Please see this question in detail.
How to characterize creep voids in ferritic steels .?
There are mainly two types of concepts employed in evaluating creep .
1)Microstructural phase evolution
2)Microvoids formation at grain boundaries (Neubauer classification)
Two images indicating microstructres of creep evolution are shown here according to the above concepts employed for ferritic steels .
My doubt is that in the first concept there is no microvoids in microstuctural evolution for creep .However second classification is mainly based on voids formation.
Whether there is microvoids formation during creep exposure in ferritic steels ?
Or else whether the decomposition of pearlite and carbide precipitation is the way to characterize creep in ferritic steel.what could be the most detrimental stage of decomposition of pearlite and carbide precipitation where the part is affected and fails subsequently .
Please see the attached pics and files for your reference . Please guide me on this .
http://www.ommi.co.uk/etd/eccc/advancedcreep/V6i1x.pdf
It depends on the duration of stay under creep conditions. As you know, the creep life duration depends mainly on the temperature (but also on the load and some factors like oxidation, ...). For instance, for a low-alloyed ferritic steel (2.25Cr-1Mo-steel), micrographies taken after short duration creep (rupture in a few hours at 883 K, stress : 70 MPa) will show only microstructural evolution (no microvoids). While the same steel will show some microvoids after stay of over 150,000 hours at 813 K and stress : 30 MPa (e.g. service aged). These sparse microvoids would have a magnitude of the order of 1 µm, so to see them one needs a scanning electron microscope. With time going, the number of microvoids increases, they build up chains, and at a later stage micro- and macro-cracks are formed as shown in the Neubauer scale. Normally, to get microvoids, the creep temperature should be higher than 0.4 Tm (Tm : melting temperature of the steel, in K).
guibert crevecoeur , Thanks for your answer
I have attached two pictures showing creep evolution morphologies , one type of evolution does not show voids at all what does it mean , if it is simply microstructural evolution whether the final stage of evolution of material is better than the 3 rd stage of second theory. Please guide..
When one is in conditions where there is no chance that microvoids will be formed, it is usually not possible to have an idea of the creep stage just on basis of a few taken micrographies. This is because the microstructural evolution is mainly the effect of temperature and not strictly connected to the load (see 1.1.1 of the document joined to the question). Also, the observed microstructure can result of heat treatment before starting with creep. So, if you observe Level F, this could mean that you are in an advanced stage of creep, but it could also mean that the microstructure is just the result of stay at temperature and the creep is actually not so far. Also, the steel might only have evolved a little bit because its microstructure was already e.g. Level D before creep start because of preliminary heat treatment (see 1.1.1 of your joined document). It depends also on the (low-alloy) steel.
One way out is experience : for the steel you are concerned with, you take micrographies at start of creep and then periodically up to rupture. This allows you to build up a comparison databank which can be further improved with the feedback from experience. But even then full reliability of interpretation on the creep stage is not guaranteed. It is not as simple as when the conditions are such that microvoids are detected.
You could also improve your databank by adding results of analysis of the carbides, at the beginning and then periodically, as inferred from 1.1.3 (p. 14) of the document joined to the question : "Carbide evolution : separation and coarsening of carbides is in general an index of material degradation due to creep exposure." The type of carbides should then also be recorded (see main carbide classes in pp. 14 and 15 in the document : M3C, M7C3, M23C6, M6C).
Another example of recording information on carbides is given in 1.1.1.1 (p. 4) of the document joined to the question : establishing a coarsening coefficient of carbides in CrMoV-steel, here VC carbide (CEGB study).
In summary, assessing the creep stage reached on basis of microstructure is not straightforward.
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