Molybdenum, element number 42 of the periodic table, lies in the table's second transition series, in Group 6A between chromium and tungsten.
It has one of the highest melting temperatures of all the elements, yet unlike most other high-melting point metals, its density is only 25% greater than iron's. Its coefficient of thermal expansion is the lowest of the engineering materials, while its thermal conductivity exceeds all but a handful of elements.
||Body-centered cubic (BCC)
||a = 3.1470 Å
|Coefficient of thermal expansion
||4.8 x 10-6 / K at 25°C
||138 W/m K at 20°C
Molybdenum-based alloys have a unique combination of properties, including high strength at elevated temperatures, high thermal and electrical conductivity, and low thermal expansion. Molybdenum metal and its alloys are the first choice in many demanding specialized applications.
Many products flow from the mining and processing of ores containing molybdenite (MoS2), including:
Chemical Mo products
Used in catalysts, polymer compounding, corrosion inhibitors, and high-performance lubricant formulations, these include pure molybdic oxide and molybdates, and lubricant grade MoS2.
Used as alloy additions to iron, steel, nickel, and titanium alloys, these include technical Mo oxide, ferromolybdenum, and Mo metal pellets.
Mo metal products
These include powder, Mo metal and Mo-base alloy mill products, and products fabricated from them.
Molybdenum is supplied as ferromolybdenum and as molybdic oxide, MoO3. In the U.S., the trend has been overwhelmingly toward the use of oxide, although the ferroalloy continues to have important applications in tool steel production and in iron and steel foundries.
Technical molybdic oxide (57% Mo, min.) is delivered in cans containing fixed amounts of molybdenum. SiO2 is the major impurity, but the product may also contain small amounts of copper, sulfur and phosphorus. Molybdic oxide is also supplied as briquettes. This product form is suitable for use in small acid or basic electric furnaces where heat times are short and may not be adequate for complete reduction and homogenization. MoO3 briquettes normally contain some carbon; its presence reduces the possibility of bath oxidation and can decrease slag volumes accordingly. The major remaining impurity is also silica.
Ferromolybdenum contains a minimum of 60% Mo. Up to 1% each of silicon and copper may be present. Product is supplied as lump, 2 in. x D, or in granular forms as fine as 8 mesh x D. Being a relatively expensive and sparingly used alloy addition, ferromolybdenum is packaged in cans or bags to prevent handling losses.
It should be mentioned that considerable quantities of molybdenum are recovered from alloy scrap and that the use of addition agents, as described below, is strongly governed by the quantity and molybdenum content of the scrap available. The scrap must be well segregated, both to recover its alloy content and to avoid contamination with Mo of steels in which its presence is undesirable.
Molybdic oxide is easily reduced to metal in electric furnace, AOD and basic oxygen practice provided enough carbon or other reducing agents are present at the time of the addition. It is not suitable for use in induction melting furnaces, vacuum practices, or as a ladle addition. Well over 90% of "new" molybdenum used in U.S. steelmaking practice is now in the form of oxide. Ferromolybdenum is used as a ladle addition for final chemistry adjustment, in high speed steel production and in ferrous foundries.
In the EF production of alloy steel, heat resisting and HSLA grades (Mo content usually less than 1%), tech oxide is added to the bath after meltdown is complete and after a preliminary chemical analysis has been taken. This allows for normal uncertainties in scrap composition and permits closer control of final molybdenum content. Adding fluorspar may help reduce slag foaming. Molybdenum recovery will usually be 95% or higher since molybdenum oxide is easily reduced in the bath. When needed, compensation for inaccuracies in the preliminary analysis can be made by adding ferromolybdenum in the ladle.
If the composition of the scrap is reasonably well known it may be more convenient to add oxide directly with the charge, leaving an allowance for final ladle trimming with ferromolybdenum, as appropriate. This is the normal practice in EAF and BOF steelmaking.
Molybdic oxide, usually in the form of briquettes, may also be used for the production of such moly-rich grades as high speed tool steels. In AOD stainless steel production, molybdenum oxide is normally added during the initial blow and ferromolybdenum is added in the final additions. Molybdenum oxide briquette recovery levels can be lower if they are added during the initial blow (87% compared with 95% when added to the vessel in the non-blow position). Electric furnace use of oxide requires some care to minimize losses due to volatilization and other causes. Nonetheless, some EF producers make as much as 70% of their molybdenum additions as oxide, with recoveries of up to 93%. The balance of the Mo addition comes from selected scrap and/or ferromolybdenum.
Careful adherence to well designed hot working procedures is necessary in order to take maximum advantage of the effects molybdenum produces in steel. This is especially important in the case of "as-rolled" HSLA plate and strip, and for moly high speed tool steels. Molybdenum slows ferrite separation from austenite to greatly enhance bainitic hardenability (helpful in HSLA steels utilizing acicular-ferrite).
For HSLA grades containing molybdenum, manganese and columbium, roughing should begin near 1205 C (2200 F). This provides the finest austenite grain size going into the finishing train.
Finish rolling may begin at controlled temperatures below 980 C (1800 F) although the Mn-Mo-Cb steels are not controlled rolled in the same sense as the Mn-V-Cb grades. Controlled cooling, using lamellar flow water and/or high pressure sprays, may be used to provide a strip temperature of 550-575 C (1020-1070 F) at the coiler. This prevents the formation of harmful coarse ferrite grains.
Molybdenum (as-rolled) dual phase steels do not require the post-rolling intercritical heat treatments needed for vanadium types, and can develop the dual phase (ferrite + martensite) structure directly off the hotmill. Close control of rolling variables is important but can be somewhat relaxed if silicon contents are high enough (~1.3%) and manganese is reduced to 0.7-0.8%. Then, slab reheat temperatures in the wide range 1150-1315 C (2100-2400 F) are acceptable. Finish rolling temperatures may be in the range 840-925 C (1550-1770 F), and coiling temperatures may vary from 450-620 C (850-1150 F) although optimum results are obtained when coiling at 565 C (1050 F).
Moly high speed steels, like other highly alloyed products, require a significant amount of hot working to break up segregated as-cast structures. Reductions in cross sectional area of 90% or more are considered mandatory. Final hot forging is performed at around 1180 C (2150 F). Powder metallurgy has gained popularity in the production of larger section high speed tool steel mill products because it reduces the amount of hot deformation needed.
Most alloy steels require higher austenitizing temperatures, high tempering temperatures and/or longer tempering times than carbon steels, and moly grades are no exception. Molybdenum steels have a tendency toward surface decarburization and protective measures such as controlled atmospheres, salt baths, borax coatings and even vacuum processes are advisable.
Both molybdenum and tungsten high speed steels require special care in heat treatment. After rough machining, parts should be given a subcritical anneal at 680-700 C (1255-1290 F) and slowly cooled to relieve machining stresses. This also improves dimensional stability during hardening. Heating for austenitizing should be performed slowly: both a preheat at 500 C (930 F) and a dwell in the range 820-840 C (1510-1545 F) are recommended. Another dwell at 1100 C (2010 F) is helpful. Thereafter, heating to the austenitizing range, 1170-1240 C (2140-2265 F) - depending on grade - should be rapid to minimize grain growth. Overheating (incipient fusion, or "burning") must be avoided, and the above precautions regarding surface protection are essential. Quenching may be direct or interrupted and should be followed by stabilization, preferably in liquid nitrogen, to help transform residual austenite. Double, or even triple, tempering in the range 510-610 C (950-1130 F) follows, but the specific temperatures chosen depends on the composition involved.
The molybdenum content of alloy steels ranges from well less than 1% in HSLA and constructional alloy grades to as much as 9% in high speed tool steels.
Moly is used in many hot rolled (0.15-0.50% Mo), hot rolled and aged (1.30-1.50% Mo) and quenched and tempered high strength steels. The yield strengths of these materials range from 350-550 MPa (50-80 ksi) for HSLA grades to as high as 960 MPa (140 ksi) in the more highly alloyed varieties. Such steels are widely used in construction, earthmoving, automotive and oil country applications. HSLA pipeline grades take advantage of the changes moly produces in a steel's stress-strain behavior, i.e., giving a continuous yield range rather than a sharply defined yield point. Whereas U-O-E pipe fabrication decreases the yield strength of conventional C-Mn-V-Cb steels by up to 70 MPa (10 ksi) due to the Bauschinger effect, acicular ferrite moly grades actually gain as much as 140 MPa (20 ksi) during the same operations. In automotive HSLA steels, moly can be used to produce the highly formable dual phase structure directly off the hot strip mill without the need for subsequent heat treatment.
Molybdenum is a strong hardenability agent and is therefore found in many AISI/SAE constructional alloy steels. Because of it potency (and relatively high cost) molybdenum content will generally be held to 0.5% or less. Heat treatable alloy steels, particularly those containing chromium, are susceptible to temper embrittlement (see Chromium), but molybdenum can reduce or even eliminate this tendency. Thus heavy section forgings, which could be vulnerable to embrittlement because they can cool slowly through the known embrittling temperature range, will be made from molybdenum-containing steel.
Molybdenum is a strong carbide former and moly steels can exhibit a secondary hardening peak. This fact, plus molybdenum's ability to strengthen ferrite even at elevated temperatures, has led to the development of a series of heat-resisting steels the best known of which are the familiar 1-1/4 Cr-1/2 Mo and 2-1/4 Cr-1 Mo grades. In addition to elevated temperature stability, these steels are resistant to hydrogen blistering. This has made them standard choices for many power boiler and heat exchanger components, and for reactors, pressure vessels and piping for the chemical and petroleum industries.
Molybdenum is added to austenitic stainless steels, e.g., Type 316, for increased corrosion resistance. Pitting and crevice corrosion resistance in marine atmospheres is especially improved, as is the ability to withstand dilute (to 20%) and concentrated (over 75%) sulfuric acid. Moly also improves the pitting resistance of martensitic stainless steels. Here, it also provides higher as-quenched hardness and better wear resistance; thus, it is found in cutlery grades such as Type 440C. The addition of molybdenum to ferritic stainless steels improves their resistance to marine atmospheres and to certain organic acids.
Molybdenum is found in a number of tool steels, where its chief function is to provide hardness (wear resistance) and strength at elevated temperatures. The most important use of moly in this field, however, is in the molybdenum or M-series high speed steels. These will contain, depending on grade, between 3.5 and 9.5% Mo, plus tungsten, chromium, vanadium and possibly cobalt.
1- International Molybdenum Association (IMOA), http://www.imoa.info
2- Ferroalloys & Alloying Additives Online Handbook, http://amg-v.com