Important to have zero colour, oxide-free welds both on the inside and outside
It is important when welding stainless steel and titanium tube and pipe joints to be able to have zero colour, oxide-free welds both on the inside and outside.
To achieve the conditions for zero colour, or oxide-free weld joints, it is necessary to remove the air around the joint to a level where the oxygen percentage is somewhere between 5 and 50 parts per million, subject to the metal and, to some extent, the application.
Removing the air from the vicinity of the weld inside and outside has to be carried out simultaneously and this is best carried out on the outside with an accessory called a Weld Trailing Shield and on the inside with an Inflatable Pipe Purging System.
The Trailing Shields are light in weight and are carried easily by the welder as the torch is moved around the seam.
As the sides are fused together, the hot metal is shielded for a very short time by the inert argon gas flowing through the welding torch and out of the ceramic gas cup.
At this time, the Trailing Shield that has also been connected to an argon supply takes over and shields the hot metal until its temperature falls below its oxidation level, thus leaving a perfectly clean weld.
In the meantime, inside the pipe, two inflatable dams tightly seal a cavity about 10” (250 mm) either side of the joint.
An argon gas supply pushes the air out and it exits through a Weld Purge Monitor which indicates when the air has achieved an oxygen level below 50 ppm, or any other level as required by the WPS (welding procedure sheet).
At that time, the welder begins his process.
Without such accessories, welding in performed in an oxidised environment, leading to loss of corrosion resistance and results in metallurgical imperfections in the joint.
Oxidised welds incur a penalty for the company, as they have to ensure that the oxidation and discolouration are totally removed by mechanical or chemical means. Such additional work can have an effect on profitability and on delivery time.
Videos showing a Trailing Shield attached to a TIG/GTAW welding torch and the set up and purging of a pipe are below.