Copper and copper alloys may be heat treated for several purposes,
described in this article.
Homogenizing is applied to dissolve and absorb segregation and
coring found in some cast and hot worked materials, chiefly
those containing tin and nickel.
Diffusion and homogenization are slower and more difficult in tin bronzes,
silicon bronzes and copper nickels than in most other copper alloys.
Therefore, these alloys usually are subjected to prolonged homogenizing
treatments before hot or cold working operations.
The high-tin phosphor bronzes (above 8% Sn) are noted for extreme
segregation. Although these alloys sometimes are hot worked,
usual practice is to roll them cold, making it necessary to first
diffuse the brittle segregated tin phase, thereby increasing
strength and ductility and decreasing hardness before rolling.
These objectives are accomplished by homogenizing at about 760oC.
Softening or annealing of cold worked metal is accomplished by heating
to a temperature that causes recrystallization and, if maximum softening
is desired, by heating well above the recrystallization temperature to
cause grain growth. Method of heating, furnace design, furnace atmosphere,
and shape of work piece are important, because they affect uniformity of
results, finish, and cost of annealing.
For copper and brass mill alloys, grain size is the standard means of
evaluating a recrystallizing anneal. Because many interreacting variables
influence the annealing process, it is difficult to predict a specific
combination of time and temperature that will always produce a given grain
size in a given metal.
Several copper alloys have been developed in which the grain size is
stabilized by the presence of a finely distributed second phase.
Examples include copper-iron alloys such as C19200, C19400 and C19500,
and aluminum-containing brasses and bronzes such as C61500, C63800,
C68800 and C69000. These alloys will maintain an extremely fine grain
size at temperatures well beyond their recrystallization temperature,
up to the temperature where the second phase finally dissolves or coarsens,
which allows grain growth to proceed.
Generally, two annealed tempers are available: light anneal, which is
performed at a temperature slightly above the recrystallization
temperature, and soft anneal, which is performed several hundred
degrees higher, at a temperature just below the point at which rapid
grain growth begins.
When annealing copper that contains oxygen, the hydrogen in the atmosphere
must be kept to a minimum to avoid embrittlement. For temperatures
lower than about 480oC, hydrogen preferably should not exceed 1%.
Stress relieving is aimed to reduce or eliminate residual stress, thereby
reducing the likelihood that the part will fail by cracking or corrosion
fatigue in service. Parts are stress-relieved at temperatures below the
normal annealing range that do not cause recrystallization and consequent
softening of the metal.
Residual stresses contribute to this type of failure, which is
frequently seen in brasses containing 15% zinc or more. Even higher-copper
alloys such as aluminum bronzes and silicon bronzes may crack under
critical combinations of stress and specific corroding, and all copper
alloys are susceptible to more rapid corrosion attack when in the stressed
Stressed phosphor bronzes and copper nickels have comparatively slight
tendencies toward stress-corrosion cracking; these alloys are more
susceptible to fire cracking, which is cracking caused when stressed
metal is heated too rapidly to the annealing temperature. Slow heating
provides a measure of stress relief and minimizes non-uniform temperature
distributions, which lead to thermal stress.
Using a high stress-relieving temperature for a short time is generally
considered best for keeping processing time and cost to a practical
minimum, even though there is usually some sacrifice in mechanical
properties. Using a lower temperature for a longer time will provide
complete stress relief with no decrease in mechanical properties.
Actually, the hardness and strength of severely cold worked alloys
will increase slightly when low stress-relieving temperatures are used.
An additional benefit of a thermal stress relieving is dimensional
stability of cold-formed parts. Also, it is often advisable to stress
relieve welded or cold formed structures. For these structures,
stress-relieving temperature is 85 to 110oC above that used for
mill products of the same alloy.
High strength in most copper alloys is achieved by cold working.
Solution treating and precipitation hardening is applied to strengthen
special types of copper alloys above the levels ordinarily obtained by
Examples of precipitation hardening copper alloys include the
beryllium coppers, some of which also contain nickel, cobalt or
chromium; the copper-chromium alloys; the copper-zirconium alloys;
the copper-nickel-silicon alloys and the copper-nickel-phosphorus alloys.
All precipitation-hardening copper alloys have similar metallurgical
characteristics: they can be solution treated to a soft condition by
quenching from a high temperature, and then subsequently precipitation
hardened by aging at a moderate temperature for a time usually not
exceeding 3 h.
The main advantages of these alloys are:
- Customer fabrication is easily performed in the soft, solution-annealed condition.
- The precipitation-hardening heat treatment performed by the fabricator
is relatively simple. It is carried out at moderate temperatures,
usually in air. Controlled cooling is not needed, and time of treatment
is not of critical importance.
- Different combinations of properties - including strength, hardness,
ductility, conductivity, impact resistance and inelasticity - can be
obtained by varying hardening times and temperatures.
The particular requirements of the application determine the type of
Age-hardenable alloys are furnished in the solution-treated condition,
in the solution treated and cold worked condition or in the age-hardened
Beryllium Coppers. Wrought beryllium coppers, C17000, C17200 AND
C17500, can develop wide ranges of mechanical properties, depending on
solution treating and aging conditions, on the amount of cold work
imparted to the alloy and on whether the alloy is cold worked after
solution treating and before aging or is cold worked after aging.
Copper-Nickel-Phosphorus Alloys. Alloys containing about 1% nickel
and about 0.25% phosphorus, typified by C19000, are used for a wide
variety of small parts requiring, high strength, such as springs, clips,
electrical connectors and fasteners. C19000 is solution treated
at 700 to 800oC. If the metal must be softened between cold
working steps prior to aging, it may be satisfactorily annealed at
temperatures as low as 620oC. Rapid cooling from the annealing
temperature is not necessary. For aging, the material is held
at 425 to 475oC for 1 to 3 h.
Chromium coppers. Chromium coppers containing about 1% Cr, such as C18200,
C18400 and C18500, are solution treated at 950 to 1010oC and
rapidly quenched. Solution treating usually is done in molten salt,
but may be done in a controlled-atmosphere furnace to prevent surface
scaling and internal oxidation. Solution treated chromium copper is
aged at 400 to 500oC for several hours to produce the desired
mechanical and physical properties. A typical aging cycle
is 455oC for 4 h or more.
Zirconium Copper. Zirconium copper C15000 (99.8Cu-0.2Zr) is solution
treated at 900 to 925oC, then quenched in water. Time at the solution
treating temperature should be minimized to limit grain growth and possible
internal oxidation by reaction of zirconium with the furnace atmosphere.
Because solution and diffusion of the zirconium occur rapidly at the
solution treating temperature, holding at temperature is not required.
Aging is done at 500 to 550oC (930 to 1020oF) for 1 to 4 h. If the material
has been cold worked, following solution treating, aging temperature may be
reduced to 375 to 475oC.
Alpha Aluminum Bronzes. The structure and consequent heat
treatability of aluminum bronze varies greatly with composition.
Single-phase (alpha) aluminum bronzes, which contain only copper
and aluminum (up to about 10% Al), can be strengthened only by
cold working. They can be softened by annealing at 425 to 760oC.