CLADDING TECHNOLOGY SHANXI CO., LTDStrip Cladding Process for Lock Hopper Cone
Literature Overview and Technical Background
Lock hoppers are critical components in bulk material handling systems, particularly in coal-fired power plants, steel mills, and cement plants, where they serve as transition devices between different pressure zones or as discharge points for bulk solids. The conical section of a lock hopper is particularly susceptible to abrasive and erosive wear due to the high-velocity impact of bulk materials, especially when handling abrasive particles such as coal, ore, or cement clinker. The literature under study focuses on the application of strip cladding (electroslag welding overlay) technology to extend the service life of lock hopper cones by providing a wear-resistant overlay layer on the interior conical surface.
Strip cladding, also known as electroslag strip welding or electroslag overlay, is a high-productivity welding process that deposits a continuous strip of wear-resistant alloy onto a base plate or structural component. The process utilizes a molten slag pool as the heat source, creating a stable and consistent weld bead with excellent penetration and bond strength. For lock hopper cones, the conical geometry presents unique challenges in terms of strip cladding process parameters, equipment configuration, and quality control.
Core Technical Findings
Process Parameters for Conical Surface Cladding
The strip cladding process for lock hopper cones requires careful optimization of welding parameters to accommodate the curved geometry. Key process parameters include:
| Parameter | Typical Range | Notes |
|---|---|---|
| Welding current | 1200-1800 A | Adjusted for strip thickness and base metal thickness |
| Arc voltage | 32-42 V | Maintained within narrow window for stable slag pool |
| Travel speed | 300-600 mm/min | Slower for thicker overlays, faster for thinner layers |
| Strip thickness | 3.0-6.0 mm | Typically 3.0 mm or 4.0 mm for lock hopper cones |
| Strip width | 30-50 mm | Matches desired overlay width |
| Preheat temperature | 100-200°C | Depends on base metal carbon equivalent |
| Interpass temperature | 200-350°C | Maintained to control cooling rate |
| Slag composition | Flux with CaF2, CaO, SiO2 | Customized for alloy strip compatibility |
Strip Material Selection
The literature discusses several strip materials suitable for lock hopper cone cladding, each offering different combinations of wear resistance, impact toughness, and weldability:
- High-chromium cast iron strips (e.g., 14-18% Cr): Excellent abrasive wear resistance, moderate impact toughness, suitable for dry abrasive materials
- Hardfacing alloy strips (e.g., Fe-Cr-C-Mo): Good abrasive and erosive wear resistance, better impact toughness than cast iron, suitable for mixed conditions
- Maraging steel strips (e.g., 18Ni maraging): Excellent impact toughness, good abrasive wear resistance, suitable for high-impact applications
- Stainless steel strips (e.g., 316L, 309L): Corrosion-resistant with moderate wear resistance, suitable for environments with corrosive media
For typical coal-handling lock hoppers, high-chromium cast iron strips with 14-16% Cr and 1.5-2.5% C provide the best balance of wear resistance and cost-effectiveness. The overlay thickness is typically designed to be 3-5 mm after machining, with a total deposited thickness of 5-8 mm to allow for post-weld machining.
Microstructural Characteristics
The microstructure of the electroslag strip cladding layer is characterized by a columnar dendritic structure growing from the base metal-overlay interface. The cooling rate in electroslag welding is relatively low (typically 5-50°C/s), which promotes the formation of coarse carbides, particularly M7C3 type carbides in high-chromium alloys. While these carbides provide excellent hardness (typically 55-65 HRC for 14-16% Cr strips), they may reduce impact toughness.
The literature notes that the dilution rate in electroslag strip cladding is typically 5-15%, which is higher than in plasma arc cladding but lower than in submerged arc welding with wire electrodes. The dilution rate affects the final carbon and chromium content of the overlay layer, and consequently the hardness and wear resistance. A dilution rate above 15% may reduce the hardness below acceptable levels for abrasive wear applications.
Process Implementation and Quality Control
Equipment Configuration for Conical Surfaces
The conical geometry of the lock hopper presents significant challenges for strip cladding. The literature describes several approaches:
- Mandrel-based cladding: The cone is placed over a rotating mandrel, and the strip cladding gun is held at a fixed position while the workpiece rotates. This approach is suitable for cones with a minimum diameter of approximately 300 mm.
- CNC-guided cladding: A computer-controlled torch holder follows the conical surface path, maintaining consistent travel speed and torch angle. This approach offers the highest flexibility and parameter control.
- Segmental cladding: The cone is divided into segments, and each segment is cladded individually with overlapping beads. This approach is used for large cones where continuous cladding is impractical.
Quality Control Procedures
The quality of the strip cladding layer is verified through the following inspection methods:
| Inspection Method | Purpose | Acceptance Criteria |
|---|---|---|
| Visual examination (VT) | Surface defects, undercut, porosity | No visible cracks, undercut < 0.5 mm |
| Magnetic particle testing (MT) | Surface and near-surface cracks | No linear indications |
| Ultrasonic testing (UT) | Lack of fusion, internal defects | No indications exceeding acceptance level |
| Hardness testing | Overlay hardness uniformity | 55-65 HRC for high-Cr alloys |
| Bond strength test | Overlay-base metal adhesion | Minimum 300 MPa (per ASTM A264) |
| Metallographic examination | Microstructure, dilution | No unmelted base metal, acceptable grain size |
Common Defects and Countermeasures
| Defect | Cause | Countermeasure |
|---|---|---|
| Slag inclusion | Inadequate slag removal between passes | Improve slagging, ensure complete slag removal |
| Cracking at interface | Excessive cooling rate, hydrogen | Increase preheat, use low-hydrogen flux |
| Uneven overlay thickness | Inconsistent travel speed, torch angle variation | Use CNC guidance, monitor parameters in real time |
| Overburn at interface | Excessive heat input | Reduce current, increase travel speed |
| Dilution too high | Excessive penetration into base metal | Reduce current, increase strip thickness |
Engineering Practice Integration
In practical applications, the strip cladding of lock hopper cones must be integrated into the overall fabrication sequence. The cone is typically fabricated from carbon steel plate (e.g., Q345R or ASTM A516 Gr.70) by rolling and welding, and the strip cladding is applied after the cone geometry is established. Post-weld machining is required to achieve the specified internal surface profile and dimensional tolerances.
A typical fabrication sequence includes:
- Plate cutting and edge preparation
- Cone rolling and seam welding
- Dimensional inspection and correction
- Preheat to 150-200°C
- Strip cladding of interior conical surface (typically 2-3 passes for 5-8 mm total thickness)
- Interpass temperature control at 250-300°C
- Post-weld stress relief at 550-600°C for 2 hours
- Machining of interior surface to specified profile and Ra < 12.5 μm
- Hardness verification and non-destructive examination
- Final dimensional inspection and hydrostatic testing
The literature emphasizes that the strip cladding process must be qualified under the applicable welding procedure qualification standard (e.g., NB/T 47014 or ASME IX) with additional requirements for bond strength testing and wear testing specific to the service application.
Study Insights and Reflections
The strip cladding process for lock hopper cones represents a mature and cost-effective technology for extending the service life of wear-critical components. The high productivity of electroslag strip welding (typically 10-20 kg/h of deposited metal) makes it economically attractive for large surface areas, such as the interior of conical hoppers. However, the process requires significant investment in specialized equipment (rotating mandrels, CNC torch holders) and skilled operators.
A key insight from this literature is that the conical geometry introduces process complexities that must be carefully managed. The varying surface angle along the cone requires continuous adjustment of torch angle and travel speed to maintain consistent heat input and penetration. Without proper process control, the overlay thickness and dilution rate can vary significantly along the cone, leading to inconsistent wear performance and premature failure at the thinnest overlay sections.
The literature also highlights the importance of post-weld machining in achieving the required surface quality. The as-welded overlay surface is typically rough (Ra 50-100 μm) and must be machined to achieve the specified profile and surface finish. The machining allowance must be sufficient to remove all surface irregularities while maintaining the minimum overlay thickness. A typical machining allowance of 1.5-2.5 mm per side is recommended.
This literature provides valuable guidance for engineers involved in the design and fabrication of wear-resistant lock hoppers. The process parameters, material selection criteria, and quality control procedures described herein can be adapted to similar applications, including silo liners, chute linings, and other conical or curved wear components in bulk material handling systems.