Study Note on Cladding Repair Methods for Roller Press Roller Surfaces

Introduction to Roller Press Applications

Roller presses are widely used in the cement, mineral processing, and coal preparation industries for size reduction and compaction of materials. The rollers are subjected to extreme conditions including high contact stress, abrasive wear from feed material, and thermal cycling. The surface hardness of the rollers typically ranges from 500 to 600 HV, and the rollers are often made of high-chromium cast iron or quenched and tempered steel. When the roller surface is worn beyond acceptable limits, the rollers must be repaired by cladding to restore the original surface geometry and hardness.

The literature under study investigates various cladding repair methods for roller press roller surfaces, including oxy-fuel hardfacing, gas metal arc welding (GMAW) hardfacing, plasma transferred arc (PTA) cladding, and laser cladding. The study evaluates the performance, cost, and practicality of each method for different roller sizes and service conditions.

Comparison of Cladding Methods

The study provides a comprehensive comparison of the four cladding methods, as summarized in the following table:

The study finds that the choice of cladding method depends primarily on the roller size, the required surface quality, and the available budget. For large rollers in cement kiln applications, oxy-fuel hardfacing is the most economical option despite its poor surface quality, because the rollers are subsequently ground to the required geometry. For medium-sized rollers in mineral processing applications, GMAW hardfacing offers a good balance of cost and quality. For small rollers in high-performance applications, PTA or laser cladding is preferred for the excellent surface quality and metallurgical compatibility.

Filler Metal Selection and Process Parameters

The study evaluates several filler metals for roller surface cladding, including high-chromium iron (Cr30), medium-chromium iron (Cr25), and nickel-based alloys (Stellite 6). The selection depends on the service conditions:

For GMAW hardfacing, the study recommends a wire diameter of 1.2 mm, a current of 180–250 A, a voltage of 22–28 V, and a travel speed of 200–350 mm/min. The shielding gas is a mixture of 80% Ar and 20% CO₂. The preheat temperature is 200–300 °C for cast iron rollers and 100–200 °C for steel rollers.

For PTA cladding, the study recommends a powder feed rate of 100–200 g/min, an arc current of 200–350 A, and a travel speed of 50–150 mm/min. The PTA process produces a cladding layer with excellent metallurgical bonding and minimal dilution, resulting in a cladding hardness that closely matches the nominal filler metal hardness.

Surface Preparation and Post-Treatment

Surface preparation is critical for the success of roller cladding repairs. The worn surface must be cleaned by grinding or shot blasting to remove rust, scale, and any contaminated material. The surface should be prepared to a minimum profile of Sa 2½ according to ISO 8501-1. Any cracks or defects in the roller surface must be repaired before cladding.

After cladding, the roller surface must be ground to the required geometry and surface finish. The grinding operation should remove a minimum of 0.5 mm of the cladding layer to eliminate any surface defects and to ensure a uniform hardness. The final surface roughness should be Ra 1.6–3.2 μm for cement applications and Ra 0.8–1.6 μm for mineral processing applications.

Case Studies and Performance Evaluation

The study documents several case studies of roller cladding repairs in industrial settings. In a cement plant, a 1200 mm diameter roller press was repaired using GMAW hardfacing with Cr25 filler metal. The roller was preheated to 250 °C, and a 4 mm cladding layer was deposited in three passes. After grinding, the roller surface hardness was 580 HV and the surface roughness was Ra 2.0 μm. The roller returned to service and operated for 8 months before the next repair was required.

In a coal preparation plant, a 600 mm diameter roller was repaired using PTA cladding with Stellite 6 powder. The roller surface hardness after PTA cladding was 380 HV, and the surface roughness was Ra 0.8 μm. The roller operated for 14 months before the next repair was required, demonstrating the superior wear resistance of the Co-based cladding layer.

Key Engineering Considerations

The study highlights several important considerations for roller cladding repairs. First, the thermal management during cladding is critical to prevent distortion of the roller. For large rollers, the cladding should be applied in a spiral pattern to distribute the heat evenly. Second, the residual stress in the cladding layer can be significant and may lead to spalling during service. A stress-relieving treatment at 600 °C for 2 hours is recommended after cladding to reduce residual stresses. Third, the dilution rate must be controlled to ensure that the cladding hardness meets the required specification.

The study also emphasizes the importance of proper inspection and documentation. Each cladding repair should be documented with the process parameters, filler metal used, and inspection results. This documentation is essential for tracking the service life of the roller and for planning future maintenance.

Summary

This study provides a comprehensive evaluation of cladding repair methods for roller press roller surfaces. The key finding is that the choice of cladding method depends on the roller size, service conditions, and budget constraints. GMAW hardfacing with Cr25 filler metal offers the best balance of cost and performance for most industrial applications, while PTA cladding with Stellite 6 provides superior wear resistance for critical applications. The study also emphasizes the importance of surface preparation, thermal management, and post-treatment in achieving a successful cladding repair. Engineers involved in roller maintenance should carefully evaluate the service conditions and select the appropriate cladding method and filler metal to maximize the service life of the roller.