I suggest you try a small experiment and you will see quite quickly why argon shielding of the back side is recommended for stainless steel. Take several pieces of stainless, thickness and exact size are irrelevant, and make two welds from on-side only; one weld with argon backing gas and one weld without an gas backing. The one with backing gas will be bright shinny (maybe a small amount of heat tint depending on the quality of your back purge) with smooth, well formed ripples. The one without backing gas will be highly irregular covered with a thick gray coating and individulal weld ripples will probably not be discernable. Cross section both welds, polish them, and look at them under a microscope. You will see that the grayish material extends into the weld metal for at least several grains in depth. The gray material is an oxide formed at high temperature and has a very high chromium content. Without backing gas you have made a highly irregular surface, posoous with penetrations into the surface and removed from solution the alloy addition, chromium, which confers general corrosion resistance - to put it simply you just provided corrosion an ideal condition in which to initiate.
While any weld will benefit from a back purge in prevention of oxidation, since stainless steels are typically used in more aggressive environments it is more important that they be back purged to prevent premature failure due to corrosion.
Depending on application and customer, argon back purging will be recommended when the chromium content reaches around 2-1/4% or greater and will not be restricted just to stainless steels.
john
OK,why SS is more suseptable than CS to get oxidised?
--- On Tue, 16/6/09, Antonio Ruscitto <ARUSCITT@metrogas.com.ar> wrote:
From: Antonio Ruscitto <ARUSCITT@metrogas.com.ar>
Subject: [MW:2447] requirement of purging in CS/SS pipes
To: materials-welding@googlegroups.com
Date: Tuesday, 16 June, 2009, 5:02 PM
Hi: I think that is for the same reason. I´ve qualificated a Welding Procedure Specification with ASTM 106 G° B, with gas purging Ar inert gas. Because that reason Regards Antonio
>>> Sukamal Naskar <sukamalbecmet@gmail.com> 11/06/2009 08:36 pm >>>
For Stainless steel we do purging ( Inert gas like Argon ) to avoid the oxidation at the root. Stainless steel is more susseptable to oxidation than carbon steel. for carbon steel root is not so much suseptable to oxidation , hence we don't use purging gas like Argon. Regards,
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