Offline Weld Overlay Device for Overall Roller Sleeves of Vertical Mills

Literature Overview

This technical report, authored by Luan Jiangtao and Gan Aijun from Nanyang Zhonglian Cement Co., Ltd. and Luan Chenchen from Sichuan University, published in 2020, presents an innovative offline weld overlay solution for the overall roller sleeves of vertical grinding mills in the cement industry. Vertical mills are critical equipment in cement production, where the grinding rollers endure severe abrasive wear from raw materials and clinker. Traditional repair methods involving in-situ welding or partial replacement of worn segments are labor-intensive, time-consuming, and often result in inconsistent surface quality. This work introduces an offline approach that removes the roller sleeve from the mill, performs comprehensive weld overlay in a controlled workshop environment, and then reinstalls the refurbished sleeve.

Core Technical Points

Wear Mechanisms and Material Requirements

The grinding rollers in vertical mills operate under extreme conditions characterized by high contact stress, abrasive sliding, and impact loading. The primary wear mechanisms include:

Abrasive wear: Caused by hard particles in the cement raw meal and clinker.
Adhesive wear: Resulting from metal-to-metal contact at high temperatures.
Impact fatigue: Leading to surface cracking and spalling over extended service.

The base material of the roller sleeve is typically a medium-carbon steel or low-alloy steel (such as 42CrMo or 40Cr), which provides adequate strength and toughness. The overlay material must offer superior wear resistance while maintaining sufficient toughness to resist impact damage. Common overlay materials include:

Overlay Material	Hardness (HRC)	Typical Application
High-carbon martensitic steel (e.g., 65Mn-based)	45–55	General abrasive wear
Cr-Mo alloy steel (e.g., 42CrMo-based)	40–50	Combined abrasion and impact
Ductile iron (high-silicon)	55–65	Severe abrasion, low impact
Tungsten carbide composite	65–75	Extreme abrasion
Hardfacing alloy (e.g., Ni-Cr-B-Si)	50–60	High-temperature abrasion

Equipment Design and Process Flow

The offline weld overlay device is designed to accommodate the large diameter (typically 600–1200 mm) and length (typically 1000–3000 mm) of roller sleeves. The equipment incorporates:

Rotating chuck system: Capable of securely clamping the roller sleeve and rotating it at controlled speeds to ensure uniform circumferential coverage.
Multi-torch configuration: Multiple welding torches arranged circumferentially to achieve complete coverage in a single rotation, significantly improving productivity.
Wire feeding and arc control: Robust wire feeding mechanisms with high feed rates to match the high deposition requirements, combined with arc stability control to maintain consistent bead quality.
Cooling system: Integrated cooling to manage interpass temperatures and prevent excessive heat-affected zone (HAZ) softening.

The typical process flow follows these steps:

Removal and inspection: The worn roller sleeve is removed from the mill and inspected for cracks, deformation, and base metal condition.
Surface preparation: Grinding to remove damaged surface layers, followed by thorough cleaning to ensure proper fusion.
Preheating: Controlled preheating to 150–250°C to reduce residual stresses and prevent cold cracking.
Weld overlay: Multi-pass welding to build up the required thickness, typically 5–15 mm depending on the wear rate and service life requirements.
Post-weld heat treatment: Stress relief annealing to reduce residual stresses and improve toughness.
Machining: Precision grinding to achieve the final dimensional accuracy and surface finish.
Quality inspection: NDT and mechanical testing to verify overlay quality.

Process Parameters

Parameter	Typical Value	Notes
Welding process	SAW or FCAW	High deposition rate required
Current	400–600 A	High current for productivity
Voltage	30–40 V	Adjusted for bead geometry
Travel speed	200–500 mm/min	Depends on deposition rate
Wire diameter	1.6–2.4 mm	Thick wire for high deposition
Number of passes	3–6	Building to required thickness
Interpass temperature	150–250°C	Controlled to prevent cracking
Post-weld heat treatment	550–650°C, 2–4 h	Stress relief

Quality Control and Defect Analysis

The weld overlay of roller sleeves is susceptible to several common defects:

Cracking: Hot cracking in the overlay layer due to high sulfur and phosphorus content in the base metal, or cold cracking in the HAZ due to high carbon equivalent and inadequate preheat. Countermeasures include strict control of base metal chemistry, adequate preheating, and selection of appropriate filler materials with low diffusible hydrogen content.
Porosity: Caused by inadequate shielding gas coverage, contaminated base metal surface, or excessive travel speed. Countermeasures include proper surface preparation, adequate gas flow rates, and optimization of travel speed.
Lack of fusion: Resulting from insufficient heat input, poor base metal cleaning, or excessive travel speed. Countermeasures include increased current, thorough surface preparation, and reduced travel speed.
Undercut: Common in vertical and overhead positions, leading to stress concentration and potential crack initiation. Countermeasures include proper torch angle control and post-weld grinding.

Non-destructive testing typically includes magnetic particle inspection (MT) for surface and near-surface cracks, ultrasonic testing (UT) for subsurface defects, and hardness profiling across the overlay thickness to verify the hardness gradient and detect HAZ softening.

Engineering Practice and Reflections

The offline approach to roller sleeve refurbishment offers several advantages over traditional in-situ methods:

Improved quality control: The controlled workshop environment allows for better process parameter control, more thorough inspection, and more reliable quality assurance.
Higher productivity: Multi-torch configurations and optimized process parameters enable significantly faster refurbishment cycles, reducing mill downtime.
Extended service life: The comprehensive overlay of the entire sleeve surface, rather than partial repair of worn areas, provides more uniform protection and extends the service interval.
Reduced operator exposure: Performing the welding in a workshop rather than inside the mill reduces operator exposure to dust, noise, and confined space hazards.

However, the offline approach also introduces logistical challenges, including the need for crane capacity to remove and reinstall the heavy roller sleeves, transportation to the workshop, and coordination with mill maintenance schedules. The FMEA (Failure Mode and Effects Analysis) approach is valuable in identifying potential failure modes at each step of the process and implementing preventive measures.

The collaboration between an industrial user (Nanyang Zhonglian Cement) and an academic institution (Sichuan University) exemplifies the productive synergy between industry and academia in addressing practical engineering challenges. The industrial partner provides the operational context and performance requirements, while the academic partner contributes metallurgical expertise and research capabilities.

Summary

The offline weld overlay device for vertical mill roller sleeves represents a significant advancement in the maintenance and refurbishment of cement grinding equipment. By combining robust equipment design, optimized process parameters, and rigorous quality control, this approach achieves higher quality, greater productivity, and longer service life compared to traditional in-situ repair methods. Engineers in the cement industry and related heavy equipment maintenance sectors should consider adopting similar offline refurbishment strategies for other large rotating components subject to severe wear. The key to success lies in the integration of metallurgical knowledge, equipment engineering, and systematic quality management.