High speed steel

High speed steel (often abbreviated HSS, sometimes HS) is a material usually used in the manufacture of machine tool bits and other cutters. It is often used in power saw blades and drill bits. It is superior to the older high carbon steel tools used extensively through the 1940s in that it can withstand higher temperatures without losing its temper (hardness). This property allows HSS to cut faster than high carbon steel, hence the name "high speed steel". At room temperature HSS and high carbon steel have an equivalent hardness; only at elevated temperatures does HSS become advantageous.

Applications

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

High carbon steel remains a good choice for low speed applications where a very keen (sharp) edge is required, such as files, chisels and hand plane blades.

Types of high speed steel

High speed steels belong to the Fe-C-X multicomponent alloy system where X represents chromium, tungsten, molybdenum, vanadium, and/or cobalt. Generally, the X component is present in excess of 7%, along with more than 0.60% carbon. (However, their alloying element percentages do not alone bestow the hardness-retaining properties; they also require appropriate high-temperature heat treatment in order to become true HSS; see History below.)

The grade type T-1 with 18% tungsten 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 and only 2% in the United States is of this type.Fact|date=February 2007

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.

M35

M35 is similar to M2, but with 5% cobalt added. The addition of cobalt increases heat resistance.

M42

M42 is a high speed steel alloy made up of roughly 8% cobalt. It is widely used in metal manufacturing because of its ability to resist wear over conventional high speed steels, allowing for shorter cycle times in production environments due to higher cutting speeds or from the increase in time between tool changes. M42 is also less prone to chipping when used for interrupted cuts and cost less when compared to the same tool made of carbide. Tools made from high speed steel and cobalt can often be identified by the letters HSS-Co.

Coatings

To increase the life of high speed steel, tools are sometimes coated. One such coating is TiN (titanium nitride). Most coatings generally increase a tool's hardness and/or lubricity. A coating allows the cutting edge of a tool to cleanly pass through the material without having the material gall (stick) to it. The coating also helps to decrease the temperature associated with the cutting process and increase the life of the tool.

urface modification

Lasers and electron beams can be used as sources of intense heat at the surface for heat treatment, remelting (glazing), and compositional modification. It is possible to achieve different molten pool shapes and temperatures. Cooling rates range from 10³ - 10⁶ K s^-1. Beneficially, there is little or no cracking or porosity formation.Boccalini and Goldenstein 2001]

While the possibilities of heat treating at the surface should be readily apparent, the other applications beg some explanation. At cooling rates in excess of 10⁶ K s^-1 eutectic microconstituents disappear and there is extreme segregation of substitutional alloying elements. This has the effect of providing the benefits of a glazed part without the associated run in wear damage.

The alloy composition of a part or tool can also be changed to form a high speed steel on the surface of a lean alloy or to form an alloy or carbide enriched layer on the surface of a high speed steel part. Several methods can be used such as foils, pack boronising, plasma spray powders, powder cored strips, inert gas blow feeders, etc. Although this method has been reported to be both beneficial and stable, it has yet to see widespread commercial use.

History

Although development of modern high speed steel began in second half of 19th century, there are documented evidences of similar grades of steel produced earlier. These include hardened steels in China in 1200s BC, wootz steel manufactured in India around 350 BC and production of Damascus and Japanese layered steel blades in years 540 AD and 900 AD.Roberts, George, et al., "Tool Steels", 5th edition, ASM International, 1998]

Following the discovery of crucible steel in 1740, in 1868 Robert Forester Mushet in England developed a steel that is considered the forerunner of modern high speed steels. It consisted of 2% C, 2.5% Mn, and 7% W. The major advantage of this steel was that it hardened when air cooled from a temperature from which most steels had to be quenched for hardening. Over the next 30 years the most important change was the substitution of chromium for manganese.

In 1899 and 1900, Frederick Winslow Taylor and Maunsel White, working with a team of assistants at the Bethlehem Steel Company at Bethlehem, Pennsylvania, USA, performed a series of experiments with the heat treating of existing high-quality tool steels, heating them to much higher temperatures than were typically considered desirable in the industry. [Kanigel 1997.] Their experiments were characterized by a scientific empiricism in that many different combinations were made and tested, with no regard for conventional wisdom or alchemic recipes, and with detailed records kept of each batch. The end result was a heat treatment process that transformed existing alloys into a new kind of steel that could retain its hardness at higher temperatures, allowing much higher speeds, feeds, and depths of cut when machining.

The Taylor-White process was patented and created a revolution in the machining industries, in fact necessitating whole new, heavier machine tool designs so the new steel could be used to its full advantage. The patent was hotly contested and eventually nullified, but the vigor of the litigation seems to have been propelled less by the merits of the case and more by the fact that many firms faced commercial extinction if they could not find a way to circumvent the patent.

The first alloy that was formally classified as high speed steel is known by the AISI designation T1, which was introduced in 1910. It was patented by Crucible Steel Co. at the beginning of the 20th century.

Although molybdenum rich high speed steels such as AISI M1 have been used since the 1930s, shortages and hence high costs of raw materials during World War II spurred the development of alloy designs with molybdenum being substituted for tungsten to produce cheaper steel. The developments in molybdenum-based high speed steel during this period made them on par with and in certain cases better than tungsten-based high speed steels. This started with the use of M2 steel (sulfurized version of M1) instead of T1 steel.The Metals Society, London, "Tools and dies for industry", 1977]

Copyediting conventions

Most copyeditors (subeditors) today would tend to choose to style the unit adjective "high-speed" with a hyphen, rendering the full term as "high-speed steel", and this styling is not uncommon (Kanigel 1997 is an example of a work edited thus). However, it is true that in the metalworking industries the styling "high speed steel" is long-established and is more commonly seen. Therefore, both can be considered acceptable variants.

References

Bibliography

cite book
last = Kanigel
first = Robert
title = The One Best Way: Frederick Winslow Taylor and the Enigma of Efficiency
publisher = Viking Penguin
date = 1997
id = ISBN 0-670-86402-1

cite journal
last = Boccalini
first = M.
coauthors = H. Goldenstein
year = 2001
month = February
title = Solidification of high speed steels
journal = International Materials Reviews
volume = 46
issue = 2
pages = 92–115 (24)
issn = 0950-6608
url = http://www.ingentaconnect.com/content/maney/imr/2001/00000046/00000002/art00002
doi = 10.1179/095066001101528411

ee also

*Tool steel
*Alloy
*Metalwork cutting tool

External links

* [http://www.materialsengineer.com/E-Alloying-Steels.htm Effects of Elements on Steel]