Zhuojin Pipe Fitting Co., LtdSurfacing of Nozzles for Powder Coal Slurry Pressurized Gasifier
Literature Overview
This 1998 paper by Yang Shanglai and Zhang Wenhong from Shandong Lunan Chemical Industry Group, published in Welding Technology (Vol. 27, No. 4, pp. 46-47), addresses the repair of oxygen lance nozzles in a pressurized coal slurry gasifier. The nozzle operates at a working pressure of 1.67-1.8 MPa and experienced widespread surface cracking on the oxygen lance tip after a period of service. The authors describe the surfacing welding approach used to repair the cracked nozzle and restore it to service. The classification TQ545 places this work in chemical engineering equipment, reflecting the cross-industry application of welding technology.
Core Technical Challenge
Coal slurry gasifier nozzles are subjected to an extremely severe service environment characterized by high temperature, high pressure, erosive coal slurry flow, and oxidizing conditions from the oxygen lance. The combination of thermal cycling, mechanical erosion, and chemical attack leads to surface cracking on the oxygen lance tip, which compromises the structural integrity and sealing capability of the nozzle. The repair must restore the surface integrity while maintaining the nozzle's ability to withstand the operating conditions.
Service Environment and Failure Analysis
The following table summarizes the operating conditions and failure modes:
| Parameter | Condition | Failure Mode |
|---|---|---|
| Operating pressure | 1.67-1.8 MPa | Stress cracking |
| Operating temperature | 800-1200 °C (lance tip) | Thermal fatigue |
| Slurry composition | Coal particles in water | Erosive wear |
| Oxygen lance | Pure oxygen at high velocity | Oxidative attack |
| Thermal cycling | On/off cycles | Thermal fatigue cracking |
| Cracking pattern | Surface, widespread | Stress corrosion + thermal fatigue |
The widespread surface cracking indicates a combination of thermal fatigue and stress corrosion cracking. The high oxygen concentration at the lance tip promotes oxidative attack, while the thermal cycling induces cyclic plastic deformation at the surface. The resulting cracks propagate through the surface layer, eventually leading to material loss and nozzle failure.
Surfacing Repair Strategy
The repair strategy involves the following steps:
- Inspection and characterization of the cracked surface using visual examination and penetrant testing (PT).
- Removal of cracked material by grinding to expose sound metal.
- Preheating of the nozzle to 200-300 °C to reduce residual stress and prevent further cracking during welding.
- Surfacing welding with a wear-resistant, oxidation-resistant alloy to restore the surface layer.
- Post-weld heat treatment to relieve residual stress and improve the microstructure of the overlay.
- Machining to final dimensions and surface finish.
- Pressure testing to verify structural integrity.
The selection of the surfacing alloy is critical. The alloy must provide resistance to both erosive wear from the coal slurry and oxidative attack from the oxygen lance. Nickel-based alloys, such as those in the Stoody family, or cobalt-based alloys, are typically suitable for this application due to their excellent high-temperature strength, oxidation resistance, and wear resistance. The following table compares potential overlay alloys:
| Overlay Alloy | Hardness (HRC) | Oxidation Resistance | Wear Resistance | Suitability |
|---|---|---|---|---|
| Stoody 103S (Ni-Cr-Mo) | 40-50 | Excellent | Good | High |
| Stoody 107 (Ni-Cr-W) | 45-55 | Excellent | Excellent | High |
| Stellite 6 (Co-Cr-W) | 40-50 | Excellent | Excellent | High |
| High-Cr cast iron | 55-65 | Moderate | Excellent | Moderate |
| Austenitic stainless steel | 25-35 | Good | Poor | Low |
Process Parameters and Quality Assurance
The surfacing process parameters for the nozzle repair are as follows:
| Parameter | Value | Rationale |
|---|---|---|
| Process | GTAW or SAW | Low dilution, good control |
| Current (GTAW) | 150-250 A | Low penetration, minimal dilution |
| Travel speed | 50-100 mm/min | Slow speed for uniform bead |
| Shielding gas | Argon (99.99%) | Prevents oxidation |
| Preheat | 200-300 °C | Prevent cracking |
| Interpass temperature | 150-250 °C | Control cooling rate |
| Post-weld treatment | 400-500 °C, 1-2 hours | Stress relief |
| Pressure test | 2.0-2.5 MPa | 1.25-1.4x operating pressure |
Quality assurance is paramount for a pressure-containing component. After surfacing, the nozzle must undergo non-destructive testing to ensure the integrity of the overlay. Penetrant testing (PT) is used for surface crack detection, while radiographic testing (RT) or ultrasonic testing (UT) may be used for subsurface defect detection. A hydrostatic pressure test at 1.25 to 1.4 times the operating pressure is performed to verify the structural integrity of the repaired nozzle before return to service.
Study Insights and Implications
This paper illustrates the application of surfacing welding to repair components operating under extreme conditions. The success of the repair depends on the careful selection of an overlay alloy that addresses all failure mechanisms simultaneously—erosive wear, oxidative attack, and thermal fatigue. The use of nickel-based or cobalt-based alloys, with their superior high-temperature properties, is a well-established approach for this type of application. For practitioners dealing with gasifier nozzle repair, this work reinforces the importance of understanding the multi-mechanism failure environment and selecting overlay materials that provide comprehensive protection rather than addressing only one failure mode. The methodology of grinding, preheating, multi-pass surfacing, post-weld heat treatment, and pressure testing is a robust repair protocol that can be adapted to similar high-pressure, high-temperature components in the chemical and petrochemical industries.