Why is Weld Purging with Backing and Trailing Gases Important?
Exposure to heat and oxygen during the welding process often results in the formation of 'temper colors' on stainless steels and certain other materials, such as titanium and its alloys, zirconium and nickel-based alloys. Temper colors are bands of colors formed around the penetration bead and adjacent parent material, resulting from a very thin, strongly adhering but easily destroyed chromium oxide layer that forms on the surface when the workpiece is exposed to oxygen. This chromium oxide layer - sometimes referred to as "sugaring" - is described as a 'passive layer' and can severely compromise the corrosion resistance of high-alloy steel materials. Weld purging with protective root-backing gases is an effective way to avoid this passive layer.
Trailing gases are used in a similar way to achieve the same protective effect - except that the trailing shield operates on the weld side of the joint, extending the protective atmosphere as the weldment cools.
The benefits of weld purging with backing and trailing gases are well documented in publications such as the Welding Journey issued by the American Welding Society (AWS).
How Does Weld Purging with Backing and Trailing Gases Work?
Complementing primary gas shielding delivered through the welding torch, back purging is achieved by passing a stream of an inert, unreactive or reducing gas over the reverse side of the weld area to control the oxygen level and therefore prevent oxidation. Root backing ensures that both sides of the weld joint are protected. While gas purging generally aims to limit the amount of oxygen in the area around the root of the weld, other gases such as nitrogen can also form compounds with some materials, and these can reduce the corrosion resistance or mechanical strength of the weld. Hence nitrogen levels must also be controlled in the case of certain back-purged welds.
In the case of secondary gas shielding, a dedicated shielding gas trail must be fitted to the welding torch. Trails with a modular design allow the welder to align the radius and length of the trail to the task at hand for more precise results. This trail then passes a secondary shielding gas over the weld bead and heat-affected zone to prevent surface oxidation. This protective shield extends the gas purging window until the material has cooled down to the point where it is no longer reactive.
What Shielding Gases are Best for Weld Purging?
Different applications and materials require different levels of purge quality and different purge gas types offer unique benefits depending on the materials being joined. Welders must be careful to select the right purge gas or purge gas mixture in the right purity and at the right flow rate to get the metallurgical and cosmetic results they want, particularly on reactive or high-value alloys. The most common purge gases used for weld shielding are argon due to its inert properties, and nitrogen due to its unreactive properties. Nitrogen has the ability to slightly improve pitting corrosion resistance, especially when used with duplex steels, but may not be suited to materials such as aluminum that are sensitive to nitride formation as this can compromise corrosion resistance. Pure helium may be popular for applications such as root protection of seam welds on large pipes and cylinders, where its low density enables full coverage of the weld area even with low flow rates.
Gas Mixes Tailored to Different Purging Challenges
In addition to offering pure argon, nitrogen and helium gases in welding grade, we also offer various hydrogen-containing backing gases in different concentrations to suit different welding process challenges. These blends mix argon and nitrogen with hydrogen. As a backing gas component, hydrogen has two essential properties. Firstly, it has a strong reducing effect. This means that hydrogen is able to bond any residual oxygen still present and, assuming sufficiently high temperatures are used, can return oxides already present to their constituent elements. In combination, nitrogen and hydrogen offer very good protection against the formation of temper colors, even under less-than-perfect conditions. Secondly, hydrogen affects the surface tension of the molten material at the weld root, which provides a very good transition to the base material.
Contact your local Linde representative to check availability of our root backing and trailing shielding gases and shield trails in your region.
FAQs
What welding methods rely on weld purging with backing and trailing gases?
Root backing and arc trailing are most commonly used in the tungsten inert gas (TIG) welding process (also known as gas tungsten arc welding or GTAW) and in metal active gas (MIG) welding, but they may also be used in other gas tungsten arc welding (GTAW) methods such as metal active gas (MAG) welding, flux-cored arc welding (FCAW) and plasma arc welding (PAW).
Is weld purging the best way to remove temper colors?
There are various methods for removing temper colors, including brushing, grinding, media blasting, pickling and electrochemical cleaning. Many of these come with speed or quality limitations. In contrast, purging with shielding gases has the advantage of preventing or greatly reducing the formation of metal oxides during the welding process rather than remedially. Purging is an economical, clean and environmentally friendly method.
What are the main weld purging techniques?
Weld purging loosely falls into three categories - pipe purging using purge dams, back purging where the shielding gas is directed at the welding zone, and chamber purging, where the workpiece is placed in an inert chamber. All methods rely on shielding gases.
What is forming gas?
Purging gases or backing gases are sometimes referred to as forming gases, particularly in Europe.
Is weld purging safe?
The volumes of inert or unreactive gases used in weld purging tend to be quite high and this significantly increases the risk of asphyxiation. It is important that welders take precautions to minimize this risk. This can include appropriate exhaust systems. We offer a broad portfolio of safety equipment and personal protective equipment (PPE), including respiratory protection and gas detectors, to help welders mitigate risks during weld purging.
