quenched in warm oil (120° to 140°F). Do not remove
the material from the oil bath until you can hold the
artificial aging usually causes an increase in hardness
work comfortably in your hand. This procedure should
result in a minimum hardness of 65 Rc. Temper to the
The following definitions will aid you in under-
standing some of the terminology used in heat-treating
Homogenizing--a mill process that uses high
temperature for prolonged periods to eliminate or
The information in this section deals primarily with
decrease segregation in castings that are to be hot- or
the heat-treating of nonferrous alloys. For specific
cold-worked. This process is usually used for copper
Metals Handbook, published by the American Society
for Metals, or any handbook that deals with
ment used to balance stress in cold-worked material
heat-treating nonferrous metals.
with decreasing mechanical strength properties induced
You can anneal most nonferrous metals by heating
by the cold-working. Nickel and nickel alloys may be
them uniformly within a certain temperature range,
subjected to this heat treatment, depending on their
soaking them within this temperature range, and then
cooling them back to room temperature. In most cases,
and the intended usage of the parts made from the
the annealing temperature for most nonferrous metals
is the temperature at which recrystallization takes place.
For example, aluminum recrystallizes at a temperature
of 300°F and copper recrystallizes at 390°F.
No matter how carefully you follow instructions for
The rate at which the metal may be cooled from the
heat-treating, you may occasionally find yourself with
annealing temperature depends upon the type of metal.
a job that just will not turn out right. To some extent you
If the metal is pure, or nearly so, or if it remains a solid
can avoid this problem by knowing in advance
solution without precipitation when cooled, any method
something about the many problems encountered in
of cooling is usually satisfactory. If, on the other hand,
heat-treating metals and alloys. The most common
the constituents precipitate during or after cooling,
heat-treating problems include (1) design problems,
furnace cooling may be necessary for complete
(2) cracking, (3) warping, (4) soft spots, (5) size
annealing to occur.
changes, and (6) spalling, and (7) blistering.
There are two types of heat treatment of nonferrous
alloys--the solution treatment and the precipitation
treatment. Some alloys require both treatments. Others
require only the solution treatment.
Although you are not usually in a position to do
The solution treatment consists of heating the alloy
much, if anything, about the design of a piece to be heat
to the temperature at which the principal constituents
treated, you should have some knowledge of the effects
go into solid solution, soaking the ahoy at this temper-
of design upon heat treatment. Unsatisfactory perform-
ature to produce a uniform structure, then cooling it at
ance of tools and structural parts is often the result of
a rate fast enough to retain the solid solution at room
poor design rather than poor materials, poor fabrication,
temperature. You should remain aware that different
or incorrect heat treatment. Errors in design cannot be
alloys have different solution temperatures.
corrected by heat treatment, but you can minimize the
problems of heat-treating a poorly designed piece if you
After a nonferrous ahoy has been solution treated,
know how to compensate for design errors when
it is in a form of supersaturated solid solution. For the
heat-treating the piece.
alloy to attain maximum hardness and strength, excess
hardening constituents that are soluble at room
When a piece of metal is removed from the heat-
temperature must precipitate from the solid solution.
treating furnace, its temperature is uniform throughout.
After the precipitation is complete and the alloy has
Whether or not this piece will cool uniformly depends
reached its full hardness and strength, it is said to be
largely upon the design. Uniform cooling is, of course,
AGE HARDENED. Alloys that do not precipitate at
a slightly inaccurate term; no piece of metal can cool