Establishing a qualitative model for rolling contact fatigue and spalling failure of back-up rolls

The demands placed on back-up rolls in hot strip mills have been investigated by literature, industrial, theoretical and experimental studies. The environmental operating conditions have been established and the demands experienced during excursions outside normal operating conditions, such as mill incidents, have also been investigated. The loads arising during normal operating conditions have been quantified by the determination of the loading profiles and the pressure distributions at the work/roll back-up roll interface. These results have been obtained by processing mill and roll schedule data using a hot strip profile coupled beam model (computer program developed by VAI Industries (UK) Ltd) and the theories of contact mechanics. The responses of the rolls to these demands have been examined in derail and the main factors affecting resistance to rolling contact fatigue identified. The tribological conditions at the work roll/back-up roll interface, the possible failure mechanisms and the rolling contact fatigue performance of current backup roll materials have been investigated by literature studies and a detailed experimental programme. The latter has included laboratory simulation on a computer controlled twin disc rolling-sliding contact testing machine with a scanning crack detection system and optical microscopy. The results of the work. in particular the observed fatigue crack morphologies and measured traction coefficients, have been used to develop a qualitative model for the rolling contact fatigue and spalling failure of backup rolls. The model has been validated by the detailed examination of spalled material from a failed back-up roll.