Vertical Mill Overlay Composite Grinding Roller Development and Application

Literature Overview

This 2013 paper by Wu Hong from Xi'an University of Architecture and Technology and Li Wenjie from Tongchuan Shengwei Building Materials Co., Ltd. addresses the development and application of overlay composite grinding rollers for vertical mill (roller press) applications in cement production. Published in the journal Cement (2013, No. 1, pp. 67-68), the study focuses on a practical engineering problem faced by cement manufacturers operating large-scale production lines. The work is particularly relevant to engineers in the cement, mining, and mineral processing industries who deal with severe wear and impact loading conditions.

Core Technical Content

The application context involves two 5000 t/d cement production lines at Tongchuan Shengwei Building Materials, utilizing MPF2116 mid-speed mills for coal grinding. The original roller sleeves were made from KMTBCr20 high-chromium cast iron, a material widely used for its excellent wear resistance. However, the study identifies several critical limitations of the monolithic high-chromium alloy approach that lead to premature failure and high maintenance costs.

Comparison of Monolithic vs. Composite Roller Sleeve Design

Technical Analysis of the Problem

The failure modes of monolithic high-chromium alloy roller sleeves in vertical mill applications are well-documented in the industry. KMTBCr20 contains approximately 20% chromium, forming a high volume fraction of primary chromium carbides (M7C3) in a martensitic matrix. While this microstructure provides excellent resistance to abrasive wear, it suffers from several critical deficiencies:

1. Low fracture toughness. The high volume fraction of brittle carbides creates interconnected crack paths, making the material susceptible to catastrophic fracture under impact loading from large feed particles.
2. Poor resistance to thermal shock. The large temperature gradients between the hot gas inlet and the cooler roller surface generate thermal stresses that, combined with the material's low toughness, promote thermal fatigue cracking.
3. Casting defects. The high carbon and chromium content of KMTBCr20 leads to significant shrinkage during solidification, resulting in porosity, shrinkage cavities, and hot tears that act as crack initiation sites.
4. Difficult repairability. The poor weldability and machinability of high-chromium cast irons make in-situ repair impractical, forcing complete roller replacement and causing significant production downtime.

Composite Design Solution

The overlay composite approach addresses these limitations by combining a tough, weldable base material with a hard, wear-resistant overlay layer. The base material, typically a low-alloy steel or medium-carbon steel, provides:

• Excellent impact resistance and fracture toughness
• Good weldability for on-site repair
• Good machinability for manufacturing
• Resistance to casting defects

The overlay layer, applied by welding, provides the wear resistance required for the grinding surface. The overlay material selection depends on the specific wear mechanism:

• Abrasive wear: High-carbon martensitic or high-chromium overlay alloys
• Impact-abrasive wear: Medium-carbon martensitic or austenitic-martensitic overlay alloys
• Thermal fatigue: Austenitic or austenitic-ferritic overlay alloys with good thermal conductivity

Engineering Practice Implications

For cement plant engineers considering roller sleeve upgrades, several practical considerations emerge:

1. Overlay process selection. The choice of overlay welding process affects the quality and performance of the composite roller. Common processes include submerged arc welding (SAW), gas metal arc welding (GMAW), and flux-cored arc welding (FCAW). SAW provides excellent penetration and low dilution, while GMAW offers good flexibility and speed. The process must be selected based on the available equipment, the required overlay thickness, and the production schedule.
2. Overlay layer thickness optimization. The overlay thickness must be sufficient to provide wear protection throughout the expected service life but not so thick as to introduce excessive residual stresses that could lead to spalling. Typical overlay thicknesses for vertical mill rollers range from 20 to 50 mm, depending on the expected wear rate.
3. Pre-welding and post-welding heat treatment. Preheating is essential to prevent cracking in the base metal, and post-weld heat treatment may be required to relieve residual stresses in the overlay layer. The heat treatment parameters must be carefully controlled to avoid softening the overlay layer.
4. Surface preparation and finishing. The overlay surface must be ground to the required surface finish and dimensional accuracy before installation. The grinding process can also help relieve surface residual stresses and improve the surface hardness profile.

Study Insights and Reflections

This paper exemplifies the practical, problem-solving approach that characterizes much of the engineering literature in the cement and mineral processing industries. The transition from monolithic high-chromium rollers to composite overlay rollers represents a significant improvement in reliability and cost-effectiveness, and the principles described are directly applicable to similar applications in mining, power generation, and material processing.

The key insight is that the optimal material for a given application is not always a single homogeneous material. Composite designs, combining materials with different properties in different zones, can achieve performance levels that exceed what any single material can provide. This principle extends beyond roller sleeves to many other engineering components, including pump impellers, valve seats, and grinding media.

The industry-academia collaboration model demonstrated in this paper is also noteworthy. The practical problems identified by the cement manufacturer were addressed through engineering research at the university, and the solutions were implemented and validated in actual production. This cycle of problem identification, research, development, and implementation is the essence of effective engineering practice.