Copper alloys that are hardened through heat treatment are
divided into two general types: those that are softened by
high-temperature quenching and hardened by lower-temperature
treatments, and those that are hardened by quenching from
high temperatures through martensitic-type reactions.
Alloys that harden during low-to-intermediate temperature
treatments following solution quenching include precipitation
hardening, spinodal-hardening and order-hardening types.
Quench-hardening alloys comprise aluminum bronzes,
nickel-aluminum bronzes, and a few copper-zinc alloys.
Quench-hardened alloys normally are tempered to improve
toughness and ductility and reduce hardness in a manner
similar to that for alloy steels.
Low-Temperature-Hardening Alloys
For purposes of comparison, Table 1 lists examples of the
various types of low-temperature-hardening alloys, as well as
typical heat treatments and attainable property levels for
these alloys.
Table 1. Heat treatment of low-temperature-hardening alloys
|
Alloy
|
Solution -Treating Temperature (a)
|
Ageing Treatment Temperature, Time
|
Hardness
|
|
°C
|
°C
|
H
|
|
Precipitation hardening
|
|
C15000
|
980
|
500-550
|
3
|
30 HRB
|
|
C17000, C17200, C17300
|
760-800
|
300-350
|
1-3
|
35-44 HRC
|
|
C17500, C17600
|
900-950
|
455-490
|
1-4
|
95-98 HRC
|
|
C18000 (b), C81540
|
900-930
|
425-540
|
2-3
|
92-96 HRB
|
|
C18200, C18400, C18500, C81500
|
980-1000
|
425-500
|
2-4
|
68 HRB
|
|
C94700
|
775-800
|
305-325
|
5
|
180 HB
|
|
C99400
|
885
|
482
|
1
|
170 HB
|
|
Spinodal hardening
|
|
C71900
|
900-950
|
425-760
|
1-2
|
86 HRC
|
|
C72800
|
815-845
|
350-360
|
4
|
32 HRC
|
|
(a)
|
Solution treating is followed by water quenching.
|
|
(b)
|
Alloy C18000 (81540) must be double aged, typically 3 h
at 540°C followed by 3 h at 425°C.
|
Precipitation Hardening Alloys
Most copper alloys of the precipitation hardening type find
use in electrical and heat-conduction applications. The heat
treatment must therefore be designed to develop the necessary
mechanical strength and electrical conductivity. The
resulting hardness and strength depend on both the
effectiveness of the solution quench and the control of the
precipitation (ageing) treatment ("age hardening" or "ageing"
is used in heat-treating practice as substitutes for the
terms "precipitation" or "spinodal hardening").
Copper alloys are hardened by elevated temperature treatment
rather than ambient temperature ageing as in the case of some
aluminum alloys. Electrical conductivity increases
continuously with time until some maximum is reached,
normally in the fully precipitated condition. The optimum
condition generally preferred results from a precipitation
treatment of temperature and duration just beyond those that
correspond to the hardness-ageing peak. Cold working prior to
precipitation ageing tends to improve the heat-treated
hardness.
In the case of lower-strength wrought alloys such as C18200
(Cu-Cr) and C15000 (Cu-Zr), some heat-treated hardness
may be sacrificed to attain increased conductivity, with final
hardness and strength being enhanced by cold working. Two
precipitation treatments are necessary in order to develop
maximum electrical conductivity and hardness in alloy C18000
(Cu-Ni-Si-Cr) because of two distinct precipitation mechanisms.
When precipitation hardening is performed at the mill, further
treatment following fabrication of parts is not required.
However, it may be desirable to stress relieve parts to remove
stresses induced during fabrication, particularly for highly
formed cantilever-type springs and intricate machined shapes
that require maximum resistance to relaxation at moderately
elevated temperatures.
Transformation Hardening
Transformation hardening strengthens certain alloys by
inducing a phase change to a harder and stronger phase.
Two-phase aluminum bronzes and some manganese bronzes are
given quench-and-temper treatments to increase strength
without unduly sacrificing ductility.
These alloys are hardened by cooling rapidly from a high
temperature to produce a martensitic type of structure, and
then are tempered at a lower temperature to stabilize the
structure and partly restore ductility and toughness.
Two-Phase Aluminum Bronzes. Binary copper-aluminum alloys
have two stable phases at room temperature when the aluminum
content is 9.5 to 16%. When other elements (most notably
about 1 to 5% iron) are added, the corresponding aluminum
content for two-phase alloys is 8 to 14%. Quenching and
tempering can strengthen any of the two-phase alloys. At
temperatures of 815 to 1010°C, the two room-temperature
phases transform to beta in the same manner that alpha plus
Fe3C in steel transforms to austenite. Rapid quenching
produces a hard, brittle structure due to formation of
metastable, ordered, close-packed hexagonal beta, which is
referred to as martensitic beta. Both oil and water quenching
are used commercially.
Tempering for 2 h at 595 to 650°C causes reprecipitation
of fine alpha in a tempered beta-martensite structure,
reducing hardness while increasing ductility and toughness.
Nickel-aluminum bronzes, although more complex, respond to
quench-and-temper treatments in a similar manner.
Nickel-bearing alloys such as C95500 and C63000
quench to a higher hardness and are more susceptible to quench
cracking in heavy and/or complex sections, making oil
quenching desirable.
Cast two-phase aluminum bronzes often are normalized by
heating to 815°C, furnace cooling to about 550°C and
then cooling in air to room temperature. This treatment
produces uniform hardness and improves machinability.
Spinodal-Hardening Alloys
Alloys that harden by spinodal decomposition are hardened by
a treatment similar to that used for precipitation hardening
alloys. The soft and ductile spinodal structure is generated
by a high-temperature solution treatment followed by
quenching. The material can be cold worked or formed in this
condition. A lower-temperature spinodal-decomposition
treatment, commonly referred to as ageing, is then used to
increase the hardness and strength of the alloy.
Spinodal-hardening alloys are basically copper-nickel alloys
with chromium or tin additions. The hardening mechanism is
related to a miscibility gap in the solid solution and does
not result in precipitation. The spinodal-hardening mechanism
results in chemical segregation of the alpha crystal matrix
on a very fine scale, and requires the electron microscope to
discern the metallographic effects. Since no crystallographic
changes take place, spinodal-hardening alloys retain excellent
dimensional stability during hardening.
Order-Hardening Alloys
Certain alloys, generally those that are nearly saturated with
an alloying element dissolved in the alpha phase, will
undergo an ordering reaction when highly cold worked material
is annealed at a relatively low temperature. Alloys C61500,
C63800, C68800 and C69000 are examples
of copper alloys that exhibit this behaviour. Strengthening
is attributed to short-range ordering of the solute atoms
within the copper matrix, which greatly impedes the motion of
dislocations through the crystals.
The low-temperature order-annealing treatment also acts as a
stress-relieving treatment, which raises yield strength by
reducing stress concentrations in the lattice at the focuses
of dislocation pileups. As a result, order-annealed alloys
exhibit improved stress-relaxation characteristics.
Order annealing is done for relatively short times at
relatively low temperatures, generally in the range from
150 to 400°C. Because of the low temperature, no special
protective atmosphere is required. Order hardening is
frequently done after the final fabrication step to take full
advantage of the stress-relieving aspect of the treatment,
especially where resistance to stress relaxation is desired.
Quench Hardening and Tempering
Quench hardening and tempering is used primarily for aluminum
bronze and nickel aluminum bronze alloys, and occasionally
for some cast manganese bronze alloys with zinc equivalents
of 37 to 41%. Aluminum bronzes with 9 to 11.5% Al, and
nickel-aluminum bronzes with 8.5 to 11.5% Al, respond
in a practical way to quench hardening by a martensitic type
reaction. Alloys higher in aluminum content generally are too
susceptible to quench cracking, whereas those with lower
aluminum contents do not contain enough high-temperature beta
phase to respond to quench treatments.
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