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Effect of Purge Gas on Cryogenic Toughness Properties of Austenitic Stainless Steel GTAW Welds
Argon Purging of SSTL LHe Jacket for SCRF Cavity
The root passes of austenitic stainless steel GTAW welds are routinely purged with an inert gas. By far the most common inert gas utilized for this purpose is argon. The intended purpose of the inert gas purge is to prevent oxidation of the weld and heat affected zone. Contamination of the weld by atmospheric oxygen can, depending upon the extent of the contamination, range anywhere from a light discoloration to heavy rough layer of oxide contamination colloquially called “sugaring”. The specification, AWS D18.2, Guide to Weld Discoloration Levels on the Inside of Austenitic Stainless Steel Tube, contains a visual guide to weld discoloration correlated to the parts per million of oxygen measured in the purge gas.
Any form of discoloration indicates that the weld and HAZ surfaces will be less corrosion resistant than the base metal. Mechanical cleaning followed by a passivation step can restore the corrosion resistance. However, frequently these welds are not accessible and, as often is the case, prevention is the better solution than treatment. At Meyer Tool our welders utilize oxygen analyzers to measure the parts per million on oxygen in our argon purge. For most applications, we do not initiate welding until the oxygen level in the purge has fallen to below 100 ppm of oxygen. Even after the root pass is completed, the purge is continued to protect the interior bead from oxidation. Standard welding grade argon is only 99.99% pure; for critical applications we will use 100% argon gas boiled off from a liquid argon dewar and do not initiate welding until the purge has reached 10 ppm.
Besides the obvious oxidation effects, a poor purge gas shield can allow oxygen to cause an increased number of inclusions in the weld metal. This is a critical factor when the weldment is to be used in a cryogenic application as it has been proven that there is a direct correlation between the number of inclusions and the fracture toughness of the weld metal at cryogenic temperatures.
Recently we had an article discussing the substitution of nitrogen gas for argon gas for the purging of austenitic stainless steel weldments brought to our attention. (Effect of Purge Gas on Properties of 304H GTA Welds). The initial driving attraction to this research is the relative cost of argon versus nitrogen gas and the economies that could be achieved by the switch. The conclusion we draw from this paper is that the proper use of nitrogen has no detrimental effects on oxidation prevention, decreases the delta ferrite content of the welds and improves mechanical properties. The ASME Code Part UHA-51 defines acceptable delta ferrite limits for weld metal and Charpy notch toughness requirements for austenitic stainless steel welds for cryogenic applications. At first glance the conclusions of this paper would indicate that using nitrogen for the purge gas would lead to improved notch toughness properties for cryogenic applications. However the paper’s focus was not on cryogenic applications and its scope did not address the mechanical properties at cryogenic temperatures. We, at Meyer Tool, can remember reading papers from the early 1990s indicating a different outcome.
Utilizing the power of the Internet we located other papers that directly address this issue. One paper from 1995 (Welding Stainless and 9% Nickel Steel Cryogenic Vessels) summarized the issue succinctly and stated “Nitrogen increases the tensile and yield strength of stainless steel welds, but decreases the low-temperature toughness.” While we find the conclusions of the newer paper intriguing for non-cryogenic applications, Meyer Tool will continue to utilize argon as our purge gas for GTAW welding of austenitic stainless steel. Staying aware of new research and remembering the conclusions of older research is one way that Meyer Tool continues to Reduce Project Risk to help our
Any form of discoloration indicates that the weld and HAZ surfaces will be less corrosion resistant than the base metal. Mechanical cleaning followed by a passivation step can restore the corrosion resistance. However, frequently these welds are not accessible and, as often is the case, prevention is the better solution than treatment. At Meyer Tool our welders utilize oxygen analyzers to measure the parts per million on oxygen in our argon purge. For most applications, we do not initiate welding until the oxygen level in the purge has fallen to below 100 ppm of oxygen. Even after the root pass is completed, the purge is continued to protect the interior bead from oxidation. Standard welding grade argon is only 99.99% pure; for critical applications we will use 100% argon gas boiled off from a liquid argon dewar and do not initiate welding until the purge has reached 10 ppm.
Besides the obvious oxidation effects, a poor purge gas shield can allow oxygen to cause an increased number of inclusions in the weld metal. This is a critical factor when the weldment is to be used in a cryogenic application as it has been proven that there is a direct correlation between the number of inclusions and the fracture toughness of the weld metal at cryogenic temperatures.
Recently we had an article discussing the substitution of nitrogen gas for argon gas for the purging of austenitic stainless steel weldments brought to our attention. (Effect of Purge Gas on Properties of 304H GTA Welds). The initial driving attraction to this research is the relative cost of argon versus nitrogen gas and the economies that could be achieved by the switch. The conclusion we draw from this paper is that the proper use of nitrogen has no detrimental effects on oxidation prevention, decreases the delta ferrite content of the welds and improves mechanical properties. The ASME Code Part UHA-51 defines acceptable delta ferrite limits for weld metal and Charpy notch toughness requirements for austenitic stainless steel welds for cryogenic applications. At first glance the conclusions of this paper would indicate that using nitrogen for the purge gas would lead to improved notch toughness properties for cryogenic applications. However the paper’s focus was not on cryogenic applications and its scope did not address the mechanical properties at cryogenic temperatures. We, at Meyer Tool, can remember reading papers from the early 1990s indicating a different outcome.
Utilizing the power of the Internet we located other papers that directly address this issue. One paper from 1995 (Welding Stainless and 9% Nickel Steel Cryogenic Vessels) summarized the issue succinctly and stated “Nitrogen increases the tensile and yield strength of stainless steel welds, but decreases the low-temperature toughness.” While we find the conclusions of the newer paper intriguing for non-cryogenic applications, Meyer Tool will continue to utilize argon as our purge gas for GTAW welding of austenitic stainless steel. Staying aware of new research and remembering the conclusions of older research is one way that Meyer Tool continues to Reduce Project Risk to help our
