The M4213 ball mills are vital equipment in cement grinding. These mills are subjected to heavy loads, frequent start-stop cycles, and prolonged service life, all of which contribute to the appearance of cracks in the welds on the mill’s body. In this case, we will discuss the issue of cracks that occurred in the corner welds between the sliding shoes and the belly plate of the mill, and the methods employed to address this problem. We will also review the post-repair conditions, which have shown satisfactory performance.
1. Problem Identification
Our manufacturing plant operates five M4213 cement ball mills, which were provided by Shenyang Cement Machinery Co., Ltd. and have been in use since 2007-2008. Over time, and with the effects of frequent start-stop cycles, alternating loads, and accumulated stresses, cracks have begun to emerge in the welds. Notably, in August 2023, a crack measuring approximately 1.7 meters long was found along the corner weld between the sliding shoe and the belly plate. This crack had propagated into the base material of the sliding shoe.
Upon inspection, magnetic particle and ultrasonic testing revealed various defects in the welds, including disjointed and substandard welds, along with multiple cracks in both the front and rear welds of the sliding shoes and belly plates. This situation necessitated an urgent repair solution.
2. Repair Approach
Given the complexity of the repairs and the difficulty of addressing the cracks, external professional contractors were consulted. While many manufacturers suggested returning the mills to the factory for repair due to the extensive deformation and residual stresses, this approach was deemed impractical due to the long downtime and high costs. Instead, we chose to work with a specialized welding and machining company based in Wuhan, which was capable of performing online repairs.
The repair process was divided into several key stages:
2.1 Pre-welding Inspection
The areas to be inspected included the corner, longitudinal, and circumferential welds on the mill body, as well as a 100mm zone around each weld. The inspection involved magnetic particle testing and ultrasonic testing, with a requirement for the welds to meet GB/T 11345 standards for ultrasonic testing.
2.2 On-Site Welding
Given the number of welds requiring repair, and the large thermal stresses associated with gas cutting and welding, we decided to perform repairs on-site. This involved fixing the cracks in the corner welds between the sliding shoes and the belly plate, as well as addressing the circumferential weld defects. Special attention was given to controlling the thermal stresses during welding to minimize distortion.
2.3 Post-Welding Inspection
After completing the welding work, a third-party service conducted ultrasonic and magnetic particle inspections to verify the quality of the welds. All repairs were conducted according to the required standards and after a 24-hour curing period.
2.4 Sliding Shoe Grinding
Following the welding repairs, radial runout values of the sliding shoes were found to exceed the allowable limits, prompting the need for online grinding. The grinding process was carried out with strict control over the grinding depth and the amount of metal dust generated to avoid contamination.
2.5 Bearing Bush Scraping
After successful grinding, the bearing bushes were subjected to a scraping process to eliminate any irregularities. The temperature of the bearings remained within acceptable limits during operation, confirming the effectiveness of the repair.
3. Post-Repair Condition
Following the repair, the ball mill has been running smoothly without any abnormal readings in current, vibration, or temperature. The mill’s performance has returned to optimal levels, and the repair measures have proven to be successful.
Furthermore, as part of ongoing maintenance, the condition of the welds on other mills was monitored, and a plan was established for periodic inspections to ensure the continued structural integrity of the ball mills.
4. Conclusion
The occurrence of cracks in the welds of the ball mill body is a common issue that arises due to the combination of heavy loads, frequent start-stop cycles, and the mill’s extended service life. By addressing these issues promptly with targeted repairs, including pre-welding inspections, on-site welding, post-welding inspections, sliding shoe grinding, and bearing bush scraping, the mill was restored to safe operational conditions.
For those facing similar issues, early detection and timely repair are critical to prevent more severe damage. The successful outcome of this repair process offers valuable insights and recommendations for maintenance of ball mills with similar issues.
Table : Summary of Repair Stages
| Stage | Description | Standards/Requirements |
|---|---|---|
| Pre-welding Inspection | Magnetic particle and ultrasonic testing of welds | GB/T 11345, GB 9443-1988 |
| On-Site Welding | Welding of crack areas with careful control of thermal stresses | JC/T 532, Process guidelines |
| Post-Welding Inspection | Third-party testing of weld integrity | GB/T 2970, Ultrasonic standards |
| Sliding Shoe Grinding | Grinding to reduce radial runout | Allowable limits: <0.4mm |
| Bearing Bush Scraping | Scraping to ensure smooth bearing operation | No abnormal temperature rise |