Allowable Stresses at High Temperature

[bob330]

Guys,

Where in the ASME code would I find allowable stres values for common austenitic SS materials at higher temperatures like 900F-1500F. Does the code mandate lower allowable stress values for the "L" grades versus the regular grades and how does 316L compare to say 321SS or 347SS?

Bob

 

[metengr]

bob330;
ASME Boiler and Pressure Vessel Code, Section II, Part D.

Yes, the L grades are indeed limited in service temeprature due to the lower creep strength.

 

[jte]

Bob-

Depends on what you mean by "ASME code." You could look at Section III Div. 1 Part NH. If you are playing in the Section VIII world then as with any other material you'd be looking at Section II Part D for temperature limits and allowable stresses. For Section VIII Div. 1 SA-240 316L has a max temp limit of 850°F while SA-240 316 (and 321 and 347) has a max temp limit of 1500°F. But keep in mind that the compressive stress chart only goes up to 1200°F for 316, 321, and 347 material. So figuring out how to support the thing may be a significant challenge... Trying to decide what is "better" among the materials will involve much more than figuring out which has the highest allowable tensile stress.

jt

 

[bob330]

Thanks,

This does not make a lot of sense, I have several separate references that show 10,000 and 100,000 hour rupture data for 316, 316L and 321 SS and the 316L is equal to or superior in rupture strength to 321SS at all the higher temps 900F-1500F and only slightly iunfereior to regular 316 grade. What's up with the 850F limit for 316L, seems quite ridiculous.

bob

 

[jte]

Bob-

Seems that most 316 or 316L material is dual grade anyway, so in your case would it make sense to specify dual grade for your application to get the L grade advantages while maintaining the "straight" grade for Code purposes?

jt

 

[metengr]

Creep deformation in metals is not = creep rupture strength (apples to oranges comparison). Use of austenitic stainless L grades is more for corrosion resistance in comparison to creep deformation resistance in elevated temperature service. Design for elevated temperature service requires a review of creep deformation AND rupture stregnth (whichever is the limiting factor).