Ball mills create an environment of high wear. This wear occurs during the crushing of various materials to reduce their particle size. Due to this environment of high wear abrasion, significant amounts of metal alloys are consumed on both the grinding balls and the shell itself. Therefore it is desirable to have alloys that have high wear rates to reduce the turnover and cost associated with these operations.
The white cast irons structure is very hard, has excellent wear resistant properties. It is for this reason they are used in high wear environments. Due to the white cast irons being made up of hard brittle carbides surrounded by a matrix of martensite a theory was proposed that the performance of these irons in a ball mill environment was dependent upon the level of "impact" the ball received. Little research has been completed in this field, but a thesis work completed by Comino started to answer if "impact" reduces the performance of white cast irons relative to low alloys steeling in a ball mill grinding application.
The type of impingement the ball experiences has a profound effect on other wear rates. These impingements can be split into two angles. High angle impingement is experienced when the ball drops from high and makes contact with either another ball or the mill liner. This type of impingement occurs when the mill is operating at high speeds. Low angle impingement or sliding is when the balls roll/slide over each other or the mill liner, this type of impingement occurs when the mill is operating at relatively low speeds.
Comino found that with an increase in high angle impingement, that is an increase in high angle impingements, the majority of white cast iron samples did not have an increase in relative weight loss. These experiments were conducted in 300mm, 500mm and 600mm ball mills.
From these results a set of experiments were developed to further analyse the wear rates of white cast irons in different impingement dominate environments. To accomplish this test were conducted in a 600mm ball mill, the preliminary test conducted used quartz as the abrasive material and a computer program was used to calculate the volume of abrasive required and the amount of water need. The two test speeds speed were 40% of critical speed and 80% of critical speed and the results gather from the BMAT were similar for the two speeds and deemed a failure.
Upon subsequent improvements in the test, with the type of abrasive being changed to basalt, the volume of abrasive being hand filled, volume of water being calculated off the volume filled by hand and the run speeds being set at the two extremes the tests were able to give better quantitate results.
The testing showed that at higher speeds the performance of white cast irons was better than low alloy steels of the same hardness. It also showed more interestingly that it performed better at the higher speeds then at the lower speeds. SEM analysis was conducted on samples to investigate which type of impingement is dominant at the extreme speeds. It was found that at low speeds (25% of critical) that low angle impingement was the dominate interaction between samples. It was found that at high speeds (85% of critical speed) high angle impingement was the dominate interaction.
The BMAT test were conducted 10 times, 4 of which were at 85% of critical speed with two test run for 6 hours and the other two run for 45 hours. All four of these test resulted in almost identical out comes, with the white cast iron performing better than the alloy steel. One 65% of critical speed test was run for 6 hours and gave very similar results to that of the 85% of critical speed. 5 test were completed for the 25% of critical speed, with two run for 6 hours, two run for 45 hours and one run for 53 hours. All of these results were almost identical with the white cast iron outperforming the low alloy steels. These results gathered here suggest that white cast irons perform better under high angle impingement dominated environments compared to low angle impingement environments.
SEM analysis was completed to investigate why the white cast irons performed better in high angle impingement environments compared to low angle impingement environments and the results are inconclusive. From intensive observations made both environments have erosion of the matrix, carbides standing proud and carbides being removed. It is almost impossible to suggest which impingement creates more cracks per unit area of damage. The quantitative data show that high angle impingements are meant to have less effect on the carbides then the low angle but this only meant to be a small amount and is why there is a lot of cross over between the two. One possible suggestion is that the low angle impingements erode the matrix's slightly more than the high angle impingements and hence leave the carbides less protected and more susceptible to cracking. From the results in this thesis however this cannot be confirmed and only postulated. More experiments and analysis is required before a final answers as to why high angle impingements outperform the others
Overall this this thesis outlines the results of White cast irons under two extreme conditions of both high and low angle impingement. Both resulting in white cast irons out performing low alloy steels, this thesis also shows that opposed to some experts belief white cast irons perform better under high angle impingements .