Which carbon steel for superheated steam application 1

[MikeG7]

Which CS for welded chamber for containment of superheated steam design temp 25 bar at +/- 350 centigrade. Of consideration is pressure cycling and corrosion.

 

[stanweld]

Pipe - SA-106B
Plate - SA-515 GR70
Forgings - SA-105
Fittings - SA-234 WPB or WPC

 

[metengr]

Probably similar to an ASME SA 517 Grade 70 carbon steel plate material.

 

[metengr]

Correction; SA 515 not SA 517 Grade 70.

 

[MikeG7]

Thanks Stanweld
I am mostly interested in plate and I also found out 515 Gr. 70 is good for this temperature due to coarse grain structure. Are there other plate grades to consider as 515 gr. 70 is not so available in stock locally.
Further, what's involved in creep analysis to check if the material is within the creep allowances if that makes any sense?
thks

 

[stanweld]

Fully "killed" SA-285C (fire box quality) or other lower strenth grades of SA-515.

 

[metengr]

If you want a listing of suitable plate materials for power boiler construction, I would recommend you review PG-6 of ASME B&PV Code, Section I. By the listing of suitable materials in Section I, creep analysis is not required because of design by rules approach rather than design by analysis.

 

[MikeG7]

If selecting A515 70, would post weld heat treatment be detrimental in as far as the original grain structure is intended to remain "coarse"? Would PWHT produce a fine grain structure and hence nullify the benefit of using this grade as opposed to a fine grade as A516 gr.70?

 

[metengr]

No, PWHT would have no effect on grain structure for the SA 515 material. Why? PWHT is typically performed below the lower transformation temperature, and as such, has no impact on grain growth. What will alter coarse grain to fine grain for the SA 515 steel is the welding process - GTAW versus SMAW or SAW.

 

[MikeG7]

Just one further point for clarification - If I use the ASME BPVC Section II Allowable stress limits, is it true to say that the design would be compliant for 0.1% creep in 100 000 hours, or in 11.4 years the maximum creep if the stress is at the maximum of the allowables would be 0.1%? Or is there more to it than that?

 

[metengr]

The allowable stresses in the time dependent regime for ASME Section II, Part D, Tables 1A and 1B are governed by the lower of 0.67 x the average stress to cause stress rupture in 100,000 hrs or the stress to cause creep deformation of 0.01%/1,000 hrs.