Metallurgical Investigation of Tie Rod used for lifting Ferro-Alloy during Steel Making
In steel making industry, bottom dispenser buckets are widely used to carry ferro-alloys, lime or other materials from one place to other or one height to other. In one of the steel making shops, such bottom dispenser buckets are used for lifting ferro-alloys which is charged further to ladle furnace. One of the ferro alloy buckets with carrying capacity up to 3 Ton failed while being lifted and crash against sidewalls of the cage and an observation platform. Fortunately, no one was reported injured due to the incident.
Upon observing the failure, it was revealed that the bucket fell due to fracture of its tie rod while lifting ferro-alloys. Visual observation reveals thermal cracks and significant thining which indicates enourmous amount of deformation that the bracket underwent prior to failure. Fractography of the fracture surface of the bracket showed dimples and these dimples have predominantly graphite nodule within them.
It suggests ductile mode of failure probably due to exposure to high temperature for prolonged duration that activated sufficient slip systems to cause plastic deformation before fracture. Chemistry of the failed tie rod complied with the specification of AISI 431 Martensitic stainless steel. And the bracket was made up of SG iron.
Micrograph of the failed bracket showed substantial bursting of the graphite nodules. These are typically observed when SG iron are subjected to very high temperature for prolonged time. It is evident that such detoriation occurred during service as graphite nodules were observed without any bursting in unused / new bracket.
SEM micrograph revealed creep voids predominantly at grain boundary triple points. Precipitation of chromium carbide was observed along grain boundaries as confirmed by EDS analysis. The above analysis reveals that tie rod and bracket failed in creep mode due to exposure to excessive (unusually high) flame for prolonged duration.
In steel making shops, Ferro alloys are added during steelmaking to achieve certain desired properties in the final product. These ferroalloys are fed to the steel in the ladle refining furnace via a conveyor which is at a suitable height. These ferro-alloys are generally lead to this height using bottom dispenser buckets. In bottom dispenser buckets (BDBs), the bottom portion has a hole at its centre which is closed by an umbrella shaped disc.
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