Offline Overlay Welding Device for Vertical Mill Roller Sleeve

Literature Overview and Background

This study focuses on the offline repair and overlay welding of roller sleeves used in vertical roller mills (VRMs), which are critical grinding equipment in cement, coal, and mineral processing industries. The roller sleeve, a heavy cylindrical component typically weighing 8–15 tons, is subjected to extreme abrasive wear from the grinding of hard materials such as limestone, coal, and fly ash. When the wear surface reaches the minimum allowable thickness, the sleeve must be either replaced or refurbished through overlay welding. The offline welding device developed in this study enables the repair of roller sleeves without removing them from the mill, significantly reducing downtime and maintenance costs.

The challenge lies in the geometry and weight of the roller sleeve: it is a large-diameter cylinder (typically 1.2–2.4 m) with a length of 1.5–3.0 m, and the wear surface is the outer cylindrical surface. Conventional repair methods require either removing the sleeve from the mill (causing 1–2 weeks of production downtime) or performing on-site welding with limited access and poor ergonomics. The offline device addresses both issues by allowing the sleeve to be welded in a dedicated workshop while the mill remains operational.

Device Design and Operational Principles

Mechanical Configuration

The offline welding device consists of a heavy-duty turntable, a welding torch carriage, a support cradle, and an integrated fume extraction system. The roller sleeve is placed on the turntable, which rotates it at a controlled speed to present the wear surface to the welding torch. The torch carriage moves axially along the sleeve length, guided by a linear rail system, to achieve uniform coverage of the entire cylindrical surface.

Welding Process and Consumables

The device is configured for submerged arc welding (SAW) with a wire-rod combination, which provides the highest deposition rate suitable for the thick overlay layers required on roller sleeves. The typical overlay thickness is 15–25 mm, applied in multiple passes with a wire-rod SAW process that deposits 3–5 mm per pass.

The consumable selection is critical for achieving the desired wear resistance. For cement grinding applications, a high-chromium martensitic or austenitic-ferritic overlay is preferred, with a hardness of 45–55 HRC. For coal grinding, a more ductile austenitic overlay with a hardness of 30–40 HRC is selected to resist thermal cracking from hot coal dust.

Process Optimization and Quality Assurance

Multi-Pass Welding Strategy

The multi-pass welding strategy is designed to achieve a uniform overlay thickness with minimal residual stress and no cracking. The strategy involves three stages: a transition layer, a build-up layer, and a wear-resistant cap layer.

The transition layer is essential because the base roller sleeve is typically a low-carbon or low-alloy steel (e.g., Q345R or 16MnR), which has a low carbon equivalent and is susceptible to cracking if directly welded with a high-carbon, high-chromium overlay. The transition layer acts as a metallurgical buffer, reducing the dilution effect and preventing the formation of brittle intermetallic phases at the interface.

Defect Analysis and Countermeasures

The study identifies and addresses the following defect modes:

1. Cracking at the transition layer interface — Caused by high dilution of the base steel into the transition layer, resulting in a high-carbon, high-hardness zone with poor toughness. Countermeasure: use a low-dilution welding technique (e.g., short arc, low travel speed) and select a transition layer composition with high austenite stability (e.g., E309L with 25% Ni).
2. Undercut at the pass boundary — Caused by insufficient overlap between adjacent passes or excessive travel speed. Countermeasure: maintain a pass overlap of at least 50% of the bead width and monitor the travel speed with a feedback loop.
3. Porosity in the overlay layer — Caused by flux contamination, moisture, or inadequate arc shielding. Countermeasure: store flux in a dry cabinet at 150–200°C, preheat the flux before use, and ensure the flux cover is maintained throughout the welding process.
4. Residual stress and distortion — Caused by the large heat input and asymmetric heating of the cylindrical sleeve. Countermeasure: use a multi-directional welding sequence (alternating between the top and bottom of the sleeve) and apply post-weld stress relief at 550–600°C for 2 hours per 25 mm of thickness.

Quality Verification Protocol

Engineering Practice and Case Studies

The offline welding device has been deployed at a major cement plant in China for the refurbishment of vertical roller mill sleeves. The plant operates three VRMs with a total annual production of 3.5 million tons of cement. The typical sleeve replacement cycle was 18–24 months, requiring 10–14 days of downtime per replacement. After adopting the offline welding device, the sleeve refurbishment cycle was extended to 30–36 months, and the downtime was reduced to 3–5 days (primarily for sleeve removal and installation, not welding).

The economic analysis shows a payback period of less than 6 months for the offline welding device investment, considering the reduction in downtime costs, material savings, and extended service life. The device also reduces the environmental impact by eliminating the need to manufacture and ship new sleeves, which involves significant energy consumption and carbon emissions.

Key Questions and Reflections

The study raises several important considerations for further development. First, the device currently requires manual loading and unloading of the roller sleeve, which involves heavy lifting and precise positioning. An integrated crane system with automated positioning would reduce the risk of injury and improve turnaround time. Second, the welding process is currently limited to cylindrical surfaces. For sleeves with internal grooves or external features (e.g., scraper rings), the torch carriage would need additional degrees of freedom to navigate complex geometries.

Third, the residual stress distribution in the refurbished sleeve after multi-pass welding is a concern. The study recommends that each refurbished sleeve undergo a comprehensive residual stress measurement using the hole-drilling method or neutron diffraction before returning to service. The residual stress should be below 100 MPa in the overlay layer and below 150 MPa in the heat-affected zone to prevent delayed cracking under operational loads.

From a standards perspective, the refurbishment process should be qualified per NB/T 47014 or ASME IX, with a specific WPS for the offline welding device. The qualification should include mechanical property testing of the overlay and HAZ, as well as non-destructive testing of the final weld. The refurbished sleeve should also undergo a hydrostatic test at 1.25 times the design pressure before returning to service, per GB/T 150 or ASME VIII Div.1 requirements.

Study Insights and Implications

This literature demonstrates that the offline welding device is a practical and cost-effective solution for refurbishing heavy-duty cylindrical components such as vertical mill roller sleeves. The key advantages are the elimination of production downtime, the reduction in material and labor costs, and the improvement in overlay quality compared to on-site repair. The multi-pass welding strategy with a transition layer, build-up layer, and wear cap layer ensures metallurgical compatibility and achieves the target wear resistance.

For engineering teams managing heavy equipment maintenance, the study provides a clear roadmap: invest in a dedicated offline welding device for high-value, high-wear components, develop a qualified welding procedure with a multi-pass strategy, and implement a rigorous quality verification protocol. The economic benefits are substantial, with payback periods typically under 12 months for plants with multiple VRMs. The study also highlights the importance of residual stress management and post-weld stress relief, which are often overlooked but critical for ensuring the long-term reliability of refurbished components.