BOILER TUBE GRADES
|Carbon steels show mild corrosion resistance and fair strength up to 1000F. However, their usage above 800F must take into account the susceptibility to graphitization. Graphitization has not been a significant problem in thicknesses encountered in boiler tubing. Use of heavy section pipe above 800F, however, is not recommended. The application of seamless and welded carbon steel tubing in boilers is restricted to a maximum temperature of 800F for rimmed and 1000F for killed steel by the ASME Boiler and Pressure Vessel Code, “Section I, Power Boilers.” The Code does not list maximum allowable stresses beyond 1000F for carbon steels.Carbon-molybdenum steels|
|Carbon-moly steels exhibit higher creep strengths than plain carbon steels, and are widely used in high-temperature boiler service. These steels nominally contain 0.5% molybdenum. When exposed to temperatures above 850-900F for long periods of time, the carbon-moly steels are also prone to graphitization. Again, the phenomenon is section-size dependent, and using pipe of this grade above 850F is not recommended. The carbide phase is not stable, and will revert to graphite. The ASME Boiler and Pressure Vessel Code, “Section I,” lists allowable stresses for carbon-moly steels up to 1000F.|
INTERMEDIATE CHROME ALLOYS
This low alloy steel exhibits graphitization resistance and greater creep strength than the carbon-moly steels. Corrosion resistance is comparable to carbon-moly. T2 has allowable stresses listed up to 1000F in the ASME Boiler Code.
|The chromium in all of the Croloys stabilizes the carbon as chromium carbides, thus making them immune to graphitization.|
|This is a 1-chromium, 1/2-molybdenum alloy which is limited to a maximum temperature of 1200F by the ASME Boiler and Pressure Vessel Code, “Section I, Allowable Stresses.” T12 is sometimes used in place of T2 tubing because of its greater strength.|
|This grade has the same creep strength properties as T12. It is more corrosion resistant than the chromium-free steels, and is fairly resistant to high-temperature oxidation because of its higher silicon and chromium contents.|
|Oxidation resistance is important because metals exposed to elevated temperatures for extended periods of time will accumulate a protective coating of scale. At some minimum temperature, the scale will become non-adherent, gradually flake, and cause solid particle erosion of turbines. However, exfoliation rarely causes failures before creep or high temperature yielding. |
Allowable stresses are listed by the ASME Boiler and Pressure Vessel Code to 1200F.
|This 2-1/4 chromium, 1 molybdenum alloy has exceptionally high creep properties, but is limited for applications to 1125F because of possible higher temperature scale exfoliation. It is listed in the ASME Boiler Code for temperatures to 1200F.|
|A 9-chromium-1 molybdenum alloy, T9 offers very good corrosion resistance with good high-temperature strength. It also has good oxidation resistance and can be used to 1200F maximum. Some times T9 is an adequate substitute for the more expensive stainless grades. The Boiler Code limits T9 to 1200F.|