Mathematical modeling of thermal stresses in basic oxygen furnace hood tubes
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INTRODUCTION
A worldwide survey of maintenance and repair problems of Basic Oxygen Furnace off-gas hoods has revealed a wide variability from one installation to another, l At one end of the spectrum lie operations where repairs required are minimal while at the other end are installations where hood problems are so severe that frequent emergency shutdowns are required to repair leaking hoods which are a hazard and hamper the productivity of the entire steelmaking operation. Statistics indicate that 30 pct of all hood failures are due to thermal stress cracking, another 30 pct are due to thinning of hood wall tubes as a result of erosion and corrosion, while the balance are related to poor water quality, localized superheating, skull build-up or other reasons. Despite the trend away from conventional tube or jacket/ panel construction to tube-membrane or improved jacket/ panel designs, there is no evidence that premature hood failure and the need for unscheduled shutdowns to repair leaking hoods has been eliminated; there is, however, an improvement in hood life. Therefore other measures are being sought to improve hood reliability. The use of metallized and refractory coatings such as tungsten carbide, 420 stainless steel, and special ceramics on regions experiencing high wear has met with some success in combating erosion. Some B.O.F. shops have adopted thicker wall tubes to delay failure resulting from wall thinning due to erosion. The increase in wall thickness can, however, lead to thermal cracking since thermal stresses increase with increasing wall thickness. Other B.O.E installations have therefore gone to thinner gauge material particularly those with jacket/panel construction. Changing the chemical composition of the hood tube material is another trend that has met with some I.V. SAMARASEKERA is Asslstant Professor, The Center for Metallurgical Process Engmeenng, Department of Metallurgical Engineering, University of British Columbia, Vancouver, BC V6T lW5, Canada Manuscript submitted April 24, 1984. METALLURGICALTRANSACTIONS B
success. Plain carbon steel is being replaced by low alloy steels and stainless steel with the intent of improving corrosion resistance. In the case of stainless steel tubing it was found that although the corrosion/erosion problems were overcome, thermal stress cracking limited hood life. -~Other measures to improve hood life include upgrading water treatment to enhance heat transfer between the tubes and cooling water since scaling has often been observed on the internal surfaces of tubes. Blowing practices are also being modified to improve hood life. A reduction in oxygen blowing rate has been found to minimize bath slopping, an event detrimental to the life of the lower hood. Although this measure is counterproductive from a steelmaking standpoint, it has in many instances helped increase overall shop productivity. Lower hot metal sihcon also reduces bath slopping and is also desirable from the standpoint of hood life. Lance height control and multlhole lances are also b
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