Part 1 The Importance of Gas Control on Weld Quality
Some form of gas protection is necessary during welding of many metals, but in particular stainless steels and titanium alloys, to maintain their physical properties and to prevent reduction in corrosion resistance. Which is the best gas and which is the best technique to use when weld purging are principle questions facing engineers.
Dr Fletcher sets out in this 4 part series, to cover all related topics including gas selection and purging equipment. He has researched published material on the subject of protective gases from a very wide range of sources. Typically this has included steel manufacturer, major users (ref.2), consumables and equipment suppliers (ref.3) independent authorities (ref.4) and many welding engineers (ref.5,6,7).
Argon, as a totally inert gas, is the most commonly used, but nitrogen and hydrogen also offer protection. Helium offers the same protection as argon but is seldom used because of the cost.
In general, nitrogen is said to offer improved pitting corrosion resistance, whilst hydrogen has two essential properties: firstly, it is able to bond any residual oxygen still present, and, assuming sufficiently high temperatures, it can return oxides already present to their constituent elements. Secondly, hydrogen affects the surface tension of the molten material at the weld root, which in turn provides a very good transition to the base material.
The Use of Nitrogen
Nitrogen is sometimes considered an inert gas. However, at the temperatures associated with welding, it may react and have an effect on weld chemistry.
The majority of major users concur that additional nitrogen in the shielding and purge gases is essential to compensate for loss of nitrogen in the weld and heat affected zones in duplex stainless steels. There is however little consensus on the optimum amount of nitrogen required.
‘..too much nitrogen will increase tungsten wear, and it is not to be recommended in the majority of cases..’ (ref. 6).
Sandvik recommends 100% nitrogen for purge gas when welding SAF 2507 super duplex steel.
Outokumpu suggest the use of 3% nitrogen in the purge gas, but claim that pure nitrogen can lead to excessive levels of austenite in the root pass.
A private communication from a USA source states they rarely use more than 3% nitrogen during purging to avoid introducing porosity.
Perhaps the best advice to date has come from the American Welding Society (ref. 8). Their conclusions are that the level of nitrogen in the shielding gas needs to be matched to the steel composition implying that different duplex steels may require different purge gas compositions. This is still rather vague however and some way from resolving all the issues for the wide range of alloy compositions that define duplex steels. It is nevertheless a positive step in the right direction.
The AWS advice is that welding of duplex stainless steels with a typical nitrogen content of 0.16% the gas should contain 1.0/1.2% N2. For steels with a typical nitrogen content of 0.25% the gas should contain 2.0/2.5% N2.
The Use of Hydrogen
A general claim is that hydrogen reacts with any free oxygen over the molten weld metal, thereby helping to prevent the formation of oxides in the molten weld metal and oxidation on the surfaces.
With respect to the employment of hydrogen in purge gas for example, API (American Petroleum Institute) suggests that benefits can be gained by using Forming Gas (a 90% N2, 10% H2 mixture) to counteract oxidation4 , but this gas is not readily available in many countries. Metrode suggests that suitable purge gases are A, N2, H2 mixtures. On the other hand BSSA (British Stainless Steel Association) states that hydrogen should not be used in the shielding or backing gas because of the possibility of hydrogen embrittlement and cracking.
On a cautionary note, however, there are limitations with regard to both nitrogen- and hydrogen- containing backing gases. These are unsuitable for use with materials such as titanium that are sensitive to gas uptake, since this can lead to embrittlement and/or porosity formation. Nor should such mixtures be used with some structural steels.
Depending on the metal being welded and the welding process used, even very minute gas impurities can have a significant effect on welding speed, weld surface appearance, weld bead coalescence, weld colour, and porosity levels.
There is always the possibility of the gas being contaminated often due to leaks in tube connections from the gas source to the purging point. Using a Weld Purge Monitor® (ref.10) to measure oxygen content in the gas will highlight this.
When to Purge
Purging should be considered on any joint design sensitive to oxidation that leaves the weld root and cap exposed to the atmosphere.
Purging is a recommended practice for welding stainless steels, low alloy steels, and most non-ferrous base metals where contamination from the welding technique may be deleterious.
Purging requires entrance and exit openings through which the purging gas can enter and leave the weld joint area at a controlled rate.
When argon is used, the gas inlet should be located lower than the exit opening to prevent entrapment of air. When helium is the major component in purge gas, the inlet gas opening should be higher than the exit. The size of the exit port should be equal to or greater than the entry to prevent pressure increase.
Generally, oxygen levels should be monitored and welding should not begin before levels less than 0.01% (100 ppm) have been achieved. This level may need to be significantly reduced for welds destined for critical appplications (ref.9). For example, major duplex steel fabricators are now specifying a maximum 50 ppm as a starting point for welding and successful titanium welding can require less than 10 ppm.
A general rule is to purge, prior to welding, at flow rates and times that will produce five to six system volume changes. The purge process should be continued after welding has been completed in order to protect the exposed weld bead until it has cooled.
Much more definitive work is now essential if fabricators are able to proceed with confidence in consistently producing welded joints capable of meeting the demanding standards imposed by service conditions. In the meantime the precautionary note from the American Welding Society (ref.8) offers the best advice with a conclusion that the shielding gas needs to be matched to the metal composition.
- Outokumpu, Sandvik
- Weir Pumps, Exxon Mobil, BP, Agip
- Metrode, ESAB, Miller Electrical, Messer Group, BOC/Linde
- American Petroleum Institute, American Welding Society. The Welding Institute, Canadian Welding Bureau,
- Seok-Hwan Jang, Corrosion Science May 2011
- Wiktorowicz and Crouch,- Welding Research Abroad, 1996 – www.aws.org
- Goswami, metallurgy, engineering codes and welding practices, stainless steel world V26 Jan 2014 (refers to duplex needing <0.5% O2 in purge gas)
- AWS CS, 10-94 Recommended Practices for Shielding Gases for Welding and Plasma Cutting.
- Taban, Kaluc and Aykan, Effect of the Purging Gas on Properties of 304H. AWS Welding Research Supplement, April 2014
- Innovation in Weld Protection. American Welding Society Spring
- Influence of Shielding Gases on the Quality of Welds
Nils E. Larson – Compressed Gas Industries, Inc.
By Dr. Michael J. Fletcher M.Sc. Metallurgy
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