A boiler tube made of ASME SA-213 Grade T2 contains 0.15% C and 0.6% Cr. The tube failed in operation with a fishmouth fracture typical of overheating. The tube had been operating normally for three years at a steam temperature of 420°C before failure occurred. A metallographic examination at various locations revealed the following microstructures:
(a) At a distance of 5000 mm from the fracture: Ferritic/pearlitic micro- structure with lightly spheroidized pearlite (heat degraded)
(b) At the thinned lip of the fishmouth: 100% martensite/bainite
(c) At a location 200 mm away from the fracture: 50% martensite/bainite with 50% heavily heat-degraded ferrite/pearlite
Explain how the microstructures above had formed. Assume that the eutectoid diagram has the following coordinates:
(i) At C = 0%, A3 = 900°C
(ii) Eutectoid point is at T = 750°C and C = 0.7%
(iii) Maximum solubility of 0.02 wt% carbon in ferrite