In the middle of the 19th century tungsten was introduced to technical applications, mainly in steel production. From then until the first quarter of the 20th century, its importance in this field steadily rose and steel became the biggest tungsten consumer. Tungsten was among the first alloying elements systematically studied and used to improve steel properties, for example hardness, cutting efficiency and cutting speed of tool steels. Different tungsten containing steels were developed in Austria, Germany, France and England, followed by high speed steels in the USA.
The addition of tungsten to construction steels has decreased since 1940 because alloying with Mo and Cr, as well as with V and Ni, yielded better performance at a lower price. From 1927, when cemented carbides (hard metals) were developed, the production of total tungsten consumed in steelmaking declined constantly to a current figure of about 20%, but nevertheless steel is today the second biggest consumer. This percentage is the average for demand worldwide but in different markets tungsten consumption of steel differs considerably, from 2% in the USA to about 10% in Europe and Japan and about 30% in Russia and China.
Tool steels are used for hot and cold forming and for cutting of materials, particularly other metals, wood or polymers. This presumes that their strength and hardness must exceed that of the materials being cut or shaped.
Tool steels are usually classified into four groups:
- High Speed Steels with W content of 1.5 to 20%
- Hot Work Steels with 1.5 to 18% W
- Cold Work Steels with 0.5 to 3 wt% W
- Plastic Mould Steels, with W content of ~1%.
HIGH SPEED STEEL
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 steel (HSS). This term describes the ability to cut metal at high Speed.
The Steel type with 18% W has not changed its composition since 1910 and was the main type used until 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 maximizes efficiently the hardness and toughness of high speed steel and maintains these properties at the high temperatures generated when cutting metals.
The main use of high speed steel continues to be in the manufacture of various cutting tools: drills, taps, milling cutters, and gear cutters, saw blades, etc. although usage for punches and dies is increasing.
Tool steel, in particular HSS, can be surface treated by nitriding, laser or plasma overlays of hard coatings (e.g.stellites) as well as by chemical or physical vapor deposition of hard carbides and nitrides. Coatings, such as TiN, TiAlN or CrAlN significantly increase tool lives but increase tool costs. Nevertheless, most tools in the higher end applications are coated today as the higher cost is well balanced by the higher productivity during machining.
PM HIGH SPEED STEEL
Only recently did the first PM High Speed Steels appear on the market. Such steels are produced by powder metallurgy. Alloy powders are fabricated by gas atomization and the powder is then consolidated by Hot Iso-static Pressing (HIP).
Such steels exhibit virtually isotropic properties due to a homogeneous and segregation-free microstructure. They show better workability and grind ability, less distortion on heat treatment, excellent surface finishing and higher productivity. PM also renders possible the production of higher alloyed steel grades than accessible by ingot metallurgy or of non-conventional variants.
PLASTIC MOLD STEEL
Such types of Plastic Mold steels have to withstand operating temperatures and pressures as well as corrosive effects of polymers and abrasive effects of fillers as occurring in the plastics processing industry. Long tool life and therefore low cost per plastic part is decisive. Good corrosion resistance and good surface finishing are critical. In particular in the latter regard, tungsten-containing PM steels have entered this market successfully. Plastic mould steels contain up to 1.2% W.
HEAT RESISTING STEEL AND CAST STEEL
In certain cases, tungsten is added when steels are used in higher temperature ranges. Heat resisting steels are either chromium nickel steels with up to 6% tungsten, or heat treatable 9-12% Cr-steels, where tungsten is added in the range of 1-2% to improve the creep resistance and long-term stability of the steels.
The main use for austenitic Cr/Ni steels is as valve steels for combustion engines, which contain around 2% W. Similar steels are used for components of steam and gas turbines, such as blades, discs, bolts and pins. Heat treatable creep resistant chromium steels can be used up to about 620°C for continuous operation in power plants. They are currently considered to possess the maximum creep strength of all heat-resistant steels for boiler components and piping systems. Such steels are also used in a slightly modified form for large cast chromium steel components in steam power plants, such as inner and outer casings or valve housings in the high pressure and medium pressure part of the turbine.
STAINLESS STEEL AND VALVE STEEL
Tungsten is added to ferritic-austenitic stainless steel. Such steel exhibits excellent corrosion resistance and strength properties. Due to their pronounced resistance against stress corrosion cracking, fatigue, pitting, crevice and erosion corrosion they are ideal materials for components in offshore, waste water, sea water desalination and chemical plants with aggressive chloride-containing media.