Oxy-Acetylene Overlay Repair of SAG Mill Sliding Bearings

Literature Overview

This study note examines the application of oxy-acetylene flame overlay welding for the repair of sliding bearings in semi-autogenous grinding (SAG) mills. SAG mills are critical equipment in mineral processing operations, where large rotating assemblies supported by sliding bearings operate under extreme loads, high speeds, and continuous vibration. The bearing journals and bearing shells are subject to severe sliding wear, scoring, and localized overheating. When these components are damaged, the downtime cost is enormous — a single SAG mill shutdown can result in losses exceeding hundreds of thousands of dollars per day. Oxy-acetylene overlay welding provides a rapid, portable repair method suitable for field conditions where arc welding equipment may be unavailable or impractical.

Core Technical Content

The literature describes the repair of SAG mill sliding bearings using oxy-acetylene flame hardfacing with nickel-based and cobalt-based alloy consumables. The key technical challenge is achieving a sound metallurgical bond between the overlay deposit and the bearing surface while maintaining the required hardness and wear resistance of the overlay.

The following table presents typical repair parameters and material selections for SAG mill bearing overlay.

The repair procedure begins with thorough assessment of the bearing damage. The damaged area must be machined to remove all affected material, creating a flat, clean surface for overlay application. For cast iron bearing housings, the machined surface should be preheated to 300–400°C using gas torches to prevent white cast iron formation at the weld zone. The preheating should be gradual and uniform to avoid thermal shock.

Process Analysis

Oxy-acetylene overlay welding differs fundamentally from arc welding in terms of heat input distribution, cooling rate, and metallurgical behavior. The flame provides a lower energy density but broader heat distribution, resulting in slower cooling rates and reduced residual stresses. This characteristic is advantageous for repairing brittle materials such as cast iron, where rapid cooling can induce cracking.

The overlay process typically involves the following sequence:

1. Surface preparation — machine the damaged area to bare metal, remove all oxide and contaminants, and ensure a flat, clean surface.
2. Preheating — apply uniform heat to the component using gas torches, maintaining the temperature within the specified range. For cast iron, a preheat of 300–400°C is recommended.
3. First pass (bonding layer) — apply a thin layer of transition material (e.g., Ni-Fe alloy or low-carbon steel) to ensure metallurgical compatibility between the base and the overlay. The bead width should be 8–12 mm with a travel speed of 100–150 mm/min.
4. Overlay passes — apply subsequent passes of the hardfacing material, maintaining an interpass temperature of 200–300°C. Each pass should overlap the previous bead by approximately 50% to ensure complete coverage.
5. Post-weld heat treatment — for cast iron components, slow cooling in insulation (vermiculite or dry sand) at a rate of 50–100°C/hour is recommended to minimize residual stresses and prevent cracking.
6. Final machining — grind the overlay surface to the required dimension and surface finish.

A critical aspect of oxy-acetylene overlay is flame control. A neutral flame with a C_V ratio of 1.0–1.1 is essential. A carburizing flame (excess acetylene) can introduce excess carbon into the overlay, increasing brittleness and reducing toughness. An oxidizing flame (excess oxygen) can burn out alloying elements from the overlay material, reducing hardness and corrosion resistance. The operator must continuously monitor the flame appearance and adjust the gas ratios accordingly.

Common Defects and Countermeasures

The defect of overlay spalling is particularly concerning in bearing applications. Spalling occurs when the overlay deposit separates from the base material during service, leading to catastrophic bearing failure. The primary cause is inadequate bonding, which can result from surface contamination, insufficient preheating, or excessive dilution in the first pass. To prevent spalling, the first pass should be applied with a narrow bead and low travel speed to ensure maximum penetration into the base material. A dilution ratio of 30–50% in the first pass is acceptable, as the primary objective is bonding rather than achieving the final overlay composition.

Engineering Practice Integration

In the context of SAG mill maintenance, oxy-acetylene overlay repair offers several practical advantages. The equipment is portable and requires only compressed gas cylinders and hoses, making it suitable for remote mining locations where electrical power may be limited. The repair can be performed in-situ without removing the bearing assembly from the mill, significantly reducing downtime.

However, the method also has limitations. The heat input is relatively high compared to arc welding, which can affect the mechanical properties of the base material over a wider zone. For precision bearing applications requiring tight dimensional tolerances, post-overlay machining is essential, and the overlay thickness must be carefully controlled to avoid excessive material removal during finishing.

A practical approach recommended in the literature is the combination of oxy-acetylene overlay for bulk material deposition followed by GTAW finishing for the final surface layer. This hybrid approach leverages the speed and portability of oxy-acetylene welding for the first 2–3 passes and the precision of GTAW for the final 0.5–1.0 mm surface layer. The GTAW finishing pass produces a smooth, dense surface with minimal spatter, ideal for subsequent grinding to the final bearing dimension.

Study Insights and Reflections

The literature provides valuable insights into the practical application of oxy-acetylene overlay welding for heavy industrial equipment repair. The method's simplicity and portability make it an indispensable tool in the maintenance engineer's toolkit, particularly for large components that are difficult to move to a workshop.

From a metallurgical perspective, the slower cooling rates associated with oxy-acetylene welding produce coarser microstructures in the overlay deposit compared to arc welding. This coarser structure can be advantageous for cast iron repairs, as it reduces the formation of brittle white cast iron at the weld zone. However, for applications requiring fine-grained, high-hardness deposits, arc welding methods (SAW or GTAW) may be preferable.

The concept of "in-situ repair" deserves emphasis. For SAG mill bearings, the ability to repair in place without disassembly is a significant operational advantage. The repair procedure should be designed to minimize thermal distortion of the bearing housing, which can affect the alignment of the mill assembly. Preheating and cooling should be performed gradually, and the component should be allowed to reach ambient temperature before reassembly.

In my professional assessment, the oxy-acetylene overlay method remains a viable and cost-effective solution for SAG mill bearing repair, provided that the process parameters are carefully controlled and the repair is followed by thorough inspection and dimensional verification. The method's effectiveness depends heavily on operator skill and experience, particularly in flame control and bead placement. Investment in operator training and qualification is therefore essential for reliable repair outcomes.