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Isn't that what H sub-grades or variants of SS primarily used for - elevated temps? please let me elaborate further...
Within the usual designations of the common austenitic grades of stainless steel, such as 304 and 316, there are "sub-grades" - "L" and "H" variants - with particular applications...
The low carbon "L" grades are useful where welding or other high temperature exposure will occur, particularly welding of medium or heavy sections... The low carbon is one way of delaying or preventing grain boundary chromium carbide precipitation (often referred to as sensitization) which can result in intergranular corrosion in many corrosive service environments... As shown in the time temperature- sensitization curves below, the precipitation of carbides occurs over time at temperatures in the range of about 450-850°C... The time for damaging precipitation to occur is highly dependant upon the amount of carbon present in the steel, so low carbon content increases resistance to this problem... Because of their application area the "L" grades are most readily available in plate and pipe, but often also in round bar... In the absence of heavy section welding, or of high temperature exposure, the corrosion resistances of the standard and "L" grades are usually identical...
Composition limits for 304 and 304L are identical in all respects except for carbon content (304L does permit up to 12.0%Ni, compared to 10.5% max for 304 - but given the cost of nickel it is usual for both grades to have close to the minimum of 8.0%, so there is no practical difference)... Neither 304 nor 304L has a minimum carbon content specified... A carbon content of 0.02% therefore fully complies with both 304 and 304L specifications... The Tensile and Yield strengths for 304 are slightly higher than that of 304L...
"H" grades are the higher carbon versions of each of the standard grades... The high carbon results in increased strength of the steel, particularly at elevated temperatures (generally above about 500°C)... Both short term tensile strengths and long term "creep" strengths are higher for these high carbon grades... "H" grades are produced primarily in plate and pipe, but may be available in some other products... Applicable grades are most commonly 304H and 316H, but high carbon versions of 309, 310, 321 and 347 are also specified in ASTM A240/A240M... The specialist high temperature grade 253MA (S30815) has no low or standard carbon version at all. As discussed above, these high carbon content grades are susceptible to sensitization if held in the temperature range of about 450-850°C... If it occurs this will result in impaired aqueous corrosion resistance... In general however, this is not an issue for a steel that is primarily intended for high temperature strength... The grades that are “stabilized” by addition of titanium (321 or 316Ti) or niobium (347) do not suffer from sensitization even after exposure at 450 – 850°C because the Ti or Nb combines preferentially with the carbon... Even the high carbon versions (321H) resist sensitization...
The high carbon version of 304 is 304H... This has the same composition specification as standard 304 except that 304H does not have the 0.10% nitrogen maximum limit which applies to both standard and "L" grades and there is a slightly different chromium content range... In addition all austenitic "H" grades must have a grain size of ASTM No 7 or coarser...
The three grades 316, 316L and 316H are exact counterparts to the 304 series... Again only the carbon contents differentiate these grades (and the nitrogen and grain size limits mentioned above)... Compositions of the alternatives are therefore as in the following table (from ASTM A240/A240M-07; for full compositions refer to the standard)... Specifications for some other products, particularly tube and pipe, have a carbon limit of 0.035% or 0.040% maximum for 304L and 316L, but are otherwise the same as for flat rolled products...
In practice, steel mills generally ensure that the "L" grade heats meet the strength requirements of the standard grades, ie. 304L and 316L will almost always have yield / tensile properties above 205 / 515MPa, so will meet both standard and "L" grade requirements...
A note about Dual Certification: It is common practice for certain products including plate, pipe and some bar to be stocked as “dual certified”... Such product is certified by the manufacturer as fully compliant with both 304 and 304L or 316 and 316L... Thus it has the resistance to sensitization expected of an “L” grade plus the higher strength of a standard grade.... Dual certified products are generally precluded from use at high temperatures (over about 500°C) because of their low carbon content, the same as other “L” products... There is also a dual certified 321 / 321H, but there is no “L” version of 321...
There are many differences between the two variants, but in order to avoid posting up another of my famous walls of text, I'm going to stop here and see if anyone else has something to add here.
![grin :grin:](/f2/epx.png)
![lol :lol:](/f2/epx.png)
![cool :cool:](/f2/epx.png)
Respectfully,
Henry