WPS qualification per ASME section 9 2

[fakhfakh]

The calculation for a pressure retaining item is based on material allowable stress which is related to the yield stress.
Why do the WPS qualification as per ASME section 9 is concerned by the tensile strength and not the yield strength ?

 

[stanweld]

The weld metal and base metal HAZ have different yield strengths than the base material. Thus it is highly difficult to measure the true yield strength during the tensile test. The basis of ASME IX is to assure that the minimum tensile strength properties of the base materials are met. The purpose of the bend tests is do assure minimum ductility. Note also that ASME Boiler Code design is based on the specified minimum tensile strength at ambient temperature of the material divided by a safety factor, not the yield strength.

 

[metengr]

To add a little to the above, the allowable stresses that are time INDEPENDENT are controlled by UTS/3.5. What this means is that the controlling parameter is ultimate tensile strength/3.5, which would occur well below 67% yield strength is reached (2/3rds of minimum specified YS). I would recommend you review ASME B&PV Code, Section II, Appendix I.

Most people do not recognize how allowable stresses are assigned. The full range of tensile properties are evaluated for the subject material at temperature. A spreadsheet is then developed comparing the UTS/3.5 to yield strength and the lower of the two values is assigned as the allowable stress.

 

[fakhfakh]

Thanks to all
But why do the Code don't deal directly with the yield strength ?

 

[metengr]

It does deal with YS, only the allowable stress is below the YS, and that is all that matters from a design by rule approach.

 

[Guest102023]

I had done a few PQRs and reduced-section tensile tests before I understood that the logic of Section IX tensile testing is to try and force failure in the weld. The weld properties are very non-uniform (besides being different from those of the base metal), so determing a 'yield strength' is not only impractical, it is impossible. You only want to have a weld that makes it over the simple hurdle of exceeding the base metal (specified minimum) UTS.