Zhuojin Pipe Fitting Co., LtdApplication of Tungsten-Chromium-Cobalt Overlay Layers in Slag Discharge and Collection Tanks
Service Environment and Wear Mechanism Analysis
This study by Liu Jianzhou and Liu Yuying from Sinopec Ningbo Engineering Co., Ltd., published in Chemical Equipment and Piping in 2011 (Vol. 48, No. 1, pp. 64-66), addresses the critical challenge of wear protection for slag discharge tanks and slag collection tanks in petrochemical and metallurgical operations. These vessels are subjected to extremely harsh service conditions that include high-temperature molten slag impact, abrasive particle erosion, thermal cycling, and chemical corrosion. Understanding the specific wear mechanisms is essential for selecting appropriate overlay materials.
The wear mechanisms in slag tanks can be categorized into several distinct types. Impact wear occurs when molten slag droplets or semi-solid slag particles strike the tank surface at high velocity, causing material removal through deformation and fracture. Abrasive wear is caused by solid slag particles embedded in the flowing slag stream that act as grinding agents against the tank wall. Thermal fatigue results from repeated heating and cooling cycles as hot slag contacts the tank surface, creating alternating compressive and tensile stresses that lead to crack initiation and propagation. Chemical corrosion occurs when reactive slag components dissolve into or react with the steel surface, weakening the material.
The combination of these wear mechanisms makes slag tank protection one of the most challenging applications in the field of overlay welding. A single overlay material rarely addresses all wear mechanisms simultaneously, and the selection must be based on a thorough understanding of the dominant wear mechanism under specific operating conditions.
Selection of Overlay Materials and Alloy Design
The study systematically evaluates different wear-resistant overlay materials suitable for slag tank applications, with particular focus on tungsten-chromium-cobalt (W-Cr-Co) alloy systems. The following table compares common overlay material types used in slag tank protection:
| Material Type | Typical Composition | Hardness (HRC) | Key Advantage | Limitation |
|---|---|---|---|---|
| High-carbon martensite | 4-5% C, 12-15% Cr | 55-62 | Cost-effective, good impact resistance | Limited thermal stability above 400°C |
| W-Cr-Co alloy | 5-8% W, 15-20% Cr, 5-10% Co | 50-60 | Excellent thermal stability, good impact resistance | Higher cost |
| Hardfacing carbide composite | WC/Co or W2C/Co | 60-70 | Very high hardness | Brittle, susceptible to impact wear |
| Austenitic Ni-Cr | 5-8% Ni, 20-25% Cr | 35-45 | Good corrosion resistance, ductile | Lower wear resistance |
The W-Cr-Co alloy system is particularly well-suited for slag tank applications because of several key metallurgical advantages. Tungsten forms extremely hard carbides (WC with hardness of approximately 1500 HV and W2C with approximately 1600 HV) that provide excellent abrasive wear resistance. The high melting point of tungsten carbides (2870 degrees Celsius for WC) ensures thermal stability at elevated operating temperatures. Chromium provides solid solution strengthening and forms protective oxide films that resist chemical attack. Cobalt acts as a binder phase that improves toughness and impact resistance while also enhancing the thermal stability of the overlay.
The microstructure of W-Cr-Co overlay alloys typically consists of a martensitic or austenitic matrix with dispersed tungsten carbides. The specific phase distribution depends on the cooling rate during welding and the exact alloy composition. In slag tank applications, a balanced microstructure with sufficient carbide volume fraction for wear resistance and adequate matrix toughness for impact resistance is desirable.
Microstructure-Property Relationships
The study emphasizes the importance of understanding how alloy elements and microstructure influence the performance of overlay materials in slag tank service. The key relationships can be summarized as follows:
- Tungsten content directly correlates with carbide volume fraction and hardness. Increasing W from 5% to 8% typically increases hardness from approximately 50 HRC to 60 HRC, but excessive tungsten can lead to brittleness and cracking.
- Chromium content affects both hardness and corrosion resistance. Chromium levels of 15-20% provide adequate solid solution strengthening and form protective Cr2O3 oxide films on the surface.
- Cobalt content influences toughness and thermal stability. Cobalt levels of 5-10% are generally optimal, providing good impact resistance without significantly reducing hardness.
- Carbon content controls carbide formation. Sufficient carbon is needed to form tungsten carbides, but excessive carbon can lead to free graphite formation and reduced toughness.
The microstructure of the overlay layer is heavily influenced by the welding process parameters, particularly the heat input and cooling rate. In slag tank applications, multi-pass overlay welding is typically used to achieve sufficient coating thickness (usually 3-6 mm). Each pass undergoes a different thermal history, which can result in microstructural variations between layers. The top layers, which cool more slowly due to the insulating effect of previously deposited layers, may develop coarser microstructures and lower hardness compared to the first layer.
Engineering Practice Considerations
From a practical engineering standpoint, several factors must be considered when applying W-Cr-Co overlay layers to slag tanks. First, the base material preparation is critical. The tank surface must be thoroughly cleaned to remove rust, scale, oil, and other contaminants that could lead to poor metallurgical bonding or inclusion defects. In some cases, the base material may need to be preheated to reduce residual stresses and prevent cracking, particularly for thick-walled tanks.
Second, the welding procedure must be carefully controlled. The heat input should be kept within a narrow range to achieve the desired microstructure and avoid excessive dilution. Too much heat input can dissolve carbides and reduce hardness, while too little can lead to incomplete fusion and poor bonding. The interpass temperature should be maintained at a level that prevents cracking without causing excessive grain growth.
Third, the overlay geometry must be designed to accommodate thermal expansion and contraction during service. Slag tanks experience significant temperature gradients, and the overlay layer must be able to accommodate these thermal strains without cracking or delaminating. In some cases, stress-relief grooves or flexible joint designs may be incorporated into the overlay pattern.
Fourth, the inspection and quality assurance procedures must be rigorous. Common defects in slag tank overlays include lack of fusion, porosity, cracking, and insufficient coating thickness. Non-destructive testing methods such as ultrasonic testing (UT) for thickness measurement, magnetic particle testing (MT) for surface cracking, and visual inspection for surface quality are essential. In critical applications, destructive testing of coupon samples may be required to verify hardness, microstructure, and bonding strength.
The study also highlights the importance of considering the entire lifecycle cost when selecting overlay materials. While W-Cr-Co alloys are more expensive than high-carbon martensitic hardfacing materials, their superior performance in high-temperature slag environments often results in longer service intervals and lower total maintenance costs. A life-cycle cost analysis should be conducted before making the final material selection, taking into account the expected service life, maintenance frequency, downtime costs, and safety implications of tank failure.
In conclusion, this study provides valuable guidance for the selection and application of W-Cr-Co overlay materials in slag tank protection, emphasizing the importance of matching the overlay material's microstructure and properties to the specific wear mechanisms and service conditions encountered in petrochemical and metallurgical operations.