Microstructural degradation of boiler tube steels under long term exposure to high temperature

Thermal microstructural degradation was investigated by isothermal annealing of samples from boiler tube steels St 35.8, 15 Mo 3, 13 CrMo 44, 10 CrMo 9 10 and X20 CrMoV 12 1 in the temperature range 600 - 780°C for up to 2 000 h. Optical and scanning electron microscopy and hardness testing were used for characterising the microstructural changes and their time-temperature dependence. The results suggest a simple and consistent time-temperature dependence for all investigated materials, expected to apply also to long term service beyond the annealing time range of the present work. This would allow assessment of the in-service thermal exposure from the observed microstructure. A collection of micrographs has been prepared for this purpose, to aid in classifying and evaluating the observed microstructural state in terms of isothermal exposure to high temperature