Evaluating the thermal strength of refractory materials on the basis of the failure temperature gradient

Abstract

The failure of parts of refractory materials used in the slide valves of steel teeming ladles is a complex physicochemical process occurring in the contact of the refractory with the metal being teemed.For several years intense searches have been made for criteria of strength most fully corresponding to the failure mechanism of a refractory, easily reproducible under laboratory conditions, and not complex to determine by calculation [1][2][3]o As a criterion of the strength of refractories this article proposes the use of the failure gradient of the temperature field, which in this case is considered as a characteristic of the material.The basis of this selection of a criterion is the following physical concepts: uniform heating of a homogeneous and isotropic solid, i.e., heating in which the gradient of the temperature field at any point of the solid is equal to zero and which with the absence of limitation of thermal deformations may not cause failures; the vector field of the temperature gradient characterizing the nonuniformity of temperature distribution in the volume of the solid clearly corresponds to the temperature field; + under the action, on a homogeneous isotropic solid, of the mass forces F(x, y, z) and the temperature field T(x, y, z), the thermal stresses satisfy the vector equation of § equilibrium in the displacements W(x, y, z): in contact with a metal, the maximum temperature gradients develop on the surface of contact and the greatest values of ~T/$n (n is the normal to the surface) occur in the first minutes after contact. Under these conditions and because of the comparatively low thermal conductivity of a refractory, the temperature boundary conditions on the other (other than the contact) surfaces of the solid are not significant and the solid may be c~nsidered as semibounded;an analysis of experience in the use of the refractory plates of slide valves in steel plants shows that a frequent reason for the failure of these parts is failure of the contact surfaces caused by the development of a multitude of surface cracks and "loosening" of the contact surface of the refractory, which opens the way for interaction of the refractory with the metal, not only on the surface, but in a surface layer with a thickness of several millimeters.All-Union Institute of Refractories.

Thermal action of molten steel on the surface layer of refractories

“…The absolute value of the extreme values may exceed by several times the failure gradient, which for the refractories considered is 25-50~ [4].…”

“…It has been shown that the occurrence of a high-gradient temperature field may lead to failure of a refractory material and the amount of the failure gradient may be considered as a criterion of its heat resistance [4]. In determination of the heat exchange with refractories it has been shown that even with significant preheating the temperature gradient on the contact surface is 90-110~ [5].…”

“…As noted earlier [4], for an evaluation of the heat resistance of the refractory material of the plates it is necessary to evaluate the character of thermal action of molten steel on their surface layer.…”