Metallurgical
Terms for the
Coating
and Heat Treating Industries
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Ultrasonic
- An NDE technique which relies on an ultrasonic beam passing through
a coating and substrate and providing a signal from the back wall
which is then detected. The height of this backwall echo depends
on the discontinuity in impedance from the sprayed coating to the
substrate. Bonding flaws can be easily seen by the weakening of
the back wall echo.
Undercooling
– Same as supercooling.
Upgrading
- In castings, the removal and repair of discontinuities to raise
the quality level of the casting beyond that which can be economically
achieved by good foundry practice.
V
Vacuum
annealing – Annealing carried out at subatmospheric pressure.
Vacuum
carburizing – A high-temperature gas carburizing process using
furnace pressures between 7 and 55kPa during the carburizing portion
of the cycle.
Vacuum
Casting - A casting in which metal is melted and poured under
very low atmospheric pressure; a form of permanent mold casting
where the mold is inserted into liquid metal, vacuum is applied,
and metal drawn up into the cavity.
Vacuum
furnace – A furnace using low atmospheric pressures instead
of a protective gas atmosphere like most heat-treating furnaces.
Vacuum furnaces are categorized as hot wall or cold wall, depending
on the location of the heating and insulating components.
Vacuum
or Low Pressure Plasma Spraying - Plasma spraying carried
out in a chamber which has been evacuated to a low partial pressure
of oxygen. It is then usually partially backfilled with argon
to avoid the possibility of forming a glow discharge.
Vacuum
Coating - Vacuum coatings processes use a vacuum (sub-atmospheric
pressure) environment and an atomic or molecular condensable vapor
source to deposit thin films and coatings. The vacuum environment
is used not only to reduce gas particle density but also to limit
gaseous contamination, establish partial pressures of inert and
reactive gases, and control gas flow. The gaseous environment
may be in the form of a partially ionized plasma. The vapor source
may be from a solid or liquid surface (physical vapor deposition—PVD),
or from a chemical vapor precursor (chemical vapor deposition—CVD).
The term "vacuum deposition" is often used instead of
PVD, particularly in the older literature. Vacuum is an environment
where the gas pressure is less than the ambient. A plasma is a
gaseous environment in which there are enough ions and electrons
for there to be appreciable electrical conductivity. Vacuum coating
is the deposition of a film or a coating in a vacuum (or low-pressure
plasma) environment. Generally the term is applied to processes
that deposit atoms (or molecules) one at a time such as physical
vapor deposition (PVD) or low-pressure chemical vapor deposition
(LP-CVD) processes or plasma-enhanced CVD (PECVD). In PVD processes,
the material being deposited comes from the vaporization of a
solid or liquid surface. In CVD processes, the material being
deposited comes from a chemical vapor precursor species that is
decomposed by reduction or thermal decomposition—mostly on a hot
surface. In some cases the material being deposited reacts with
the gaseous environment or a codeposited species to form a film
of a compound material such as an oxide, a nitride, carbide, or
a carbonitride. In CVD processing, the use of a plasma to fragment
the chemical vapor precursor in the vapor phase allows the decomposition
or reduction processes to proceed at lower temperatures than with
thermal activation alone. PECVD can be performed at pressures
as low as those used in PVD processing (low-pressure PECVD, LP-PECVD),
where the precursor vapor is decomposed mainly in the plasma.
In some cases a hybrid deposition process of PVD and LP-PECVD
is used to deposit alloys, composites, or compounds. An example
is metal carbonitrides where the carbon comes from a chemical
vapor precursor such as acetylene; the nitrogen comes from a gas;
and the metal from evaporation, sputtering, or arc vaporization
of a solid or liquid surface.
Vacuum
nitrocarburizing – A subatmospheric nitrocarburizing process
using a basic atmosphere of 50% ammonia / 50% methane, containing
controlled oxygen additions of up to 2%.
Vanadium (V) - inhibits grain growth during heat treating
while improving strength and toughness of hardened and tempered
steels. Additions up to .05% increase hardenability whereas larger
amounts tend to reduce hardenability because of carbide formation.
Vanadium is also utilized in ferrite/pearlite microalloy steels
to increase hardness through carbonitride precipitation strengthening
of the matrix.
Vector plot - A way of displaying results from a CFD analysis,
where the velocity of each particle is visualized with an arrow.
Since the arrow represents "velocity" it has a direction
and magnitude (the length of the arrow indicates the relative
magnitude)
Veins
- A discontinuity on the surface of a casting appearing as a raised,
narrow, linear ridge that forms upon cracking of the sand mold
or core due to expansion of the sand during filling of the mold
with molten metal.
Vickers
hardness test – A microindentation hardness test employing
a 136° diamond pyramid indenter (Vickers) and variable loads,
enabling the use of one hardness scale for all ranges of hardness
– from very soft lead to tungsten carbide. Also known as diamond
pyramid hardness test.
W
Walking-beam
furnace – A continuous-type furnace consisting of two sets of
rails, one stationary and the other movable. Only the work being
processed has to be heated because trays or fixtures are not needed.
Warpage
- Deformation other than contraction that develops in a casting
between solidification and room temperature; also, distortion
occurring during annealing, stress relieving, and high-temperature
service.
Wash
- A casting defect resulting from erosion of sand by metal flowing
over the mold or corded surfaces. They appear as rough spots and
excess metal on the casting surface. Also called cuts.
Washburn
Core - A thin core which constricts the riser at the point
of attachment to the casting. The thin core heats quickly and
promotes feeding of the casting. Riser removal cost is minimized.
Water
quenching – A quench in which water is the quenching medium.
The major disadvantage of water quenching is its poor efficiency
at the beginning or hot stage of the quenching process.
Wear
- Loss of material from a surface by means of relative motion
between it and another body. Third bodies i.e. grit
Welding
- A process used to join metals by the application of heat. Fusion
welding, which includes gas, arc, and resistance welding, requires
that the parent metals be melted.
Welding,
Autogenous - Method of uniting two pieces of metal by melting
their edges together without solder or any added welding metal,
as by the thermite process that employs a medium of finely divided
aluminum powder and oxide or iron by which a temperature of some
2982.2°C (5400°F) is obtained.
Welding
Electrode - A metal or alloy in rod or wire forms used in
electric arc welding to maintain the arc and at the same time
supply molten metal or alloy at the point where the weld is to
be
accomplished.
Welding
Flash - Skin exposed too long to the ultraviolet rays of welding
or melting arcs will burn as in a sunburn. Though temporary blindness
can result, it is not permanent, as is popularly believed.
Welding Process - The joining of materials by the application
of heat or friction.
Welding Stress - That stress resulting from localized heating
and cooling of metal during welding.
White layer
– Compound layer that forms as a result of the nitriding
process.
Wire spraying
- A thermal spray process whereby the supply for the coating material
is fed into the gun in the form of a continuous wire.
Work hardness
- Hardness developed in metal resulting from cold working.
X
X-Ray Diffraction
(XRD) - A surface science technique for crystalline structure
analysis of materials. Widely used both in the research and industrial
area. analyzed depth up to 10 micrometers.
X-ray Photoelectron
Spectroscopy (XPS) - An electron beam technique applied for
near-surface elements identification and chemical state analysis.
Widely used in thin film science. Analysis depth: a few Angstroms.
Y
Yield point - The first stress in a material, usually less
than the maximum attainable stress, at which an increase in strain
occurs without an increase in stress. If there is a decrease in
stress after yielding, a distinction may be made between upper
and lower yield points.
Yield strength – The stress
at which a material exhibits a specified deviation from proportionality
of stress and strain. An offset of 0.2% is used for many metals.
Compare with tensile
strength.
Z
Zinc (Zn)
- Thermal spray coatings of zinc or zinc alloys (e.g. Zn/Al, Zn/Sn)
provides galvanic corrosion protection.
Zinc Plating
- The electro deposition of zinc or zinc alloys (e.g. Zn/Ni, Zn/Sn)
to provide galvanic corrosion protection.
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