Why are there cracks in copper pipes when welding?

The causes of cracks and pinholes in copper tube welding are as follows:

1. Improper operation will lead to excessive brazing temperature.

2. Copper tube has its own defects.

There are two situations that deserve special attention.

First, the copper tube was damaged in the process of production and processing, and some parts showed tiny cracks or deep cracks.

Internal defects of pipe wall. When this happens in the brazing area, microcracks or defects will expand with the grain growth during brazing heating, and even penetrate the pipe wall. Although this coincidence is rare, it does happen.

Second, the excessive content of some trace elements in copper tube materials leads to brazing cracks. The author once encountered serious brazing cracks in T2 copper tube due to high oxygen content.

The process operation is normal, but after brazing, there are obvious macro cracks in the copper tube, and more seriously, it breaks at the welding place. After metallographic analysis, it was found that the grain boundary in the heat affected zone widened (photo 5), and then it was determined that the oxygen content was several times higher than the normal value. Oxygen exists at the grain boundary in the form of cuprous oxide in copper, and reacts with hydrogen in propane at high welding temperature to generate water vapor, which cracks the copper tube from the inside of the structure.

At present, copper tubes made of No.2 phosphorus deoxidized copper are widely used in refrigeration equipment. Due to the deoxidation of phosphorus, the oxygen content in copper is greatly reduced, thus avoiding the above problems. Besides oxygen, copper also contains some other impurity elements.

It is also harmful to exceed the normal level. For example, bismuth, lead and sulfur generate low-melting compounds in copper, and most of them gather at grain boundaries, which reduces the high-temperature resistance of copper and even leads to the danger of cracks at normal brazing temperature.