Tungsten melting point: 3410°C, the highest of any metal, and why x-ray tubes depend on it

Tungsten’s melting point: the canonical 3410°C

Tungsten is element 74 on the periodic table. Its pure-metal melting point is 3410°C, though some sources cite 3422°C depending on purity and measurement method. This is the highest melting point of any metal in existence.

In practical ARRT terms, the canonical answer is 3410°C. Study materials that cite 3370°C or 3390°C are using outdated chemistry databases. Modern references (NCBI Bookshelf, Radiopaedia, Bushong) confirm 3410°C.

Why does a melting point matter to radiography? Because the x-ray tube anode is a furnace. At the focal spot, electrons from the cathode slam into the tungsten target at near light speed. When an electron hits a tungsten nucleus, one of two things happens: the electron either bounces off elastically (leaving its kinetic energy behind as heat), or it decelerates and radiates energy as a photon. The ratio is brutal: 99.8% of the energy becomes heat, and only 0.2% becomes x-rays.

At the instant a clinical exposure fires, the focal spot reaches temperatures around 2500°C. That’s hot enough to melt copper (1084°C), vaporize gold (2856°C), and incinerate most alloys. Tungsten, with its 3410°C melting point, is the only metal that routinely survives this punishment.

Common wrong answer (3370°C) and why it sticks

A significant number of study materials cite 3370°C as the melting point of tungsten. This was correct data in early periodic tables, but has been superseded by more precise measurement.

The persistence of 3370°C in study guides is because:

1. It comes from older chemistry references (some textbooks were printed before 1990 and never updated).
2. It “looks close enough” to 3410°C that students don’t question it.
3. Free online periodic table sites sometimes serve cached or user-edited data.

If you see 3370°C on an old practice exam, or 3422°C on a chemistry site, understand that 3410°C is the modern ARRT-canonical value. The exam will use 3410°C.

Why x-ray tubes need 3410°C tolerance

To understand why tungsten is non-negotiable, consider the energy budget during a single exposure.

A typical chest x-ray technique is 100 kVp, 5 mAs (killovolts, milliamperes-seconds). Those 5 mAs represent 5,000 millicoulombs of electron charge flowing from the cathode to the anode over the exposure time. Each electron carries 100 keV of kinetic energy (100,000 electron volts). The total energy delivered to the anode is therefore:

5 mA × 0.001 s × 100 keV = 500 joules (if the exposure is 1 millisecond).

On a rotating anode, this energy is spread across the focal track (a ring on the rotating disk), so the peak temperature at the focal spot depends on:

• The size of the focal spot (smaller = hotter)
• The speed of anode rotation (faster = cooler, because fresh tungsten surface replaces the heated surface)
• The heat capacity and conductivity of tungsten

Modern rotational anode tubes reach focal-spot temperatures of 2000°C to 2500°C during fluoroscopy or rapid-sequence radiography. This is dangerously close to the melting point, which is why:

1. Fluoroscopy equipment has heat-unit limits (e.g., max 400,000 heat units in 5 minutes).
2. Cine acquisition protocols limit frame rates and exposure per frame.
3. Angiography tubes are equipped with more efficient cooling systems.

Tungsten’s high melting point is a safety margin. If tungsten had a melting point of, say, 2600°C (like molybdenum), even brief high-speed fluoroscopy would create pitting and tube failure.

Tungsten alloy: rhenium and the W-Re anode

Pure tungsten is mechanically brittle. Under repeated heating and cooling cycles (thermal cycling), pure tungsten develops microcracks that grow and eventually cause anode failure.

To solve this, manufacturers alloy tungsten with rhenium. The standard is a 90% tungsten, 10% rhenium (W-Re) alloy. Rhenium is element 75, sits right next to tungsten on the periodic table, and has a similar melting point (3186°C). The alloy retains tungsten’s high melting point while gaining ductility: the rhenium acts as a binder, preventing cracking under thermal stress.

W-Re anodes are more durable than pure tungsten and are now the standard in all modern x-ray tubes. When the ARRT discusses “tungsten anodes,” it almost always means W-Re.

Key properties of the W-Re anode:

• Composition: 90% tungsten, 10% rhenium
• Melting point: essentially unchanged (still ~3400°C)
• Ductility: improved (less cracking during thermal cycling)
• Cost: higher than pure tungsten, but worth it for durability
• Focal-spot size tolerance: tighter (W-Re allows smaller focal spots because the alloy is more homogeneous)

The anode disk itself is mounted on a molybdenum or graphite stem for heat dissipation. Molybdenum has a melting point of 2610°C and is a good thermal conductor. Graphite conducts heat well and can tolerate even higher temperatures, but is more brittle and less commonly used.

What happens when the anode overheats

If the focal spot is driven beyond safe thermal limits, progressive damage occurs:

Thermal pitting

Heat causes micro-vaporization of tungsten at the focal track surface. Tiny craters form (pitting). The craters increase the effective focal-spot size (making the image less sharp). Pitting is usually reversible if cooling is allowed, but if the tube is abused repeatedly, the craters deepen.

Cracking

Rhenium-alloyed tungsten is more resistant to cracking than pure tungsten, but it can still fail under extreme thermal cycling. A crack in the anode weakens the focal track and can propagate during subsequent exposures.

Melt spots

In severe overheating, the focal track locally exceeds the melting point. Small patches of tungsten melt and reflow, creating fused zones. These are permanent damage and significantly degrade tube performance.

Anode shaft bearing failure

Heat conducted away from the anode passes through the bearing assembly (a ball bearing or sleeve bearing). Excessive heat can overheat the bearing oil or lubricant, causing the bearing to seize. This stops anode rotation, which concentrates all subsequent heating onto a single spot, accelerating catastrophic failure.

Complete tube failure

If the anode is damaged beyond repair, or if the bearing seizes, the tube is no longer serviceable. The entire x-ray tube must be replaced (a costly procedure, typically $5,000-$15,000 depending on the tube type).

Other roles of tungsten in the x-ray tube

Tungsten appears in two other places in the tube besides the anode disk:

Filament cathode

The cathode is a heated filament made of tungsten wire, typically 0.1 to 0.2 mm in diameter. When current flows through the filament, it heats to 2200°C and releases electrons by thermionic emission (the same process as an old incandescent light bulb). Tungsten is chosen for the filament because it has a high melting point, low vaporization rate, and good electron emission efficiency at high temperature.

Focusing cup

The focusing cup is a molybdenum or tungsten electrode (sometimes called the “grid” or “control electrode”) that surrounds the filament. It applies a small negative voltage (focused) or slightly positive voltage (defocused) to concentrate or spread the electron beam. If it’s made of tungsten, it’s only there because molybdenum is slightly preferred for this role, but tungsten works.

Why this matters on the ARRT

The ARRT tests tungsten properties in the x-ray equipment section of the Image Production domain. Common question patterns:

1. Fact questions: “What is the melting point of tungsten?” Answer: 3410°C.
2. Why questions: “Why is tungsten used in x-ray tube anodes?” Answer: It has the highest melting point of any metal, allowing it to tolerate the 99.8% heat conversion at the focal spot.
3. Thermal stress questions: “What happens to the anode when the focal spot reaches 2500°C?” Answer: It approaches the melting point limit; further heating causes pitting, cracking, or melt spots.
4. Alloy questions: “What is a tungsten-rhenium anode?” Answer: A 90% W / 10% Re alloy that maintains high melting point while improving ductility and crack resistance.

The exam may also ask you to compare tungsten to other elements (e.g., “Why not use molybdenum?” Answer: molybdenum’s melting point of 2610°C is too low). Know tungsten’s melting point cold, and understand why that number matters physically.

For a deeper look at x-ray tube construction, electron-nucleus interactions, and characteristic vs. bremsstrahlung radiation, see our chapter on x-ray equipment and photon interactions. For how focal-spot size and heat affect image quality, the image acquisition and technique chapter covers technique factors and their thermal limits.

Quick reference table

Property	Value	Why it matters
Melting point (pure)	3410°C (3422°C in some sources)	Highest of any metal; allows focal spot to reach 2500°C without melting
Atomic number (Z)	74	High Z improves bremsstrahlung efficiency; more x-ray photons per electron
Density	19.3 g/cm³	High density concentrates energy in a small focal spot
Thermal conductivity	174 W/(m·K)	Conducts heat away from focal spot to the stem; slower than copper but adequate
Coefficient of thermal expansion	4.5 × 10⁻⁶ /K	Low; resists warping under thermal cycling
W-Re alloy composition	90% W, 10% Re	Rhenium improves ductility and crack resistance; melting point unchanged
Common stem material	Molybdenum (melting point 2610°C) or graphite	Conducts heat efficiently away from the anode disk
Focal-spot operating temperature	~2000 to 2500°C	Well below melting point, but dangerously close during high-speed imaging

ARRT exam tip

If you only memorize one fact from this page: tungsten’s melting point is 3410°C, the highest of any metal. This is why it is the only practical choice for x-ray tube anodes. The focal spot operates at approximately 2500°C, a margin of safety of only ~900°C. Any hotter and the anode fails.

Study cards that cite 3370°C, 3390°C, or any other value are outdated. The ARRT uses 3410°C as the canonical answer. Modern tungsten reference data confirms this.

For students starting an ARRT prep plan from scratch, our Curriculum walks through the four ARRT domains in plain English, with x-ray equipment fundamentals in the equipment chapter and free ARRT practice questions in every category.