CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay of Coal Water Slurry Pressurized Gasifier Nozzles
Literature Overview
Coal water slurry (CWS) gasifier nozzles are subjected to one of the most severe combined degradation environments encountered in industrial practice: high-temperature oxidation, severe abrasive erosion from solid particles in the slurry, and thermal fatigue from cyclic temperature changes. This literature review and study note focuses on the weld overlay solutions developed for these nozzles, examining the selection of overlay materials, welding process parameters, and the performance of the resulting overlay layers under simulated gasifier conditions.
Overlay Material Selection and Design Rationale
The selection of overlay materials for CWS gasifier nozzles is governed by the requirement to resist both erosion and oxidation simultaneously. The literature identifies three principal material systems that have been evaluated:
| Material System | Typical Composition | Hardness (HV) | Erosion Resistance | Oxidation Resistance | Cost |
|---|---|---|---|---|---|
| High-Cr Fe-based (Cr25–30) | Cr25Mo4, Cr26Ni16 | 350–450 | Good | Excellent | Moderate |
| Ni-based (Inconel 625) | Ni-22Cr-9Mo-3Nb | 250–300 | Fair | Excellent | High |
| High-Cr Co-based (Stellite 6) | Co-28Cr-6W | 400–500 | Excellent | Good | Very High |
| Hardfacing alloy (Cr-C-Mo) | Cr15C3Mo2 | 600–800 | Excellent | Poor | Low |
The most successful approach, as documented in the literature, involves a multi-layer overlay strategy. The first layer (bond layer) is a nickel-based alloy (e.g., Inconel 625 or Alloy 600) applied to ensure metallurgical compatibility with the base steel (typically Cr-Mo steel such as 12Cr1MoV or 15CrMo). The subsequent wear layers consist of high-chromium iron-based alloys or Co-Cr alloys, depending on the specific erosion and oxidation severity at different nozzle locations.
Welding Process Parameters and Defect Control
The welding process parameters for nozzle overlay are critical because the geometry of the nozzle creates significant challenges for weld access and heat input control. The literature recommends the following process parameters for GTAW (TIG) and SAW overlay:
- GTAW overlay: Current 120–180 A, voltage 14–18 V, travel speed 3–6 cm/min, shielding gas Ar with 2–5% H2 or He for improved penetration. Wire feed rate 0.5–0.8 m/min.
- SAW overlay: Current 400–600 A, voltage 25–35 V, travel speed 8–15 cm/min, flux with RE deoxidizers for improved cleanliness.
- Preheat temperature: 150–250°C for Cr-Mo base steels to prevent cold cracking.
- Interpass temperature: Maintain below 250°C to minimize intergranular sensitization of the bond layer.
Common defects observed in nozzle overlay include:
- Hot cracking in the nickel-based bond layer, caused by low melting point eutectics at grain boundaries. Countermeasures include controlling sulfur content in the filler metal (<0.01%) and using a slightly lower current with faster travel speed.
- Undercut and incomplete fusion at the nozzle root, caused by poor electrode accessibility. Countermeasures include using a smaller diameter electrode and a specialized welding positioner.
- Porosity in the overlay layer, caused by inadequate shielding or contaminated base metal. Countermeasures include pre-cleaning the base metal with a wire brush and using a trailing shield for GTAW.
Performance Testing and Field Results
The literature reports erosion testing results showing that the multi-layer overlay system (Inconel 625 bond layer + Cr25Mo4 wear layer) achieves an erosion resistance 3–5 times that of the base 12Cr1MoV steel under simulated CWS conditions. The overlay layer thickness is typically maintained at 3–5 mm, with a minimum residual thickness of 1.5 mm before replacement.
Field experience from several gasification plants confirms that the overlay nozzles last 1.5–2.5 times longer than unprotected nozzles, with the primary failure mode shifting from erosion to thermal fatigue cracking at the overlay-base metal interface. This interface cracking is attributed to the mismatch in thermal expansion coefficients between the overlay and base metal, and can be mitigated by using a ductile nickel-based bond layer.
Study Insights and Engineering Recommendations
This literature provides a comprehensive framework for overlay design of CWS gasifier nozzles, and the key insight is that no single overlay material can simultaneously address all degradation mechanisms. The multi-layer approach, combining a ductile bond layer with a hard, oxidation-resistant wear layer, represents the optimal engineering compromise. For future improvements, I recommend investigating laser cladding as an alternative to arc welding, as it offers lower heat input, reduced dilution, and the ability to deposit functionally graded layers with a gradual transition in composition from the base metal to the wear layer. This could further extend the service life of gasifier nozzles and reduce the frequency of costly shutdowns for nozzle replacement.