Effect of microstructural modifications at high temperature on the work of fracture of magnesia-graphite refractories

S. GASS ; A.TOMBA MARTINEZ; P. GALLIANO; N. BELLANDI; C. BAUDÍN

Santiago

Congreso; 16th Biennial Worldwide Congress on Refractories UNITECR 17; 2017

ALAFAR

Graphite containingrefractories are highly resistant to mechanical and thermal cycles. Themechanical parameter that determines the resistance of the materials to crackdevelopment under subcritical conditions is the work of fracture. Thisparameterdepends on the microstructure of the materials and,particularly, onthe nature of the matrix. Asfabricated,unfired MgO-C and MgO-C-antioxidant brickshave very low porosity (< 5 %), determined by the characteristics of the rawmaterials and the type of binder. The loss of volatile components of the binders,the inherent instability of the bricks caused by the carbothermal reduction ofMgO that yields CO and Mg(g), and the oxidation of residual carbon from bindersand of graphite flakes lead to increases in porosity during thermal treatments.Moreover, new phases that depend on the atmosphere could be formed at hightemperature due to reactions between the components of the brick. Therefore,the nature of the matrix of the graphite containing refractories experiencessignificant changes during use. These changes will determine the mechanicalperformance of the materials. In this work, non-commercial MgOC andMgO-C-antioxidant model materials have been used to determine the relationshipsbetween the microstructure and the work of fracture. Main composition was 83-85wt.% of magnesia (fused/sinter: 70/30) and 12 wt.% of graphite flakes. Theeffect of two different organic binders, phenolic resin and a new eco-binder(chemically modified pitch) has been evaluated. Fully microstructural evaluationof the as received and thermally treated (up to 1400ºC) materials has beendone. The work of fracture has been determined using controlled fracture tests.Relationships between the microstructure and the obtained work of fracture valuesare presented and the effect of the different binders and the antioxidant is established.