304L use at 649 Celsius vs 304

[Quentin Nguyen]

Hi guys,
I'm currently dealing with an ASME VIII-1 - 2021 vessel with a design temperature of 649 C.
The material required is stainless steel. So as in ASME II-D - 2021, 304L has max. temp at 649 C while 304 has a G12 note for C>=0.04% for temp. >550 C.
That is kind of conflict, don't you think?
I need to clear that point before I can officially buy 304L for all parts for this 649 C design temp.
Please help me on that. Thank you!

 

[EdStainless]

Looks like an oversite to me.
The 304 has higher allowed stress correct?
They don't want you using the higher stress limits with 304 that is actually 304L.

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P.E. Metallurgy, consulting work welcomed

 

[Quentin Nguyen]

Thanks for your reply.
Yes, the 304 has higher allowable stress. But to use at temp. over 550 C it must have C>=0.04%
But 304L has limited temp. at 649 C. The temp. from 550 to 649 is in conflict.
I don't quite understand that

 

[Geoff13]

At 550°C the 304 has an allowable of 93.3 MPa (with Note G5) whereas the 304L has an allowable of only 49.5 MPa (also Note G5). In addition the 304 doesn't reach its time-dependent properties until 565°C whereas the 304L has already passed its 480°C transition.

Presumably the committee feels the C > 0.04% is needed to maintain the higher 304 allowables above 550°C.

What allowable stress you need for your design will decide if you can use 304L, or require the 304 with C > 0.04%.

 

[EdStainless]

I have worked with end users where 480C is the break between 304L and 304H.
Largely because it is nearly impossible in the US to get 304 with C above L and below H.
Just as a side note, even though N raises the strength of 304L a lot it does not contribute to creep properties.
High strength 304L is an illusion above 400C.

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P.E. Metallurgy, consulting work welcomed

 

[Quentin Nguyen]

So for my case, design temp. at 649 C should use 304/316 with C>=0.04% or use H grade. I did the strength calculation for 304L and I can afford the thickness of it. 304L even though the code permits but it doesn't meet the requirement.
Is that true?
Thank you!