Welding/Aerospace Tubing Weld
QUESTION: We need to weld 17-4 PH stainless steel (1" OD, .035" wall) condition H900 to 17-7 PH stainless steel tube (same dimensions)condition H1025. What would be the best weld method to use? Also, do the two parts need to be passivated before the weld or is passivating the entire until after the weld enough? The completed part is for an aerospace application.
ANSWER: If the part is to be post-weld heat treated then you should TIG weld with 17-4 PH filler.
If the part is NOT to be post-weld heat treated, then Inconel 625 or Hastelloy W are recommended as TIG fillers.
Be careful to avoid excessive dissolution of the filler metal into the parent metal as it can result in cracking.
You will need a full back purge of the weld joint and a post-weld passivation, either with a pickling paste, or the far less toxic electro-passivation using citric acid.
You can learn about electro-passivation at http://citrisurf.com/
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QUESTION: To clarify: TIG weld would be better than GMAW, PAW, etc.? AND, should the parts being welded be passivated BEFORE AND AFTER the weld or is just AFTER sufficient?
ANSWER: GMAW is never used for duplex stainless steel, that I have ever heard of.
Plasma Arc Welding is almost never done manually anymore, and has become a strictly robotic process for extremely difficult materials.
Duplex Stainless does not qualify.
You obviously are unclear on what passivation is.
Stainless steels are predominantly Iron. Corrosion of the Iron is held in check by the Nickle and Chromium in the stainless steel.
During the weld process, liquefication and solidification can cause free disassociated iron particles to float to the surface of the metal. If these iron particles are not removed they can cause rust to form on the welds.
Attacking the surface of the metal with an acid removes these Iron particles and restores the Nickle and Chromium protection to the weld.
You can use a strong acid, such as Nitric, Sulphuric, or Hydrofluouric, or a weak acid such as Citric, coupled with electricity.
Heat treating the duplex stainless does not liquefy and solidify it, so it does not create these Iron particles, therefore there is no need for passsivation before welding.
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QUESTION: I am very clear on what passivation is and the comment about heat treating was from an answer not my question. We are welding (and apparently EBW is the best) machined end caps (17-4 PH) to a tube (17-7 PH). The component drawings call out for passivation after machining and rightly so. However, when welded to the tube it becomes an assembly that will need to be passivated after welding. My question is: do the individual components need to be passivated prior to the weld considering the entire assembly will be passivated after?
F/U Question: This is for an aircraft. Do you know what the best weld spec to call out would be? Our current drawings call out for AMS-STD-2219 but that spec doesn't cover EBW.
OK to answer the passivation question...
I don't see what you would gain by passivating before the Electron Beam Weld.
The only areas that would be affected would be the mating surfaces and those are going to be fused together.
The rest of the part will be passivated after the weld.
As to the weld spec I am afraid you are above my pay grade there.