Material Reference
Alloy Steel Flanges & Fittings
When the service temperature climbs past where carbon can hold its strength, or the hydrogen partial pressure makes carbon a liability, chrome-moly alloy is the answer. We supply the F11, F22, and F91 grades that cover refinery, power, and petrochemical high-temperature service, plus the A350 LF forgings that handle the cold end of the spectrum.
Chrome-Moly Grades
Chrome-moly steels are low alloy steels with chromium for oxidation and corrosion resistance and molybdenum for elevated temperature creep strength. The standard chrome-moly grades for forged flanges all live under ASTM A182.
A182 F11 - 1.25Cr / 0.5Mo
The entry point of the chrome-moly family. F11 is used where service temperature pushes carbon past its comfortable rating but does not justify the cost of F22 or F91. Typical applications include moderate-temperature steam piping, hydrotreater service, and the lower- temperature crude unit lines in refineries. Common in Class 2 hardness and Class 3 normalized-and-tempered conditions.
A182 F22 - 2.25Cr / 1Mo
F22 is the high-temperature workhorse. With more chromium and molybdenum than F11, it holds its strength well above 900F and is the standard for hydroprocessing service in refineries, where elevated temperature meets high hydrogen partial pressure. F22 in V or W modified form (with added vanadium) shows up in heavy-wall hydrocracker piping where Nelson chart limits matter.
A182 F91 - 9Cr / 1Mo Modified
F91 is the modern high-temperature alloy for supercritical and ultra-supercritical steam service. The grade pushes allowable stresses at temperature well above F22, which lets designers use thinner walls and lighter flanges in power plant main steam and hot reheat piping. F91 demands careful welding and heat treatment - get the post weld heat treatment wrong and the toughness suffers badly.
Low Temperature Alloy - The A350 LF Grades
The other end of the alloy spectrum is low temperature service. A350 LF2 is sometimes grouped with carbon and sometimes with alloy, depending on how the project organizes its material classes. LF2 is a carbon steel forging with impact testing at -50F, which makes it the standard for cold-climate gas processing.
A350 LF3 carries roughly 3.5 percent nickel and is qualified to -150F, which puts it in the deep cryogenic carbon range used heavily on LNG and ethylene service. Below LF3 the next steps are 9 percent nickel plate or austenitic stainless.
For the full breakdown on LF grades and the related cryogenic family, see the cold-service section of our carbon steel page.
Where Alloy Steel Belongs
Chrome-moly alloy steels show up in three core sectors. In refineries, they cover hydroprocessing, catalytic reformer, and crude vacuum tower service where temperature and hydrogen push carbon past its limits. In power generation, they dominate the main steam, hot reheat, and superheater piping of fossil fuel and nuclear plants. In petrochemical service, they are common on cracking furnaces, ethylene production, and high-temperature process headers.
Each of those duties demands the right grade, the right heat treatment condition, and matched welding procedures. We can quote the flange and document the heat number; the welding procedure is on you and your fabricator.
Why Alloy Over Carbon
Three reasons drive the alloy decision. First, creep resistance - once metal temperature climbs past 800F the allowable stress on carbon drops sharply, while chrome-moly grades hold their numbers much further up the curve. Second, hydrogen service - the Nelson chart sets carbon limits low relative to F11, F22, and the V-modified grades, so high hydrogen partial pressure pushes the spec up the alloy ladder. Third, oxidation resistance at temperature, where chromium oxide scale outperforms iron oxide.
If your line list shows design temperature above 800F, design pressure above Class 600, or any meaningful hydrogen partial pressure, alloy steel is usually the right starting point.
Chrome-Moly Quoted The Same Day
F11, F22, F91, in standard B16.5 or large-bore configurations. Send the spec and we will source it.