Equilibrium and non Equilibrium study of the Fine Pearlite Formation in two Cast High Speed Steels after solidification and thermal treatment

Two High Speed Steels (HSS) have been studied both having chemical compositions
closed one to another. Both HSS grades exhibited similar matrix/carbides microstructure in the
industrial as-cast conditions, except for M7C3 eutectic carbides only observed in one grade and fine
pearlite only found in the other grade.
Equilibrium diagrams were obtained from Thermodynamical simulations to enhanced predicted
phase transformations whereas Differential Thermal Analysis was performed on both alloys in order
to determine their crystallization behaviour in non equilibrium conditions.
Light microscopy and Electron microscopy associated with microprobe analyses were done to
characterize the microstructure of both alloys.
Thermal treatments with related bulk hardness assessments were carried out to enhance conditions
for pearlite formation at the end of the casting route.
In continuous cooling conditions obtained from DTA simulations, the matrix of both as-cast HSS
grades is composed of eutectic carbides, martensite and retained austenite, these phases exhibiting
similar chemical compositions. The introduction of an intermediate isothermal stage in the solid
state within the DTA cooling sequence leads to the formation of a fine pearlite phase mainly located
in the vicinity of M2C eutectic carbides on one HSS grade. The origin of this fine pearlite is
discussed in this paper.