Recovery of High-Temperature Creep-Resistant Substructure in Rutile

Abstract

Journal of The Americanlining are exposed to temperatures and atmospheres sufficient to cause formation of a dense layer during the latter portion of the blow and during turndown for sampling and tapping. Either dissolution of a previously formed layer or of MgO condensed from the vapor phase into the unsaturated liquid slag would account for the apparent lack of a dense layer in the reaction zone of brick removed from the BOF. However, the point to bear in mind is the overall similarity in microstructural features of laboratory and service specimens.A review of all information on the relative wear rates of burned impregnated and pitch-bonded brick indicates that the most significant difference that would account for the higher wear rate of the pitch-bonded brick in high-wear areas is the very porous weak zone at the refractory-slag interface. This zone results from internal oxidation-reduction reactions and occurs independent of formation of a dense layer. If the dense layer forms. it bridges coarse particles; the strength of the interface depends on the degree to which the coarse particles are anchored in the carbon-bonded matrix.When the dense layer can form, its retention in service for burned and pitch-bonded brick is contingent on MgO saturation of the slag. In this respect, process parameters of the operation should avoid large volumes of low-lime/silica slags. VII. SummaryThe laboratory data and theoretical analysis of the present study show that internal oxidation-reduction reactions will occur in all types of carbon-bearing BOF refractories under steelmaking conditions. The rate of these reactions increases with increasing temperature and decreases with increasing pMg and/or pC0 at constant temperature. Above 2700°F in an oxidizing atmosphere, a dense MgO layer forms and slows the rate of internal reactions. Subsequent reaction appears to be limited by diffusion across the dense layer. This layer dissolves quickly in low-lime/silica slags undersaturated with Ceramic Society-Bell et al. Vol. 55, No. 1 MgO but is quite impervious to high-lime/silica slags easily saturated with MgO. Porosity and low-lime/silica material accumulate in the decarburized region behind the hot face independent of formation of a dense layer. The alteration in the distribution of porosity and impurities must result in a decrease in high-temperature strength of the affected area adjacent the hot face. This phenomenon would account for the faster rate of wear of pitch-bonded brick in high-wear areas of the BOF. , "Carbon-ME0 Reactions in BOF Refractories," Amer. Ceram. SOC. Bull., 50 [7] B. Brezny and R. A. Landy, "Effects of Heat and Silicate

Creep recovery of rutile under reduced stress

“…Migration of subboundaries was observed illustrating the same gen eral features previously noted by Bell et al [6]. Optical micrographs.…”

“…A similar study of the recovery of creep resistant substructure In rutile single crystals _• cr^ -where cr^ = 10,000 psi, and cr^ = 500 psi) was made by Bell, Krishnamachari and Jones [6]. They obtained an activation energy of 135 kcal/mole for the recovery process.…”

“…(7) where is the activation energy for self diffusion. For pure metals at high temperatures, is nearly equal to the activation energy for self diffusion, AH^ [13]" For alloys and ionic materials aH^ in many cases is nearly equal to the activation energy for diffusion of one of the elements, usually the one exhibiting the lowest diffusivity [16].…”

“…] have also revealed creep re covery. Kennedy [30] concluded that the recovery process provides the nu clei for recrystal1izatlon» Bull [7] reported that the activation energy for recovery Is within 2 percent nf the v/aIua for grain boundary migra tion reported by Gifklns [18]. In dilute Al-Mg solid solution, however.…”

“….05- The dependence of the recovery parameter n on the recovery time t^ was shown by Bell et al [6] to follow the relation n = K tr^'^7 (23) at zero reduced stress, where K is a constant.…”