This thesis is on a project concerning the development of a new ball mill test to evaluate impact fatigue spalling (IFS) in white cast iron alloys. This project aims to determine the suitability of laboratory ball mills in producing appreciable IFS in white cast iron alloys.
This project was executed through the following stages:
1. Experiment A & B: Determine the conditions to produce IFS and assess the statistical scatter in 600mm ball mill.
2. Experiment C: Investigate the phenomenology of IFS and determine the mechanism of IFS in a wide range of alloys in the 600mm ball mill.
3. Experiment D: Obtain a correlation between microstructure and microhardness and weight loss performance – Experiment D.
4. Experiment E: Investigate the phenomenology of IFS in a range of alloys in the 1800mm ball mill.
Experiment A revealed that the 600mm ball mill was able to produce IFS damage in the specimens. Optical microscopy revealed evidence of subsurface cracks running parallel to the surface in all three samples. The results from the experiment A prompted work to be done to assess statistical scatter in weight loss performance – Experiment B. The results showed that there is significant specimen variability in both Mag A (AR) and Mag C (AR). However, the test interval variability was found to be quite low. Hardness testing on the Mag A specimens indicated that the specimen with the lowest hardness was the outlier, which performed the best (lowest weight loss). Experiment C revealed a reasonably clear ranking in weight loss performance between the six different ACCs, with Mag A (AR) performing the worst, followed by Mag B (1120), Mag B (AR), Mag C (AR), Mag C (1120) and Mag A (1120).
As before, the specimen variability was large, particularly in the Mag A (AR) specimens. From experiment D, the micrographs revealed that alloys with a softer matrix (pearlite or austenite) performed better (lower weight loss). Mag A (AR), which was the worst performing, has a fully martensitic matrix. This was backed up by the hardness results, which confirmed that the hardest alloy was Mag A (AR). Additionally, the calculated austenite volume fraction using delta H cryo were in good agreement with the micrographs and hardness.
Experiment E revealed that the IFS phenomenology trend in the 1800mm ball mill test is different compared to the 600mm ball mill test. The weight loss trend for most specimens was quite linear and there was no clear acceleration phase. Additionally, the absolute weight loss of the specimens is about a third of the absolute weight loss of specimens in the 600mm ball mill, at the same test time (15 hours). It was identified that the reason for the lower absolute weight loss seen in the specimens is because of the lower number of impacts experienced. This thesis succeeded in showing that the laboratory ball mills can be used to produce reasonable IFS damage in white cast iron ball specimens. A performance ranking between different ACCs was also obtained. Additionally, a reasonable correlation between weight loss performance and microhardness and microstructure, was obtained. To reduce statistical variability of test and specimens, the recommendations outlined in the report should be considered in the next phase of test development.