The ranges of complete and limited metastability of carbide M7C3 in Fe − C − Cr system
Keywords:
non-equilibrium four-phases reaction, metastable diagram Fe − C − Cr, stable carbide M23C6, metastable carbide M7C3, nranges of complete metastability, ranges of limited metastabilityAbstract
Formulation of the problem. The big importance for such practical questions of metal science as working out new materials with metastable phases, their conduct under heat treatment and exploitation , has prognos of ranges of their possible transformation in the stable condition, as it allows considerably reduce the time of search. Since system Fe − C − Cr is the base for essential part of white wear-resistaunt cast irons it was selected for analysis. To determine ranges of possible transformation of metastable carbide M7C3 into stable M23C6 is necessary clearly define with the metastable version of the triple diagram. Objective. The basis of selection one of the versions Fe − C − Cr diagram as a metastable for comparison it with the stable one and determination the ranges of transformation of metastable carbide M7C3 in stable M23C6. Main part. The big amount versions of triple diagram Fe − C − Cr can be separate for stables and metastables. Cast alloys structure at a first look tended to agree with metastable version, and homogenized agreed with the stable one. The debatable for the selection of metastable version is the question about presence and exteut of range L + M23C6. In conseqienceof presence such a range is reaction L + M23C6 → α + M7C3. However in fact four-phases reaction with both this carbides in going in another direction. The presence of non-equilibrium reaction L + M7C3 → α + M23C6 allows to determine real especialities of some high-chromium alloys microstructure and select as a metastable the version in which two-phases range L + M23C6 is absence. Than the comparison of stable phase diagram with the selected metastable version allows to determine the ranges of possible transformations of metastable carbide in stable carbide M23C6. Conclusions. Scheme of forming non-equilibrium four-phases reaction is shown and it explains the especialities of real cast structure. It is allowed select metastable version of Fe − C − Cr diagram and determine the ranges of complete and limited metastability of carbide M7C3 according to carbide M23C6.
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