I am performing heat treatment on as-cast 0.85 wt% carbon Cr-Mo steel. The heat treatment is being done at 925 degree C for a soaking time of 8 hours. The aim is to achieve complete dissolution of carbides to obtain a fully pearlitic structure. However, there seems to undissolved carbides even after heat treatment. But I am confused if they are actually cementite network or proeutectoid ferrite or M7C3 carbides? Is there any chance of formation of carbides during the cooling stage?
If your steel is hypereutectoid then there won't be any chance of forming proeutectoid ferrite. If there is any continuous network along the prior austenite grain boundaries it must be proeutectoid cementite. And you must provide the information about the composition of the alloy and the 'M' in 'M7C3'. Check if the annealing temperature is above the dissolution temperature of that carbide. You can try with micro/nanohardness to get sure if it is proeutectoid cementite or not.
As correctly pointed out by Arka Mandal , at this composition proeutectoid cementite+pearlite will form rather than proeutectoid ferrite. Since you are, however, close to the eutectoid point, there is a small chance of local compositional inhomogeneity, such that certain regions are depleted in carbon, and form proeutectoid ferrite. This should not ordinarily happen with such a long soaking time though.
As for metallographic distinction of the phases, etching with picral should be adequate. Ferrite will be lightly coloured, typically appearing light brown/straw coloured. Cementite will remain white, while its interface with the matrix will be appear dark.
Distinction between cementite and other carbides would require compositional analysis or diffraction. Still, for this heat treatment condition, cementite would form a network along grain boundaries, while other carbides would be present as individual particles.
First of all if you deviate away from eutectoid point you end having hypoeuctectoid microstructure ( proeutectoid ferrite formation) and hypereuctectoid microstructure( proeutectoid cementite formation). Which means for a hypereutectoid composition, the driving force is readily available once the temperature is below the solvus temperature of austenite and cementite phase field enabling cementite to precipitate out at the grain boundaires. Untill the eutectoid temperature is reached the entire grain boundaires forms a 'network of cementite' which is undesirable from the mechanical viewpoint. Since the cementite is brittle in nature cracks initiate and propagates along the grain boundaires. Thus the failure mode is different for both the off-eutectoid compositions. Better way to quantify is to do XRD and meausre the volume fraction of 2 phases.
Thank you all for the valuable feedback. I found that austenitizing the steel at 960 degree celsius for a holding time of 8 hours does not lead to complete carbide dissolution, i.e. Cr7C3 (M7C3) carbides still exist in the pearlitic structure. Is there any reason as to why this happens? Is there a chance of carbide precipitation during furnace cooling from 960 degree celsius
Never furnace cool these steels unless you want to precipitate carbides (in some applications that is desired). 500-750C is the typical temperature range (would vary with you exact composition) that you should avoid.
Air cooling is much better to obtain pearlite.
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