ABSTRACT: The causes of oxidation and cracking of red copper tubes after annealing and the improvement measures are analyzed and discussed. The well vacuum protection atmosphere is used to carry out bright annealing instead of the traditional process, the phenomenon of oxidation and cracking of red copper tube after annealing is improved remarkably.

Keywords: Copper Tube; Annealing; oxidation; cracking; Bright Annealing; MAP classification number: TM623 document identification number: A article number: 11-5004(2018)07-0262-2

Because of its good conductivity, heat conductivity and economy compared with oxygen-free copper, t2 copper has become the preferred raw material for making copper tube terminals of locomotive cable connection accessories. When purchasing, in order to facilitate transportation, avoid possible piling deformation during transshipment and storage, the state of delivery shall be hard state (Y) . In order to ensure good processing plasticity, it is necessary to carry out annealing before pressing, remove internal stress, eliminate work hardening and obtain good plasticity and grain structure, so as to ensure that cracking does not occur during pressing. This paper mainly discusses the phenomenon of serious oxidation and cracking on the surface of copper tubes after annealing in box-type resistance furnace, analyzes the causes of the phenomenon, and through improving equipment, adopting well-type vacuum furnace, pre-vacuuming and adding protective atmosphere, the phenomenon of serious oxidation and cracking has been significantly improved after test and comparison.

1. Cause analysis of annealing oxidation and cracking

The main chemical compositions of T2 copper are shown in Table 1, and the mechanical properties are shown in Table 2.

Before the improvement of annealing equipment, a small box-type resistance furnace was used to carry out high temperature rapid heating (700 °C, 15 minutes of heat preservation) annealing. The cooling method was water cooling, each furnace in about 500 ~ 1000 pieces of between the stowage mode for mixed stacking. According to this process, the processed products have the defects of serious oxide scale, coarse grain, pressing cracking and so on.

1.1 analysis of the cause of oxidation

Copper annealed in reducing atmosphere is easy to react with oxygen in air to produce Cuo, the chemical equation is: 2CU + O2 = = 2cuo, commonly known as oxidation phenomenon. When heating in box-type resistance furnace, because it is not in vacuum, the medium in the furnace is mainly air. After heating at high temperature, the red copper tube will react with oxygen, Carbon Dioxide, water vapor and so on in the air to form oxide film. Because the surface of copper pipe is polluted by oil before charging, after continuous annealing, the impurities of oil fume, dust and carbon black accumulated in the furnace can not be discharged circularly, resulting in more and more serious oxidation. In addition, the high temperature (annealing temperature is about 700 °C in our box-type resistance furnace) and the rapid oxidation of copper after contact with air are also the main reasons for the surface oxidation of copper tubes.

1.2 cause analysis of cracking

The main cause of cracking is due to annealing temperature is too high, to obtain a coarse grain structure, dissolved in the metal and the residual hydrogen and Cuprous oxide on the grain boundary to reduce the role of the following formula: CU2O + H2 = 2cu + H2o, the resulting moisture escapes outwards beyond the Grain Boundary Strength, resulting in cracks, or "hydrogen embrittlement" . In addition, because the charging mode is free mixing and the charging quantity is in pursuit of maximum production efficiency, the copper tubes are heated unevenly in the furnace, resulting in incomplete annealing and residual stress between grains.

2. Improvement Measures

The oxidation phenomenon causes the hidden trouble in the electric connection, reduces the conductivity, causes the electric connection breakdown, seriously affects the normal operation of the locomotive. Cracking phenomenon produces more scrap rate, so that enterprises bear more economic losses. In order to completely improve the serious oxidation and cracking phenomenon, the annealing process needs to be improved. At first, we use the method of laying charcoal powder in the box resistance furnace and burning it to get carbon dioxide protective atmosphere. Although the effect is obvious, it can not be used in batch because of the inconvenient operation and serious pollution. After that, the traditional box annealing was replaced by the pit furnace pre-vacuum and protective atmosphere annealing process, which finally eliminated the annealing oxidation and cracking that had puzzled the enterprise for many years. The main structure of the pit-type vacuum annealing furnace is composed of furnace chamber, furnace liner, temperature control cabinet and vacuum pump. The main working principle of the pit-type vacuum annealing furnace is to pre-vacuum the furnace liner to-0.1 MPA, then fill 0.02 MPA ~ 0.04 MPA nitrogen through the inlet of the furnace liner cover, as a protective atmosphere, thus achieving bright annealing. One of the important parts of the round furnace liner material for 310S high-temperature stainless steel, 10mm wall thickness of the liner, its flange has a silicone rubber seal ring. The flange of the tank mouth is 30mm thick, made of 310S high temperature resistant stainless steel, and the bottom of the tank is oval head. The heating element is made of CR20NI80 NI-CR resistance band, which is divided into upper, middle and lower zones and distributed in the furnace. At the same time, the furnace adopts all-fiber energy-saving lining, compared with laying firebrick, it saves more than 20% electricity, reduces the energy consumption, reduces the furnace weight, the heat storage is small, and the temperature rises quickly. The closed cooling tower provides circulating water cooling for the sealing ring of the furnace cover, which extends the service life of the sealing ring and ensures the vacuum degree and sealing effect. The cover of the furnace is equipped with an exhaust valve, which is used to adjust the pressure in the heating process. The gas pressure in the furnace is guaranteed to be below 0.1 MPA. In order to ensure uniform heating of copper tubes in the furnace, taking into account the convenience of charging and discharging, annealing process is used to load the copper tubes on a plate one by one so as not to collide with each other and ensure uniform heating. The multi-layer charging ensures the working efficiency of the single furnace to the maximum. When exiting the furnace, the whole furnace workpiece can be hoisted out only by hoisting the column. At the same time, in order to ensure the continuity of production, the next furnace workpiece can be hoisted into the furnace when exiting the furnace, making full use of the residual heat in the furnace, shorten the heating time, to achieve multi-shift continuous operation.

3. Well vacuum annealing process

3.1 process step 

(1) charging. Put The workpiece into the annealing fixture, lift it into the furnace, cover the furnace and tighten the screws. The furnace is then hoisted into the hearth. 

(2) vacuum. Connect the vacuum pump to the air inlet, pre-vacuum when the pressure in the furnace chamber is close to-0.1 MPA, then stop the pumping, and record the time of vacuum pumping, the time of vacuum pumping is less than 6 minutes, if it exceeds the need to check the equipment. Keep pressure for 5 minutes, observe the pressure gauge must not change, otherwise leak detection is required. 

(3) nitrogen. When the pressure gauge indicates that it is between 0.02 MPA and 0.04 MPA, the filling can be stopped. Observation pressure gauge must not be changed, otherwise leak detection is required.

 (4) heat and hold. Set the heating temperature from 450 °C to 500 °C for 120 minutes. During the heating period, the pressure gauge should be observed, and the pressure should be controlled to ensure that the pressure in the furnace is under 0.1 MPA and not less than 0.05 MPA, so as to avoid the explosion of the furnace caused by the excessive expansion of the gas pressure in the furnace with the increase of the furnace temperature, or the air pressure is too low to cause air to flow back into the furnace to oxidize the annealed parts. The cover can not be opened until the furnace is cooled to less than 50 °C and the workpiece can be removed.

4. Validation

After adopting the well-type furnace pre-vacuuming and adding the protective atmosphere annealing process, the oxidation phenomenon has been completely improved (Fig. 1, Fig. 2) , which saves a pickling process for the enterprise. Cracking has also decreased significantly, from 15% of the original failure rate of cracking down to 2% .

5. Conclusion

Cracking directly leads to the scrap of products, the scrap rate is about 5% ~ 20% of the total amount of products every year, and oxidation affects the surface quality of copper tubes and the electrical conductivity of the terminals. Through the above improvement measures, the cracking phenomenon has been reduced to 2% , oxidation phenomenon has been thoroughly improved, product quality has been greatly improved. 

Source: Chinanews.com, by Hu Min