Surfacing Repair of Large Support Rolls in Metallurgical Equipment

Literature Overview and Background

The paper by Cao Ye, Zhao Qinghua, and Liu Chengmin (2006), published in Welding (No. 7, pp. 33-36), addresses the comprehensive challenge of surfacing repair of large support rolls used in hot rolling mills. Support rolls are critical components in rolling mills, bearing the loads from work rolls and transferring them to the mill housing. These rolls are subjected to severe conditions including high temperatures, heavy loads, thermal cycling, and abrasive wear, making them prone to surface degradation that requires periodic repair. The authors emphasize that successful repair is a systematic engineering endeavor involving three key factors: material selection, process control, and equipment capability.

Core Technical Approach

The paper presents a holistic view of support roll repair, recognizing that no single factor alone can ensure successful repair. The three pillars of material selection, process control, and equipment capability must be addressed simultaneously and in a coordinated manner.

Material Selection for Support Roll Surfacing

The selection of surfacing material for support rolls is governed by the specific service conditions:

• Hardfacing alloys: For rolls experiencing significant abrasive wear, hardfacing alloys containing carbides (Cr7C3, WC, Mo2C) or borides provide enhanced wear resistance. Common electrode types include D256, D257, and D260 for carbide-based hardfacing, and D277 for boron-based hardfacing.
• High-speed steel alloys: For rolls requiring a combination of wear resistance and thermal fatigue resistance, high-speed steel-based surfacing alloys (such as D321, D322) are often selected. These alloys contain high levels of tungsten, molybdenum, vanadium, and chromium, providing excellent hot hardness and wear resistance.
• Nickel-base alloys: For rolls in service with corrosive atmospheres or where thermal fatigue is the dominant failure mode, nickel-base surfacing alloys (such as D208, D209) offer superior resistance to thermal cracking and oxidation.
• Multi-layer approaches: A common strategy is to use a transition layer (such as D260 or D256) followed by a hardfacing surface layer (such as D257 or D277), optimizing both fusion quality and surface performance.

Microstructural Considerations

The microstructure of the surfacing deposit is critical for achieving the desired performance. In carbide-forming alloys, the morphology and distribution of carbide particles significantly influence wear resistance. Fine, uniformly distributed carbides provide superior abrasive wear resistance, while coarse or segregated carbides can act as crack initiation sites. The cooling rate, which is influenced by heat input and base material thermal conductivity, is a key factor in controlling carbide morphology.

In high-speed steel-based alloys, the precipitation of fine carbides during cooling provides the primary strengthening mechanism. The cooling rate must be controlled to ensure adequate precipitation without excessive grain coarsening. Post-weld tempering is often required to stabilize the microstructure and reduce residual stresses.

Process Control Strategies

Preheating and Interpass Temperature

Large support rolls are typically made of high-alloy steel with high carbon equivalent, making them susceptible to cracking during welding. Preheating to 200-400°C is generally required to reduce the cooling rate in the heat-affected zone and minimize the risk of cold cracking. The interpass temperature should be maintained within a controlled range (typically 150-300°C) to prevent excessive grain coarsening while ensuring adequate plasticity for stress relief.

Welding Sequence

The welding sequence for large cylindrical rolls is critical for controlling distortion and residual stress distribution. A common approach is to use a spiral or helical pattern, starting from one end and progressing to the other, with overlapping passes to ensure uniform coverage. The sequence should be designed to minimize the accumulation of residual stresses in any particular direction and to promote uniform thermal distribution around the roll circumference.

Post-Weld Heat Treatment

Post-weld heat treatment is essential for large support rolls to relieve residual stresses and stabilize the microstructure. A typical PWHT cycle involves heating to 550-650°C for a holding time of 2-4 hours per inch of thickness, followed by controlled cooling. The PWHT temperature must be carefully selected to avoid over-tempering the surfacing alloy, which could reduce hardness and wear resistance.

Equipment Capability Requirements

The authors emphasize that even with optimal material selection and process parameters, the absence of appropriate equipment can lead to repair failure. Key equipment requirements include:

• Power source: Must provide stable current and voltage output with minimal ripple, as fluctuations in current can cause arc instability and inconsistent penetration.
• Wire/strip feed system: Must maintain constant feed speed with minimal variation, as speed fluctuations directly affect heat input and dilution rate.
• Torch positioning: Must maintain consistent torch angle and stand-off distance throughout the welding pass, which is particularly challenging on large cylindrical surfaces.
• Flux delivery system: Must ensure uniform flux coverage over the entire weld seam, preventing arc exposure and slag inclusion.
• Preheating and temperature monitoring: Must provide uniform preheating of the roll and continuous monitoring of preheat and interpass temperatures.

Engineering Practice Integration

The paper's emphasis on the three-factor approach—material, process, and equipment—reflects a mature engineering philosophy that recognizes the interdependence of these factors. In practice, failures in support roll repair are often traced to the neglect of one or more of these factors. For example:

• Selecting a hardfacing alloy with insufficient thermal fatigue resistance leads to cracking during service.
• Using excessive heat input without adequate preheating causes cracking in the heat-affected zone.
• Employing equipment with poor feed speed stability results in inconsistent deposit composition and properties.

FMEA-Based Approach to Repair Planning

Applying Failure Mode and Effects Analysis (FMEA) to support roll repair planning can systematically identify and mitigate potential failure modes:

Key Questions and Reflections

The paper raises an important point about the systemic nature of repair operations. In many industrial settings, repair is treated as a simple welding operation, with minimal attention to the broader engineering context. However, the authors correctly identify that successful repair requires a coordinated approach that integrates metallurgical knowledge, process engineering, and equipment capability. This perspective is particularly relevant for large, critical components such as support rolls, where repair failure can result in significant production losses and safety risks.

A question that arises from this work is the role of surface preparation in repair quality. The paper does not extensively discuss surface preparation methods, such as machining, grinding, or thermal cleaning, which are critical for ensuring good fusion between the surfacing layer and the base material. In practice, inadequate surface preparation is a common cause of repair failure, and its importance should be emphasized in repair procedures.

Study Insights and Implications

This paper provides a valuable framework for approaching the repair of large, critical components in metallurgical equipment. The three-factor approach—material, process, and equipment—offers a structured methodology that can be applied to other repair scenarios. For engineers involved in equipment maintenance and repair, the key takeaway is that successful repair is not simply a matter of applying a suitable welding consumable; it requires a comprehensive engineering approach that considers all aspects of the repair process. The emphasis on equipment capability is particularly noteworthy, as it highlights the often-overlooked importance of process equipment in achieving consistent repair quality.