Metallurgical Terms
P
P
Chemical symbol for phosphorus.
Pack Annealing
(See Box Anneaiing)
Pack Carburizing.
(See Carburizing).
Parkerizing.
Chemical treatment producing a complex phosphate coating on iron and steel
articles to improve their corrosion resistance. It is usually followed by a
final protective coating of oil, paint or enamel.
Passivity
This applies to a metal or alloy which is normally chemically active; it is
said to be passive when its behaviour resembles that of a n7ore noble metal.
Patented Steel
(See Patenting)
Patending
Heating to a suitable temperature well above the transformation range,
followed by cooling in air or in a bath of molten lead or salt maintained at a
suitable temperature, to produce a structure which will facilitate subsequent
cold working and give the desired mechanical properties in the finished state.
In wire making, the process is applied to medium or high carbon steel between
drawings.
Pb
Chemical symbol for lead, from the Latin plumbum
Pd
Chemical symbol for palladium
Pearlite
The lamellar conglomerate of ferrite and cementite which constitutes the
eutectoid in the iron-carbon equilibrium system. It results from the
transformation of austenite at or below Ar1 and is so called from the
mother-of-pearl lustre given by an etched surface when viewed under the
microscope. This pearly appearance is due to the fine and regular alternation of
the two constituents. The lamellar arrangement of ferrite and cementite produces
a tough structure and is responsible for the mechanical properties of the
unhardened steels. Peariite is present in small quantities in low carbon steels
and increases in quantity as the carbon increases, until in a plain carbon
steel, containing about 0•83 % of carbon, the structure consists entirely of
pearlite.
Peltier Effect
The phenomenon whereby heat is liberated or absorbed at a junction where an
electric current passes from one metal to another.
PencilTest
A small ingot cast from a sample of liquid steel to provide test material
for analysis, for carbon or hydrogen, for example.
Peritectic
A reaction when an alloy freezes which takes place between a liquid and a
solid phase to produce another solid phase.
Permanent Extension
(See Permanent Set).
Permanent Mould Die Casting
(See Die Casting);
Permanent Set.
(a) An extension remaining after the load has been removed from a test piece
when the elastic limit has been exceeded (see Elastic Limit).
(b) Permanent deflection of any structure after being subjected to a load.
Permeability
(a) The ratio B/H for a magnetized material, where B is the flux density
produced and H is the magnetizing force.
(b) In sand, a measure of that physical property which permits gas to pass
through the moulded mass of sand.
Ph Value
A method of expressing differences in the acidity or akalinity of solutions.
It is the logarithm to the base 10 of the reciprocal of the concentration of the
hydrogen ions in gram molecules per litre in an aqueous solution. Thus, water
has a pH value of 7; acidic solutions are less than 7, and basic solutions are
higher than 7.
Phase Diagram
(Thermal Equilibrium Diagram) A graph showing the limits of composition and
temperature of the various phases present in an alloy system under equilibrium
conditions. A phase is the name given to a constituent which can be
distinguished from the remainder of an alloy by physical and chemical methods
such as etching, and in the solid may be a solid solution, an element or an
inter-metallic compound.
Photoelasticity
A property of transparent substances which enables the presence of strain to
be detected by examination in polarized light. If models of engineering
structures are made of such a substance, the stress distribution in the
structures may be deduced.
Photomicrograph
A photographic reproduction of a microstructure.
Pickling
The process of chemically removing scale or oxide from metal objects to
obtain a chemically clean surface. This is usually effected by immersion in an
acid bath. For steel, the acids generally employed are sulphuric and
hydrochloric to which is added an inhibitor of organic material such as glue in
small quantities, often about 0•05 %, or slightly more, to reduce or restrain
the attack of the acid on the metal and so effect the removal of the scale with
a minimum loss of metal. Inhibitors may be of a foaming or non-foaming type, the
object of the former being to produce a blanket of foam on the surface of the
pickling bath with the object of reducing acid spray caused by the bursting of
bubbles of hydrogen at the surface. Scale may also b removed by molten caustic
soda baths with additions of sodium hydride or other compounds. This treatment
is followed by water quenching and if required a brief acid dip. Materials
otherwise difficult to pickle may often be rapidly and efficiently de-scaled in
this way.
Pickling Brittleness
(See Acid Brittleness)
Pig.
A mass of metal (e.g., cast iron, copper or lead) cast in a simple shape for
transportation or storage, and subsequently re-melted for purification,
alloying, casting into final shape or into ingots for rolling.
Pig Iron
Crude iron produced by the reduction of iron ore in a blast furnace and cast
into pigs which are used for making steel or cast iron. The principal impurities
are carbon from 2•5 to 5%, and varying amounts of silicon, manganese, sulphur
and phosphorus. The composition varies according to the type of ore, smelting
practice, and the purpose for which the iron is to be used.
Pigging Back
The addition of pig iron to the bath of the open hearth furnace to ensure a
satisfactory boil or to increase the carbon content to the desired amount. The
reverse process is that of oreing down, in which ore is fed in to effect
oxidation of the carbon.
Pinch Pass
(See Skin Pass)
Pinholes
Minute gas cavities fairly uniformly spaced throughout a casting and due to
the liberation of occluded gas. They are found chiefly in light alloy castings.
Pipe
An axial cavity produced in the ingot by the contraction of the metal on
freezing. When the pipe is in the top or open part of the ingot it is known as
primary pipe and when it appears in the lower part it is described as secondary
pipe.
Pitting Corrosion.
Passive metals are resistant to a wide variety of corrosive media but under
certain conditions breakdown of the passive film may occur at various points.
Depending on the conditions, this may lead to a complete breakdown of the
passive film or the attack may concentrate at these initial areas, leading to
pitting.
Plain Carbon Steel
(See Carbon Steel)
Planetary Mill
A rolling mill in which there are several small diameter work rolls
supported in a cage round each backing roll. The hot slab is fed in under
pressure and reduced to strip; very large reduction can be attained in a single
pass.
Plasma
Gas heated to a very high temperature and confined so that the constituent
molecules break down into electrically charged particles of high energy. It is
formed in the electric are and if suitably confined and directed by
electromagnetic means can be used as a source of intense heat. Temperatures up
to 17,000˚C may be attained. The plasma are can be used for spraying metals and
ceramics, for metal cutting and welding.
Plastic Flow
The deformation of metals by the mechanism of slip along definite geometric
plane within the crystals.
Poisson's Ratio
The ratio of the transverse contraction per unit dimension of a bar of
uniform cross-section to its elongation per unit length, when subjected to a
tensile stress.
Its value in steel is of the order of 0•28.
Poling
Stirring molten metal, either in a furnace or in a ladle with a pole of
green wood, the heat distilling off the volatile products which stir up the
metal, and, together with the charcoal formed, help to reduce any oxide present.
Pot Annealing
(See Box Annealing).
Pot Quenching
Quenching carburized parts directly from the carburizing box or pot.
Powder Metallurgy
The art of producing metal powders and forming them into coherent objects
Individual, mixed, or alloyed metal powders, with or without the inclusion of
non metallic constituents, are pressed or moulded into objects which may be
simultaneously or subsequently heated to produce a coherent mass, either without
fusion or with the fusion of a low melting constituent only. The product may be
used directly or forged to improve its qualities.
Precipitation Hardening
(See Ageing).
Precision Casting
A metal die is used to make a wax (Lost Wax process) or frozen mercury (Mercast)
pattern. This is invested with a refractory slurry forming a mould which after
drying and baking can be used for casting into. The pattern melts and runs out
when the mould is warmed. Large numbers of very small accurate castings can be
made in this way. The Lost Wax process has been used only comparatively recently
for steel; it is sometimes known as the Cire Perdue process and was used for
bronze and precious metals by the ancient Egyptians and Benvenuto Cellini among
others.
Press Forging
(See Forging).
Pressure Die Casting
The usual die casting process in which the molten metal is forced into
highly finished moulds under considerable pressure by plungers, air pressure or
combined methods.
Pressure Welding
(See Welding)
Primary Metals
(See Secondary Metals)
Primary Pipe
(See Pipe)
Process Annealing
A process, commonly applied in the sheet and wire industries, in which an
iron-base alloy is heated to a temperature close to, but below, the lower limit
of the transformation range, and subsequently cooled. This process is applied
for the purpose of softening for further cold-working.
Producer Gas
Made by passing a mixture of air and steam through an incandescent bed of
coal or coke. The gas, consisting mainly of carbon monoxide, hydrogen and
nitrogen, together with tarry hydrocarbons if made from coal, leaves the gas
producer at about 600˚C and has a calorific value of about 150 BTU's/cu. ft.
Projection Welding
(See Welding).
Proof Resilience
(See Resilience)
Proof Stress
The stress which is just sufficient to produce a permanent elongation equal
to a specified percentage of the original gauge length. It can be determined
(a) by direct measurement of the gauge length after various loads have been
applied to the test piece and removed, or
(b) from the load-elongation curve by drawing a line parallel to the straight
portion of the curve and distant from it by an amount representing the required
permanent elongation, thus determining the load at which the line cuts the
curve.
Proportional Limit
(See Elastic Limit)
Pt
Chemical symbol for platinum.
Puddling Process
A process for the production of wrought iron, invented about 1780. The
hearth of the furnace was lined with iron oxide, both the lining and the oxides
added during the process being known as fettling. The process was carried on by
melting pig iron in an oxidizing atmosphere and the silicon, manganese and part
of the phosphorus were oxidized during the melting down and in the interval
preceding the boil. The boil removed still more phosphorus and the greater part
of the carbon. During the final stage, the melting point rose and the metal
became pasty. The iron was then gathered together into balls which were taken to
the hammer and worked into blooms to expel as much slag as possible, after which
the blooms were rolled into puddle bars.