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dual rated 304/304L

dual rated 304/304L

dual rated 304/304L

(OP)
Hi everyone,
I have recently performed an ASME code calculation (using software) on a pressure vessel.  I thought that it was odd that the vessel would pass for 304 but not for 304L.  This of course raised the question of what the difference is between the two at say 1150 degrees F.  My question isn't really what the difference between them is, rather what are the properties of the dual rated version, and if there is such a clear dividing line of 0.03% carbon content, how can the material perform the function of both?  Also, my purchasing manager has stated that he cannot find pure 304 anywhere, that we must use 304/L or go to a different # stainless, is this true?

RE: dual rated 304/304L

Dual certified materials are not some mystical version of materials.  It just happens to meet the requirements of two grades that have significant overlap in the ranges specified for each grade.  304 and 304L are just one of many examples.  It is entirely acceptable to buy dual certified to 304 and 304L and use it exclusively as either grade.  The Codes only require that you follow a consistent set of rules.  For example, you treat it as 304 or you treat it as 304L.  You may not mix and match the properties in the design, fabrication or rerection of the unit.

To answer your question about what are the properties of a dual certified material...  they are as reported on the mill test report.  And by coincidence, they meet the range limits stated for two grades of material.

Joe Tank

RE: dual rated 304/304L

Almost all "304" plate is dual certified.
It may be 0.03 max carbon stuff dual certified 304L/304. This, meanins it has decent corrosion resistance as-welded and the tensile & yield properties of 304, at least near room temperature.
1150F is in the creep range, for which you need dual certified 304/304H. This has a minimum of 0.04% carbon for strength, plus I think both grain size and minimum annealing temperatures. It is widely available.
Dual certification is the common practice. The mill does not want to melt two or three different chemistries, i.e. 304L, 304 and 304H so they make two different chemistries do the work of these three. It is the magic of precise chemistry control by refining the melt in an Argon-Oxygen Decarburization vessel, where a little nitrogen brings up the yield strength of 304L to the requirements of 304. Great for room temperature but the dual-certified "L"
grade is not usually considered strong enough for use in the creep-limited range.
Clear as mud?  

RE: dual rated 304/304L

For simplicity, I've limited my understanding of dual certification to this practical summary: It's 304L material with controlled amounts of Nitrogen to bump up the yield to match 304ss.

It's not a material for high temperature / creep range service.

Not sure that answers the question! :)


Just a note of warning - we had a minor incident where we ordered ASTM spec'd 304ss SW fittings, and gave the supplier the ok to supply dual certified to make up the numbers. The storeman picking the materials however, threw in some 304L non dual certified into the mix - to make up the numbers. We ended up accepting the material but slightly derating the DP of the line to suit.

RE: dual rated 304/304L

Material manufacturers indeed often work with dual certified stainless steels 304L/304 (also 316L/), which is allowed.
This means:
- a maximum carbon content of 0.03% --> limited by 304L, interesting for welding
- Tensile/yield test results --> limited by 304, interesting for calculations (thinner thk's --> material saving)

BUT:
- Indeed, lower carbon also leads to poor creep properties, (for 304L I would stay below 425°C).
- Could the lower carbon also have an influence on the chemical resistance for certain fluids (I don't know, some one else)???

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