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Bright annealing process for copper alloy strip and wire

Jan. 08, 2022

Common heat treatment methods for copper alloys are homogeneous annealing, stress-free annealing, recrystallization annealing, solution and aging treatment. In order to prevent oxidation during processing, to save pickling costs and to obtain a bright surface, it is permitted to anneal copper alloy strips, wires and coils in a protective atmosphere or in a vacuum furnace, i.e. bright annealing.

Large amounts of O2, CO2 and H2O in the air oxidize the surface of copper alloys, which must be pickled before further processing. Heating in a protective atmosphere reduces the oxygen content in the furnace and greatly improves the surface quality of the annealed copper alloy. Bright annealing process does not require pickling equipment, no environmental pollution, no health hazards, reduced metal loss, cost savings, and greatly extends the service life of copper alloy strips, wires and coils.


Bright annealing process for copper alloy strip and wire

 

Protective gases

The common protective gases are O₂, CO₂, CO, H₂, H₂ O and N₂. Among them, N2 can be regarded as an inert gas at the heat treatment temperature and does not participate in the chemical reaction, while O2, CO2 and H2O are oxidizing gases and CO and H2 are reducing gases. The main components of copper surface oxidation in the reaction are O2 and H2O. oxygen reacts with copper and zinc to form metal oxides. The equation is 4Cu + O₂ ==== 2Cu₂O.

The very small amount of oxygen in the protective atmosphere is sufficient to oxidize copper and zinc. The oxygen content in the bright treatment furnace for copper alloys must be less than 1 ppm, otherwise the surface of the alloy will oxidize. This is because water vapor can oxidize copper alloys containing zinc, aluminum, lead, tin and beryllium when heated, and the lower the temperature, the more obvious the oxidation. Therefore, the furnace atmosphere must be kept below -60°C.

The main protective atmospheres used for heat treatment of copper alloys are: high-purity nitrogen, purified exothermic atmosphere, nitrogen, ammonia decomposition gas; pure hydrogen, etc. Among them, high-purity nitrogen itself has no reducing ability, purifying exothermic atmosphere and nitrogen-based atmosphere in the reducing atmosphere CO and H2 less, low reduction potential, not suitable for bright treatment. Copper alloy. The current protective atmosphere is mainly ammonia decomposition gas and pure H2. 75% of the ammonia decomposition gas is H2 and the remaining 25% is N2. This is because H2 has good reducing properties and excellent heat transfer than nitrogen, high speed temperature consistency and rapid cooling also increases productivity accordingly; with the change of heat transfer effect, the temperature difference in the charge is reduced and the sticking phenomenon is reduced. The very low density of hydrogen can greatly reduce the unit energy consumption and hot air circulation resistance, and reduce the noise of the strong circulation motor of the furnace table, which is kept below 85dBA.

 

Lubricant

Lubricant also plays an important role in achieving good bright annealing results. Firstly, it must evaporate completely and cannot remove oxygen from the spotless heating process, otherwise oxygen will react with the hydrogen in the protective gas to form vapor and lower the reduction potential. Secondly, mineral oils and emulsions are used as lubricants in the cold rolling of copper strip. The emulsion is characterized by good cooling effect and can obtain a large amount of trace pressure to make high-speed rolling possible and improve productivity, but the emulsion has the defect that impurities are easily eroded, so the emulsion rolled strip must be annealed in a short time, otherwise it will be corroded. At present, low-viscosity mineral oil is known as the main lubricant because of less impurities and easy volatilization by heat.

 

Bright annealing equipment

The bright annealing processing equipment for copper alloy strips, wires and coils is mainly a hooded annealing furnace. The process is not only to obtain a smooth surface and suitable mechanical properties, but also not to have sticky glue, which requires higher performance and structure of the annealing equipment.

✳ Good uniformity of furnace temperature

The annealing equipment must have good furnace temperature uniformity in order to accurately control the annealing temperature of copper alloys. The hooded annealing furnace has a strong convection circulation system, effective circulation fan, large air volume, high wind pressure, fast wind speed, good heat exchange effect, furnace temperature uniformity less than ±5℃, so that all furnace materials get uniform mechanical value and process value. At the same time, it shortens the annealing time and improves the productivity.

✳Good sealing

The working load space is an all-metal housing. With the help of a water-cooled rubber seal between the furnace bed flange and the inner cover flange, the circulating fan unit achieves an absolute vacuum-sealed space. There is no mechanical seal at the blower shaft and no possibility of leakage. Therefore, the dew point of the protective atmosphere can be maintained at -60°C during the entire annealing process, which makes it possible to bright anneal copper alloys.

First the vacuum should be evacuated and then nitrogen purging should be sent to make the atmosphere in the working space as pure as possible, i.e. with the lowest possible oxygen content. The work space is tested for tightness and leak points are found so that there is no chance for air and hydrogen to mix. This vacuum process is essential for copper alloy wire and tubes. During the annealing process, the entire heating phase is cleaned by a protective atmosphere rather than a vacuum. This is because the protective atmosphere can remove evaporated lubricant more effectively than vacuum to ensure a clean surface on the annealed workpiece.

✳ Unique combined cooling system

Typically, each hooded furnace is equipped with two chambers, a heating hood and a cooling hood. For optimum efficiency, the cooling time of the furnace must be shorter than the heating time to allow sufficient time for vacuum replacement at the end of the next cycle of cooling, unloading, loading and annealing.

Equipped with a highly efficient strong convection circulation fan, the convection heat transfer rate is greatly increased and the discharge time during heating, holding and cooling annealing is greatly reduced. In addition, the combined air/water cooling system, not only at the beginning of cooling, the blower of the cooling cover draws in air to spray on the surface of the inner cover, which has cooled the inner cover to below 200℃, then the water spraying device starts to work, spraying water on the inner cover until the end of cooling. Not only prolong the service life of the inner cover, but also greatly reduce the cooling time.

 

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