NICKEL-CHROMIUM STEEL, an alloy of iron, nickel, chromium and carbon. Nickel strengthens the alloy and increases its toughness. Chromium unites first with the carbon to form com plex carbides, and forms a material that, after quenching even at a slow rate, is hard both at the surface and inside. The two elements in one steel seem to intensify these respective effects, and after proper heat treatment produce unexampled alloys, strong, hard, tough, with great resistance to shock, penetration and repeated stresses. Nickel-chromium steel for armour plate is made as follows : a steel ingot containing 0.40% carbon, 3.75 to 4.o nickel and 1.5 to 2.0 chromium after forging into a thick slab is placed in a carbonizing furnace, covered with a pile of char coal and heated for several days until the steel at the surface has absorbed an additional 1% carbon, gradually tapering off to the original content at a depth of about i inches. It is later given a complex heat treatment designed to produce maximum toughness at the back and maximum hardness at the carbonized surface. Even more intricate and skilful heat treatment is re quired for armour-piercing shells, with their glass hard piercing point (often protected by a soft wrought iron cap) and a tough body which will retain its shape and contents of high explosive even after passing through a 12 in. plate. Such alloy steels must be very carefully made of clean raw materials—acid open-hearth and electric furnaces are preferred (although American armour is usually basic steel). Since chromium is oxidized readily, and its oxides are quite detrimental to the steel, the ferro-chromium is added near the end of the heat after the bath has been thoroughly deoxidized. Ingots are cast of cool metal, stripped and placed in soaking pits as soon as possible. Air-hardening steels such as these are "tender" in the ingot ; surface cracks are frequent and must be carefully chipped out of the billet. Reheating of large masses must be very deliberate to avoid interior cracks.
Nickel-chromium steels now hold first place in use in the United States for important parts which are to be case-hardened, or for highly stressed forgings. About 21 times as much nickel as chromium seems to withstand best the normal variations in com mercial heat treatments. Some 25 analyses current in the United States fall within the following ranges :— forgings, because it can be water quenched from a fairly wide temperature range and is somewhat cheaper than the 3-1-% nickel steel (q.v.). After a deliberate drawing operation such forgings machine easily and uniformly. Principal uses are for automobile steering knuckles, connecting rods, crank and drive shafts, axes, chisels and locomotive and car axles. Physical properties of
medium size forgings after the respective draws are: the alloy than their progenitors, whose general excellence is so well known that they are frequently quoted as a standard of com parison for newer alloy combinations. In addition to the steels mentioned below, nickel-chromium steels (q.v.), with or without molybdenum, are used in large tonnages. Nickel is also an essen tial component of high nickel-chromium stainless and heat resist ing steels and alloys.
Nickel (a) forms a solid solution with either gamma or alpha iron ; (b) strengthens and toughens this solid solution; (c) slows down the speed of the carbon reactions, and reduces the tempera ture at which they occur. These effects are a desirable combination in the use of nickel steels for case-hardening purposes. The follow ing represent the wide range of these steels used in the U.S.:— The excellence of these steels is more than is indicated in the above table, for they have great toughness against repeated shock.
Heat-treated castings have given satisfaction when made of about 0-45% carbon, 2.6 nickel and 0-75 chromium. They can compete with cast manganese steel for street railroad crossings and special track-work because they are about as durable under traffic, cheaper and can be machined in the usual way. Joints can be welded satisfactorily with thermit and the battered points rebuilt by fusion welding. An important European use of such material in hardened and tempered condition is for railroad car couplings.
While a demand to reduce alloy costs has required as low an alloy in heat treated machine parts as will give the required strength, toughness, and hardness (supplemented by furnace and heat treating practices to give proper grain size control), the com bination of nickel and chromium is again found in high alloy steels to resist various chemical reagents, atmospheric corrosion, and high temperature oxidation. Chromium is the principal ennobling element, and about 18% is the minimum for severe services. Eight per cent or more nickel causes this alloy to become austen itic (see STEELS, ALLOY), ductile and not hardenable by heat treatment. This austenitic alloy is unstable in certain temperature ranges, that is, its microstructure changes enough to impair its excellent qualities. To mitigate this action, carbon is kept very low and molybdenum is frequently added. The latter also im proves the creep resistance (strength at high temperature). These high nickel-chromium steels are not only pleasing and useful for decorative and domestic purposes, but are indispensable to many chemical and food processes, paper manufacturing, and petroleum refining. (See ALLOYS ; IRON AND STEEL.) (E. E. T.)