Surfacing Treatment Technology for Ball-Free Mill Rotor Disc

Literature Overview

This paper by Li Kunshan from the School of Mechanical Engineering, University of Jinan, published in Materials Protection (2002, Vol. 35, No. 8, pp. 60-60), presents a technical case study on surfacing treatment applied to ball-free mill rotor discs. The study addresses the wear problem in ball-free mills (also known as pin mills or rod mills) through application of appropriate surfacing technology to extend component service life.

Core Technical Content

Ball-Free Mill Operating Environment

Ball-free mills represent an important class of grinding equipment used in cement, mineral processing, and chemical industries. Unlike ball mills that use spherical grinding media, ball-free mills employ cylindrical or rod-shaped media that create both impact and shear grinding actions. The rotor disc (or grinding table) in these mills operates under:

• Continuous abrasive contact with grinding media and material
• High centrifugal loading during operation
• Impact loading from falling media
• Varying material hardness and abrasiveness
• Continuous rotation creating complex stress states

The wear pattern on rotor discs is typically characterized by:

• Central wear zone with highest material removal
• Radial wear grooves from media movement
• Edge wear from material spillage
• Selective wear exposing substrate below surface layers

Surfacing Technology Selection

For ball-free mill rotor discs, surfacing technology selection must consider:

Common surfacing approaches for this application include:

• Submerged arc surfacing with hardfacing wire (high deposition rate, suitable for thick deposits)
• Shielded metal arc surfacing with hardfacing electrode (flexible, suitable for field application)
• Flame spraying (surface preparation required, limited to thin coatings)
• Plasma transfer arc surfacing (good quality control, moderate deposition rate)

Performance and Wear Resistance

The surfacing treatment applied to ball-free mill rotor discs must provide:

• Surface hardness of 50-60 HRC minimum for effective abrasion resistance
• Adequate toughness to prevent spalling under impact loading
• Uniform surface profile to maintain consistent grinding action
• Resistance to adhesive wear from material transfer

The paper demonstrates that appropriate surfacing treatment significantly extends rotor disc service life, reducing both replacement frequency and production downtime. The economic benefit is particularly significant for large mill installations where rotor disc replacement requires extensive disassembly and realignment.

Engineering Practice Integration

In my experience with grinding mill maintenance, the following practical considerations are important for surfacing applications on rotor discs:

1. Surface preparation: The worn surface must be thoroughly cleaned and prepared before surfacing. This typically involves:

• Removal of loose scale and contamination
• Grinding of worn surface to remove damaged material
• Creation of appropriate surface profile for weld metal adhesion
• Degreasing to ensure flux or shielding gas effectiveness

1. Dimensional considerations: Rotor discs must maintain precise dimensional accuracy for proper mill operation. Surfacing must account for:

• Original design thickness for wear allowance
• Required flatness tolerance for grinding efficiency
• Runout tolerance for vibration control
• Post-surfacing machining requirements

1. Thermal distortion control: Large rotor discs are susceptible to thermal distortion during surfacing. Countermeasures include:

• Backing plate or chucking support during surfacing
• Symmetric surfacing pattern to balance thermal input
• Controlled interpass temperature to limit thermal gradients
• Post-surfacing stress relief if distortion is observed

1. Consumable selection strategy: The choice between impact-resistant and abrasion-resistant consumables depends on the specific wear mechanism:

• Abrasion-dominant service: High-carbide consumables (Cr-C, Ni-Cr-C)
• Impact-dominant service: Tougher consumables with lower hardness
• Mixed service: Balanced consumables providing both properties

Key Reflections and Study Insights

This paper, though brief (single page), addresses a practically important application of surfacing technology in heavy industry. The ball-free mill rotor disc represents a component where surfacing repair or protective coating provides significant economic benefit through extended service life.

The 2002 publication date reflects an era when surfacing technology for grinding mill components was still developing, with much reliance on empirical approaches. Modern practice has evolved significantly with:

• Better understanding of wear mechanisms in different grinding applications
• More sophisticated consumable formulations with tailored properties
• Improved process control through automated surfacing systems
• Advanced characterization techniques for evaluating surfacing quality

The fundamental principle remains unchanged: appropriate surfacing technology, correctly applied, can extend the service life of critical grinding mill components by 2-5 times compared to unprotected surfaces, providing significant economic benefit through reduced maintenance frequency and improved production continuity.

The paper's contribution lies in documenting practical surfacing solutions for a specific industrial application, providing reference data for engineers facing similar challenges in grinding mill maintenance. While the technical details are limited by the brief format, the practical demonstration of successful surfacing application provides valuable confidence for similar implementations.

This case illustrates the broader principle that surfacing technology serves as a versatile tool for component life extension across diverse industrial applications, from high-value precision components to heavy-duty industrial equipment, provided that appropriate consumable selection and process control are applied to match the specific service requirements.