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*BOF hood [#v8aeaf8c] **1.BOF hoods system [#sc409258] >In the BASIC oxygen furnace (BOF) process, oxygen is blown under pressure into a furnace bath containing hot metal, steel scrap and fluxes to produce steel. BOF hood systems are designed to capture gases and particulates generated in the furnance, cool the effluents and convey them for cleaning prior to discharge into the atmosphere. A schematic of a typical closed-hood system is shown in [[Fig.1>#fig1]]. >A worldwide survey of BOF hood systems completed in 1981 revealed that 30 % of all hood failures result from thermal stress cracking. Another 30 % are caused by thinning of hood wall tubes because of erosion or corrosion mechanisms. The remaining hood failures are related to poor water quality, localized superheating, skull buildup on the furnace lip ring or other reasons. &br; CENTER:&aname(fig1);&attachref(BOFhoo72.jpg);&br; &br; **2.Circumstances of repair [#hbba4c4b] **2.Failure mechanisms [#s2f54794] >The main bridge was completed on 12 April 1961. On the morning of 10 July, 1962, that is to say one of the coldest days of the Melbourne, winter-the temperature was 30 F-, one span collapsed. >Thermal fatigue cracking occurs when the hood wall is repeatedly cycled in response to rapid changes in effluent gas temperature. Thermal expansion and contraction of the hood wall create fluctuating tensile and compressive stress in the hood. **3.Types of structure [#y739dbec] >Typically, thermal fatigue produces circumferential cracks along the crown of a tube or hot side of a panel. Cracking may initiate on the gas-side, water-side or on both surfaces simultaneously. >The bridge consists of two 2,300-ft-long and is of welded, deck-girder, suspended-span construction, with spans up to 160 ft. The cross section is shown in [[figure 3>#fig3]]. The superstructure consisted of reinforced concrete decking cast in site. The steel used conforms closely with British Standard Specification 968. ( Steel conforming to BSS 968 has a minimum yield point of 55,000 psi and an allowable basic design stress of 26,000 psi in normally used in Australia.) **3.Hood repair [#i2e91678] >Hood repairs are commonly made on an emergency basis, as scheduling allows, so that the BOF can return to normal operations as soon as possible. During repair operations, cracks are quickly patch welded. Panels of tubes are often replaced only after it has been determined that the area is not repairable because of the extent of cracking damage or wall thinning ([[Fig.2>#fig2]]). >However, there was report that cracks originated at the toes of repair welds ([[Fig.3>#fig3]]). Therefore, when possible, it is recommended that a tube or panel be replaced rather than weld repaired. &br; CENTER:&aname(fig3);&attachref(King_h37.jpg);&br; CENTER:&aname(fig2);&attachref(BOFhoo73.jpg);&br; &br; CENTER:Fig.3 Cross section. CENTER:Fig.2 Welded repair and thermal fatigue cracking on gas-side surface. &br; &br; CENTER:&aname(fig3);&attachref(BOFhoo74.jpg);&br; &br; CENTER:Fig.3 Welded repairs and fatigue cracking on gas-side surface. &br; **4.Details of loading [#z570b7cc] **4.Case [#qde28128] >Fluctuating loads due to vehicles live load. -Ⅰ．Water system : Open-recirculation water system was used in this excess air hood. >One span collapsed under the load of s semi-trailer vehicle with a total weight of 47tons; a load within the permissible limits for the bridge. -Ⅱ．Circumstances of Repair : >It was reported that many of the lower hood panels were replaced every 18 months to minimize unscheduled repair. However, two lower panels were failing repeatedly in less than one year. **5.Description of damage [#ve1ca17a] -Ⅲ．Crack location : Membrane carbon steel tube hood panels. >These panels were located in a portion of the lower hood, away from the highest heat flux zone. >Numerous circumferential cracks were present on the gas-side surface, with at least one through-wall crack ([[Fig.4>#fig4]]). Inspection of the internal surface revealed a heavy water-deposit and transverse cracking. >The external cracks were straight and oxide-filled ([[Fig.5>#fig5]]) and the internal cracks initiated at the base of pit ([[Fig.6>#fig6]]). >The collapsed span was made up of four 100 ft long suspended girders, and each girder fractured at a point 16 ft from the northern end of the span. The structure sagged 18 in. Inch-wide cracks spread acrossed the deck. -Ⅳ．Cause of fatigue crack : Abnormally large temperature fluctuations. >During a blow when peak gas temperatures were achieved, reduced heat transfer across the tube wall created excessive metal temperatures. Between heats, this area would cool significantly because this panel did not receive residual radiant heat from the furnace. >The heavy buildup of water-side deposit and corrosion product contributed to the premature failure. >The cause of the failure was given as brittle fracture, at a point on each girder where a change in flange section occurred. The night temperature contributed to the collapse. The flanges of the girders were thickened locally by welded-on cover plates ([[Fig.4>#fig4]]), and each fracture was at the point of termination of one of these cover plates ([[Fig.5>#fig5]]). The cover plates were welded manually. -Ⅴ．Repair method applied : To help the life of these hood panels, Improved water quality and reduced internal corrosion would be recommended. >Under bead cracking was observed in welds at ends of tension-flange cover plates. It was suggested that the cause of the cracks might have been due to a lack of preheating at these points or to the presence of excess hydrogen or inadequate precautions for the difficulties of welding. &br; CENTER:&aname(fig4);&attachref(BOFhoo75.jpg);&br; &br; CENTER:Fig.4 Thermal fatigue cracking on gas-side surface. &br; &br; CENTER:&aname(fig4);&attachref(King_h38.jpg);&br; CENTER:&aname(fig5);&attachref(BOFhoo76.jpg);&br; &br; CENTER:Fig.4 Flange of a girder with cover plate. CENTER:Fig.5 Thermal fatigue cracking on external surface. &br; &br; CENTER:&aname(fig5);&attachref(King_h39.jpg);&br; CENTER:&aname(fig6);&attachref(BOFhoo77.jpg);&br; &br; CENTER:Fig.5 Clack and fracture location. CENTER:Fig.6 Corrosion fatigue cracking on water-side surface. &br; **6.Repair method applied [#dcdd4de9] >The recommended repair was to pre-stress the girders with steel cables or other devices within the depth of the present structure so that flanges liable to cracking under tension would be subjected to compressive stresses. **Reference [#lc48dd2a] >'''Engineering News Record,'''Sept., 20, 1962. >'''Engineering,''' Sept., 21, 1962. >Nishimura T. and Miki C., Fracture of Steel Bridges Caused by Tensile Stress, '''J. Japanese SCE,''' Nov., 1975. (In Japanese) >Hargrave R. E. Common failure mechanisms in BOF hoods. '''Iron and Steel Engineer,''' Nov., 1996,22-28. &br; &br;
