ABSTRACT: A large number of cracks appeared in the welded joint of a red copper pipe and a low carbon steel pipe near the side of the red copper pipe, by means of macroscopic analysis, chemical composition analysis, scanning electron microscope observation and metallographic examination, the cracking reason of the welded parts was analyzed. The results show that the excessive oxygen content in the raw material of red copper pipe and the high welding temperature lead to the weakening of grain boundary, which results in the superimposition of microstructure stress and thermal stress at the welded joint, when the superimposed stress is greater than the bearing capacity of the material, it will cause the grain boundary cracking, which will lead to the cracking of the welded parts. Finally, the improvement measures are put forward on the basis of analysis.

Keywords: Red Copper Tube and low carbon steel tube welded parts; cracking; oxygen content; grain boundary weakening. Classification: TG457.6 DOCUMENT SYMBOL: B Article Number: 1001-4012(2010)06-0385-03

Copper pipe is widely used in air conditioning and refrigeration industry because of its excellent plasticity, heat conductivity and corrosion resistance. The copper tubes used as heat exchange and transfusion pipelines are usually connected with other materials such as liquid storage tanks. The product is made of TP2(m) red copper pipe and low carbon steel pipe by flame brazing. After the welding, it is found that the product cracks near the side of the red copper pipe and the reject rate reaches 30% . In order to find out the reason of the product cracking and improve the product quality, the author has carried on the physical and chemical examination and analysis to it.

 1. Physical and chemical tests 1.1 macro analysis

Fig. 1 is the macro-morphology of the cracked product. The crack is located on the side of the welded joint of Red Copper Tube and low carbon steel tube. The surface of red copper tube is orange skin net shape.

1.2 chemical composition analysis

The samples were taken from raw materials and cracked products of copper tubes for chemical composition analysis. The raw material sample is the same batch of unwelded copper pipe. The sample location of cracked product after welding is at a as shown in Fig. 1. As can be seen from Table 1, the oxygen content of the raw material of red copper pipe is higher than the standard value

The phosphorus content of Red Copper Tube near the weld is obviously lower, but the iron content is slightly higher. The low phosphorus may be due to the melting of the compounds at the grain boundary, and the iron may come from the low carbon steel pipe. The impurity elements such as lead and bismuth were not found in red copper tube, so the possibility of hot embrittlement and cold embrittlement caused by low melting point eutectic or brittle compound distributed on the grain boundary could be ruled out.

1.3 metallographic examination

The metallographic specimen at the B position shown in Fig. 1 was observed by optical microscope. The microstructure of the specimen was found to have serious intergranular cracks and some of the grain boundaries had point-like holes, as shown in Fig. 2. Fig. 2 microstructure of cracked copper tube

1.4 Sem analysis

The fracture surface was observed by Hitachi S3400N tungsten filament scanning electron microscope after cleaning. As can be seen from Fig. 3, the fracture surface is dominated by dimple morphology, and there are obvious void-like defects, secondary intergranular cracks and oxidation in local area. Among them, Fig. 3(a) is a low magnification fracture surface, and macroscopically, the fracture surface with no obvious plastic deformation can be observed to have a slight wall thickness contraction, which is a characteristic of one-time fracture of ductile materials, it is found that there are holes and pits in the section, and there are more intergranular secondary cracks in the local section of the cracked copper tube, the fracture feature is dimple, see Fig. 3(c) After further enlargement of the fracture surface, tiny pores and oxidation were observed on the grain boundary, as shown in Fig. 3(d) .

2. Synthesis

The results of chemical composition analysis show that the oxygen content of red copper tube is 0.018% , which is higher than the upper limit of oxygen content of TP2 copper in GB/t 5231-1985(0.010%) , from 0.024% to

0.002% . Metallographic examination showed that there were serious intergranular cracks in the microstructure of cracked copper tubes, and some holes were observed in the local grain boundaries. The results of SEM show that the fracture surface is mainly dimple, and there are holes and oxidation in the fracture surface, and there are secondary intergranular cracks in the outer wall of cracked copper tube, and there are tiny holes in the grain boundary, which have the character of intergranular ductile fracture. The reason is that superheat causes Cu3P compound to dissolve in the grain boundary of Red Copper Tube, and the grain boundary to creep at high temperature and form intergranular ductile crack. The physical property data of the two materials are shown in Table 2[3] . It can be seen that the thermal conductivity of red copper and low carbon steel is quite different, the welding temperature on one side of the copper tube will be too high, which will lead to overheat or overheat, which will cause the melting of CU3P compound distributed on the grain boundary and cause the weakening of the grain boundary.

The crack on the side of the cracked copper tube belongs to one-time crack with high stress. The intergranular crack is mainly caused by the melting of intergranular compound and the intergranular crack. When the welding temperature is above 800 °C, it is easy to react with the reducing gases such as H2, Co, C2H2 and so on. When the stress exceeds the strength of the material, the oxygen in the Red Copper Tube will accumulate on the grain boundary, this is consistent with the large number of small dimples observed by scanning electron microscopy. Under the combined action of the grain boundary cracking and the hole between the grain boundary, the welding parts finally cracked on the side of the copper tube.

3. Conclusions and recommendations

Because the oxygen content in the raw material of red copper pipe exceeds the standard and the welding temperature is too high, the grain boundary is weakened, which causes the superimposition of the microstructure stress and the thermal stress at the welding joint of red copper pipe and low carbon steel pipe, when the superimposed stress is greater than the bearing capacity of the material, it will cause the grain boundary cracking, which will lead to the cracking of the welded parts. It is suggested to strengthen the control of raw material composition, especially the oxygen content, to control and improve the welding process, to fully preheat the side of low carbon steel pipe, and to reduce or avoid the influence of welding temperature and welding time on the quality of welded parts. The rejection rate of welded joints produced by the above measures is greatly reduced. 

Source: Chinanews.com, by Shang Yanwei