Zhuojin Pipe Fitting Co., LtdStudy Note on Surfacing Repair Technology for Disc Shear Blades
Literature Overview
The paper by Liu Xiaogang, Wang Binwu, and Qin Xuedong, published in Coal Mine Machinery (Vol. 26, No. 11, 2005, pp. 108–109), addresses the surfacing repair of failed disc shear blades used in coal mine operations. Funded by the Guilin Science and Technology Project (20040104-3), the study was conducted at Guilin Aerospace Polytechnic. The authors investigate the feasibility of restoring worn or damaged disc shear blades through overlay welding, followed by tempering heat treatment, and evaluate the resulting mechanical properties and microstructure.
Core Technical Approach
The fundamental problem addressed is that disc shear blades in coal mining environments suffer from severe abrasive wear and edge chipping, leading to frequent replacement and high operational costs. The authors propose a surfacing repair strategy using overlay welding to rebuild the cutting edge, followed by a controlled tempering cycle to optimize hardness and wear resistance.
The key process parameters investigated include:
| Parameter | Typical Range | Purpose |
|---|---|---|
| Surfacing wire composition | High-carbon martensitic steel | Hardness and wear resistance |
| Welding current | 150–250 A (SMAW) | Penetration and dilution control |
| Interpass temperature | < 150°C | Prevent excessive grain growth |
| Tempering temperature | 400–550°C | Reduce residual stress, improve toughness |
| Tempering time | 2–4 h | Complete stress relief |
| Cooling rate | Furnace cool | Avoid secondary cracking |
Microstructural Analysis and Key Findings
The study employed metallographic examination to characterize the as-welded and post-tempered microstructures. The as-welded overlay layer typically exhibits a martensitic structure with retained austenite, resulting in high hardness but also high brittleness and residual stress. The tempering treatment transforms retained austenite into tempered martensite and carbides, achieving a balance between hardness and toughness.
Key observations from the microstructural analysis:
- The as-welded layer shows a lenticular martensite morphology with hardness reaching 55–62 HRC.
- After tempering at 450–500°C, the microstructure transitions to tempered martensite with dispersed carbides, maintaining hardness at 48–55 HRC while significantly improving impact toughness.
- The heat-affected zone (HAZ) of the base steel shows minimal microstructural change due to the low interpass temperature maintained during multi-pass surfacing.
Engineering Practice and Cost-Benefit Analysis
The authors conclude that the surfacing repair approach offers superior economic performance compared to replacing blades with new conventional shear blades. The cost reduction is attributed to:
- Elimination of full blade replacement, utilizing the existing base material.
- Improved service life due to the optimized microstructure after tempering.
- Reduced downtime for blade replacement in continuous mining operations.
From a practical standpoint, this approach aligns with the PDCA cycle: the Plan phase involves selecting appropriate surfacing consumables and defining the tempering parameters; the Do phase executes multi-pass surfacing with strict interpass temperature control; the Check phase verifies hardness, microstructure, and wear resistance; and the Act phase refines parameters based on field performance data.
Critical Reflections and Practical Implications
While the study demonstrates clear benefits, several practical considerations deserve emphasis. First, the dilution rate between the surfacing layer and the base steel must be carefully controlled to avoid excessive softening of the overlay. Second, the tempering cycle must be matched to the specific surfacing alloy composition—over-tempering can reduce hardness below acceptable levels for cutting applications, while under-tempering leaves residual stresses that may cause cracking during service. Third, the surface quality after surfacing must be ground or machined to ensure proper blade geometry and cutting performance.
In my experience with similar repair operations on mining equipment, the success of surfacing repair depends heavily on base material preparation. Thorough cleaning, edge beveling, and preheating are often overlooked but are critical for achieving sound metallurgical bonding. The study's conclusion that surfacing repair outperforms conventional replacement in both cost and performance is well-supported, provided the process discipline is maintained throughout.