Introduction to boiler tube grades

Boiler tubes are critical components in high-temperature and high-pressure applications, particularly within power generation systems. These tubes must withstand extreme conditions, including high temperatures, pressure, and corrosive environments, while maintaining structural integrity and performance. To meet these demands, various grades of steel are utilized, each offering distinct properties tailored to specific operating conditions. The following sections outline the characteristics, advantages, and limitations of different boiler tube grades, including carbon steel, carbon-molybdenum steels, intermediate chrome alloys, and austenitic stainless steels. These materials are classified and regulated by standards such as the ASME Boiler and Pressure Vessel Code to ensure safety and reliability in their respective applications.

Carbon Steel (SA178)

Carbon steel tubes offer mild corrosion resistance and fair strength up to 1000°F. However, when used above 800°F, they may be susceptible to graphitization, though this issue is not prominent in the thicknesses commonly used in boiler tubing. For heavy section pipes, usage above 800°F is not recommended. According to the ASME Boiler and Pressure Vessel Code, Section I, Power Boilers, the use of seamless and welded carbon steel tubing in boilers is limited to a maximum of 800°F for rimmed steel and 1000°F for killed steel. Beyond 1000°F, the Code does not specify allowable stresses for carbon steels.

Carbon-Molybdenum Steels (SA209)

Carbon-molybdenum steels, typically containing 0.5% molybdenum, have higher creep strength compared to plain carbon steels, making them suitable for high-temperature boiler applications. However, these steels can also undergo graphitization if exposed to temperatures above 850-900°F for extended periods. This issue is dependent on section size, and it is advised not to use pipes of this grade above 850°F due to the instability of the carbide phase, which can revert to graphite. The ASME Boiler and Pressure Vessel Code, Section I, lists allowable stresses for carbon-molybdenum steels up to 1000°F.

Intermediate Chrome Alloys

SA213-T2
This low alloy steel provides resistance to graphitization and offers greater creep strength than carbon-moly steels. Its corrosion resistance is similar to carbon-moly steels, and the ASME Boiler Code lists allowable stresses for T2 up to 1000°F. The presence of chromium stabilizes the carbon as chromium carbides, preventing graphitization.

SA213-T12
This alloy, containing 1% chromium and 0.5% molybdenum, is limited to a maximum temperature of 1200°F by the ASME Boiler and Pressure Vessel Code, Section I, Allowable Stresses. T12 is sometimes preferred over T2 due to its superior strength.

SA213-T11
T11 has similar creep strength properties as T12 but offers better corrosion resistance and is more resistant to high-temperature oxidation due to higher silicon and chromium content. Oxidation resistance is crucial because metals exposed to high temperatures for extended periods develop a protective scale. However, at certain temperatures, this scale may become non-adherent, leading to exfoliation and potential solid particle erosion in turbines. The ASME Boiler and Pressure Vessel Code lists allowable stresses for T11 up to 1200°F.

SA213-T22
This alloy contains 2.25% chromium and 1% molybdenum, providing exceptionally high creep properties. However, its application is limited to 1125°F due to potential high-temperature scale exfoliation. The ASME Boiler Code lists T22 for temperatures up to 1200°F.

SA213-T9
Containing 9% chromium and 1% molybdenum, T9 offers excellent corrosion resistance, good high-temperature strength, and oxidation resistance, making it suitable for use up to 1200°F. T9 can be an adequate substitute for more expensive stainless grades, and the ASME Boiler Code limits its use to 1200°F.

Stainless Steels – Austenitic Stainless Steels

Austenitic stainless steels are covered in the ASME Boiler and Pressure Vessel Code with two sets of allowable stresses due to their relatively low yield strength. The higher allowable stress values are intended for temperatures where the usage is limited by short-time tensile properties. These stresses exceed 62.5% but do not exceed 90% of the yield strength, where small amounts of plastic deformation can be expected. These higher stress values are typically used for super-heater and reheater tubing. The Boiler Code lists maximum allowable stresses depending on the specific austenitic stainless steel grade.

SA213-T304
Variations of this 18% chromium, 8% nickel grade include 304L, 304LN, 304H, and 304N, all offering excellent corrosion and oxidation resistance along with high strength. The low carbon grades maintain high strength by controlling nitrogen content. T304 has higher carbon content and a minimum solution annealing temperature to ensure good long-term elevated temperature strength, with limitations up to 1650°F under oxidizing conditions. The ASME Boiler Code, Section I, lists allowable stresses for T304 up to 1500°F.

SA213-T316
Similar to T304, T316 includes molybdenum, enhancing its resistance to pitting and crevice corrosion, and providing better corrosion resistance and creep strength. Variations include 316L, 316LN, 316H, and 316N.

SA213-T321 and T347
T321 and T347 are stabilized variations of T304, using titanium and columbium respectively, along with proper heat treatment, to ensure good long-term strength at elevated temperatures. High-carbon versions like T321H and 347H, similar to 304H, were developed with higher carbon contents and specified minimum solution annealing temperatures. T309 (25% chromium, 13% nickel) and T310 (25% chromium, 20% nickel) provide the maximum resistance to oxidation and corrosion among stainless steels. However, these steels contain ferrite, making them more susceptible to sigma phase.