Abstract
The service life of a hot working roll is limited by fire cracking caused on its surface by repeated thermal stress. A testing apparatus was developed for the purpose of the estimation of fire cracking characteristics of iron and steel.
In this study, fire cracking characteristics of an actual slabbing mill roll is investigated and the relation between the crack length and its number and also the effect of maximum heating temperature are studied.
The results may be summarized as follows:
(1) Fire cracking characteristics of the portion from where a premature breakage of a slabbing mill roll was happened was inferior. The lamellar pearlitic structure was indicated at this portion by microstructual examination. The microstructure of the other part of the roll was globular pearlitic. This difference of microstructure was supposed to be caused by a mis-control during the heat treatment. The unfavorable lamellar pearlitic structure is a cause of inferior fire cracking characteristics.
(2) Distribution of fire cracking characteristics was investigated from the surface to interior of a slabbing mill roll. Fire cracking characteristics of the internal portion is inferior because of the increase of lamellar pearlite and segregation of carbon. By a re-heattreatment, the microstructure was improved, but the improvement of the inferiority caused by the segregation was difficult.
(3) Effect of a crack on the stress of its neighbourhood diminishes at the portion where is about twenty times of crack depth off from the crack. Consequently the deeper the crack grows, the fewer the cracking.
(4) Effect of maximum heating temperature was studied on a Cr-Mo steel between 300 and 700°C. The maximum crack depth was observed at 600°C. At the temperature above 600°C, the constraint upon the specimen surface given by the regidity of specimen itself becomes week and the thermal stress would be lowered.
