Abstract

Graphite containing refractories are highly resistant to mechanical and thermal cycles. The mechanical parameter that determines the resistance of the materials to crack development under subcritical conditions is the work of fracture. This parameter depends on the microstructure of the materials and, particularly, on the nature of the matrix. As-fabricated, unfired MgO–C and MgO-C-antioxidant bricks have very low porosity (<5 %), determined by the characteristics of the raw materials and the type of binder. The loss of volatile components of the binders, the inherent instability of the bricks caused by the carbothermal reduction of MgO that yields CO and Mg(g), and the oxidation of residual carbon from binders and of graphite flakes lead to increases in porosity during thermal treatments. Moreover, new phases that depend on the atmosphere could be formed at high temperature due to reactions between the components of the brick. Therefore, the nature of the matrix of the graphite containing refractories experiences significant changes during use. These changes will determine the mechanical performance of the materials. In this work, non-commercial MgO-C and MgO-C-antioxidant model materials have been used to determine the relationships between the microstructure and the work of fracture. Main composition was 83-85 wt% of magnesia (fused/sinter: 70/30) and 12 wt.% of graphite flakes. The effect of three different organic binders, phenolic resin, a new eco-binder and a chemically modified pitch has been evaluated.  Fully microstructural evaluation of the “as received” and thermally treated (up to 1450ºC) materials has been done. The work of fracture has been determined using controlled fracture tests. Relationships between the microstructure and the obtained work of fracture values are presented and the effect of the different binders and the antioxidant is established.