Practical recommendations for Purging Welds in Pipe

When welding on austenite Cr-Ni-steel, the welding seam and the adjacent material will oxidate if it is exposed to oxygen.
Oxidated surfaces are then no longer resistant to corrosion.
The afterwards required removal of oxide layers with mechanical means, e.g. grinding, also destroys the passive layer on the surface. This leads to a further reduction of corrosion resistance.
The inhibition of tarnish is achieved by the use of protective gas in combination with technical appliances to hold off the oxygen.
A remaining oxygen content of more than 0.1 % prevents a homogeneous composition of the welding seam and causes chromium to oxidate. Therefore, these materials should only be used in the welding process if the remaining oxygen is less than 70 ppm.
Additionally, the protective gas should be applied until all heated areas have cooled down to at least 180 °C.
The protective gas should be used in mulitpass welding, depending on the welding method and the heat input per unit lenght of weld, up to a welding seam thickness of 10-12 mm minimum.
When welding preheated chromium steel with more than 1.25 % chrome, the formation of chromic oxides has to be avoided. This can be achieved with a remaining oxygen content of less than 1000 ppm.
High preheating temperatures of up to 300 °C favour the formation of chromic oxides. Therefore a further reduction of the oxygen content is recommended.


The remaining oxygen content is influenced by the following criteria:
1.) The supplied welding and forming gases contain a remaining oxygen content
2.) There is an uptake and diffusion of oxygen through
a.) ring conduits/ supply lines
b.) hoses of various materials, connections, sealings, interior of weldings machines
c.) TIG and plasma hoses
d.) pressure reducing valves
e.) purging systems
f.) yet to be welded welding seams

Important: The charging volume of purging systems are to be as small as possible. This does not only improve the welding quality but also saves time and gas by allowing scavenging times of just 1 to 2 minutes.

Because the oxygen concentration caused by the criteria mentioned above could possibly be higher then specified, continuous controlling with the help of oxygen indicators is required.

In formation processes or in the protection of root faces, we recommend to avoid the use of flammable gas

Flammable gas compositions are given if the concentration of hydrogen lies between 4% and 75% in mid-air.
When welding on large-sized pipes or containers, a gas composition such as the one mentioned above could lead to severe explosions.
For nitrogen-hydrogen gas mixtures containing a hydrogen concentration of more than 10%, the EN 439 (formerly known as DIN 32526) requires flashing. However, flashing is only possible under certain conditions. As the hydrogen flame is hardly visible in daylight, injuries would be likely to happen.

The material used for our sealings will resist temperatures of up to 330 °C.
However, direct exposure to flames of extremely high temperatures will destroy the material.
When protective gases are used for welding on large-sized pipes, containers or small rooms, the oxygen content is being surpressed. Therefore, an additional supply of oxygen has to be provided.
The use of an oxygen indicator is strongly recommended.

forming gases

The usual forming gases are:
1.) Argon as an inert gas
2.) Nitrogen as a semi-inert gas
3.) Mixtures of either argon and hydrogen or nitrogen and hydrogen, both as reducing protective gas

Protective Gas Materials
Argon
all metals, austenite Cr-Ni-steel, austenite ferretic steel (Duplex), gas-sensitive materials such as titanium, zirconium and molybdenum, hydrogen-sensitive materials such as high-tensile fine-grained steel, copper and copper alloys, aluminium and aluminium alloys as well as other non-ferretic metals, ferretic Cr-steel
nitrogen; compositions of nitrogen and argon
austenite Cr-Ni-steel Duplex and Superduplex steel
compositions of nitrogen and hydrogen
austenite Cr-Ni-steel (which is not stabilised by titanium) various kinds of steel except high-tensile fine-grained steel
compositions of argon and hydrogen
austenite Cr-Ni-steel nickel and nickel-based alloys
 
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