AISI 304 vs AISI 304L - identifying which is which in a production environment

No. And most 304 and 304L are dual certified making it even more difficult.

There are portable optical emission spectrometers that could be used to detect carbon content.

Remember that all 304L meets the 304 specs.
OES or LIBS are the only portable methods that will detect Carbon.
And that is what would be needed.
What does it matter?
If you can't keep traceability in your process, you have much bigger problems (a broken quality systetm).

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P.E. Metallurgy, consulting work welcomed

Thanks for all the answers!

I really appreciate your help.
It was purely a hypothetical question, mostly dictated by my curiousity.
I will read more about portable carbon detecting devices.

Stay safe!

Do read about "spark test" too.

DHURJATI SEN
Kolkata, India

Most forged and wrought material (fittings, flanges, pipe, tubing) are dual grade meaning that the material meets the carbon content of the L grade and the strength of the straight grade. The real risk is with welding consumables.

The optical emission spectrometer is the thing that is used to do the "spark test", but such testing is best avoided.

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In fact, according to the content of chromium and nickel, it is all 304 stainless steel, and the composition is 18% chromium (Cr) and 8% nickel (Ni), but the main difference is the carbon content.
304L is an ultra-low carbon stainless steel, the carbon content is reduced to below 0.03%, which can avoid intergranular corrosion, and in theory, the effect of stress corrosion resistance is stronger than that of 304, but the effect is not obvious in practical application.
The hardness, tensile strength and yield strength of 304 and 304L are different. The view that 304L can be used instead of 304 is wrong.