Sep 12, 2026Procurement Guides
Boiler Tube Grades and Standards: A Reference Guide
A practical guide to boiler tube grades and standards: carbon, carbon-molybdenum, Cr-Mo and austenitic stainless, with ASME temperature limits.
Quick answer: boiler tubes are grouped into carbon steel, carbon-molybdenum steel, low-alloy chromium-molybdenum steel and austenitic stainless steel, and the grade you choose is set mainly by steam temperature, oxidation resistance and creep strength. Carbon and carbon-molybdenum tubes are limited to roughly 800F to 1000F, chromium-molybdenum grades such as T11, T12 and T22 cover the middle range up to about 1200F, and austenitic grades such as 304H, 316H, 321H and 347H are used where higher temperature strength or better oxidation resistance is required. Always confirm the allowable stress for the exact grade and temperature in the applicable ASME Boiler and Pressure Vessel Code section.
Why Boiler Tube Grade Selection Matters
A boiler tube works under pressure, at temperature and in contact with combustion products and steam, and those three conditions drive the material choice. Carbon steel is economical and strong but loses strength as temperature rises and can suffer graphitization. Alloy additions of molybdenum, chromium and nickel push the usable temperature range higher and improve oxidation and corrosion resistance. Because allowable stress falls as temperature climbs, grade and design temperature must be considered together.
Carbon and Carbon-Molybdenum Boiler Tubes
Carbon steel has mild corrosion resistance and moderate strength and is normally considered suitable up to about 1000F. Above roughly 800F, graphitization susceptibility must be taken into account. Per ASME Boiler and Pressure Vessel Code, Section I (Power Boilers), seamless and welded carbon steel tubes are limited to 800F for boiling steel and 1000F for killed steel, with no allowable stress listed above 1000F.
Carbon-molybdenum steel, nominally about 0.5 percent molybdenum, offers higher creep strength than plain carbon steel and is widely used in high-temperature boilers. It is also prone to graphitization when held above about 850F to 900F for long periods. Section I lists allowable stresses for carbon-molybdenum steel up to 1000F.
Low-Alloy Chromium-Molybdenum Grades
Low-alloy Cr-Mo steels improve graphitization resistance and creep strength, because chromium stabilizes carbon as chromium carbide. T2 is a low-alloy grade with better graphitization resistance and creep strength than carbon-molybdenum steel and similar corrosion resistance, with allowable stresses listed to 1000F.
T11 and T12 are 1 percent chromium, 1/2 percent molybdenum grades. T12 is often used in place of T2 because it is stronger, and is limited to a maximum of 1200F per Section I. T22, a 2-1/4 percent chromium, 1 percent molybdenum grade, has very high creep properties but is commonly limited to about 1125F because of scaling risk, while allowable stresses are listed below 1200F.
T9, a 9 percent chromium, 1 percent molybdenum grade, combines good corrosion resistance, high-temperature strength and oxidation resistance up to 1200F. In some duties it can replace more expensive stainless grades. Boiler codes limit T9 to 1200F.
Temperature limits are not suggestions. The allowable stress for the exact grade at the exact design metal temperature is what makes a boiler tube safe, and that value comes from the applicable code table.
Austenitic Stainless Boiler Tubes
Austenitic stainless steels have two sets of allowable stresses in the ASME Boiler and Pressure Vessel Code because of their relatively low yield strength. The higher values are based on short-term tensile properties, exceed 62.5 percent of yield but not 90 percent, and permit small amounts of plastic deformation. They are typically used for superheater and reheater piping.
The 18 percent chromium, 8 percent nickel grade 304 has variants including 304L, 304LN, 304H and 304N, each offering excellent corrosion and oxidation resistance and high strength. In the low-carbon grades, strength is maintained by controlling nitrogen. The higher-carbon 304H uses the lowest solution annealing temperature to secure good long-term high-temperature strength, and 304 grades are limited to 1650F under oxidizing conditions, with Section I allowable stresses listed up to 1500F.
T316 is similar to T304 but has better corrosion resistance and creep strength; molybdenum raises its resistance to pitting and crevice corrosion, with variants such as 316L, 316LN, 316H and 316N. T321 and T347 are T304 variants with comparable minimum tensile properties, stabilized by titanium and niobium plus heat treatment. For long-term elevated-temperature strength, T321H and T347H, like 304H, use higher carbon and specified minimum solution annealing temperatures.
Among stainless steels, T309 and T310 offer the greatest oxidation and corrosion resistance with good high-temperature properties, though their ferrite content makes them more prone to sigma phase in long-term high-temperature service.
How SPAI STEEL Supports Boiler and Alloy Tube Projects
SPAI STEEL, operated by Hunan Spai Pipe Industry Co., Ltd under SP Group, has served buyers since 1996 and exports to more than 100 countries. It offers carbon steel, stainless steel and alloy pipe and tube to GB, API, ASTM, ASME/ANSI, BS, DIN, JIS and GOST standards, with carbon steel production in Tianjin Jinghai and stainless and alloy production in Wenzhou Longwan.
FAQ
What is the most common boiler tube material?
Carbon steel is the most common starting point for lower-temperature boiler service, while carbon-molybdenum and low-alloy Cr-Mo grades such as T11, T12 and T22 are common as steam temperature rises. Austenitic grades such as 304H, 316H, 321H and 347H are specified for higher-temperature or more corrosive duty.
At what temperature does graphitization become a concern?
Graphitization becomes a concern above roughly 800F in carbon steel and above roughly 850F to 900F in carbon-molybdenum steel, depending on section size, which is why the ASME code limits those materials accordingly.
Why do austenitic stainless steels have two allowable stress values?
Because austenitic grades have relatively low yield strength, the code provides a higher set of allowable stresses based on short-term tensile properties. Those higher values are typically used for superheater and reheater piping, where limited plastic deformation is acceptable.
Can alloy boiler tubes replace stainless tubes?
In some duties, such as T9 service up to 1200F, an alloy grade can replace a more expensive stainless grade. The decision depends on temperature, oxidation and corrosion conditions, and on the allowable stress listed for that grade.
Send your grade, size, design temperature and applicable code to our engineering team for a confirmed boiler tube and alloy tube recommendation.
