No allowable stress for alloy 20. 1

[kevlar49]

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.

 

[EdStainless]

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.

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

 

[metengr]

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.

 

[pwtomlin]

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

 

[ApC2Kp]

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.

 

[metengr]

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.