CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay Repair Technology for Disc Shear Blades
Literature Overview
This study, published in China Surface Engineering in 2006 by Sui Xiangrong, Shen Fenggang, Wang Qingbao, Zhang Di, and Nie Zhenhua from the Welding Research Institute of China Metallurgical Group Building Research Institute, addresses the weld overlay repair technology for disc shear blades used in steel strip processing. The research focuses on the development and optimization of a weld overlay process that can restore the cutting edge of worn or damaged disc shear blades, extending their service life and reducing the cost of blade replacement.
Technical Background and Application Context
Disc shear blades are critical components in the processing of steel strips, where they are used to cut the strip to the required width. The blades are subjected to severe wear, impact, and abrasion during operation, and they typically require replacement or repair after a certain number of cutting cycles. The replacement of blades is costly, and the development of an effective repair technology can significantly reduce the operating cost of the steel processing line.
The disc shear blade is typically made from a high-carbon steel or a tool steel, such as Cr12MoV or D2, which provides the necessary hardness and wear resistance for the cutting operation. However, the cutting edge of the blade is susceptible to wear, chipping, and cracking, which can affect the quality of the cut and the performance of the blade. The weld overlay repair technology aims to restore the cutting edge by depositing a layer of wear-resistant material on the worn surface, which can then be ground to the required geometry.
The selection of the overlay material is critical for the success of the repair. The overlay material should have the following properties:
- High hardness: To provide wear resistance and extend the service life of the blade.
- Good weldability: To ensure a strong bond with the base material and prevent cracking during welding.
- Low dilution: To maintain the composition and properties of the overlay material.
- Good machinability: To allow the overlay layer to be ground to the required geometry after welding.
Common overlay materials for disc shear blade repair include:
| Material Type | Composition | Hardness (HV) | Application |
|---|---|---|---|
| High-carbon steel | C > 2%, Cr, Mo | 600-800 | General wear resistance |
| Hardfacing alloy | Cr, Mo, V, W | 800-1000 | Severe wear conditions |
| Carbide composite | WC, Co, Cr | 1000-1500 | Extreme wear resistance |
| Nickel-based alloy | Ni, Cr, B, Si | 400-600 | High-temperature wear |
| Cast iron | C, Si, Mn, Cr | 500-700 | Low-cost repair |
Weld Overlay Process Selection
The selection of the weld overlay process depends on the geometry of the blade, the required overlay thickness, and the production requirements. The following processes are commonly used for disc shear blade repair:
| Process | Heat Input | Dilution | Typical Application |
|---|---|---|---|
| Submerged Arc Welding (SAW) | High (20-50 kJ/mm) | High (30-50%) | Thick overlay layers; high productivity |
| Shielded Metal Arc Welding (SMAW) | Medium (5-15 kJ/mm) | Medium (20-40%) | Small repairs; field repair |
| Gas Metal Arc Welding (GMAW) | Medium (5-20 kJ/mm) | Medium (20-35%) | Multi-pass overlay; good control |
| Flux-Cored Arc Welding (FCAW) | Medium (10-25 kJ/mm) | Low-Medium (15-30%) | High deposition rate; good wetting |
| Plasma Transferred Arc (PTA) | Low-Medium (2-10 kJ/mm) | Low (5-15%) | Thin overlay; low dilution; high quality |
| Laser Cladding | Very Low (0.5-2 kJ/mm) | Very Low (2-10%) | Thin overlay; precise control; high quality |
For disc shear blade repair, the SAW process is often used for the initial build-up of the overlay layer due to its high deposition rate and low cost. The GMAW or FCAW process may be used for subsequent passes to achieve the required overlay thickness and quality. The PTA or laser cladding process may be used for the final pass to ensure a low-dilution, high-quality overlay layer with excellent wear resistance.
The welding parameters for the disc shear blade repair are critical for the quality of the overlay layer. The following parameters should be optimized:
| Parameter | Recommended Range | Effect on Quality |
|---|---|---|
| Current (A) | 200-400 (SAW); 100-250 (GMAW) | Too high: excessive dilution, burn-through; Too low: poor wetting, LOF |
| Voltage (V) | 25-35 (SAW); 20-30 (GMAW) | Too high: wide weld bead; Too low: narrow weld bead, poor fusion |
| Travel speed (mm/min) | 200-400 (SAW); 300-600 (GMAW) | Too fast: incomplete fusion; Too slow: excessive heat input |
| Wire feed speed (mm/min) | 300-600 (GMAW) | Too fast: excessive deposition; Too slow: poor wetting |
| Shielding gas (GMAW) | Ar + CO2 (80/20 or 90/10) | Affects arc stability, penetration, and spatter |
Microstructure and Mechanical Properties
The microstructure of the weld overlay layer is a critical factor in determining the wear resistance and service life of the repaired blade. The microstructure is influenced by the composition of the overlay material, the welding process parameters, and the cooling rate. The following microstructural features are commonly observed in weld overlay layers:
| Feature | Description | Effect on Properties |
|---|---|---|
| Dendrites | Tree-like structures formed during solidification | Can act as crack initiation sites if coarse |
| Eutectic carbides | Hard carbides formed at the interface between dendrites | Provide wear resistance |
| Martensite | Hard, brittle phase formed in high-carbon steels | High hardness, low toughness |
| Austenite | Soft, ductile phase; can transform to martensite on cooling | Provides toughness; can be tempered |
| Ferrite | Soft, ductile phase; can be strengthened by alloying | Provides ductility; can be strengthened by grain refinement |
The hardness of the overlay layer is a key indicator of its wear resistance. The hardness should be sufficient to provide the required wear resistance, but not so high that it leads to brittleness and cracking. The typical hardness range for a disc shear blade overlay layer is 600-900 HV, depending on the application and the required service life.
The toughness of the overlay layer is also important, as the blade is subjected to impact loading during the cutting operation. The toughness can be evaluated using the Charpy V-notch test or the fracture toughness test (KIC). The toughness should be sufficient to prevent cracking during the cutting operation, but not so high that it reduces the wear resistance.
Engineering Practice and Quality Control
The implementation of the weld overlay repair technology for disc shear blades requires a comprehensive quality control program to ensure the reliability and performance of the repaired blade. The following quality control measures are recommended:
- Surface preparation: The worn surface of the blade should be thoroughly cleaned and prepared before welding to remove contaminants such as oil, rust, and scale that can cause porosity and lack of fusion.
- Process qualification: The welding process should be qualified in accordance with the applicable code or standard to ensure that the process is capable of producing a quality overlay layer.
- Welder qualification: The welder should be qualified to ensure that they have the skills and knowledge to produce a quality weld overlay.
- Non-destructive examination (NDE): The overlay layer should be examined using NDE methods such as magnetic particle testing (MT) or penetrant testing (PT) to detect surface defects such as cracks, porosity, and lack of fusion.
- Hardness testing: The hardness of the overlay layer should be measured to ensure that it meets the required specification.
- Grinding and finishing: The overlay layer should be ground to the required geometry and surface finish after welding to ensure that the blade is ready for use.
The quality of the repaired blade should be verified by a trial run on the steel processing line before it is put into regular service. The trial run should be conducted under controlled conditions to evaluate the performance of the repaired blade and to identify any potential issues.
Key Questions and Reflections
The study raises several important questions regarding the weld overlay repair technology for disc shear blades. First, the long-term performance of the repaired blade is not fully addressed. The wear resistance and service life of the repaired blade may be different from that of a new blade, and this should be evaluated through field trials and long-term monitoring. Second, the cost-effectiveness of the repair technology should be evaluated against the cost of blade replacement. The repair technology should be economically viable to be adopted by the steel processing industry.
From a practical standpoint, the challenge lies in balancing the need for high-quality weld overlay with the economic constraints of production. The implementation of rigorous quality control measures may increase the cost of repair, but the potential savings from extended blade life and reduced downtime justify the investment.
Study Insights and Implications
This research provides a valuable contribution to the understanding of the weld overlay repair technology for disc shear blades. The detailed analysis of the welding process, microstructure, and mechanical properties provides a practical guide for engineers involved in the repair of disc shear blades. The emphasis on process qualification, NDE, and quality control highlights the importance of a systematic approach to ensuring the reliability and performance of the repaired blade.
For engineers involved in the maintenance and repair of steel processing equipment, this study reinforces the need for a comprehensive understanding of the welding process and the importance of quality control in ensuring the long-term integrity of the repaired blade. The findings also highlight the need for continued research into the long-term performance of the repaired blade and the development of advanced welding techniques that can further improve the quality and reliability of the weld overlay layer.