Thermal-shock resistance and nature of crack resistance of alumina material with a layer-granular structure

Abstract

The thermal-shock resistance of refractories, as a rule, determines their working capacity in various high temperature equipment.It is especially important to increase the thermal-shock resistance of refractories that operate in high speed and rapid cooling units when the level of the stress changes is relatively high. According to the usual criteria [i] the spalling resistance of refractories depends on the nature of the crack resistance -the critical coefficient of the stress rate KIe ~ the specific destruction workYF--and also on the kinetic features of the crack dissemination.The greater the difficulty of initiating and spreading the cracks in the structure of the material then the greater its spalling resistance.In this case, it is important to prevent unstable growth of cracks.Retardation of the spread of cracks can be achieved in layer-granular materials [2]. This occurs as a result of the change in the trajectory of the crack in relation to the direction of the action of the maximum stresses and the reduction thereby in the intensity of the stresses at the peak of the crack.We made a complex study of the crack resistance, the kinetics of destruction, and the thermal-shock resistance of aluminosilicate material with a layered-granular structure [2]. In the porous layers we used technical alumina whose particles have a spherolitic structure. The volume proportion of porous layers in the granules was 30%. For comparison we studied the characteristics of aluminosilicate material with a granular structure obtained by pressing a batch whose composition was controlled with regard to technical specifications ShchEM-.0742.025 TU, and by subsequently sintering the green articles at 1400~ This material is used as a refractory during the firing of ferrite articles.Thermal-shock resistance was assessed on specimens measuring 60 x 80 x 7 m~n according to the number of thermal cycles before destruction in high-speed heating and cooling conditions (heating to temperatures of 1400~ at a rate of 200 deg/min, and cooling in nitrogen at a rate of 150~The study of the cracking resistance and the destruction kinetics was done with the double torque method on flat specimens with a notch (Fig. I). The specimens measuring W = 59 mm, d = 8 mm, d n = 5.5 mm, ~0 = i0 mm were loaded with a distance between the external and internal supports W m = 15 mm, and with a deforming rate of 8.10 -7 m/sec and room temperature on the Instron-lll5 machine.The coefficient of loading intensity KI, MPa-m l/2, was calculated from the equation