A case study on the fracture of sinter machine pallet axles

Abstract

A sinter machine produces sinter at 1200–1400°C to supply raw materials for blast furnace ironmaking. The sinter machine is composed of a series of pallet cars which are constrained on rails. Each pallet car has four axles which connect the pallet car and four running rollers. In sinter production, the pallet axles experience periodic thermal expansion and periodic load changes under high temperature, high moisture, corrosive atmosphere and heavy dust. After serving for about seven years, pallet axles of one of ArcelorMittal sinter machines started breaking, causing unscheduled production delay. The fracture of the axles is in the orientation of top and bottom, and the cracks are deeper in upper portions than in lower portions of axle cross sections. The axles have been studied through chemical analysis, strength analysis, stress analysis, lifetime analysis via deterministic mechanics and lifetime distribution analysis. It has been found that the axles are breaking in fatigue-crack mechanism. The fatigue cracks are resulted from effect of thermal expansion on net tensile stress of the axles. Considering harsh working environment and dynamic loading, lifetime analysis, using deterministic mechanics, indicates that originally installed pallet axles are approaching the end of their lifetime. Weibull distribution function can very well describe lifetime distribution of pallet axles. Accordingly, about 50% of originally installed axles will be gone before next two and half years, and more than 90% of them will break before next six and half years. An intelligent replacement strategy is recommended. In a scheduled maintenance time, carefully inspect all axles using ultrasonic crack detectors, and replace those axles whose expected lifetime is shorter than next scheduled maintenance date. To replace steel SAE 1045 with steel SAE 4040 for fabricating new pallet axles may significantly increase lifetime of the new pallet axles.