Heat Treating of Copper Alloys,Heat Treating of Copper Alloys,Heat Treating of Copper Alloys
HEAT-TREATING PROCESSES that are applied to copper and copper alloys include homogenizing, annealing, stress
relieving, solution treating, precipitation (age) hardening, and quench hardening and tempering.
Homogenizing
Homogenizing is a process in which prolonged high-temperature soaking is used to reduce chemical or metallurgical
segregation commonly known as coring, which occurs as a natural result of solidification in some alloys. Homogenizing
is applied to copper alloys to improve the hot and cold ductility of cast billets for mill processing, and occasionally is
applied to castings to meet specified hardness, ductility, or toughness requirements.
Homogenization is required most frequently for alloys having wide freezing ranges, such as tin (phosphor) bronzes,
copper nickels, and silicon bronzes. Although coring occurs to some extent in brasses, -aluminum bronzes, and
copper-beryllium alloys, these alloys survive primary mill processing and become homogenized during normal process
working and annealing. Rarely is it necessary to apply homogenization to finished or semifinished mill products.
A characteristic of high cooling rates is the uneven distribution of the alloy elements in the interior of the dendritic
microstructure. These differences increase with higher cooling rates and greater differences in composition between melt
and solid phase at the onset of crystallization. This difference may be equalized in some alloys by long-time
homogenization as a result of diffusion processes taking place in the solid phase.
The time and temperature required for the homogenization process vary with the alloy, the cast grain size, and the desired
degree of homogenization. Typical soak times vary from 3 to over 10 h. Temperatures normally are above the upper
annealing range, to within 50 °C (90 °F) of the solidus temperature.
Homogenization changes the mechanical properties: ultimate tensile strength, hardness, and yield (proof) strength all
slowly decrease, whereas elongation at fracture and necking increase by as much as twice the initial value. Figure 1 shows
a typical example of these changes taking place at a homogenizing time of 4 h for alloy C52100, a wrought phosphor
bronze alloy containing nominally 92% Cu, 8% Sn, a small amount of phosphorus, and trace amounts of several other
elements.

The normal precautions that apply to annealing should be used for the homogenization of any particular alloy. The
furnace atmosphere should be selected for the control of both surface and internal oxidation. Where there is appreciable
danger of liquefying segregated phases, the materials, particularly castings, should be well supported and heated slowly
through the final 100 °C (180 °F).
Typical applications of homogenization are:
· Alloy C71900 (copper-nickel-chromium) billets: 1040 to 1065 °C (1900 to 1950 °F) for 4 to 9 h, to
prevent cracks, seams, and excessive wood fiber structure in extrusions
· Alloy C52100 and C52400 (phosphor bronzes, 8 and 10% Sn): 775 °C (1425 °F) for 5 h, to reduce
embrittlement in billets and slabs that are to be cold rolled
· Alloy C96400 (cast 70Cu-30Ni): 1000 °C (1830 °F) for 2 h under a protective atmosphere and then
cooled to 400 °C (750 °F), followed by air cooling

Annealing
Annealing is a heat treatment intended to soften and to increase the ductility and/or toughness of metals and alloys.
Annealing is applied to wrought products, during and after mill processing, and to castings. The process includes heating,
holding, and cooling, and a proper process description should include heating rate, temperature, time at temperature,
atmosphere, and cooling rate where each may affect results.

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