Wear-Resistant Cladding of Toothed Rolls for Sintered Hot Ore Crushing

Literature Overview and Industrial Context

This 2003 study by researchers from Handan Iron and Steel Group Sintering Plant and CITIC Heavy Industries addresses a critical wear problem in the iron and steel industry: the rapid degradation of toothed rolls used for crushing sintered hot ore. Sintered hot ore is a key intermediate product in steelmaking, produced by agglomerating iron ore fines and burning the mixture to create a porous, friable material suitable for blast furnace charging. The crushing of sintered hot ore is performed using toothed rolls, which are heavy-duty rotating rollers with protruding teeth designed to grip and fracture the material.

The operating environment for toothed rolls is exceptionally harsh: the rolls operate at elevated temperatures (200-400°C), experience high impact loads from the falling material, and are subjected to severe abrasive wear from the hard, angular particles of sintered ore. Without effective wear protection, the roll teeth can lose 50-80% of their original profile in as few as 200-500 operating hours, necessitating frequent replacement and causing significant production losses.

Wear Mechanisms and Material Requirements

Understanding the specific wear mechanisms is essential for selecting the appropriate cladding material and process. The toothed rolls for sintered hot ore crushing experience a combination of wear mechanisms that must be addressed simultaneously:

The dominant wear mechanism is abrasive wear, accounting for approximately 60-75% of the total material loss. However, the contribution of impact and thermal wear is significant and must not be neglected in the material design. The cladding material must therefore be designed as a multi-functional surface layer that addresses all relevant wear mechanisms simultaneously.

Material Selection for Toothed Roll Cladding

The cladding materials evaluated for this application typically fall into two categories:

High-chromium cast iron based cladding:

• Composition: 12-28% Cr, 2.5-3.5% C, with Mo, V, or Ni additions
• Microstructure: Martensite matrix with M7C3 carbides
• Hardness: 55-65 HRC (as-cast), 60-70 HRC (after heat treatment)
• Advantages: Excellent abrasion resistance, good impact toughness, lower cost
• Disadvantages: Limited thermal stability above 400°C

Tungsten carbide composite cladding:

• Composition: 60-70% WC in a cobalt or nickel matrix
• Microstructure: Uniform WC particles in binder phase
• Hardness: 80-90 HRC (1500-2000 HV)
• Advantages: Exceptional abrasion resistance, good thermal stability
• Disadvantages: Higher cost, more brittle, requires careful process control

For the sintered hot ore crushing application, high-chromium cast iron cladding is generally the preferred choice due to its favorable balance of hardness, toughness, and cost. The impact loading conditions require a material with adequate toughness to resist chipping and spalling, which WC-Co composites may not provide without careful design.

Cladding Process Selection and Implementation

The cladding of toothed rolls requires a process that can handle the large diameter (typically 800-1500 mm) and heavy weight (10-50 tons) of the rolls, while achieving uniform coverage of the tooth surfaces. Several processes have been considered for this application:

For the toothed roll application, a combination of SAW and FCAW is typically employed. The bulk of the cladding deposit is applied using SAW for productivity, while the tooth tips and critical transition areas are finished with FCAW for better control and surface quality.

Process Parameters for Toothed Roll Cladding

The preheating of the roll body is critical to prevent cracking in the heat-affected zone and the cladding layer. The large mass of the roll means that heat dissipation is significant, and without adequate preheating, the cooling rate can exceed the critical value for cracking in high-carbon cladding materials. A preheat temperature of 200-300°C is typically sufficient to reduce the cooling rate below the critical threshold.

The cladding sequence is planned to minimize distortion and residual stress. The roll is typically cladded in sections, with the welding sequence arranged to balance the thermal input around the circumference. Back-step welding or symmetric welding patterns are employed to reduce angular distortion.

Quality Control and Performance Evaluation

The quality of the cladded toothed rolls is assessed through a comprehensive inspection program:

The hardness profile of the cladding layer is typically examined by taking cross-sections at multiple locations around the roll circumference and measuring hardness at intervals from the surface to the base metal. A well-executed cladding process should show a uniform hardness distribution across the cladding layer, with a gradual transition to the base metal hardness at the interface.

Field performance evaluation is conducted by measuring the wear rate of the cladded toothed rolls over a defined operating period. The wear rate is calculated as the volume of material removed per unit of material processed, expressed in units of mm³ per ton of ore crushed. Target wear rates for high-chromium cladded rolls are typically below 0.5 mm³/ton, compared to 2-5 mm³/ton for unclad steel rolls.

Engineering Practice and Operational Considerations

The successful implementation of wear-resistant cladding on toothed rolls requires close coordination between the fabrication shop and the operating plant. Several practical considerations must be addressed:

• Roll shipping and handling: Cladded rolls must be protected during transport to prevent damage to the cladding surface. The cladding layer is susceptible to mechanical damage, and any gouging or denting must be repaired before the roll is returned to service.
• Roll installation: The cladded rolls must be installed with proper alignment to avoid eccentric loading that could cause uneven wear or premature failure.
• Operating conditions: The cladding performance is optimized for specific operating conditions. Changes in ore composition, moisture content, or feed rate can significantly affect the wear rate and may require adjustment of the cladding material or process.
• Maintenance and repair: Localized damage to the cladding layer, such as chipping of individual teeth, can often be repaired by local re-cladding using the same or a compatible material.

The economic analysis of cladding versus replacement is a critical factor in the decision to apply wear-resistant cladding. For toothed rolls with a high replacement cost and significant downtime associated with replacement, cladding typically provides a favorable return on investment. The cladding cost is generally 30-60% of the cost of a new roll, while the service life extension is 3-8 times, resulting in a net cost reduction of 50-70% per unit of ore processed.

Key Insights and Technical Reflections

This study provides valuable insights into the application of wear-resistant cladding to heavy-duty mining and processing equipment. Several key observations emerge:

• The multi-mechanism wear environment of sintered hot ore crushing requires a material design that addresses abrasive, impact, and thermal wear simultaneously, rather than optimizing for a single wear mechanism.
• The combination of SAW for bulk deposition and FCAW for finishing provides an effective balance of productivity and quality for large roll cladding applications.
• Preheating and interpass temperature control are critical process parameters that must be rigorously controlled to prevent cracking in high-carbon cladding materials.
• The economic case for cladding is compelling when the replacement cost and downtime of heavy rolls are considered, making cladding a cost-effective solution for extending equipment life.

The methodology of this study—combining materials science, welding engineering, and operational experience—provides a template for addressing similar wear problems in other heavy industrial applications. The systematic approach to wear mechanism analysis, material selection, process optimization, and quality control can be adapted to other equipment types and operating conditions.

In conclusion, the wear-resistant cladding of toothed rolls for sintered hot ore crushing demonstrates the significant potential of surface engineering to extend the life of heavy-duty industrial equipment. The combination of appropriate material selection, rigorous process control, and systematic quality assurance enables the achievement of wear-resistant performance that substantially improves the economics of mining and processing operations.