Molybdenum (Mo)
Molybdenum is a refractory metal with the sixth-highest melting point of all elements (2,623°C). Approximately 80% of molybdenum is used as an alloying element in steels and superalloys, where it dramatically improves high-temperature strength, creep resistance, and pitting corrosion resistance. It is the element that differentiates 316 from 304 stainless steel.
Properties
- Density
- 10,280 kg/m³
- Melting Point
- 2,623 °C
- Boiling Point
- 4,639 °C
- Thermal Conductivity
- 138 W/(m·K)
- Electrical Resistivity
- 5.34 × 10⁻⁸ Ω·m
- Tensile Strength
- 500–700 MPa
- Yield Strength
- 350–550 MPa
- Hardness
- 150–250 HV
- Elongation
- 10–25%
- Crystal Structure
- BCC
Common Grades
- Mo 99.95%: Pure molybdenum. Furnace elements, heat shields, sputtering targets, glass melting electrodes.
- TZM (Mo-0.5Ti-0.1Zr): High-strength molybdenum alloy. Rocket nozzles, isothermal forging dies, X-ray anodes.
- FeMo (60–75% Mo): Ferromolybdenum. Primary alloying addition for steels and stainless steels.
- MoS₂ (Molybdenum disulfide): Solid lubricant (like graphite). Bearings, fasteners, high-temperature lubrication.
Applications
- Steel Alloying: 316 stainless steel (2–3% Mo), CrMo pressure vessel steels, HSLA steels, tool steels
- High Temperature: Furnace elements (up to 1,900°C in reducing atmosphere), heat shields, sintering boats
- Chemical Processing: Catalyst in petroleum refining (hydrodesulfurization), chemical reactor components
- Aerospace: Rocket nozzles (TZM), turbine disc alloys, heat sinks for electronics
FAQ
Why does 316 stainless steel contain molybdenum?
The 2–3% molybdenum in 316 stainless steel dramatically improves resistance to pitting and crevice corrosion, especially in chloride environments. Mo stabilizes the passive film in the presence of chloride ions. This is why 316 is specified for marine, chemical processing, and pharmaceutical applications where 304 would fail.
What is the PREN value and how does molybdenum affect it?
PREN (Pitting Resistance Equivalent Number) = %Cr + 3.3×%Mo + 16×%N. Molybdenum has a coefficient of 3.3 — meaning 1% Mo addition is equivalent to 3.3% Cr for pitting resistance. This is why even 2–3% Mo in stainless steel makes such a dramatic difference in corrosion performance.
What is the TZM alloy used for?
TZM (Mo-0.5%Ti-0.1%Zr) is a molybdenum alloy with significantly higher recrystallization temperature and creep strength than pure Mo. It maintains useful strength to 1,400°C, making it essential for isothermal forging dies, rocket nozzles, X-ray rotating anodes, and high-temperature furnace components.