Tungsten
Tungsten is one of the so-called refractory metals, along with molybdenum, niobium, rhenium, and tantalum. These materials are known primarily for their high melting points.
In fact, tungsten has the highest melting point of all known metals, at 6192°F (3422°C), along with a number of other very useful characteristics, including:
- High Density
- Extreme hardness
- High tensile strength
- High electrical resistance
- Resistance to oxidation and creep
- The lowest vapor pressure of all metals
Tungsten has long served industry based on its unique properties — sometimes because of just one unique property, sometimes based on a combination.
Tungsten Wire
How Tungsten Wire Is Fabricated?
To get from powder to wire, tungsten is subjected to pressing, sintering, swaging, drawing, and annealing at elevated temperatures in hydrogen atmospheres.
Using tungsten powder, the wire fabrication process generally goes like this:
- First comes pressing—-tungsten power begins to take the bar form from which wire will eventually be produced. Here, tungsten powder that has been milled, sifted, blended, and weighed is put into a tool-steel breakaway mold and loaded into a hydraulic press, where it is compacted into a bar held together by wax and pressure.
- In pre-sintering, the bar — which is still quite fragile — is placed into a high-temperature hydrogen atmosphere furnace, where the material begins to consolidate until it reaches 60% to 70% of theoretical density. This ensures that the bar is strong enough to be clamped and sintered.
- In sintering, the bar is treated with electric current in a hydrogen atmosphere. This generates heat and causes the bar’s density to increase to 93%; at the same time, the bar shrinks and tungsten crystals begin to form. At this point, the as-sintered bar is now very strong but brittle at room temperature.
- In swaging, the bar is repeatedly heated to make it malleable, lubricated, and passed through a swager — basically, rotating ball peen hammers — to incrementally reduce the diameter of the bar with each pass. This step elongates the crystals and creates a fibrous structure that will provide ductility and strength in the final product.
- Now the tungsten is ready for drawing — the point where the swaged wire is lubricated and drawn through a series of tungsten carbide or diamond dies to further reduce the diameter. The drawing process also further elongates the wire’s fibers and increases its tensile strength.
However, the wire ultimately does retain many of the valuable characteristics of tungsten, such as its:
- High melting point
- High tensile strength
- Low vapor pressure
- Low coefficient of thermal expansion
But it has properties that tungsten doesn’t have, such as the ability to stretch to a very fine point, finer than a hair. This is what has led tungsten wire into many areas of application, such as replacing conventional steel wire for diamond cutting wire, the preferred material for making micro-mechanical wire ropes used in modern surgical robots, yarn for cut-proof gloves, etc.