CLADDING TECHNOLOGY SHANXI CO., LTDApplication of Tungsten-Chromium-Cobalt Overlay Layer in Slag Ladle
Literature Overview
The 2011 paper by Liu Jianzhou and Liu Yuying, published in "Chemical Equipment and Piping" (化工设备与管道), describes the application of a tungsten-chromium-cobalt (W-Cr-Co) overlay layer for protecting slag ladles in the petrochemical industry. The authors are affiliated with Sinopec Ningbo Engineering Co., Ltd., one of China's leading petrochemical engineering companies. This work addresses a practical and severe wear problem in the refining and chemical industry where molten slag and ash handling equipment is subjected to extreme thermal and abrasive conditions.
Service Environment and Failure Analysis
Slag ladles in petrochemical plants are used to transfer molten slag, coke ash, and other high-temperature particulate materials from furnaces and reactors to disposal or recycling areas. The service conditions are exceptionally severe:
| Parameter | Typical Value |
|---|---|
| Slag temperature | 1200–1500°C |
| Ladle operating temperature | 800–1200°C |
| Slag density | 2.8–3.5 g/cm³ |
| Abrasive particle size | 0.5–15 mm |
| Thermal cycling rate | 5–20 cycles/day |
| Slag chemical composition | SiO₂, Al₂O₃, CaO, Fe₂O₃, plus minor impurities |
The primary failure mechanisms in slag ladles include:
- Abrasive wear from molten slag particles impacting and sliding across the ladle surface
- Thermal fatigue from repeated heating and cooling cycles causing surface cracking
- Chemical attack from reactive slag components (particularly SiO₂ and alkali metals) dissolving into the overlay surface
- Spalling from thermal stress-induced delamination of the overlay layer
W-Cr-Co Overlay Alloy Design
The tungsten-chromium-cobalt system was selected for several compelling reasons:
- Tungsten forms extremely hard WC (hardness 1500–2000 HV) and W₂C carbides that provide exceptional abrasion resistance at elevated temperatures. Tungsten also has an extremely high melting point (3422°C), providing thermal stability.
- Chromium forms Cr₇C₃ and Cr₃C₂ carbides that contribute to hardness and provide oxidation resistance through the formation of protective Cr₂O₃ scales.
- Cobalt serves as a binder metal that dissolves into the austenitic matrix, providing solid solution strengthening and, critically, improving the hot hardness retention of the overlay. Co is also essential for forming Co-W solid solutions that resist thermal softening.
| Component | Content (wt%) | Function |
|---|---|---|
| W | 20–35 | Refractory carbide formation, hot hardness |
| Cr | 8–15 | Oxidation resistance, secondary carbides |
| Co | 8–20 | Binder matrix, hot strength retention |
| C | 3.0–5.0 | Carbide formation |
| Fe | Balance | Matrix base |
Overlay Process and Manufacturing
The overlay was applied using multi-pass submerged arc welding (SAW) or gas metal arc welding (GMAW) with flux-cored wire. The manufacturing sequence was as follows:
- Base preparation: The ladle steel surface was ground to remove rust, scale, and any previously failed overlay material. A chamfer of 30° × 3 mm was machined at the edges to facilitate overlay termination.
- Preheating: The ladle was preheated to 300–400°C using induction heating or flame heating to reduce thermal gradients and minimize cracking risk.
- Overlay application: Three to five passes were applied, with each pass approximately 2–3 mm thick. The interpass temperature was maintained between 200–350°C.
- Post-weld treatment: Stress relief annealing at 600–700°C for 2 hours reduced residual stresses without significantly softening the overlay.
- Surface finish: The overlay surface was ground to a smooth finish (Ra ≤ 6.3 μm) to reduce slag adhesion and facilitate material flow.
Performance Results
The W-Cr-Co overlay demonstrated the following performance characteristics:
| Property | W-Cr-Co Overlay | Conventional Q345R Steel | Improvement Factor |
|---|---|---|---|
| Hardness at 20°C (HRC) | 58–65 | 20–25 | N/A |
| Hardness at 800°C (HV) | 350–420 | 120–150 | 3× |
| Abrasive wear life | 18–24 months | 3–5 months | 5–6× |
| Thermal fatigue cracks | Minimal (isolated) | Extensive network | Qualitative |
| Chemical attack depth | <0.1 mm after 12 months | 2–5 mm after 12 months | Significant |
The extended service life of 5–6 times represents a substantial economic benefit, considering that slag ladle replacement requires significant downtime for the associated process equipment.
Microstructural Analysis
The overlay microstructure consists of:
- Matrix: Austenitic (Co-W solid solution) with 10–20% retained martensite formed during air cooling
- Primary carbides: WC (5–15 μm), Cr₇C₃ (3–8 μm), and Co₃W (2–5 μm)
- Secondary carbides: Fine Cr₃C₂ and (Cr,W)₇C₃ particles (0.5–2 μm) dispersed in the matrix
- Grain size: 30–60 μm equiaxed structure
The retained martensite provides additional hardness contribution but also introduces brittleness. This is acceptable in slag ladle applications where the primary loading is abrasive rather than impact. For applications where impact loading is significant (such as ladle dumping), the overlay should be tempered to reduce brittleness.
Engineering Practice Considerations
Several practical lessons emerge from this application:
- Overlay thickness matters: For slag ladles, a minimum overlay thickness of 8–10 mm is recommended. Thinner overlays (<5 mm) tend to be penetrated by thermal cracks before the overlay is worn through, leading to premature failure.
- Termination quality: The edges of the overlay are the most common initiation sites for spalling. Proper termination with a tapered profile and adequate overlap (minimum 20 mm beyond the ladle edge) is essential.
- Inspection intervals: Visual inspection of the overlay should be conducted monthly, with thickness measurement by ultrasonic testing (UT) every 6 months. When the remaining overlay thickness falls below 3 mm, re-overlay should be performed before failure occurs.
- Compatibility with base material: For ladles made of high-strength low-alloy steel (such as Q345R or 16MnR), the thermal expansion mismatch with the W-Cr-Co overlay is manageable. However, for ladles made of austenitic stainless steel, the coefficient of thermal expansion is closer to that of the overlay, reducing thermal stress at the bond interface.
Key Reflections
This case study illustrates the practical engineering challenge of matching overlay alloy properties to specific service conditions. The W-Cr-Co system excels in high-temperature abrasive wear applications but would be inappropriate for applications requiring high ductility or resistance to thermal shock. The cobalt binder is particularly effective at maintaining hardness at elevated temperatures because the Co-W solid solution does not undergo the same degree of thermal softening as Fe-Cr systems.
From a broader perspective, this application exemplifies the value of surface engineering in extending the service life of critical equipment. Rather than designing the entire ladle from expensive wear-resistant materials, applying a localized overlay to the wear zones achieves the same functional result at a fraction of the material cost. This philosophy of targeted surface protection is applicable across the petrochemical industry for equipment such as cyclone separators, ash handling chutes, and furnace linings.
The 2011 publication date of this paper is notable — the technology described remains relevant today, though newer approaches such as plasma transferred arc (PTA) cladding and laser cladding of W-Cr-Co alloys now offer finer microstructural control and reduced dilution. However, for large-area applications such as slag ladles, conventional arc welding methods remain the most cost-effective and scalable approach.