Metallurgical Terms
G
Ga
Chemical symbol foe Gallium
Gamma Iron
The allotropic form of iron existing between the A, and A, transformation
points, which in pure iron occur at 910˚C and 1405˚C; with the addition of
carbon or other alloying elements the position of these points varies. Gamma
iron has a face centred cubic lattice and is non-magnetic (See Allotropy). Gamma
iron containing carbon or other elements in solution is known as austenite.
Gas Carburizing
The introduction of carbon into the outer layers of low carbon steel by
heating in a current of gas rich in carbon compounds, such as carbon monoxide
and/or hydrocarbons. (See Carburizing).
Gas Pocket
(See Blowholes)
Gas Welding
(See Welding)
Gate
(a) The end of a runner where the metal enters the casting. The term
sometimes applied to the entire assembly of connected channels, to the pattern
parts which form them or to the metal which fills them, whilst sometimes it is
restricted to mean the first or main channel.
(b) In drop forging, that portion of the die which is cut out to permit a short
connection between the impression and the outer edge of the dies.
Gauge Length
(a) The specified length marked on the parallel portion of a tensile test
piece on which the elongation is measured.
(b) The active length in the torsion test piece.
Gauss
The c.g.s. unit of magnetic induction. (See Magnetic Flux Density).
Ge
Chemical symbol for Germanium
Ghost
A faint light-coloured streak, just visible at certain angles, under
suitable lighting conditions on a freshly machined or etched surface. It is due
to local segregation in the wrought steel and often has a high proportion of
phosphorus in solution and excess of sulphide inclusions.
Grain Size Control
A special deoxidation technique producing steel having an austenitic grain
size within a specified range in accordance with the McQuaid Ehn grain size
scale. It is achieved by the addition of aluminium to the liquid steel.
Grain Size Measurement
An etched section is compared with standard charts, or the number of grains
intersected by a line of standard length is counted, as in the Snyder and Graff
method. The result is often reported as an arbitrary number in the A.S.T.M.
system the grain size number N is given by the formula: number of grains per sq.
in. at x 100 = 2N-1 Grain size may also be measured by comparing the fracture
surface with the standard specimens as in the Shepherd test. Usually the grain
size refers to the austenite grains which formed in the preceding austenizing
treatment.
Graphitizing
An annealing process applied to certain iron-base alloys, such as cast iron,
or to some steels with high carbon and silicon contents, by which the combined
carbon is wholly or in part transformed to graphitic or free carbon.
Gravity Die Casting
(See Die Casting)
Grey Iron
(See Cast Iron)
Grinding Cracks
Cracks caused by incorrect grinding, producing local heating and rapid
cooling. They generally occur in the grinding of hardened material such as high
carbon or case hardened steels, and appear in the form of network.
Growth
(a) The swelling and weakening experienced by cast iron on repeated heatings
in oxidizing atmospheres.
(b) In powder metallurgy, the increase in dimensions of a compact which may
occur during sintering, although normally the tendency is for the compact to
shrink.