In terms of fire side corrosion, the ultra supercritical steam temperatures cause the outside tube metal temperature to exceed the melting temperature for some eutectic salts formed by coal ash, and these molten salts will flux away the protective oxide layer of normal ferritic tube materials. As a result, special austenitic alloys, such as HR3C ( modified 310 SS), are used for the final superheater and final reheater tubes in the boiler.
For thick walled vessels and pipeline components, the designer needs to adjust his standard analyisis of pipline axial growth ( + earthquake + other loads) with special consideration of the transient Id vs OD temperature gradient that occurs during a fast startup or shutdown of the plant. The transient temperature gradient thru the wall of the thick walled pipe causes thermal stresses to be generated, and these concentrate to high values at drilled bore holes ( for vents, drains, instrumentation), at tees and wyes, and at weld interfaces between thick valves and thinner pipes. Validation of the design at these particular locations now should involve a 3D FEM model of these highest stressed components and a ftaigue analysis conducted using modern fatigue methods ,which now include fracture mechanics due to the recognition that a pre-existing crack always exists at every welded interface.
As indicated earlier, the relatively lower yield stress and lower themal diffusivity of SS would suggest that a SS pipeline would experience much shorter fatigue life than an equivalent ferritic pipeline. But then again, one could make the false assumption that the plant only starts up once and runs continuously for 50 yrs, and blame fatigue cracking on operator error/ feedwater quality/ incorrect zip code.
European mfrs had for years needed to deal with a very difficult ferritic alloy, (X20 ?), and had evolved a very disciplined approach to welding this nasty alloy. When P91 become available in the 1980's, it was relatively simpler to weld than ( X20), and the installed techical capabilities of the qualified EU mfr's was easily able to cope with P91. The US, on the other hand, had evolved from the opposite extreme, being used to facile procedures associated with alloy P22. The QC procedures and technical capabilities installed at most mon & pop foundries was not ( and maybe is not) adequate to reliably deal with the QC requirements of P91.
