PQR and Backing Gas

May somebody help me?

To weld a stainless steel pipe, I understand that it is necesary to use backing gas (inside the pipe). Bit i it possible to weld without this backing gas?

Some suppliers inform me that it has an extra-cost. I don´t understand the reasons?

In the PQR, doesn`t say anithing about backing gas.

Thanks in advance


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A backing gas is only necessary if the weld is made from one side. What happens is that the underside of the weld suffers oxidation during welding if left unprotected by a backing gas. If the oxidation can be removed by grinding or machining, then no problem. The oxidation will reduce its corrosion resistance and resistance to contamination, and it will look poor. However large stainless ducts are often not purged because of the cost of gas, as you can't see the inside nobody worries.

For more info on purging:-

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Regards John

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John Dyson

understand the

This requires a 4 part answer.

Part 1: For the ASME IX code, the deletion of a backing gas is an essential variable for some materials, but this does not apply to P-Number 8 materials (i.e. 300-series stainless steel) [refer to ASME IX QW-408.9]. Therefore, if the PQR did not include a backing gas then the production welding can be done without backing gas.

Part 2: The above would apply if the PQR was done as a single-sided weld. If the PQR was done as a double-sided joint but the production welding will be a single-sided joint, then common sense (but not ASME IX) would indicate that the PQR is not completely appropriate.

Part 3: Because of some deficiencies in ASME IX, most user specifications would require inert gas backing for stainless steel applications. For example, refer to API Recommended Practice 582 ("Welding Guidelines for the Chemical, Oil, and Gas Industries"), which states (para. 7.3) " Back purging is required for the GTAW and GMAW processes for welding materials having a nominal chromium content greater than 2-1/4% unless the joint is ground or back gouged to sound metal." This would require back purging for all types of stainless.

Part 4: Is it a good idea to weld stainless steel without the back purge? The answer is "no", particularly if the stainless is specified for the corrosion resistance of the internal operating fluid.

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thanks for your help and congratulations for your web!!.

We are talking about 2" / 4" pipes. No very big diameters.

Would you please, let me know (approximatly) the different between the time than I should spend with and without backing gas?

Muy supplier refuse to weld with backing because they think it will take them time x 3. I believe they are completly wrong !!.


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