Metallurgical investigation of failure of coal mill drag chain pin

The present paper reports metallurgical investigation of fractured connecting pins of drag chain conveyors used for coal conveying from raw coal hopper to grave gate in coal mill of a cement plant. The failure analysis of two fractured pins was carried out; the location of the fracture was near the end of the pins. Both the failed pins had reduced cross section in the immediate vicinity of the fractured surface. The chemical composition of the pins confirmed to En-19 specifications. The hardness and metallographic studies indicated that the pins were induction hardened at the surface whereas the core of the pins had tempered martensitic microstructure. The visual observations of the failed pins confirmed entrapment of fine to heavy coarse coal particles on the pin surface causing mild to severe polishing wear with subsequent reduction in the cross section. The metallographic studies showed non-uniformity in the induction hardening and undesirable coarse martensitic microstructure at the core. The analysis of the fractured surface and fractographic studies by SEM indicated fatigue fracture due to bending stresses and mild to moderate torsional stresses. The proper induction hardening to ensure required case depth and use of En-24 steel for connecting pins has been suggested to further improve life of the connecting pins.