temperature limitation for use of low-carbon stainless steel

The trade off in the "L" grades is reduced sensitization for lower strength (both yield and tensile), at least at room temp., 5000 to 8000 psi according to my data. So, if it is true, as you say, that the drop off vs. temp is the same for regular and "L"'s, then the "L"'s are going to hit the softening critical point first. There is another forum specifically on stainless steels (forum 1135) that might handle your question better.

To obfuscate the issue, I have seen special B&PV Sec.VIII code cases approved for use of 304L up to 1600F. Now we have "dual spec" TP304/304L that has 304 mechanicals and 304L metallurgy. Go figure.

The issue is carbon content and it's affect on creep-rupture strength of austenitic stainless steel. The higher the carbon content, the higher the creep strength of the austenitic stainless steel. The Code committee’s take a conservative approach in limiting the low carbon grades of stainless steel to lower temperatures as a result of reduced creep-rupture and increased creep deformation rate.

We still have the paradox of the dual certification as posted by ccw.
All the 304 SS used by our site is purchased to a restricted Carbon (0.015% max) along with restriction on S, P, and Si. The latter 3 can be waived depending on the end use, but the carbon spec. is never waived.
The problem arises in that nearly all the pipe and plate comes on site with the dual specification of 304/304L. based on evidently on tensile and yield strength at RT. The carbon meets our specification of 0.015% max. There is no way we would used this material at the higher temperature for the reasons put forward by [/]metengr[/b]. The design review has always caught this but everyone is waiting for some to get by. I seen it come close to getting into service.

Another problem has cropped up with pipe from a well know pipe manufacturer. Not only is the pipe dual certified it is dual standard in every way except the MTR.

1 3/4" x 0.083 A/SA213 A269 304/304L A&P Ht #
The problem arises when the MTR only covers ASTM A213 only. Same Ht #

As I posted concerning fasteners, the majority of the people handling these materials have no clue to relevance and meaning of the markings.

the issue can perhaps be viewed most clearly considering the ASME B31.3 allowable stress tables, which provide allowable stresses for low carbon austenitic stainless steel to as high as 1500F.

304 can be provided in a range of carbon spanning from L grade to H grade (low carbon to high carbon grades).

If you have type 304 stainless steel, there is a footnote to the allowable stress table that requires a minimum carbon content (0.04% min) to use the allowable stresses at temperatures above 1000F. If the material you have does not satisfy that min carbon content, you need to use the allowable stresses at temperatures above 1000F for the low carbon grade of the material, 304L. The difference in allowable stresses has to do with differences in creep properties, at these higher temperatures.

If you have dual stamped material, the metalurgy has been tweeked to provide the required room temperature strength of the straight grade, while complying with the low carbon content requirement for the L grade. Thus, it meets both specifications. For this material, you would use the allowable stresses for the straight grade to 1000F, and the allowable stresses for the L grade at temperatures above 1000F.

for B$PV fabrication, you will choose one spec only at time of design, even if the material is dual
you can not mix its properties,
gb

These days most of the oil companies restrict the use of 304/304L for offshore applications.