Grade 9 vs Grade 91 chromoly

[atomicbob]

We are fabricating ASME Code pressure vessels specified to be 9 chrome. We had used Gr 91 in our calculations and drawings for the plate materials, as it was the only grade of 9cr in our software (Advance Pressure Vessel).

The material we received is grade 9, and it was rejected as non-conforming by the client inspector. I believe the gr 9 should be acceptable, but we have been unable to find the stress curves for gr 9 plate in the ASME books, and can't run calculations.

Can anyone give me any direction here?

Thanks

 

[metengr]

atomicbob;
The stress lines for Grade 9 are in Section II, Part D of the ASME B&PV Code. You assumption for using Grade 91 stress values as Grade 9 is incorrect, and should not have been used. You need to re-run the stress calcs using the correct stress values.

 

[davefitz]

Well the good news it was caught in time. Now , the question is , if they are not familiar with the difference between gr 9 and gr 91, doesn't it follow that they probably would not fabricate the gr 91 vessels right the first time?

 

[metengr]

Yep.

 

[atomicbob]

Thanks Metengr! I knew the stresses must be there. It didn't make any sense to me. We'll keep looking.

 

[atomicbob]

Well, I seem to be back to where I started. Back in section II part D, Table 1A.

9Cr-1Mo isn't shown in plate form.

What am I missing?

 

[metengr]

atomicbob;
Look at ASME SA 387 Grade 91. For Grade 9, SA 336 forged plate.

 

[atomicbob]

Thanks. I see that one, but my MTR's are for SA-387 Gr 9 Cl 2.

 

[atomicbob]

Ah ha! Said the blind man.

SA-387 Gr 9 is not recognized by Sect 8, Div. 1 as pressure vessel material per Table UCS-23.

Thanks for your help guys. I'll buy new shells and heads and then likely have some questions about welding and stress relieving Gr 9 to Gr 91 joints.