High-speed steel

High-speed steel (HSS or HS) is a subset of tool steels, commonly used in tool bits and cutting tools.

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, in their generally recommended heat treatment, HSS grades generally display high hardness (above HRC60) and abrasion resistance (generally linked to tungsten and vanadium content often used in HSS) compared with common carbon and tool steels.

History

Although development of modern high speed steel began in the second half of the 19th century, there is documented evidence of similar levels of steel produced earlier. These include hardened steels in China in 13th century 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.^[1] High speed properties of those steels would be mostly coincidental, and the result of local iron ores containing natural traces of tungsten or other favorable alloying components.

In 1868 the English metallurgist Robert Forester Mushet developed Mushet steel, considered to be the forerunner of modern high speed steels. It consisted of 2% carbon (C), 2.5% manganese (Mn), and 7% tungsten (W). The major advantage of this steel was that it hardened when air cooled from a temperature at which most steels had to be quenched for hardening. Over the next 30 years the most significant change was the replacement of manganese (Mn) with chromium (Cr).^[2]

In 1899 and 1900, Frederick Winslow Taylor and Maunsel White, working with a team of assistants at the Bethlehem Steel Company at Bethlehem, Pennsylvania, US, performed a series of experiments with the heat treating of existing high-quality tool steels, such as Mushet steel; heating them to much higher temperatures than were typically considered desirable in the industry.^[3][4] 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, and rate of cutting when machining.

The Taylor-White process^[5] 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.^{[citation needed]}

The first alloy that was formally classified as high-speed steel is known by the AISI designation T1, which was introduced in 1910.^[1] It was patented by Crucible Steel Co. at the beginning of the 20th century.^[2]

Although molybdenum rich high-speed steels such as AISI M1 have been used since the 1930s, material shortages and high costs caused by World War II spurred development of less expensive alloys substituting molybdenum for tungsten. The advances in molybdenum-based high speed steel during this period put them on par with and in certain cases better than tungsten-based high speed steels. This started with the use of M2 steel instead of T1 steel.^[2][6]

Types

High speed steels are alloys that gain their properties from either tungsten or molybdenum, often with a combination of the two. They belong to the Fe–C–X multi-component alloy system where X represents chromium, tungsten, molybdenum, vanadium, or cobalt. Generally, the X component is present in excess of 7%, along with more than 0.60% carbon. The alloying element percentages do not alone bestow the hardness-retaining properties; they also require appropriate high-temperature heat treatment to become true HSS; see History above.

In the unified numbering system (UNS), tungsten-type grades (e.g. T1, T15) are assigned numbers in the T120xx series, while molybdenum (e.g. M2, M48) and intermediate types are T113xx. ASTM standards recognize 7 tungsten types and 17 molybdenum types.^[7]

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

In general the basic composition of T1 HSS is 18% W, 4% Cr, 1% V, 0.7% C and rest Fe. Such HSS tool could machine (turn) mild steel at speed only up to 20~30 m/min (which was quite substantial in those days)

M2

M2 is molybdenum based high-speed steel in tungsten–molybdenum series. The carbides in it are small and evenly distributed. It has high wear resistance. After heat treatment, its hardness is the same as T1, but its bending strength can reach 4700 MPa, and its toughness and thermo-plasticity are higher than T1 by 50%. It is usually used to manufacture a variety of tools, such as drill bits, taps and reamers. Its decarburization sensitivity is a little bit high.^[9]

M36

M36 is similar to M2, but with 8% cobalt added. The addition of cobalt increases heat resistance. M36 is also known as HSSE or HSS-E.

M42

M42 is a molybdenum-series high-speed steel alloy with an additional 8% cobalt. It is widely used in metal manufacturing because of its superior red-hardness as compared to more 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 costs less when compared to the same tool made of carbide. Tools made from cobalt-bearing high speed steels 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.

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 to 10⁶ K s⁻¹. Beneficially, there is little or no cracking or porosity formation.^[2]

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.^[2]

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.^[2]

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 speed steels also found a market in fine hand tools where their relatively good toughness at high hardness, coupled with high abrasion resistance, made them suitable for low speed applications requiring a durable keen (sharp) edge, such as files, chisels, hand plane blades, and high quality kitchen, pocket knives, and swords.

References

External links

• Effects of Elements on Steel
• A Comparison Table of Different High Speed Steel (HSS) Grade Standards