316/316L Dual Certifications

[mexkimo]

Can anyone tell how a 316L forging can have the higher yield and tensile strengths as 316. Do suppliers make three different metals now? Or is there a method to make the low carbon stainless stronger?

 

[ERT]

mexkimo,

The 300 series stainless steels readily work harden. Obviously, forging involves some cold-work so you should expect forged 316L to be much stronger than as annealed.

I don't have data handy on cold-worked 316L. But I know that 302 can increase UTS from 36,000 psi to 180,000 psi by cold-working.

So the condition is more important than the minor differences between 316 and 316L.

Regards,

ERT

 

[mexkimo]

ERT,

Thanks for your prompt response. You are more than likely correct about the conditioning of the forging. I was just wondering if this conditioning is typical for most low carbon stainless nowadays, and that even if one were to specify a 316L material, that the new manufacturing processes will pretty much give you the higher UTS values.

I am designing a piping system and I needed to list the design pressure for this system using a 316L flange. 316L flanges fall under the ASME B16.5 group 2.3 materials and have a lower pressure-temp rating than that of a 316 (group 2.2)forging.

I have been buying dual stamped flanges 316/316L for about three years now, and was interested in how this dual certification was achieved. With the dual certs we achieve the higher pressure-temp rating and at the same time can reduce the rusting out of welds on piping systems.

 

[moltenmetal]

Dual certs simply mean that the material meets both the low carbon content of the L grade and the tensile/yield strength of the plain grade. What this means is that the high strength/yield of a particular dual certified material sample is not simply due to excess carbon content, but is a result of better alloying/quality control in the manufacture of the stainless steel. These days it's tough to find material that isn't dual certified.

 

[EdStainless]

The strength also is impacted by factors not reflected in the composition. The first is the nitrogen content. A little bit goes a long way.
The other issue is how well annealed the material is. When you cold work material to high strength and then under-anneal it it sure looks strong.
The catch is that neither of these factors helps creap strength or stress rupture. They only increase the low temperature properties.

There is also a pattern with the older specifications. If you look at new specs for higher alloyed grades you will see that the actual properties are not much above the spec minimums. The older specs however show a large difference. Given old steel making methods there was much more variation in chemistry and in properties. When these grades are made with todays methods the results are consistantly higher properties.
One side result of this is to give the 'old' grades what amounts to higher saftey factors than the new grades.


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