High-speed steel

Last updated

High-speed steel (HSS or HS) is a subset of tool steels, commonly used as cutting tool material.

Contents

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 Rockwell hardness 60) and abrasion resistance (generally linked to tungsten and vanadium content often used in HSS) compared with common carbon and tool steels.

History

In 1868 English metallurgist Robert Forester Mushet developed Mushet steel, considered 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). [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, US, performed a series of experiments with heat treating existing high-quality tool steels, such as Mushet steel, heating them to much higher temperatures than were typically considered desirable in the industry. [2] [3] Their experiments were characterised by a scientific empiricism in that many different combinations were made and tested, with no regard for conventional wisdom or alchemic recipes, and detailed records kept of each batch. The 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 [4] was patented and created a revolution in machining industries. Heavier machine tools with higher rigidity were needed to use the new steel to its full advantage, prompting redesigns and replacement of installed plant machinery. The patent was contested and eventually nullified. [5]

The first alloy that was formally classified as high-speed steel is known by the AISI designation T1, which was introduced in 1910. [6] 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 had seen some use since the 1930s, it was the material shortages and high costs caused by WWII that 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. [1] [7]

Types

High speed steels are alloys that gain their properties from a variety of alloying metals added to carbon steel, typically including tungsten and molybdenum, or a combination of the two, often with other alloys as well. [8] 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.

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. [9]

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.

A sample of alloying compositions of common high speed steel grades (by %wt) [10] [11] (impurity limits are not included)
Grade C Cr Mo W V Co Mn Si
T10.65–0.804.00-181-0.1–0.40.2–0.4
M10.80481.51.0---
M20.85456.02.0---
M71.0048.751.752.0---
M350.924.356.41.85-0.35
M421.103.759.51.51.158.0--
M500.8544.25.101.0---


Molybdenum High Speed Steels (HSS)

Combining molybdenum, tungsten and chromium steel creates several alloys commonly called "HSS", measuring 63–65 Rockwell "C" hardness.

M1
M1 lacks some of the red-hardness properties of M2, but is less susceptible to shock and will flex more.
M2
M2 is the "standard" and most widely used industrial HSS. It has small and evenly distributed carbides giving high wear resistance, though its decarburization sensitivity is a little bit high. 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. 1.3343 is the equivalent numeric designation for M2 material identified in ISO 4957.
M7
M7 is used for making heavier construction drills where flexibility and extended drill life are equally important.
M50
M50 does not have the red-hardness of other grades of tungsten HSS, but is very good for drills where breakage is a problem due to flexing the drill. Generally favored for hardware stores and contractor use. It is also used in high-temperature ball bearings. These steels are obtained by alloying tungsten, chromium, vanadium, cobalt and molybdenum with steel.

Cobalt High Speed Steels (HSS)

The addition of cobalt increases heat resistance, and can give a Rockwell hardness up to 70 Min. [12]

M35
M35 is similar to M2, but with 5% cobalt added. M35 is also known as Cobalt Steel, HSSE or HSS-E. It will cut faster and last longer than M2. [13]
M42
M42 is a molybdenum-series high-speed steel alloy with an additional 8% cobalt. [12] It is widely used in metal manufacturing industries 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. [13]

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, as well as cooling rates ranging from 103 to 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]

Forming

HSS drill bits formed by rolling are denoted HSS-R. Grinding is used to create HSS-G, cobalt and carbide drill bits. [14]

Applications

The main use of high-speed steels continues to be in the manufacture of various cutting tools: drills, taps, milling cutters, tool bits, hobbing (gear) cutters, saw blades, planer and jointer blades, router bits, 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 damascus kitchen knives and pocket knives.[ citation needed ]

High speed steel tools are the most popular for use in woodturning, as the speed of movement of the work past the edge is relatively high for handheld tools, and HSS holds its edge far longer than high carbon steel tools can.[ citation needed ]

See also

Related Research Articles

Alloy Mixture or metallic solid solution composed of two or more elements

An alloy is an admixture of metals, or a metal combined with one or more other elements. For example, combining the metallic elements gold and copper produces red gold, gold and silver becomes white gold, and silver combined with copper produces sterling silver. Combining iron with non-metallic carbon or silicon produces alloys called steel or silicon steel. The resulting mixture forms a substance with properties that often differ from those of the pure metals, such as increased strength or hardness. Unlike other substances that may contain metallic bases but do not behave as metals, such as aluminium oxide (sapphire), beryllium aluminium silicate (emerald) or sodium chloride (salt), an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, opacity, and luster. Alloys are used in a wide variety of applications, from the steel alloys, used in everything from buildings to automobiles to surgical tools, to exotic titanium alloys used in the aerospace industry, to beryllium-copper alloys for non-sparking tools. In some cases, a combination of metals may reduce the overall cost of the material while preserving important properties. In other cases, the combination of metals imparts synergistic properties to the constituent metal elements such as corrosion resistance or mechanical strength. Examples of alloys are steel, solder, brass, pewter, duralumin, bronze, and amalgams.

Stellite is a range of cobalt-chromium alloys designed for wear resistance. The alloys may also contain tungsten or molybdenum and a small but important amount of carbon.

Carbon steel Steel in which the main interstitial alloying constituent is carbon

Carbon steel is a steel with carbon content from about 0.05% up to 2.1% by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states:

Tool steel

Tool steel refers to a variety of carbon steel and alloy steel that are particularly well-suited to be made into tools. Their suitability comes from their distinctive hardness, resistance to abrasion and deformation, and their ability to hold a cutting edge at elevated temperatures. As a result, tool steels are suited for use in the shaping of other materials. With a carbon content between 0.5% and 1.5%, tool steels are manufactured under carefully controlled conditions to produce the required quality. The presence of carbides in their matrix plays the dominant role in the qualities of tool steel. The four major alloying elements that form carbides in tool steel are: tungsten, chromium, vanadium and molybdenum. The rate of dissolution of the different carbides into the austenite form of the iron determines the high-temperature performance of steel. Proper heat treatment of these steels is important for adequate performance. The manganese content is often kept low to minimize the possibility of cracking during water quenching.

Titanium carbide

Titanium carbide, TiC, is an extremely hard refractory ceramic material, similar to tungsten carbide. It has the appearance of black powder with the sodium chloride crystal structure.

Tempering (metallurgy) Process of heat treating used to increase toughness of iron-based alloys

Tempering is a process of heat treating, which is used to increase the toughness of iron-based alloys. Tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to some temperature below the critical point for a certain period of time, then allowing it to cool in still air. The exact temperature determines the amount of hardness removed, and depends on both the specific composition of the alloy and on the desired properties in the finished product. For instance, very hard tools are often tempered at low temperatures, while springs are tempered at much higher temperatures.

SAE steel grades

The SAE steel grades system is a standard alloy numbering system for steel grades maintained by SAE International.

Alloy steel is steel that is alloyed with a variety of elements in total amounts between 1.0% and 50% by weight to improve its mechanical properties. Alloy steels are broken down into two groups: low alloy steels and high alloy steels. The difference between the two is disputed. Smith and Hashemi define the difference at 4.0%, while Degarmo, et al., define it at 8.0%. Most commonly, the phrase "alloy steel" refers to low-alloy steels.

Eglin steel (ES-1) is a high-strength, high-performance, low-alloy, low-cost steel, developed for new generation of bunker buster type bombs, e.g. the Massive Ordnance Penetrator and the improved version of the GBU-28 bomb known as EGBU-28. It was developed in collaboration between the US Air Force and the Ellwood National Forge Company.

Cold saw

A cold saw is a circular saw designed to cut metal which uses a toothed blade to transfer the heat generated by cutting to the chips created by the saw blade, allowing both the blade and material being cut to remain cool. This is in contrast to an abrasive saw, which abrades the metal and generates a great deal of heat absorbed by the material being cut and saw blade.

Cemented carbide Type of composite material

Cemented carbide is a hard material used extensively as cutting tool material, as well as other industrial applications. It consists of fine particles of carbide cemented into a composite by a binder metal. Cemented carbides commonly use tungsten carbide (WC), titanium carbide (TiC), or tantalum carbide (TaC) as the aggregate. Mentions of "carbide" or "tungsten carbide" in industrial contexts usually refer to these cemented composites.

Mushet steel, also known as Robert Mushet's Special Steel (RMS) and, at the time of its use, self-hardening steel and air-hardening steel, is considered to be both the first tool steel and the first air-hardening steel. It was invented in 1868 by Robert Forester Mushet. Prior to Mushet steel, steel had to be quenched to harden it. It later led to the discovery of high speed steel.

Cobalt-chrome

Cobalt-chrome or cobalt-chromium (CoCr) is a metal alloy of cobalt and chromium. Cobalt-chrome has a very high specific strength and is commonly used in gas turbines, dental implants, and orthopedic implants.

Annular cutter

An annular cutter is form of core drill used to create holes in metal. An annular cutter cuts only a groove at the periphery of the hole and leaves a solid core or slug at the center.

DIN 1.2344 tool steel is a tool steel grade standardised for hot working. The main feature of this grade is the combination of alloyed elements of chromium, molybdenum and vanadium, Cr-Mo-V, which provides a high wear resistance to thermal shock. It is well known as for its great strength, and heat resistance. It is heavily used for die casting in the cold heading field. The presence of high vanadium in DIN 1.2344 can handle the abrasion at both low and high temperatures. It always provides a uniform and high level of machinability. This tool steel is mostly used for aluminum, magnesium and zinc die casting.

Havar, or UNS R30005, is an alloy of cobalt, possessing very high mechanical strength. It can be heat-treated. It is highly resistant to corrosion and is non-magnetic. It is biocompatible. It has high fatigue resistance. It is a precipitation hardening superalloy.

USAF-96 is a high-strength, high-performance, low-alloy, low-cost steel, developed for new generation of bunker buster type bombs, e.g. the Massive Ordnance Penetrator and the improved version of the GBU-28 bomb known as EGBU-28. It was developed by the US Air Force at the Eglin Air Force Munitions Directorate. It uses only materials domestic to the USA. In particular it requires no tungsten.

Shock resisting steels are a class of tool steels designed to resist breakage by shock. Under the AISI classification system there are seven types, labeled S1 to S7.

References

  1. 1 2 3 4 5 6
    • Boccalini, M.; H. Goldenstein (February 2001). "Solidification of high speed steels". International Materials Reviews. 46 (2): 92–115 (24). doi:10.1179/095066001101528411.
  2. Kanigel, Robert (1997). The One Best Way: Frederick Winslow Taylor and the Enigma of Efficiency . Viking Penguin. ISBN   0-670-86402-1.
  3. Misa, Thomas J. (1995). A Nation of Steel: The Making of Modern America 1865–1925 . Baltimore and London: Johns Hopkins University Press. ISBN   978-0801860522.
  4. "taylor-white process". Webster's Revised Unabridged Dictionary. MICRA, Inc. Retrieved 13 April 2013.
  5. "The High-Speed Tool-Steel Patent Decision". Electrochemical and Metallurgical Industry. 7. March 1909. The famous patent suit of the Bethlehem Steel company against the Niles-Bement-Pond Company for infringement of two fundamental patents of F. W. Taylor and M. White (668,369 and 668,270, both of Feb. 19, 1907,) has been decided in favor of the defendant... The decision of the court emphasizes that there is no new composition of steel invented by Taylor and White...
  6. Roberts, George (1998) Tool Steels, 5th edition, ASM International, ISBN   1615032010
  7. The Metals Society, London, "Tools and dies for industry", 1977
  8. American Machinist. McGraw-Hill. 1908.
  9. High Speed Steel (HSS) Archived 1 April 2010 at the Wayback Machine , Retrieved 17 May 2010.
  10. "Properties of Tool Steel AISI T1" . Retrieved 17 March 2008.
  11. "high speed tool data - ICS Cutting Tools". www.icscuttingtools.com.
  12. 1 2 "M42 High Speed Steel" (PDF). Retrieved 15 April 2020.
  13. 1 2 "Cobalt Steel Cutting Tools | Regal Cutting Tools". www.regalcuttingtools.com.
  14. "Drill bits buying guide". advice.manomano.co.uk.