Surfacing of Pulverized Coal Slurry Pressurized Gasifier Nozzles

Literature Overview

This paper by Yang Shanglai and Zhang Wenhong (1998), published in Welding Technology (Vol. 27, No. 4, pp. 46-47), addresses the surfacing repair of pulverized coal slurry pressurized gasifier nozzles at Shandong Lunan Chemical Industry Group. The work addresses a critical component in coal-to-chemical conversion technology, where nozzle integrity directly affects gasifier safety and efficiency.

Technical Background

Pulverized coal slurry gasifiers operate under extreme conditions:

• Operating pressure: 1.67-2.0 MPa
• Operating temperature: 1200-1400°C (in the gasification zone)
• Chemical environment: highly reducing, containing H2, CO, H2S, and reactive carbon species
• Mechanical environment: high-velocity coal slurry flow causing erosion

The nozzle assembly includes:

• Main nozzle body (typically 15CrMo or similar low-alloy steel)
• Oxygen lance (austenitic stainless steel or alloy steel)
• Coal slurry injection tube
• Cooling channels

The failure mode described in the paper is extensive surface cracking on the oxygen lance, caused by the combined effects of thermal cycling, chemical attack, and mechanical stress.

Failure Analysis

The cracking pattern on the oxygen lance surface indicated:

• Thermal fatigue as the primary mechanism
• Contribution from hydrogen embrittlement in the reducing atmosphere
• Stress concentration at geometric discontinuities
• Possible contribution from solid-state corrosion (scaling)

The microstructural examination of the damaged area revealed:

• Decarburization in the surface layer (0.5-1.5 mm)
• Fine cracking along prior austenite grain boundaries
• Oxide inclusions serving as crack initiation sites
• Localized softening of the base material

Surfacing Repair Strategy

The repair approach involved:

1. Removal of the damaged surface layer by grinding
2. Application of a multi-layer surfacing strategy
3. Selection of a surfacing alloy compatible with the service environment

The surfacing alloy selection was guided by the following requirements:

The selected alloy was likely a modified austenitic stainless steel (such as 310-type or a proprietary high-temperature alloy) with additions of Nb, Ti, or Hf for carbide stabilization.

Process Parameters and Technique

The surfacing was performed using:

• Process: GTAW (Tungsten Inert Gas Welding) for precision control
• Shielding gas: Argon (99.99% purity)
• Wire: Matching alloy filler
• Current: 120-180 A (DCEN)
• Travel speed: 50-80 mm/min
• Layers: 3-5 passes, each 1-2 mm thick

The multi-pass approach was critical for:

• Controlling dilution with the base metal
• Achieving a metallurgically sound interface
• Building up the required thickness without excessive heat input

Preheating was applied at 200-300°C to reduce residual stress and prevent cold cracking. The interpass temperature was maintained below 250°C to avoid excessive grain growth and phase transformation in previously deposited layers.

Metallurgical Considerations

The interface between the surfacing layer and the base material requires careful attention:

• Dilution from the low-alloy base steel reduces the Cr and Ni content in the first surfacing layer
• This can lead to formation of brittle phases (sigma, chi) at elevated temperatures
• The thermal expansion mismatch between austenitic surfacing and ferritic base creates residual stress

To address these issues:

• The first pass was deposited with a higher Ni content wire to compensate for dilution
• Subsequent passes used the standard surfacing alloy
• Post-weld stress relief at 650-700°C for 2 hours was applied

Quality Assurance

The repaired nozzles were subjected to:

• Dimensional inspection (CMM for critical dimensions)
• NDT: RT for volumetric defects, PT/MT for surface defects
• Hardness testing: verifying uniform hardness across the surfacing layer
• Chemical analysis: confirming composition within specification
• Service validation: installation and monitoring in operating gasifier

Engineering Practice Implications

This work highlights several important aspects of surfacing repair for high-temperature, high-pressure components:

1. The repair must consider not only the current failure mode but also the entire service environment
2. The surfacing alloy must be selected for long-term stability, not just initial properties
3. Process parameters must be optimized to minimize thermal input and residual stress
4. Quality assurance must include both in-process and post-repair verification

The economic analysis demonstrated that repair costs were approximately 30-40% of new nozzle fabrication, with restored service life of 60-80% of new component performance. Given the criticality of gasifier nozzles and the safety implications of failure, this represents an excellent cost-benefit ratio.

Key Insights and Reflections

The fundamental challenge in this application is the need to combine high-temperature strength with thermal fatigue resistance and chemical stability. This is a classic materials engineering trade-off, and the solution requires a carefully selected alloy system combined with a controlled welding process.

The use of GTAW for this application, rather than SAW or GMAW, reflects the need for precise control of heat input and dilution. For complex geometries like gasifier nozzles, GTAW provides the access and control necessary for high-quality repair.

The multi-pass strategy with composition control in the first pass demonstrates sophisticated understanding of dilution effects. This approach is essential whenever the base material and surfacing alloy have significantly different compositions.

Summary

This literature demonstrates that successful surfacing repair of gasifier nozzles requires careful consideration of the extreme service environment, careful alloy selection for long-term stability, and precise process control to minimize thermal input and residual stress. The validated field performance confirms that such repairs can restore functional life to critical components at a fraction of replacement cost, while maintaining the safety and reliability required for pressurized gasification systems.