The three principal factors in the manufacturing of steel grinding balls used in mining industry are the material, the forging process and the heat treatment. If this parts fail during drop ball test with severe spalling and fractures. What do you think about wich can be the reason? Can you help me with a paper about this type of piece, please?
Forged steel grinding media ball are generally made out of high carbon such as C-98 or high carbon chromium grade 103Cr.After post forging annealing subjected to quenching and tempering which must result in martensite and unstable retained austenite.Hardness achieved is more than 60 HRC.Post heat treatment ball must be subjected to MPI, DPT and Drop ball test (DBT).DBT indicates impact toughness of the balls.In case balls are getting cracks/spalling during DBT it is due to low impact stength of balls and can be improved by following corrective heat treatment;
1. Cryogenic treatment after quenching at subzero temperature to transform retained austenite to martensite.
2. Do double tempering follwed by cryogenic treatment
Please refer atteched article for better understanding
Dear Mr Shivraj SINGH Kasana.
Many, many thanks for the paper attach. In relation with your proposals about point 1 and point 2 the difficulty to apply is that type of piece have a little price, and if people add more process the piece price are not competitive.
In case cryogenic treatment and double tempering not possible ,please try following;
1.Reduce carbon content and add little amount of chromium
2. To improve impact toughness dissolve primary carbides in cast ingot/bloom by high temperature homogenization before start of forging
3.Low temperature stress relieve for longer duration
Regards
Shivraj Singh Kasana
In the case of practical production units where you might have a batch over 500 tons of forged steel balls failing severely during the DBT (fracture or spalling), the best solution would be to re-temper them. This will drop the hardness, to say, 54-58 HRc, impact toughness will be improved but wear resistance compromised. If the balls' target environment is e.g SAG mill or large diameter balls mills, there are no two ways about it, you have to re-temper them to improve impact toughness which is much needed in the severe impact loading environments of such mills.
However, the longtime solution would be to take a holistic investigation approach in which you look at the process from its mouth to tail i.e. from steel-making process (cleanliness of the steel, inclusions type&morphology, chemical composition etc), then you also investigate the forging bar manufacturing process, which is usually drawing or rolling (reheating temperatures and thermo-mechanical properties of the bar feed into the forging plant). Then the forging process itself i.e forging temperatures, ball shape, heat-treatment, micro-structures, retained austenite etc. Finally you make an effort to quantify the impact loading energy&stress into the balls during the DBT and relate it to the established mechanical properties of the ball.
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