Pipeline welding with automatic welding machine can greatly improve the welding production and one-time qualified rate of welded pipe joint. However, during welding, it is inevitable that there will be porosity. During welding, the bubbles in the molten pool fail to escape during solidification and remain to form porosity. We can tell that the cause for the production of welding porosity is that the gas fails to overflow from the weld pool before the weld crystallizes. The gas forming porosity comes from the gas melted in the base metal and the steel core of the electrode or the gas produced by the coating during melting. Oil, rust and scale on the base metal decompose after heating and from the atmosphere.

There are factors which cause porosity as following:

1. The shielding gas is not pure.

2. Air flow from outside welding booth, damp welding consumables.

3. The paint on both sides of pipe joint is not cleaned.

4. Before welding, the air in the pipe is not drained.

5. Excessive sulfur content in base metal.

6. There is arc magnetic blow.

7. Too much spatters on the nozzle, this will reduce the gas flow.