Overlay Welding Repair Technology for Vertical Roller Mill Liner Plates

Literature Overview

This paper, published in China Cement (Issue 9, 2006, pp. 63–65) by authors from Tangshan Jidong Cement Co., Ltd. and Tangshan Metallurgical Mining Machinery Factory, presents a practical engineering case study on the overlay welding repair of vertical roller mill (VRM) liner plates in a cement grinding system. The study addresses the significant economic challenge of liner plate wear in cement production, where the continuous grinding of abrasive cement clinker and raw materials causes severe wear on the mill disc and roller liner surfaces. The paper discusses the repair methodology, including the Smith company's overlay welding technology, and evaluates the effectiveness of the repair in terms of production capacity restoration.

Vertical Roller Mill Operating Environment and Wear Mechanisms

Vertical roller mills are critical equipment in modern cement production, used for both raw material grinding and clinker grinding. The operating environment is extremely demanding:

The wear mechanisms in VRM liner plates include:

• Abrasive wear: The dominant mechanism, caused by the sliding and rolling of abrasive particles against the liner surface. This produces directional wear patterns and surface grooving.
• Impact wear: Caused by the impact of material particles and the rolling pressure between the mill rollers and disc.
• Fatigue wear: Cyclic loading leads to subsurface crack initiation and spalling.
• Corrosive-abrasive wear: In wet grinding or high-temperature environments, chemical attack can accelerate wear.

Overlay Welding Repair Methodology

The repair process described in the paper follows a systematic approach:

Pre-Repair Preparation

1. Inspection and assessment: The worn liner plate is inspected to determine the extent of wear, the remaining base material thickness, and the presence of cracks or fatigue damage.
2. Surface preparation: The worn surface is prepared by grinding or machining to remove the fatigue layer, oxidation scale, and any contaminated material. This step is critical because welding onto a contaminated or fatigued surface leads to poor bond quality and early failure.
3. Crack repair: Any cracks found during inspection are repaired by grinding out the crack and filling with appropriate weld metal before the overlay welding.

Overlay Welding Process

The paper references the Smith company's overlay welding technology, which typically involves:

• Welding process: Submerged arc welding (SAW) or flux-cored arc welding (FCAW) are commonly used for overlay welding of VRM liner plates due to their high deposition rates and good penetration.
• Electrode/wire selection: The overlay material is typically a high-chrome cast iron (e.g., A2, A5 grade) or a high-manganese alloy, chosen for its excellent abrasion resistance.
• Welding parameters: Controlled heat input, appropriate welding speed, and proper travel technique are essential to achieve a dense, defect-free overlay with minimal dilution.
• Multi-pass welding: For thick overlay layers, multiple passes are applied with controlled interpass temperature to minimize cracking and ensure uniform composition.

Post-Weld Processing

After overlay welding, the surface is typically machined to the required dimensions and profile. The machining also helps to remove any surface defects and ensure dimensional accuracy for proper mill operation.

Performance Evaluation and Economic Analysis

The paper reports that the repaired liner plates restored the mill's production capacity (台时产量 - hourly output) to acceptable levels. The key performance indicators include:

The economic benefits of overlay welding repair compared to complete liner plate replacement are substantial:

• Cost savings: Repair costs are typically 30-50% of the cost of new liner plates.
• Downtime reduction: Repair can be performed in-situ or with minimal disassembly, reducing mill downtime.
• Extended service life: Properly executed overlay welding can restore liner plates to near-original condition, extending service life by 12-24 months.

Engineering Practice Considerations

Quality Control

The quality of the overlay welding repair is critical to its long-term success. Key quality control measures include:

• Visual inspection (VT): All welds are visually inspected for surface defects, undercut, and porosity.
• Magnetic particle testing (MT): Used to detect surface and near-surface cracks in the overlay and heat-affected zone.
• Hardness testing: The overlay hardness should be verified to ensure the correct material was deposited and proper cooling rates were achieved. Typical overlay hardness for high-chrome iron is 50-60 HRC.
• Dimensional inspection: The machined surface must meet the required profile and tolerance specifications for proper mill operation.

Common Defects and Countermeasures

Study Insights and Implications

This paper, while relatively brief, addresses a critical practical issue in cement production that has significant economic implications. The overlay welding repair of VRM liner plates is a well-established practice in the cement industry, but the quality and effectiveness of the repair depend heavily on proper surface preparation, appropriate material selection, and skilled welding execution. The reference to Smith company's technology highlights the importance of using proven, standardized repair procedures rather than ad-hoc approaches. For maintenance engineers in cement plants, this work reinforces the value of overlay welding as a cost-effective repair strategy that can significantly extend the service life of critical grinding components. The key lesson is that the success of overlay welding repair is not just about depositing material—it is about creating a metallurgically sound bond between the repair material and the base substrate, which requires careful attention to surface preparation, welding parameters, and post-weld treatment.