High-speed welding and cutting of stainless steel bipolar ...
High Speed Steel
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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 exten- sively 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 tempera- ture, 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. 1 History Although development of modern high speed steel be- gan in the second half of the 19th century, there is docu- mented evidence of similar levels of steel produced ear- lier. These include hardened steels in China in 13th cen- tury 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 forerun- ner 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 hard- ened 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 Maun- sel White, working with a team of assistants at the Bethlehem Steel Company at Bethlehem, Pennsylvania, US, performed a series of experiments with the heat treat- ing 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 characterised by a scientific em- piricism in that many different combinations were made and tested, with no regard for conventional wisdom or al- chemic 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 necessitat- ing 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. The first alloy that was formally classified as high-speed steel is known by the AISI designation T1, which was in- troduced 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 short- ages and high costs caused by World War II spurred de- velopment of less expensive alloys substituting molybde- num 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] 2 Types High speed steels are alloys that gain their properties from either tungsten or molybdenum, often with a com- bination 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 percent- ages do not alone bestow the hardness-retaining proper- ties; 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 recog- nize 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 the remainder Fe. Such a HSS tool could machine (turn) mild steel at speeds of up 1
description
Explanation of High speed steels structure
Transcript of High Speed Steel
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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 issuperior to the older high-carbon steel tools used exten-sively through the 1940s in that it can withstand highertemperatures without losing its temper (hardness). Thisproperty allows HSS to cut faster than high carbon steel,hence the name high-speed steel. At room tempera-ture, in their generally recommended heat treatment, HSSgrades generally display high hardness (above HRC60)and abrasion resistance (generally linked to tungsten andvanadium content often used in HSS) compared withcommon carbon and tool steels.
1 History
Although development of modern high speed steel be-gan in the second half of the 19th century, there is docu-mented evidence of similar levels of steel produced ear-lier. These include hardened steels in China in 13th cen-tury BC, wootz steel manufactured in India around 350BC and production of Damascus and Japanese layeredsteel blades in years 540 AD and 900 AD.[1] High speedproperties of those steels would be mostly coincidental,and the result of local iron ores containing natural tracesof tungsten or other favorable alloying components.In 1868 the English metallurgist Robert Forester Mushetdeveloped Mushet steel, considered to be the forerun-ner 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 hard-ened when air cooled from a temperature at which moststeels had to be quenched for hardening. Over the next 30years the most signicant change was the replacement ofmanganese (Mn) with chromium (Cr).[2]
In 1899 and 1900, Frederick Winslow Taylor and Maun-sel White, working with a team of assistants at theBethlehem Steel Company at Bethlehem, Pennsylvania,US, performed a series of experiments with the heat treat-ing of existing high-quality tool steels, such as Mushetsteel; heating them to much higher temperatures thanwere typically considered desirable in the industry.[3][4]Their experiments were characterised by a scientic em-piricism in that many dierent combinations were madeand tested, with no regard for conventional wisdom or al-chemic recipes, and with detailed records kept of eachbatch. The end result was a heat treatment process that
transformed existing alloys into a new kind of steel thatcould retain its hardness at higher temperatures, allowingmuch higher speeds, and rate of cutting when machining.The Taylor-White process[5] was patented and created arevolution in the machining industries, in fact necessitat-ing whole new, heavier machine tool designs so the newsteel could be used to its full advantage. The patent washotly contested and eventually nullied.The rst alloy that was formally classied as high-speedsteel is known by the AISI designation T1, which was in-troduced 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 asAISI M1 have been used since the 1930s, material short-ages and high costs caused by World War II spurred de-velopment of less expensive alloys substituting molybde-num for tungsten. The advances in molybdenum-basedhigh speed steel during this period put them on par withand in certain cases better than tungsten-based high speedsteels. This started with the use of M2 steel instead of T1steel.[2][6]
2 TypesHigh speed steels are alloys that gain their propertiesfrom either tungsten or molybdenum, often with a com-bination of the two. They belong to the FeCX multi-component alloy system where X represents chromium,tungsten, molybdenum, vanadium, or cobalt. Generally,the X component is present in excess of 7%, along withmore than 0.60% carbon. The alloying element percent-ages do not alone bestow the hardness-retaining proper-ties; they also require appropriate high-temperature heattreatment to become true HSS; see History above.In the unied numbering system (UNS), tungsten-typegrades (e.g. T1, T15) are assigned numbers in theT120xx series, while molybdenum (e.g. M2, M48) andintermediate types are T113xx. ASTM standards recog-nize 7 tungsten types and 17 molybdenum types.[7]
The addition of about 10% of tungsten and molybdenumin total maximises eciently the hardness and toughnessof high speed steels and maintains those properties at thehigh temperatures generated when cutting metals.In general the basic composition of T1 HSS is 18% W,4% Cr, 1% V, 0.7% C and the remainder Fe. Such aHSS tool could machine (turn) mild steel at speeds of up
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2 7 REFERENCES
to 20~30 m/min (which was quite substantial at the time).
2.1 M2M2 is molybdenum based high-speed steel in tungstenmolybdenum series. The carbides in it are small andevenly distributed. It has high wear resistance. After heattreatment, its hardness is the same as T1, but its bend-ing strength can reach 4700 MPa, and its toughness andthermo-plasticity are higher than T1 by 50%. It is usuallyused to manufacture a variety of tools, such as drill bits,taps and reamers. Its decarburization sensitivity is a littlebit high.
2.2 M35M35 is similar to M2, but with 5% cobalt added. Theaddition of cobalt increases heat resistance. M35 is alsoknown as HSSE or HSS-E.
2.3 M42M42 is a molybdenum-series high-speed steel alloy withan additional 8% cobalt. It is widely used in metal manu-facturing industries because of its superior red-hardnessas compared to more conventional high-speed steels, al-lowing for shorter cycle times in production environmentsdue to higher cutting speeds or from the increase in timebetween tool changes. M42 is also less prone to chippingwhen used for interrupted cuts and costs less when com-pared to the same tool made of carbide. Tools made fromcobalt-bearing high speed steels can often be identied bythe letters HSS-Co.
3 CoatingsTo increase the life of high-speed steel, tools are some-times coated. One such coating is TiN (titanium nitride).Most coatings generally increase a tools hardness and/orlubricity. A coating allows the cutting edge of a tool tocleanly pass through the material without having the ma-terial gall (stick) to it. The coating also helps to decreasethe temperature associated with the cutting process andincrease the life of the tool.
4 Surface modicationLasers and electron beams can be used as sources of in-tense heat at the surface for heat treatment, remelting(glazing), and compositional modication. It is possibleto achieve dierent molten pool shapes and temperatures.Cooling rates range from 103 to 106 K s1. Benecially,there is little or no cracking or porosity formation.[2]
While the possibilities of heat treating at the surfaceshould be readily apparent, the other applications begsome explanation. At cooling rates in excess of 106 K s1eutectic microconstituents disappear and there is extremesegregation of substitutional alloying elements. This hasthe eect of providing the benets of a glazed part with-out the associated run in wear damage.[2]
The alloy composition of a part or tool can also bechanged to form a high speed steel on the surface of alean alloy or to form an alloy or carbide enriched layer onthe surface of a high speed steel part. Several methodscan be used such as foils, pack boronising, plasma spraypowders, powder cored strips, inert gas blow feeders, etc.Although this method has been reported to be both bene-cial and stable, it has yet to see widespread commercialuse.[2]
5 ApplicationsThe main use of high-speed steels continues to be in themanufacture of various cutting tools: drills, taps, millingcutters, tool bits, gear cutters, saw blades, planer and join-ter blades, router bits, etc., although usage for punchesand dies is increasing.High speed steels also found a market in ne hand toolswhere their relatively good toughness at high hardness,coupled with high abrasion resistance, made them suit-able for low speed applications requiring a durable keen(sharp) edge, such as les, chisels, hand plane blades, andhigh quality kitchen, pocket knives, and swords.High speed steel tools are the most popular for use inwoodturning, as the speed of movement of the work pastthe edge is relatively high for handheld tools, and HSSholds its edge far longer than high carbon steel tools can.
6 See also Crucible Industries List of steel producers
7 References[1] Roberts, George (1998) Tool Steels, 5th edition, ASM In-
ternational, ISBN 1615032010
[2] Boccalini, M.; H. Goldenstein (February 2001).Solidication of high speed steels. Interna-tional Materials Reviews 46 (2): 92115 (24).doi:10.1179/095066001101528411.
[3] Kanigel, Robert (1997). The One Best Way: FrederickWinslow Taylor and the Enigma of Eciency. VikingPenguin. ISBN 0-670-86402-1.
History Types M2 M35 M42
Coatings Surface modification Applications See also References External links Text and image sources, contributors, and licensesTextImagesContent license
