What is Molybdenum?
Molybdenum (symbol Mo, atomic number 42) is a refractory metal with the sixth highest melting point of all elements: 2,623°C (4,753°F). Only tungsten, tantalum, osmium, rhenium and carbon surpass it. Its name comes from the Greek molybdos ("like lead"), because its ore was confused with graphite and galena.
What makes molybdenum special is its combination of high melting point + low density + excellent machinability. Compared to tungsten, molybdenum has half the density (10.2 vs 19.25 g/cm³) and is much easier to machine, allowing creation of insert electrodes with mirror surface finish for high-precision electrical contact welding.
Discovery History
Molybdenum was discovered in 1778 by Swedish chemist Carl Wilhelm Scheele, who determined that molybdenite (MoS₂) was a mineral distinct from graphite. However, Scheele could not isolate the metal due to lack of an adequate furnace.
In 1781, his compatriot Peter Jacob Hjelm finally isolated metallic molybdenum by reducing molybdenum oxide with carbon. Fun fact: Scheele also discovered oxygen, chlorine and barium, but credit always went to others - earning him the nickname "Scheele the unlucky one."
The "Big Bertha" Cannon and Molybdenum
During World War I, Germany discovered that adding 1-2% molybdenum to steel dramatically improved its high-temperature resistance. The famous 43-ton "Big Bertha" cannon used molybdenum steel instead of conventional steel, withstanding the extreme heat of continuous firing. This military discovery drove the entire alloyed steel industry.
Molybdenum on the Moon
The Soviet Luna 24 mission (1976) brought samples from the lunar Sea of Crises. Scientists found a molybdenum grain measuring 1 × 0.6 micrometers inside a pyroxene fragment - evidence that this metal exists beyond Earth.
14th Century Japanese Knives
Although molybdenum was officially "discovered" in 1778, Japanese blacksmiths were unknowingly using it in the 14th century. Certain iron ore deposits in Japan contained natural molybdenum, producing exceptionally hard and corrosion-resistant steel knives.
Industrial Uses of Molybdenum
Alloyed Steels (80%)
Most molybdenum is used in high-strength steels, stainless steels and superalloys for jet engine turbines.
Lubricants
Molybdenum disulfide (MoS₂) is a solid lubricant for extreme pressures, used in engines and heavy machinery.
Furnace Electrodes
Its high melting point makes it ideal for electrodes in glass furnaces operating above 1,700°C.
Insert Electrodes
RWMA Class 14 electrodes (Refractory Metal Faced Electrodes) for electrical contact and terminal welding with mirror finish.
Why Molybdenum for Resistance Welding?
In resistance welding, molybdenum RWMA Class 14 is used when tungsten (Class 13) is too brittle or difficult to machine. Molybdenum offers:
- Mirror surface finish - critical for silver contact welding without marks
- Better machinability - tighter tolerances than tungsten
- Lower density - lighter electrodes, less inertia
- Does not contaminate copper - ideal for terminals and connectors
- TZM for intensive production - higher hardness and thermal fatigue resistance
"According to AWS J1.3/J1.3M:2020-AMD1, RWMA Class 14 molybdenum electrodes must have a minimum purity of 99.9% Mo, minimum hardness of 85 HRA, electrical conductivity of 30% IACS (17.4 MS/m), and ultimate compressive strength of 88 ksi (607 MPa)."
— AWS J1.3/J1.3M:2020-AMD1, An American National Standard
AWS J1.3 Class 14 Minimum Requirements
| Property | Minimum Value | SI Units |
|---|---|---|
| Composition | 99.9% Mo min. | — |
| Hardness | 85 HRA | — |
| Electrical Conductivity | 30% IACS | 17.4 MS/m |
| Ultimate Compressive Strength | 88 ksi | 607 MPa |
| ISO 5182 Equivalent | B13 or Mo | |
| ASTM Standard | ASTM B387 Type 360/361 | |
Source: AWS J1.3/J1.3M:2020-AMD1, Table 3 — Minimum Requirements for Group B Refractory Metals and Refractory Metal Composites.