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No allowable stress for alloy 20.

No allowable stress for alloy 20.

No allowable stress for alloy 20.

(OP)
Does anyone know why B31.3 does not list allowable stress values for alloy 20 N08020 beyond 800 F.  Is this because of metallurgical embrittlement?

Want to replace a valve in fully-regnerative catalytic reformer regen gas service that was 12Cr originally.  Want to do it with alloy 20 Cb-3 b/c no 12Cr (410 or CA15, CA6NM) available.  Design temperature is 1000F.

RE: No allowable stress for alloy 20.

I think that the B&PVC gives values to 1000F.  The high Ni makes the alloy stable to higher temps.
What makes alloy 20 an alternate for 410?  Sounds like a real jump to me.

= = = = = = = = = = = = = = = = = = = =
Still trying to help you stop corrosion.
formerly Trent Tube, now Plymouth Tube
eblessman@plymouth.com
or edstainless@earthlink.net

RE: No allowable stress for alloy 20.

N08020 is not permitted above 800 deg F in the 2006 Edition of the ASME B&PV code, see Section II, Part D, Table 1-B. I am not aware of any embrittlement issue. However, there might be a code case to allow exposure to 1000 deg F. I would guess that at the time it was code approved, the creep strength of this material limited its use above 800 deg F.

RE: No allowable stress for alloy 20.

I'm surprised to hear there's no CA15 or CA6NM available.

RE: No allowable stress for alloy 20.

kevlar49,

The ASME B31.3 is somewhat eccentric in how it handles Alloy 20 - the N8020 has 800 F limit, and is grouped with the stainless steel materials.  The cast version CN7M is grouped with the nickel materials and had 1000 F limit.  
The composition of rolled material is somewhat different than the cast material, but does it warrant the different group or temperature limits?

If you are considering a valve substitution, then most likely the valve body would be cast CN7M, limit 1000 F.  

RE: No allowable stress for alloy 20.

Most likely, the cast version has higher carbon (0.07) versus 0.02 for wrought. Higher carbon means higher creep strength. There are reasons for limits.

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