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Purging 304
3

Purging 304

Purging 304

(OP)
Today I found myself confused as to IF ss piping (304 specifically) needed to be purged prior to welding. There is a project in the shop, pre-fab of a vapor piping, 2" sch40 304 ss for a local food plant ( no inspection required) and it is not being purged. Also, the first pass is fusion welded (no penetration whatso ever) and My boss's point of view is that the purging is not required because it is for vapor, and that the client know's that it is not being purged prior to welding; He is telling me also that there are 30 year old installations that were welded the same way and are functional ( he went into detail about engineering economics as to say that It would not be cost effective to purge). I strongly dissagree, but this may be because of my ignorance to some aspects of ss. I have studied guides of ss welding, I am a Certified Welding Inspector  and I don't remember ever wleding without purging on SS

Appreciate any responses

regards

Raul

RE: Purging 304

2
Purging with Argon the interior of stainless piping before Gas Metal Arc Welding or Gas Tungsten Arc Welding is needed to avoid oxidation and welding defects on the backside of the welds. Purging need only be local, by using suitable partitions.
As you do not specify the welding process to be adopted, in case it is Shielded Metal Arc Welding, the backside could be protected by putting in place granulated flux before welding.
Without purging or fluxing, the back side of the welds will result oxidized and possibly porous or otherwise defective. If inferior quality is accepted then OK.
304 piping is prone to intergranular attack, after sensitization due to welding. Better would be to select 304L or 321, and weld using 347.

http://www.welding-advisers.com/

RE: Purging 304

I have seen people do it both ways, but purging is prefered (but an added expense)

If it is relatively low pressure you could save A LOT of money by switching to thin wall SS tubing (we use .065" wall for 2") BUT if you go that route the purging becomes more important since the weld is not nearly as thick, and you will need to verify the pressure ratings. (consider it next project).

$/lb SS prices are so insane that saving material is very beneficial. You could probably get polished tubing for less than the sch40 pipe.

RE: Purging 304

While the subject of wall thickness is being discussed, have you ever calculated the actual wall required for low pressure service?  The standard sch10 pipe or sanitary sizes are way overkill.
The problem is availability.  There is some sch5 used, but not much.  That is what Victaulic uses for PressFit I believe.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.
http://www.trent-tube.com/contact/Tech_Assist.cfm

RE: Purging 304

As much as I hate such a joint here is a way out.  Not perfect but will come much closer to what I  would consider an acceptable joint.

http://www.oxfordalloys.com/stainlesssteel.htm#3

Another method, that I’ll get clobbered for, is that you can purge with Nitrogen while still using Argon on the Torch.   You also  might try a little welding flux (Solar) on the id of the pipe while purging with N2.

http://www.solarflux.com/

Also be careful with thin wall pipe and tubing as they can create many problems by themselves unless you have a dedicated work force.    

RE: Purging 304

Nitrogen back purge is fine with me.  It sure is beter than air.

The only time that I seen probelms is utility lines is when they change plant operating conditions.  If these vent lines ever start seeing any condensate then the wellds will begin corroding.

In light wall almost all welding is automatic, clamp on orbital heads.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.
http://www.trent-tube.com/contact/Tech_Assist.cfm

RE: Purging 304

Without purging, the root pass will be highly oxidized; however, the joint may still be acceptable for the service condition. Because you are not adding filler metal for the root pass, underbead cracking of the root pass is a strong possibility since the root will be fully austenitic. If the vapors are corrosive as I would expect them to be be since the material selected is 304 SS, premature failure should be expected.

If the purchaser has no specifications governing the weld and failure of the weld cannot cause injury to others and the weld is expected to last beyond the warranty period, you may wish to make the weld as your boss stated. There are other methods of making these welds that would be cheaper than the methods you have chosen and would provide a superior weld.

RE: Purging 304

I am involved in the specifications for high purity gas supply systems for semiconductor plants.  These systems are 316L stainless tubing welded with automatic welding systems.  They are required to be purged with essentially no welding discoloration.  The alloy specs are optimized for welding compatibility.  The welding specs require full penetration welds, with considerable additional detail about acceptable weld results.

The participants in this industry have done some characterization of the tubing surfaces by surface analysis (Auger, ESCA, etc.), corrosion testing, and a little actual testing in gases of interest to the industry.  Some of these gases are very highly corrosive (chlorides, bromides, etc.) WHEN MOIST, as little as a few ppm moisture levels for the most aggressive gases.  Well maintained defect-free systems that have not been exposed to moisture containing aggressive gases show no evidence of corrosion after many years of life.  Welding discoloration has been shown to be more susceptable to onset of corrosion.

Although this information is probably well beyond your requirements I can excerpt some of this information for you, or you can find it on the SEMI website under standards.

http://wps2a.semi.org/wps/portal/_pagr/118/_pa.118/755

I agree with DesignerMike's suggestion to utilize tubing rather than pipe, and suggest looking into automated butt welding systems.

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