Metallurgical Failure Analysis of a Steel Wire Rope Used at a Copper Smelting Facility

Detailed metallurgical failure analysis was carried out on a failed 25-mm steel wire rope. Chemical analysis, mechanical testing and microstructural analysis have shown the original material of the wires to comply with relevant standards and to be suitable for the present application. The main failure mode was found to be brittle intergranular fracture. This was promoted by recrystallization and grain coarsening which has also lowered mechanical strength of the rope. This microstructural modification itself is the result of wires experiencing relatively high temperatures (in excess of 300 degrees C) caused by inter-strand and inter-wire sliding/rubbing as well as dry sliding against sheaves, in particular the equalizer sheave. Rubbing/sliding actions have caused appreciable amounts of wear and wire breaking. The present analysis revealed the root cause of the present failure to be an inappropriate design and/or selection of the sheaving system including a small equalizer sheave-to-rope diameter ratio, large rope entry angle and small groove angle. These factors were responsible for causing substantial rope rotation and open twisting leading to the damage patterns explained above coupled with core strand shortening relative to outer strands. Dry sliding of the rope against equalizer sheave apparently helped augmenting the rope deterioration process. The present failure is believed to have started by failure of the core strand followed by failure of the outer strands.