GRAPHITIZATION: Formation of graphite in iron or steel. Where graphite is formed during solidification, the
phenomenon is called primary graphitization; where formed later by heat treatment, secondary graphitization.
GRAY (Gy): The SI unit of absorbed dose. One gray is equal to an absorbed dose of 1 Joule/kilogram (or 100 rads).
GRID (RT): An assembly of strips of metal, opaque to X-rays, assembled edgewise and interleaved with material of low
absorption, to be placed between the object and the screen or film, in order to reduce the effects of scattered radiation
from the object.
GRID RATIO (RT): The ratio of the depth of the opaque strips of a grid, measured in the direction of the primary
beam, to the spacing between them.
GRINDING CRACKS: See CRACKS, GRINDING.
GRINDING STRESS: Residual stress, generated by grinding, in the surface layer of work. It may be tensile,
compressive, or both.
GRIT BLAST: See SANDBLAST.
GROSS POROSITY: In weld metal or in a casting, pores, gas holes or globular voids that are larger and in greater
number than obtained in good practice.
H AND D CURVE (RT): See CHARACTERISTIC CURVE.
HAIRLINE SEAM: See SEAM.
HALATION (RT): The fogging of a film emulsion due to reflection and dispersion of the radiation within the
emulsion. This is generally apparent at locations of heavy exposure.
HALF-LIFE (RT): The time in which half the atoms in a radioactive substance disintegrate. Half-lives vary from
millionths of a second to billions of years.
HALF-LIFE (BIOLOGICAL): The time required for a biological system, such as a man or an animal, to eliminate, by
natural processes, half the amount of a substance that has entered it.
HALF-VALUE LAYER (RT): The thickness of a material that transmits 50 percent of the radiation incident upon it.
In exponential attenuation, the half-value layer is related to the linear attenuation coefficient and the mean free path.
HALF-VALUE PERIOD (RT): See HALF-LIFE.
HALF-WAVE RECTIFIED AC (MT): Alternating current which passes through a rectifier in such a manner that the
reversing half of the cycle (negative) is blocked out completely. It is pulsating unidirectional current. It differs from
HALL DEVICE (MT): An element composed generally of a semiconductor material which exhibits a relatively large
output voltage across the edges of the element in a directional mutually perpendicular to current flowing through the
material and a magnetic field at right angles to the current flow.