ASME Sec 9 vs API RP 582 for Backpurging SS

[Nashanas]

Hello,

According to ASME Sec 9 QW 408.9: The backpurging is required for a certain set of materials only. So e.g P8 materials are not required to have backpurging.

According to API RP 582 Section 7.3 backpurging is required for GTAW and GMAW for all materials above 2.25% of Cr. So P8 materials with Cr% > 2.25 are supposed to be backpurged.

Why is API more strict about back purging then ASME. Is there a metallurgical reason?

 

[weldstan]

The need for backpurging is also based on service conditions which could adversely affect corrosion resitance of a non backpurged weld. ASME IX is not concerned with corrosive service conditions; yhat is up to the Owner/Engineer.

 

At root this is a metallurgical matter. A molten root in a SS weld exposed to air will oxidize vigorously, leading to an undesirable final outcome. Appearance and corrosion resistance will be terrible.

You know it is Cr that imparts corrosion resistance to SS surfaces, but Cr is in fact an enthusiastic dance partner with O2.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

The only time that SS alloys are not backpurged is when all that matters is the external appearance.
Look at AWS D18.2.
Sec 9 is basically saying that it does not effect the mechanical properties of the welds. Corrosion resistance is your responsibility not the Code's.

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P.E. Metallurgy, consulting work welcomed

 

[weldstan]

In the majority of refinery applications, 5Cr, 9Cr and higher chromium alloys are useed in very corrosive environments.