Quenching

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Coke being pushed into a quenching car, Hanna furnaces of the Great Lakes Steel Corporation, Detroit, Michigan, November 1942 ArthurSiegelcoke1.jpg
Coke being pushed into a quenching car, Hanna furnaces of the Great Lakes Steel Corporation, Detroit, Michigan, November 1942

In materials science, quenching is the rapid cooling of a workpiece in water, oil or air to obtain certain material properties. A type of heat treating, quenching prevents undesired low-temperature processes, such as phase transformations, from occurring. It does this by reducing the window of time during which these undesired reactions are both thermodynamically favorable, and kinetically accessible; for instance, quenching can reduce the crystal grain size of both metallic and plastic materials, increasing their hardness.

Contents

In metallurgy, quenching is most commonly used to harden steel by inducing a martensite transformation, where the steel must be rapidly cooled through its eutectoid point, the temperature at which austenite becomes unstable. In steel alloyed with metals such as nickel and manganese, the eutectoid temperature becomes much lower, but the kinetic barriers to phase transformation remain the same. This allows quenching to start at a lower temperature, making the process much easier. High-speed steel also has added tungsten, which serves to raise kinetic barriers, which among other effects gives material properties (hardness and abrasion resistance) as though the workpiece had been cooled more rapidly than it really has. Even cooling such alloys slowly in air has most of the desired effects of quenching; high-speed steel weakens much less from heat cycling due to high-speed cutting. [1]

Extremely rapid cooling can prevent the formation of all crystal structure, resulting in amorphous metal or "metallic glass".

Quench hardening

Quench hardening is a mechanical process in which steel and cast iron alloys are strengthened and hardened. These metals consist of ferrous metals and alloys. This is done by heating the material to a certain temperature, depending on the material. This produces a harder material by either surface hardening or through-hardening varying on the rate at which the material is cooled. The material is then often tempered to reduce the brittleness that may increase from the quench hardening process. Items that may be quenched include gears, shafts, and wear blocks.

Purpose

Before hardening, cast steels and iron are of a uniform and lamellar (or layered) pearlitic grain structure. This is a mixture of ferrite and cementite formed when steel or cast iron are manufactured and cooled at a slow rate. Pearlite is not an ideal material for many common applications of steel alloys as it is quite soft. By heating pearlite past its eutectoid transition temperature of 727 °C and then rapidly cooling, some of the material's crystal structure can be transformed into a much harder structure known as martensite. Steels with this martensitic structure are often used in applications when the workpiece must be highly resistant to deformation, such as the cutting edge of blades. This is very efficient.

Process

The process of quenching is a progression, beginning with heating the sample. Most materials are heated to between 815 and 900 °C (1,500 to 1,650 °F), with careful attention paid to keeping temperatures throughout the workpiece uniform. Minimizing uneven heating and overheating is key to imparting desired material properties.

The second step in the quenching process is soaking. Workpieces can be soaked in air (air furnace), a liquid bath, or a vacuum. The recommended time allocation in salt or lead baths is up to 6 minutes. Soaking times can range a little higher within a vacuum. As in the heating step, it is important that the temperature throughout the sample remains as uniform as possible during soaking.

Once the workpiece has finished soaking, it moves on to the cooling step. During this step, the part is submerged into some kind of quenching fluid; different quenching fluids can have a significant effect on the final characteristics of a quenched part. Water is one of the most efficient quenching media where maximum hardness is desired, but there is a small chance that it may cause distortion and tiny cracking. When hardness can be sacrificed, mineral oils are often used. These oil-based fluids often oxidize and form a sludge during quenching, which consequently lowers the efficiency of the process. The cooling rate of oil is much less than water. Intermediate rates between water and oil can be obtained with a purpose formulated quenchant, a substance with an inverse solubility which therefore deposits on the object to slow the rate of cooling.

Quenching can also be accomplished using inert gases, such as nitrogen and noble gasses. Nitrogen is commonly used at greater than atmospheric pressure ranging up to 20 bar absolute. Helium is also used because its thermal capacity is greater than nitrogen. Alternatively argon can be used; however, its density requires significantly more energy to move, and its thermal capacity is less than the alternatives. To minimize distortion in the workpiece, long cylindrical workpieces are quenched vertically; flat work pieces are quenched on edge; and thick sections should enter the bath first. To prevent steam bubbles the bath is agitated.

Often, after quenching, an iron or steel alloy will be excessively hard and brittle due to an overabundance of martensite. In these cases, another heat treatment technique known as tempering is performed on the quenched material in order 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.

History

There is evidence of the use of quenching processes by blacksmiths stretching back into the middle of the Iron Age, but little detailed information exists related to the development of these techniques and the procedures employed by early smiths. [2] Although early ironworkers must have swiftly noticed that processes of cooling could affect the strength and brittleness of iron, and it can be claimed that heat-treatment of steel was known in the Old World from the late second millennium BC, [3] it is hard to identify deliberate uses of quenching archaeologically. Moreover, it appears that, at least in Europe, 'quenching and tempering separately do not seem to have become common until the 15th century'; it is therefore helpful to distinguish between 'full quenching' of steel, where the quenching is so rapid that only martensite forms, and 'slack quenching', where the quenching is slower or interrupted, which also allows pearlite to form and results in a less brittle product. [4]

The earliest examples of quenched steel may come from ancient Mesopotamia, with a relatively secure example of a fourth-century BC quench-hardened chisel from Al Mina in Turkey. [5] Book 9, lines 389-94 of Homer's Odyssey is widely cited as an early, possibly the first, written reference to quenching: [2] [6]

as when a man who works as a blacksmith plunges a screaming great axe blade or adze into cold water, treating it for temper, since this is the way steel is made strong, even so Cyclops' eye sizzled about the beam of the olive.

However, it is not beyond doubt that the passage describes deliberate quench-hardening, rather than simply cooling. [7] Likewise, there is a prospect that the Mahabharata refers to the oil-quenching of iron arrowheads, but the evidence is problematic. [8]

Pliny the Elder addressed the topic of quenchants, distinguishing the water of different rivers. [9] Chapters 18-21 of the twelfth-century De diversis artis by Theophilus Presbyter mentions quenching, recommending amongst other things that 'tools are also given a harder tempering in the urine of a small, red-headed boy than in ordinary water'. [2] One of the fuller early discussions of quenching is the first Western printed book on metallurgy, Von Stahel und Eysen , published in 1532, which is characteristic of late-medieval technical treatises.

Modern scientific study of quenching began to gain real momentum from the seventeenth century, with a major step being the observation-led discussion by Giambattista della Porta in his 1558 Magia Naturalis . [10]

Mechanism of heat removal during quenching

Heat is removed in three particular stages:

Stage A: Vapor bubbles formed over metal and starts cooling

During this stage, due to the Leidenfrost effect, the object is fully surrounded by vapor which insulates it from the rest of the liquid.

Stage B: Vapor-transport cooling

Once the temperature has dropped enough, the vapor layer will destabilize and the liquid will be able to fully contact the object and heat will be removed much more quickly.

Stage C: Liquid cooling

This stage occurs when the temperature of the object is below the boiling point of the liquid.

See also

Related Research Articles

Metallurgy Domain of materials science that studies the physical and chemical behavior of metals

Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are called alloys. Metallurgy encompasses both the science and the technology of metals; that is, the way in which science is applied to the production of metals, and the engineering of metal components used in products for both consumers and manufacturers. Metallurgy is distinct from the craft of metalworking. Metalworking relies on metallurgy in a similar manner to how medicine relies on medical science for technical advancement. A specialist practitioner of metallurgy is known as a metallurgist.

Steel Metal alloy made by combining iron with other elements

Steel is an alloy of iron with typically a few tenths of a percent of carbon to improve its strength and fracture resistance compared to iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant need typically an additional 11% chromium. Because of its high tensile strength and low cost, steel is used in buildings, infrastructure, tools, ships, trains, cars, machines, electrical appliances, and weapons. Iron is the base metal of steel. Depending on the temperature, it can take two crystalline forms : body-centred cubic and face-centred cubic. The interaction of the allotropes of iron with the alloying elements, primarily carbon, gives steel and cast iron their range of unique properties.

Differential heat treatment

Differential heat treatment is a technique used during heat treating to harden or soften certain areas of a steel object, creating a difference in hardness between these areas. There are many techniques for creating a difference in properties, but most can be defined as either differential hardening or differential tempering. These were common heat treating techniques used historically in Europe and Asia, with possibly the most widely known example being from Japanese swordsmithing. Some modern varieties were developed in the twentieth century as metallurgical knowledge and technology rapidly increased.

Heat treating Process of heating something to alter it

Heat treating is a group of industrial, thermal and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve the desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, carburizing, normalizing and quenching. Although the term heat treatment applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally, heating and cooling often occur incidentally during other manufacturing processes such as hot forming or welding.

Martensite

Martensite is a very hard form of steel crystalline structure. It is named after German metallurgist Adolf Martens. By analogy the term can also refer to any crystal structure that is formed by diffusionless transformation.

Austenite Metallic, non-magnetic allotrope of iron or a solid solution of iron, with an alloying element

Austenite, also known as gamma-phase iron (γ-Fe), is a metallic, non-magnetic allotrope of iron or a solid solution of iron, with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of 1000 K (727 °C); other alloys of steel have different eutectoid temperatures. The austenite allotrope is named after Sir William Chandler Roberts-Austen (1843–1902); it exists at room temperature in some stainless steels due to the presence of nickel stabilizing the austenite at lower temperatures.

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 3.8 per cent by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states:

Carburizing Heat treatment process in which a metal or alloy is infused with carbon to increase hardness

Carburising, carburizing, or carburisation is a heat treatment process in which iron or steel absorbs carbon while the metal is heated in the presence of a carbon-bearing material, such as charcoal or carbon monoxide. The intent is to make the metal harder. Depending on the amount of time and temperature, the affected area can vary in carbon content. Longer carburizing times and higher temperatures typically increase the depth of carbon diffusion. When the iron or steel is cooled rapidly by quenching, the higher carbon content on the outer surface becomes hard due to the transformation from austenite to martensite, while the core remains soft and tough as a ferritic and/or pearlite microstructure.

Case-hardening

Case-hardening or surface hardening is the process of hardening the surface of a metal object while allowing the metal deeper underneath to remain soft, thus forming a thin layer of harder metal at the surface. For iron or steel with low carbon content, which has poor to no hardenability of its own, the case-hardening process involves infusing additional carbon or nitrogen into the surface layer. Case-hardening is usually done after the part has been formed into its final shape, but can also be done to increase the hardening element content of bars to be used in a pattern welding or similar process. The term face hardening is also used to describe this technique, when discussing modern armour.

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.

Hardenability Depth to which a metal is hardened after being submitted to a thermal treatment

The hardenability of a metal alloy is the depth to which a material is hardened after putting it through a heat treatment process. It should not be confused with hardness, which is a measure of a sample's resistance to indentation or scratching. It is an important property for welding, since it is inversely proportional to weldability, that is, the ease of welding a material.

Cryogenic hardening is a cryogenic treatment process where the material is cooled to approximately −185 °C (−301 °F), usually using liquid nitrogen. It can have a profound effect on the mechanical properties of certain steels, provided their composition and prior heat treatment are such that they retain some austenite at room temperature. It is designed to increase the amount of martensite in the steel's crystal structure, increasing its strength and hardness, sometimes at the cost of toughness. Presently this treatment is being used on tool steels, high-carbon, high-chromium steels and in some cases to cemented carbide to obtain excellent wear resistance. Recent research shows that there is precipitation of fine carbides in the matrix during this treatment which imparts very high wear resistance to the steels.

Hardening is a metallurgical metalworking process used to increase the hardness of a metal. The hardness of a metal is directly proportional to the uniaxial yield stress at the location of the imposed strain. A harder metal will have a higher resistance to plastic deformation than a less hard metal.

In metallurgy and materials science, annealing is a heat treatment that alters the physical and sometimes chemical properties of a material to increase its ductility and reduce its hardness, making it more workable. It involves heating a material above its recrystallization temperature, maintaining a suitable temperature for an appropriate amount of time and then cooling.

Hardened steel

The term hardened steel is often used for a medium or high carbon steel that has been given heat treatment and then quenching followed by tempering. The quenching results in the formation of metastable martensite, the fraction of which is reduced to the desired amount during tempering. This is the most common state for finished articles such as tools and machine parts. In contrast, the same steel composition in annealed state is softer, as required for forming and machining.

Japanese swordsmithing

Japanese swordsmithing is the labour-intensive bladesmithing process developed in Japan for forging traditionally made bladed weapons (nihonto) including katana, wakizashi, tantō, yari, naginata, nagamaki, tachi, nodachi, ōdachi, kodachi, and ya (arrow).

Martempering is also known as stepped quenching or interrupted quenching. In this process, steel is heated above the upper critical point and then quenched in a salt, oil, or lead bath kept at a temperature of 150-300 °C. The workpiece is held at this temperature above martensite start (Ms) point until the temperature becomes uniform throughout the cross-section of workpiece. After that it is cooled in air or oil to room temperature. The steel is then tempered. In this process, Austenite is transformed to martensite by step quenching, at a rate fast enough to avoid the formation of ferrite, pearlite or bainite.

Austempering

Austempering is heat treatment that is applied to ferrous metals, most notably steel and ductile iron. In steel it produces a bainite microstructure whereas in cast irons it produces a structure of acicular ferrite and high carbon, stabilized austenite known as ausferrite. It is primarily used to improve mechanical properties or reduce / eliminate distortion. Austempering is defined by both the process and the resultant microstructure. Typical austempering process parameters applied to an unsuitable material will not result in the formation of bainite or ausferrite and thus the final product will not be called austempered. Both microstructures may also be produced via other methods. For example, they may be produced as-cast or air cooled with the proper alloy content. These materials are also not referred to as austempered.

Mangalloy Alloy steel containing around 13% manganese

Mangalloy, also called manganese steel or Hadfield steel, is an alloy steel containing an average of around 13% manganese. Mangalloy is known for its high impact strength and resistance to abrasion once in its work-hardened state.

Thermomechanical processing, is a metallurgical process that combines mechanical or plastic deformation process like compression or forging, rolling etc. with thermal processes like heat-treatment, water quenching, heating and cooling at various rates into a single process.

References

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  2. 1 2 3 Mackenzie, D. S. (June 2008). "History of quenching". International Heat Treatment and Surface Engineering. 2 (2): 68–73. doi:10.1179/174951508x358437. ISSN   1749-5148.
  3. Craddock, Paul T. (2012). "Metallurgy in the Old World". In Silberman, Neil Asher (ed.). The Oxford companion to archaeology. Volume 1 of 3 (2nd ed.). New York: Oxford University Press (published 2012-10-12). pp. 377–380. ISBN   9780199739219. OCLC   819762187.|volume= has extra text (help)
  4. Williams, Alan (2012-05-03). The sword and the crucible : a history of the metallurgy of European swords up to the 16th century. History of Warfare. Volume 77. Leiden: Brill. p. 22. ISBN   9789004229334. OCLC   794328540.|volume= has extra text (help)
  5. Moorey, P. R. S. (Peter Roger Stuart) (1999). Ancient mesopotamian materials and industries : the archaeological evidence . Winona Lake, Ind.: Eisenbrauns. pp.  283–85. ISBN   978-1575060422. OCLC   42907384.
  6. Forbes, R. J. (Robert James) (1972-01-01). Studies in ancient technology. Metallurgy in Antiquity, part 2. Copper and Bronze, Tin, Arsenic, Antimony and Iron. 9 (2d rev. ed.). Leiden: E.J. Brill. p. 211. ISBN   978-9004034877. OCLC   1022929.
  7. P. R. S. Moorey, Ancient Mesopotamian Materials and Industries: The Archaeological Evidence (Winona Lake, Indiana: Eisenbrauns, 1999), p. 284.
  8. R. K. Dube, 'Ferrous Arrowheads and Their Oil Quench Hardening: Some Early Indian Evidence', JOM: The Journal of The Minerals, Metals & Materials Society, 60.5 (May 2008), 25-31.
  9. John D. Verhoeven, Steel Metallurgy for the Non-Metallurgist (Materials Park, Ohio: ASM International, 2007), p. 117.
  10. J. Vanpaemel. HISTORY OF THE HARDENING OF STEEL: SCIENCE AND TECHNOLOGY. Journal de Physique Colloques, 1982, 43 (C4), pp. C4-847-C4-854. DOI:10.1051/jphyscol:19824139; https://hal.archives-ouvertes.fr/jpa-00222126.