Fatigue curves 2

[PVchemEng]

Fatigue curves

in Appendix 5 of ASME VIII div.2 I can find design fatigue curves for low alloy for temperature not exceeding 700°F.

If the peak of Temperature cycle exceed 700°F, what's the right approach to use?
And what's the right approach to use if the peak of Temperature cycle reach the creep range?

Thanks

 

[metengr]

PVchemEng;
That is right because above 700 deg F you are in time dependent properties for this material, and this creates a new set of problems for analysis. Above 700 deg F most low alloy steels are in the creep-fatigue regime, and this must be dealt with separately.

 

[davefitz]

refer to 2007 rev of asme sect VIII part 2

par 6.1.6.3 recommends using part 10 of api 579-1 for parts operating in the creep regime.


In boiler situations, the most severe fatigue stresses occur during a cold startup , where the temperature may rane from 100 F to 1050 F. Based on the older german boiler code TRD 301 annex 1, the governing fatigue properties are based on some weighted medium temperature of the thermal cycle, weighted toward the high end , perhaps 0.75* Tmax + 0.25 * Tmin, so a 700 F limit on fatigue properties is not an issue. Creep fatigue interaction needs to be taken into account, but there is no final academic agreement on how to properly account for this that I know of. There are published political compromises in some codes to allow the design process to move forward, but the matter is not yet technically resolved.