MECHANICAL AND THERMAL CHARACTERIZATION OF MgO-C BRICKS AND THERMOMECHANICAL STRESS RESISTANCE ANALYSIS

P.G: GALLIANO; E. BENAVIDEZ; S.E. GASS; E. BRANDALEZE; A.G. TOMBA MARTINEZ

Aachen

Congreso; 59th International Colloquium on Refractories 2016; 2016

The European Centre for Refractories (ECREF)

MgO-C bricks are widely used in the steel industry. Steelmaking requires chemical, thermal and mechanical extreme conditions producing refractory wear through several processes. Different types of degradation suffered by these materials are due to chemical corrosion, thermomechanical stresses, and abrasion. Mechanical and thermal properties of these materials are related to different processing issues, including the percentage, size and quality of their raw materials. Three different qualities of commercial MgO-C bricks (named A, B, and C) were studied in order to obtain information about their composition, microstructure, texture as well as thermal, mechanical and thermal shock properties. In this work, the following analyses were performed on as-received materials: (i) bulk density and apparent porosity, (ii) mineralogical composition, (iii) microstructural, (iv) ignition loss at 1000°C, and (v) thermal and thermogravimetric behaviour in air up to 1000°C. Moreover, the refractories were tested in the range of temperature where thermal shock damage is prone to occur ( 1200°C): (i) dilatometry up to 1200°C (in air) and 1500°C (in argon), (ii) static compressive stress-strain curves at room temperature and 1200°C (in argon), and (iii) impulse excitation tests at room temperature. From the analysis of as-received bricks, the presence of metallic antioxidants was identified in all of them. According to the phase counting performed on optical images of the microstructure, the matrix/aggregates ratio was determined. Besides, the sintered and the electrofused magnesia grains were distinguished. The contents of both the organic-binder and the graphite were corroborated by the loss on ignition and from thermogravimetric data.Mechanical and thermal properties were obtained from the mechanical and dilatometric tests. The static Young´s modulus (E), and the fracture strength (F) and strain (F) were extracted from the stress-strain curves. At room temperature, A and B bricks presented the higher values of fracture strain. This fact was associated to the higher content of graphite present in these materials. At 1200°C, the values of fracture strength of bricks A and C increased twice relative to those measured at room temperature. This was attributed to the presence of products from reactions where antioxidants were involved. The very low content of antioxidants in brick B, and the increased porosity at higher temperatures, would be responsible for the slightly decrease of F at 1200°C in this material.From the ultrasonic non-destructive test, the Poisson's ratio () was obtained. On the other hand, the thermal expansion coefficients () were taken from the dilatometric curves in argon. Then, two thermal shock parameters, the resistance to crack initiation (R) and the resistance to crack propagation (R´´´) were calculated from the experimental values of thermomechanical properties (E, F, , and ). Analysing the values of both parameters (R and R´´´), brick B (at room temperature) and brick A (at 1200°C) were the most suitable for applications where the cracks generated by thermal shock must be avoided. Furthermore, brick A (at 25°C) and brick B (at 1200°C) were the materials having the higher resistance to crack propagation. Thermal shock tests are currently in process to validate these results.