METAL PROPERTIES, CHAR, USES, AND CODES - OD1643 - LESSON 1/TASK 1
repeated or alternating stresses too small to produce a permanent
deformation when applied statically.
(12) Corrosion Fatigue. Failure by corrosion fatigue is a fatigue
failure in which corrosion has lowered the endurance limit by the
formation of pits which act as centers for the development of fatigue
cracks.
Moreover, when any protective film that has been placed on
the metal is broken by fatigue stresses, corrosion spreads through
the cracks in the film and produces pits which act as stress raisers.
If a metal member exposed to fatigue is also exposed to corrosive
agencies, such as a damp atmosphere or oil that has not been freed
from acid, the stress necessary to cause failure is lowered. It is
interesting to note that the unit stress of an extremely strong heat-
treated alloy steel that is subjected to corrosion fatigue will be no
greater than that of a relatively weak structural steel.
The
importance of protecting the surfaces of fatigue members against
corrosion by galvanizing, plating, etc., is obvious.
(13) Machinability.
Machinability is the ease or difficulty with
which a material lends itself to being machined.
(14) Hardness.
Hardness is the ability of a material to resist
penetration and wear by another material. It takes a combination of
hardness and toughness to withstand heavy pounding. The hardness of
a metal is directly related to its machinability, since toughness
decreases as hardness increases.
Steel can be hardened by heat-treating it.
The object of heat-
treating steel is to make the steel better suited, structurally and
physically, for some specific application. If additional information
pertaining to heat-treatment of steel is desired, see TM 9-237 and FM
9-24.
3.
Categories of Metals
a.
General. All metals fall within one of two categories. Either
they contain iron and are considered ferrous metals, or they contain
no iron and are considered nonferrous metals.
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