GTAW - backing gas is an essential variable in stainless steel welding P-No. 8?

Only read in the title, you may face a lot questions from Client or Client’s Representative. Which type of purging/backing gas for GTAW (P-No. 8) such backing for fillet weld, backing gas for root/hot pass up to 6.0 mm, etc.

What essential variables and supplementary essential variables?

If changed in beyond limits may have signification effect on properties and toughness of weld product, see QW-105 of ASME IX 2017.

In ASME IX, GTAW welding variables are given in Table QW-256, point to QW-408.9: deletion backing gas or change composition gas.

This apply for groove weld in P-no. 41 through 49, P-No.-10I, 10J, 10K , P-No. 51 through P-No. 53 and P-No. 61 through P-No. 62

For P-No. 8 such Type 316, 304, … see grouping in table QW/QB-422. These materials were not listed in Subsection QW-408.9. So, the backing/purging gas for groove weld of austentic stainless steel P-No. 8 is not essential variable.

What project documents details?

Most project specifications and recommendation particles are required backing gas type and its composition when welding from one side or prevent oxidization which cause of rejection.

Which standard guides to visual acceptance?

  1. See project specification for stainless steel, or
  2. NORSOK standard M-601 Annex B, Acceptable oxidations of welds in stainless steels
  3. API RP 582 : 2016, see Section 7.

Conclusion:

  1. Running PQR should have purging gas and specific maintenance packing gas at least 3 passes or more.
  2. WPS should show all welding variables, I recommend to bring all welding variables to your company’s standard format, read a sample here.

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