Mainly for the sake of morbid curiosity I'll mention one of my favorites. Steel balls that are used in grinding mills (SAG mills) apparently are heated and their surfaces hardened while grinding, and stresses induced in the steel ball itself. When taken out of the mill and allowed to cool, they sometimes explode (and I'd imagine a large, exploding ball of steel would be a tad dangerous). Such explosions may have even killed people. Here's a discussion of exploding SAG mill steel balls... (NOTE: Side benefit is you get to keep a straight face while discussing exploding balls).

ruble3 (Mining)30 Mar 05 08:23 Glad to be here! That particular incident happened in the early nineties-I hadn't worked in process plants before & was skeptical until I heard it from the horses mouth -still not sure why it happens - only refernce I could find was a case on a legal website where a family sued the mill after a worker was killed by an exploding ball - they blamed a 'manufacturing defect' ?? -still looking

tomrivet (Chemical)1 Jun 05 00:59 We have exploding balls quite a bit. The current thoughts are that the outside of the ball gets work hardend at a high temp (say 40C). Then the ball shrinks as it cools...

arunmrao (Materials)1 Jun 05 13:01 The balls explode due to builtup internal stresses. There is a volumetric expansion as some of the austenite has not completely transformed into martensite during heat treatment. It is this residual austenite which causes explosion of the balls. I have seen crazy things happening,imagine on a dark night!!

 

TurinShroud (Mining)1 Jun 05 16:59 I work at a copper mine that has a SAG mill with 5" balls. When we go in the mill for liner inspections, you can feel balls exploding underneath your feet. Last year my boss got cut in the ear by one.

I always thought that the balls exploded due to the internal stress generated by the outside cooling (and shrinking) faster than the inside. If we wait long enough before entering the SAG mill, the balls are no longer cooling and stop exploding.

Just yesterday, one of the operators said the balls can explode even after a long time. I had never heard of the balls exploding after they have cooled down. Has anyone else out there ever heard of the balls exploding after a long time?

If the exlosion is due to differential cooling, I just don't see how they could explode after they have been out the SAG mill for more than a few hours

Mainly for the sake of morbid curiosity I'll mention one of my favorites. Steel balls that are used in grinding mills (SAG mills) apparently are heated and their surfaces hardened while grinding, and stresses induced in the steel ball itself. When taken out of the mill and allowed to cool, they sometimes explode (and I'd imagine a large, exploding ball of steel would be a tad dangerous). Such explosions may have even killed people. Here's a discussion of exploding SAG mill steel balls... (NOTE: Side benefit is you get to keep a straight face while discussing exploding balls).

ruble3 (Mining)30 Mar 05 08:23 Glad to be here! That particular incident happened in the early nineties-I hadn't worked in process plants before & was skeptical until I heard it from the horses mouth -still not sure why it happens - only refernce I could find was a case on a legal website where a family sued the mill after a worker was killed by an exploding ball - they blamed a 'manufacturing defect' ?? -still looking

tomrivet (Chemical)1 Jun 05 00:59 We have exploding balls quite a bit. The current thoughts are that the outside of the ball gets work hardend at a high temp (say 40C). Then the ball shrinks as it cools...

arunmrao (Materials)1 Jun 05 13:01 The balls explode due to builtup internal stresses. There is a volumetric expansion as some of the austenite has not completely transformed into martensite during heat treatment. It is this residual austenite which causes explosion of the balls. I have seen crazy things happening,imagine on a dark night!!

TurinShroud (Mining)1 Jun 05 16:59 I work at a copper mine that has a SAG mill with 5" balls. When we go in the mill for liner inspections, you can feel balls exploding underneath your feet. Last year my boss got cut in the ear by one.

I always thought that the balls exploded due to the internal stress generated by the outside cooling (and shrinking) faster than the inside. If we wait long enough before entering the SAG mill, the balls are no longer cooling and stop exploding.

Just yesterday, one of the operators said the balls can explode even after a long time. I had never heard of the balls exploding after they have cooled down. Has anyone else out there ever heard of the balls exploding after a long time?

If the exlosion is due to differential cooling, I just don't see how they could explode after they have been out the SAG mill for more than a few hours