Metallurgical Terms

C

C
Chemical symbol for Carbon

Ca
Chemical symbol for Calcium

Carbide
A chemical combination of carbon with iron or any other elements, e.g., Fe3C (cementite). Metallic carbides are hard and brittle; certain of them, of which the principal are tungsten carbide and titanium carbide, are the chief constituents of the hard metals used for cutting tools.

Carbon Case Hardening
The term is usually applied to the complete process of carburizing or cyaniding low carbon steels, with or without one or more further heating to promote grain refinement, followed by quenching in order to produce a hard case. The high carbon content of the exterior of the steel so treated renders the surface hard and wear-resistant, whilst the low carbon content of the core leaves it tough and ductile.

Carbon Steel
Steel whose properties are determined primarily by the percentage of carbon present. It is usual to consider that besides iron and carbon, such steels may contain a maximum of manganese up to 1•5 %, silicon up to 0•5%, sulphur and phosphorus up to 0•1%, nickel up to 0•40%, chromium up •to 0•30 %, molybdenum up to 0•13 %, copper up to 0•25 %, tungsten, cobalt, aluminum up to 0•10 %, and niobium, tantalum, titanium, vanadium, zirconium up to 0•05%. These alloying elements in such quantities are regarded as residual elements, but their deliberate addition in substantial amounts will put the steel in the alloy steel category.

Carbonitriding
A case hardening process in which steel is heated in an atmosphere containing both carbon and nitrogen. Steels of the case hardening type are normally used, and since the absorption of nitrogen depresses Ac1, and increases hardenability the temperature and alloy content need not be as high as those used in case carburizing. A typical treatment would be: heat at 825˚C for two hours in a carburizing gas to which about 10% anhydrous ammonia has been added, and quench in oil; but many variations of these conditions can be employed to give the required depth and hardness of case.

Carburizing
The introduction of carbon into the surface layer of steel having low carbon content (case hardening steel). It may be affected by heating in a solid, liquid or gaseous carbon-containing medium, which at high temperatures provides a supply of carbon for absorption by the material being carburited. By controlling the temperature and time of treatment, the concentration of carbon in the surface of the steel and the depth of penetration may be varied. In the original process of box- or pack-carburizing, the steel is heated to the necessary temperature in a solid carburizing compound, usually a mixture of hardwood charcoal and an oxide or carbonate of the alkali’s or alkaline earths. Gas carburizing gives better control over the carbon content of the case. (See Carbon Case Hardening).

Case
The surface layer of steel which has been made substantially harder than the interior or core by a process of case hardening.

Case Hardening
A method of hardening the surface of a metal whilst keeping the interior soft. The term is usually applied to carbon case hardening but nitriding and flame hardening is also case hardening processes.

Case Hardening Steel
Low carbon steel suitable for carbon case hardening.

Cast Iron
Iron which has been cast into shape and which contains a total carbon content varying between about 1•8% and 4•5%, the carbon being present in excess of the amount which can be retained in solid solution in austenite at the eutectic temperature. In addition to carbon, there are also present, varying amounts of silicon, manganese, sulphur and phosphorus. Cast irons can be divided into several types. In Grey cast iron all or part of the carbon is in the form of graphite flakes which are responsible for the inherently poor shock resistance and relatively low strength. In Spheroidal graphite (or nodular) iron the graphite is in spheroidal or nodular form. This is achieved by adding controlled amounts of magnesium or cerium before casting. Mechanical strength and shock resistance are greatly increased. White cast iron has its carbon almost entirely in combination with iron as cementite (Fe3C) and is consequently very hard, brittle and practically unmachinable. When broken the surface is white. Malleable cast iron is made from white iron by annealing at about 850˚C to remove carbon (white-heart process) or to convert the cementite to rosettes of graphite (black-heart process). It is more ductile than either grey or white iron. Alloy cast iron contains alloy additions such as nickel, chromium and silicon in amounts sufficient to give special properties such as improved resistance to corrosion or abrasion.

Cast Steel
(a) A term, originally applied to crucible steel to distinguish it from shear steel, and still used for high carbon tool steel.
(b) The term also covers steel which has solidified from the molten condition in a mould and hence undergoes no further change of shape, e.g., by forging or rolling, except for such minor modifications as may be involved in machining.

Caustic Embrittlement
A type of intercrystalline stress corrosion cracking produced below the liquid level in mild steel and low-alloy steel boilers. Failure is at riveted joints or crevices where concentration of caustic can occur associated with a high internal or external stress. Welded boilers are normally free from this type of attack.

Cb
Chemical symbol for Columbium (see Niobium)

Ce
Chemical symbol for Cerium

Cementation
(a) The process of introducing elements into the outer layer of metal objects by means of high temperature diffusion.
(b) (Converting process). In this process, best quality Swedish wrought iron bars were packed in layers separated and surrounded by charcoal in pots, which were slowly heated to a temperature of about 1100˚C and maintained at that temperature for a period of 7 to 10 days according to the desired carbon content, and then allowed to cool slowly, the whole process occupying about three weeks. During this operation, carbon provided by the charcoal, diffused into the iron, and some of it, reacting with the slag content of the wrought iron, liberated carbon monoxide which formed blisters on the surface of the bar, hence the name blister bar, and blister steel; or converted or cemented bar, or cemented steel, or plated bar. Some six of these were then piled together, placed in a clip, raised to a white heat, and hammered into a faggot or small bloom. This was known as single shear steel. For the production of double shear steel the faggot was nicked, bent back on itself, re-heated and hammered down again to its original size. The process is now obsolete; the last cementation furnace in the country is now preserved at the Corporate Laboratories of the British Steel Corporation, Sheffield.

Cemented Bar
(See Cementation)

Cement Carbides
(See Hard Metals)

Cementite
The iron carbide (Fc3C) constituent of steel and cast iron. It is hard, brittle and crystalline and contains 6•67 % of carbon by weight.

Centrispinning (centrifugal casting)
The introduction of liquid metal into a rapidly rotating mould so that the metal is directed by centrifugal force to take up the shape of the mould.

Chaplets
Metal supports used to hold the cores in position in the mould. They subsequently melt and become part of the casting.

Charpy Test
A notched-bar impact test in which a test piece 55mm long by I0mm square is supported at both ends, and having a notch in the middle of its length, is struck behind the notch by a striker carried on a pendulum. The energy absorbed in fracture is obtained from the height to which the pendulum rises. A notch with a depth of 2mm and rounded at the bottom to a diameter of 2mm is known as a Mesnager notch. A notch produced by drilling a hole usually 2mm diameter and center 4mm below one face with a saw cut from that face to the hole, is known as a keyhole notch. Other notches may be used, such as the Izod V-notch. Chill.
(a) The portion of an ingot which is contained within the ingot mould proper, that is the part which is used and which excludes the feeder head and well.
(b) Metal inserts placed in a sand mould to control the process of solidification and thus improve the soundness of a casting.

Chill Crystals
The thin layer of very fine crystals forming the outside of an ingot and produced by the rapid cooling effect of the mould on the liquid metal.

Chromizing
The production of a corrosion and heat-resisting surface layer by the high temperature diffusion of chromium into iron or steel.

Cire Perdue Process
(See Precision Casting)

Clad Steels
Two dissimilar types of sheets or plates bonded together. The covering metal usually possesses greater corrosion resistance, as when mild steel is clad with stainless steel.

Cleavage Planes
Planes of easy fracture related to the crystal structure. They are not necessarily related to the boundaries of the crystal, and are found in both minerals and metals.

Clink
A large crack in a block of metal caused by uneven contraction or expansion during cooling or heating.

Close Packed Hexagonal
An arrangement of atoms in crystals. (See Space Lattice).

Cluster Mill
A mill in which slender work rolls are each backed-up by more than one roll to prevent deflection of the work roll in the direction of rolling. The backing rolls may themselves be supported by other backing rolls.

Co
Chemical Symbol for Cobalt

Coefficient of Elasticity
(See Elasticity)

Coefficient of Expansion
It is the ratio of the change in length, area or volume per degree to the corresponding value at a standard temperature. For example, the coefficient of linear expansion of mild steel at 20˚ to 100˚C is 0•000011 per ˚C.

Coercive Force
The demagnetizing force which has to be used to remove the residual magnetism of a substance.

CO2 Process
A method of bonding moulding sand. The sand, mixed with about 4% sodium silicate solution, is moulded and then treated with carbon dioxide at room temperature.
Action between the sodium silicate and the carbon dioxide causes the sand to become firmly bonded.

Cogging (Roughing)
The action of reducing a hot ingot to blooms or slabs, It is the first stage in forging or rolling. An ingot which is reduced under a hammer is said to be hammer cogged to distinguish it from one reduced under a pair of rolls in a cogging mill which is said to be roll cogged.

Coining
The forming of metals under pressure in closed dies so that an imprint of the die is left on the surface of the work piece. The operation is usually performed cold.

Cold Cracking
Cracks in cold, or nearly cold, metal, due to excessive internal stress caused by contraction. Such cracks may be brought about by the fact that the mould is too hard or by the unsuitable design of a casting.

Cold Drawing
The process of reducing the cross-sectional diameter of tubes or wire by drawing through dies without previously heating the material.

Cold Heading
A process for making the heads of rivets and bolts by plastic flow in dies. Wire or bar stock is used and by the application of pressure on the end, the diameter is increased and the flow confined in dies.

Cold Lap
Wrinkled markings on the surface of an ingot representing incipient freezing of the surface of the metal, and due to too low a casting temperature. The surface is liable to contain oxide at the base of the wrinkles and subsequent hot working may lead to cracks, rokes or double skins.

Cold Rolling
Rolling of metal or steel at room temperature to provide a smooth surface finish and/or enhanced tensile strength. (See Cold-Working).

Cold Short
Lack of ductility when worked in the cold. Cold Shut.
(a) The freezing over of the top surface of an ingot before the mould has been filled, due to an interruption of the stream of metal.
(b) A casting imperfection due to metal entering the mould by different gates or sprues, failing to unite on meeting.

Cold Welding
(See Welding)

Cold Working
Altering shape by plastic deformation below the annealing or re-crystallization temperature. This treatment may consist of cold rolling, hammering, drawing, pressing, heading, spinning, extruding, or swaging, usually at room temperature. The hardness and tensile strength are progressively increased with the degree of cold work, whilst the ductility and impact values are lowered. Where this increased hardness is not desired, the material is annealed between the cold working operations or it may be subjected to a final annealing. Steels containing 0•7-0•85% carbon may be cold worked until they possess a tensile strength of 1850 N/mm2 (120 tons per square inch), while piano wires may attain 2300 N/mm2 (150 tons per square inch) or higher.

Columnar Crystals
Crystals which during solidification have gown preferentially, in one direction, owing to a temperature gradient, thus developing one axis which is longer than any other.

Combined Carbon
Carbon present in steel or cast iron which is in the form of carbide such as iron carbide (Fe3C), as distinct from graphite and temper carbon.

Constitutional Diagram
A diagram representing the limits of temperature and composition within which the different phases or constituents occurring in an alloy system are stable.

Contact Corrosion
The corrosion occurring when two dissimilar metals are in contact in the presence of a liquid which can serve as an electrolyte. Owing to the different solution pressures of the two metals, electric potentials similar to those operating in an ordinary voltaic cell are set up and the less noble of the metals is liable to be attacked.

Continuous Casting
A method of producing cast slabs or billets in long lengths using water-cooled moulds of very short length. The castings are continuously withdrawn through the bottom of the moulds while teeming of the metal is proceeding.

Contraction Cavities
Voids formed when the supply of molten metal fails at certain points. In well designed castings, the bulk of the contraction that accompanies the solidification is concentrated in the feeder heads and risers, from which molten metal flows to compensate for contraction in the casting proper. In steel ingots the contraction may result in the formation of pipe if feeding is inadequate.

Controlled Atmosphere
A gas or mixture of gases in which steel articles are heated to produce or maintain a specific surface condition. The atmosphere may be made by partially burning town gas in a separate combustion chamber, in some cases over a catalyst. The resulting mixture may be cooled to remove water vapour before passing into the furnace, for scale-free heating. Hydrocarbons, such as propane, may be added to produce a carburizing atmosphere. Ammonia is used to produce an atmosphere for nitriding, or for the bright annealing of stainless steel.

Converted Bar
(See Cementation)

Converter
(See Bessemer process & Tropenas Converter)

Converting Furnace
(See Cementation)

Converting Pots
(See Cementation)

Converting Process
(See Cementation)

Cooling and Heating Curves
Graphs obtained by plotting time against temperature for a metal cooling or heating under constant conditions. Changes of rate resulting from the absorption or evolution of heat indicate phase changes.

Core
(a) The inner and unaffected part of a case hardened steel which is softer than the surface layer or case.
(b) The inner portion of rolled rimming steel.
(c) A solid mass of sand or loam forming part of the mould.
(d) That part of a magnetic circuit around which the winding is placed.

Coring
The microscopic segregation developed by the progressive freezing of zones successively richer in one metal when a liquid solution of two metals is solidifying to form a solid solution.

Corner Crack
A longitudinal crack at or near the corner of an ingot. The tendency of an ingot to crack in this way can be reduced by suitably modifying the design of the ingot mould.
The crack is generally associated with corner segregation.

Corrosion Fatigue
Corrosion combined with alternating or repeated stress accelerates the fatigue. The severity of the action depends upon the range and frequency of the stress, the intensity of the corroding conditions, and the time. The fracture is similar to a fatigue fracture, but may exhibit a characteristic discolouration, dark at the origin and grading away to a lighter shade with increasing distance from it.

Cr
Chemical symbol for Chromium

Crater Crack
At the finish of a welding run the molten metal may form a small crater owing to contraction on freezing. The crack which often forms in this crater is known as a crater crack.

Cratering
The formation of a cavity on the face of a cutting tool near its tip as a result of wear during machining.

Creep Test
A method of determining the plastic deformation of metals under a definite load at a definite temperature. Various criteria are used for assessing the behavior of materials in a creep test, namely
• as a definite creep rate, e.g., between X and Z hours (e.g., 24 -72 or 25-35) or creep rate at 1,000 hours
• Total deformation in a certain period of time. Owing to the lengthy nature of such testing where the service may extend to 10 years, or longer, considerable use is made of the extrapolation of creep test results obtained for relatively short times.

Crevice Corrosion (Shielding)
This type of attack occurs in crevices as formed by washers, sleeves or bearings, etc., or at the edge of a protective coating. It is due to a concentration of corrosive media or by depletion in oxygen resulting in an aeration cell.

Critical Cooling Rate
The term, as applied to steel, represents the slowest rate of cooling which allows the formation of martensite. In many cases this amounts to the rate of cooling which will just suppress the austenite/pearlite transformation. (See also Transformation Range).

Critical Points
The points of temperature at which changes of phase occur in steel. They are marked by the liberation of heat, decalescence, during cooling, and by absorption of heat, decalescence, on heating, thus resulting in halts or arrests on the cooling or heating curves. In steel there are several such points, the temperature for which depends largely on the composition of the steel. (See also Transformation Range).

Critical Temperature
(a) The temperature at which some change occurs in a metal or alloy during heating or cooling, i.e., the temperature at which an arrest or critical point is shown on heating or cooling curves.
(b) The temperature at which alpha iron loses its magnetic properties, i.e., about770˚C.
(c) The temperature above which a given gas cannot be liquified.

Croning Process
(See Shell Moulding)

Cross Rolling
Rolling in a direction transverse to the longitudinal direction of the original ingot. Cross rolling is also used to increase the width of spoon blanks before forming the spoon bowl.

Crucible Steel Process
In this method steel was originally made by melting blister bar or wrought iron, charcoal and ferro-alloys in crucibles holding about 25 kg. It was the first process to produce steel in a molten condition, hence the product was called cast steel. It has been replaced by the high frequency process.

Cryogenic
A term used for low temperatures, e.g., cryogenic steel is for sub-zero use.

Crystal Boundaries
The surfaces of contact between adjacent crystals in a metal. Anything not soluble in the crystals tends to be situated at the crystal boundaries, but in the absence of such a phase, the boundary between two similar crystals is simply the region where the orientation changes.

Crystalline Fracture
(See Fracture)

Cu
Chemical Symbol for Copper

Cup and Cone
A type of fracture occurring in tensile test pieces from steels possessing reasonable ductility, and containing no local abnormality, where the necking occurs after maximum stress. The fracture gives one surface with a cup-like contour, and the other in the form of a cone, fitting the cup. This is accepted as evidence of toughness and uniformity of structure.

Cupola
The furnace commonly used for the production of iron castings in the foundry. It is a straight shaft furnace, consisting of a circular shell of steel plates lined with fire- brick, open at the top and bottom. The shaft stands on a bed plate with a central opening closed by two hinged cast iron doors which are protected from the heat of the furnace by a bed of sand. These doors support the charge and thus at the end of the heat, they can be opened to allow the residue of iron, coke and slag to drop out. The bed plate is supported on cast iron legs which stand on a foundation of concrete or brick. The charge, consisting of alternate layers of iron and coke, with limestone as flux, is introduced through the charging door situated about half way up the stack. Surrounding the stack, near the bottom, is the wind box through which air, often preheated, is driven to the tuyeres, which may be arranged in two or more layers. The breast, an opening in the bottom of the furnace in which the fire is lighted, consists of fireclay and is rebuilt after each heat. The tap hole is situated in the breast and a projecting spout below guides the molten iron into the ladle. The slag hole is slightly below the tuyeres, but at a higher level than the tap hole.

Cuppiness
An internal defect in wire-drawing which leads to fracture of the cup and cone type. It may be caused by excessive cold work (overdrawing) without adequate intermediate annealing or by segregation where the hard centre, being less ductile than the surface, tears and starts the fracture.

Curie point
The temperature at which alloys become magnetic on cooling or non-magnetic on heating.

Cyaniding (Cyanide Hardening)
The process consists of introducing carbon and nitrogen into the surface of steel by heating it to a suitable temperature (e.g., 760˚-845˚C) in a molten bath of sodium cyanide, or a mixture of sodium and potassium cyanide, diluted usually with sodium carbonate, and quenching in water or oil. The process is used where a very thin case of high hardness is required. An immersion of 5 to 10 minutes usually suffices.