Types of Boiler Tubes Grades

Various Grades of Boiler Tubes-

CARBON STEEL

Carbon steels have moderate corrosion resistance and a fair strength up to 1000 degrees Fahrenheit. Furthermore, its use above 800F must take into account its graphitization susceptibility. In the thicknesses observed in the boiler tubes, graphitization was not a severe issue. However, thick section pipe should not be used above 800 degrees Fahrenheit. The ASME Boiler and Pressure Vessel Code, "Section I, Power Boilers," limits the use of seamless and welded carbon steel pipe in boilers to a maximum temperature of 800F for pipes and 1000F for steel calmed. For carbon steels, the Code does not specify maximum allowed stresses beyond 1000F. Steels containing carbon and molybdenum.

CARBON-MOLYBDENUM STEELS

Molybdenum carbon steels are utilized in high-temperature boilers because they have a stronger creep resistance than plain carbon steels. These steels have a molybdenum content of 0.5 percent. Carbon-molybdenum steels are susceptible to graphitization when exposed to temperatures above 850900F for extended periods. Again, the phenomenon is dependent on the section size, and using this grade of pipe above 850F is not suggested. The carbide phase will revert to graphite if it is not stable. Allowable stresses for carbon-moly steels up to 1000F are listed in Section I of the ASME Boiler and Pressure Vessel Code.

 INTERMEDIATE CHROME ALLOYS

Graphitization resistance and creep strength are higher in this low alloy steel than in carbon-moly steels. The corrosion resistance of this alloy is comparable to that of carbon-moly. The ASME Boiler Code lists acceptable strains for T2 up to 1000F.

All of the Croloys' chromium stabilizes the carbon as chromium carbides, making them resistant to graphitization.

Because of the possibility of higher temperature scale exfoliation, this 2-1/4 chromium, 1 molybdenum alloy is only suitable for applications up to 1125F. For temperatures up to 1200 degrees Fahrenheit, it is listed in the ASME Boiler Code.

T9 is a 9chrome1 molybdenum alloy that has good corrosion resistance and high thermal resistance. It also has strong oxidation resistance and can withstand temperatures up to 1200 degrees Fahrenheit. T9 can sometimes be used in place of more expensive stainless steel grades. T9 is limited to 1200F per the Boiler Code.

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