Wear-Resistant Hardfacing Technology for Vertical Roller Mill Rollers and Grinding Tables

Literature Overview

This paper by Zhang Kunmou, Zheng Guoliang, and Peng Xinqiao, published in the New Century Cement Herald (2005, Vol. 11, No. 6), addresses the wear problem in vertical roller mills (VRM), which have become increasingly popular in cement grinding due to their significant energy savings. The authors, representing Kunshan Huifeng Wear-Resistant Industrial Co., Ltd., a specialist in wear-resistant hardfacing technology, discuss the challenges posed by irregular wear patterns on mill rollers and grinding tables, the resulting increase in roller-table gap, declining operational efficiency, rising energy consumption, and ultimately the forced shutdown for maintenance. This paper provides practical insights into hardfacing technology for VRM components, drawing on the authors' industrial experience.

Core Technical Points on VRM Component Wear

Vertical roller mills operate under extreme conditions: high pressure (typically 80-150 bar), high temperature (up to 150-200°C in the grinding zone), and continuous abrasive contact with cement clinker and other grinding media. The rollers and grinding table are subjected to a combination of abrasive wear, adhesive wear, and fatigue spalling. The irregular nature of the wear pattern is particularly problematic because it leads to uneven contact between the roller and table, which in turn causes vibration, noise, and further accelerated wear.

Wear Mechanisms in VRM Components

The following table summarizes the primary wear mechanisms affecting VRM rollers and grinding tables:

The authors correctly identify that the irregular wear pattern is the most challenging aspect of VRM component wear. Unlike uniform wear that can be predicted and managed through regular maintenance, irregular wear creates localized high-stress areas that accelerate further degradation. This is a classic example of a positive feedback loop in wear engineering, where an initial small deviation leads to progressively worse conditions.

Hardfacing Alloy and Process Selection for VRM Applications

For VRM rollers and grinding tables, the hardfacing requirements are more demanding than for roller presses due to the higher pressures and more severe thermal cycling. The following table presents typical hardfacing specifications for VRM applications:

The selection of hardfacing alloy must balance hardness (for wear resistance) against toughness (for resistance to spalling and cracking). In VRM applications, a high-carbon chromium alloy is typically preferred for the primary wear surfaces, while a medium-carbon transition layer is applied between the base material and the hardfacing layer to reduce residual stresses and improve adhesion.

Welding Process Considerations

The welding process used for VRM component hardfacing must be capable of handling thick overlay builds (15-25 mm) while maintaining consistent quality. Submerged arc welding (SAW) is the most commonly used process for this application due to its high deposition rate and ability to produce thick, uniform overlay layers. However, the process parameters must be carefully controlled to minimize residual stresses and prevent cracking.

Typical SAW Parameters for VRM Hardfacing

The preheat and interpass temperature control is critical for preventing cold cracking in the weld metal and heat-affected zone. For high-carbon base materials such as 42CrMo, a preheat of at least 200°C is typically required. The interpass temperature should be maintained within a narrow range to avoid excessive grain growth while still allowing sufficient stress relief between passes.

Maintenance and Repair Strategy

The paper emphasizes that VRM component wear management requires a proactive maintenance strategy rather than reactive repair. The following maintenance protocol is recommended:

1. Scheduled inspection of roller and table surfaces at defined intervals (typically every 2000-4000 operating hours) using ultrasonic thickness measurement and visual inspection.
2. Wear rate monitoring by plotting overlay thickness versus operating hours to establish a predictive maintenance schedule.
3. Surface profiling to detect irregular wear patterns and identify potential root causes (misalignment, feed distribution issues, etc.).
4. Timely re-hardfacing before the overlay is worn through to the transition layer or base material.
5. Post-weld heat treatment to relieve residual stresses and improve the toughness of the hardfacing layer.

Common Defects and Countermeasures

Engineering Practice Integration

In practical VRM operations, the hardfacing of rollers and grinding tables is often performed during scheduled maintenance shutdowns. The process typically involves grinding the worn surface to a uniform profile, cleaning and preparing the surface, applying the transition layer, building up the hardfacing layer in multiple passes, and finally machining the surface to the required profile and finish. The entire process can take 3-7 days depending on the size of the component and the required overlay thickness.

The authors' experience suggests that the key to successful VRM hardfacing is not just the welding process itself, but the entire maintenance workflow. This includes proper planning of the maintenance schedule, availability of qualified welders and equipment, quality control of consumables, and thorough inspection of the finished overlay. A well-executed hardfacing program can extend the service life of VRM components by 3-5 times compared to unhardfaced surfaces, significantly reducing maintenance costs and downtime.

Study Insights and Implications

The most significant insight from this paper is the recognition that irregular wear patterns in VRM components are a systemic problem that requires a holistic approach to solve. Simply applying a harder overlay layer is not sufficient if the root causes of irregular wear (such as misalignment or feed distribution problems) are not addressed. The paper's emphasis on maintenance and repair strategy, rather than solely on welding technology, reflects a mature understanding of how hardfacing performs in real industrial environments.

For engineers working with VRM systems, the key takeaway is that hardfacing technology must be integrated into a comprehensive wear management program. This includes proper alloy selection, process control, maintenance scheduling, and root cause analysis of wear patterns. Only through such an integrated approach can the full potential of hardfacing technology be realized in VRM applications.