CLADDING TECHNOLOGY SHANXI CO., LTDOverlay Welding of Coal-Water Slurry Pressurized Gasifier Nozzles
Literature Overview
Coal-water slurry pressurized gasifiers are core equipment in integrated coal gasification plants, operating at extreme conditions of high temperature (up to 1400°C in the gasification zone), high pressure (6.5-8.3 MPa), and highly erosive coal-water slurry containing abrasive particles. The gasifier nozzles, which inject the coal-water slurry into the gasification chamber, are subject to severe erosion-corrosion attack at the inlet end, where the slurry velocity can exceed 60 m/s. This literature addresses the overlay welding technology applied to the nozzle surfaces to extend their service life, examining process selection, material compatibility, and quality assurance strategies.
The nozzle assembly typically consists of a carbon steel or low-alloy steel body with a nickel-based alloy or cobalt-based alloy overlay layer at the erosion-prone end. The overlay must provide resistance to both mechanical erosion by abrasive coal particles and chemical corrosion by the high-temperature reducing atmosphere containing H₂S, CO, and H₂O.
Core Technical Analysis
Material System and Overlay Design
The selection of overlay material is governed by the dual requirement of erosion resistance and corrosion resistance. The following table summarizes the typical material systems employed:
| Overlay Material | Hardness (HV) | Erosion Resistance | Corrosion Resistance | Typical Application |
|---|---|---|---|---|
| Inconel 625 | 220-280 | Excellent | Excellent | High-temperature gasifier nozzles |
| Stellite 6 (Co-Cr-W) | 400-500 | Superior | Good | Abrasive slurry nozzles |
| Inconel 718 | 350-400 | Very good | Excellent | High-pressure nozzles |
| Hastelloy C276 | 200-250 | Good | Superior | Corrosive environments |
| Nickel 200 | 150-200 | Moderate | Excellent | Low-erosion conditions |
For coal-water slurry gasifier nozzles, Inconel 625 is the most commonly specified overlay material due to its excellent combination of high-temperature strength, corrosion resistance in reducing atmospheres, and good weldability. The overlay layer thickness is typically specified at 3-6 mm, with a minimum of 3 mm to provide adequate erosion allowance over the intended service interval.
Process Selection and Parameters
Given the geometry of gasifier nozzles — typically cylindrical tubes with diameters ranging from 50 to 150 mm and wall thicknesses of 8-15 mm — the process selection must accommodate the confined working space and the need for high deposition rates.
| Process | Deposition Rate | Layer Quality | Equipment Cost | Suitability |
|---|---|---|---|---|
| PTA (Plasma Transferred Arc) | 0.5-1.5 kg/h | Excellent, low dilution | High | Preferred for Inconel 625 overlay |
| GTAW (TIG) | 0.1-0.3 kg/h | Good, low dilution | Low | Suitable for thin overlay or repair |
| SAW (Submerged Arc) | 1.5-3.0 kg/h | Moderate, higher dilution | Moderate | Suitable for thick overlay on large nozzles |
| Laser Cladding | 0.3-0.8 kg/h | Excellent, very low dilution | Very high | Premium applications |
The plasma transferred arc (PTA) process is the industry standard for gasifier nozzle overlay due to its ability to achieve very low dilution rates (typically 5-10%), ensuring that the overlay layer retains its full alloy composition and corrosion resistance properties. The PTA process also produces a dense, pore-free overlay with excellent metallurgical bonding to the base metal.
Typical PTA process parameters for Inconel 625 overlay on a gasifier nozzle are as follows:
| Parameter | Value |
|---|---|
| Plasma current | 180-250 A |
| Arc voltage | 22-28 V |
| Travel speed | 150-300 mm/min |
| Powder feed rate | 400-800 g/min |
| Shielding gas | Argon (99.99%) |
| Layer thickness per pass | 0.3-0.5 mm |
| Number of passes | 8-20 (to achieve 3-6 mm total) |
| Interpass temperature | Below 150°C |
Quality Assurance and Inspection
The quality of the overlay layer is critical to the safe operation of the gasifier. The following inspection regime is typically applied:
- Visual inspection (VT): Each layer is inspected for surface cracks, undercut, and porosity. Any visible defects must be ground out and repaired before the next layer is applied.
- Magnetic particle testing (MT): Applied to the final surface to detect surface-breaking cracks and lack of fusion. The overlay layer must be free of linear indications exceeding 1 mm in length.
- Ultrasonic testing (UT): Applied to the overlay-base interface to detect subsurface defects, lack of fusion, and delamination. The acceptance criteria follow ASME V or equivalent standards.
- Hardness testing: The overlay layer hardness must be within the specified range (typically 200-300 HV for Inconel 625) to confirm the correct material composition.
- Chemical analysis: A spot check of the overlay composition is performed to verify that the dilution rate is within acceptable limits and that the alloy elements are present at the specified concentrations.
Engineering Practice Integration
In practice, the overlay welding of gasifier nozzles is performed during the fabrication stage of the gasifier, before assembly into the vessel. The nozzle is typically mounted on a rotary fixture that allows continuous rotation during the PTA process, ensuring uniform overlay thickness around the circumference.
A critical consideration is the thermal management of the nozzle during multi-pass overlay. The interpass temperature must be kept below 150°C to prevent excessive grain growth and to avoid sensitization of the nickel-based alloy. This is achieved through active cooling of the nozzle body using compressed air or water-cooled fixtures, while the nozzle head (the overlay zone) is maintained at a controlled temperature.
The metallurgical bonding between the overlay and the base metal is achieved through a series of transition layers. The first pass is deposited at a low current to ensure adequate penetration into the base metal, followed by subsequent passes at progressively higher currents to build up the overlay thickness. The dilution rate in the first pass is typically higher (15-20%), but the overall dilution rate across all passes is controlled to be below 10% by ensuring that the majority of the overlay thickness is deposited at low dilution.
A common challenge encountered in production is the formation of micro-cracks in the overlay layer, particularly in the last few passes. This is attributed to the high thermal conductivity of the nickel-based alloy, which causes rapid cooling and high residual stresses. Countermeasures include the use of a backing layer of pure nickel (Ni 200) deposited at a low current before the Inconel 625 overlay, and the application of a post-overlay stress relief treatment at 700-800°C for 1-2 hours.
Key Reflections
The overlay welding of coal-water slurry gasifier nozzles exemplifies the application of advanced welding technology to solve a severe erosion-corrosion problem in a high-value industrial asset. The selection of PTA as the preferred process is well-justified by its ability to produce high-quality overlay layers with minimal dilution, which is essential for maintaining the corrosion resistance of the nickel-based alloy in the harsh gasifier environment.
A key insight from this literature is the importance of the transition layer strategy. The direct deposition of Inconel 625 on a carbon steel base can result in cracking due to the high dilution and the formation of brittle iron carbides at the interface. The use of a nickel backing layer effectively decouples the metallurgical incompatibility between the base and the overlay, providing a ductile buffer zone that accommodates the thermal expansion mismatch.
The inspection requirements for gasifier nozzle overlay are stringent, reflecting the criticality of the component. The combination of MT, UT, and hardness testing provides a comprehensive quality assurance package, but the engineer must recognize that no single NDT method is sufficient. The overlay layer may appear sound on the surface while harboring subsurface defects that would lead to premature failure in service.
Summary
The overlay welding of coal-water slurry pressurized gasifier nozzles is a technically demanding application that requires a sophisticated understanding of nickel-based alloy metallurgy, PTA process technology, and quality assurance methodology. The successful implementation of this technology depends on careful process parameter control, rigorous inspection, and attention to thermal management during multi-pass deposition. Engineers should approach this application with a systematic approach that integrates materials science, process engineering, and quality management to ensure the reliable and durable protection of these critical gasifier components.