Field Surfacing Repair of Metal Matrix Composite Ceramic Grinding Disc

Literature Overview

This paper by Ni Junjie and Yang Wei from the Zhengzhou Machinery Research Institute, published in New Century Cement Herald (2013, Vol. 19, No. 5, pp. 80-81), documents a successful field repair of a metal matrix composite (MMC) ceramic grinding disc used in a raw material vertical mill using ZD903-O surfacing wire. The repair involved three intermittent surfacing passes to restore the original dimensions, achieving excellent metallurgical bonding between the surfacing layer and the ceramic-reinforced metal matrix substrate.

Core Technical Content

Background: Metal Matrix Composite Grinding Discs

Metal matrix composite (MMC) grinding discs represent an advanced wear-resistant technology in cement grinding mills, where the grinding table surface is reinforced with embedded ceramic elements (typically alumina or silicon carbide) in a metal matrix. The RM57/28 type raw material vertical mill grinding disc combines the advantages of:

• Metal matrix: provides toughness, thermal conductivity, and dimensional stability
• Ceramic inserts: provide exceptional abrasion resistance through high hardness (typically 85-92 HRA for alumina)
• Synergistic effect: the metal matrix absorbs impact energy while ceramic elements resist abrasive wear

The failure mode of these grinding discs is progressive wear of the metal matrix between ceramic inserts, eventually exposing the ceramic elements and reducing grinding efficiency. When the disc surface becomes worn beyond acceptable limits, complete replacement is expensive and time-consuming, making field repair an attractive alternative.

Surfacing Repair Process and Results

The repair was performed using ZD903-O surfacing wire (a hardfacing consumable designed for severe abrasive wear) through three intermittent surfacing passes. The key results include:

The most significant finding is that the wear resistance of the repaired surface exceeded that of the original MMC grinding disc. This result, while initially counterintuitive, can be explained by the metallurgical characteristics of the ZD903-O deposit, which likely contains a higher volume fraction of hard carbide phases compared to the original MMC matrix.

Metallurgical Bonding Analysis

The paper highlights two critical metallurgical aspects of the successful repair:

1. Intermetallic bonding formation: ZD903-O wire was able to form effective intermetallic bonds with the metal matrix of the MMC disc, ensuring mechanical integrity of the repair. This is significant because surfacing over ceramic-reinforced substrates presents unique challenges due to the thermal expansion coefficient mismatch between the ceramic inserts and the metal matrix.
2. Thermal stress management: The intermittent surfacing strategy (three separate passes with cooling intervals) was crucial for managing the thermal stresses induced by the surfacing process. The ceramic inserts, with their low thermal expansion coefficient compared to the metal matrix, create localized stress concentrations during heating and cooling. Intermittent deposition allows partial stress relaxation between passes.

Process Rationale

The success of this repair can be attributed to several factors working together:

• Heat input management: The intermittent surfacing strategy limited cumulative thermal input, preventing excessive thermal distortion and stress buildup
• Stress release strategy: Cooling between passes allowed elastic stress relaxation and partial plastic deformation, reducing residual stress accumulation
• Consumable compatibility: ZD903-O was selected for its ability to bond effectively with the metal matrix while providing superior wear resistance
• Dimensional restoration approach: Three passes allowed controlled build-up to the required thickness without excessive dilution or thermal cycling in any single pass

Engineering Practice Integration

From a practical standpoint, this case study demonstrates several important principles for field repair of advanced composite components:

1. Field repair feasibility: The successful repair at the site (rather than in a workshop) demonstrates that advanced surfacing techniques can be applied in industrial field conditions with appropriate equipment and consumable selection.
2. Wear performance improvement: The finding that the repaired surface outperforms the original in wear resistance opens possibilities for planned component upgrade during repair, effectively extending service life beyond the original design specification.
3. Thermal management for composite substrates: When surfacing over components containing dissimilar materials (ceramic inserts in metal matrix), the thermal expansion mismatch must be carefully managed through process parameter selection and deposition strategy.
4. Economic justification: Field repair of grinding discs eliminates the need for costly downtime and replacement, with repair costs typically representing only 10-20% of new component costs while restoring functionality.

Key Reflections and Study Insights

This paper represents an important practical contribution to the field of industrial component repair, demonstrating that advanced composite materials can be successfully repaired through surfacing technology when appropriate process strategies are employed.

The finding that thermal effects on embedded ceramic elements were minimal is particularly noteworthy. In theory, the large thermal expansion coefficient mismatch between alumina (7.5 × 10⁻⁶/°C) and steel (12-13 × 10⁻⁶/°C) should create significant thermal stresses during surfacing. The successful outcome suggests that the intermittent surfacing strategy effectively managed these stresses, and that the metal matrix surrounding the ceramic inserts provided adequate stress buffering.

The concept of "surpassing original performance" through repair is philosophically significant. It challenges the traditional view that repair is merely restoration to original condition and opens the possibility of planned component improvement during maintenance activities. This approach could be extended to other industrial components where repair is performed regularly.

The relatively brief nature of this paper (only 2 pages) is a limitation, as more detailed information on welding parameters, microstructural analysis, and quantitative wear testing would strengthen the technical contribution. However, the practical demonstration of successful field repair of MMC components provides valuable confidence for similar applications in cement, mining, and other heavy industry sectors.