Technical Data

Metallurgical Terms

Ga.

Chemical symbol for gallium.

Gamma Iron.

The allotropic form of iron existing between the A₃ and A₄ transformation points, which in pure iron occur at 910° 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).

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.

Ge.

Chemical symbol for Germaniun.

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 frequently by the addition of aluminium to the liquid steel.