Metallurgical Terms
D
Damping Capacity
This has been described as “elastic hysteresis” and “internal friction”, but
the word damping more suitably indicates the effect obtained when a material
having the property in question is subjected to vibrations which are not
maintained by external energy, e.g., if a steel bar supported at one end is
struck with a hammer it will vibrate for varying lengths of time depending upon
the stress imposed and the composition and condition of the steel, but at a
uniformly decreasing rate owing to the internal friction of the material which
dissipates the energy as heat and sound.
Decalescence
(See Critical Points)
Decarbutization
The loss of carbon at the surface of steel as a result of heating in an
atmosphere which converts the carbon of the steel to a gaseous form.
Deep Drawing
The forming of hollow bodies from metal sheet by forcing the sheet through a
die by means of a punch.
Delta Ferrite
The allotropic form of iron which in the pure metal exists between 1405"C
and the melting point, 1539˚C. The atoms are arranged in the body centred cubic
structure as in alpha iron and it is often present in high chromium stainless
steels at room temperature as a separate phase.
Dendrite
A tree-like crystal formation. Metal crystals grow by branches developing in
certain directions from the nuclei. Secondary branches are later thrown out at
intervals by the primary ones and in this way a skeleton crystal, or dendrite,
is formed. The interstices between the branches are finally filled with solid
which in a pure metal is indistinguishable from the skeleton. In many alloys,
however, the final structure consists of skeletons of one composition in a
matrix of another, giving what is called a cored structure. (See Goring).
Deoxidation
The final operation in the production of "killed" steels, when elements such
as silicon or aluminum are added to stabilize the dissolved oxygen. This
prevents it reacting with the carbon to form carbon monoxide which would form
blowholes during subsequent solidification.
Deseaming
(See Scarfing)
Descaling
(a) The removal of scale from the surface of steel by mechanical or chemical
means, e.g., by sand- or shot-blasting, tumbling, flame descaling, or pickling.
(See pickling).
(b) The removal of scale from the inner surface of boilers and tubes.
Diamond Pyramid Hardness Test
(See Vickers Hardness Test)
Die
(a) A metal block used in stamping operations. It is pressed down on to a
blank of sheet metal on which the pattern or contour of the die surface is
reproduced.
(b) An internally threaded steel block provided with cutting edges for producing
screw threads.
(c) In drop forging, steel blocks usually supplied in pairs with part of the
impression in each block, the lower die being attached to the anvil whilst the
upper one is attached to the hammer itself and moves up and down with it.
(d) In wire-drawing, a small plate or body containing one or more tapering holes
called die holes.
(e) A tool having a prepared hole through which tubes are pulled in cold
drawing.
(f) In punching or piercing, the bottom tool into which the punch is guided
giving clearance between punch and die, which results in shearing of the metal.
(g) In resistance welding, a member usually shaped to the work contour to clamp
the parts being welded and conduct the welding current.
(h) A device used in forge welding primarily to form the work while hot and
apply the necessary pressure.
(i) In powder metallurgy, the part or parts making up the confining form in
which a powder is pressed.
Die Casting
(a) The process of pouring metals into final shapes in metal moulds. If the
metal is forced into the mould the process is known as pressure die casting, but
otherwise it is known as gravity- or permanent mould-die casting. The process is
usually applied to alloys having relatively low melting points, e.g., tin, zinc,
lead and aluminum or magnesium base alloy. The advantages of this process are
the high precision of the castings so produced and the high rates of production
which may be achieved.
(b) The product of the die casting process.
Diffusion
The process whereby the molecules in a solution move from regions of high
concentration towards those of lower concentration until complete homogeneity is
attained. This process is rapid in gases, and moderate in liquids. The migration
of atoms within a solid is brought about by thermal activation. Examples of this
include nitriding, carbon case hardening and cementation which involve diffusion
into the surface of a solid whilst decarburization involves diffusion from the
surface.
Direct Reduction
The scarcity of blast furnace coke has led to the development of processes
in which low carbon iron is reduced directly from ore. For example iron ore is
heated in a kiln in contact with reducing gas yielding metallic pellets suitable
for charging into an electric are furnace.
Dislocation
A discontinuity in the crystal lattice of a metal. The movement of such
discontinuities under stress may be used to explain slip, creep, plastic
yielding, etc.
Dolomite
A naturally occurring mineral; when calcined, a mixture of lime and
magnesia, used as a basic furnace refractory.
Dozzle
A refractory feeder head raised to white heat and inserted in the top of a
crucible steel ingot mould. It is filled with molten steel which flows downwards
to fill the cavity formed by the contraction of the solidifying ingot.
Draw
(a) A term used in the U.S.A. for temper.
(b) To remove the pattern from the mould.
(c) An external contraction effect, analogous to piping, which is sometimes seen
on the surface of a casting.
(d) A form of porosity due to insufficient venting at the corners of castings.
Draw Plate
A steel plate pierced with small holes, through which wire is drawn during
its manufacture.
Drawing
(a) Drawing out metal rods into wire, in the cold state, by pulling through
a series of consecutively smaller holes (dies) in steel plates (draw plates).
(b) Forging to produce the greatest movement in the longitudinal direction.
(c) In the solidification of castings, the production of shrinkage cavities by
reason of improper design, insufficient feeding, too low a casting temperature.
(d) In the U.S.A. synonymous with tempering.
Drop Forging
(a) A forging operation in which a metal shape is formed by repeated blows
from a drop or steam hammer on to a bar or billet placed between a pair of dies.
The upper die containing half-the impression of the desired shape is fixed in
the tup whilst the lower die, containing the other half of the impression, is in
the stationary anvil. The force exerted by the hammer causes plastic flow in the
metal which imparts a tough fibrous structure to the steel, the die being
designed to ensure the most advantageous plastic flow of the material so that
the grain follows the contour of the forging with as little interruption as
possible. The process is economical only when a large number of reproductions
are required, as the cutting of the dies is expensive. Where a change, not only
of form but of the section of the material takes place, the product is a drop
forging; if only a change of form occurs the product is a drop stamping.
(b) The metal shape made by the drop forging process. In such forgings the grain
flow or fibre is of importance.
Drop Test
A strength test for steel tyre which consists of dropping the tyre on to a
rail from a specified height (depending on the tyre diameter), or alternatively,
of dropping a heavy weight on to the tyre held in an upright position.
Dry Cyaniding
A process for case hardening steel by heating it to a temperature of between
625˚ and 850˚C in an atmosphere containing a carburiting gas with ammonia added
in controlled amounts. The combined action of the carbon and nitrogen thus added
to the steel permits a wide range of case and core characteristics by direct
quenching or by cooling slowly in the atmosphere as required.
Ductility
The property of metals which permits deformation to occur without fracture.
Ductile metals show considerable elongation under tensile stress and ultimately
fail by necking with consequent rapid increase in stress locally.
Dye Penetrant Inspection
Used for detecting surface porosity or cracks, particularly in non-magnetic
substances. The part to be examined is cleaned and coated with a dye which
penetrates any small cracks or openings. The surface is then wiped clean and
coated with a white powder. The dry powder soaks up the dye which is still held
in the defects and thereby indicates their position.
Dynamic Strength
Resistance to loads suddenly applied or to pulsating loads. (See Impact,
Fatigue).