(Chemical symbol: Mo). A hard, silver-white metal, molybdenum is a significant alloying element in the production of engineering steels, corrosion resistant steels, tool steels, and cast irons. Atomic number 42; atomic weight, 95.95; melting point, 2620°C
(4748°F); boiling point 4804°C (8680o F), and specific gravity 10.2.
Molybdenum does not occur free in nature but is obtained from molybdenite (MoS2) and from wulfenite (PbMoO4). The metal is prepared by the reduction of the oxide with carbon, usually in an electric furnace. Small additions of molybdenum to steel promote relatively uniform hardness and strength even throughout heavy sections. In addition, molybdenum contributes the following properties:
(1) Increases the resistance of commonly used engineering steels to softening after tempering.
(2) Increases the strength and creep resistance of low- and high-alloy steels at elevated temperatures.
(3) Improves the resistance to corrosion by pitting in engineering steel, and improves the general corrosion resistance of chromium and chromium-nickel
corrosion resistant steels.
(4) Tends to retard intergranular corrosion in austenitic chromium-nickel steels if some delta ferrite is present.
In addition, molybdenum contributes other properties when added to steel. Molybdenum does not cause the formation of a tightly adherent scale on steel when
the steel is heated in an oxidizing atmosphere for hot working. This assists in subsequent cleaning operations when the steel is pickled or sand blasted; a tight scale is more difficult and costly to remove.
When added to cast iron, molybdenum improves the high-temperature strength and toughness. It is neither a graphitizer nor a strong carbide stabilizer in cast iron. The addition of molybdenum to gray cast iron increases the tensile strength; this is attributable to direct solid solution effect in ferrite and to retardation of transformation of austenite.