Zhuojin Pipe Fitting Co., LtdOverlay Welding of Continuous Casting Hot Shearing Blades
Literature Overview and Industrial Motivation
The paper by Xu Hengjun, Jiang Jianmin, Xiong Dijing, and Li Hui'e, published in "New Technology and New Process" (1997, No. 3, pp. 32–33), addresses the design and fabrication of hot shearing blades for continuous casting steel billets using overlay welding. Hot shearing blades are critical components in the continuous casting process, responsible for cutting red-hot steel billets at temperatures exceeding 1000°C. The blades must withstand extreme thermal cycling, abrasive contact with hot steel, and mechanical impact during the shearing operation. The conventional approach of manufacturing blades entirely from high-alloy tool steel such as 3Cr2W8 is effective but expensive and time-consuming. The authors propose an alternative approach using a cost-effective quenched and tempered medium-carbon low-alloy steel as the base material, with a high-performance overlay layer applied to the cutting edge. This approach extends blade life by 2–3 times compared to conventional all-alloy blades while reducing production costs.
Metallurgical Design and Material Selection
Base Material Selection
The authors selected a quenched and tempered medium-carbon low-alloy steel as the base material for the blade body. This material provides adequate toughness and strength to withstand the mechanical loading during shearing, while being significantly less expensive than high-alloy tool steel. The typical composition of the base steel includes 0.4–0.5% carbon, 1.0–1.5% manganese, and small amounts of chromium, molybdenum, and vanadium. After quenching and tempering, the base material achieves a hardness of 30–38 HRC, which provides sufficient strength and impact resistance for the blade body.
Overlay Material Design
The overlay material for the cutting edge was designed to provide high hardness, thermal fatigue resistance, and wear resistance at elevated temperatures. The optimal composition was determined through laboratory trials and field testing. The overlay material contains 2.0–2.5% carbon, 5–6% chromium, 1–2% tungsten, and 0.5–1.0% vanadium, producing a high-carbon martensitic microstructure with dispersed carbides. The resulting overlay layer achieves a hardness of 60–65 HRC, which provides excellent resistance to abrasive wear at the cutting edge. The tungsten and vanadium carbides are particularly effective at resisting thermal fatigue cracking, which is a common failure mode for hot shearing blades.
Dilution Rate Control
The dilution rate between the base metal and the overlay layer is a critical parameter that affects the hardness and wear resistance of the overlay. The authors report a dilution rate of 10–15%, which is considered optimal for achieving the target hardness and microstructure. Excessive dilution reduces the carbon and alloy content of the overlay, leading to lower hardness and reduced wear life. The dilution rate was controlled by using a single-layer overlay with a thin bead profile and a high alloy content in the consumable.
Welding Process and Performance Evaluation
The overlay welding was performed using shielded metal arc welding (SMAW) with a specialized overlay electrode. The welding parameters were optimized to minimize dilution and ensure good fusion with the base metal. The following table summarizes the key welding parameters and performance results:
| Parameter | Value | Method/Note |
|---|---|---|
| Base material hardness | 30–38 HRC | After quenching and tempering |
| Overlay material composition | 2.0–2.5% C, 5–6% Cr, 1–2% W, 0.5–1.0% V | Electrode design |
| Overlay hardness | 60–65 HRC | Rockwell hardness test |
| Dilution rate | 10–15% | Metallographic analysis |
| Welding current | 100–150 A | SMAW |
| Preheat temperature | 150–200°C | Reduce cracking risk |
| Blade life improvement | 2–3 times | Field trial comparison |
| Cost reduction | Significant | Base material savings |
The field trials demonstrated that the overlay-welded blades achieved a service life of 2–3 times that of conventional all-alloy blades made from 3Cr2W8. The improved life was attributed to the superior hardness and thermal fatigue resistance of the overlay layer, combined with the adequate toughness of the base material. The overlay-welded blades also performed well in repair applications, where worn or damaged blades could be restored by applying a new overlay layer to the cutting edge.
Quality Control and Defect Analysis
The quality of the overlay layer was verified through metallographic examination, hardness testing, and field performance evaluation. Metallographic analysis revealed a fine-grained martensitic microstructure with dispersed carbides, confirming the effectiveness of the alloy design. The overlay layer showed good fusion with the base metal, with no cracks, pores, or lack of fusion defects. The dilution rate was verified to be within the target range of 10–15%, ensuring adequate hardness and wear resistance.
Common defects in overlay welding of hot shearing blades include cracking, porosity, and excessive dilution. Cracking is primarily caused by high carbon content in the overlay and rapid cooling. Countermeasures include proper preheating, controlled cooling rate, and low-hydrogen consumables. Porosity can be caused by contaminated base metal or flux, and is mitigated by thorough surface cleaning. Excessive dilution reduces the hardness of the overlay and is controlled by optimizing welding parameters and consumable composition.
Reflections and Engineering Implications
The paper by Xu et al. presents a practical and economically attractive solution to the problem of hot shearing blade wear in continuous casting. The approach of using a cost-effective base material with a high-performance overlay layer is a classic example of the "functionally graded material" concept, where different materials are used in different regions of a component to optimize performance and cost. The overlay layer provides the necessary hardness and wear resistance at the cutting edge, while the base material provides the required toughness and strength for the blade body.
One of the key insights from this paper is the importance of dilution rate control. In overlay welding, the dilution rate directly affects the composition and properties of the overlay layer. A dilution rate that is too high reduces the hardness and wear resistance, while a dilution rate that is too low may lead to poor bonding and cracking. The authors' systematic approach to optimizing the dilution rate through consumable design and welding parameter selection is a valuable lesson for engineers working on similar overlay welding applications.
The paper also highlights the economic benefits of overlay welding. By using a less expensive base material and applying a thin overlay layer of high-performance alloy, the overall cost of the blade is significantly reduced while the service life is extended. This approach is particularly attractive for components that experience localized wear, where the entire component does not need to be made from the most expensive material.
Summary
The overlay welding approach for continuous casting hot shearing blades is an effective and cost-efficient solution to the problem of blade wear in high-temperature shearing applications. The paper by Xu et al. demonstrates that a carefully designed overlay material, combined with a suitable base material and optimized welding parameters, can achieve a service life of 2–3 times that of conventional all-alloy blades while reducing production costs. The approach is applicable to other components that experience localized wear at elevated temperatures, and the principles of dilution rate control and metallurgical design discussed in the paper are broadly relevant to overlay welding engineering.