Abstract
Tie rod is a part of lifting and lowering mechanism of skirt of steel making vessel. The design stress on tie rod was1000 MPa and working temperature is 650–700 °C. Tie rods were approximately 1.9 m (1900 mm) long and 65 mm in diameter. The presently used EN24 material has average life of 6 months. The tie rod thinned as the diameter reduced to 45 mm and length increased upto 2055 mm. The unaffected microstructure showed martensite with bainite and the thinned region shows grain boundary chromium carbide precipitates which indicated microstructural degradation due to high temperature exposure over long period. The grain boundary precipitates is indication of creep which led to precipitate-induced void formation, and their growth and propagation. The steady state creep rate was calculated for 6 months with a change in length of 0.175 m and it is found to be 1.12 × 10−8 m/s which is 3 order magnitude lower than the creep resistant steel. The yield strength and UTS deteriorated due to creep and found to be much lower than the designed stress. At 650 °C which is working temperature of tie rod with a time of 105 h and the rupture strength was compared for different materials. Recommended martensitic steel AISI 431 rupture strength at the same condition is around 100 MPa. The comparison of two materials at 650 °C showed better service life for recommended material.
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