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Ga
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Chemical symbol for Gallium. |
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Galvanic Action
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When iron and steel are subject to conditions of aqueous corrosion
the incidence and rate at which the corrosion takes place will alter
if the steel is coupled with other metals or alloys that are also
exposed to the electrolyte. Copper, brass, bronze, lead and nickel
are more "noble" and act as auxiliary cathodes to the steel and
accelerate its anodic dissolution, that is, its corrosion.
Magnesium, zinc and zinc-base alloy are nearly always less noble and
tend to divert the attack from the steel to themselves. The galvanic
relationship of various metals is an important factor affecting
corrosion. |
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Gamma Iron
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The allotropic form of iron existing between the temperature 910°C
and 1400°C is known as Gamma Iron. It has a face centered cubic
lattice and is non-magnetic. Gamma iron containing carbon or other
elements in solution is known as austenite. |
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Gas Carburising
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A heat treatment method used in the case- hardening of steel. Carbon
is absorbed into the outer layers of the components by heating in a
current of gas, rich in carbon compounds. The process is more
versatile than some other methods as the depth of the case and the
limiting carbon content of the case can be controlled by the
composition of the atmosphere, the dew point and the temperature. |
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Gauge Length
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Used in the mechanical testing of steel, it is the length marked on
the parallel portion of a tensile test piece from which the
elongation is measured. |
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Gauge Plate
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An alloy tool steel supplied in flat and square section with the
surfaces ground to close limits. It is also known as Ground Flat
Stock and is used for the manufacturing of gauges, punches, dies,
jigs, templates etc. |
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Ge
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Chemical symbol for Germanium. |
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Grain
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An individual crystal in a polycrystalline metal or alloy; it may or
may not contain twinned regions and sub grains. |
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Grain Flow
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Fiber like lines appearing on polished and etched sections of
forgings, which are caused by orientation of the constituents of the
metal in the direction of workings during forging. Grain flow
produced by proper die design can improve required mechanical
properties of forgings. |
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Grain Growth
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Growth of some grains at the expense of others, resulting in an
overall increase in average grain size. |
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Grain Size Control
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When a steel is austenitised by heating to above the critical range,
time is required for the production of a homogeneous structure
during which there is a tendency towards grain growth. Although
subsequent hot and cold working affect the grain size, it is
originally controlled at the steel making stage by the addition
of aluminium. |
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Grain Size Measurement
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Grain size is normally quantified by a numbering system. Coarse 1-5
and fine 5-8. The number is derived from the formula N=2n-1 where n
is the number of grains per square inch at a magnification of 100
diameters. Grain size has an important effect on physical
properties. For service at ordinary temperatures it is generally
considered that fine grained steels give a better combination of
strength and toughness, whereas coarse grained steels have better
machinability. |
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Granular Fracture
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A type of irregular surface produced when metal is broken,
characterized by a rough, grain like appearance as differentiated
from a smooth, silky or fibrous type. It can be sub classified into
transgranular and intergranular forms. This type of fracture is
frequently called crystalline fracture, but the inference that the
metal broke because it "crystallized" is not justified, because all
metals are crystalline when in the solid state. Contrast with
fibrous fracture, silky fracture. |
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Graphitising
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An annealing process applied to cast iron and steels with a high
carbon and high silicon content by which the combined carbon is
wholly or in part transformed to graphitic or free carbon. |
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Grey Iron
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Also known as flake iron on account of all or part of the carbon
content being in the form of graphite distributed through the metal
as flakes. |
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Grinding
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A machining process:- (a) to shape components that are too hard to
be machined by conventional methods such as hardened tool steels and
case or induction hardened components. (b) to obtain a high degree
of dimensional accuracy and surface finish on a component. |
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Grinding Cracks
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Cracks can arise from incorrect grinding and appear in the form of a
network. They are caused by the generation of high heat and rapid
cooling in the area of contact and they mostly occur when grinding
fully hardened material such as tool steel. |
