Vertical Roller Mill Liner Weld Overlay Repair Technology

Literature Overview and Technical Context

This 2006 study by Zhang Xinsheng, Gong Jinhua, Li Huansheng, and Chang Yongli from Tangshan Jidong Cement Co., Ltd. and Tangshan Metallurgical Mining Machinery Factory addresses the weld overlay repair of vertical roller mill (VRM) liners in cement grinding operations. Published in the journal China Cement, this research reflects the practical engineering challenges faced in the cement industry, where vertical roller mills are widely used for grinding raw materials and clinker.

Vertical roller mill liners are subjected to extreme operating conditions characterized by high-impact loading from rotating grinding rollers, abrasive wear from cementitious materials, and sustained compressive forces. The liner wear rate can reach 2–5 mm per month depending on the material being ground and the operating conditions, making periodic repair or replacement a significant maintenance cost. Weld overlay repair offers a cost-effective alternative to complete liner replacement, extending service life while reducing downtime and material costs.

Core Technical Analysis

Material Selection and Metallurgical Requirements

The selection of overlay materials for VRM liner repair is governed by the specific wear mechanisms present in the grinding environment. The primary wear mechanisms include abrasion from hard cement particles, impact from roller compression, and adhesive wear from material transfer.

The metallurgical compatibility between the base liner material (typically Q345 or similar low-alloy steel) and the overlay material is a critical consideration. Mismatched thermal expansion coefficients and dilatation characteristics can lead to cracking at the interface during thermal cycling. The dilution rate in the first overlay pass must be carefully controlled to ensure adequate hardness in the final layer while maintaining sufficient toughness to resist cracking.

Welding Process Selection and Parameters

The welding process selection for VRM liner repair must balance deposition rate, quality, and practicality for field conditions. The following table compares commonly used processes:

For VRM liner repair, GMAW and FCAW are most commonly employed due to their excellent field applicability, reasonable deposition rates, and manageable equipment requirements. The typical welding parameters for overlay repair include:

• Current: 250–400 A for GMAW, 300–500 A for FCAW
• Voltage: 25–35 V for GMAW, 28–38 V for FCAW
• Travel speed: 200–400 mm/min
• Wire feed rate: 4–8 m/min for GMAW, 5–10 m/min for FCAW
• Shielding gas: CO₂ or 80% Ar + 20% CO₂ for GMAW
• Preheat temperature: 100–200 °C for high-carbon overlay materials
• Interpass temperature: Maximum 250–350 °C to prevent excessive grain growth

Repair Strategy and Quality Control

The repair strategy for VRM liners involves several key considerations:

1. Wear assessment: The worn areas must be precisely identified and mapped to determine the required overlay volume and geometry. Ultrasonic thickness gauging and visual inspection are commonly used methods.
2. Surface preparation: The worn surface must be ground or machined to remove the work-hardened layer and provide a clean, sound substrate for welding. A bevel or groove preparation may be required for deep wear areas to ensure adequate fusion.
3. Multi-pass strategy: Thick overlay layers should be deposited in multiple passes to manage dilution, reduce residual stresses, and achieve the desired hardness profile. The first pass typically uses a transition material to reduce cracking susceptibility, while subsequent passes use the final overlay material.
4. Post-weld treatment: Depending on the overlay material, post-weld heat treatment may be required to relieve residual stresses, refine grain structure, or achieve the desired microstructure. For high-carbon overlay materials, controlled cooling or tempering treatment is often necessary.
5. Quality verification: Post-repair inspection should include visual examination, magnetic particle testing for surface cracks, and hardness testing to verify overlay layer properties.

Engineering Practice Integration

In cement mill operations, the downtime cost associated with liner replacement or repair is a major economic factor. A typical VRM liner replacement can require 2–5 days of downtime, during which the entire grinding circuit is non-productive. Weld overlay repair can reduce this downtime to 1–2 days for partial repairs, significantly improving plant availability.

The economic comparison between repair and replacement depends on several factors:

The decision to repair or replace should be based on a comprehensive analysis that includes the remaining useful life of the liner, the extent and location of wear, the availability of skilled welders and equipment, and the production schedule requirements.

Study Insights and Implications

This research from the cement industry provides practical insights into the challenges and solutions for weld overlay repair of heavy-duty grinding equipment. The emphasis on field-applicable welding processes, practical material selection, and systematic quality control reflects the engineering reality of maintenance operations where conditions are often less than ideal. For engineers involved in cement plant maintenance, this work offers a valuable framework for developing repair procedures that balance quality, cost, and schedule constraints. The broader lesson is that weld overlay repair technology, when properly applied, can significantly extend the service life of critical components while reducing overall maintenance costs, demonstrating the economic and technical value of welding engineering in industrial maintenance applications.