ASTM 335 P11 and P12 : differences?

[kclim]

What is the difference between the properties of P11 (1 1/4 Cr 1/2 Mo) and P12 (1 Cr 1/2 Mo) steels? In what applications would one be preferrential to the other?

As far as I can gather, apart from the Chromium discrepancy, there are very few differences in composition (I think the silicon range was different in both). P11 is roughly equivalent to BS 3604 621 while P12 is equivalent to 622.

Also of some interest is that the corresponding ISO standard (2604) grade TS 32 is equivalent to P12, although there is no equivalent ISO grade for P11.

Would anyone care to offer an explanation?

Regards

 

[metengr]

There are subtle differences between these alloys. The P12 contains 1% Cr-0.5% Mo versus P11, which contains 1.25% Cr - 0.5% Mo. The main difference between these alloys is the allowable stress values, per ASME Section II, Part D;

For P12 @ 700 deg F, the allowable stress is 15.8 Ksi
P11 @ 700 deg F, the allowable stress is 15.5 Ksi

For P12 @ 1000 deg F, the allowable stress is 7.2 Ksi
P11 @ 1000 deg F, the allowable stress is 6.3 Ksi

My take on the other difference is that the increase in chromium for P11, gives a slight edge in terms of scaling/oxidation resistance. The molybdenum is what gives both alloys increased strength at elevated temperature.

 

[metengr]

The P11 at 700 deg F is 15.1 Ksi not 15.5 Ksi (my typo). Also, after reviewing some of my boiler tube material I would suspect that the boiler manufacturers back in the 1950's/1960's had research departments that continually liked to tweak boiler tube materials for higher operating pressures and temperatures. As such, as chromium content and moly contents changed, the boiler OEM's took advantage and aggressively marketed these alloys under such names a Croloy 1/2, 1, 1.25, 1.75, 2, etc.

 

[stanweld]

P11 has better defined application in hydrogen service and is specified by the petrochemical industry in hydrogen applications.

 

[kclim]

Thank you for your answers. Which of the two grades (P11, P12) is more 'standard' ?(i.e. which would be easier to obtain?)

 

[metengr]

It depends on your location in the world. I work in the US power generation business, and P11 or T11 is not readily available, in comparison to say, P22 or T22. I really don't use P12 or T12 in any of our boiler steam circuits so I can't comment on availability in the US.

 

[stanweld]

In the USA, P11 is more readily available. All of the steam genetating power plants that we have built in the past 8 years have had P11 piping systems. We have had to replace P12 piping with P11 in a number of recent repairs of boilers due to lack of availability of P12 from local Warehouse pipe suppliers. If adequate time is available, we have also been able to locate P12.

We have had some P12 piping associated with steam turbines manufactured by Siemens and Alstom (European supplied).

 

[unclesyd]

Depending on your location and as stated above both materials are in short supply. If you are looking at any quantity of either material you had better get preproject monies and get some commitments on price and delivery dates.
Don't be surprised at either.