Introduction
Hot work tool steels are steels capable of withstanding high abrasion, heat and pressure conditions that prevail in manufacturing units that perform processes such as forming, shearing and punching of metals at high temperatures of 480 to 760°C (900 to 1400°F). These steels have wear resistance up to 540°C (1000°F).

Hot work tool steels are designated as group H steels and they have 0.35% to 0.45% carbon, 6% to 25% chromium, with vanadium, molybdenum, and tungsten as the other alloying elements. Tungsten is primarily used in hot forming tool steels due to its high temperature strength, toughness and resistance to grain growth.

Overview
Tungsten hot-work steels constitute the H21 to H26 types of hot work steels. These steels have similar characteristics as those of other of high-speed steels. The hot work steel type H26 has low carbon content when compared to that of T1 high speed steel. The primary alloying elements of tungsten hot-work steels include tungsten, carbon, chromium and vanadium.

Properties
Tungsten hot work steels have high alloy content, which enhances their heat resistance. The high alloy content also makes the tungsten steels brittle and unsuitable for the water-cooling process. Breakage of tungsten hot work steels can be reduced if they are preheated to operating temperatures prior to use. Tungsten steels have normal working hardness of 45 to 55 HRC. Thermal shock resistance and toughness of these steels can be improved by reducing the carbon content. In such cases, it is necessary to adjust the tungsten and vanadium content also as these two reduce the hardenability of steel by trapping large amount of carbon in the form of carbides.

Scaling can be reduced by quenching tungsten hot-work steels in oil or hot salt. Tungsten hot-work steels are resistant to distortion when they are air- hardened, and have higher hardening temperature when compared to chromium hot-work steels.

Applications
Tungsten hot work steels find applications in the following areas:

•Manufacturing mandrels and extrusion dies for high temperature applications, such as extrusion of brass, nickel alloys, and steel
•Hot-forging dies of rugged design.

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Background
200 million tungsten bullets a year, using an ounce of tungsten each. That makes more than 5,500 t, or one eighth of existing annual tungsten consumption in the world.
 
Green Bullets
In 1999 the US Army began manufacturing "green" bullets. The bullets - which are used primarily for shooting practice during peace time - are as deadly to humans as their predecessors but less deadly to the Earth.

Lead bullets, which the Army currently uses, tend to bioaccumulate in the environment, often ending up in sediments, surface water, and groundwater, according to A Multimedia Strategy for the Management and Reduction of Lead Hazards released by the US Environmental Protection Agency Region 5. Accumulated lead can adversely affect wildlife and people who get their drinking water from a contaminated source, according to the report. Lead slugs are such a water quality hazard that the federal district court in New York ruled that spent lead shot is a "pollutant" as defined by the Clean Water Act.

The lead slugs the Army uses in traditional 5.56 mm bullets have been bioaccumulating at shooting ranges, forcing several to close. These slugs will be replaced with environmentally friendly tungsten-based slugs, according to Wade Bunting, project manager for environmental armament technologies at the Picatinny Arsenal in New Jersey. Not only is tungsten more environmentally "benign" than lead, but ozone depleting chemicals and volatile organic compounds have been eliminated from the bullet manufacturing process - resulting in pollution prevention and money savings, he says. Although tungsten is more expensive than lead, the cleanup of the manufacturing process actually will result in savings of $0.01 to $0.05 per round, or $5 million to $20 million per year, he explains.

The bullets also will allow several indoor and outdoor shooting ranges that closed because lead concentrations became a human health or environmental hazard to reopen, cutting down the costs of transporting troops to far away, still-operational shooting ranges and eliminating associated pollution he adds.

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Background
Silver/Tungsten alloys containing between 25 and 50% silver are used for electrical contacts. These materials are produced via powder metallurgical techniques due to their widely different melting points.

Tungsten oxides and tungstates form on the surface of these metals. This can lead to increasing contact resistance over time.

If more arc resistance is required then these materials can withstand or if contact resistance becomes a problem, silver/tungsten carbide materials offer an alternative.
 
Key Properties
Silver/Tungsten metals combine the high thermal and electrical conductivities of silver with the arc resistance of tungsten.
 
Applications
As mentioned above, silver/tungsten materials are used for electrical contacts. Typically they are used in heavy duty devices subject to high currents. The presence of the refractory material tungsten, reduces the chances of welding and improves resistance to arc erosion. Optimal compositions are found by balancing conductivity and non-welding properties.

Devices that utilise silver/tungsten materials include:

•         Circuit breakers (often in the 50-100Amp range)

•         Relays that require good arc resistance
 

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Background
Tungsten was among the first alloying elements systematically used - as early as the middle of the 19th century - to improve steel properties. It has been one of the most important alloying constituents in tool steels and constructional steels and was added to enhance the properties of hardness, cutting efficiency and speed of tools.

These highly alloyed steels are used primarily in working, cutting and forming of metal components. They must thus possess high hardness and strength, combined with good toughness, over a broad temperature range.
 
Tungsten Alternatives
During World War II, a shortage of tungsten and an increasing demand for tools forced US and European steel makers to find a substitute, and molybdenum was chosen to replace tungsten in varying percentages. This was also cost efficient as the price was lower and the atomic weight is only half that of tungsten (1% Mo is roughly equivalent to 2% of W).

Tool Steels
Tool steels are usually classified in four groups:

•         Ledeburitic Cr-steels with less than 1% W - normally used for producing thread rolls and dies

•         Cold work steels with 0.5 - 3% W, of which cutting tools for instance are made

•         Hot work steels with 1.5 - 9% W, for dies and extrusion tools working up to 500°C and more

•         Steels for plastic moulding which does not include W-bearing steel grades

High Speed Steels
When tool steels contain a combination of more than 7 % tungsten, molybdenum and vanadium, along with more than 0.60% carbon, they are referred to as high speed steels (HSS).

This term is descriptive of their ability to cut metals at the "high speeds" in use through the 1940’s.

The T-1 type with 18% W has not changed its composition since 1910 and was the main type used up to 1940, when substitution by molybdenum took place. Nowadays, only 5-10% of the HSS in Europe is of this type and only 2% in the USA.

The addition of about 10% of tungsten and molybdenum in total maximises efficiently the hardness and toughness of high speed steels and maintains these properties at the high temperatures generated when cutting metals.

The main use of high speed steels continues to be in the manufacture of various cutting tools: drills, taps, milling cutters, gear cutters, saw blades, etc., although usage for punches and dies is increasing.

Coating
In 1979, the Western World’s production of HSS reached 120,000 tons/year, which has never been achieved again. The production in 1996 was 70,000 tons. One important reason for this is the world-wide rapid spread of coating techniques like the PVD procedure, e.g. the coating of HSS tools with a thin layer of TiN and other types of coating.

Coating increases the life of drills by up to 10 times, or it enables the cutting speed (productivity) to be doubled while the lifetime remains the same as for uncoated.

Another reason for declining consumption in HSS is the increasing switch-over to cemented carbide tools.

Heat Resisting Steels
In certain cases, where corrosion resistant steels are used in higher temperature ranges, tungsten is added. Heat resisting steels are chromium/nickel steels with up to 6% tungsten. The main use is as valve steels for combustion engines, containing around 2% W. Such steels are, for instance, used for the valves on the outlet side of automotive engines, where the red-hot hardness has to be combined with high temperature corrosion resistance.

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