Tor Marlow Barka discusses the advantages of using auxiliary gas shield when welding both titanium and stainless steel
For decades titanium welding was performed employing either a gas shielded process with inert gas purge and auxiliary shielding to protect material behind weld torch, or chamber welding. The purpose is to avoid air reacting with the hot metal surface until the temperature is low to prevent unacceptable oxidation.
Titanium alloys have been used in aircraft and the space industry in the USA since the 1950s, and they spread to other industries as time passed. In Norway titanium was introduced with the oil industry in the 1970s. As welding of titanium required auxiliary shielding, creative souls made their own versions of the Auxiliary Gas Shield or fixed gas shielding. That was also the case in Norway, but we have only one commercial producer, TiTech Production (TiTech Pro), distributing its product with trademark Trailing Shield.
Why Auxiliary Gas Shield?
The welding torch provide a gas flow shielding the arc and metal transfer. The gas is getting ionised and produces an arc-plasma for energy transfer which in turn are melting the metal. Argon is the most common shielding gas for TIG-welding, but in case of lack of fusion, adding 30% Helium will increase energy transfer.
Torch shielding gas protects the weld pool and adjacent hot metal surface from air. But by very reactive metals like titanium the torch gas may not give sufficient protection. By welding, the torch is continuously moving away from the hot weldment, allowing the hot titanium surface to combine with oxygen and nitrogen from the air. While at room temperature up to 200°C air combine with titanium forming a silver bright coloured protective oxide, high temperature oxides may adversely affect properties.
We believe that you may even increase heat input to some extent within existing WPQR range of approval, or document by a new WPQR. Increased heat input implies increased welding productivity and still have a silver bright weldment surface due to uniform gas distribution and the cooling effect.
Auxiliary Gas Shield only for titanium?
Except from construction steel, Auxiliary Gas Shield may have the same positive effect on welding other metal alloys. Stainless steel alloys are like titanium getting oxidised in air at ambient temperatures. 11-12% chromium steel alloys form a corrosion resistant chromium oxide layer at room temperature. The ability to protect the steel from more aggressive environment will improve by adding more chromium and molybdenum. But at elevated temperatures like obtained by welding, exposed in air the chromium oxide layer will grow thicker, extracting chromium from the steel surface affecting the corrosion resistant properties of high chromium alloys like super duplex and 6Mo stainless steel, and even for 316 and duplex stainless steel. Depending on requirements, the oxide and adjacent metal surface need to be removed.
Like for titanium welding, welding of stainless steel will have similar oxide colour challenges (ref. Norsok M-601). Auxiliary Gas Shield may leave a silver bright weld after finishing welding, with no need for brushing or pickling.
In many cases stainless steel brushing may be sufficient to remove the poor oxide layer. But in cases like robotised and mechanised welding, the efficiency will be affected by need of removing oxide also between welding layers. In general, brushing will make a stop in welding progress. Auxiliary Gas Shield will produce a bright weldment both during welding and after finished welding. No post weld treatment is needed. As with stainless steel, other metal alloys may have advantage in using auxiliary gas shield for welding.
First commandment in general is always to start with a clean weld joint and 99,996% pure gas. Humidity contains hydrogen, and grease consist of hydrogen and carbon, all having a detrimental effect especially on titanium. Even Auxiliary Gas Shields cannot compensate for a contaminated shielding gas or weld groove.
Welding produces heat that for stainless steel and many other exclusive metal alloys are not only affecting surface properties but also heat affected zone (HAZ) in metal beside the weld deposit. In construction steel we normally wish slow cooling, but in non-hardenable stainless steels, hardenable aluminium alloys and other metal alloys we wish a rapid cooling not to disturb the metal structure and properties. Auxiliary Gas Shield may have a good influence on cooling the weldment including HAZ, for better mechanical and corrosion properties.
Auxiliary Gas Shields for other reasons than oxidation or cooling weldments?
Site/open air welding and in big halls sometimes with draft, the gas shielded welding may have challenges with proper gas protection of the welding arc. The shielding gas may be blown away or disturbed by turbulence causing different kinds of weld defects. Auxiliary Gas Shield creates a protected local environment around the welding process that keeps the shielding gas in place, including the zone behind the weld torch.
Auxiliary Gas Shield might also have good influence on welders’ work environment, such as air pollution and ergonomic aspects. There is less need for brushing, grinding or pickling after welding - which is beneficial for welders' health.
Tor Marlow Barka is a senior welding engineer and metallurgist with TiTech Pro