Influence of High Temperature Heat Treatment on in situ Transformation of Mo-rich Eutectic Carbides in HSS and Semi-HSS Grades

Alloys of the complex system Fe-Si-C-Cr-C-X, where X is a strong carbide
forming-element are well known to exhibit interesting mechanical properties, including wear
and abrasion resistances. Such a tribological behavior is mainly due to the presence of
carbides especially those obtained during the solidification route and that are known as
primary or eutectic carbides. It may therefore be interesting to determine the relative stability
of primary carbides when considering thermal and thermomechanical treatments performed at
a temperature high enough to allow either the homogenization of the matrix or the occurrence
of a desired grain size. This thermal stage is often required to produce tailored microstructures
that can lead to improved mechanical properties. In this work a series of thermal treatments
performed on samples originated form casting foundry parts were done. Raw materials are
both HSS and semi-HSS grades used in application where wear resistance is needed. Thermo-
Calc® (TC) simulations and Differential Thermal Analysis (DTA) were performed to
determine the crystallization behavior and the subsequent solid state transformations of the
studied alloys respectively in equilibrium and in non equilibrium conditions. Light and
Scanning Electron Microscopies were done together with hardness measurements in order to
enhance metallurgical features of the heat treated samples. Image analysis yielded the
determination of carbides volume fractions. It appears from microstructural analyses and
carbides quantification that Mo-rich eutectic carbides undergo in situ phase transformations
during heat treatments. In fact Mo-rich M2C carbides transform themselves into MC, M6C
and M3C, through a so-called budding phenomenon. Such a phenomenon is the evidence of a
preferential migration of some atoms that escape from the parent M2C carbide to diffuse
further away from their initial site with increasing time and temperature. The stable or
metastable nature of eutectic carbides is also discussed from DTA and TC results, as M2C
carbides found in both as-conditions and DTA samples were not predicted by equilibrium
conditions.