About one stable and three metastable eutectic microconstituents in the Fe-W-C system

摘要

The Fe-W-C system has been the subject of numerous investigations and the findings have led to quite different interpretations in the iron-rich corner, justifying the need for new experimental studies. In the present study, some 12 compositions are prepared using both the powder metallurgy technique and the casting of melted alloys. DTA experiments, long time annealing supplemented by the phase separation technique and controlled solidification experiments are carried out. The micrographs of the corresponding specimens reveal significant sedimentation of WC carbides, an observation which must be carefully taken into account in interpreting the micrographs. The liquid/solid equilibria between 1423 and 1543 K are investigated and the corresponding partial isothermal sections of the Fe-W-C system are compared to the calculated phase equilibria based on thermochemical models. Four ternary eutectic microconstituents gamma/M_6C/ cementite, gamma/WC/cementite, gamma/M_6C/graphite and gamma/WC/graphite, are observed by experiments. The microstructures are characterised and the four eutectic temperatures are established both experimentally and by calculations. The experimental eutectic temperatures lie within a range of 10 K but well below the temperatures predicted by thermodynamic modelling and the calculated temperatures within a range of 20 K. This explains why metastable microconstituents can so readily be formed.