Heat treating

Many material properties are achieved by states that arise on the way to equilibrium without corresponding to this final state. This is done by controlled heating and cooling with heat treatment. During heat treatment, only solid-state reactions take place, which are usually thermally activated (diffusion). The exception is martensitic transformation (diffusionless transformation by crystal shearing).

Heat treatment includes the following processes:

  • Heating
  • Holding at one temperature (homogenising)
  • Cooling

The material to be treated passes through different temperatures at different times (temperature-time curve) and is then cooled at different rates in different media (water, oil, salt, inert gas, air) depending on the material and process. The range here extends from furnace cooling to abrupt quenching in order to adjust certain technological properties (chemical concentration, strength, hardness, toughness, microstructure, etc.).

Heat treatment
Soft annealing (recrystallisation)
Stress relief annealing

Heat treatment

Heat treatment processes for copper and copper alloys include homogenisation, soft annealing (recrystallisation annealing), stress-relief annealing and age hardening, consisting of solution annealing, quenching with subsequent tempering. Due to the diversity of copper-based materials and the dependence of temperatures and times of individual heat treatments on the composition of the material, its microstructure and its strength condition due to cold deformation, etc., only global details of the individual heat treatments can be given here. Some reference values for heat treatment temperatures can be found here in the tables or in manuals, data sheets and publications. The heat treatments are preferably carried out in electrically heated furnaces with temperature control accurate to ± 2.5 °C, but also in gas-heated furnaces. Depending on the material and requirements, the annealing is carried out under normal atmosphere, under protective gas or in vacuum, whereby the protective gas is adjusted to be neutral, oxidising or reducing depending on the requirements. The heat treatment can be carried out either discontinuously for batches in pot furnaces, bell furnaces or chamber furnaces or continuously in – mainly horizontally arranged – continuous furnaces. In the case of spring strips, which have to meet the highest requirements in terms of uniformity of structure, accuracy (tolerances) and flatness of the strip, the heat treatment in the strip levitation furnace (type: Junkers) is carried out almost exclusively under inert gas.


Homogenisation annealing is only carried out on castings or hot-formed semi-finished moulds at the semi-finished product plants or foundries before delivery to the further processor. The heat treatment is carried out to compensate for segregations or local differences in composition. Alloys with the alloying elements tin (tin bronze with a wide solidification range) and copper-nickel alloys are mainly affected. Homogenisation annealing is carried out at relatively high temperatures with long annealing times and is therefore quite expensive.

The wrought alloy CuSn8 in particular tends to segregate strongly and is preferably homogenised at 760 °C before cold rolling in order to dissolve the brittle high-tin phase. Homogenisation annealing is not necessary for other copper materials that are hot and cold rolled, or are intermediately annealed (soft annealed) between individual cold forming operations.

For age-hardenable copper alloys, solution annealing above the solubility limit of the alloying element is also referred to as homogenising.

Soft annealing (recrystallisation)

If, through cold forming, the strength of a material, with simultaneously decreasing elongation at fracture, takes on such high values that it cannot be cold formed any further, soft annealing must be carried out. In such an annealing, the soft state characterised by the formation of new crystals (recrystallisation) is generally aimed for. Recrystallisation depends on the grain size of the microstructure, the degree of deformation of the preceding cold working and the recrystallisation temperature. However, if the cold forming is too low, there is a risk of coarse grain formation during annealing. The correlations can be shown in a three-dimensional diagram and explain why no recrystallisation temperature can be specified for a particular material, but only a temperature range. The temperatures to be applied depend on the annealing time. Normally, a short annealing time will be applied at high temperatures and a long annealing time at low temperatures. However, if a recrystallisation diagram is available, the recrystallisation temperature can be specified for a given degree of deformation and known grain size for a given annealing time. Recrystallisation is preceded by material recovery. This is understood to be a relaxation of the material without new grain formation. Grain enlargement follows recrystallisation. It is a growth of energetically favourable grains at the expense of less favourable ones.

Stress relief annealing

Stress relief annealing reduces the stresses present in the semi-finished product or workpiece due to cold forming without causing too great a drop in strength. Stress relief annealing is most important for copper materials that tend to stress corrosion cracking, e.g. for copper-zinc alloys. Since, in addition to a corrosion medium, the presence of internal or external tensile stresses is required for stress corrosion cracking to occur, the risk of corrosion can be significantly reduced by lowering the manufacturing-related tensile stresses. Furthermore, thermal stress relieving mitigates the risk of internal tensile stresses being released by further processing, which can lead to warping of the material. Furthermore, internal stresses add up with the external stress, so that under certain circumstances the load-bearing capacity of the semi-finished part or the workpiece is impaired. For stress relief annealing, the temperature and annealing time are selected so that the recrystallisation threshold is not reached. Material recovery takes place, structural irregularities heal and stresses are relieved without significantly reducing the mechanical strength properties. Thermal stress relieving should take place after cold working has been completed, which can introduce internal stresses into the workpiece. For example, machining can leave internal stresses in the finished machined workpiece.

The standardised copper casting materials are stress-relieved at 250 °C, with the exception of the copper-aluminium casting alloys. The annealing time is 1 h per 25 mm wall thickness. The stress relieving temperature of the copper-aluminium casting alloys is higher at 315 °C.

Annealing of copper and times for heat treatments

It is difficult to give general times for heat treatments. Basically, there is a correlation between time and temperature, i.e. the same result can be achieved with a short time and high temperature (used in continuous furnaces) as with a long time and low temperature. The times also depend on:

  • On the oven (temperature distribution, temperature uniformity);
  • On the number of parts in the oven and the charging;
  • On the size or mass of the individual parts.

As a rule of thumb for the standard processes (discontinuous in a pot, bonnet or chamber oven) can be applied:

  • Small parts 30-60 min.
  • Large parts 1-3 h.

All times apply from the time when the entire batch is heated through (!). Annealing should be carried out under protective gas (nitrogen is sufficient for copper materials) to avoid scaling. The cooling can be done as desired, with copper materials there are no hardening effects as with steel, not even with rapid cooling. Rapid cooling can possibly lead to distortion.

Reference values for the heat treatment of non-hardenable copper materials

(all values in °C)

MaterialStress relief annealingSoft annealingHomogenisation
Copper and silver alloyed copper100 - 150400 - 500 (not applicablet)
Brass (CuZn)250 - 300450 - 600 (not applicable)
Bronze (CuSn), wrought materials 200 - 300 475 - 675ca. 700
Bronze, cast materials200 - 450(not used)ca. 650
Nickel silver (CuNiZn)250 - 400580 - 650 (not applicable)
CuAl alloys (Al-Bronze) *250 - 300 ca. 600 (not applicable)
CuNi alloys280 - 500620 - 900 (not applicable)

* In the case of heterogeneous alloys, additional targeted adjustment of strengths by heat treatments between 450-950 °C, see literature

Recommended times and temperatures for soft annealing of some selected copper-based materials

AbbreviationTemperature [°C]Time [h]
Cu-ETP / Cu-FRHC300 - 650 1)0,5 - 3
Cu-OF425 - 650 0,5 - 3
Cu-PHC / Cu-HCP350 - 6500,5 - 3
Cu-DHP350 - 6500,5 - 3
CuZn15425 - 6500,5 - 3
CuZn30450 - 6750,5 - 3
CuZn33425 - 7000,5 - 3
CuZn37450 - 600 2)0,5 - 3
CuZn36Pb3425 - 6000,5 - 3
CuZn39Pb2425 - 6000,5 - 3
CuZn39Pb3425 - 6500,5 - 3
CuZn40Pb2425 - 6500,5 - 3
CuZn31Si1500 - 6000,5 - 3
CuZn38Mn1Al500 - 6500,5 - 3
CuZn37Mn3Al2PbSi 500 - 6500,5 - 3
CuSn4500 - 700 3)0,5 - 3
CuSn6500 - 700 3)0,5 - 3
CuSn8500 - 700 3)0,5 - 3
(CuSn6Zn6)500 - 700 3)0,5 - 3
CuNi18Zn20600 - 750 4)0,5 - 3
CuNi18Zn27600 - 750 4)0,5 - 3
CuNi9Sn2600 - 7000,5 - 3
CuNi10Fe1Mn625 - 7500,5 - 3
(CuNi44Mn1)650 - 8500,5 - 3
CuAg0,10400 - 6500,5 - 3
CuAg0,10P400 - 6500,5 - 3
CuFe2P650 - 7000,5 - 3
CuSP425 - 6500,5 - 3
CuTeP425 - 6500,5 - 3
CuZn0,5425 - 6000,5 - 3
CuBe2720 -7600,5 - 3
CuBe2Pb 720 -7600,5 - 3
CuCo2Be920 -9600,5 - 3
CuNi2Si725 - 7600,5 - 3
CuCr1Zr600 - 8000,5 - 3
CuZr850 - 9650,5 - 3

* 1) When annealing in a reducing atmosphere, keep the temperature below 450 °C to avoid hydrogen sickness.
2) It is important not to exceed the specified maximum temperature in order not to impair the cold formability of the material.
3) Alloys are susceptible to cracking during annealing and should be thermally stress relieved beforehand.
4) The alloys are to be annealed in an oxidising atmosphere. They are susceptible to cracking and should be thermally stress relieved beforehand.
( ) Alloys no longer included in EN

Recommended annealing times and temperatures for stress relief annealing of selected copper-based wrought materials

AbbreviationTemperature [°C]Time [h]
Cu-ETP / Cu-FRHC150 - 200 1)1
Cu-OF150 - 2001
Cu-PHC / Cu-HCP150 - 2001
Cu-DHP150 - 2001
CuZn15200 - 3001
CuZn30200 - 3001
CuZn33200 - 3001
CuZn37200 - 3001
CuZn36Pb3200 - 3001
CuZn39Pb2200 - 3001
CuZn39Pb3200 - 3001
CuZn40Pb2200 - 3001
CuZn31Si1250 - 3501
CuZn38Mn1Al300 - 4301
CuZn37Mn3Al2PbSi350 - 4501
CuSn4 200 - 3001
CuSn6200 - 3001
CuSn8200 - 3001
(CuSn6Zn6)200 - 3001
CuNi18Zn20300 - 4001
CuNi18Zn27300 - 4001
CuNi9Sn2250 - 4001
CuNi10Fe1Mn280 -4501
(CuNi44Mn1)300 - 4001
CuAg0,10250 - 300 1)1
CuAg0,10P 250 - 3001
CuFe2P200 - 3001
CuSP150 - 2001
CuTeP150 - 2001
CuZn0,5200 - 3001
CuBe2250 - 3001
CuBe2Pb250 - 3001
CuCo2Be350 - 4201
CuNi2Si350 - 4501
CuCr1Zr300 - 3501
CuZr350 - 4001

* 1) When annealing in a reducing atmosphere, keep the temperature below 450 °C to avoid hydrogen sickness.
( ) Alloys no longer included in EN

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