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Up Topic Welding Industry / Technical Discussions / Need Help Welding Tungsten
- - By - Date 07-02-2004 22:32
Hello, I'm new to this board and am looking for somebody with experience in welding Tungsten parts with a heli-arc setup. The company I work for closed down our branch out on the east coast and brought it out to the west coast to continue operations. The problem is that we lost our experienced welder and are on a pretty steep learning curve. We are welding tungsten heating elements but the welds are coming out extremely ugly and do not look like they are structurally sound. I'm after any techniques, tips, tricks, or any tidbits of information on how to get a good solid weld on tungsten parts that will be structurally sound. Right now the welds almost look like they are cold welds. There is a guy by the name of Bud who used to work at our company and is a member of the American Welding Society, so if you're out there Bud, please reply! Thanks for any help!

Paul
Parent - - By GRoberts (***) Date 07-02-2004 23:10
I have never welded tungsten myself, but I would imagine it to be difficult as welding it with GTAW (heliarc) is using tungsten to weld tungsten. You would obviously have to weld in DCENM (reverse) polarity, so that the electrode doesn't melt instead of the part being welded. However, if you come to your wits end using GTAW, I would suggest trying plasma arc welding. The arc is much hotter, and I would guess, makes it easier to weld tungsten that has such a high melting point.
Parent - - By swnorris (****) Date 07-06-2004 12:46
Hi Paul,

I agree with GRoberts that direct current, electrode negative is needed for the GTAW process. Welding is best done using argon or helium shielding in a gas tight welding chamber capable of maintaining a high purity atmosphere, or in a high vacuum to prevent contamination, because oxygen and nitrogen may be present in the metal, or they could be absorbed from the ambient atmosphere during welding.
Prior to welding, the surfaces must be clean and free of dirt, grease, oil, oxides, and other foreign matter, because any of these can inhibit wetting, flow, or fusion, and will also contaminate the metal. The components should first be degreased with a suitable and safe solvent. They should then be cleaned by any one of the following methods: immersion in 20% potassium hydroxide solution (boiling), electrolytic etching in 20% potassium hydroxide solution, chemical etching in 50 volume % HNO3 - 50 volume % HF solution, immersion in molten sodium hydroxide, or immersion in molten sodium hydride. In any case, the cleaning media should be thoroughly removed by rinsing with clean water, and then the component dried with hot air.
Fixturing should provide minimum restraint on a weldment, and components should be preheated to above the transition temperature of the metal to avoid cracking from the thermal stresses.
Welds should be made using procedures that produce good fusion and give a narrow, minimum width heat affected zone with a minimum input of heat. Runoff tabs are helpful when terminating a weld to avoid crater cracks in the joint. Proper grinding of the areas to be welded improves arc starting, reduces arc wandering and improves penetration. Grinding the weld surface to remove bead ripples, surface contamination, or both, may improve weld joint ductility.
Tungsten is sensitive to the rate of loading and to stress concentration. The ductile to brittle transition range is shifted upward by increasing the strain rate. Welded joints are very notch sensitive, and where possible, the weld surface should be finished smooth and faired gradually into the base metal. Notches at the weld root should be avoided.
Weldments should be stress relieved promptly at a temperature below the recrystallization temperature of the base metal. Incidently, there is no evidence that the mechanical properties of welds in tungsten are improved by heat treatment, however, heating to just below the recrystallization temperature of the base metal will relieve residual stresses, which could be beneficial by reducing the likelihood of cracking during subsequent handling.
Also, you may want to contact your local consumables rep. They should provide technical support that should assist you in resolving your problems.
Parent - - By - Date 07-08-2004 17:02
"Welded joints are very notch sensitive, and where possible, the weld surface should be finished smooth and faired gradually into the base metal."

What exactly do you mean by "notch sensitive"?? I have heard this from two people now but I'm not exactly sure what it means.
Parent - By swnorris (****) Date 07-08-2004 19:03
Hi Paul,

Notch sensitive means that tensile stresses applied along a small inside radius of curvature will easily produce a crack. Notch sensitive is the opposite of notch tough. For example, if you were to grind a notch across a piece of flat bar and subject the bar to a tensile test, after it is pulled, if the notch does not reduce the breaking stress, the metal is notch tough and can be used at a particular stress level. If the notch reduces the breaking stress, the metal is notch sensitive. The more the breaking stress is reduced, the more notch sensitive the bar is. The higher strength alloys are most often lower in ductility and may not be able to stretch and relieve stresses around notches without failure.
Notches are basically angular, u, or v shaped indentations or irregularities in the base or weld metal. Small notches such as these can cause locally high stresses, and a sudden fracture can occur if the alloy can't stretch to relieve such high stresses. The elimination of notch potential in the weld metal can be greatly reduced by smoothing the welds and carefully feathering the weld toes into the base metal.
Up Topic Welding Industry / Technical Discussions / Need Help Welding Tungsten

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