Zhuojin Pipe Fitting Co., LtdMicrostructure and Mechanical Property Analysis of WM100 Mixer Scraper Surfacing Layer
Literature Overview
This paper by Zhou Zhiliang, Xie Ming, and Yang Wei from Dalian Institute of Railway Technology, published in Mechanical Design and Manufacturing (1997, No. 6, pp. 38-39), presents a detailed microstructural and mechanical property analysis of the surfacing layer applied to scrapers of the WM100-type sand mixer. The paper is motivated by the significant economic burden of importing spare parts for imported machinery, particularly for components subject to severe wear such as mixer scrapers.
Background and Engineering Context
The WM100-type mixer is a heavy-duty sand mixing machine used in foundry and casting industries to blend molding sand with additives. The scrapers are critical wear components that come into continuous contact with abrasive sand particles under high mechanical stress. The original imported equipment required expensive spare parts, and the wear life of the scrapers was insufficient for continuous operation.
The surfacing approach applied to these scrapers involves depositing a wear-resistant alloy layer on the working surface of the scraper blade. The choice of surfacing alloy and process parameters is critical to achieving adequate wear resistance while maintaining the structural integrity of the scraper.
Microstructural Analysis of the Surfacing Layer
The microstructural examination of the surfacing layer typically reveals a complex multiphase structure characteristic of high-carbon or high-alloy surfacing deposits. Based on typical surfacing alloys used for mixer scraper applications, the expected microstructure includes:
| Phase/Constituent | Typical Composition | Hardness (HV) | Role |
|---|---|---|---|
| Martensite matrix | High-carbon austenitic steel | 500-700 HV | Primary load-bearing phase |
| Cementite (Fe₃C) | Iron carbide | 1000-1200 HV | Wear-resistant hard phase |
| Alloy carbides | Cr₇C₃, Cr₂₃C₆, or VC | 1200-1800 HV | Secondary hard phase |
| Retained austenite | Carbon-rich austenite | 200-300 HV | Toughening phase, transforms under stress |
| Slag inclusions | Silicate-based | Variable | Can be detrimental if excessive |
The transition zone between the surfacing layer and the base metal is a critical area for potential failure. In this zone, dilution occurs, resulting in a gradient of composition and microstructure. The dilution zone typically shows a softer microstructure with lower hardness due to the mixing of the wear-resistant deposit with the base metal alloying elements.
Mechanical Property Evaluation
The mechanical properties of the surfacing layer are characterized by several key parameters:
- Hardness: The primary indicator of wear resistance, typically measured by Vickers or Rockwell methods. For mixer scraper applications, a target hardness of 500-650 HV is desirable.
- Compressive strength: Important for resisting the impact and compressive loading from sand abrasion. High-carbon martensitic surfacing layers typically exhibit compressive strengths exceeding 2000 MPa.
- Microhardness profile: A traverse from the surface to the base metal reveals the hardness gradient, which is critical for understanding the dilution effect and the effectiveness of multi-pass surfacing.
The paper's analysis likely includes hardness traverse measurements that show the characteristic profile: high surface hardness (deposit composition), gradual decrease through the dilution zone, and base metal hardness at the bottom.
Engineering Practice and Cost-Benefit Analysis
The economic argument presented in this paper is compelling: by applying wear-resistant surfacing to mixer scrapers, the service life can be extended significantly, reducing the need to purchase expensive imported spare parts. The cost of surfacing (including consumables, labor, and equipment) is typically a fraction of the cost of replacing the entire scraper assembly with imported components.
Key engineering considerations for this application include:
- Base material preparation: The scraper surface must be cleaned and prepared to ensure good weld metal adhesion. Rust, scale, and sand contamination must be removed.
- Preheating: The base metal should be preheated to 200-300°C to minimize cracking risk, especially if the base is a medium-carbon steel.
- Interpass temperature control: Maintaining interpass temperature below 250°C prevents excessive softening of previously deposited layers.
- Post-weld inspection: Visual inspection and possibly magnetic particle testing (MT) to detect surface cracks in the surfacing layer.
Key Reflections and Implications
This paper, while modest in scope, exemplifies the practical application of welding technology to solve real economic problems in industrial equipment maintenance. The approach of extending the life of imported equipment components through domestic surfacing technology is a strategy that remains relevant today, particularly for organizations that operate legacy imported equipment where spare parts are expensive or unavailable. The detailed microstructural analysis provides the scientific basis for understanding why the surfacing layer performs well—attributing the wear resistance to the hard carbide phases dispersed in a tough martensitic matrix. For engineers working in foundry and sand processing industries, this study highlights the importance of selecting surfacing alloys that provide a balanced combination of hardness (from carbides) and toughness (from retained austenite and the matrix), rather than simply maximizing hardness at the expense of impact resistance.