TIG: Tungsten INERT Gas welding.
In Houston a good TIG welder goes for about $15.00 to $25.00 an hour.
Not that hard and certainly not that big a deal.
Stay safe; stay armed.
Posted on 08/12/2002 9:26:37 AM PDT by RightWhale
Gas Tungsten Arc Welding process used to repair cracks
NASA-KSC FACT SHEET
Posted: August 9, 2002
The welding process that will be used to repair flow liner cracks found in the orbiter fleet is a Gas Tungsten Arc Welding technique that results in a very pure weld with minimal impurities in the final product. This type of technique is typically used for the welding of oxygen sensitive materials. In the case of the orbiter flow liner cracks, the weld is a full penetration weld that will need to go completely through the metal even though the welder will only have access to one side. This increases the difficulty of the process.
Since cleanliness is critical to the Shuttle Main Propulsion System, advantages of Gas Tungsten Arc Welding over conventional arc welding are the lack of weld spatter and minimal debris generation. This type of weld leaves no slag after the weld pass, and minimal debris on the backside of the repair where access is extremely limited. The liquid hydrogen lines require a 400 micron cleanliness level, meaning that no particles bigger than 400 microns (0.016 inches) can be observed in the line after the repair process. Safeguards and protective measures have been put in place, and a final inspection after completion of the work will verify lines are clean and ready for flight.
The limited access in the repair area creates a number of additional challenges for the welder. Due to the argon gas inerting process necessary for the weld repair, the welder will be instrumented with an oxygen sensor to ensure he is able to breath adequately. Another oxygen sensor will be placed near the weld to verify that no oxygen is in the immediate area of the welding as it is taking place.
The detailed weld process for the repair of the flow liners has been developed specifically for this repair at the Marshall Space Flight Center, Huntsville, Ala. The process consists of several autogenous (no filler material) heat passes to reduce and eliminate any residual stresses created by the actual filler weld pass.
The flow liners on Atlantis, Endeavour and Discovery are crafted from Inconel 718, a nickel and iron super alloy that has high strength and is very resistant to the extreme ranges of temperatures in Main Propulsion System fuel flow lines during ascent. The flow liners themselves are 0.05 inches thick. Flow liners on Columbia, the first orbiter built, are made from Corrosion Resistant Stainless Steel 321 (CRES), which has similar performance characteristics.
Because of limited technical data available for Gas Tungsten Arc Welding of welding on the specific material used in Atlantis, Endeavour and Discover flow liners (Inconel 718), an empirical test program was developed and performed at Marshall to both certify the welder and the repair process.
More than 100 "coupon" samples were created with slots stamped in them just as the actual part. These coupons were then run on a cyclic load-testing device, capable of inducing millions of fatigue load cycles in only a couple of days, until they developed cracks of a similar size and location as the actual parts on the orbiters. These cracked coupons then were subjected to a variety of weld repair techniques, and placed back into the testing machine to attempt to create the crack again. The weld repair technique, which performed as well as the parent material in the cyclic load testing, was selected as the process for this repair. Additional coupons were run through the entire process and tested at many different load values to establish a base line technical data, and to familiarize the welder with the repair process. The testing program is ongoing and will be repeated for material used for Columbia's flow liners.
To further ensure the viability of the flow liners, the holes in the flow liner have been carefully polished to relieve any of the residual stress resulting from the stamping of the holes into the liner and to reduce the probability of micro cracks growing into actual cracks. This polishing effort is similar to efforts taken in the polishing of internal parts in racing engines. Racing engines also are subjected to very high stresses, and it has been demonstrated that polishing eliminates surface irregularities, which in turn decreases their susceptibility to cracking and failures.
Throughout the welding operation and following its completion, a number of non-destructive inspections and evaluations using X-Rays, Ultrasonic Testing, Eddy Currents and Borescopes will ensure that the final repair is consistent with the parent material and with the coupons from the test program.
Sounds like there was one person qualified to perform this weld. That would be Jerry Goudy. It is often the case that one welder is trained and able to perform a specialty weld. Was he paid $50 per weld?
If NASA has available one welder certified to do this weld, and if it costs $10 million per day the Shuttles sit idle, will they quibble? Top welders live for this.
NASA has the tendency to assume its audience knows nothing beyond third-grade arithmetic. Maybe this approach is justified to a degree in press releases. But if you go to their tech papers, it is pretty hard-core stuff. You're right, though, the article is kind of elementary for the average FreeRepublic reader.
No doubt. NASA may have tested 10 welders for this operation, and found one that could do it every time. Maybe this one has a special trick he uses to guarantee consistency and even the engineer [there's always an engineer somewhere in the woodwork] might not pick up on it.
I posted the article to point out that one welder [an assumption, there might be several] is putting the $20 billion shuttle fleet back in service. Each mission costs $ 0.5 billion or more, and the schedule is slipping day by day until service is restored. Flights have already been delayed going on two months at the cost of $ millions at least, and who knows what the welder[s] is being paid, but it is a bargain. I could probably do 90% of the weldments myself, screwing up only 10%, but they need 100%.
It's pretty common to qualify just one individual or a small group to do a particular weld as needed, instead of keeping the qualifications up to date on a metal/joint that you don't run into all the time. This keeps you from having your welders spend all their time in the qual booth. A job comes up for some goofy joint design, you have your welders practice a bit, then weld a qualification joint.
TIG: Tungsten INERT Gas welding.
In Houston a good TIG welder goes for about $15.00 to $25.00 an hour.
Not that hard and certainly not that big a deal.
Stay safe; stay armed.
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