“High temperature mechanical behavior of MgO-C refractories. Stress-strain curves”

G.A. ROHR,; A.G. TOMBA M.; A.L. CAVALIERI; L.F. MARTORELLO; P.G. GALLIANO

Dresden, Alemania

Congreso; 10th Biennial Worldwide Congress on Refractories UNITECR’07; 2007

The German Refractories Association (GRA) y Fédération Européenne des Fabricants of Produits Réfractaries (PRE)

A deep understanding of the high temperature mechanical behavior of MgO-C refractories is required in order to optimize their performance in the steelmaking vessels. A suitable tool for that purpose is the determination of stress-strain curves at in-service temperatures and under controlled atmosphere. The information that is provided by these laboratory tests is essential to understand the refractory materials behavior during the vessel campaign. Furthermore, the obtained thermomechanical data of the MgO-C refractories is need for any type of stress calculations of both the refractory lining and vessel shell. In this work, three types of commercial bricks of MgO-C that are used in the steel industry (A1, A2 and B) were evaluated. Characterization of these materials included crystalline phase determination by X-ray diffraction (XRD), pycnometric and apparent density measurements (DIN IN 993-1), open, closed and true porosity determination (DIN IN 993-1), differential thermal analysis (DTA) and thermogravimetric analysis (TGA), cold crushing strength (CCS, DIN IN 993-5 and ASTM C133) and microstructural analysis by optical and scanning electron microscopy (SEM). Results showed that all the studied materials contain MgO as periclase and C in graphite form as their main crystalline phases, in addition to small percentage of Si, Al and Mg. A1 and A2 are pitch-bonded materials with a higher percentage of sinter magnesia, whereas B is a resin-bonded brick and contains a greater amount of electrofused MgO. The average magnesia aggregate diameter in A1 and A2 is greater (2.13 and 1.73 mm, respectively) than in B (1.35 mm). C-content was found to be between 12 and 13%. Weight losses of 2-5 % at 500°C, and 8-10 % at 900°C were determined, being the resin bonded material the one that showed lower weight losses at low temperatures. Density values were around 3 g/cm3 in all materials, and apparent porosity range was found between 3.5 and 4.6 %. B samples showed a higher cold crushing strength value (29±2 MPa) than those from A1 and A2 (21±2 and 15±2 MPa, respectively). The stress-strain curves were obtained through compression tests on cylindrical specimens (diameter: 27 mm; height: 40 mm) obtained by cutting and machining of bricks. Results were obtained at room temperature, 600°C and 1000°C under atmosphere control (flowing N2 gas). An Instron 8501 servo-hydraulic machine was used with a capacitive extensometer, suitable for axial strain measurements at high temperatures (±0.6mm). A constant displacement rate of 0.1 mm/min was used up to the specimen failure. From these stress-strain curves, the following mechanical parameters were obtained: elastic limit, failure strain, failure stress (mechanical strength, sR) and tangent Young´s modulus (Et). All the materials exhibited quasi-fragile behavior in the testing conditions. Nevertheless, B showed a more fragile behavior with higher elastic modulus (Et: 10-70 GPa) and higher mechanical resistance (sR: 20-60 MPa). Materials A1 and A2 showed a similar behavior, being their Et between 3-15 GPa and their sR in the range of 4-20 MPa. Microstructural observation of the specimens after failure showed that the fracture occurred mainly through the bonding phase.  Results also showed a significant degradation of the mechanical properties at 600°C, which was mainly attributed to the thermal transformations that occur in the carbonous matrix of these materials.