Tor Marlow Barka,
Sr. Welding Engineer (IWE) – Sr. Metallurgist (Civ.Eng),
Independent author – Senior Consultant
AUXILIARY GAS SHIELD:
A bright idea – a bright weld
Why not try Auxiliary Gas Shield (gas shield extension) by welding of stainless steel and other metal alloys? Welding titanium there is no question. This article is based on welder’s response and metallurgical and welding technological assessments of real and possible advantages using auxiliary gas shield by welding both titanium and stainless steel.
Historical flashback
For decades titanium welding is performed employing either 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 to react with hot metal surface until temperature is low to prevent unacceptable oxidation.
Titanium alloys are used in aircraft and space industry in USA since 1950’s, and spread to other industries as time passed. In Norway titanium was introduced with the oil industry in the 1970’s. As welding of titanium required auxiliary shielding, creative souls made their own versions of Auxiliary gasshield or fixed gas shielding. That was also the case in Norway, but we have only one commercial producer, TiTech Production AS (TiTech PROTM), distributing their product with trademark TRAILING SHIELD®.
Auxiliary Gas Shield may be defined as a device providing the zone behind the welding torch with shielding gas to protect the hot surface from oxidising, and prevent embrittlement or poor corrosion resistance.
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.
The colour of the affected surface depends on oxide layer thickness (just like the refraction of light in oil contamination on water). From 300-400°C the colour of the un-shielded surface becomes straw yellow which is normally acceptable. Some standards (ISO 15614-5 and Norsok M-601) may also allow a narrow band of intensive colours close to the limits of the gas shield. Temperatures from 400°C and above results in a thicker oxide layer starting with dark blue, then purple, brown or even light-blue and grey-blue colours, which indicates exposure to air at elevated temperatures for too long time. This is not acceptable because the properties of the titanium surface are adversely affected as oxygen and nitrogen at elevated temperatures may dissolve directly into base material, potentially causing embrittlement. This is stated in a TWI report which conclude: “It is conventional for auxiliary gasshield to extend coverage down to a metal temperature of 250°C, to prevent the formation of coloured oxide films”.
Titanium Grade 2 welded by Glenn Hovet with second generation Trailing Shield® 2.0™ from TiTech PRO™
According to Norsok M-601, coloured oxide shall not be removed before visual inspection because the colours are telling the welding history, and will be decisional for acceptance or not. The oxide may be removed after finished welding by brushing with a clean stainless steel brush, also between weld layers. Clean brush will not leave eighter iron contamination nor oil/grease (i.e. carbon and hydrogen) affecting the properties. Angle grinder is not recommended as it might add iron contamination and leave grinding marks on the surface. Before starting welding drying, oxide brushing and degreasing will remove moisture and hydrogen which may cause porosity and embrittlement of weld deposit. Even a fingerprint is said to lead to porosity in weld deposit.
It’s essential auxiliary gasshield is distributing gas flow without turbulence, producing a uniform gas protection of the weld for optimal shield, like Generation 2 of the Norwegian TRAILING SHIELD® do, according to international welders’ and welding engineers response. We are proud of Norway being in front in auxiliary gas shield development.
We believe that you may even increase heat input to some
316 Stainless steel welded by Glenn Hovet with second generation Trailing Shield® 2.0™ from TiTech PRO™
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 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 gasshield 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 Shieldscannot 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, like air pollution and ergonomic aspects.
Less need for brushing, grinding or pickling after welding is profitable for welders health.
Auxiliary Gas Shield may give the welder physical support by uncomfortable weldments, like outside corner welds and other cases with poor ergonomic support.
Auxiliary Gas Shield for other welding processes than TIG?
Automatised and mechanised welding processes normally are performed by MIG/MAG or plasma welding. Auxiliary Gas Shields are perfect tailor-made for these welding processes, and may rise productivity as there is less need for interrupting welding sequence because of oxide cleaning. MIG/MAG and plasma are effective welding processes that may produce higher heat input than TIG, and thus meet a greater challenge to make acceptable weldments due to oxide colours. Auxiliary Gas Shieldsmay tackle the challenge by providing prolonged gas protection and cooling of metal surface.
Auxiliary Gas Shieldsfor other sizes, other forms and welders’ other needs?
According to the Norwegian producer they may help you designing the perfect Trailing shield® for your purpose.
Economic aspects
Auxiliary Gas Shield are low cost and long lasting. The rubber skirts withstand the hot welding arc and the components are designed for purpose.
Applying Auxiliary Gas Shieldsin other disciplines than titanium welding may meet resistance as they might be looked at as un-convenient.But if the welder finds advantages by using Auxiliary Gas Shields, it might be a successful history, like all the positive responses received form satisfied customers.
