What are the differences between A335 P11 and A335 P22? For example, the allowable operating temperature for a design life of pipes. Thanks in advance.
The ASTM material standard has description of the differing chemical compositions. Physical properties of strength and the upper temperature limits are result of the increased chrome and molybdenum components. See specific piping codes for maximum allowable design temperatures. Different welding filler materials will have separate welding procedures. There are also higher pre-heat temperatures and postweld heat treatment temperatures. The manufacturing work of the higher strength material has its increased costs. The relative cost differences may no longer be as significant with more use of the P22 material grade.
ASTM A-335, P11 is listed in B31.3 Appendix A as a low/intermediate alloy steel with 1 1/4 Cr, 1/2 Mo
ASTM A-335, P22 is listed in B31.3 Appendix A as a low/intermediate alloy steel with 2 1/4 Cr, 1 Mo
You should check the notes 72 and 75 for use of P22 at high temperatures.
Several years ago the B31 Codes reduced the allowable stress of P11 due to "lessons learned" regarding creep-fatigue interaction of this material at high temperatures.
At temperatures above 300 degrees the P11 material's allowable stress decreases much faster than does P22 materials allowable stress.
I think it is also important to discuss the history of spectacular failures that have occured with seam-welded P11 and P22.
If you do some research on the matter, you can find incidents where high-energy piping (P11 and P22) has failed in power plant applications. Much of the defective piping was installed in the 1960s and had "marginal" logitudinal seam welds that eventually failed through a creep mechanism.
