F/X of Austenite temperature on Creep Properties of T11

[SMF1964]

Does anyone have any thoughts on the following:

The possibility of adjusting the creep properties of a ferritic steel (1.25Cr-0.5Mo) by only adjusting the temperature at which the steel is austenitized?

 

[Metalguy]

IIRC the creep strength of steel increase somewhat as the grain size gets larger, so a high temp norm. and/or aust. should help. I don't have any numbers avail. but they surely exist.

 

[metengr]

SMF1964;
As metalguy indicated, you will have increased creep rupture strength with a coarse grain structure versus a fine grained structure. However, there is a trade-off and that is creep rupture ductility. Typically, as creep rupture strength increases, the rupture ductility decreases.

You might recall that General Electric manufactured a "C" grade of turbine rotor steel that was intentionally heat treated at a high austenitizing temperature to enhance creep rupture strength, thru a coarse grained tempered martensite/bainite microstructure. The problem that developed in service was dangerously low toughness and significantly reduced creep rupture ductility.

Keep in mind that creep rupture strength and creep deformation are two different concepts. Most T11 tubing in boilers undergoes ferrite grain growth and spheroidization of carbides, over time, because of constant exposure to elevated temperature service (reheaters and superheaters). I believe the reason for tube longevity (tubing that has been in service for over 200,000 operating hours) is related to the tube material being able to accommodate strains from local creep deformation over time. If the tube were fabricated to increase creep rupture strength but sacrifice creep rupture ductility, you would have significant cracking problems at butt welds and welded attachments early on in service because the tube material would crack instead of being able to absorb local variations in creep strain.