Purging/backing gas (argon) during welding.

[Khandaker Imamul Hasan]

Dear Expert,

Could any one inform me the

Q-1. Why Purging/backing gas (argon) is required during welding of SS(TP 304L, 316L, 321…)?

Generally we do purging for SS welding

1.        to prevent oxidation and

2.       backing for good root profile.

Could you tell me what type of oxidation (chemical reaction) occurs if we don’t use purge gas?

Q-2.  Why we don’t Purging for A335 P11(1.25 Cr + 0.5 Mo)?

Generally we do purging for those low alloy steel which Cr content is above 2,such as A335 P22(2.25 Cr + 1 Mo).

Could you tell me why purging is not recommended for P11 alloys (chemical reaction/composition will be more appreciable)?

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Best Regards

Khandaker Imamul Hasan

[Diganta Sarma]

 
Dear Mr Hasan,
When subjected to high temperatures and atmospheric oxygen, many metals tend to intense oxidation. The oxides generally appear as temper color, for example, with stainless steel or titanium materials. Temper colors will greatly impair the corrosion resistance of such metals. Furthermore, severe oxidation will impair the formation of weld roots. Thus, in many cases, the root side has to be protected against oxygen in order to ensure optimum corrosion resistance. The careful exclusion of atmospheric oxygen can prevent oxidation and temper colors.
 

As little as 30 ppm of residual oxygen in the purged gas may be enough to create temper colours which results in decreasing the pitting corrosion resistance of stainless steels . At levels below 50 ppm, formation of temper colours is usually less common; 100 ppm are generally considered a limit when processing stainless steels. Above this value, increased formation of temper colours can be expected. Purging, i.e. covering the weld root and adjacent parent material with a shielding gas in order to protect the penetration bead from becoming contaminated by coming into contact with atmospheric gases during and after welding, stands in contrast to the other methods, since formation of temper colours can be prevented or greatly reduced from the outset.

 

Trust these answers your query. Let me know if you would like additional information

 

Diganta Sarma

Linde Gases

 

 

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Date: Wed, 10 Oct 2012 23:53:46 +0600
Subject: [MW:15545] Purging/backing gas (argon) during welding.
From: kimamu...@gmail.com
To: material...@googlegroups.com

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[kishor kaku]

Dear Sir,

 

What is the limit of Cr Content for Purging?

 

Regards,

 

kishor

[pankaj...@wipro.com]

Dear Diganta sarma,

Why purging is not done to CS? Why to SS only

 

Pankaj Kolhe

QC

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[prem nautiyal]

CS does not contain elements like Cr which have high affinity to oxygen for forming oxides like SS.   Regards   Prem Nautiyal --- On Fri, 10/12/12, pankaj...@wipro.com <pankaj...@wipro.com> wrote:

[kannayeram gnanapandithan]

For Cr content less than / equal to 3% not require purging

Pandithan
In-Charge/WT
Cethar Ltd

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