[updated:LAST EDITED ON Apr-01-08 AT 02:04 PM (CDT)]WHAT "H" GRADES ARE & WHY THEY ARE USED "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%C2°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 sensitisation if held in the temperature range of about 450-850%C2°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 %E2%80%9Cstabilised%E2%80%9D by addition of titanium (eg 321 or 316Ti) or niobium (eg 347) do not suffer from sensitisation even after exposure at 450 %E2%80“ 850%C2°C because the Ti or Nb combines preferentially with the carbon. Even the high carbon versions (eg 321H) resist sensitisation ref http://www.atlasmetals.com.au/files/ASM_Tech_Notes/TN8-L%20H%20and%20Standard%20grades%20rev%20May%202007.pdf hope this help