Heat Treating of Copper and Copper Alloys

Copper and copper alloys may be heat treated for several purposes, described in this article.

Homogenizing

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 760^oC.

Annealing

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 480^oC, hydrogen preferably should not exceed 1%.

Stress Relieving

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 condition.

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 110^oC above that used for mill products of the same alloy.

Precipitation Hardening

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 cold working.

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 hardening treatment.

Age-hardenable alloys are furnished in the solution-treated condition, in the solution treated and cold worked condition or in the age-hardened condition.

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 800^oC. If the metal must be softened between cold working steps prior to aging, it may be satisfactorily annealed at temperatures as low as 620^oC. Rapid cooling from the annealing temperature is not necessary. For aging, the material is held at 425 to 475^oC 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 1010^oC 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 500^oC for several hours to produce the desired mechanical and physical properties. A typical aging cycle is 455^oC for 4 h or more.

Zirconium Copper. Zirconium copper C15000 (99.8Cu-0.2Zr) is solution treated at 900 to 925^oC, 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 550^oC (930 to 1020^oF) for 1 to 4 h. If the material has been cold worked, following solution treating, aging temperature may be reduced to 375 to 475^oC.

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 760^oC.