Product Technology – Terminology

AGE HARDENING

A process causing structural change which may occur gradually in certain metals and alloys at room temperature or more rapidly at higher temperatures. The effects of Age Hardening are caused by the precipitation from a super saturated solid solution after rapid cooling from a high temperature, to give an increase in the hardness of the material. Where the ageing occurs at an elevated temperature the process is often referred to as Precipitation Hardening. Typical age hardening metals and alloys include Aluminium, Duralanium (alloy of Copper and Aluminium), Magnesium, Nickel, Titanium and some stainless steels.

ALLOTROPIC

To exist in several different crystalline structures, hence allows heat treatment to control properties, or to change the lattice structure in the ‘Solid’ state only (may give rise to an exothermic reaction) i.e. Iron.

ALPHA IRON (Ferrite)

Body centered cubic form of iron below 910oC which is magnetic and as such can be inspected with Magnetic Particle Inspection.

ANISOTROPIC

Properties vary with orientation or direction of testing

ANNEALING

Performed by heating the material to a temperature of around 850 oC for Steels and allowing any carbide to be taken in to solution. On cooling slowly in a furnace any stresses which were present due to cold working, welding etc. are removed to give a softer more machinable re-crystallised structure.

SUB CRITICAL ANNEAL (Stress Relieving)

Is carried out at a temperature below that at which any carbide are taken into solution. As such recrystallisation does not take place which results in a material not quite having the properties of a full anneal.

AUSTENITE (Gamma Iron)

Face centered cubic form of iron above 910oC which is non-magnetic. By adding Manganese, Nickel and Chromium to steel a stable form of Austenite is formed at room temperature - i.e. Austenitic Stainless Steels.

BECKING (Mandrel Forging)

A process which produces rings by forging discs whose centre has been punched away on a mandrel bar and working the wall thickness between the mandrel bar and the hammer.

BLOOM

An intermediate product which has been rolled or forged down from an ingot usually square in section which is to be further worked. The machinery which is used to create this product is housed in an area usually designated a blooming mill or cogging mill. Another intermediate product is the rectangular slab which is worked in a slabbing mill.

BLOW HOLE

Rounded or Elongated (Wormhole) smooth walled gas filled cavities in solid metals formed either by the trapping of gas evolved during solidification of the metal or by gas or steam from a moulds surface.

BRAZING

The process of joining two pieces of metal by fusing a layer of brass or other non-ferrous metal between the adjoining surfaces.

CARBURISING

The introduction by absorption of carbon into the surface layer of steel having low carbon content. Also known as Case Hardening. The carbon may be obtained from either a solid liquid or gaseous carbon containing Iridium. After carburising the surface of the product is capable of heat treatments such as hardening.

CHAPLETS

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

CHILLS

These are metal inserts which are placed on the inside of a sand mould to control the process and direction of solidification and as such improve the integrity and soundness of a casting.

CHILL CRYSTALS

The first crystals which form very quickly on the inside surface of a metal ingot mould and have the smallest structure of all the crystals in the ingot.

CLINK

A form of cracking occurring due to a rapid thermal gradient, so called due to the sound created as the material spontaneously cracks, usually associated with large forgings or high strength materials when rapidly cooled for high (working) temperatures.

COGGING

The production of slabs or blooms by either forging or rolling from an ingot. This is the first rough stage of working.

COLD CRACKING

Cracks in cold or nearly cold material due to excessive internal stress caused by contraction. This may be due in part to the unsuitable design of a mould and subsequent casting.

COLD DRAWING

A process of reducing the cross sectional diameter of tubes and wire with a subsequent increase in harden ability by drawing the material though dies without previously heating the material.

COLD ROLLING

Rolling metal or thin sheet at a temperature below the recrystallisation to provide a smooth surface and/or enhance tensile strength.

COLD SHUT

Any area within a casting where two areas show incomplete fusion. Examples include, unfused Chaplets and Chills.

COLD WORKING

Working below re-crystallisation temperature.

CONTINUOUS CASTING

The production of cast slabs or billets of long lengths by withdrawing from water cooled moulds solidifying metal which is continually being added to.

CORNER CRACK

Formed by bad mould design allowing grains to join together at 90 Deg to each other. Segregation is often associated with corner cracking and can be reduced with better mould design.

CRATER CRACK

The small crater which occurs at the finish of a weld run due to contraction/shrinkage.

CREEP CRACKING

Continuous deformation of a material under constant load.

CURIE POINT

The temperature at which alloys become non-magnetic on heating.

CUPPING

Production of seamless pipe via pressing.

DEOXIDISATION

The operation which changes any dissolved oxygen into non-metallic inclusions by the addition of Aluminium (Al2O3,) or silicon (Si20). This also prevents the carbon which is present from forming carbon monoxide which gives rise to blow holes.

DIE CASTING (Chill Casting)

The process of teeming metals probably under pressure into two half metal moulds to generate the final product. Generally used for lower melting point alloys such as Zinc, Lead and Aluminium to give a highly precise component.

DROP FORGING (Impact Forging)

The process of using repeated blows on metal between two dies, each of which contains half the impression of the desired shape.

EXCESS PENETRATION BEAD

Weld filler material deposited which extends beyond the parent metal surface/ thickness in the root, but within specified limits in the acceptance criteria.

EXTRUSION

The action of forcing a material though a restricted orifice (die) under pressure, causing a greatly elongated section with the cross sectional shape of the die used.

FATIGUE

The effect on a metal of repeated cycles of stress. Fracture may result from the development of a crack which propagates under the repeated stress.

FEEDER HEAD

An extra portion into which liquid metal can be poured. This extra metal is available to fill the cavities below when shrinkage occurs in the casting.

FETTLING

The first cleaning process by which castings, and closed die forgings etc, have excess material removed and may be used to improve the overall manufactured surface profile.

FLAKES (Hair Line Cracks)

Occur due to entrapment of hydrogen on cooling. Casting in a vacuum or below a slag coating can reduce the possibility of flakes occurring, as can suitable heat treatment after production.

FLAME HARDENING

This is a method of local hardening by which the steel is heated by an oxyacetylene torch which transverses the material at a pre- determined rate, related to the depth of hardening. Quenching is often carried out by a jet of water immediately following the heating torch.

FLASH

This is the metal which is squeezed out of a pair of forging dies which is excess to filling the two dies. In drop forgings this excess usually fills a flash line within the dies themselves at a junction of the parting line.

FIN

Similar to flash above but usually occurs on a casting again on the parting line between two halves of the mould.

FORGING

The art of working parts at a temperature above the recrystallisation point between the hammer and anvil, either by sharp successive blows of short duration (hammer forging) or by a hydraulic press where pressure is applied for longer periods of time (press forging).

FRETTING

When two surfaces press against each other whilst slightly moving, heat which is built up between them allows small particles from each surface to become stuck (welded) together and eventually break free. This type of debris removal can be a starting place for fatigue to occur.

GATE

This is the end of the runner section which connects to the mould and allows liquid metal to enter the mould itself. It is an area which gives a change in velocity and as such turbulent flow within the casting cavity. This site is favorable for the formation of micro shrinkage and porosity.

GATING SYSTEM

Refers to the feeding system which feeds a number of individual ingot moulds from one centre down gate.

GRINDING CRACKS

Fine shallow cracks caused by local overheating and cooling. Generally a loss of coolant is the reason for an irregular network of cracks.

HARD FACING

A method of improving wear resistance by the introduction of a hard protective coating of surface metal such as stellite, metal carbides or inconel, etc.

HARDENABILITY

This is the property which determines depth and distribution of hardness due to quenching from a high temperature. It is generally a function of carbon content and composition and is related to the formation of Martensite.

HETEROGENIUS

A non uniform structure.

HOMOGENIUS

A uniform structure.

HOT SHORTNESS

Brittleness in metals with a chance of cracking at certain temperatures when under stress due to the loss of ductility near the melting point.

HOT SPOTS

Highest temperature area after teeming, ideally kept close to the feeders it is the most likely area for defects to occur.

HOT TEAR

This crack manifests itself in castings near to or at a change in section due to the stresses where by differing sections cool and contract at different rates. Hot tears can occur at any time once solidification has taken place.

HOT TOP

Extension of the ingot mould, used to retain heat in the upper part of the ingot as it solidifies an exothermic powder is usually applied, this will reduce the formation of secondary piping.

INCLUSIONS

Usually due to deoxidisation using Aluminium and Silicon to produce the non metallic oxides Silicon oxide and Aluminium oxide. Also caused by manganese which combines with sulphur in preference to iron forming manganese sulphide. All inclusions can cause a reduction in ductility fatigue strength and tensile strength.

INDUCTION HARDENING

This process can surface harden or fully harden by heating the material to a temperature above the transformation range with an alternating magnetic field and then quenching immediately.

INGOT

A casting made in a cast iron ingot mould which is to be reworked by rolling or forging.

INTERCRYSTALLINE CORROSION (Intergranular Corrosion)

Also known as Weld Decay due to this phenomena occurring within the heat affected zone of austenitic welds. After heating within the range 500 – 800 Deg C. If the material is subjected to a corrosive atmosphere chemical attack can take place. This is due to the above temperature allowing carbon to be deposited at grain boundaries as chromium carbide and so depleting the austenite adjacent to the boundaries of chromium and leaving it susceptible to attack. The addition of small quantities of niobium or titanium will form carbides in preference to chromium and will therefore not deplete the austenite of its chromium making it less susceptible to intergranular corrosion cracking.

ISOTROPIC

Composed of equiaxed grains.

LACK OF FUSION

A lack of bonding between 2 or more materials in a weld, this may be generated by incorrect welding conditions such as, too low a current, too fast travel, incorrect edge preparation.

LACK OF PENETRATION

Lack of bonding of the original material and the weld metal in the root area; causes could be through using too large a diameter electrode, having too large a root face, having too narrow a root gap or too low a current.

LAMELLAR TEARING

Found in the rolled plate of configuration weld joints such as T, K or Y joints caused due to lack of through thickness ductility which causes a tear to occur along the line of weakness plains present in the plate.

LAP

This defect occurs when metal is folded on the surface when forging without being welded up on further working.

MALLEABILITY

The property which enables a metal to be mechanically deformed under compression such as by hammering or rolling without cracks occurring.

MARTENSITE

The hardest decomposition product of austenite, which is formed due to quenching from high temperatures at a rate which is greater than its critical cooling rate and as such produces a brittle hard substance called martensite. Martensite is too brittle to be of use and materials are generally required to have further heat treatment processes to remove the structure.

MOULD

The cavity usually in two parts placed together into which molten metal is poured to produce a cast product.

NITRIDING

A surface hardening process by which the metal is heated within an ammonia atmosphere for a number of hours. The steels which are affected by this process must contain amounts of Aluminium, Chromium, Molybdenum, Vanadium and tungsten which will form stable hard nitrides.

NORMALISING

This process involves heating the material to above the transformation (recrystallisation) temperature and holding for a length of time prior to air cooling. Unlike annealing a stress free structure is not achieved due to the faster rate of cooling but internal stresses are relieved and grain size is refined.

PIN HOLES

Minute gas cavities generally in light alloy castings due to the liberation of gases which had been initially absorbed by the materials making up the alloy.

PIPE

The shrinkage cavity which occurs on top of an ingot. Any cavity which is oxidized and open to the atmosphere is called Primary Pipe and if it is formed subsurface called Secondary Pipe. Both types can be reduced by using a feeder head.

PRECISION CASTING (Lost Wax or Investment)

Refractory slurry forms the mould by flowing around an exact wax replica of the part to be made. After heating and subsequent removal of the wax the mould can be filled with molten metal to give a precise copy of the wax pattern.

QUENCH CRACK

A fracture resulting from thermal and transformation stresses induced during rapid cooling by immersing a hot part into a quenching medium such as oil, water, brine, etc.

RESIDUAL STRESS

The stress which exists in parts by external loading such as cold working or phase change. Stresses are also induced by processes such as castings and welding which transform from liquid to solid.

RIMMING STEEL

Steel which has not been deoxidised and the resultant oxygen reacts with carbon to form CO and CO2 gases during solidification this results in the formation of blow holes.

RISER

The Riser on a casting acts as a reservoir (along with the feeder) head but also allows gases which are formed to escape minimising porosity and blow holes. It is also an indication that the casting cavity is full.

ROLLING

A process similar to forging except that the material is elongated and reduced in section between two rolls revolving in opposite directions. Rolling can also be carried out below the recrystallisation temperature and as such is referred to as cold rolling.

SCALE

The oxidised surface of steel produced during hot working at elevated temperatures. The scale consists of stable iron oxides such as Fe2O3 and Fe3O4.

SEAMS

Elongated indications along rolled bar material where the surface has been pinched together without being welded together. They can also occur from oxidized surface blow holes which have been stretched and elongated through working the material.

SEASON CRACKING

This occurs in severely cold worked materials particularly copper and brass resulting from a combination of corrosion and internal stresses.

SEGREGATION

This is the heterogeneous (non-uniform) distribution of impurities or alloying elements. Not only is it dependent on the chemical composition but also on the cooling rate. For example close to the surface of an item the impurities become trapped within the rapidly growing crystals. Further below the surface where cooling is slower the segregates tend to form together during the V and A type ghosting segregates.

SHELL MOULDING

A mixture of very fine sand and a resin bonding are mixed together over a heated metal pattern at about 250oC. In this way a thin walled half pattern is formed, which when used in a pair can be filled with molten metal.

SHOT BLASTING

A method of cleaning steel surface by abrasion, where steel shot is blasted onto the surface. The surface can become slightly work hardened with an increase in fatigue strength, due to the peening effect of the shot removing sharp edges etc on the surface of the material, however this can/ will also close small crevices and is not advisable prior to penetrant inspection without an acid etch taking place.

SKELP

Plate prepared to be made into pipe.

SOLIDIFICATION

‘Directional solidification’ solidifies from one end of a casting to the other. ‘Progressive solidification’ solidifies from the outside of the casting to the centre.

SPATTER (Welding)

Globules of molten metal thrown out of the weld pool onto the parent metal remote from the weld. Causes could be using too high a current using contaminated consumables which give rise to explosions within the weld pool, or magnetic arc blow if using DC techniques on ferritic material.

STAINLESS STEELS

Ferritic stainless steel – magnetisable/non hardable, used for general cutlery consists of low carbon, 13% Cr e.g. AISI 403 ≥ 10 – 15°C 11.5 – 13% Cr

Martensitic stainless steel – magnetisable/ may be hardened, used for cutting knives consists of high carbon, 13% +Cr e.g. AISI 440 0.6 – 1.2% C 16 – 18% Cr

Austenitic stainless steel – non-magnetic/ non-hardenable, a general purpose stainless consists of low carbon, 17 – 19Cr, 8 – 10% Ni e.g. ≤ 0.15C, 17 – 19% Cr, 8 – 10% Ni 18% Cr, 8% Ni, 2.5% Mo this is often used for corrosion resistance in sea water environments.

Duplex stainless steels – are a combination of both ferritic and austenitic structures which are used for high temperature applications up to 600°C (typically).

STRESS CORROSION

Also referred to as environment sensitive cracking. Deterioration in mechanical properties through the simultaneous action of static stress and an exposure to a corrosive atmosphere. It is often accompanied by cracking which can be either transgranular or intergranular.

STRESS RELIEF

A process where by residual stress is reduced by heating within the range 600 – 650 Deg C for carbon steel and holding for sufficient time to allow internal stress to be released by creep. This will be followed by a controlled cooling to prevent further stresses being induced. Stress Relief is typically carried out after welding, cold working, casting, etc.

SUBLIMATION

Changes from a solid directly to a gas.

SUPERHEAT

Additional heat above that necessary for melting.

TEMPERING

This is the process of heating hardened or mechanically worked steel at some temperature below the transformation temperature to remove brittleness and improve toughness so that the material can be usefully used. On heating bright steel between the temperature of 200 Deg C and 400 Deg C the correct temper temperature may be indicated by the color of the oxide layer which forms on the surface.

TUBE

Generally a seamless hollow cylinder. If there is a seam joint it is generally referred to as a Pipe.

VACUUM DEGASSING

The process of casting steel in a vacuum vessel or refining the steel to remove gaseous products. Any molten stream of metal which is introduced into the degassing vessel will have any gases pulled out the stream. The molten steel can also be stirred using an inert gas such as argon or helium which purges any gas out of the melt. One of the benefits of degassing is that non metallic inclusion content is much reduced.

WORK HARDENING

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

WELDING

A process by which two pieces of metal are joined by heat or pressure, or both with or without additional filler metal, so that recrystallisation takes place across the joint. Usually there is local fusion and heat for the process is obtained in a number of different ways, such as an electric arc which may be struck between an electrode and the metal to be joined or electrical resistance. Below are some of the more common techniques:

Oxy-gas welding (oxy acetylene)

Manual metal arc welding

Metal inert gas welding

Metal active gas welding

Tungsten inert gas welding

Plasma arc welding

Submerged arc welding