CLADDING TECHNOLOGY SHANXI CO., LTDStellite Alloy Weld Overlay Technology on Hopper Cone Components
Application Background and Technical Challenges
Hopper cones, also known as lock hopper cones or cyclone cones, are critical components in coal-fired boilers and fluidized bed combustion systems, where they serve as the interface between the combustion chamber and the ash removal system. These components are subjected to extremely harsh operating conditions, including temperatures ranging from 800 °C to 1200 °C, abrasive wear from fly ash and slag particles traveling at high velocities, and thermal cycling that induces significant fatigue stresses. The use of Stellite alloy weld overlay layers is a well-established engineering solution for extending the service life of hopper cones, but the successful implementation requires careful attention to material selection, welding procedure, and quality control.
Stellite alloys, particularly Stellite 6 (ASTM B396 Alloy 6) and Stellite 21, are cobalt-chromium-tungsten alloys that exhibit exceptional resistance to high-temperature wear, corrosion, and thermal shock. The primary challenge in applying Stellite overlay layers to hopper cones lies in the combination of the complex geometry, the thick overlay requirement (typically 6–12 mm), and the stringent quality requirements for defect-free performance.
Stellite Alloy Selection and Microstructural Characteristics
The selection of the appropriate Stellite alloy for hopper cone overlay depends on the specific operating conditions, including temperature range, abrasion severity, and the presence of corrosive species. The following table compares the key properties of the two most commonly used Stellite alloys:
| Property | Stellite 6 (Alloy 6) | Stellite 21 |
|---|---|---|
| Composition (Co-Cr-W) | Co-27Cr-6W-3Fe | Co-28Cr-12W-2Fe |
| Hardness (as-welded, HRC) | 38–42 | 42–46 |
| Hardness (after aging, HRC) | 48–52 | 52–56 |
| Service temperature limit | 1100 °C | 1200 °C |
| Thermal conductivity (W/m·K) | 11 | 9 |
| Coefficient of thermal expansion (10⁻⁶/°C) | 13.5 | 14.2 |
| Typical application | General wear/corrosion | High-temperature abrasion |
The microstructure of as-welded Stellite 6 consists of a dendritic gamma (γ) matrix with interdendritic carbides, primarily M7C6 and M23C6 type. The carbide morphology and distribution are critical for wear resistance, and the welding procedure must be designed to minimize carbide coarsening and segregation. After aging at 900–950 °C for 2–4 hours, the microstructure transforms to a uniform distribution of fine M6C carbides within the gamma matrix, which provides optimal hardness and wear resistance.
For hopper cone applications where temperatures exceed 1000 °C, Stellite 21 is preferred due to its higher tungsten content and superior thermal stability. However, Stellite 21 exhibits greater susceptibility to hot cracking during welding, requiring more careful control of heat input and interpass temperature.
Welding Process Selection and Procedure Development
The welding process selection for Stellite overlay on hopper cones is governed by the geometry, the required overlay thickness, and the available equipment. The following processes are most commonly employed:
- Gas tungsten arc welding (GTAW / TIG): Suitable for thin overlay layers (1–3 mm) on small or geometrically complex areas. Provides excellent control over heat input and dilution.
- Submerged arc welding (SAW): The preferred process for thick overlay layers (6–12 mm) on large flat or gently curved surfaces. Enables high deposition rates and consistent quality.
- Electroslag welding (ESW): Used for very thick overlay layers (10–20 mm) on large hopper cone sections. Provides uniform microstructure and low dilution.
- Plasma transferred arc (PTA) powder cladding: Increasingly used for high-quality overlay layers with minimal dilution and excellent surface finish. Suitable for critical applications.
- Flux-cored arc welding (FCAW): Used for field repair and maintenance applications where portability is required.
The welding procedure specification (WPS) for Stellite overlay must be qualified in accordance with NB/T 47014 or ASME IX, with specific attention to the following parameters:
| Parameter | GTAW | SAW | ESW | PTA |
|---|---|---|---|---|
| Heat input (kJ/mm) | 5–15 | 30–60 | 80–150 | 15–30 |
| Interpass temperature (°C) | 150–250 | 150–300 | N/A (single pass) | 150–250 |
| Preheat (°C) | 100–150 | 150–200 | 200–300 | 100–150 |
| Travel speed (mm/min) | 20–50 | 100–200 | 50–100 | 30–80 |
| Dilution factor (%) | 10–20 | 5–15 | 2–8 | 5–15 |
The dilution factor is a critical parameter that directly affects the final composition and properties of the overlay layer. For Stellite 6 overlay, the dilution should be kept below 15% to ensure adequate cobalt and chromium content. The dilution can be controlled by using a transition layer of lower alloy content, by reducing the heat input, or by using a multi-layer approach with the first layer providing dilution control and subsequent layers providing the final alloy composition.
Quality Control and Defect Prevention
The quality of Stellite overlay layers on hopper cones is paramount, as any defect can lead to premature failure in the harsh operating environment. The quality control strategy should follow a systematic approach encompassing all stages of the manufacturing process:
- Pre-weld inspection: Verification of base metal chemistry, geometry, and cleanliness. The base metal surface must be free of rust, scale, and contaminants that could promote porosity or lack of fusion.
- In-process monitoring: Real-time monitoring of welding parameters including voltage, current, travel speed, and wire feed rate. Any deviation from the qualified procedure must be documented and investigated.
- Post-weld NDT: Comprehensive non-destructive testing including visual inspection (VT), magnetic particle testing (MT), ultrasonic testing (UT), and radiographic testing (RT). The acceptance criteria for Stellite overlay layers are typically more stringent than for structural welds, with zero tolerance for cracks and porosity.
- Mechanical testing: Hardness testing at multiple depths and locations, tensile testing of coupon specimens, and intergranular corrosion testing where applicable.
Common defects in Stellite overlay layers include:
| Defect Type | Root Cause | Prevention Strategy |
|---|---|---|
| Hot cracks | Excessive sulfur/phosphorus, high heat input | Control consumable chemistry, reduce heat input |
| Cold cracks | Hydrogen embrittlement, high restraint | Preheat, low hydrogen consumables, PWHT |
| Porosity | Contaminated base metal, improper gas shielding | Surface preparation, adequate shielding |
| Lack of fusion | Insufficient heat input, poor joint preparation | Increase heat input, proper bevel preparation |
| Carbide segregation | Excessive cooling rate, improper solidification | Optimize heat input, consider post-weld aging |
Engineering Case Study: Coal Boiler Hopper Cone Overlay
A case study involving the overlay of Stellite 6 on a 1.8-meter diameter hopper cone for a 1000 MW coal-fired boiler provides valuable insights into the practical challenges and solutions. The original hopper cone, fabricated from 16MnR carbon steel with a 12 mm wall thickness, experienced premature wear failure after only 8 months of operation, with the cone surface showing severe erosion and thinning in the ash flow path.
The overlay repair was performed using a two-layer SAW procedure: a transition layer of 3 mm using a nickel-iron alloy filler (ENi-Fe) to minimize dilution, followed by a 9 mm Stellite 6 overlay layer. The welding was performed in a horizontal position with the cone oriented to allow gravity-assisted slag removal. The procedure included:
- Preheat temperature: 200 °C applied uniformly over a 100 mm radius around the weld area.
- Interpass temperature: Maintained between 150 °C and 250 °C using infrared thermometers.
- Heat input: 45 kJ/mm for the transition layer, 55 kJ/mm for the Stellite layer.
- Post-weld aging: 950 °C for 4 hours in a controlled atmosphere furnace.
Post-overlay NDT revealed a single linear indication of 12 mm length and 0.2 mm width in the second Stellite pass, which was traced to a minor slag inclusion. The defect was removed by grinding, repaired with a qualified repair procedure, and re-inspected to full acceptance. The overlay layer achieved a uniform hardness of 50–52 HRC after aging, with a dilution factor of 12% confirmed by optical emission spectroscopy (OES) analysis.
After 24 months of operation under identical conditions, the overlaid hopper cone showed minimal wear, with a surface erosion rate of less than 0.5 mm per year, representing a 15-fold improvement over the original unprotected cone. Metallographic examination of a coupon taken from the worn area confirmed that the Stellite overlay layer retained its microstructural integrity with no evidence of carbide coarsening or cracking.
Study Insights and Recommendations
The successful application of Stellite alloy weld overlay on hopper cone components requires a holistic approach that integrates material science, welding engineering, and quality management. The key lessons from the technical literature and engineering practice are as follows:
First, the dilution control is the single most important factor in achieving the desired overlay properties. The transition layer approach, using a nickel-iron or austenitic stainless steel filler, effectively reduces the dilution from the carbon steel base metal and ensures that the final Stellite layer achieves the target composition and properties.
Second, the post-weld aging treatment is not optional but essential for Stellite 6 overlay layers. The as-welded microstructure contains coarse interdendritic carbides that are detrimental to wear resistance, and the aging treatment transforms these into fine, uniformly distributed M6C carbides that provide optimal hardness and toughness.
Third, the quality control strategy must be rigorous and systematic, with particular attention to the detection and repair of linear defects. The operating environment of hopper cones is extremely demanding, and any defect in the overlay layer can serve as a crack initiation site that leads to catastrophic failure.
The future of Stellite overlay technology lies in the development of advanced processes such as laser cladding and high-velocity oxygen fuel (HVOF) spraying, which offer the potential for even lower dilution, finer microstructure, and improved bond strength. However, for the foreseeable future, conventional welding processes remain the most practical and cost-effective solution for hopper cone overlay applications, provided that the procedure is carefully designed and rigorously executed.