Ferromolybdenum: Alloying Principles, Grades and Steelmaking Use
Leave a message
What Ferromolybdenum Is and What the Alloy Really Does
Ferromolybdenum, normally written FeMo, is a master alloy in which molybdenum is carried into liquid steel on an iron base. The point of the iron is practical rather than metallurgical: pure molybdenum melts at about 2,623 °C, so a lump of pure metal dropped into a ladle at 1,600 °C would simply sink and sit there while the heat was cast. Combining molybdenum with iron produces a lump that the steel bath can dissolve within the tap-to-cast window, at a molybdenum content the melter can weigh and calculate with confidence.
The commercial designation of the alloy states its nominal molybdenum content: FeMo55, FeMo60, FeMo70. Delivery is usually in 10-50 mm lumps, with 3-10 mm or 0-3 mm material available where a ladle-injection or cored-wire route is used. In Chinese production the alloy itself is specified under GB/T 3649, delivery, packing and certification follow GB/T 3650, sampling and sample preparation follow GB/T 4010 or ISO 3713, and the wet-chemical and instrumental methods used to prove the analysis belong to the GB/T 5059 series.
So the "principle" of ferromolybdenum has two halves, and buyers need both. The first half is why molybdenum is worth adding at all: what it does to hardenability, carbide formation, tempering resistance, creep strength and corrosion resistance. The second half is why the alloy is made and sized the way it is, because that determines how much of the molybdenum you paid for actually reports in the final ladle analysis. A purchase order that fixes only the molybdenum percentage and ignores density, sizing and solution behaviour buys a certificate, not a result.
Metallurgical Principles: What Molybdenum Does Inside Steel
Molybdenum is a strong carbide-forming element, though a comparatively mild one when ranked against titanium, niobium, vanadium and tungsten. In low-alloy steels it first partitions to cementite, then, during tempering in the 500-600 °C range, it forms its own alloy carbides - principally M6C of the Fe3Mo3C type, together with M2C and molybdenum-rich M23C6. Those carbides are fine, slow to coarsen and stable to well above the temperature at which plain cementite has already spheroidised. The practical consequence is secondary hardening: a tempered molybdenum steel can hold hardness at 550 °C where an unalloyed steel of the same carbon level has long since softened.
The second principle is hardenability. Per unit of added weight molybdenum is one of the most efficient hardenability elements available. Additions of only 0.15-0.30% Mo delay the pearlite and bainite transformations enough that heavy sections can be through-hardened in oil or forced air, which is why molybdenum appears in thick-wall pressure components, large forgings and heavy machinery parts rather than only in highly alloyed grades. Molybdenum also interacts usefully with chromium: in 1.25Cr-0.5Mo and 2.25Cr-1Mo steels the molybdenum both raises elevated-temperature strength and suppresses reversible temper embrittlement, the grain-boundary segregation of phosphorus, tin, antimony and arsenic that otherwise makes a slowly cooled Cr-Mo weldment brittle at ambient temperature.
The third principle is solid-solution strengthening and creep resistance. Molybdenum atoms are substantially larger than iron atoms and raise the lattice friction that dislocations must overcome; because molybdenum also raises the recrystallisation temperature and slows self-diffusion, it is a standard addition in steels that must carry load for 100,000 hours at 450-550 °C. The fourth is corrosion: molybdenum stabilises the passive film in chloride environments, and its contribution is captured in the pitting resistance equivalent number, PREN = %Cr + 3.3×%Mo + 16×%N. That single expression explains why 2-3% Mo separates a 316L grade from a 304 grade in a chloride service, and why molybdenum-bearing austenitic and duplex grades are chosen for seawater-adjacent duty.
There is a fifth effect that is often overlooked and works the other way. Molybdenum is not a high-temperature oxidation element. Its oxide MoO3 is volatile above roughly 700-800 °C, so a molybdenum-bearing steel scales faster, not slower, in air at elevated temperature. Resistance to scaling comes from chromium, silicon and aluminium. This is exactly where many popular explanations of ferromolybdenum go wrong, and a buyer who accepts the error will specify the wrong material for a furnace application.
Commercial Grades and Chemical Composition
The grades traded internationally are defined by their molybdenum content and by the residuals that would harm a clean steel: silicon, carbon, phosphorus, sulphur, copper and, where the steelmaker is producing low-aluminium grades, aluminium. The table below sets out the ranges normally offered. The binding figures for any shipment are those of the mill certificate issued against GB/T 3649 and the agreed purchase order, and it is normal for a contract to tighten one or two residuals beyond the general grade range.
| Grade | Mo, % | Si, % max | C, % max | P, % max | S, % max | Cu, % max | Normal use |
|---|---|---|---|---|---|---|---|
| FeMo55 | 55.0-60.0 | 2.0 | 0.15 | 0.05 | 0.10 | 0.50 | General steelmaking where the molybdenum charge is large and residual tolerance is generous |
| FeMo60 | 60.0-65.0 | 2.0 | 0.10 | 0.05 | 0.10 | 0.50 | Cr-Mo pressure vessel and boiler steels, structural and pipeline grades |
| FeMo65 | 65.0-70.0 | 1.5 | 0.10 | 0.05 | 0.10 | 0.50 | Lower charge weight per tonne of steel; tool steels and alloy steels |
| FeMo70 | 65.0-75.0 | 1.5 | 0.10 | 0.05 | 0.10 | 0.50 | Stainless and nickel-base melting, where freight and charge space are the constraint |
Two residuals deserve more attention than they usually receive. Copper is not removed by any normal steelmaking step, so a FeMo made from a copper-bearing scrap source will carry that copper into every heat it touches; for a steel with a 0.20% Cu ceiling the difference between 0.10% and 0.50% Cu in the alloy is real. Aluminium is not a standard grade-defining element in the same way, but alumino-thermic FeMo inevitably retains some aluminium, and heats intended for clean-steel applications - thin-plate, wire, bearing or forging grades - often specify a low-aluminium variant with the residual agreed in the contract rather than left to the general grade.
How Ferromolybdenum Is Made: the Aluminothermic Route
Almost all ferromolybdenum is produced by aluminothermic reduction, a self-sustaining exothermic reaction between molybdenum oxide and aluminium. The route begins with molybdenite concentrate, essentially MoS2 at roughly 45-50% Mo, which is dead-roasted in a multiple-hearth furnace at about 600-650 °C to drive off sulphur and yield technical molybdenum oxide containing 57-62% Mo. That oxide, not the sulphide, is the feed to the reduction step.
The oxide is blended with aluminium granules in a ratio slightly above stoichiometric, an iron source in the form of mill scale, iron oxide or clean steel turnings, a fluorspar flux, and crushed recycled FeMo fines. The mix is charged into a refractory-lined reactor and ignited. Once started, the reduction of molybdenum oxide by aluminium releases enough heat to melt both the metal and the alumina slag without external power, and the reactor contents reach a temperature of the order of 2,000 °C. Because the metal is far denser than the slag, the FeMo collects as a pool on the reactor bottom while a calcium-aluminate slag floats above it.
After solidification the regulus is stripped of slag, crushed, screened to the ordered size range and sampled. Several operating levers decide the quality of the result. Excess aluminium improves slag fluidity and metal-slag separation but raises the aluminium residual in the alloy; too little aluminium leaves molybdenum in the slag as oxide or as trapped prills and cuts yield. Fluorspar controls slag viscosity and therefore the cleanliness of the metal-slag interface. Furnace and reactor practice, cooling rate and the amount of recycled fines in the charge all move the recovery, which for a well-run operation generally sits in the low-to-mid 90s as a percentage of the molybdenum charged. A separate silicothermic or electro-silicothermic route exists for grades where a very low aluminium residual matters more than power consumption.
The reason the process is aluminothermic rather than carbothermic is chemical: carbon reduction of molybdenum oxide at steelmaking temperature would produce molybdenum carbide and a carbon-contaminated alloy, which defeats the purpose of a master alloy intended for low-carbon steels. Aluminium is chosen because alumina is exceptionally stable, so the reaction is strongly exothermic and the aluminium oxide leaves as a liquid slag that can be separated cleanly.
Physical Behaviour That Drives Addition Practice
FeMo has a density of roughly 9-9.5 g/cm³, close to that of the molybdenum it carries and well above the 7.0 g/cm³ of liquid steel. That single fact governs how the alloy must be added. A lump charged onto a slag cover will not float or disperse; it will punch through the slag, reach the bath bottom and remain there until it dissolves. The alloy's melting range is far above steelmaking temperature, so it does not melt as a bulk solid - it dissolves progressively into the melt, and the rate depends on lump size, bath superheat, and whether the bath is stirred.
Timing: add during tapping, into the tapping stream, so that the turbulence of the falling stream keeps the particles in contact with fresh steel instead of letting them settle into a cold corner of the ladle.
Superheat: dissolution consumes heat locally; a bath with 30-40 °C of superheat above its liquidus absorbs a normal FeMo addition without trouble, while a tight, cold heat can produce a molybdenum-rich skull on the ladle bottom.
Stirring: argon bubbling or electromagnetic stirring after the addition homogenises the bath and prevents a bottom-rich layer from being cast into the first slab.
Sizing: 10-50 mm lumps suit ladle addition; 0-3 mm fines dissolve almost instantly but are easily lost to the slag or to dust extraction, so they belong to injection or briquetting routes rather than to open ladle charging.
Recovery of molybdenum is high because molybdenum has a much lower affinity for oxygen than aluminium, titanium, boron or rare earths. Under normal ladle practice, 95-98% of the molybdenum charged reports in the ladle analysis, and the small remainder is lost as fines carried into the slag or as dust. This is why molybdenum is normally specified and priced on a contained-metal basis and why the melter's calculation can be made with a narrow recovery assumption - unlike aluminium or titanium, where recovery swings widely with oxygen content and holding time.
Where the Molybdenum Goes: Applications by Steel Family
The tonnage picture is dominated by a handful of families, and each uses molybdenum for a different reason. The table below links the application to the typical molybdenum level and to the specification that governs it.
| Steel family | Typical Mo, % | Why molybdenum is present | Governing specification |
|---|---|---|---|
| Carbon-molybdenum boiler plate | 0.45-0.60 | Elevated-temperature strength and creep resistance without chromium | ASTM A204 Gr. A/B |
| 1.25Cr-0.5Mo plate | 0.45-0.65 | Creep strength plus resistance to temper embrittlement in thick sections | GB/T 713 15CrMoR; ASME SA387 Gr. 11 |
| 2.25Cr-1Mo plate | 0.90-1.10 | Hydrogen-attack resistance and high-temperature strength in reactor service | GB/T 713 12Cr2Mo1R; ASME SA387 Gr. 22 |
| Austenitic stainless | 2.00-3.00 | Pitting and crevice corrosion resistance in chloride media | ASTM A240 316L; GB/T 4237 |
| Hot-work tool steel | 1.10-1.75 | Secondary hardening and thermal fatigue resistance | GB/T 1299 4Cr5MoSiV1 |
| High-speed steel | 4.50-5.50 | Red hardness through stable molybdenum-rich carbides | GB/T 9943 W6Mo5Cr4V2 |
| High-strength structural and linepipe | 0.10-0.30 | Acicular ferrite promotion and strength in heavy wall | Mill chemistry, not mandated by the product standard |
Beyond these families, molybdenum enters alloy cast iron and indefinitely chilled rolls as a carbide stabiliser at roughly 0.3-1.0%, where it refines the carbide network and raises wear resistance. It is also a major alloying element in solid-solution-strengthened nickel-chromium-molybdenum alloys, which carry 8-10% Mo to combine chloride resistance with high strength. And it appears in small but critical amounts in bearing steels, spring steels and maraging-type grades, where the carbide-forming and hardening behaviour matter more than the tonnage.
A buyer sourcing FeMo for a Cr-Mo plate order should work backwards from the product specification rather than from the alloy grade. If the heat is a 2.25Cr-1Mo plate with 0.90-1.10% Mo and a 0.05% maximum phosphorus, the FeMo charge must be computed on contained molybdenum, and the phosphorus and copper contributions of the alloy must be checked against the residual budget of the finished steel. Charging FeMo60 at 1.7% of heat weight to reach 1.0% Mo, for example, also brings in roughly 0.085% phosphorus from a 0.05% P alloy if the recovery is 100% - a figure that must be modelled before the heat is tapped, not discovered on the product analysis.
Buying, Sampling and Acceptance
Ferromolybdenum is a heterogeneous product: a drum contains lumps that cooled at different rates and, because the metal is dense, molybdenum-rich and slag-rich fractions can segregate during filling and transport. Sampling therefore decides whether the certificate means anything. GB/T 3650 governs delivery, packing and the certificate of analysis; GB/T 4010 and ISO 3713 describe how increments are taken and how the laboratory sample is prepared. In practice a sound lot sample is built from increments taken from several drums, at both top and bottom, crushed to pass a defined screen and then divided down, so that the assay represents the lot rather than the top of one drum.
Acceptance checks that catch most disputes are straightforward:
Molybdenum assay against the grade range, by the methods of the GB/T 5059 series, with a second laboratory used as umpire if buyer and seller differ beyond the agreed tolerance.
Residuals - silicon, carbon, phosphorus, sulphur and copper - compared with the grade limits and with any tighter contract figure.
Size distribution - the percentage passing the lower screen and retained on the upper screen, since oversize slows dissolution and undersize is lost to the slag.
Moisture and fines - wet alloy is both a weighing error and a safety issue, because aluminium-bearing ferroalloys react with water to release hydrogen.
Packing integrity - drum weight, lining, and whether the lot has been stored under cover.
On the commercial side, ferromolybdenum is quoted per kilogram or per pound of contained molybdenum rather than per tonne of alloy, which makes grade comparison simple: divide the price per tonne by the molybdenum percentage to get the contained-metal cost, then add the freight and packaging difference. Because the alloy is dense, freight per tonne of contained molybdenum falls as the grade rises, which is why FeMo70 can be the cheaper purchase in a distant market even at a higher nominal price. Under customs tariff codes the product falls in the 7202.70 heading for ferromolybdenum. Storage should be dry and covered, drums should not be stacked beyond their rated load, and fines should be kept out of drains and away from moisture.
Common Misconceptions Worth Correcting
Molybdenum does not improve scaling resistance. It improves strength retention and corrosion resistance in aqueous chloride service, but its oxide volatilises at high temperature, so molybdenum-bearing steels are in fact less oxidation resistant than plain carbon steel in a furnace atmosphere. High-temperature oxidation duty calls for chromium, silicon or aluminium additions, not molybdenum.
Molybdenum does not work by raising thermal conductivity. The alloying rationale is carbide stability, hardenability and solid-solution strengthening; thermal conductivity in steel is dominated by carbon level and microstructure, and molybdenum additions generally lower it slightly.
A higher grade is not automatically a cheaper source of molybdenum. Compare contained-metal cost, not alloy price, and include recovery, freight and residual penalties.
Molybdenum does not make a steel easier to weld. Every point of molybdenum raises hardenability, so a 2.25Cr-1Mo weldment needs preheat, controlled interpass temperature and often post-weld heat treatment. Weldability is a function of the whole chemistry and the procedure, not of the alloying element alone.
FeMo cannot be added at any convenient moment. Charged onto a slag cover it sinks and may never fully dissolve; charged into a cold bath it forms a bottom skull. Timing, superheat and stirring are part of the specification in practice even when they are not written on the order.
FAQ
Q: What is the molybdenum content of FeMo60 ferromolybdenum?
FeMo60 is a 60% nominal grade, normally delivered in the 60.0-65.0% Mo range, with silicon at 2.0% max, carbon at 0.10% max, phosphorus at 0.05% max, sulphur at 0.10% max and copper at 0.50% max. The certificate issued against GB/T 3649 and the agreed order is the binding statement for a specific lot.
Q: Why is molybdenum added as ferromolybdenum instead of as pure molybdenum?
Pure molybdenum melts near 2,623 °C, far above steelmaking temperature, so it would not dissolve within the tap-to-cast window. The iron base lowers the melting range and the density difference still lets the lump sink through slag into the bath, where it dissolves with recovery typically above 95%.
Q: What recovery should be assumed when FeMo is added to the ladle?
With addition into the tapping stream, adequate superheat and argon stirring, 95-98% of the charged molybdenum normally reports in the ladle analysis. Losses come from fines carried into the slag, dust extraction and incomplete dissolution in an underheated heat.
Q: Does molybdenum improve oxidation resistance at high temperature?
No. MoO3 is volatile above roughly 700-800 °C, so molybdenum-bearing steels scale faster than plain carbon steel in air. Molybdenum is chosen for creep strength, tempering resistance and aqueous corrosion resistance; scaling resistance comes from chromium, silicon and aluminium.
Q: What lump size of ferromolybdenum should be ordered?
10-50 mm is the standard ladle-addition size. Coarser material dissolves too slowly for a short tap-to-cast cycle, while 0-3 mm fines dissolve quickly but are easily lost to slag or dust and are better suited to injection or briquetting.
Q: How is a ferromolybdenum lot sampled and accepted?
Increments are taken from several drums at top and bottom, crushed and divided following GB/T 4010 or ISO 3713, and analysed by the methods of the GB/T 5059 series. Acceptance also covers residuals, size distribution, moisture, packing weight and the certificate of analysis required by GB/T 3650.
