Surfacing Welding Technology for Roller Press Squeeze Rollers in Cement Industry

Literature Overview

This paper by Zhao Yunfeng and Li Wenqing (2000), published in Welding (Hanshan), addresses the surfacing welding technology for squeeze rollers used in roller presses within the cement manufacturing industry. The authors represent Tangshan Cement Machinery Factory and the Heilongjiang Provincial Building Materials Planning and Design Research Institute, respectively. The work addresses a practical engineering challenge in the cement industry: the restoration and enhancement of squeeze roller surfaces that experience severe wear during the grinding and crushing of cement clinker and raw materials. The paper provides practical guidance on welding material selection, process parameters, and quality control for the surfacing of these critical components.

Core Technical Content

Application Context

Roller presses are widely used in the cement industry for the grinding of clinker, raw meal, and coal. The squeeze rollers (also called grinding rollers) are the primary wear components in these machines, subjected to extreme conditions:

• High compressive loads: The rollers experience very high contact pressures during the grinding process, often exceeding 100 MPa.
• Abrasive wear: The cement materials being ground are highly abrasive, causing rapid surface wear.
• Thermal cycling: The grinding process generates significant heat, subjecting the roller surface to thermal cycling.
• Impact loading: Uneven material feed can cause impact loading on the roller surface.

These conditions result in rapid wear of the roller surface, necessitating periodic restoration through surfacing welding. The original roller material (typically medium carbon steel or alloy steel) does not have adequate wear resistance, making overlay welding with wear-resistant materials essential.

Surfacing Material Selection

The selection of surfacing materials for squeeze rollers is critical and depends on the specific operating conditions:

For cement industry applications, high-carbon martensitic and austenitic surfacing materials are most commonly used. The high-carbon martensitic materials provide good wear resistance at moderate cost, while the austenitic materials offer superior wear resistance in more severe conditions.

Process Parameters

The surfacing welding of squeeze rollers typically employs shielded metal arc welding (SMAW) or submerged arc welding (SAW) with appropriate welding consumables. The key process parameters include:

The preheating requirement is particularly important for squeeze rollers, which are typically made of medium carbon steel or low-alloy steel. Without adequate preheating, the high carbon equivalent of the base metal can lead to cold cracking in the heat-affected zone.

Interpretation of Technical Points

Wear Mechanism Analysis

Understanding the wear mechanism is essential for selecting the appropriate surfacing material. In roller press applications, the wear is a combination of:

• Abrasive wear: The primary wear mechanism, caused by the hard particles in the cement material sliding across the roller surface. The wear rate depends on the hardness of the surfacing material relative to the abrasive particles.
• Adhesive wear: Material transfer between the roller surface and the grinding material, particularly under high contact pressures.
• Fatigue wear: Surface cracking and material removal due to cyclic contact loading, leading to spalling of the surface layer.
• Corrosive wear: In some environments, moisture and chemical attack can accelerate wear, particularly if the surfacing material is not corrosion-resistant.

The selection of surfacing material should be based on the dominant wear mechanism. For abrasive wear, high hardness is the primary requirement. For fatigue wear, a combination of hardness and toughness is needed. For adhesive wear, a hard surface with good adhesion resistance is essential.

Microstructural Considerations

The microstructure of the surfacing layer directly determines its wear resistance:

• Martensitic structure: High-carbon martensite provides high hardness (60-65 HRC) but limited toughness. Suitable for moderate wear conditions.
• Austenitic structure with carbides: The retained austenite provides toughness while the hard carbides (Cr7C3, Cr23C6) provide wear resistance. The carbide morphology (size, shape, distribution) is critical.
• Composite structure: A combination of hard and soft phases can provide a balance of wear resistance and toughness, improving fatigue life.

The welding process parameters significantly influence the microstructure. The cooling rate determines the phase transformation in the weld metal, while the dilution from the base metal affects the chemical composition and hence the microstructure of the overlay.

Quality Control Requirements

The quality of the surfacing layer is critical for the performance and service life of the squeeze roller. Key quality control measures include:

The dilution testing is particularly important, as excessive dilution from the base metal can significantly reduce the hardness and wear resistance of the overlay. The dilution rate depends on the welding process, parameters, and the number of layers.

Engineering Practice Integration

The practical application of surfacing welding for squeeze rollers involves several important considerations:

• Surface preparation: The worn surface must be thoroughly cleaned and prepared before welding. Rust, scale, and contaminated material must be removed by grinding or machining. The surface should be roughened to promote good metallurgical bonding.
• Welding sequence: For large rollers, a systematic welding sequence is essential to minimize distortion and residual stress. The sequence should ensure uniform heat input distribution and avoid localized overheating.
• Post-weld treatment: Depending on the surfacing material, post-weld heat treatment may be required. For martensitic materials, tempering may be needed to improve toughness. For austenitic materials, solution treatment may be required to optimize the microstructure.
• Surface finishing: After welding, the surface must be machined or ground to the required dimensional tolerances and surface finish. The grinding process should be controlled to avoid introducing residual stresses that could reduce fatigue life.
• Service monitoring: After installation, the service life of the surfaced roller should be monitored. The wear rate can be measured periodically to predict the remaining service life and plan for the next restoration cycle.

Case Study Considerations

In cement plant operations, the restoration of squeeze rollers is a recurring maintenance activity. The frequency of restoration depends on:

• The abrasiveness of the material being ground
• The operating conditions (load, speed, feed rate)
• The quality of the surfacing layer
• The maintenance practices (lubrication, alignment, etc.)

A well-executed surfacing operation can extend the service life of a squeeze roller by several times compared to the unoverlaid condition. The cost-effectiveness of surfacing versus roller replacement depends on the specific operating conditions and the availability of replacement rollers.

Key Questions and Reflections

Several aspects of this technology warrant further consideration:

1. Welding consumable selection: The paper provides general guidance on welding material selection but does not provide detailed recommendations for specific operating conditions. A more comprehensive guide correlating material properties with operating conditions would be valuable for engineers.
2. Long-term performance data: The paper focuses on the welding process itself but does not provide extensive data on the long-term performance of the surfaced rollers in service. Long-term performance data, including wear rate measurements over extended periods, would be valuable for predicting service life and optimizing restoration schedules.
3. Alternative technologies: Beyond conventional arc welding surfacing, alternative technologies such as thermal spray (HVOF, plasma spray), laser cladding, and electric arc surfacing (EAS) offer different advantages. A comparison of these technologies for squeeze roller applications would be valuable.
4. Process automation: The welding of large rollers is often performed manually, which can lead to inconsistent quality. The development of automated or semi-automated welding systems for roller surfacing could improve quality consistency and productivity.

Study Insights and Implications

This paper provides practical guidance on the surfacing welding of cement industry squeeze rollers, addressing a common maintenance challenge in the cement manufacturing sector. The work emphasizes the importance of material selection, process parameter optimization, and quality control in achieving satisfactory service life.

The paper reflects the practical orientation of the cement industry, where cost-effectiveness and reliability are paramount. The recommended surfacing materials and processes are proven technologies that can be implemented with existing equipment and skills, making them accessible to cement plant maintenance teams.

The work also highlights the importance of understanding the wear mechanism in selecting the appropriate surfacing material. This fundamental understanding is essential for optimizing the performance of the surfaced roller and extending its service life.

From a broader perspective, the surfacing of squeeze rollers is a representative example of the wider application of overlay welding technology in the cement and mineral processing industries. Similar challenges arise in the restoration of kiln linings, ball mill liners, and other wear components. The principles established in this work can be applied to these related applications, contributing to the overall improvement of maintenance practices in the cement industry.