Backing gas

By Lawrence (*****)

Date 03-27-2007 14:47 Edited 03-27-2007 14:52

Tam,

D17 is sort of a double edged sword. It is a slim doccument compared to some other welding codes which can be a real benefit, however, It does not speak directly to many specific craftsmanship issues because it has an expectation that if you are working on aircraft componants you have a some pretty extensive expertise. This can make it a challenge for folks in your situation.

Again, the standard practice manuals belonging to the OEM's for the engines you are overhauling ought to give you some insight into purge gasses.

There are 20 to as many as 40 different alloys contributing to an aircraft powerplant these days, some of those alloys are going to be overhauled with different procedures than when they were fabricated. There is no simple answer that will speak to all backpurging situatrions even though most stainless and superalloys will require argon backing on full pen welds.

There are forumlas for calculating purge gas for pipe that can be found in almost any welding textbook. ASM Volume 6 will be a big benefit on materials behavior also.

For overhauling liners and saddles, compressors, HPT, LPT boss replacements and other pressure vessles and there attachments you will need to fabricate your own custom argon backing fixtures, some will be integrated into your restraining fixtures others will simply fit the conture of the back of your componant.

Some of those argon backups will need to be plated to keep copper from migrating to the HAZ during welding operations (high cobalt haynes 25 being just one example of a copper sensitive material found in hot sections)....

You might also ask very specific material related questions here on the forum and get great advice too. But there is no simple instruction that is going to cover everything.

An expert consultant could set your quality program on it's feet and teach you how to keep it going.

By TAMPALUIS (**)

Date 03-27-2007 15:24

Thanks Lawrence,

Do you think that reviewing our repair and overhaul aircraft manuals will be a great help to found instructions and information about that (also about weld fabrication generally)?

regards

By Lawrence (*****)

Date 03-27-2007 16:35 Edited 03-27-2007 16:40

The overhaul repair manuals, not fabrication manuals (class 1 doccuments) are going to provide essencial information for componants based on part number... (this is important as as engines are produced over a span of years and same componants may be fabricated from different materials or have minor geometry changes) These doccuments should provide filler metals for weld repairs based on part #.

The class 1 repair doccuments may stipulate that welding be done per (standard practice manual XXX or General Notes XXX) The standard practice manuals will have the filler wire cross reference charts for dissimilar metals to assist with engingeneering variance authorities and more general information related to things like argon purge, electrode selection and things of that nature. This is why the standard practice manuals are so important for overhaul and depot level repairs. They work together.

GE and Pratt both have excellent examples of standard practice manuals that are specific to overhaul welding... The major U.S air carriers also have their own Standard Practice Manuals which they have developed over the decades to use in their overhaul facilities, they are often based on the OEM data with specific inputs from their own engineering authority and experience.

By Bill M

Date 03-28-2007 13:45

Have you reviewed AWS C5.10-94 "Recommended Practices for Shielding Gases for Welding and Plasma Cutting"? There is a section in there about purging and backing gas as well.