The first part of Dr. Fletcher's four-part series on welding examines the critical role of gas control in maintaining weld quality, particularly for stainless steels and titanium alloys. Proper shielding and purging gases are essential to protect the weld from oxidation and maintain corrosion resistance.
The study explores various gas options:
Different recommendations exist for nitrogen levels in welding duplex stainless steels, with varying advice from manufacturers and organisations like the American Welding Society (AWS). The study highlights the need for precision in gas selection and the importance of monitoring gas purity, as even small impurities can significantly impact weld quality.
Proper purging techniques, such as controlling gas flow and monitoring oxygen levels, are also crucial. The study underscores the need for further research to develop consistent guidelines, though current AWS recommendations remain valuable for welders.
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The Importance of Gas Control on Weld Quality
Welding certain metals, particularly stainless steels and titanium alloys, necessitates gas protection to maintain their physical properties and prevent corrosion resistance from reducing. One of the critical questions for engineers is which gas and purging technique to use to ensure optimal weld quality.
Dr. Fletcher explores this topic in a four-part series covering gas selection and purging equipment. His research draws from various sources, including steel manufacturers, major users, consumable and equipment suppliers, independent authorities, and welding engineers.
Gas Selection for Welding
Argon, an utterly inert gas, is the most commonly used shielding gas. However, nitrogen and hydrogen also offer protection, with helium providing similar benefits to argon, albeit at a higher cost.
The Use of Nitrogen
Nitrogen is sometimes viewed as an inert gas, but it can react and influence weld chemistry at welding temperatures. For duplex stainless steels, additional nitrogen in the shielding and purge gases is essential to compensate for nitrogen loss in the weld and heat-affected zones.
However, there is little consensus on the optimal nitrogen levels:
The American Welding Society (AWS) advises that the nitrogen level in the shielding gas should match the steel composition, with specific recommendations for different duplex steels. This guidance is a step forward but lacks precision for the diverse range of duplex steel alloys.
The Use of Hydrogen
Hydrogen in the purge gas can help prevent oxide formation by reacting with free oxygen in the molten weld metal. The American Petroleum Institute (API) suggests that Forming Gas (90% N2, 10% H2) can counteract oxidation, but this gas is not widely available. However, the British Stainless-Steel Association (BSSA) advises against using hydrogen in the shielding or backing gas due to the risk of hydrogen embrittlement and cracking.
Hydrogen—and nitrogen-containing gases are unsuitable for materials like titanium, as they can cause embrittlement or porosity, and they should not be used with certain structural steels.
Gas Purity and Contamination Control
Even minute gas impurities can significantly affect welding speed, surface appearance, weld bead coalescence, colour, and porosity levels. Gas contamination often occurs due to leaks in tube connections from the gas source to the purging point. Using a Weld Purge Monitor® to measure oxygen content can help detect such contamination.
When and How to Purge
Purging should be considered for any joint design sensitive to oxidation, especially where the weld root and cap are exposed to the atmosphere. It is particularly recommended for stainless steel, low alloy steel, and most non-ferrous metals. Proper preparation involves creating entrance and exit openings for the purging gas to enter and exit the weld joint area at a controlled rate.
The gas inlet should be lower than the exit opening for argon to prevent air entrapment. When using helium, the inlet should be higher than the exit. The exit port should be at least as large as the entry to prevent pressure buildup.
Testing and Monitoring
Oxygen levels should be monitored during welding, and welding should not begin until levels below 0.01% (100 ppm) are achieved. In some cases, such as welding duplex steel or titanium, the oxygen level may need to be reduced to as low as ten ppm. Purging should continue after welding to protect the exposed weld bead until it has cooled.
Conclusions
More definitive research is needed to help fabricators consistently produce welded joints that meet demanding service standards. Until then, the American Welding Society's advice to match the shielding gas to the metal composition remains the best guideline.
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