I am trying to help someone who builds chromoly race car frames. He has run into folks who insist chromoly tubing (.040-.090 wall thickness, 3/4"-1 1/2" diameter) should be back purged to protect the root side of the tubing from contamination. Does anybody know if there would be any advantage to this on 4130 tubing joints? I've never heard of it being done on this material in this application. Thanks!
I'm in the airframe welding business and while I'm sure it wouldn't hurt to back purge 4130 tube welds, I've never heard of it being done in this field. If it's good enough for airframes, I'd say it would also work on race car frames.
Do the "folks" who testify to that fact, have evidence of the advantage of purging this material? I've seen aluminum purged with the same intent. It only drives the cost of that weld - up.
The intent of a practical weld is to reduce the cost of machining, casting, forging, etc. and to reduce costs encountered by material joining/fabrication. This could be compared to the $$ lost with oversized welds or over-welding.
To put some teeth into the excellent logic DGXL presents here,
You could refer to Mil Std 1595, This is the governing doccument for fusion welding of missles and aerospace componants. Also the ASM Handbook Volume 6 is another excellent source for supporting doccumentation.
I have never run into engineering specifications that require argon purge for 4130.
The real question to be addressed in most cases is post-weld heat treatment/stress relief.
Lawrence
Primarly back purging prevents oxidation at the weld root. This oxidation reduces corrosion properties in the case of stainless steel.
But, back purging can assist the welder in obtaining a good root weld. Reducing the oxygen content in the atmosphere, the surface tension of the molten metal can be reduced, resulting in better wetting properties.
Altough this is not very common in constructional chromoly steel tubing.
We repair & manufacture new sprintcar chassis' here in Australia using 4130 exclusively and we don't internally purge during welding. To the best of our knowledge none of the major sprintcar chassis' manufacturers in the USA purge either.
Cost wise, it would send prices skyrocketing in an already competitive field.
Regards Andrew.
FYI: Lincoln has a FAQ section on chrome-moly tubing welding.
http://www.lincolnelectric.com/knowledge/articles/content/chrome-moly.asp
On much higher alloyed Cr-Mo, (e.g. 5%Cr) there can be oxidation problems in the root of the weld that makes obtaining good penetration difficult. (Although not impossible.) On the lower Cr levels, this is not really a factor.
I would not recommend purging.
Regards
Niekie Jooste
A little more insight on this subject. I used to GTAW tubular helicopter frames fabricated from 4130. They are all still in the air and corrosion or any other metallurgical problem is not an issue.
If it's good enough for a rotorcraft @ 20,000 feet, it should be satisfactory for your application. I showed a copy of this post to several friends and they all had the same response - "What for?"
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