Development and Application of Overlay Welded Composite Grinding Rollers for Vertical Mills

Literature Overview and Industrial Context

This study, authored by Wu Hong from Xi'an University of Architecture and Technology and Li Wenjie from Tongchuan Shengwei Building Materials Co., Ltd., was published in the journal "Cement" in 2013. The research addresses a critical component in the cement industry: the grinding rollers used in vertical roller mills (VRMs). These rollers are subjected to extreme abrasive wear from the continuous grinding of cement clinker, and their service life directly impacts the operational efficiency and cost of cement production. The development of overlay welded composite grinding rollers represents a practical engineering solution to extend roller life and reduce replacement frequency.

Vertical roller mills are widely used in modern cement plants for the grinding of clinker, raw materials, and coal. The grinding rollers, typically made of medium-carbon steel or low-alloy steel, experience severe abrasion from the interaction with the grinding table and the material being ground. Traditional grinding rollers, without overlay protection, may last only 3-6 months before requiring replacement or refurbishment. The overlay welding of wear-resistant materials onto the roller surface provides a cost-effective means to extend service life by 2-5 times, depending on the application and operating conditions.

Overlay Welding Process and Consumable Selection

The overlay welding process for grinding rollers typically employs submerged arc welding (SAW) or flux-cored arc welding (FCAW) due to the high deposition rate and deep penetration characteristics of these processes. The consumable selection is critical and must be tailored to the specific abrasion mechanism encountered in the cement grinding application.

The overlay layer composition is typically a high-carbon, high-chromium martensitic alloy, which provides a combination of high hardness and adequate toughness. The carbon content is generally in the range of 1.5-2.5 wt%, and the chromium content is 10-15 wt%, which promotes the formation of hard M7C3 and M23C6 carbides. The volume fraction of carbides in the overlay layer is typically 30-50%, which is the primary contributor to the wear resistance.

The welding procedure for grinding roller overlay is carefully designed to ensure good bonding and minimal residual stress. The base roller is typically preheated to 200-300 degrees Celsius to reduce the risk of cracking. The overlay is applied in multiple passes, with the first pass serving as a transition layer between the base metal and the wear-resistant overlay. The interpass temperature is maintained at 150-250 degrees Celsius to prevent excessive hardness in the heat-affected zone.

Performance Evaluation and Service Life Assessment

The performance of the overlay welded grinding roller is evaluated through a combination of laboratory testing and field trials. Laboratory testing includes hardness profiling across the overlay layer, metallographic examination of the microstructure, and wear testing using standardized methods such as the pin-on-disc test or the dry sand rubber wheel test. Field trials involve monitoring the roller in actual cement grinding service and measuring the wear rate over a defined period.

The wear rate of the overlay layer is typically expressed in terms of material loss per unit time, measured in millimeters per 1000 hours of operation. For cement grinding applications, a well-designed overlay layer should exhibit a wear rate of less than 0.5 mm per 1000 hours, compared to 1.5-3.0 mm per 1000 hours for uncoated steel rollers. The improvement in service life is directly proportional to the reduction in wear rate and can translate to significant cost savings in terms of reduced replacement frequency and downtime.

Common defects in overlay welded grinding rollers include cracking at the bond line, spalling of the overlay layer, and excessive dilution. Cracking is typically caused by high residual stresses, excessive heat input, or inadequate preheating. Spalling is often related to poor bonding quality or thermal fatigue from repeated heating and cooling cycles. Excessive dilution reduces the hardness and wear resistance of the overlay layer and can be mitigated by optimizing the welding parameters and using a transition layer.

Engineering Practice and Quality Control

For engineers implementing overlay welded grinding rollers in cement plants, a systematic approach to quality control is essential. The following practices are recommended:

1. Base roller preparation: The roller surface must be cleaned and ground to remove rust, scale, and contaminants. The surface roughness should be within acceptable limits to ensure good bonding.
2. Welding procedure qualification: The welding procedure should be qualified in accordance with relevant standards, including mechanical property testing, microstructural examination, and wear testing of the deposited layer.
3. In-process monitoring: During production welding, the welding parameters should be monitored and recorded to ensure consistency. The interpass temperature should be measured and maintained within the specified range.
4. Post-weld inspection: Non-destructive testing, including visual inspection, magnetic particle testing, and ultrasonic testing, should be performed to detect surface and subsurface defects.
5. Hardness verification: Hardness measurements should be taken at multiple locations across the overlay layer to verify uniformity and compliance with specifications.

Study Insights and Implications

This research by Wu Hong and Li Wenjie demonstrates the practical value of overlay welding technology in extending the service life of critical industrial components. The development of composite grinding rollers for vertical mills represents a successful example of academic research translated into industrial application, with direct economic benefits to cement manufacturers. The study also highlights the importance of collaboration between academia and industry in solving real-world engineering problems.

For engineers in the cement and mining industries, the overlay welded composite grinding roller offers a proven solution to the challenge of abrasive wear. The key to success lies in the careful selection of consumables, the optimization of welding parameters, and the implementation of rigorous quality control procedures. Future developments in this area may include the application of advanced overlay techniques such as laser cladding and plasma transferred arc welding, which offer finer microstructural control and potentially superior wear resistance.