The NASS glossary of terms

  • Acid Steel

    Steel produced in a furnace with an acid lining, i.e. consisting of a siliceous refractory and under a siliceous slag. With an acid slag, carbon, silicon and manganese only are removed so that the pig iron must not contain sulphur and phosphorus in percentages exceeding those permissible for the specification being made. Most steel manufactured today is in furnaces with basic linings.

  • Air-Hardening Steel

    Sometimes referred to as self- hardening steel. A steel that becomes fully hardened when cooled in air from above its critical point and does not require rapid quenching by oil or water. The risk of distortion is greatly reduced by air hardening. High Speed Steel was one of the earliest examples of this type of

  • Allotropy

    The property possessed by certain elements to exist in two or more distinct forms that are chemically identical but have different physical properties. In the case of iron the crystal structure has one form at room temperature and another at high temperature. When heated above 910oC the atomic structure changes from body centred cubic to face centred cubic but reverts again when cooled. The allotropy of iron modifies

  • Alloy Steel

    A steel to which one or more alloying elements other than carbon have been deliberately added (e.g. chromium, nickel, molybdenum) to achieve a particular physical property.

  • Alpha Iron

    The body centred cubic form of iron which, in pure iron, exists up to 910oC.

  • Annealing

    Heating steel to, and holding at a suitable temperature, followed by relatively slow cooling. The purpose of annealing may be to remove stresses, to soften the steel, to improve machinability, to improve cold working properties, to obtain a desired structure. The annealing process usually involves allowing the steel to cool slowly in the furnace.

  • Arc Furnace

    A steel melting furnace in which heat is generated by an arc between graphite electrodes and the metal. Both carbon and alloy steels are produced in electric arc furnaces and scrap rather than molten metal is used as the base material. Furnaces with capacities up to 200 tonnes are now in use.

  • Austempering

    Quenching from a temperature above the transformation range to a temperature above the upper limit of martensite formation, and holding at this temperature until the austenite is completely transformed to the desired intermediate structure, for the purpose of conferring certain mechanical properties.

  • Austenite

    The solid solution of carbon in gamma (face centered cubic) iron.

  • Austenitic Steels

    Steels containing high percentages of certain alloying elements such as manganese and nickel which are austenitic at room temperature and cannot be hardened by normal heat-treatment but do work harden. They are also non-magnetic. Typical examples of austenitic steels include the 18/8 stainless steels and 14% manganese steel.

  • B

    Chemical symbol for Boron.

  • Bainite

    An acicular aggregate of ferrite and carbide particles formed when austenite is transformed on cooling at temperatures in the intermediate (200-450oC) range, i.e. above the martensite and below the pearlite range.

  • Balanced Steel

    Steels in which the deoxidisation is controlled to produce an intermediate structure between rimmed and killed steel. Sometimes referred to as semi- killed steels, they possess uniform properties throughout the ingot and amongst their applications are boiler plate and structural sections.

  • Base Metal

    A metal which oxidises when heated in air, e.g. lead, copper, tin, zinc, as opposed to noble metals such as gold and platinum.

  • Basic Steel

    Steel produced in a furnace in which the hearth consists of a basic refractory such as dolomite or magnesite, as opposed to steel melted in a furnace with an acid lining. The basic process permits the removal of sulphur and phosphorous and in this respect is superior. Present day BOS and electric arc furnaces use basic linings.

  • Be

    Chemical symbol for Beryllium.

  • Bend Test

    Bending tests are carried out to ensure that a metal has sufficient ductility to stand bending without fracturing. A standard specimen is bent through a specified arc and in the case of strip, the direction of grain flow is noted and whether the bend is with or across the grain.

  • Bessemer Process

    A method of producing steel, first introduced in the last century, where air is blown under pressure through molten iron to remove the impurities by oxidation. The development of the process has led to the present day Basic Oxygen Steel-making plants that account for bulk production of commercial quality steels in the UK.

  • Bi
  • Chemical symbol for Bismuth.

  • Billet
  • A section of steel used for rolling into bars, rods and sections. It can be a product of the ingot route, or increasingly today produced directly by continuous casting.

  • Blast Furnace
  • A tall, cylindrical, refractory lined furnace for the production of pig iron or hot metal for direct conversion into steel.

  • Bloom
  • A large square section of steel intermediate in the rolling process between an ingot and a billet. Blooms are now also being produced by the continuous casting process eliminating the necessity of first producing an ingot.

  • Boron Steels
  • The addition of boron in the range 0.0005- 0.005% to certain steels increases the hardenability. A range of boron steels is now listed in the current BS 970 and are widely used for the production of cold headed fastenings.

  • Brazing
  • Brazing is a method of joining metal parts together by fusing a layer of brass between the adjoining surfaces. A red heat is necessary and a flux is used to protect the metal from oxidation.

  • Bright Annealing
  • An annealing process that is carried out in a controlled atmosphere furnace or vacuum in order that oxidation is reduced to a minimum and the surface remains relatively bright.

  • Bright Drawing
  • The process of drawing hot rolled steel through a die to impart close dimensional tolerances, a bright, scale free surface, and improved mechanical properties. The product is termed bright steel.

  • Brinell Hardness Test

    The Brinell hardness test for steel, involves impressing a ball 10 mm diameter, of hard steel or tungsten carbide, with a loading of 3000 kilogrammes into the steel surface. The hardness of the steel is then determined by measurement of the indentation. For steels with a hardness over 500 BHN the Vickers test is more reliable.

  • Calcium

    In the form of calcium silicide acts as a deoxidizer and degasifier when added to steel. Recent developments have found that carbon and alloy steels modified with small amounts of calcium show improved machinability and longer tool life. Transverse ductility and toughness are also enhanced.

  • Carbon

    Carbon is an essential element in steel, it is added in specific amounts to control the hardness and strength of the material. In general, increased carbon content reduces ductility but increases tensile strength and the ability of the steel to harden when cooled rapidly from elevated temperatures.

  • Carbon Steel

    A steel whose properties are determined primarily by the amount of carbon present. Apart from iron and carbon, manganese up to 1.5% may be present as well as residual amounts of alloying elements such as nickel, chromium, molybdenum, etc. It is when one or more alloying elements are added in sufficient amount that it is classed as an alloy steel.

  • Carbo-Nitriding

    A case-hardening process in which steel components are heated in an atmosphere containing both carbon and nitrogen.

  • Carburising

    The introduction of carbon into the surface layer of a steel that has a low carbon content. The process is carried out by heating the components in a solid liquid or gaseous carbon containing medium. The depth of penetration of carbon into the surface is controlled by the time and temperature of the content. The process is carried out by heating the components in a solid liquid, or gaseous carbon containing medium. The depth of penetration of carbon into the surface is controlled by the time and temperature of the treatment. After carburising it is necessary to harden the components by heating to a suitable temperature and quenching.

  • Case-Hardening

    The process of hardening the surface of steel whilst leaving the interior unchanged. Both carbon and alloy steels are suitable for case-hardening providing their carbon content is low, usually up to a maximum of 0.2%. Components subject to this process, particularly in the case of alloy steels, have a hard, wear-resistant surface with a tough core.

  • Cast Iron

    A definition can be applied that Cast Iron is an alloy of iron and carbon in which the carbon is in excess of the amount that can be retained in solid solution in austenite at the eutectic temperature. Carbon is usually present in the range of 1.8% to 4.5%, in addition, silicon, manganese, sulphur and phosphorus are contained in varying amounts. Various types of cast iron are covered by a British Standard classification and includes grey, malleable and white irons. Elements such as nickel, chromium, molybdenum, vanadium can be added to produce alloy cast irons.

  • Cast Steel

    A term originally applied to crucible steel and sometimes today used to describe tool steels. The term is misleading and is falling into misuse. It can also be applied to steel castings made by pouring molten steel into a mould but which are not subject to further forging or rolling.

  • Cb

    Chemical symbol for Columbium.

  • Ce

    Chemical symbol for Cerium.

  • Cementite

    An iron carbide (Fe3C) constituent of steel. It is hard, brittle and crystalline. Steel which has cooled slowly from a high temperature contains ferrite and pearlite in relative proportions varying with the chemical composition of the steel. Pearlite is a lamellar structure of ferrite and cementite.

  • Charpy Test

    A test to measure the impact properties of steel. A prepared test piece, usually notched, is broken by a swinging pendulum. The energy consumed in breaking the test piece is measured in Joules. The more brittle the steel the lower the impact strength. Izod is a similar and more widely used impact test in this country. Both are quoted in the current edition of BS 970.

  • Chromium

    When used as an alloying element, chromium increases the hardenability of steel and in association with high carbon gives resistance to wear and abrasion. Chromium has an important effect on corrosion resistance and is present in stainless steels in amounts of 12% to 20%. It is also used in heat-resisting steels and high duty cast irons.

  • Co

    Chemical symbol for Cobalt.

  • Cobalt

    An alloying element used in tool, magnet and heat resisting steels. Together with tungsten and molybdenum, cobalt is used to form the super high speed steels. It improves the red hardness value of the steel, that is, it enables the steel to resist softening at a high temperature or in the case of a cutting tool to hold its edge under severe conditions.

  • Coefficient of Expansion

    The ratio of change in length, area, or volume per degree to the corresponding value at a standard temperature.

  • Cogging

    An intermediate rolling process when a hot ingot is reduced to a bloom or slab in a cogging mill.

  • Cold Drawing

    The process of reducing the cross sectional area of wire, bar or tube by drawing the material through a die without any pre-heating. Cold drawing is used for the production of bright steel bar in round square, hexagonal and flat section. The process changes the mechanical properties of the steel and the finished product is accurate to size, free from scale with a bright surface finish.

  • Cold Working

    Altering the shape or size of a metal by plastic deformation. Processes include rolling, drawing, pressing, spinning, extruding and heading, it is carried out below the recrystallisation point usually at room temperature. Hardness and tensile strength are increased with the degree of cold work whilst ductility and impact values are lowered. The cold rolling and cold drawing of steel significantly improves surface finish.

  • Contact Corrosion

    When two dissimilar metals are in contact without a protective barrier between them and they are in the presence of liquid, an electrolytic cell is created. The degree of corrosion is dependent on the area in contact and the electro-potential voltage of the metals concerned. The less noble of the metals is liable to be attacked, i.e. zinc will act as a protector of steel in sea water whereas copper or brass will attack the steel in the same environment.

  • Continuous Casting

    A method of producing blooms, billets and slabs in long lengths using water cooled moulds. The castings are continuously withdrawn through the bottom of the caster whilst the teeming of the metal is proceeding. The need for primary and intermediate mills and the storage and use of large numbers of ingot moulds is eliminated. The continuous casting process is also used in the production of cast iron, aluminium and copper alloys.

  • Controlled Atmosphere

    A gas or mixture of gases in which steel is heated to produce or maintain a specific surface condition. Controlled atmosphere furnaces are widely used in the heat treatment of steel as scaling and decarburisation of components is minimised by this process.

  • Core

    In the case of steel this refers to a component that has been case-hardened where the centre is softer than the hard surface layer or case. It can also be applied to the central part of a rolled rimming steel.

  • Corrosion Fatigue

    Fatigue that arises when alternating or repeated stress combines with corrosion. The severity of the action depends on the range and frequency of the stress, the nature of the corroding condition and the time under stress.

  • Cr

    Chemical symbol for Chromium.

  • Creep

    The form of plastic deformation that takes place in steel held for long periods at high temperature. Methods of creep testing involve the determination of strain/time curves under constant tensile load and at constant temperature.

  • Critical Cooling Rate

    The slowest rate of cooling from the hardening temperature which will produce the fully hardened martensitic condition.

  • Critical Point

    This generally refers to a temperature at which some chemical or physical change takes place. These transformations cause evolution of heat on cooling or absorption of heat on heating and appear as discontinuities or arrest points in the heating and cooling curves. The temperatures vary with the carbon content of the steel and the rate of cooling.

  • Critical Temperature

    The temperature at which some phase change occurs in a metal during heating or cooling, i.e. the temperature at which an arrest or critical point is shown on heating or cooling curves.

  • Crystalline Fracture

    A type of fracture that appears bright and glittering, it having formed along the cleavage planes of the individual crystals. Normally an indication that brittle fracture has occurred.

  • Cu

    Chemical symbol for Copper.

  • Cyanide Hardening

    A process of introducing carbon and nitrogen into the surface of steel by heating it to a suitable temperature in a molten bath of sodium cyanide, or a mixture of sodium and potassium cyanide, diluted with sodium carbonate and quenching in oil or water. This process is used where a thin case and high hardness are required.

  • Decalescence

    A term used in reference to the absorption of heat without a corresponding increase in temperature, when steel is heated through the critical points (phase changes).

  • Decarburisation

    The loss of carbon from the surface of steel as a result of heating in a carbon weak atmosphere. During the rolling of steel hot surfaces are exposed to the decarburising effects of oxygen in the atmosphere and as a result the surface is depleted of carbon. In steels where the components are to be subsequently heat treated it is necessary to remove the decarburised surface by machining.

  • Delta Iron

    When pure or practically carbon-free iron is cooled from above its melting point it solidifies at about 1535oC as delta iron having a body-centred cubic lattice structure, which persists down to about 1400oC. On further cooling it undergoes an allotropic change to gamma iron which has a face-centred cubic lattice and is non-magnetic.

  • Deoxidation

    Elements such as silicon and aluminium when added to molten steel react to form stable oxides and reduce the amount of dissolved oxygen. The solubility of oxygen in steel is reduced as temperature is lowered during solidification and the excess oxygen combines to form carbon monoxide. If the molten metal is not deoxidised the effervescence produced by the evolution of carbon monoxide during solidification would result in blow holes and porosity. Steel treated in this way is termed, “Killed Steel”.

  • Descaling

    It is necessary to remove the scale from hot rolled bars or coil before bright drawing. This is normally carried out by shot blasting or pickling in acid. Other methods of descaling steel products include sand blasting, flame descaling and tumbling.

  • Deseaming

    A process of burning out defective areas on the surface of ingots, blooms or billets. The condition of the surface is such that it can then be rolled or forged into a satisfactory product.

  • Diamond Pyramid Hardness Test

    This test, more commonly known as the Vickers test, finds greater use in the laboratory than the workshop. It employs a pyramid shaped diamond with an included angle of 136o which is impressed into the specimen using loads of 5 to 120 kg making a small square impression. This test is used for finished or polished components because the impression can be very small. The diamond pyramid hardness number is obtained from a calculation based on measuring the diagonals of the impressions in the steel.

  • Die

    The term die is most commonly used in tooling, i.e. press tools “punch and die” but there are many other types of die, e.g. thread cutting dies, forming dies, forging dies, die-casting dies, etc. The term when applied to steel often refers to drawing dies through which hot rolled wire and bar are drawn to produce the finish and dimensional accuracy that is required for bright steel.

  • Dislocation

    A discontinuity in the crystal lattice of a metal. The movement of dislocations under stress may be used to explain slip, creep, plastic yielding, etc.

  • Dolomite

    A natural carbonate of calcium and magnesium generally used as a flux in blast furnaces.

  • Drawing

    The process of pulling metal wire, rods, or bars through a die with the effect of altering the size, finish and mechanical properties. In the USA, it is a term used for tempering.

  • Drop Forging

    An operation in which a metal shape is formed by forcing hot metal into impressions formed in solid blocks of hardened alloy steel, the forging dies. The dies are made in halves, one attached to the rising and falling block of the drop forge and the other to the stationary anvil. Drop forgings are widely used in the automotive industry for crankshafts, stub-axles, gears, etc.

  • Ductility

    The property of metal which permits it to be reduced in cross sectional area without fracture. In a tensile test, ductile metals show considerable elongation eventually failing by necking, with consequent rapid increase in local stresses.

  • Dye Penetrant Inspection

    A method for detecting surface porosity or cracks in metal. The part to be inspected is cleaned and coated with a dye which penetrates any flaws that may be present. The surface is wiped clean and coated with a white powder. The powder absorbs the dye held in the defects indicating their location.

  • Elastic Limit

    The maximum stress that can be applied to a metal without producing permanent deformation. When external forces act upon a material they tend to form internal stresses within it which cause deformation. If the stresses are not too great the material will return to its original shape and dimension when the external stress is removed.

  • Elasticity

    The property which enables a material to return to its original shape and dimension.

  • Electrical Steels

    Steels which are characterised by their magnetic properties and are intended for the manufacture of electrical circuits. They are supplied in the form of cold rolled sheet or strip, generally less than 2mm thick and up to 1500mm wide. Grain orientated steels have preferential magnetic properties in the direction of rolling and non- grain orientated steels have similar magnetic properties both transversely and in the direction of rolling.

  • Electroslag Refining

    A specialised steel making process in which a rolled or a cast ingot in the form of an electrode is remelted in a water cooled copper mould. The melting is activated by resistive heat generated in a conductive slag. The resulting product has a similar basic chemical composition to the original ingot, but is characterised by high purity and low inclusion content. Typical applications include high integrity components for the aerospace industry.

  • Elevated Temperature Drawing

    A process of drawing steel bars at elevated temperatures (normally 250-300oC) which under optimum conditions produce steels that have higher tensile and yield strengths than those cold drawn with the same degree of reduction. The process is little used in the United Kingdom.

  • Elongation

    A test to measure the ductility of steel. When a material is tested for tensile strength it elongates a certain amount before fracture takes place. The two pieces are placed together and the amount of extension is measured against marks made before starting the test and is expressed as a percentage of the original gauge length.

  • End Quench Test

    More commonly referred to as Jominy Test it is used to determine the hardening ability of steel.

  • Equiaxed Crystals

    Crystals, each of which has axes approximately equal in length. These are normally present in the centre of a steel ingot.

  • Equilibrium

    A diagram constructed from thermal and other data showing the limits of composition and temperature within which the various constituents or phases of alloys are stable.

  • Etching

    Treatment of a prepared metal surface with acid or other chemical reagent which, by differential attack, reveals the structure.

  • Eutectic

    A mixture of two or more constituents which solidify simultaneously out of the liquid at a minimum freezing point.

  • Eutectoid

    A mixture of two or more constituents which forms on cooling from a solid solution and transforms on heating at a constant minimum temperature. A eutectoid steel contains approximately 0.83% carbon.

  • Extrusion

    The production of a section by forcing a billet to flow through a die. Often used for producing complex sections, the process is used with both hot and cold metal. Seamless tubes are produced by forcing a hot billet to flow through a die over a mandrel positioned centrally in the die.

  • F

    Chemical symbol for Fluorine.

  • Face Centred Cubic Lattice

    An arrangement of atoms in crystals in which the atomic centres are disposed in space in such a way that one atom is located at each of the corners of the cube and one at the centre of each face. Steel in the face-centred cubic arrangement is termed austenite.

  • Fatigue

    The effect on metal of repeated cycles of stress. The insidious feature of fatigue failure is that there is no obvious warning, a crack forms without appreciable deformation of structure making it difficult to detect the presence of growing cracks. Fractures usually start from small nicks or scratches or fillets which cause a localised concentration of stress. Failure can be influenced by a number of factors including size, shape and design of the component, condition of the surface or operating environment.

  • Fatigue Limit

    The maximum value of the applied alternating stress which a test piece can stand indefinitely.

  • Fatigue Testing

    Fatigue tests are made with the object of determining the relationship between the stress range and the number of times it can be applied before causing failure. Testing machines are used for applying cyclically varying stresses and cover tension, compression, torsion and bending or a combination of these stresses.

  • Fe

    Chemical symbol for Iron.

  • Ferrite

    The solid solution of carbon in body-centered cubic iron, a constituent of carbon steels.

  • Ferritic Steel

    Term usually applied to a group of stainless steels with a chromium content in the range of 12- 18o and whose structure consists largely of ferrite. Such steels possess good ductility and are easily worked but do not respond to any hardening or tempering processes. Types of applications include automotive trim and architectural cladding.

  • Ferro Alloys

    Alloys of iron with chromium, manganese, silicon, tungsten, molybdenum or vanadium. Used in steelmaking as a means of introducing these alloying elements into the cast or as deoxidisers.

  • Fettling

    The removal of sand adhering to castings by hammering, tumbling or shot blasting.
    Fin In rolling mill practice a fin is a projection extending from the side of rolled sections. It causes considerable trouble and is the result of overfill. The fin, formed when the bar or shape is fed through one pass, is likely to be rolled back into the bar at the next pass. It is rarely encountered in modern rolling mills.

  • Flame Hardening

    A surface hardening process in which heat is applied by a high temperature flame followed by quenching jets of water. It is usually applied to medium to large size components such as large gears, sprockets, slide ways of machine tools, bearing surfaces of shafts and axles, etc. Steels most suited have a carbon content within the range 0.40-0.55%.

  • Flash

    A fin that arises from metal in excess of that required to fill the final impression in a forging die and is exuded from the parting line between the dies similarly it can arise at the mould joint in a casting.

  • Forging

    A process of working metal to a finished shape by hammering or pressing and is primarily a “hot” operation. It is applied to the production of shapes either impossible or too costly to make by other methods or needing properties not obtainable by casting. Categories of forgings include Hammer, Press, Drop or

  • Fracture

    Fractures are often described by the appearance of the surface of the break in a piece of steel. Crystalline is bright and glittering, failure having developed along the cleavage planes of individual crystals and can be typical of brittle material. A silky fracture has a smooth dull grain indicative of ductile material such as a mild steel. In tensile testing fractures are described by shape, e.g. cup and cone.

  • Freecutting Steels

    Steels which have had additions made to improve machinability. The most common additives are sulphur and lead, other elements used include tellurium, selenium and bismuth.

  • Ga

    Chemical symbol for Gallium.

  • Galvanic Action

    When iron and steel are subject to conditions of aqueous corrosion the incidence and rate at which the corrosion takes place will alter if the steel is coupled with other metals or alloys that are also exposed to the electrolyte. Copper, brass, bronze, lead and nickel are more “noble” and act as auxiliary cathodes to the steel and accelerate its anodic dissolution, that is, its corrosion. Magnesium, zinc and zinc-base alloy are nearly always less noble and tend to divert the attack from the steel to themselves. The galvanic relationship of various metals is an important factor affecting corrosion.

  • Gamma Iron

    The allotropic form of iron existing between the temperature 910oC and 1400oC is known as Gamma Iron. It has a face centred cubic lattice and is non-magnetic. Gamma iron containing carbon or other elements in solution is known as austenite.

  • Gas Carburising

    A heat treatment method used in the case- hardening of steel. Carbon is absorbed into the outer layers of the components by heating in a current of gas, rich in carbon compounds. The process is more versatile than some other methods as the depth of the case and the limiting carbon content of the case can be controlled by the composition of the atmosphere, the dew point and the temperature.

  • Gauge Length

    Used in the mechanical testing of steel, it is the length marked on the parallel portion of a tensile test piece from which the elongation is measured.

  • Gauge Plate

    An alloy tool steel supplied in flat and square section with the surfaces ground to close limits. It is also known as Ground Flat Stock and is used for the manufacturing of gauges, punches, dies, jigs, templates etc.

  • Ge

    Chemical symbol for Germanium.

  • Grain Size Control

    When a steel is austenitised by heating to above the critical range, time is required for the production of a homogeneous structure during which there is a tendency towards grain growth. Although subsequent hot and cold working affects the grain size, it is originally controlled at the steel making stage by the addition of aluminium.

  • Grain Size Measurement

    Grain size is normally quantified by a numbering system. Coarse 1-5 and fine 5-8. The number is derived from the formula N=2n-1 where n is the number of grains per square inch at a magnification of 100 diameters. Grain size has an important effect on physical properties. For service at ordinary temperatures it is generally considered that fine grained steels give a better combination of strength and toughness, whereas coarse grained steels have better machinability.

  • Graphitising

    An annealing process applied to cast iron and steels with a high carbon and high silicon content by which the combined carbon is wholly or in part transformed to graphitic or free carbon.

  • Grey Iron

    Also known as flake iron on account of all or part of the carbon content being in the form of graphite distributed through the metal as flakes.

  • Grinding

    A machining process:- (a) to shape components that are too hard to be machined by conventional methods such as hardened tool steels and case or induction hardened components. (b) to obtain a high degree of dimensional accuracy and surface finish on a component.

  • Grinding Cracks

    Cracks can arise from incorrect grinding and appear in the form of a network. They are caused by the generation of high heat and rapid cooling in the area of contact and they mostly occur when grinding fully hardened material such as tool steel.

  • H

    Chemical symbol for Hydrogen.

  • Hard Metal Facing

    A method of increasing the wear resistance of a metal by the deposition of a hard protective coating. Alloys such as Stellite or a metallic carbide are most often used for the coating.

  • Hard Metals

    A group of materials more commonly known as cemented carbides. They consist of mixtures of one or more of the finely divided carbides of tungsten, titanium, tantalum and vanadium embedded in a matrix of cobalt or nickel by sintering. Widely used for cutting tools where for many applications they have replaced conventional high speed steels.

  • Hardenability

    The property that determines the depth and distribution of hardness when steel is heated to a given temperature and then quenched (more precisely it may be defined as an inverse measure of the severity of cooling conditions necessary to produce on continuous cooling a martensitic structure in a previously austenitized steel i.e. to avoid transformations in the pearlitic and bainitic ranges). The lower the cooling rate to avoid these transformations, the greater the hardenability. The critical cooling rate is largely a function of the composition of the steel. In general the higher the carbon content, the greater the hardenability, whilst alloying elements such as nickel, chromium, manganese and molybdenum increase the depth of hardening for a given ruling section.

  • Hardening

    Increasing the hardness of steel by heat treatment. This normally implies heating the steel to a required temperature and quenching in a suitable medium, e.g. oil or water.

  • Hardness

    The hardness of steel is generally determined by testing its resistance to deformation. A number of methods are employed including Brinell, Vickers and Rockwell. The steel to be tested is indented by a hardened steel ball or diamond under a given load and the size of the impression is then measured. For steel there is an empirical relationship between hardness and tensile strength and the hardness number is often used as a guide to the tensile strength, e.g. 229 Brinell = 772N/mm2 (50 tons/

  • Heat

    In steel making terms this is often used to define the batch or cast produced from a single melting operation.

  • Heat Treatment

    A process where solid steel or components manufactured from steel are subject to treatment by heating to obtain required properties, e.g. softening, normalising, stress relieving, hardening. Heating for the purpose of hot-working as in the case of rolling or forging is excluded from this definition.

  • High Speed Steel

    The term `high speed steel’ was derived from the fact that it is capable of cutting metal at a much higher rate than carbon tool steel and continues to cut and retain its hardness even when the point of the tool is heated to a low red temperature.

  • Hooke’s Law

    This states that “within the limits of elasticity the strain produced by a stress of any one kind is proportional to the stress”. The stress at which a material ceases to obey Hooke’s Law is known as the limit of proportionality.

  • Hot Quenching

    Cooling in a medium, the temperature of which is substantially higher than room temperature.

  • Hot Work

    The rolling, forging or extruding of a metal at a temperature above its recrystallisation point.

  • Hydrogen

    An undesirable impurity if present in steel and a cause of fine hairline cracks especially in alloy steels. Modern vacuum treatment eliminates this problem.

  • Hyper-Eutectoid Steel

    A steel that contains more than 0.83% carbon which with appropriate heat treatment consists of pearlite and cementite.

  • Hypo-Eutectoid Steel

    A steel that contains less than 0.83% carbon and which in annealed condition has a structure of ferrite and pearlite.

  • I

    Chemical symbol for Iodine.

  • Impact Test

    A test designed to give information on how a specimen of a known material will respond to a suddenly applied stress, e.g. shock. The test ascertains whether the material is tough or brittle. A notched test piece is normally employed and the two methods in general use are either the Izod or Charpy test. The result is usually reported as the energy in ft.lbs. or KJ. required to fracture the test piece.

  • In

    Chemical symbol for Indium.

  • Inclusion Count

    A method of assessing the number and size of non-metallic inclusions present in metal.

  • Inclusions

    Usually non-metallic particles contained in metal. In steel they may consist of simple or complex oxides, sulphides, silicates and sometimes nitrides of iron, manganese, silicon, aluminium and other elements. In general they are detrimental to mechanical properties but much depends on the number, their size, shape and distribution.

  • Induction Hardening

    A widely used process for the surface hardening of steel. The components are heated by means of an alternating magnetic field to a temperature within or above the transformation range followed by immediate quenching. The core of the component remains unaffected by the treatment and its physical properties are those of the bar from which it was machined, whilst the hardness of the case can be within the range 37/58 Rc. Carbon and alloy steels with a carbon content in the range 0.40/0.45% are most suitable for this process.

  • Ingot

    The mass of metal that results from casting molten steel into a mould. An ingot is usually rectangular in shape and is subsequently rolled into blooms and billets for rods, bars and sections and slabs for plates, sheet and strip. With the increasing use of the continuous casting process the ingot route is less used as the molten steel is now directly cast into a bloom or billet.

  • Ingot Mould

    The receptacle into which molten steel is poured to form an ingot. After solidification the steel is suitable for subsequent working, i.e. rolling or forging.

  • Intercrystalline Corrosion

    Chromium-nickel austenitic stainless steels are prone to this form of corrosion when they are welded and subsequently in contact with certain types of corrosive media. When heated within a temperature range of 450-800oC precipitation of the chromium carbides takes place at the grain boundaries in the area of the weld and these areas no longer have the protection of the chromium on the peripheries of the grains. This type of corrosion is also known as Weld Decay and Intergranular Corrosion. The most common way to avoid the problem is to select a grade of steel that is very low in carbon i.e. 0.03% or less, or one that is stabilised with niobium or titanium.

  • Interrupted Quenching

    Rapid cooling to a selected temperature by quenching in a suitable medium, usually molten salt, holding at the temperature for an appropriate time and then cooling to room temperature. This process is used to minimise the risk of distortion.

  • Iron

    The term iron, as used in the chemical or scientific sense of the word, refers to the chemical element iron or pure iron and is the chief constituent of all
    commercial iron and steel.

  • Isothermal Annealing

    Heating to and holding at a temperature above the transformation range, then cooling to and holding at a suitable temperature until the austenite to pearlite change is complete.

  • Isothermal Transformation Curve

    Also known as the Time Temperature Transformation Curve. If a small piece of steel is heated sufficiently slowly for it to become austenitic and then plunged into a salt bath and held at a constant temperature below the upper critical point for a definite time followed by rapid quenching, it is possible by examination to determine the extent to which the transformation of the austenite has occurred. By taking a number of specimens of the same steel and treating them in the same way, but varying the holding temperature and time the behaviour of the steel with time and temperature can be studied. The information obtained can be plotted as time- temperature transformation curves which is useful in heat treatment practice, particularly for martempering and austempering.

  • Izod Impact Test

    A test specimen, usually of square crossed section is notched and held between a pair of jaws, to be broken by a swinging or falling weight. When the pendulum of the Izod testing machine is released it swings with a downward movement and when it reaches the vertical the hammer makes contact with the specimen which is broken by the force of the blow. The hammer continues its upward motion but the energy absorbed in breaking the test piece reduces its momentum. A graduated scale enables a reading to be taken of the energy used to fracture the test piece. To obtain a representative result the average of three tests is used and to ensure that the results conform to those of the steel specification the test specimens should meet the standard dimensions laid down in BS 131.

  • Jominy Test

    A method for determining the hardenability of steel. The Jominy test is covered by BS 4437:1987. A standard test piece 25mm x 100mm is heated to a pre- determined temperature and quenched by a jet of water sprayed onto one end. When the specimen is cold, hardness measurements are made at intervals along the test piece from the quenched end and the results are plotted on a standard chart from which is derived the hardenability curve. BS 970 contains hardenability curves for many of the steels in the Standard. Properly carried out, this test will illustrate the effect of mass upon a chosen steel when heat treated and indicate if the steel is of a shallow, medium or deep hardening type.

  • Joule

    A unit of energy. One joule is equal to the energy expended in one second by one ampere against the resistance of one ohm. In the mechanical testing of steel it is the unit used in the Charpy V notch impact test.

  • K

    Chemical symbol for potassium.

  • Kaldo Process

    A method of producing steel from molten iron, using an inclined rotating converter and a water cooled oxygen lance inserted through the converter mouth. Originating in Sweden, this process is no longer in use in the UK.

  • Killed Steel

    The term indicates that the steel has been completely deoxidised by the addition of an agent such as silicon or aluminium, before casting, so that there is practically no evolution of gas during solidification. Killed steels are characterised by a high degree of chemical homogeneity and freedom from porosity.

  • Knoop Hardness Test

    A micro hardness test in which an elongated pyramidical diamond is pressed into the surface.

  • La

    Chemical symbol for Lanthanum.

  • Lap

    A defect appearing as a seam on a rolled bar. Laps are rolled over pieces of material that arise when a bar is given a pass through the rolls after a sharp overfill or fin has been formed, causing the protrusion to be rolled into the surface of the product. The presence of oxides usually prevents the lap welding to the original bar surface, so that in subsequent cold working it is carried through as a longitudinal crack.

  • L-D Process

    An oxygen steel making process named after the towns in Austria, Linz and Donawitz, where it was first developed. It is a modified Bessemer process, steel is produced in a solid bottom converter by injection of oxygen into the molten iron bath from a water cooled lance inserted through the converter mouth. Present day BOS (basic oxygen steelmaking) plants are developments of the L- D Process.

  • Leaded Steels

    When added to steel, lead does not go into solution but exists in a very finely divided state along the grain boundaries. It greatly assists machinability as it acts as a lubricant between the steel and the tool face. Lead is normally added in amounts between 0.15-0.35% and when combined with similar amounts of sulphur, optimum machinability is attained as in such steel as BS 970 230M07 Pb.

  • Li

    Chemical symbol for Lithium.

  • Limiting Range of Stress

    The greatest range of stress that a metal can withstand for an indefinite number of cycles without failure. If exceeded, the metal fractures after a certain number of cycles, which decrease as the range of stress increases.

  • Limiting Ruling Section

    The maximum diameter of cross section of a bar or component in which certain specified mechanical properties are achieved after heat treatment.

  • Limits

    A term used to determine a minimum and maximum. In a mechanism, it should denote the minimum and maximum sizes for each part, between which the parts will function properly in conjunction with each other and outside of which they will not. The words “limits” and “tolerances” are often interchanged, “tolerance” represents the difference between the minimum and maximum limits.

  • Limits of Proportionality

    The stress (load divided by original area of cross section of the test piece) at which the strain (elongation per unit of gauge length) ceases to be proportional to the corresponding stress. It is usually determined from a load-elongation diagram, obtained by plotting extensometer readings and is the stress at which the load-elongation line ceases to be straight.

  • Liquid Carburising

    A widely used method of case-hardening steel that eliminates scaling and the tendency to decarburisation and results in clean components. Sodium cyanide is the common media for this process, usually heated within the range of 900-930oC. It is advisable to pre-heat the components in neutral salts to avoid a temperature drop resulting from immersing cold components into the cyanide. After carburising, either single quench hardening or refining and hardening and tempering is carried out.

  • Machinability

    Simply defined as a measure of the ease with which a metal can be machined satisfactorily.

  • Macrostructure

    The general crystalline structure of a metal and the distribution of impurities seen on a polished or etched surface by either the naked eye or under low magnification of less than x10.

  • Magnetic Crack Detection

    The bar or component to be tested is magnetised by passing a heavy current through it or by making it the core of a coil through which a heavy current is passed. Cracks or inclusions cause the magnetic flux to break the surface forming free magnetic poles. When the component is sprayed with a suspension of finely divided magnetic particles they collect at the free poles to visibly show the presence of defects.

  • Malleability

    It can be defined as the property of a metal to be deformed by compression without cracking or rupturing. The load may be applied slowly or suddenly and will determine whether the material will be suitable for forging or rolling into thin sheet.
    Manganese One of the most important constituents of steel in which it fulfils a number of functions. It acts as a mild de-oxidising agent. It combines with the sulphur present to form globular inclusions of Manganese Sulphide which are beneficial to machining. It increases tensile strength and the hardenability of steel.

  • Martempering

    A heat treatment involving austenitisation followed by step quenching, at a rate fast enough to avoid the formation of ferrite, pearlite or bainite to a temperature slightly above the Ms point. Soaking must be long enough to avoid the formation of bainite. The advantage of martempering is the reduction of thermal stresses compared to normal quenching. This prevents cracking and minimises distortion.

  • Martensite

    The hard constituent produced when steel is cooled from the hardening temperature at a speed greater than its critical cooling rate. Martensite is an acicular phase when seen in the microstructure of steel.

  • Mass Effect

    A term used to signify the effect of size and shape during heat treatment, since it is the rate of cooling of a piece of steel which determines the properties resulting from the hardening and quenching process.

  • Matrix

    The mass or principal constituent (e.g. iron in the case of steel) in which other constituents are embedded.
    Maximum Stress In the testing of the strength of steel a sample is machined into a standard test piece and is stretched in a tensile testing machine until it breaks. The results are expressed in N/mm2 and is the value of the maximum load reached in the test divided by the original cross sectional area of the specimen.

  • McQuaid EHN Grain Size Test

    A method of assessing grain size. It consists of a test piece at 927oC for 8 hours by slow cooling and subsequent microscopical examination. The grain size is measured at x100 magnification and compared to standard charts, the figures range from No. 1 – very coarse, to No. 8 – very fine.

  • Meehanite

    A trade name applied to a certain type of cast iron.

  • Melting Point

    The temperature at which a solid begins to liquefy.

  • Mg

    Chemical symbol for Magnesium.

  • Micron

    A unit of length equal to one millionth of a metre (0.001mm).

  • Microstructure

    The structure that is observed when a polished and etched specimen of metal is viewed in an optical microscope at magnifications in range of approximately x25 to x1500.

  • Mn

    Chemical symbol for Manganese.

  • Mo

    Chemical symbol for Molybdenum.

  • Modulus of Elasticity

    When a material is subjected to an external load it becomes distorted or strained. With metals, provided the loading is not too great, they return to their original dimensions when the load is removed, i.e. they are elastic. Within the limits of elasticity, the ratio of the linear stress to the linear strain is termed the modulus of elasticity or more commonly known as Young’s Modulus.

  • Molybdenum

    Its use as an alloying element in steel increases hardenability and in low alloy steels reduces the risk of temper brittleness. When added to stainless steels it increases their resistance to corrosion. It is also used in high speed steels.

  • N

    Chemical symbol for Nitrogen.

  • Na

    Chemical symbol for Sodium.

  • Nb

    Chemical symbol for Niobium.

  • Ni

    Chemical symbol for Nickel.

  • Nickel

    One of the most widely used alloying elements in steel. In amounts 0.50% to 5.00% its use in alloy steels increases the toughness and tensile strength without detrimental effect on the ductility. Nickel also increases the hardenability, thus permitting the steel to be oil- hardened instead of water quenched. In larger quantities, 8.00% and upwards, nickel is the constituent, together with chromium, of many corrosion resistant and stainless austenitic steels.

  • Niobium

    Also known as columbium. Niobium is a strong carbide forming element which is added to certain 18/8% chromium-nickel stainless steels as a stabiliser to prevent inter-granular corrosion arising from welding.

  • Nitriding

    A case hardening process that depends on the absorption of nitrogen into the steel. All machining, stress relieving, as well as hardening and tempering are normally carried out before nitriding. The parts are heated in a special container through which ammonia gas is allowed to pass. The ammonia splits into hydrogen and nitrogen and the nitrogen reacts with the steel penetrating the surface to form nitrides. Nitriding steels offer many advantages: a much higher surface hardness is obtainable when compared with case-hardening steels they are extremely resistant to abrasion and have a high fatigue strength.

  • Nitrogen

    Nitrogen is a gas that forms approximately 79% by volume or 77% by weight of the atmosphere. It can combine with many metals to form nitrides and is thus applied to the case-hardening of steel, the usual source for this purpose being ammonia.

  • Noble Metals

    Metals such as gold, silver and platinum which are resistant to corrosion by all but the most powerful acids.

  • Non Destructive Testing

    Those forms of testing that do not result in permanent damage or deformation to the part being tested. Typical examples are magnetic crack detection, ultrasonic inspection, X-Ray inspection and gamma radiography.

  • Non Magnetic Steels

    Austenitic steels such as the 14% manganese steels and the 303 type 18/8% chromium-nickel stainless steels.

  • Normalising

    A heat treatment process that has the object of relieving internal stresses, refining the grain size and improving the mechanical properties. The steel is heated to 800-900oC according to analysis, held at temperature to allow a full soak and cooled in still air.

  • Notched Bar Test

    A test to determine the resistance of a material to a suddenly applied stress, i.e. shock. A notched test piece is employed in an Izod or Charpy machine and the results are recorded in ft.lbs. or Joules.

  • O

    Chemical symbol for Oxygen.

  • Occlusion

    A term applied, in the case of metals, to the absorption or entrapment of gases.

  • Oil Hardening Steel

    Used to describe tool or alloy steels where oil is used as the quenching medium in the hardening process.

  • Open Hearth Furnace

    Developed in the middle of the last century, the open hearth or Siemens-Martins process, as it is known, accounted for a major proportion of UK steel
    production until the early 1970′s. For economic and quality reasons it has been replaced by the Electric Arc Furnace and the Basic Oxygen Steelmaking process. There are no open hearth furnaces in use in Britain today but they are still in use in Russia and Eastern Europe.

  • Orange Peel Effect

    An effect that arises on the surface of steel sheets when they are stretched beyond their elastic limit.

  • Ore

    An ore is a material that contains a metal in such quantities that it can be mined and worked commercially to extract that metal. The metal is usually contained in chemical combination with some other element in addition to various impurities.

  • Os

    Chemical symbol for Osmium.

  • Overheating

    Failure of tools and components in heat treatment can arise through overheating. This may be caused due to quenching from a temperature too high for the type of steel involved. Overheating is evidenced by cracking, grain-coarseness, erratic surface hardness and pitting.

  • Oxidation

    A common form of chemical reaction which is the combining of oxygen with various elements and compounds. The corrosion of metals is a form of oxidation, rust on iron for example is iron oxide.

  • Oxy-Acetylene Welding

    A process for joining two pieces of metal in which the required high temperature is obtained by the combustion of acetylene gas and oxygen. The gases are thoroughly mixed in the nozzle or tip of the welding torch to ensure perfect combustion. The weld may be formed directly between two adjoining surfaces, but usually metal from a welding rod is fused in between the surfaces of the joint.

  • Oxygen

    Oxygen is one of the chief constituents of the atmosphere of which it forms approximately one fifth. It is odourless and invisible. Although oxygen itself does not burn it is extremely efficient in supporting combustion, nearly all other chemical elements combine with it under evolution of heat. It has many uses in industry and is essential to the BOS (Basic Oxygen Steelmaking Process).

  • P

    Chemical symbol for Phosphorus.

  • Parkerising

    A chemical treatment applied to ferrous metals to improve their corrosion resistance. The process is based on a manganese phosphate solution which produces a fairly thick coating. This can subsequently be painted or impregnated with oil. Patenting A heat treatment process often applied to high carbon wire. The steel is heated to a suitable temperature well above the transformation range, followed by cooling in air or a bath of molten lead or salt. A structure is produced suitable for subsequent cold drawing and which will give the desired mechanical properties in the finished state.

  • Pb

    Chemical symbol for Lead.

  • Pd

    Chemical symbol for Palladium.

  • Pearlite

    A lamellar constituent of steel consisting of alternate layers of ferrite (alpha-iron) and cementite (iron Carbide Fe3C) and is formed on cooling austenite at 723oC. This produces a tough structure and is responsible for the mechanical properties of unhardened steel.

  • pH Value

    A method of expressing differences in the acidity or alkalinity of a solution. A figure of 7 is regarded as neutral, figures below this indicate the decree of acidity and above alkalinity.

  • Phosphorus

    An element that forms 0.12% of the earth’s crust, chiefly in the form of phosphates. Its presence in steel is usually regarded as an undesirable impurity due to its embrittling effect, for this reason its content in most steels is limited to a maximum of 0.050%.

  • Pickling

    A process to chemically remove scale or oxide from steel to obtain a clean surface. When applied to bars or coils prior to bright drawing, the steel is immersed in a bath of dilute sulphuric acid heated to a temperature of around 80oC. An inhibitor is added to prevent attack and pitting of the cleaned metal. After pickling, a washing process takes place followed by immersion in a lime-water bath to neutralise any remaining acid. For environmental reasons shot blasting has largely replaced pickling.

  • Pig Iron

    The product of the blast furnace. The term was derived from the method of casting the bars of the pig iron in depressions or moulds formed in the sand floor adjacent to the furnace. These were connected to a runner (known as a sow) and when filled with metal the runner and the numerous smaller moulds were supposed to resemble a litter of suckling pigs, hence the term pig iron.

  • Pinch Pass

    A term applied when, after annealing, sheet or strip is lightly rolled with the object of preventing stretcher lines or kinks on subsequent cold working.

  • Pipe

    A defect that arises during the solidification of steel in the ingot mould. As steel contracts on solidification a central cavity forms in the upper portion of the ingot, if this is not completely removed before rolling into bars a central defect known as “pipe” results. The risk of piping is considerably reduced on continuously cast steel due to molten steel being available to fill any shrinkage cavity.

  • Poisson’s Ratio

    If a square bar is stressed in a testing machine in the direction of its length so that the length increases, there is a contraction in each opposite direction, which produces a decrease in the thickness of the bar. The ratio between the contraction at right angles to a stress and the direct extension is called the Poisson’s ratio. Its value in steel is in the order of 0.28.

  • Pot Quenching

    Quenching carburised parts directly from the carburising pot or box.

  • Powder Metallurgy

    A method of producing components by pressing or moulding metal powders which may be simultaneously or subsequently heated to produce a coherent mass.

  • Pre-Heating

    Used in the hardening process. Tools are pre- heated before heating to the final temperature, this is particularly important in tools of complex shape to prevent distortion or cracking. Pre-heating reduces the time of exposure to the hardening temperature and helps to minimise scaling and decarburisation.

  • Projection Welding

    A welding process that uses small projections on one or both components of the weld to localise the heat and pressure, the projections collapse when the weld is made.

  • Proof Stress

    The stress that will cause a specified small, permanent extension of a tensile test piece. Commonly the stress to produce 0.2% extension is quoted in N/mm2 for steel. This value approximates to the yield stress in materials not exhibiting a definite yield point.

  • Quenching

    Rapid cooling from a high temperature by immersion in a liquid bath of oil or water. Molten salts may also be used.

  • Quenching Crack

    A fracture, often termed a hardening crack, which arises from thermal stresses induced during rapid cooling.

  • Ra

    Chemical symbol for Radium.

  • Radiography

    A method of non-destructive testing. Internal examination of a metallic structure or component is carried out by exposing it to a beam of X-Ray or gamma radiation. Internal defects can be seen on a screen or recorded on film.

  • Rb

    Chemical symbol for Rubidium.

  • Re

    Chemical symbol for Rhenium.

  • Re-crystallisation

    The re-arrangement of crystals in cold worked metal brought about by heating so that the deformed crystals are absorbed by newly-formed crystals and the effects of work hardening are removed. Also occurs when steel is heated through the transformation range and when steel is hot worked.

  • Red Hardness

    A term sometimes associated with high speed steel because it has the property of retaining sufficient hardness for cutting metals even when heated to a temperature high enough to cause a dull redness. The tungsten content has a significant influence on this property.

  • Reduction of area

    The percentage decrease in the cross- sectional area of a tensile test piece caused by wasting or necking of the specimen. It is expressed as a percentage of the original area of the test piece and is a measure of ductility.

  • Refining

    (a) The removal of impurities and metallic oxides from the molten bath by the reaction of the slag and other additions.
    (b) A heat treatment process with the object of refining or making the grain size of the steel uniform.

  • Residual Stress

    The stress which exists in an elastic solid body in the absence of, or in addition to, the stresses caused by an external load. Such stresses can arise from deformation during cold working such as cold drawing or stamping, in welding from weld metal shrinkage, and in changes in volume due to thermal expansion.

  • Rh

    Chemical symbol for Rhodium.

  • Rockwell Hardness Testing

    A method for testing the hardness of metals by determining the depth of penetration of a steel ball or a diamond sphero-conical indentor. The value is read from a dial and is an arbitrary number related to the depth of penetration. For testing hard steels, a sphero-conical diamond is used with a 150 kg load, the result is read from the black scale on the dial and is prefixed with the letter C. A hardened tool steel would typically give a reading of 62Rc. For softer metals Scale B is used with a 1/16″ diameter steel ball and a standard load of 100 kgs.

  • Rolling

    The process of shaping metal by passing it between rolls revolving at the same peripheral speed and in opposite directions. In steel there are a number of different types of rolling mill for processing the ingot to its finished shape. These are variously known as Cogging mills, Slabbing mills, Billet mills, Bar mills and Strip mills, which produce plate, sections, bars, sheet and strip. Cold rolling of previously hot rolled strip is carried out to produce strip that is accurate to size and with a smooth bright polished surface.

  • Rolling Lap

    A fault arising from the overfilling or mis- alignment of rolls, the result is a bulge on the bar which is rolled into the metal and is lapped over. It remains throughout subsequent working and appears as a longitudinal crack.

  • Ru

    Chemical symbol for Ruthenium.

  • Ruling Section

    More accurately termed limiting ruling section. One of the most important factors associated with the choice of steel for a given purpose is to ensure that the desired mechanical properties are obtained throughout the section when the material has been heat treated. The limiting ruling section determines the maximum diameter or cross-section of a bar or component in which the specified properties can be achieved by a given heat treatment. The analysis of the steel also has an important bearing on this.

  • S

    Chemical symbol for Sulphur.

  • Salt Bath

    A method of heating steel using a bath of molten salts. Salt baths give uniform heating and prevent oxidation, they are used for hardening, tempering or quenching. The type of salt used depends on the temperature range required. For hardening, sodium cyanide, sodium carbonate and sodium chloride are in common use.

  • Sb

    Chemical symbol for Antimony.

  • Scale

    The oxidised surface of steel produced during hot working, as in rolling, and by exposure to air or steam at elevated temperature.

  • Scarfing

    Also termed deseaming. It is a process for burning out defective areas on the surface of ingots or semi-finished products such as billets so that the product is suitable for subsequent rolling or forging.

  • Scrap

    It forms the basic raw material for making steel by the electric arc process. Steel offers ecological advantages as it can be recycled enabling the discarded car of today to appear as part of a new model tomorrow. Scrap is sorted and graded before use and the necessary elements are added during the steel making process to achieve the desired specifications.

  • Se

    Chemical symbol for Selenium. Seams A surface defect caused during the steel making process. Seams are generally formed from blow holes in the ingot, non metallic inclusions, or stresses arising during the solidification stage. They appear as longitudinal discontinuities in the bar.

  • Secondary Hardness

    An increase in hardness which sometimes occurs when hardened steel is re-heated. It can be caused by the transformation of retained austenite to martensite or by the precipitation of alloy carbides.

  • Segregation

    A term applied to the concentration and partial separation of one or more elements from solution during solidification of liquid steel in an ingot mould. Sulphur and phosphorus tend to segregate to a greater extent than other elements which can have a particular adverse effect on machinability in high sulphur free- cutting steels. Modern steel making and continuous casting have largely overcome this problem.

  • Selenium

    An element that closely resembles sulphur in its properties. The main use in steel is as a freecutting additive but due to high cost its use is limited to stainless steel. One of the benefits being the ability to obtain a very good surface finish on machined components.

  • SG Iron

    An abbreviation for Spheroidal Graphite Cast Iron. As the name implies, graphite is present in spheroidal form instead of flakes and compared with Grey Cast Iron it has higher mechanical strength, ductility and increased shock resistance.

  • Shearing Test

    The test applied to metal to determine the stress required to fracture it across its section.

  • Sherardizing

    A process developed in Britain in 1904 by Sherard Cowper-Coles. It is a method of producing a protective zinc coating on iron and steel products.

  • Shore Scleroscope

    An instrument that measures the hardness of a sample in arbitrary terms of elasticity. A diamond tipped hammer is allowed to fall freely down a graduated glass tube on to the sample under test. The hardness is measured by the height of the rebound. In another form the rebounding hammer actuates the pointer of a scale so that the height of the rebound is recorded.

  • Spinning

    The formation of sheet metal blanks into hollow circular shapes. This is carried out on a lathe with forming tools which service to press and shape the metal. Annealing may be needed during and/or after the operation to remove the effects of work hardening.

  • Spot Welding

    A process for joining steel sheets. The two parts are held between electrodes and the heat generated at the interface between the sheets causes local welding when pressure is applied.

  • Spring Steel

    The steels used for spring making depend on the application and type of spring. They range from plain carbon grades in the range 0.5% to 1.00% C. to Chromium, Chromium-Vanadium, Nickel-Chromium-Molybdenum, Silico- Manganese and Silicon-Manganese-Chromium-Molybdenum types. Full details can be found in BS5770.

  • Stabilisation

    A term applied to a number of processes: a) A type of heat treatment to relieve internal stresses: b) The retarding or prevention of a particular reaction by the addition of a stabilising element c) A thermal and/or mechanical treatment given to magnetic material in order to increase the permanency of its magnetic properties or condition.

  • Stainless Steel

    Can be defined as a group of corrosion resisting steels containing a minimum 10% chromium and in which varying amounts of nickel, molybdenum, titanium, niobium as well as other elements may be present. An Englishman, Harry Brearley, is generally acknowledged to be the pioneer who developed stainless steels for commercial use.

  • Steel

    Generally defined as a metallic product whose principal element is iron and where the carbon content is not more than 2%. (The presence of large quantities of carbide forming elements may modify the upper limit of the carbon content.)

  • Strain Ageing

    The gradual changes in physical and mechanical properties, in particular hardness and tensile strength, which takes place following cold rolling or deformation. At atmospheric temperatures, this may take place over a number of weeks but can be accelerated by heating.

  • Strain Hardening

    The loss of ductility and gain in hardness resulting from strain ageing.

  • Stress Relieving

    A heat treatment including heating and soaking at a suitable temperature (e.g. 600-650oC) followed by cooling at an appropriate rate in order to reduce internal stresses without substantially modifying the steel’s structure. This treatment may be used to relieve stresses induced by machining, quenching, welding or cold working.

  • Stress Strain Curve

    A graph in which stress (load divided by the original cross sectional area of the test piece) is plotted against strain (the extension divided by the length over which it is measured).

  • Sub-Critical Annealing

    Heating to, and holding at, some point below the critical temperature. Subsequent cooling may be in air. This form of heat treatment has a variety of uses depending on the temperature and specification of the steel, its purpose is often to soften the material.

  • Sub-zero Treatment

    A low temperature treatment carried out after quenching on hardened steel to transform the retained austenite into martensite. It involves immersing the component in a bath of solid carbon dioxide at a temperature of minus 70-80oC.

  • Sulphur

    Generally regarded as an impurity in steel as it can have detrimental effects on strength, ductility and weldability as well as producing hot and cold shortness. Its content in most steels is limited to a maximum of 0.050%. Sulphur is beneficial to machining and is added to freecutting steels in amounts up to 0.35% with the manganese content increased to overcome any detrimental effects.

  • Surface Hardening

    A method of hardening the surface of steel to increase its wear resistance. Depending on the analysis of the steel, the following treatments can be employed: Case-hardening, Nitriding, induction hardening, Flame hardening.

  • Swaging

    A method of forming or reducing steel or other metals to a desired shape by a series of blows rapidly applied by dies or hammers. The process is applied to wires, rods and tubes and can be used for a variety of pointing, tapering, sizing and reducing operations.

  • Swarf

    The particles of metal arising from machining or grinding operations, much of it finds its way to the steel maker for remelting.

  • Ta

    Chemical symbol for Tantalum.

  • Tantalum

    A rare metal of silver white colour having excellent corrosion resistance and a high melting point. It is widely used for chemical process equipment and specialised aero-space and nuclear applications.

  • Te

    Chemical symbol for Tellurium.

  • Tellurium

    Its main use in the steel industry is as an additive in leadbearing freecutting steels to further improve their machinability. Its presence in the steel is either within the manganese sulphide particles, where it is partially soluble, or as particles combined with lead or manganese. For certain applications it offers significant improvements in machinability but the added cost is a factor that should be taken into account.

  • Temper

    A term to which a number of definitions can be applied. These include: a) The operation of tempering b) The degree of hardness left in a steel bar after quenching and tempering c) The grading of the hardness of low carbon cold rolled strip, e.g. Hard, Half Hard, Quarter Hard, Skin Passed, Soft d) An indication of the amount of carbon present in a tool steel, e.g. razor temper, file temper, die temper, etc.

  • Temper Brittleness

    The loss in impact resistance that is present in some low and medium carbon alloy steels when tempered in the range of 350oC – 600oC. It is revealed by the notched bar impact test but not the tensile test.

  • Temper Colours

    Before the use of instruments such as pyrometers, colours were used to judge temperatures when hardening and tempering. For example, on carbon tool steel where the tempering range may typically be from 200oC to 350oC, the colours change with the rise in temperature giving Light Straw at around 210oC, Purple at 275oC, and Grey at 330oC. The practice still continues in workshops where controlled heat treatment facilities are not available.

  • Temper Rolling

    A light pass given to annealed cold rolled strip to prevent the formation of kinks and stretcher strain markings on subsequent cold working. Also termed Pinch pass and Skin pass.

  • Tempering

    A heat treatment applied to ferrous products after hardening. It consists of heating the steel to some temperature below the transformation range and holding for a suitable time at the temperature, followed by cooling at a suitable rate. The object of tempering is to decrease hardness and increase toughness to produce the desired combination of mechanical properties.

  • Tensile Strength

    The maximum load applied in breaking a tensile test piece divided by the original cross-sectional area of the test piece. Originally quoted as tons/ it is now measured as Newtons/ Also termed Maximum Stress and Ultimate Tensile Stress.

  • Tensile Test

    A standard test piece is gripped at either end by suitable apparatus in a testing machine which slowly exerts an axial pull so that the steel is stretched until it breaks. The test provides information on proof stress, yield point, tensile strength, elongation and reduction of area.

  • Thomas Process

    The Continental name for the basic Bessemer steel making process, now superseded by modern day BOS plants.

  • Ti

    Chemical symbol for Titanium.

  • Time Temperature Transformation Curve

    An isothermal transformation diagram showing the relationship between temperature and the time taken for the decomposition of austenite when the transformation occurs at constant temperature.

  • Tin

    When present in steel it is an undesirable impurity which gives rise to temper brittleness. When used as a coating on steel, it has a good resistance to corrosion for many applications.

  • Titanium

    Small amounts added to steel contribute to its soundness and give a finer grain size. In austenitic stainless steels it acts as a carbide stabiliser and is used to prevent intercrystalline corrosion, commonly termed “weld decay”. Titanium carbide is also used with tungsten carbide in the manufacture of hard metal tools.

  • Tolerances

    The amount of variation permitted on dimensions or surfaces. The tolerance is equal to the difference between the maximum and minimum limits of any specified dimension.

  • Tool Steel

    A generic term applied to a wide range of steels, both plain carbon and alloy. It includes steels suitable for various types of cutting tools, press tools, hot and cold heading dies, moulds for plastics and die- casting, extrusion tools, hand tools, etc.

  • Torsional Strength

    The resistance of a bar to twisting. Closely related to its shear strength.

  • Toughness

    The ability of a metal to rapidly distribute within itself both the stress and strain caused by a suddenly applied load, or more simply expressed, the ability of a material to withstand shock loading. It is the exact opposite of “brittleness” which carries the implication of sudden failure. A brittle material has little resistance to failure once the elastic limit has been reached.

  • Transformation Range

    The temperature range within which austenite forms and ferrite or carbide progressively dissolves while ferrous alloys are being heated. Also the temperature range within which austenite decomposes to form ferrite and carbide on cooling.

  • Transformation Temperature

    The temperature at which a change in phase occurs or the limiting temperature of a transformation range. These critical points are denoted by symbols, e.g. Ac1 the temperature at which austenite begins to form on heating. There are 12 principal temperatures to which symbols are applied.

  • Transition Temperature

    The temperature at which a transition from ductile to brittle fracture takes place in steel. It is usually determined by making a series of Charpy impact tests at various temperatures, the transition temperature is usually taken as the point where 50% of the fracture is brittle.

  • Transverse Strength

    A measurement of strength when the load is applied across the longitudinal flow of the grain of a metal. Certain impurities such as sulphur have a detrimental effect on the transverse strength. This can be minimised by the inclusion modification process.

  • Transverse Test

    A test taken at right angles to the principal direction of rolling or forging.

  • TTT Curve

    An abbreviation of Time Temperature Transformation Curve.

  • Tufftriding

    A form of surface hardening, the process involves nitrogen but does not achieve the hardness of conventional nitriding.

  • Tungsten

    When used as an alloying element it increases the strength of steel at normal and elevated temperatures. Its “red hardness” value makes it suitable for cutting tools as it enables the tool edge to be maintained at high temperatures. In conjunction with other alloying elements it finds applications in heat resisting and other severe service conditions.

  • U

    Chemical symbol for Uranium.

  • Ultimate Analysis

    In chemistry, this is a quantitative analysis in which percentages of all elements in the substance are determined.

  • Ultimate Tensile Strength

    The highest load applied in breaking a tensile test piece divided by the original cross- sectional area of the test piece.

  • Ultrasonic Inspection

    A means of locating defects in steel. When acoustic energy in the ultrasonic range is passed through steel, the sound waves tend to travel in straight lines, rather than diffusing in all directions as they do in the audible range. If there is a defect in the path of the beam it will cause a reflection of some of the energy, depleting the energy transmitted. This casts an acoustic shadow which can be monitored by a detector placed opposite the transducer or energy source. If the acoustic energy is introduced as a very short burst, then the reflected energy coming back to the originating transducer can also be used to show the size and depth of the defect. Ultrasonic techniques can be used to detect deeply located defects or those contained in the surface layer. Skill and experience are required in interpreting the results portrayed on the cathode ray tube.

  • Unkilled Steel

    Steel which has been insufficiently deoxidised and evolves gas during solidification with the formation of blow-holes.

  • Upsetting

    Working a piece of steel so that its length is shortened and its cross-sectional area is increased. Its effect is to increase ductility in the radial and tangential directions.

  • Uranium

    A white malleable metal which is softer than steel. Its specific gravity is 18.7, it melts at a temperature of 2400oC.

  • V

    Chemical symbol for Vanadium.

  • Vacuum Arc Remelting

    A process used for producing advanced steels to the most demanding and critical specifications, particularly in such areas as aerospace applications. The steel is first produced to a very close analysis and the resulting ingot is slowly remelted in a Vacuum Arc Remelting furnace for up to 14 hours. Such steels are, by necessity, expensive to manufacture.

  • Vacuum Degassing

    A ladle of molten metal is placed within a chamber which is then evacuated. This reduces the gas content, particularly hydrogen, as well as reducing non- metallic inclusions. Modern secondary steel making processes using Vacuum Arc Degassing units that include automated stirring and control of temperature and chemical analysis, ensure a consistent and high quality product.

  • Vanadium

    Steels containing vanadium have a much finer grain structure than steels of similar composition without vanadium. It raises the temperature at which grain coarsening sets in and increases hardenability where it is in solution in the austenite prior to quenching. It also lessens softening on tempering and confers secondary hardness on high speed steels. Vanadium is used in nitriding, heat resisting, tool and spring steels in conjunction with other alloying elements.

  • Vickers Hardness Test

    A method of determining the hardness of steel whereby a diamond pyramid is pressed into the polished surface of the specimen and the diagonals of the impression are measured with a microscope fitted with a micrometer eye piece. The rate of application and duration are automatically controlled and the load can be varied.

  • W

    Chemical symbol for Tungsten, from wolfram.

  • Welding

    The process of joining together two pieces of metal so that bonding accompanied by appreciable interatomic penetration takes place at their original boundary surfaces. The boundaries more or less disappear at the weld, and integrating crystals develop across them. Welding is carried out by the use of heat or pressure or both and with or without added metal. There are many types of welding including Metal Arc, Atomic Hydrogen, Submerged Arc, Resistance Butt, Flash, Spot, Stitch, Stud and Projection.

  • Whiskers

    Thin hair-like growths on metal that are barely visible to the naked eye, they are stronger than the metals from which they are formed, probably because they are free from defects.

  • White Annealing

    A heat treatment process carried out on pickled steel with the objective of eliminating the hydrogen that has entered the steel during the pickling operation and thus removing any tendency to hydrogen embrittlement.

  • Widmanstatten Structure

    A microstructure resulting when steels are cooled at a critical rate from extremely high temperatures. It consists of ferrite and pearlite and has a cross-hatched appearance due to the ferrite having formed along certain crystallographic planes.

  • Wolfram

    The alternative name for tungsten.

  • Woody Fracture

    A fracture that is fibrous or woody in appearance due to the elongation of the individual grains. This may be accentuated by the presence of slag or by a banded structure. It is grey and dull and is characteristic of ductile but non-homogeneous material such as wrought iron.

  • Work Hardening

    The increase in hardness and strength produced by cold plastic deformation or mechanical working.

  • Wrought Iron

    A commercial iron that has little use today and has been replaced by mild steel. It was commonly produced by the puddling process. The temperatures employed in its production are too low to render it fluid, it is heated until it forms a pasty mass then it is squeezed or forged. The process does not lend itself to removal of impurities so it contains an appreciable quantity of slag. It will not respond to any heat treatment designed to increase the hardness or strength.

  • X-Ray Crystallography

    X-ray photographs of metals are a means of providing information which in many cases cannot be obtained by microscopic methods. The lines produced by each element, or phase are characteristic, and their general pattern enables the crystalline structure to be identified. The scale of the pattern can be used to determine accurately the size of the unit cell and, therefore, the distance apart of the individual atoms. From the relative intensity of the lines it is possible to deduce the distribution throughout the unit cell, the various types of atoms in an alloy or the degree of preferred orientation in the material.

  • Yield Point

    Can be defined as the point where a tensile test piece begins to extend permanently. If the load is reduced to zero, the test piece will not return to its original length.

  • Yield Strength

    The stress at which general plastic elongation of the test piece takes place. This point is well defined in hardened and tempered or annealed structures but can be ill defined in “as drawn” structures.

  • Young’s Modulus

    Within the limits of elasticity, the ratio of the linear stress to the linear strain is termed the modulus of elasticity or Young’s Modulus and may be written Young’s Modulus, or E =(Stress/Strain) It is this property that determines how much a bar will sag under its own weight or under a loading when used as a beam within its limit of proportionality. For steel, Young’s Modulus is of the order of 205000 N/mm2.

  • Zinc

    Zinc is a metallic chemical element it has a white colour with a bluish tinge. It has a high resistance to atmospheric corrosion and a major use is as a protective coating for iron and steel sheet and wire. Galvanised sheets are a prime example. The melting point of zinc is 419oC.

  • Zirconium

    Acts as a deoxidising element in steel and combines with sulphur.

  • Zn

    Chemical symbol for Zinc.

  • Zr

    Chemical symbol for Zirconium.