Properties of Metals
WORKING OF METALS
WORKING OF METALS
Heat treatment and working of the metal are discussed as metallurgical processes
used to change the properties of metals. Personnel need to understand the effects
on metals to select the proper material for a reactor facility.
STATE how heat treatment affects the properties of heat-
treated steel and carbon steel.
DESCRIBE the adverse effects of welding on metal including
types of stress and method(s) for minimizing stress.
Heat treatment of large carbon steel components is done to take advantage of crystalline defects
and their effects and thus obtain certain desirable properties or conditions.
During manufacture, by varying the rate of cooling (quenching) of the metal, grain size and grain
patterns are controlled. Grain characteristics are controlled to produce different levels of hardness
and tensile strength. Generally, the faster a metal is cooled, the smaller the grain sizes will be.
This will make the metal harder. As hardness and tensile strength increase in heat-treated steel,
toughness and ductility decrease.
The cooling rate used in quenching depends on the method of cooling and the size of the metal.
Uniform cooling is important to prevent distortion. Typically, steel components are quenched in
oil or water.
Because of the crystal pattern of type 304 stainless steel in the reactor tank (tritium production
facility), heat treatment is unsuitable for increasing the hardness and strength.
Welding can induce internal stresses that will remain in the material after the welding is
completed. In stainless steels, such as type 304, the crystal lattice is face-centered cubic
(austenite). During high temperature welding, some surrounding metal may be elevated to
between 500F and 1000F. In this temperature region, the austenite is transformed into a body-
centered cubic lattice structure (bainite). When the metal has cooled, regions surrounding the
weld contain some original austenite and some newly formed bainite. A problem arises because
the "packing factor" (PF = volume of atoms/volume of unit cell) is not the same for FCC crystals
as for BCC crystals.