Inert gas purging to prevent discoloring of the weld underbead and loss of corrosion resistance is relatively common during fabrication of stainless steels. For the welder however there is very little quantitative published information available with respect to quality of purge gas and types of purge techniques. This review proposes maximum oxygen levels in the purge gas to ensure absence of contamination. It also critically analyses commercially available products which have been proven to reduce the time and cost of the purging process.

High-quality joints in stainless steels between cylindrical sections such as tubes and pipes can only be made by ensuring that atmospheric gases are eliminated during the welding process.

The presence of gases, and in particular oxygen, around the molten weld can lead to wide-ranging defects. Discoloration is unsightly and in some instances might produce metallurgical imbalance. Gross oxidation inevitably results in reduction in mechanical properties and can cause catastrophic loss of corrosion resistance. Nitrogen contamination can result in brittleness. Gases in the weld may give rise to cracking during or after cooling.

Basic principles — Weld root quality when making tubular joints can be ensured by applying appropriate safeguards that are based on removal of air from the fusion zone and the provision of inert gas. This is achieved by ‘gas purging’.

Purging gases — The most commonly used purging gases in the USA are commercial quality argon and helium. Purge gas flow rate and pressure need to be established and once selected they should be included in the formal welding procedure.

Variation in purge gas quality may arise during welding and it may be desirable to apply continuous gas monitoring, especially to control oxygen and moisture content. For this purpose dedicated oxygen weld purging monitors are available commercially.

Fig. 1. A Weld Purge Monitor specifically designed for measuring oxygen in weld purge gas. These instruments are capable of measuring oxygen levels down to 0.001% (10 ppm).

Purging procedure — The first requirement is to provide gas entry and exit points. Gas is fed through one end seal with an exit hole at the other end to prevent an undesirable build-up of pressure. Argon has a greater density than air and the gas inlet should be at a lower elevation than the bleed end so that air is expelled effectively from the pipe bore. The converse applies for helium that is lighter than air.

Total purging — On small pipes and tubes, where the internal volume is small, the cost of continuous total purging may not be significant. Under these circumstances wooden or plastic discs simply taped to the tube ends will be adequate.

When total purging is impractical, perhaps because the pipe volume is large or because access is difficult, alternative containment techniques are available.

Fig. 2. A schematic section of the purge system in position in a pipeline. For oxygen measurement, gas may be sampled through a tube placed at the joint (A), between the barrier and the pipe (B) or through the barrier (C).

Water-soluble products — A low-cost and effective solution to providing gas coverage is to make discs from water-soluble material and seal the inside the section needing protection. In the event that water-soluble film is not available, paper and tape can be used, although generally this is much less suitable. They should not be placed in position until after any pre-weld heat treatment and be far enough apart, typically 500 mm, to avoid thermal damage during welding. Purge gas is introduced into the area between the soluble dams by means of a hypodermic tube through the weld joint line or by penetrating one of the film dams with a purging hose.

On small diameter pipes an effective dam can be produced simply by crumpling paper and pushing it into the pipe bore. Soluble pastes are also available and can be convenient for small diameters.

On completion of the welding operation the dam material can be removed by passing water into the pipe and allowing time for it to dissolve the barrier medium.