The selection of materials for heater tubes is based on the design temperature and pressure of the tubes and the corrosivity of the process. The economics associated with the materials can not be overlooked as well. Suitable materials are evaluated looking at the total installed cost, including availability of the material, fabrication and heat treatment requirements.
Carbon steel, Cr-Mo steels, and austenitic stainless steels are common tube metallurgies. Carbon steel is limited to the low temperature applications. Many companies choose to limit carbon steel to applications below 800°F (427°C) to prevent problems from spheroidization and graphitization. The addition of chromium and molybdenum improve high-temperature strength, resistance to spheroidization, and resistance to oxidation and some corrosion mechanisms. Austenitic stainless steels are often used for tube applications where temperatures exceed about 1300°F (704°C) or the corrosivity of the process requires its use.
The common tube materials, corresponds to ASTM tube or pipe specification, and the maximum design metal temperature limits are per API Std. 530. The design metal temperature is the upper limit of the reliability of the rupture strength. Tube wall calculations per API Std. 530 should be completed to determine tube life at these temperatures. Other factors such as hydrogen partial pressure and resistance to oxidation often result in lower temperature limits.
Boiler tubes are generally carbon steel, 1-1/4Cr-1/2Mo and 2-1/4Cr-1Mo steel. Typically, the governing criterion is the oxidation rate of the material being evaluated. Carbon steel is often used in the water-filled and steam-generating tubes where the metal temperature is below 800°F (427°C). Tubes used in the steam superheat section can be higher alloys for improved strength and resistance to external oxidation. Again, the selection depends on the metal temperature and operating stress of the tube.