Reason for difference of max. service temperature for SS316 & SS316L according to ASME II 1

[doggie87]

Checking the max. allowable service temperature for SA 240 TP316L and SA 240 TP316 on ASME II.

And, quite surprising that 316 L has max. service temperature of 454 C (850 F) while 316 has max. service temperature of 816 C (1500 F).

The main difference of 316 L and 316 is the carbon composition. (0.08 for SS316 and 0.03 for SS316 L).

The main reason by having lower carbon content is to avoid carbide formation with chromium at elevated temperature during welding.

Due to the low carbon content, SS 316L has lower strength but not much difference with SS 316. But, in return, this has resulted in lower max. service temperature.

It is highly appreciated if some one could explain this discrepancy from material science point of view!! Many Thanks.

 

[metengr]

The simple answer is that the higher carbon content promotes ambient temperature and elevated temperature creep strength in austenitic stainless steels.

 

[EdStainless]

Remember that those are max carbon levels, you can get 316 straight grade with 0.015% C.
This is why it almost always dual certified material.
The code does not typically consider issues of metallurgical stability, just suitable properties at the given temperatures.
Most modern SS has some N in it. This raises the strength even with very low C levels.
The problem is that N does not improve creep strength as much as C does.
If you are working at high temp then you need to order 316H which has a min C in the spec.

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P.E. Metallurgy, Plymouth Tube

 

[doggie87]

Thanks for the explanation.

SS 316 L material is chosen for a max service temperature condition beyond the allowable value to ASME II (454 C).

Note that the material purchase is based on ASME II specification although the material is used for the structure part and NOT for the pressure part.

Also, it appeared that ASME is more stringent than API 530, i.e. the limiting design metal temperature (815 C vs 454 C).

Quite unclear whether SS 316 L is allowable to be used for structure part, even at the service condition higher than the allowable max. service condition.

 

[EdStainless]

I should add that in the old days (when there was a price premium on 'L') people would order straight grade with 0.03% max C.
So they would get the benefits of 'L' without having to pay for it.
Today, in the US and Europe there is no such thing as 316 straight grade. It is all with 'L' or 'H', with the 'L' material all being dual certified as 316/316L. This material is commonly used to the temperature (and stress) limits of the straight grade material.

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P.E. Metallurgy, Plymouth Tube