Zhuojin Pipe Fitting Co., LtdOverlay Welding Repair of Rolling Mill Guide Plate Heads
Literature Overview
This 1994 paper by Wan Weiguo from the Steel Research Institute of Maanshan Iron and Steel Company, published in Welding, addresses the hardfacing repair of guide plate heads in a wire rod rolling mill. The guide plate head is a consumable component that experiences severe abrasive wear from the continuous contact with hot wire rod. The paper documents the failure analysis of different guide plate head materials and presents an overlay welding repair solution.
The guide plate in a wire rod rolling mill serves to center and guide the rod as it passes through the roll gap. The guide plate head, which is the portion in direct contact with the rod, is subject to continuous abrasive wear from the hot steel surface. The failure mode is characterized by deep grooves worn into the guide plate surface, which leads to loss of centering accuracy and eventual failure of the guide function.
Failure Analysis of Conventional Materials
The paper provides a valuable comparative analysis of different guide plate head materials, which is directly relevant to material selection in similar applications.
| Material | Service Life | Failure Mode | Cost |
|---|---|---|---|
| Cast iron | Low | Poor wear resistance | Low |
| High chromium-nickel alloy | Low | Brittle fracture, spalling | High |
| 45 steel | Half a shift | Deep groove wear | Moderate |
The cast iron guide plate head has poor mechanical properties and insufficient wear resistance for this application. The high chromium-nickel alloy, while offering excellent wear resistance, is excessively brittle and prone to fracture under the impact loading conditions. The 45 steel, which was the default material used by the plant, provides adequate toughness but insufficient wear resistance, lasting only half a shift before failure.
This failure analysis illustrates a fundamental principle in tribology: wear resistance and toughness are often inversely related. Materials with high hardness tend to be brittle, while materials with high toughness tend to be soft. The optimal material for the guide plate head must balance these competing requirements based on the specific loading conditions.
Overlay Welding Repair Strategy
The overlay welding repair strategy proposed in this paper involves applying a hardfacing overlay to the 45 steel guide plate head. This approach combines the toughness of the 45 steel base with the wear resistance of the hardfacing overlay, creating a composite structure that outperforms either material alone.
The overlay welding procedure involves the following steps:
- Surface preparation: The worn surface of the guide plate head is machined to remove all damaged material and expose sound base metal. The surface is cleaned to remove scale, oil, and contamination.
- Preheating: The 45 steel base is preheated to 200–300 °C to reduce the cooling rate and minimize the risk of cracking. The preheat temperature is lower than for cast steel because 45 steel has a lower carbon equivalent.
- Overlay deposition: The hardfacing overlay is applied using a suitable electrode. The overlay composition should provide a hardness of at least 50 HRC to resist abrasive wear from the hot rod. The overlay thickness should be 3–5 mm to ensure adequate wear resistance after machining.
- Post-weld machining: The overlay surface is machined to the required geometry. The machining allowance must account for the possibility of surface defects in the overlay.
- Quality verification: Hardness testing confirms that the overlay meets the specified minimum hardness. Visual inspection and magnetic particle testing verify the absence of surface defects.
Electrode Selection
The selection of the hardfacing electrode is critical to the success of the repair. The electrode must provide the following characteristics:
- Adequate hardness at operating temperature (typically 800–1000 °C for hot rolling)
- Sufficient toughness to resist impact and thermal shock
- Good weldability with the 45 steel base
- Low dilution sensitivity to maintain hardness after base metal mixing
Common electrode types for this application include chromium carbide-based electrodes, which provide excellent abrasive wear resistance, and cobalt-based electrodes, which offer superior high-temperature hardness retention. The choice between these two types depends on the specific operating conditions and cost considerations.
Engineering Practice Considerations
The economic analysis of this repair is compelling. The guide plate head is a high-consumption item in a wire rod mill producing 200,000 tons per year. With a 45 steel guide plate head lasting only half a shift, the annual consumption is extremely high. The overlay welding repair can extend the service life by several times, significantly reducing the annual cost of guide plate heads.
The repair cycle must be carefully managed. The overlay welding repair should be performed before the guide plate head reaches a critical wear depth, as excessive wear can damage the underlying base metal and compromise the weld integrity. A wear monitoring system should be implemented to track the wear rate and schedule repairs proactively.
Quality Control Procedures
The quality control procedures for this repair should include:
| Inspection | Method | Acceptance Criteria |
|---|---|---|
| Surface quality | Visual inspection | No cracks, porosity, undercut |
| Surface defects | Magnetic particle testing | No indications |
| Overlay hardness | Rockwell C hardness test | Minimum 50 HRC |
| Overlay thickness | Micrometer measurement | Minimum 3 mm after machining |
| Dimensional accuracy | Gauge or coordinate measurement | Within specified tolerance |
The hardness requirement of 50 HRC is a minimum value. In practice, the hardness should be verified at multiple locations across the overlay surface to ensure uniform deposition. Variations in hardness can indicate uneven dilution or inconsistent welding parameters.
Study Insights
This paper provides a practical demonstration of how overlay welding can solve a material selection problem that cannot be addressed by material substitution alone. The 45 steel base provides the necessary toughness, while the hardfacing overlay provides the necessary wear resistance. This composite approach is superior to any single material and represents a sophisticated application of welding technology.
The paper also highlights the importance of failure analysis in maintenance engineering. By systematically evaluating the performance of different materials, the engineers identified the root cause of the problem and developed a targeted solution. This analytical approach is more effective than trial-and-error experimentation and provides a framework for solving similar problems in other applications.
The economic impact of this repair is substantial. In a mill producing 200,000 tons of wire rod per year, the guide plate head consumption represents a significant operating cost. The overlay welding repair reduces this cost by extending the service life of each guide plate head, thereby reducing the frequency of replacement and the associated downtime.
From a broader perspective, this work exemplifies the philosophy of design-for-maintenance. Rather than accepting the limitations of a single material, the engineers used welding technology to create a composite component that combines the best properties of multiple materials. This approach can be applied to many other components in the rolling mill, including roll necks, bearing seats, and other wear-critical surfaces.
The paper by Wan Weiguo is a valuable reference for engineers working on rolling mill maintenance. It demonstrates that overlay welding is a versatile and cost-effective technology that can solve complex tribological problems in heavy industry. The systematic approach to material selection, process development, and quality control provides a model for engineering excellence in maintenance operations.