ABSTRACT: After annealing treatment of Red Copper, mechanical property test and metallographic structure analysis were carried out. The results show that annealing treatment can improve the structure and properties of Red Copper. Key words: Red Copper; Annealing Treatment; microstructure; Mechanical Properties classification number: TG146.1 + 1 Document identification number: A article number: 1008-3944(2006)03-0129-03

Red Copper (pure copper) has excellent conductivity, thermal conductivity and plasticity and good corrosion resistance. It is widely used in electronic industry, instrument and high voltage switch industry. In industrial production, the requirements of surface hardness and internal grain size of red copper parts are very strict, and the usual annealing process is difficult to meet the requirements. The high-voltage Switching Element Foshan means (as shown in figure 1) . In the forging process, because of the particularity of the working condition, the manufacturer has strict requirements on the structure performance and Hardness Index: Qualified Components require the structure with fine grain and hardness above HB70. Some defects, such as coarse grain and insufficient hardness, will appear in the conventional annealing process, which will seriously affect the service life and work efficiency of the components. In order to solve this problem, the annealing process of red copper at various temperatures was studied. The metallographic structure of the sample was observed, the hardness of the sample was measured, the hardness curve was made, and the microstructure and hardness change rule of T2 red copper annealed at different temperature were obtained, the microstructure of red copper has been studied and analyzed, and the optimum annealing process has been found by considering both grain size and hardness, so as to make good preparation for the production of the finger parts.

1.Material Preparation and test methods 

(1) the chemical composition of the test material and equipment using copper T2 as the test material is shown in Table 1.

The copper bars of 12 ~ 15 samples were drawn and deformed by 50% , 28 of which were divided into 7 groups. After annealing treatment, two of each group were tested for mechanical properties, two of them were tested for metallography, and the average value was taken. Test Equipment: Olympus (PMG3) Optical metalloscope and IAS-4 Image Analyzer System; box resistance furnace; brinell hardness Tester (HB3000) .

(2) test methods the annealing process is shown in Table 2.

2.Test Results and analysis 

(1) mechanical properties

The brinell hardness of the specimen was measured. The results are shown in Table 3 and Fig. 2. It is shown from the diagram that the hardness of red copper decreases after annealing treatment, in which the hardness curve is steep in the region of 300 °C ー360 °C and decreases rapidly with the increase of annealing temperature, and the hardness curve begins to slow down in the region of 360 °C ー550 °C, the hardness curve is approximately parallel to the temperature axis in the region of 550 °c ー720 °C, that is, the effect of annealing temperature on the hardness of the material is weak.

The main factor affecting the hardness of red copper by deformation annealing is the grain size. Because of the effect of fine grain strengthening, the finer the grain, the higher the strength. In the model of grain boundary dislocation packing, there is a reaction or back stress on the dislocation source in the middle of grain after the dislocation packing, which increases with the number of dislocation reaction and increases to a certain value, can make the dislocation source stop action. The dislocation source in the center of the coarse grain is still emitting the dislocation source when the dislocation source in the center of the fine grain has been forced to stop, because the dislocation source in the reaction grain is close to the dislocation source. As a result, there are more stacking dislocations around the coarse grains, resulting in greater stress concentration, which makes it easier for the dislocation sources of the neighboring grains to start, so the yield strength of the coarse grains is lower, that is to say, the plastic deformation of the coarse grains begins under the lower external force, thus the strength is lower.

(2) The metallographic specimen was corroded by 3% FECL 3 + 10% HCL aqueous solution. The results are shown in Fig. 3. The results show that the effect of holding time on grain size is small at low annealing temperature and large at high annealing temperature. Therefore, annealing at high temperature should be as far as possible to shorten the holding time to avoid coarse grains. In order to avoid recrystallization texture, the cold deformation degree before annealing should not be more than 40% ~ 60% . The higher the cold deformation degree, the higher the annealing temperature, the more obvious the recrystallization texture. The microstructure of 400 °C annealing is much different from that of 700 °C annealing. The microstructure of the former is much finer than that of the latter, which is suitable for pressure processing. The main factors that affect the grain size are annealing temperature and holding time. The actual grain size depends on the specific heating temperature and holding time, but mainly depends on the heating temperature. At a certain heating temperature, the grains grow with the holding time, but grow up to a certain size, the grains grow up very slowly. At the same maximum heating temperature, the faster the heating speed, the smaller the grain size. This is because the higher the heating speed, the higher the transformation temperature, the higher the nucleation rate, the smaller the initial grain size, and the faster the heating speed, the shorter the holding time, the less the grain size.

3.Conclusion

Annealing treatment can change the grain size of purple copper structure, thus affecting the mechanical properties of Red Copper, improve its strength and hardness. Because of the different annealing temperature, the microstructure and hardness of red copper are different. The hardness at low annealing temperature is high, and the microstructure is fine and dense, so it is not easy to be processed under pressure. Our aim is to find an optimal annealing temperature to ensure that the grain size is small and hardness is moderate. The suitable temperature range of hardness can be found from the curve of hardness As can be seen from the diagrams and metallographs, annealing at 390 °C is the preferredtemperature for the pre-treatment of red copper during forging.

 Source: Chinanews.com, by Guo Guizhong

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