technique for assessing service temperatures of carbon and low alloy steel plant
components is to note the degree of spheroidisation of iron and other carbides.
This technique is especially useful when there is doubt about the reliability of
is well known that the amount of spheroidisation is dependent upon both time and
temperature. Toft and Marsden demonstrated that there are basically six stages
of spheroidisation and using a Sherby Dorn type parametric equation, showed that
it was possible to get a reasonable correlation with a "weighted average"
Fortunately, because the creep
rupture life correlates reasonably well with this weighted average, the method
is valuable in monitoring the remanent life of plants.
In-situ surveys can be done using a
surface replica technique.
When assessing steels, the effects
of stress relief which induces some spheroidisation have to be considered.
Furthermore, in the case of low alloy steels containing significant amounts of
chromium and molybdenum, the as-received microstructures can contain significant
amount of massive carbides and bainite which will influence spheroidisation