Difference between F91 and F22?? 3

[manees]

Hi,

What is the difference between F91 and F22.?
Where does these materials used i.e application??

 

[hokie66]

Is there a joke here somewhere? I would say F69 is the answer.

 

[stanweld]

manees,
Because I know a number of Mechanical Engineers without a clue about alloy steels and their spplications, the following is given:
Differences in chemistry, strength, creep strength. Review SA-182, and ASME II Part D.
Applications: both in power boilers. F22 also in petrochem hydrogen service, hydrocracker reactors, other high temp service.

 

[davefitz]

Manees,

Let me count the ways...I think there is too much written on this subject to summarize in this blog. Try a google search on the subject, and read the papers thoroughly. Recent summaries published by Jeff Henry ( metallurgist for Struct Integ) are a good place to start.ASTM A182 specification is one source for the forging process differences.

P22 was developed for widespread use in the electric Power steam generating industry immediately after WW2, and was the mainstay alloy used for the main steam lines from 1950-1990, for design temperatures 950-1100 F. Simple to fabricate welded and forged components, the crystal structure was ferrite and perlite, and forging techniques were configured to avoid the formation of martensite and bainite, ie, slow cooling following heat treatment. The relatively low allowable stresses implied thick walled main steam lines were required as thick as 6" thk on supercritical units, and this implied the thermal stresses that were generated during startups and shutdowns were in excess of that permitted for a daily cycling unit. Basically, your grandfather's base-loaded coal fired unit uses P22 in the main steam line. Thin welds were inherently ductile, and there was little risk of developing cracks in the fabrication shop .

In 1962, Timken bearing company developed a new alloy for bearings, but it never became commercially successful, and its chemical makeup matches what we now call P91. Circa 1977, during the developement of the Clich River Breader reactor, it was determined that the thermal stresses that would be generated in P22 or 304H piping during a liquid sodium fast transient would be in excess of allowable , and thus there was a pressing need for a much stronger alloy in the temperature range of 1000-1150 F. CE rediscoved the Timken alloy, then called "modified 9 chrome". The higher allowable stress at those elevated temperatures permitted much thinner walled pipes , and since the thermal sress varies by the -2 power of the wall thickness, a doubling of the allowable stress implies 4 times less thermal stress . For other reasons, though,the breeeder reactor was shelved, and the P91 we now use is a useful remnant of that project.

P91/ F91, when properly fabricated to ensure the ASME-assumed strength properties are retained , has a tempered martensite grain structure with finely dispersed strengthening elements. Incorrect fabrication procedures, including weld procedures, forging and bending procedures, and post fabrication heat treatment , can contribute to the development of a grain structure that is not" tempered martensite with finely dispersed strengthening elements", and the result is a component that is not as strong as the designer assumed. There are many ways to make a simple mistake that basically ruins the component. In terms of forging, for example, the final N+T should be followed by a fast cooldown from 1975 F , at a rate in excess of -10 F/minute in the temperature range of 1975 - 1300 F, or else the material fill form ferrite instead of martensite. Unfortunatley, most older shops still use the old forging procedures for F22, wehrein they cool the components as slowly as possible- such a procedural error would ruin a F91 component. The bottom line is that the F91 component should be fabricated by a shop which has experience with this alloy and has their procedures reviewed and approved by a metallurgist familair with all aspects of the material.

F91 is used for valve bodies , elbows, tees, wyes as steam line components for power plants that have design temperatures of 980-1100F and are expected to cycle daily. The thinner wall ( compared to F22) lowers the thermal stress during transients by a factor of about 4, and also lowers the number of flexibility loops needed and the cost of supports.


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