Fig. 16 100,000-h creep-rupture strength of various steels used in boiler tubes. TB12 steel has as much as five times the 100,000-h creep-rupture strength of conventional ferritic steels at 600 °C (1110 °F). This allows an increase in boiler tube operating temperature of 120 to 130 °C (215 More

Fig. 19 Oxygen pitting along the outside-diameter surface of boiler tubes from a fire-tube boiler. (a) Through-wall pitting due to oxygen pitting. (b) Oxygen pitting had penetrated approximately 80% of the boiler tube wall thickness on this sample. More

Fig. 9 (a) Short-term and (b) long-term overheating of boiler tubes. Long-term overheating usually is caused by creep as the microstructure of the material degrades at temperature over time. Grains do not deform, but voids develop at grain-boundary junctions and grow and coalesce over time More

Fig. 9 Microstructures of specimens from carbon steel boiler tubes subjected to prolonged overheating below Ac 1 . (a) Voids (black) in grain boundaries and spheroidization (light, globular), both of which are characteristic of tertiary creep. 250×. (b) Intergranular separation adjacent More

Fig. 10 Typical microstructures of 0.18% C steel boiler tubes that ruptured as a result of rapid overheating. (a) Elongated grains near tensile rupture resulting from rapid overheating below the recrystallization temperature. (b) Mixed structure near rupture resulting from rapid overheating More

Fig. 12 Plots of scale thickness versus temperature for two sizes of boiler tubes and two values of heat flux. (a) and (b) The effect of scale thickness on the temperature gradient across the scale. (c) and (d) The effect of scale thickness on the temperature of the metal at the outer surface More

Fig. 20 Hydrogen damage (dark area) in a carbon steel boiler tube. The tube cross section was macroetched with hot 50% hydrochloric acid. More

Fig. 15 Creep-rupture strengths of various boiler tube steels at 600 °C (1110 °F). Source: Ref 21 More

Fig. 37 Short-term and rapid overheating of a steel boiler tube (reheater, superheater, or similar—source unknown) resulted in a longitudinal “fish-mouth” rupture. The tube had experienced elevated temperatures (455 to >730 °C, or 850 to >1350 °F) where the metal strength is markedly More

Fig. 24 Cross section through a studded carbon steel boiler tube, showing reduction in dimensions of the studs that occurs in operation. Note the loss of wall thickness in the tube around the entire fireside circumference, including the crotch of the tube near the membranes. More

Fig. 44 Corrosion-fatigue cracks in carbon-steel boiler tube originated at corrosion pits. Corrosion products are present along the entire length of the cracks. 250× More

Fig. 14 Most of the damage in a boiler tube is related to loss of wall thickness due to corrosion. Creep damage occurs late in life due to stress increase. More

Fig. 18 Metallographic mount of failed steel boiler tube sample exhibiting corrosion fatigue. Source: Ref 53 More

Fig. 2 Classification of boiler-tube damage mechanisms More

Fig. 8 Microstructure of a carbon steel boiler tube subjected to prolonged overheating below Ac 1 showing (a) decomposition of pearlite into ferrite and spheroidal carbides (original magnification: 400×) and (b) spheroidization of carbide and grain-boundary voids characteristic of tertiary More

Fig. 11 Photograph of thin-lip rupture in a boiler tube caused by rapid overheating More

Fig. 12 Typical microstructures of carbon steel boiler tube that ruptured as a result of rapid overheating. (a) Elongated grains near rupture resulting from rapid overheating below the recrystallization temperature. (b) Mixed structure near rupture resulting from rapid overheating between Ac 1 More

Fig. 69 Photograph of failed boiler tube showing polishing of the outer surface More

Fig. 73 (a) Close-up view of a boiler-tube fracture surface indicating initiation of fatigue cracks from multiple locations on the right-side portion of the tube. Left side is the fast-fracture zone, indicative of failure under overload condition. (b) Ratchet marks indicative of crack More

Fig. 7 Failure wheel for boiler tube damage mechanisms. Underlined mechanisms are always secondary in this system. More