THERMAL TREATMENT OF GRINDING BALLS AT NEW BALL-ROLLING SHOP
Experimental data test of heat treatment are presented in conditions of shop production of increased hardness grinding balls on EVRAZ-NTMK, commissioned in 2018. On example of balls Ø 120 mm production from the 55G grade steel billet, by heat treatment changing temperature-time parametersm, was obtained products corresponding to 3 grade hardness according to GOST 7524-2015, with intended purpose 2 grade hardness steel. Particular attention is paid to possibility of making 5th grade hardness balls, with ensured surface and bulk hardness. Experimental studies were carried out on steel grades Ш-3Г and 75ХГФН. The most critical in terms of final properties formation are: the temperature and the nature of its distribution over workpiece surface at ball-rolling mill outlet, the cooling time, the workpiece temperature, before being fed into quenching drum, the cooler temperature and dwell time into quenching drum. It was obtained that all modes, which considered ones satisfy bulk hardness criterion, this indicates a sufficient all steels hardenability. However, on surface and macrosections of balls, which highers alloyed steel, cracks were discovered, that spread deep into the metal at 45 to 50 mm depth, which will inevitably lead grinding media during operation to destruction. The possibility of mass production 5th grade hardness balls is ensured by exact observance of heat treatment temperature-time parameters and is conditioned by equipment complex high automation, control and all technological parameters correction in the on-line mode. Conditions for implementation of grinding balls heat treatment technological modes on the new ball rolling shop of JSC "EVRAZ-NTMK" allows to obtain high grades hardness balls on the steel with lower proprties, which shows its significant potential.