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Weld purging. is the act of removing, from the vicinity of the joint, oxygen, water vapour and any other gases or vapours that might be harmful to a welding joint as it is being welded and immediately after welding.〔Sewell, Ron. "Recent Advanced in Weld Purging." Welding Journal 1 June 2015. Print.〕 Stainless steels, duplex steels, titanium-, nickel- and zirconium- alloys are sensitive to the presence of air, oxygen, hydrogen, water vapour and other vapours and gases that may combine with the hot metal as it is being joined. Such gases may combine with the metal to form undesirable compounds that may reduce corrosion resistance or may be instrumental in creating cracks or other structural defects in metals. Weld Purging is generally necessary for the first weld run when joining to separate parts. This sealing weld will be called a “root run” when it takes more than one run or (pass) to fully seal the root area from above.〔Fletcher, Michael. "Choosing an Effective Weld Purging Technique." Stainless Steel World. 1 Apr. 2013. Web.〕 Once the root run has been completed, it is possible to stop the purging process unless the welding engineer has specified that purging should be continued for the second and third passes for example in case the root weld becomes hot enough to oxidise in the air that will have replaced the purge gas. ==Purging methods== Unwanted gas is generally removed by flushing with an inert gas. Argon is generally used for this purpose but helium is an alternative depending on gas cost and availability. Nitrogen has been used as a purge gas but is unsuitable for some stainless steels.[ The most common way to remove gas from the weld zone is to flush it away with an inert gas.〔Fletcher, Michael. "Major Advances in Weld Purge Technology." Energy Global. 1 Oct. 2013. Web. Another method of purging is to enclose the metal parts completely in a vacuum chamber and evacuate it, prior to backfilling with inert gas for the welding process.〔Fletcher, Michael. "Repeatable Results." World Pipelines 2012: 139-142. Print.〕 When purging with inert gases, it is important to enter the gas very slowly.〔Sewell, Ron. "Methods of Weld Root Purging for Pipe Welding." Welding Journal 1 Aug. 2000. Print.〕 With Argon, which is heavier than air, it should be entered from the bottom of the enclosed space, where it will move slowly across the base area, be it the bottom of a pipe or tank, and then move slowly upwards displacing the air upwards out of a release hole at the top. This is where a Weld Purge Monitor®, oxygen sensing instrument, should be placed. Helium however, which is also an inert gas should be entered from the top of the enclosed space, where it will first flood across the upper limits and push the trapped air downwards and out through an exit where the oxygen level can be measured until it reaches the required level. The purpose of slow gas entry, typically 5 – 7 litres/m (….CFH), is to avoid turbulence, which would otherwise lengthen the purge process considerably. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Weld purging」の詳細全文を読む スポンサード リンク
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