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.
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.
BOILER TUBE GRADES
STAINLESS STEELS – Austenitic stainless steels
Austenitic stainless steels are presented in the ASME Boiler and Pressure Vessel Code with two sets of allowable stresses. The reason for this is their relatively low yield strength. The higher allowable stress values were determined at temperatures where the usage would be restricted by the short-time tensile properties.
The Boiler Code lists maximum allowable stresses for varying temperatures depending on the individual austenitic stainless grade.
Variations of this 18 chromium, 8 nickel grade include 304L, 304LN, 304H and 304N. Each of these offers excellent corrosion and oxidation resistance along with high strength.
High strengths are maintained in the low carbon grades by controlling the nitrogen content.
T304 has higher carbon and a minimum solution annealing temperature to assure good long-time elevated temperature strengths. T304 grades are limited to 1650F under oxidizing conditions. Section I of the ASME Boiler Code lists allowable stresses up to 1500F.
T316 is similar to T304, but offers better corrosion resistance and creep strength. The molybdenum addition to 316 increases its resistance to pitting and crevice corrosion.
T321 and 347 are variations of T304 and have comparable minimum tensile properties. These two grades are stabilized with additions of titanium and columbian respectively, along with proper heat treatment.
Of all the stainless steels, T309 (25 chromium, 13 nickel) and T310 (25 chromium, 20 nickel) offer the maximum resistance to oxidation and corrosion. They also offer good high-temperature properties. Since these steels contain ferrite, however, they are more susceptible to sigma phase.
Related Industry Knowledge
- Introduction to stainless Steel tubes
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- Classification of stainless steel tubes
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