REFERENCE. Molybdenum, Publication No. 592, 1925, Department of Mines, Canada. (W. WA.) Molybdenum of the metal molybdenum as an addi tion to other alloy steels and as an alloy in plain carbon steels has become widespread in both England and America since 1920. Prior to the commercial development of a large deposit of molybdenite in the Colorado mountains, the metal was too rare and costly for such purposes, although enough was known of its metallurgy to indicate many interesting possibilities. Shortly after unlimited supplies be came available, it was found that small amounts (about 0•30%) added to a large number of the then popular engineering alloy steels im proved the hardenability—that is, increased the depth of hardening, or, alternatively, permitted a milder quench for the same hardness depth—and widened the range of effective heat treatment tempera tures, without interfering with other desirable mechanical properties such as machinability or forgeability. Molybdenum in steel has a very strong tendency to form stable carbides—a tendency even stronger than that of chromium or tungsten—and that portion of the metal which is dissolved in austenite at high temperature markedly slows down the transformations on cooling. As indicated in the articles IRON AND STEEL and STEELS, ALLOY, the stable carbides hamper grain growth prior to quenching, thus rendering the steel fine-grained and tough, while the sluggish transformation rates lend depth to the hardening. Molybdenum carbides are also slow to precipitate from the quenched microstructure unless the tempering heat is higher and longer than usual, a practice which relieves internal strains in the metal (a prime cause of brittleness). Alloy steels with molybdenum added therefore require more deliberate and higher heat treatments, but in return they possess unusual toughness at equivalent hardness levels.
These properties are well exhibited in the medium carbon ranges of nickel-chromium steels (roughly 0.40% carbon, 2% nickel, 0.75% chromium, 0.30% molybdenum). Izod impact strengths of soft-lb. or more are possessed after tempering at 550'C, when the tensile strength is about 175,000lb. per sq.in. and the Brinell hardness 35o. Fatigue strength is high. Depth hardening characteristics are ex cellent—strength, yield point, and hardness four inches below the surface being within s'io of the values at the surface of a properly quenched and tempered forging. American mass production plants
are finish-machining these heat treated forgings at extraordinary hard ness levels, Brinell 400 or over.
Sluggish accumulation of carbides on reheating quenched steel, as mentioned above, is responsible for dimensional stability under heat and load—that is, resistance to creep in high temperature boilers and to relaxation in bolts and studs. Low carbon steels of firebox quality to which has been added 0.5% or more molybdenum have therefore been much used in high temperature, high pressure service. A plain o•15% carbon steel will creep I% in io,000hr. at F, under a load of 6,200lb. per sq.in.; adding o.5% molybdenum increases this to 15,800lb., whereas a further addition of 1.25% chromium will raise the corresponding creep strength to 24,00o pounds. For really severe services high alloys are required for surface stability, and in them molybdenum also confers resistance against slow plastic deformation.
A family of carbon-molybdenum steels for engineering purposes has been developed by extensive co-operative work between the Ameri can steel industry and a progressive automobile builder. These are utilized in the same way as the conventional chromium steels, chromium-nickel steels, and others—that is, for axles, gears, shafts, and highly stressed forgings. Steel production is none too easy, as the purchasers insist on grain size and "normality" specifications (the latter having to do with the microstructural appearance of carburized samples).
Molybdenum has also been successfully substituted for tungsten in high speed tool steels. Early attempts to do this failed because such steels hardened with soft surfaces, and while this could be ground off of lathe tools, it prevented the manufacture of intricate cutters or broaches. A coating of borax prevents this action, as well as heat treatment in salt baths recently developed for the necessary high tem perature. Consequently tool steels where half or more of the tungsten is replaced have come on the market, considerably cheaper than con ventional high speed, yet equal in performance. (See HIGH SPEED