Some Consideration on the Premature Failure of Fully Hardened Roll

Abstract

Four high cold strip mill work rolls are fully hardened forged steel rolls which are made of high carbonchromium steel with an ordinary composition of 0.8-1.15 percent carbon and 0.8-2.6 percent chromium, subjected to water quenching and low temperature-stress relief at 100-250°C. They have residualstresses caused by the above mentioned heat treatment, so they are very liable to have troubles in rollingoperations.
Major troubles occurring in the early period of the life of these rolls are fracture taking with time lag afterquenching in storage and fatigue fracture originating at the center bore surface in rolling operation. Thesetroubles are, it seems, ascribable to their manufacturing conditions. However, as the occurrence of thesefractures is not so frequent, the differences in properties between fractured rolls and sound rolls have not beenclarified yet. Common understanding was that delayed fracture would most likely bethe result of quenchcracking due to heavy residual stresses developed during the quenching process of the roll, while the fatiguefracture might be caused by defects existing in the vicinity of the bore.
A study on the manufacturing processes, properties and residual stresses of the roll has produced thefollowing result which conflicts with the conventional understanding.
1) The phenomenon of a fracture occurring in the roll with time lag after quenching is a delayedfailure caused by the hydrogen in the material under the influence of tensile residual stresses. It is nota simple quench cracking. And the occurrence of the fracture is very frequent in the winter time.
This is not because the temperature was low, but because fractured rolls which had been east in summer whenhumidity was high and hydrogen content in molten steel was increased accordingly, were subjected toquenching treatment in cold winter.
The delayed failure is likely to happen in a roll whose steel contains 3.5ppm or more of hydrogen attapping.
Some rolls of this steel may incur fracture, but others do not, possibly because of nonuniform distributionof hydrogen, excessive segregation and varied situation of micro defects from which fracture may originate.
2) Fatigue fracture at bore takes place even if there is no defect. The cause of fracture is that fatiguestrength is lowered by tensile residual stress acting as a mean stress against working stress in rolling operation.And it is inferred that tensile residual stress at bore is caused through insufficient cooling in bore underquenching treatment.