Zhuojin Pipe Fitting Co., LtdOxy-Acetylene Surfacing Repair of Autogenous Mill Sliding Bearing
Literature Overview
This 2000 paper by Zhang Qibin from the Qidaogou Iron Mine Beneficiation Plant, published in Mining Machinery (矿山机械), Volume 28, Issue 10, documents the use of oxy-acetylene flame surfacing to repair a sliding bearing (bearing bush) of an autogenous grinding mill. The work is classified under TG455 (welding processes) and represents practical tribological repair technology in mineral processing applications.
Core Technical Content
Autogenous mills are among the most heavily loaded machines in mineral processing circuits. The sliding bearings that support the mill shell and pinion gear experience continuous sliding friction under heavy radial loads, often exceeding several hundred kilonewtons. The bearing bush material—typically a copper-based or lead-based alloy (e.g., CuSn10, CuAl10Fe5Ni5, or Babbitt alloy)—develops progressive wear, scoring, and sometimes partial melting under boundary lubrication conditions.
Oxy-acetylene flame surfacing offers several advantages for this application:
- Low heat input: Compared to arc welding, flame heating produces a more gradual thermal gradient, reducing the risk of thermal cracking in brittle bearing alloys.
- Portability: Oxy-acetylene equipment is lightweight and can be deployed in confined spaces around large mill assemblies.
- Material compatibility: A wide range of surfacing alloys can be applied using flux-cored wire or powder, allowing tailoring of the tribological surface layer.
Surfacing Process Details
| Process Parameter | Value / Range | Notes |
|---|---|---|
| Flame type | Neutral or slightly carburizing | Prevents excessive oxidation of base |
| Preheat temperature | 200–300 °C | Reduces thermal shock to base material |
| Surfacing wire | CuSn10 or CuAl10Fe5Ni5 | Matches bearing alloy composition |
| Wire diameter | 2.5–4.0 mm | Standard flux-cored surfacing wire |
| Layer thickness | 0.5–1.5 mm per pass | Thin layers to minimize dilution |
| Final layer hardness | 200–280 HV | Optimized for sliding friction |
| Post-weld cooling | Controlled slow cooling | Prevents hot cracking in copper alloys |
Metallurgical Considerations
The critical metallurgical challenge in bearing bush surfacing is achieving a sound metallurgical bond between the surfacing alloy and the base material. Copper-based alloys have high thermal conductivity, which means the weld pool solidifies quickly, increasing susceptibility to solidification cracking. The following measures are essential:
- Flux selection: A proper flux (typically borax-based) must be applied to protect the molten pool from oxidation and to promote wetting of the base surface.
- Interlayer control: Multiple thin layers with adequate interlayer heating ensure complete fusion without overheating the base material.
- Microstructural inspection: After repair, metallographic examination should confirm the absence of unmelted inclusions, microcracks, or segregation at the interface.
Common Defects and Countermeasures
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Hot cracking | Rapid solidification, impurity segregation | Preheat, reduce sulfur/phosphorus in consumables |
| Poor fusion | Insufficient base preheat, incorrect flux | Increase preheat, ensure flux coverage |
| Porosity | Flux breakdown, gas entrapment | Proper flux application, controlled wire feeding |
| Excessive dilution | Too thick layer, high heat input | Reduce layer thickness, lower flame temperature |
| Residual stress | Thermal gradient during cooling | Stress relief annealing at 500–600 °C |
Engineering Practice Insights
From an engineering practice perspective, this repair method is particularly valuable when the bearing housing cannot be easily dismounted from the mill assembly. In many beneficiation plants, the autogenous mill represents a critical production bottleneck, and unplanned downtime is extremely costly. The ability to perform in-situ surfacing repair—potentially with the mill partially disassembled—can save days of shutdown time compared to sending the bearing housing to a foundry for re-machining.
However, the success of such repairs is contingent upon proper surface preparation. The worn bearing surface must be thoroughly cleaned, and any cracks or delamination must be removed by machining before surfacing. A roughness profile of Ra 3.2–6.3 μm on the prepared surface is generally acceptable for flame surfacing, as the molten wire will flow into micro-asperities to create mechanical interlocking.
Reflections
This paper, though brief, captures a fundamental principle of welding repair: the selection of welding process is dictated not only by material compatibility but also by geometric accessibility, heat sensitivity, and production constraints. Oxy-acetylene flame surfacing, while an older technology, remains relevant in situations where arc welding equipment is impractical or where the thermal sensitivity of the base material demands gentle heating. In modern practice, such repairs would be validated by dimensional accuracy checks (roundness, taper) and hardness profiling across the repaired zone to ensure uniform tribological performance.