The Effect of Mold Flux on Reheat Scale -- Austenitic Stainless Steel Slabs

In the present study, the characteristics of the subscale were investigated with respect to alloy composition, oxidizing environment, and surface treatment. Samples were tested isothermally under simulated reheating conditions with regards to time, temperature and oxidizing gas composition. Findings suggest that water vapor is a key variable in dictating the scale growth mechanisms during reheating of type 304 stainless steel. The volatilization of CrO 2(OH)2 in the presence of water vapor is one factor which can prevent stainless steel from forming a continuous external eskolaite layer; mold flux can influence this behavior.;FactSage calculations were done to compare the maximum equilibrium solubilities of major oxide species in mold flux and the maximum equilibrium partial pressure of volatilized chromium oxide species in atmospheres with and without water vapor content. FactSage results also helped to model expected local equilibrium oxide formation as a function extent of oxidation. From that work, a mechanism was developed to explain deep subscale formation. A mechanism is proposed to explain how mold flux avoids deep subscale formation due to its low solubility of chromium oxide, promoting the formation of a continuous eskolaite oxide layer by preventing chromium loss by volatilization.;A series of Fe-xCr binary alloys with x ranging from 14 to 24 wt% were used to investigate the isolated effects of chromium content on oxidation behavior and scale morphology as a function of surface treatment and oxidizing environment. Type 304L stainless steel was also tested. These tests were specifically aimed at investigating the effect of chromium volatilization on continuous external eskolaite formation during high temperature oxidation and to elucidate the mechanism involved in how the presence of mold flux influences this behavior. The samples were oxidized either in a wet atmosphere (laboratory air and 18%H 2O content simulated reheating atmosphere) or dry N2-O 2 gas mixtures at 1250°C, for two hours. The post-oxidation microstructures were investigated by scanning electron microscopy (SEM) with backscattered electron (BSE) imaging and the energy dispersive spectroscopy (EDS) technique was used to measure chemical compositions. X-ray diffraction (XRD) was also used to verify the dominant phases present. A binary Fe-15Ni alloy was also tested in air at 1280°C to test for any direct effect mold flux might have (in the absence of chromium) on elimination of the network of nickel enriched metal inherent in subscale.;For oxidized Fe-15Ni, the presence of mold flux worsened the oxidation attack, supporting the idea that the effect of mold flux is not to promote oxidation of nickel (from the unoxidized metallic networks) as such. Fe-Cr alloys oxidized in wet air revealed the formation of a continuous external eskolaite layer for an 18%Cr and 19%Cr steel in the flux treated case, where none was observed in the non-flux treated case, supporting the proposed mechanism regarding the ability of the molten flux to promote the growth of a protective Cr2O3 layer.;Dry gas oxidation tests reveal that volatilization of chromium oxide is likely a major contributor in preventing protective continuous eskolaite formation at high temperature. This was shown when deep subscale formation was avoided for a 304L sample oxidized in a dry N2-20%O2 mixture, resulting in the formation of a continuous external eskolaite layer. Furthermore, eskolaite formation was observed for Fe-14Cr and Fe-18Cr alloys oxidized in dry gas where none formed in wet air. The change in chromium depletion profiles in wet environments versus dry environments supports the idea of increased chromium loss due to volatilization.;Results from this work support the proposed mechanism explaining how enhanced chromium loss can occur due to volatilization in water containing gas and how mold flux affects this behavior. Mold flux serves to prevent chromium loss by acting as a solubility barrier to chromium transport through the molten silicate, hence preventing chromium loss due to volatilization at the gas/silicate interface. (Abstract shortened by UMI.).