Study Note on Overlay Composite Grinding Roller Development for Vertical Mills

Literature Overview

This paper by Wu Hong from Xi'an University of Architecture and Technology and Li Wenjie from Tongchuan Shengwei Building Materials Co., Ltd., published in Cement (2013, Issue 1, pp. 67-68), describes the development and application of overlay composite grinding rollers for vertical mills in cement production lines. The study addresses the challenges encountered with monolithic KMTBCr20 high-chrome alloy roller sleeves in MPF2116 medium-speed mills used in two 5000 t/d production lines.

Problem Analysis

The original roller sleeves were cast from KMTBCr20 high-chrome alloy, a material known for its excellent wear resistance due to the presence of hard carbide particles (M7C3 type) in an austenitic or martensitic matrix. However, this material presents several significant challenges in the demanding environment of vertical mill grinding:

• Poor impact resistance: The high-chrome alloy is inherently brittle and susceptible to cracking under the severe impact loading from large feed particles.
• Casting defects: Monolithic castings inevitably contain porosity, shrinkage cavities, and segregation that act as stress concentrators and crack initiation sites.
• Cracking and fracture: Under cyclic loading and thermal cycling, the brittle matrix initiates and propagates cracks, leading to catastrophic failure.
• Uneven wear: The carbide distribution in the casting is non-uniform, leading to differential wear rates and uneven grinding surface geometry.
• Poor machinability: The hard carbide particles make the material extremely difficult to machine for dimensional accuracy.
• Poor weldability: The high carbon equivalent and brittle carbides make the material susceptible to cracking during welding repair.
• Low reusability: The combination of poor weldability and poor machinability limits the ability to rebuild worn surfaces, increasing lifecycle costs.

Composite Design Approach

The solution adopted was a composite roller design that combines the wear resistance of the high-chrome alloy with the toughness of a ductile base material. The composite structure typically consists of:

The overlay welding process used for the surface layer is selected based on the specific wear mechanism encountered in the vertical mill. For the grinding of cement clinker and raw materials, the wear mechanism is a combination of abrasive wear from hard particles and impact wear from particle collisions. The overlay material must therefore possess both high hardness and adequate toughness to resist both wear mechanisms.

Welding Process and Quality Control

The overlay welding process for the composite roller involves several critical steps:

1. Surface preparation: The core surface must be thoroughly cleaned and prepared to ensure good metallurgical bonding with the overlay layer. This may involve machining, grinding, or shot blasting.
2. Preheating: Due to the potential for thermal stresses between the core and overlay materials, preheating is essential to reduce the cooling rate and minimize cracking risk.
3. Multi-pass welding: Multiple overlay passes are typically applied to build up the required thickness while maintaining good dilution control.
4. Post-weld treatment: Stress relief annealing is recommended to reduce residual stresses and improve the toughness of the overlay layer.

Quality control measures include:

• Visual inspection for surface defects, cracks, and undercut.
• Magnetic particle testing (MT) or dye penetrant testing (PT) for surface and near-surface cracks.
• Hardness testing to verify the overlay layer hardness meets specifications.
• Impact testing of the overlay material to confirm adequate toughness.
• Dimensional inspection to ensure the grinding surface geometry is within tolerance.

Engineering Practice and Performance Results

The composite overlay roller design has demonstrated significant improvements over the monolithic high-chrome alloy rollers. The key benefits include:

• Improved impact resistance: The ductile core absorbs impact energy and prevents crack propagation.
• Reduced cracking: The gradual property transition eliminates the sharp interface that acts as a crack initiation site.
• Enhanced wear life: The overlay surface provides consistent wear resistance across the entire grinding surface.
• Improved reusability: Worn surfaces can be ground away and re-overlay welded, extending the roller service life.
• Cost reduction: The combination of longer service life and reusability significantly reduces the total cost of ownership.

Key Reflections

This case study illustrates the fundamental principle of composite material design in engineering: combining materials with complementary properties to achieve a performance that neither material can provide alone. The FMEA approach is evident in the analysis, where the failure modes of the monolithic roller (cracking, fracture, uneven wear) were identified and addressed through the composite design.

The practical significance of this work extends beyond the specific application. The approach of using overlay welding to create composite surfaces on wear parts is widely applicable across industries including mining, power generation, and material processing. Engineers should consider composite overlay solutions whenever a component is subjected to combined wear mechanisms that cannot be addressed by a single material.