Why 304L permitted in creep temp range in ASME Sec VIII Divn 1

[bksree]

Hi
In the 2013 version of ASME Sec VIII Divn 1, the max allowed temp (for which properties are available) for SS304L is 650C while that for SS 316L is only 475C (as in earlier versions). SS 316L contains Mo which gives it creep strength while SS 3104L does not have it. Then why is SS 304L permitted in creep temp. range (i.e. above 425C)
Can someone elaborate.

TIA

 

[metengr]

The molybdenum addition in 316 stainless steel is for increased pitting corrosion resistance and not elevated temperature creep strength. The creep strength is still mostly governed by carbon content.

 

[viper3982]

bksree,

As mentengr said, the reason of adding the moly is all about corrosion resistance.

But a clarification on why you'd see different values.
1.) read Section 2 Part D A-201

Also,

The key lies in the "L" part of the 304L/316L.

The regular (non-low-carbon) steel has a large change in the chemical structure when heating: When you heat it up past about 800F, you tend to get a phenomena called 'chrome carbide precipitation' where the carbon and chromium melt together and form Cr3C2. It precipitates and removes the chromium from grain boundaries, making it susceptible to inter-granular stress corrosion cracking.
To fix this problem, they added a requirement to keep carbon below .04% (thus the L) Low-carbon alloy prevents the precipitate from forming; however the issue of this is kind of what you were implying: yield-point run-out.

When you deal with 304vs316, you have different chromium allowances, this slightly different chromium allowance allows a very marginal band to play with before encountering chromium carbide precip.

Thus, you see a very slightly difference in the max temperatures...you can still see the drastic drop after the ~800F

r/