Molybdenum – an alloying addition to steel
Molybdenum (Mo) is one of the key alloying elements used in steels, influencing their structure and mechanical properties. Its unique properties improve the strength, corrosion and creep resistance, and hardenability of steel.
Solubility of molybdenum in iron
It has limited solubility in various allotropic forms of iron:
- In γ iron (austenite) – up to about 3%.
- In austenite containing about 0,30% carbon – up to about 8%.
- In α iron (ferrite) – up to 32%, with the solubility decreasing with decreasing temperature.
Molybdenum – physicochemical properties
| Parameter | Older data (encyclopedia) | Contemporary data (IUPAC / CRC 2023) |
| Chemical symbol | Mo | Mo |
| Atomic number Z | 42 | 42 |
| Atomic mass [u] | 95,94 | 95,95 |
| Group in the periodic table | 6 (transition metals) | 6 (transition metals) |
| Electron configuration | [Kr] 4d⁵ 5s¹ | [Kr] 4d⁵ 5s¹ |
| Density (20 °C) [g/cm³] | 10,2 | 10,28 |
| Melting point [°C] | 2600 ° C | 2623 ° C |
| Boiling point [°C] | 4600 ° C | 4639 ° C |
| Hardness (Mohs) | ~ 5,5 | 5,5 |
| Color and appearance | silvery white, metallic, hard | silvery white, metallic, hard |
| Young's modulus (E) [GPa] | no data | ~ 329 |
| Shear modulus (G) [GPa] | no data | ~ 126 |
| Bulk modulus (K) [GPa] | no data | ~ 230 |
| Electrical conductivity [MS/m] | no data | ~18,7 (32% IACS) |
| Thermal conductivity [W m⁻¹ K⁻¹] | no data | ~ 138 |
| Specific heat cp [J kg⁻¹ K⁻¹] | no data | 251 |
| Enthalpy of fusion [kJ/mol] | no data | ~ 37,5 |
| Enthalpy of vaporization [kJ/mol] | no data | ~ 598 |
| Electronegativity (Pauling) | ~ 1,8 | 2,16 |
| Stable isotopes | no data | ⁹²Mo, ⁹⁴Mo, ⁹⁵Mo, ⁹⁶Mo, ⁹⁷Mo, ⁹⁸Mo, ¹⁰⁰Mo |
Chemical characterization
- Molybdenum is a refractory metal that is resistant to melting and creep at high temperatures.
- The surface is quickly covered with a thin layer of MoO₃, which partially protects against corrosion.
- It dissolves in steel, increasing its resistance to pitting corrosion and crevice (especially in environments with Cl⁻ ions).
- In tool steels it increases hardness and resistance to tempering.
- Creates stable carbides (Mo₂C) that increase abrasion resistance.
- In catalysis (e.g. petrochemistry) used in the form of sulfides (MoS₂).
- Impact on steel
Like chromium, it is a ferrite-forming additive and exhibits a strong affinity for carbon—more so than chromium and tungsten. Despite its high price, its unique properties make it widely used in various grades of steel, including:
- Structural steels (0,2–0,3% Mo)
- Cold work tool steels and hot (0,2–0,5% Mo)
- In high-speed steels (approx. 5% Mo)
- Acid-resistant stainless steels (2–4% Mo)
Key effects of molybdenum addition
- Corrosion resistance – especially in environments containing chlorides and acids, which makes it indispensable in stainless steels.
- Improved creep strength – increases the resistance of steel to long-term loads at high temperatures.
- Reduction of tempering brittleness – prevents the development of temper brittleness in hardened and tempered steels.
- Increased hardenability – it is superior to chromium and tungsten in this aspect, which makes molybdenum steels they more easily obtain a uniform hardened structure.
- Reduction in the amount of retained austenite – crucial for case-hardening steel i hardening surface.
- Reduced tendency to overheat molybdenum steels are less susceptible to excessive austenite grain growth.
- Resistance to hardness loss during tempering – delays the processes of carbide secretion and coagulation.
Molybdenum – the use of molybdenum steels
It is widely used in structural, tool, and specialty steels. It is used in:
- Heat-resistant steels – e.g. alloys containing additional chromium and vanadium, used in the energy and aviation industries.
- Hot work tool steels and high-speed steels – improves durability and resistance to high temperatures.
- Stainless steel i acid-resistant – improves resistance to acids, especially sulfuric and acetic acids.
- Steels for components operating at high temperatures – thanks to the ability to increase creep resistance.
Molybdenum Limitations and Challenges
Despite its numerous advantages, it also has some limitations:
- High price – limits its use in some low-cost steels.
- Reduced resistance to oxidation – at high temperatures it forms volatile MoO₃ oxides, which may limit its use in heat-resistant steels.
- Limited solubility in ferrite at low temperatures – which affects the microstructure of some steel grades.
The future of molybdenum steels
As technology advances, the demand for molybdenum in modern steels is growing. High-alloy martensitic steels, for example, are precipitation-hardened by the precipitation of intermetallic compounds containing this element. In the future, its importance in materials engineering can be expected to continue to grow, particularly in applications requiring high creep, corrosion, and fatigue resistance.