Ferromolybdenum Alloy Composition, Grades and Steelmaking Use

What Ferromolybdenum Is and Where It Sits in the Supply Chain

Ferromolybdenum is a master alloy in which molybdenum is combined with iron so that the element can be introduced into a steel or iron melt in a controlled, low-loss form. Commercial material carries a nominal molybdenum content between roughly 55% and 75%, with the balance being iron plus limited amounts of silicon, aluminium, carbon, sulphur, phosphorus and copper. It is sold as crushed and screened lumps produced by aluminothermic reduction of roasted molybdenite concentrate, and it enters the melt shop either at tapping or during ladle treatment.

The industry uses an iron-based carrier rather than pure molybdenum metal for practical reasons. Pure molybdenum melts at about 2,623 °C, far above any steelmaking temperature, and its density of roughly 10.2 g/cm³ makes it slow to dissolve even after it finally liquefies. A ferroalloy with a lower melting range, a density close to that of liquid steel and an assay that can be verified by chemical analysis dissolves predictably in the ladle, so recovery of the alloying element stays high and the heat analysis stays inside its specification window.

For a buyer, ferromolybdenum is therefore judged on three things at once: the molybdenum assay, which determines how much alloying value each tonne actually carries; the impurity window, which decides whether the heat will still meet its carbon, sulphur and phosphorus limits; and the physical condition of the lumps, which decides whether the material dissolves in the bath or floats in the slag and reports to the wrong place.

Grade Chemistry and Typical Specification Limits

Ferromolybdenum is bought against a grade table rather than a single number. The grade name indicates a nominal molybdenum content, and the delivered assay sits inside a window around that figure because the reduction product is sorted by analysis after crushing. In the GB/T 3649 family the grades are designated by nominal molybdenum content, and within each grade the quality classes differ mainly in carbon, sulphur, phosphorus and copper limits. The table below shows the windows that are commonly written into purchase orders for the three grades that dominate trade.

Grade	Mo (%)	C max (%)	Si max (%)	S max (%)	P max (%)	Cu max (%)	Typical use
FeMo55, tight class	55.0 - 60.0	0.10	1.0	0.08	0.05	0.50	General alloy steels, cast iron
FeMo55, standard class	55.0 - 60.0	0.20	1.5	0.10	0.08	0.80	Structural and HSLA steels
FeMo60, tight class	60.0 - 65.0	0.10	1.0	0.08	0.05	0.50	Cr-Mo pressure vessel steels
FeMo60, standard class	60.0 - 65.0	0.15	1.5	0.10	0.06	0.80	Tool steels, stainless steels
FeMo70	65.0 - 75.0	0.10	0.8	0.06	0.04	0.30	Superalloy feedstock, high-Mo grades

Read the table as a starting point for negotiation, not as a substitute for the ordered specification. What matters commercially is that the molybdenum window is narrow enough that the melt shop can calculate the addition weight with confidence. A spread of five points of molybdenum between the bottom and top of the window is normal and is absorbed by trimming the addition; a spread of fifteen points forces the melt shop to analyse every lump lot separately.

The impurities behave in different ways in the furnace. Carbon is the first limit to check when the alloy is destined for low-carbon steels, because every 0.01% of carbon in the alloy becomes part of the carbon balance of the heat. Silicon and aluminium are residues of the reduction process and act as deoxidants, which is usually harmless in a rimmed or semi-killed practice but not acceptable where inclusion control is tight. Sulphur and phosphorus concentrate in the steel and affect toughness and hot ductility, so tight-class material is specified for plate and forging grades with impact requirements. Copper comes from scrap charged to the reduction, and because copper cannot be removed in steelmaking it is capped tightly for any alloy that also carries residual-element limits. Tin, antimony and arsenic are not normally quoted on ferromolybdenum certificates but are requested for superalloy feedstock, where they are counted as tramp elements against the melt's residual budget.

The molybdenum in the alloy is not present as free metal. It is bound in iron-molybdenum intermetallic phases such as Fe₂Mo and Fe₃Mo₂ together with molybdenum-rich solid solution, which is why the material is hard, brittle and easy to crush but slow to dissolve if the lumps are too coarse.

How Ferromolybdenum Is Produced

Production runs in two stages. First, molybdenite concentrate is roasted in air to drive off sulphur and produce technical molybdic oxide, a grey-green powder that is the actual feed to the reduction. Second, the oxide is reduced by an aluminothermic reaction in a sand-lined reactor. Aluminium is the main reductant; ferrosilicon and sometimes silicon metal are added to supplement the heat balance and to control the silicon content of the product, while iron units in the form of clean scrap or mill scale set the final molybdenum concentration. Fluorspar and lime are charged as slag formers so that the alumina produced by the reaction stays fluid and separates from the metal.

Because the reaction is strongly exothermic and self-propagating once ignited, the charge design is the whole art of the process. An excess of aluminium leaves aluminium in the alloy and makes the slag viscous; too little reductant leaves molybdenum in the slag as oxide and lowers the yield. Coarse oxide and coarse reductant give a slower, cooler reaction with poor metal-slag separation, which shows up later as slag-rich lumps with low apparent density. Good practice produces a sound ingot-like block with a clean, glassy slag on top, which is stripped after cooling, then crushed in jaw and roll crushers, screened into commercial size ranges, and blended into lots that are sampled for analysis.

That history explains why two lots with identical certificates can behave differently in the ladle. Slag-rich lumps, fines and oxidized surface all analyse correctly on a small sample but dissolve poorly, and the operator compensates by adding more alloy, which disturbs the carbon and silicon balance.

Lump Size Ranges and Packing

Standard commercial size ranges are 10-50 mm for converter and ladle addition, 10-100 mm for electric furnace charging, 3-10 mm for small induction furnaces and for cored-wire or injection practice, and 0-3 mm fines that are sold at a discount or briquetted. Each range is specified with an oversize and undersize allowance, and the allowance is checked by sieving a sample rather than by eye.

Packing follows the general requirements of GB/T 3650 for ferroalloys: steel drums of 100 kg or 250 kg, or one-tonne bulk bags with a moisture barrier liner, marked with grade, lot number, net weight and the producing unit. Ferromolybdenum does not react with water and is not a hazardous good in normal transport, but it must be kept dry, because a wet fines fraction promotes hydrogen pickup and makes weighing unreliable, and it must be stored away from acidic or alkaline liquids that would attack the fines.

Melting Behaviour, Density and Recovery in the Ladle

A 60% ferromolybdenum alloy becomes liquid well below the melting point of pure molybdenum, in the region of 1,600 to 1,800 °C depending on composition, and its density in the range of about 9.0 to 9.5 g/cm³ for sound lumps is high enough to sink through a steelmaking slag instead of floating on it. These two properties together are the reason the alloy can be added to the ladle during tapping and still report almost all of its molybdenum to the steel.

Molybdenum has a low affinity for oxygen compared with silicon, manganese or aluminium, so it is not lost to the slag by oxidation in the way that titanium or boron would be. Recovery of 95-98% is typical for correctly sized lump added to a tapping stream, with the loss coming from fines carried into the slag, from slag-rich lumps that never fully dissolve, and from material left in the ladle as undissolved heels. Where a very high molybdenum specification is required, or where the heat is small, the alloy is added in the furnace before tapping and given time to dissolve under a lime slag.

Two operating rules follow from the physics. First, do not add ferromolybdenum after the heat has been fully deoxidized and the slag has been thickened, because the alloy then sits on a viscous slag instead of penetrating it. Second, do not compensate for coarse size by increasing the addition weight; the undissolved remainder stays in the ladle and the next heat inherits it, which is how unexplained molybdenum drifts appear in a sequence.

Where the Molybdenum Ends Up: Applications by Steel Family

High-strength low-alloy and linepipe steels. Additions of about 0.10-0.30% Mo combined with niobium refine the microstructure and raise strength without sacrificing toughness, and molybdenum also improves the resistance of the heat-affected zone to softening during welding. Linepipe grades made to API Spec 5L at X65 and X70 PSL2 levels routinely carry molybdenum as part of this balance.

Chromium-molybdenum pressure vessel steels. The 1.25Cr-0.5Mo and 2.25Cr-1Mo families specified in ASME BPVC Section II Part A as SA387 Grade 11 and Grade 22, and in GB/T 713 as 15CrMoR-type grades, rely on molybdenum for creep strength at 400-550 °C, for resistance to hydrogen attack and for hardenability in thick sections. This is the most quality-sensitive ferromolybdenum market, and tight-class low-carbon material is normally specified.

Tool and die steels. Hot-work grades of the 4Cr5MoSiV1 type contain around 1.5% Mo, which raises temper resistance so that the die retains hardness after repeated exposure to hot workpieces. Cold-work grades use molybdenum together with chromium for through-hardening and carbide stability.

Stainless and corrosion-resistant steels. The 316L family specified in ASTM A240/A240M carries 2.00-3.00% Mo, which raises resistance to pitting and crevice attack in chloride environments. Molybdenum also improves the passivation behaviour of duplex and super-duplex grades and is a principal reason their pitting resistance equivalent number is higher than that of 304 or 316 base compositions.

Cast iron and alloyed irons. Molybdenum refines the graphite structure, raises hardness and improves high-temperature strength in automotive and roll applications, and is added to irons as ferromolybdenum rather than as oxide because recovery from the oxide in a cupola is unreliable.

Superalloy and molybdenum metal feedstock. Nickel-base alloys such as Alloy 625, UNS N06625, contain about 9% Mo, and the master alloy used to charge them must be low in copper, tin and antimony. Separately, ferromolybdenum is a route to molybdenum metal powders and to molybdenum products such as foil, sheet and sintering boats, where the alloy is purified further after reduction.

Sampling, Analysis and Acceptance

Acceptance starts with sampling. Sampling and sample preparation for ferroalloys follow ISO 3713 and, in Chinese practice, GB/T 4010 for preparing samples for chemical analysis, both of which require that the sample represent the lot rather than a convenient lump. In practice this means taking increments across the lot, crushing to a defined particle size, riffling down to a laboratory sample, and keeping a retained sample sealed and identified by lot number for at least the warranty period.

Molybdenum is normally determined by inductively coupled plasma optical emission spectrometry after dissolution, or gravimetrically where a referee result is needed. Carbon and sulphur are determined by combustion infrared absorption, phosphorus and copper by ICP, and silicon and aluminium by ICP or X-ray fluorescence after fusion. The certificate of analysis should state the method and the lot number it refers to; a certificate without a lot number cannot be tied to the material in the bag and is not evidence.

Physical acceptance is equally important and is often skipped. Check the sieve analysis against the ordered size range, check for visible slag on broken faces, check that the lumps are sound rather than porous, and weigh a sample of bags to confirm the declared net weight. A lot that passes on chemistry but fails on density will cost more in the ladle than any saving on the purchase price.

Common Purchasing Mistakes

Buying on molybdenum assay alone. Two lots at the same assay can differ by a factor in dissolution behaviour, and the cheapest material per molybdenum unit is frequently the most expensive per tonne of steel made to specification.

Ignoring the carbon window. Ferromolybdenum added to a low-carbon steel carries its carbon with it, and a 0.20% carbon alloy can push a tight heat over its limit when a 0.10% grade would not.

Accepting excessive fines. Fines blow out of the ladle or dissolve into the slag; a size range with a large undersize fraction is a hidden price increase.

Treating the certificate as the lot. Certificates describe the sample, not the bags; without lot marking and a retained sample there is no traceability if the heat fails.

Mixing grades in storage. FeMo55 and FeMo60 look identical and are frequently stored side by side; without clear marking the wrong grade is charged and the molybdenum calculation is wrong by five points.

Forgetting moisture and weighing discipline. Bulk bags stored outdoors gain weight in the liner; payment on delivered weight without a dry-weight clause transfers that water to the buyer.

FAQ

Q: What molybdenum content does commercial ferromolybdenum normally contain?
Most traded material falls between 55% and 75% Mo, with the 60% grade the volume leader. The exact window is fixed by the grade table in the purchase order, and the delivered assay is reported on the certificate of analysis per lot.

Q: Why is ferromolybdenum used instead of pure molybdenum metal?
Pure molybdenum melts at about 2,623 °C and is denser than steel, so it dissolves slowly and unevenly. The master alloy melts in the region of 1,600-1,800 °C, has a density close to liquid steel and dissolves during tapping with recovery of 95-98%.

Q: What recovery can be expected when ferromolybdenum is added to the ladle?
With correctly sized 10-50 mm lump added to the tapping stream, 95-98% is normal. Losses come from fines in the slag, slag-rich lumps and undissolved heels, and they rise sharply when the size range is coarse or the slag is viscous.

Q: Does molybdenum burn out of the steel like titanium or aluminium?
No. Molybdenum has a much lower affinity for oxygen, so it is not oxidized from the bath under steelmaking conditions. This is why the alloy can be charged early in the furnace as well as added at tapping.

Q: Which impurity limits matter most in ferromolybdenum?
Carbon, sulphur and phosphorus decide whether the heat meets its analysis; copper, tin and antimony matter when the steel is also held to residual-element limits; aluminium and silicon affect deoxidation and inclusion control in clean-steel practice.

Q: How should a delivered lot of ferromolybdenum be checked?
Verify lot marking and net weight, sieve a sample against the ordered size range, inspect broken faces for slag and porosity, and compare the certificate of analysis with a retained sample prepared under ISO 3713 and GB/T 4010 practice.