pitting of the surface), high strength to carry the normal loads, moderate cost of the steel with its moderate alloy content, cheap heat treatment, and a quiet running gear. "A soft, tough core to resist impact," once a prime requisite of a successful gear, is not contained within these specifications, as it has become amply evident that if design and shape are correct, a transmission gear set will operate normally without much impact. Toughness is achieved by fine grain size rather than high tempering after quench; indeed, tempering is done at the minimum temperature to avoid letting down the hardness at the surface. Other 0.40 to 0.50% carbon fine-grained steels widely used in this improved practice con tain 1.75% nickel and 0.25% molybdenum; 1% chromium; 1%

chromium with 0.18% vanadium. Here is a good example of that interchangeability of alloy steels mentioned in the preceding sec tion; choice may depend more on personal preference than on demonstrated advantages.

Austenitic Steels.

Hadfield's cast 0% manganese steel, dis covered in 1882, was remarkable in that it did not harden on quenching. It is, in fact, an austenitic alloy, in the sense that the expected change from austenite solid solution to hard marten site does not occur at 20o° C. but the quenched austenite is very stable at room temperature. Cobalt, but especially nickel, has similar effects. One result of this is that the most useful of the stainless steels, known as 18-8 and containing 18% chromium and 8% nickel, is unhardenable by quenching.

One characteristic of these commercial austenitic steels is that the austenite is not entirely stable. The small plastic movements in high manganese steel, associated with battering or attempted cutting, permit the martensitic reaction to occur in those re stricted regions, causing intense hardness—hence their wear re sistance in rough service. This is a favourable reaction, but many of the 18-8 steels are put into petroleum stills operating between about 450° and 85o° C., and this temperature gives enough ther mal agitation to the atoms so that chromium carbides form from the austenite solid solution, unfortunately not in a cloud through out the mass to "key" and strengthen the metal, but localized at the grain boundaries, seriously depleting the surrounding matrix of its chromium necessary for corrosion resistance. This deterior ation of 18-8 (even by the heat of welding) is prevented by intro ducing minor amounts of strong carbide-forming elements, like molybdenum, titanium or columbium. The latter two, especially, form carbides that are so insoluble in austenite that they act as inert non-metallic inclusions. (See also IRON AND STEEL; ALLOYS; and the various types of steels under their own headings.) BIBLIOGRAPHY.-Edgar C. Bain, The Alloying Elements in Steel (1939) ; Sir Harold Carpenter and J. M. Robertson, Metals (vol. II, 1939). Comprehensive monographs critically appraising the literature on the various alloys are being issued by Alloys of Iron Research, New York. (E. E. T.)