Metallurgical Terms
L
La
Chemical symbol for Lanthanum
Lap
(a) A defect appearing as a seam on the surface of forged or rolled
products. It is caused by a portion of the steel being folded over on itself and
failing to be welded up on further rolling or forging. Laps may arise from
careless working, defective grooves in rolls or from sharp fins or corners on
the ingot (Cf. Cold Lap)
(b) A piece of soft metal, wood or leather, usually in the form of a rotating
disc which is charged with an abrasive or polishing powder.
(c) The extent to which one plate of a lap joint covers the other.
Lap-Weld
A term applied to a weld formed by lapping two pieces of metal and then
pressing or hammering. It is applied in particular to the longitudinal joint in
tubes produced by a welding process in which the edges of the skelp are beveled
or scarfed so that when they are overlapped they can be welded together. The
product is known as lap-welded or lap-weld pipe.
Latent Heat
Thermal energy expended in changing the state of a body without raising its
temperature, e.g., ice to water at 0˚C, or water to steam at 100˚C, or alpha to
gamma iron at the A, point.
L-D Process (Austrian)
A modified Bessemer process where steel is produced in a solid bottom
converter by injection of oxygen into the molten iron bath from a water-cooled
lance inserted through the converter mouth.
Ledeburite
The eutectic of the iron / iron carbide system. It freezes at about 1130˚C
and is composed of austenite and cementite containing about 4.3% carbon. During
cooling, the austenite may transform to ferrite and cementite. It is typically
found in cast iron.
Li
Chemical symbol for Lithium
Lime (CaO)
Added to the bath during steel making, in order to obtain a slag of the
required composition.
Limit of Proportionality
The stress (load divided by original area of cross-section of the test
piece) at which the strain (elongation per unit of gauge length) ceases to be
proportional to the corresponding stress. In practice it is determined by
inspection of a load-elongation diagram, obtained by plotting extensometer
readings, and is the stress at which the load-elongation line ceases to be
straight.
Limiting Creep Stress
A term, not often used now, denoting the maximum stress at which a material
will not creep by more than a certain amount within the working life of the
part.
Limiting Range of Stress
The greatest range of stress that a metal can withstand for an indefinite
number of cycles without failure. If exceeded, the metal fractures after a
certain number of cycles which decrease as the range of stress increases. When
the mean stress is zero, half this range is the fatigue limit.
Liquid Carburizing
(See Surface Hardening)
Liquidus
A line in a binary phase diagram or a surface on a ternary phase diagram,
representing the temperatures under equilibrium conditions at which freezing
begins during cooling, or melting is completed on heating, i.e., the line or
surface above which all the alloys in the system are completely molten.
Lithium (Li)
One of the alkali metals. Lithium-containing gas may be used to form a
protective atmosphere in annealing furnaces.
Load-Extension Curve
A line plotted from the results of a tensile test, with loads as ordinates
and elongations of the gauge length as abscissae, thus relating the extension of
the material under test to the applied load. (See also Stress Strain Curve).
Lost Wax Process
(See Precision Casting)
Luders Lines
(See Stretcher Strains)