Hardfacing Materials for Slag Mill Roller Surfaces

Literature Overview

This paper by Zhang Haiyan, Wei Wei, Yang Wei, and Zhang Yongsheng from Zhengzhou Institute of Mechanical Research (published in New Century Cement Herald, 2021, Vol. 27, No. 4, pp. 65-66) addresses the development of a specialized open-arc welding wire (designated ZD904-O) for hardfacing the roller surfaces of vertical slag mills. Slag grinding in cement production subjects mill rollers and grinding tables to extremely severe abrasive wear due to the low grindability of ground granulated blast furnace slag (GGBFS). The authors developed a high-carbon, high-alloy wire that produces a hardfacing deposit with excellent abrasion resistance and controlled stress-relief cracking to prevent spalling.

Application Context

Vertical roller mills (VRMs) are widely used in cement production for grinding raw materials and slag. The grinding process involves:

• Roller diameter: 1.5-2.5 meters
• Grinding pressure: 10-20 MPa
• Slag abrasivity: High (Mohs hardness 5-7 for slag particles)
• Operating temperature: 80-150°C (hot gas drying)
• Impact loading: Moderate to high
• Wear rate: 0.5-2.0 mm/month without protection

Ground granulated blast furnace slag (GGBFS) is particularly challenging because it contains sharp, angular particles with high hardness that cause rapid abrasive wear. The low grindability of slag means higher grinding pressures are required, further accelerating wear on the roller surfaces.

Material Development

Wire Composition Design

The ZD904-O wire was designed with the following composition philosophy:

The composition achieves a balance between three competing requirements:

1. Hardness: High carbon and alloy content for maximum carbide volume fraction
2. Toughness: Sufficient retained austenite and ductile matrix to resist impact
3. Spalling resistance: Controlled microcracking to relieve residual stress

Carbide System Analysis

The hardfacing deposit contains three types of alloy carbides:

The volume fraction of carbides in the deposit is approximately 40-55%, distributed in a eutectic pattern with the martensitic matrix. The MC carbides (formed by Mo) are the hardest phase and provide the primary resistance to abrasive wear, while the M₇C₃ carbides provide a secondary level of hardness.

Microstructural Innovation: Stress-Release Cracking

A key innovation of the ZD904-O wire is the deliberate introduction of fine "stress-release cracks" in the hardfacing deposit. This is achieved through:

1. High carbon content: Promotes formation of retained austenite that transforms on cooling, creating internal stresses
2. Differential thermal expansion: Carbides and matrix have different CTE values, generating microstresses
3. Rapid cooling: Open-arc welding on a large mass base provides fast heat extraction

These microcracks (typically 5-20 μm wide, spaced 50-200 μm apart) serve a critical function:

• Stress relief: They relieve the high compressive and tensile residual stresses in the deposit
• Crack arrest: They prevent the formation of large cracks that would lead to spalling
• Energy absorption: They provide additional toughness through crack branching

This is analogous to the controlled cracking in refractory materials, where fine cracks improve thermal shock resistance without compromising structural integrity.

Performance Characteristics

Process Parameters

Engineering Application

The ZD904-O wire has been successfully applied to:

1. Vertical slag mill rollers: Roller surface hardfacing for 8-12 mm thickness
2. Grinding tables: Table surface protection against abrasive wear
3. Separator vanes: Wear protection for classifier components
4. Feed chutes: Abrasive material transfer surfaces

The application procedure follows a standard protocol:

• Surface preparation: Shot blast to Sa 2.5, remove existing worn layer
• Edge preparation: V-groove or J-groove, 3-5 mm deep
• Preheating: Induction or oxy-fuel preheat to 150-200°C
• Hardfacing: Multi-pass welding with ZD904-O wire
• Inspection: UT or MT for cracks, hardness spot checks
• Machining: Final grinding to required profile

Study Insights

This 2021 paper reflects the maturity of hardfacing technology in the cement industry. The deliberate engineering of "stress-release cracks" represents a sophisticated understanding of the relationship between microstructure, residual stress, and service performance. Rather than treating cracking as a defect to be eliminated, the authors recognize it as a beneficial feature that can be controlled and exploited.

The development of a specialized wire (ZD904-O) rather than adapting a general-purpose hardfacing alloy demonstrates the principle that application-specific materials outperform generic solutions. The slag grinding application has unique requirements—high abrasion resistance combined with moderate impact resistance and spalling resistance—that cannot be met by standard high-chromium cast iron hardfacing alloys.

The economic impact is substantial: a single vertical slag mill roller set can cost $50,000-$100,000 to replace, and downtime for replacement can cost $5,000-$10,000 per day. Extending roller life from 2-3 months to 6-10 months represents savings of hundreds of thousands of dollars annually per mill.

The open-arc (FCAW) process was selected for its high deposition rate and automation potential, which is critical for the large surface areas involved in roller hardfacing. The flux-cored wire provides consistent composition and eliminates the need for external flux handling.