| Iron alloy phases |
|---|
| Austenite (γ-iron; hard) |
| Types of Steel |
| Plain-carbon steel (up to 2.1% carbon) |
| Other Iron-based materials |
| Cast iron (>2.1% carbon) |
High speed steel (often abbreviated HSS) 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.
[edit] 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.
[edit] 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.[citation needed]
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.
| Grade | C | Cr | Mo | W | V | Co |
|---|---|---|---|---|---|---|
| T1 | 0.75 | - | - | 18.0 | 1.1 | - |
| M2 | 0.95 | 4.2 | 5.0 | 6.0 | 2.0 | - |
| M7 | 1.00 | 3.8 | 8.7 | 1.6 | 2.0 | - |
| M42 | 1.10 | 3.8 | 9.5 | 1.5 | 1.2 | 8.0 |
[edit] 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 CoHSS.
[edit] 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.
[edit] Surface 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.[1]
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.[1]
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.[1]
[edit] History
Until the 19th century, steelmaking was all art and no science. Recipes and methods were at the discretion of each master, who was something like a chef or an alchemist. During that century, the sciences of chemistry and metallurgy developed, and near the end of the century, the science component began to supplant the art component as the majority share of the steelmaking profession.
In the latter half of the 19th century, a steel was developed by Mushet in England 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.[1]
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.[2] 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. Thus high speed steel was not a new alloy, but rather a product of a new, higher-temperature heat treatment process applied to already-known alloys.
The Taylor-White process was patented and created a revolution in the machining industries, in fact necessitating whole new, heavier machine tool designs to be used to 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 arguments boiled down to the idea that "we steelmakers already knew all about alloys and all about heat, so there is nothing novel about Taylor-White steel." The speciousness of this idea is apparent: if anyone had really known how to double feeds and speeds, they would have already been doing it.
The first alloy that was formally classified as high speed steel is known by the AISI designation T1. It was patented by Crucible Steel Co. at the beginning of the 20th century.[1]
Although molybdenum rich high speed steels such as AISI M1 have been used in the USA since the 1930s, shortages of raw materials during World War II, spurred the development of alloy designs with lower alloy contents and cheaper alloying elements. This was mainly achieved by substituting for tungsten and vanadium. This allowed M2 steel to overtake T1 steel in the 1950s.[1]
[edit] 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.
[edit] References
[edit] Bibliography
Kanigel, Robert (1997). The One Best Way: Frederick Winslow Taylor and the Enigma of Efficiency. Viking Penguin. ISBN 0-670-86402-1.
Boccalini, M.; H. Goldenstein (February 2001). "Solidification of high speed steels". International Materials Reviews 46 (2): 92-115 (24). ISSN 0950-6608.
[edit] See also
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