Research On Creep Deformation Of Ductile Cast Iron Cooling Staves Of Blast Furnace

The life of the cooling stave is the fundamental factor determining the long campaign ofblast furnace, slow creep deformation can occur to the cooling stave due to high temperatureload and impact of continuous thermal stress, which may cause cracks in the stave and thecooling water pipes, resulting in damage and even failure of the stave. Therefore, it is veryimportant for prolonging the campaign of blast furnace to quantitatively analyze creepdeformation of cooling staves at high temperature.In this paper, high-temperature tensile creep tests were firstly conducted on the materialof QT400-18ductile cast iron used for ductile cast iron cooling staves; to obtain the creepcurves A, m and n of the ductile cast iron at different temperatures. In combination of the hottests of ductile cast iron cooling staves, creep deformation of the stave body and coolingpipes were analyzed by the ABAQUS coupled thermal/structural calculation method toprovide a theoretical basis for improving structural design of ductile cast iron cooling stavesand controlling creep deformation. The main idea of this paper:(1) With the parameters determined by the hot test of ductile cast iron cooling stave asthe thermal boundary conditions, the temperature and thermal stress distribution of the stavein bared condition and slag adhering condition were calculated by numerical simulation.Based on the calculation results and in combination of the yield strength of the ductile castiron, the temperature levels of the high-temperature tensile creep test of ductile cast iron weredetermined as250℃,300℃and350℃, and the stress levels of the high-temperature tensilecreep test of ductile cast iron were determined as220MPa and300MPa.(2) According to the determined test scheme, a high-temperature tensile creep test wasconducted on the ductile cast iron material used in cast iron stave to obtain the creep curvesof ductile cast iron at different temperatures. The creep parameters of ductile cast iron atdifferent temperatures were obtained by multiple linear regressions of the test data. Theresult showed that: when the test temperature was lower than400℃, the first creep stage wasmore significant under higher stress than lower stress, the creep rate and the creepdeformation of the specimen was greater, the stress played the leading role in creepdeformation; when the test temperature higher than400℃, even though the stress was very small, the creep deformation of the specimen was very great, the temperature played theleading role in creep deformation.(3) The effects of gas temperature, slag skin thickness and edge contact pressure oncreep deformation of staves were analyzed by the coupled thermal/structural method. Theresult showed that: the higher gas temperature, the greater the creep deformation of the stavewas; the slag skin reduced the creep deformation of the stave; at a constant gas temperature,when the edge contact pressure was lower than50MPa, the stave bulged toward the hotsurface to form an "arch"; when the edge contact pressure was higher than100MPa, thestave bulged toward the cold surface to form an "arch". Therefore when installing the staveon the furnace, the expansion clearance between the stave and the furnace shell shall not beless than6mm, the expansion clearance between the adjacent staves shall not be less than7mm, the reserved expansion clearance in the height direction shall not be less than9mm,and the cast iron stave shall be made with certain arc matching the shape of the furnacehousing.(4) The calculation result of creep deformation of water pipes inside the stave showedthat: the cooling water pipe inside the cast iron staves not only bended toward the hot surface,but also deviate from the both ends in the width direction. It is noted that depression occurredat the contact part of water inlet and outlet and the furnace shell, indicating the bulging partof the pipe toward the hot surface tends to be squeezed by the stave body and the water inletand outlet is the part of the pipe susceptible to breakage. In order to reduce the pipedeformation and prevent the pipe from breaking, at least a clearance of1.6mm shall be keptbetween the water pipe and the furnace shell opening, and the most appropriate clearancebetween the water pipe and the stave body shall be0.092mm.