Surfacing Materials for Slag Mill Roller Surface in Cement Industry

Literature Overview

This paper, published in 2021 in the "New Century Cement Herald" by Zhang Haiyan, Wei Wei, Yang Wei, and Zhang Yongsheng from Zhengzhou Machinery Research Institute, addresses the challenge of developing surfacing materials for the roller surfaces of vertical mills used in slag grinding in the cement industry. The authors developed a specialized open-arc welding wire, designated ZD904-O, specifically formulated for slag mill roller surfacing. The paper reports on the design rationale, microstructure, wear resistance, and practical application of this surfacing material, highlighting its ability to address the long-standing problem of rapid wear and spalling of roller surfaces in slag grinding operations.

Core Technical Points

Slag Grinding Challenges

Vertical mills (also called roller mills or tube mills) are widely used in the cement industry for grinding raw materials, clinker, and slag. Slag grinding presents unique challenges compared to other grinding operations because slag has a relatively low grindability and a high abrasiveness. The hard, angular slag particles exert severe abrasive and impact loading on the mill rollers and grinding table, leading to rapid wear and frequent maintenance requirements. Traditional surfacing materials for roller surfaces often fail to provide adequate wear resistance and toughness under these conditions, resulting in spalling, chipping, and premature failure of the surfacing layer.

ZD904-O Wire Design

The ZD904-O welding wire was developed through a systematic approach to alloy design. The key design principle was to achieve a high carbon and high alloy composition that would produce a weld metal rich in hard carbides while maintaining sufficient toughness to resist spalling under impact loading. The wire composition was optimized to produce a weld metal containing a mixture of M3C2, MC, and M7C3 carbides, each of which contributes to wear resistance through different mechanisms. The M3C2 carbides provide general wear resistance, the MC carbides offer extreme hardness and resistance to abrasive wear, and the M7C3 carbides provide a balance of hardness and toughness.

Stress-Relieving Cracks

One of the most interesting aspects of the ZD904-O surfacing material is the intentional introduction of fine "stress-relieving cracks" in the surfacing layer. These microcracks are not defects but are a deliberate metallurgical feature designed to prevent catastrophic spalling of the surfacing layer. The fine cracks act as stress concentrators that arrest larger crack propagation, effectively segmenting the surfacing layer into small, independent regions that cannot easily delaminate from the base material. This approach is analogous to the use of controlled microcracking in some ceramic and composite materials to improve fracture toughness.

Technical Parameter Summary

Engineering Practice Integration

The development of the ZD904-O wire represents a targeted solution to a specific industrial problem: the rapid wear of vertical mill roller surfaces in slag grinding operations. The authors report that the wire was developed based on the actual requirements of cement industry customers, who had been experiencing persistent problems with roller surface wear and spalling. The wire was tested in actual service conditions, where it was applied to mill rollers using open-arc (flux-cored or solid wire) welding processes.

The application of the ZD904-O wire requires careful attention to welding process parameters, including preheating temperature, interpass temperature, and post-weld cooling rate. The high carbon and alloy content of the wire makes the weld metal susceptible to cracking if the cooling rate is too rapid or if the heat input is too low. Conversely, excessive heat input can lead to coarse microstructures and reduced hardness. The authors likely optimized these parameters through a combination of laboratory testing and field trials.

The practical benefits of using the ZD904-O wire include extended roller surface life, reduced maintenance frequency, and lower overall cost of ownership for the vertical mill. In the cement industry, where mill availability directly affects production capacity and profitability, even modest improvements in roller surface life can translate into significant economic benefits.

Key Questions and Reflections

Several aspects of this work merit further investigation. First, the paper does not provide detailed comparative data between the ZD904-O wire and conventional surfacing materials used for slag mill rollers, which would help quantify the performance improvement. Second, the long-term service performance of the ZD904-O surfacing layer, including its behavior under thermal cycling and varying moisture conditions in the slag, would be valuable to document. Third, the paper could benefit from a discussion of the weldability of the ZD904-O wire, including its susceptibility to porosity, slag inclusion, and hot cracking, which are common issues with high-carbon surfacing materials.

The concept of intentional microcracking in surfacing layers is particularly interesting and deserves further exploration. While this approach has been applied in the ZD904-O wire, it could potentially be extended to other high-carbon surfacing materials used in different industries, such as mining, aggregate processing, and power generation. The key challenge is to control the crack density and spacing to achieve the desired balance between stress relief and wear resistance.

Study Insights and Implications

The development of the ZD904-O wire exemplifies the importance of tailoring surfacing materials to specific application requirements rather than relying on generic hard-facing alloys. The slag grinding application demands a unique combination of properties: high hardness for abrasive wear resistance, sufficient toughness to resist impact spalling, and the ability to withstand thermal cycling without cracking. The ZD904-O wire achieves this balance through a carefully designed high-carbon, high-alloy composition that produces a multi-phase carbide microstructure with intentional microcracking for stress relief. For engineers in the cement and mineral processing industries, this paper provides a valuable case study in surfacing material development for demanding wear applications. The broader implication is that surfacing material design should be driven by a thorough understanding of the service environment, including the nature of the wear mechanisms, the loading conditions, and the environmental factors, rather than by generic hardness values alone.