Study Note on Overlay Welding of Pulverized Coal Slurry Pressurized Gasifier Nozzles

Literature Overview

This paper by Yang Shanglai and Zhang Wenhong (1998), published in Welding Technology (Volume 27, Issue 4, pp. 46-47), addresses a critical repair challenge in the petrochemical gasification industry. The authors, from Shandong Lunan Chemical Industry Group Company, investigated the overlay welding repair of pulverized coal slurry pressurized gasifier nozzles that developed extensive surface cracking after a period of service. The nozzles operated at pressures of 1.67-1.87 MPa and were subjected to extreme thermal and erosive conditions. The study presents a practical solution for restoring these critical components to service.

Core Technical Content

Pulverized coal slurry gasifier nozzles are among the most demanding components in the petrochemical gasification process. These nozzles inject pulverized coal slurry into a pressurized gasification chamber where it reacts at extremely high temperatures (1300-1500°C) with oxygen and steam to produce syngas. The primary oxygen injection port (一次氧喷头) experiences particularly severe conditions:

• Thermal cycling: Repeated heating and cooling during gasification cycles
• Erosive wear: High-velocity coal slurry flow causes severe erosion
• Thermal stress: Temperature gradients between the hot gas and the cooler nozzle body
• Chemical attack: Exposure to reactive gases and molten slag
• Mechanical stress: Operating pressure of 1.67-1.87 MPa

Failure Mechanism

The paper identifies surface cracking as the primary failure mode of the primary oxygen injection port. This cracking likely results from a combination of:

1. Thermal fatigue: Repeated thermal cycling causes cyclic plastic deformation at the surface
2. Erosion-corrosion synergy: The erosive action of coal slurry exposes fresh metal to chemical attack
3. Residual stress: Manufacturing and service stresses concentrate at surface defects
4. Microstructural degradation: Prolonged exposure to high temperatures causes grain growth and phase transformations

Overlay Welding Solution

The overlay welding repair approach addressed the surface cracking problem by:

1. Removal of damaged material: Grinding or machining away the cracked surface layer
2. Application of overlay weld: Depositing a wear-resistant, crack-resistant alloy layer
3. Restoration of geometry: Rebuilding the nozzle surface to original dimensions
4. Performance verification: Testing the repaired nozzle for service readiness

Overlay Material Selection

The selection of overlay material for gasifier nozzle repair requires consideration of multiple performance criteria:

Process Considerations

For this application, the following process considerations are critical:

1. Preheating: Essential to reduce thermal gradients and minimize cracking in both the substrate and overlay
2. Interpass temperature: Must be carefully controlled to prevent excessive HAZ softening or cracking
3. Welding sequence: Must minimize constraint and allow for thermal expansion
4. Heat input: Must balance penetration with dilution control
5. Post-weld treatment: Stress relief is essential to reduce residual stresses

Engineering Practice Implications

Connection to Pipe and Fitting Manufacturing

The overlay welding challenges encountered in gasifier nozzle repair are analogous to several applications in the steel pipe manufacturing industry:

FMEA Analysis for Gasifier Nozzle Overlay Welding

Applying Failure Mode and Effects Analysis (FMEA) to the overlay welding repair of gasifier nozzles:

Quality Verification Requirements

For safety-critical components like gasifier nozzles operating under pressure, comprehensive quality verification is mandatory:

1. Visual inspection: 100% inspection of all overlay weld surfaces
2. Magnetic particle testing (MT): 100% coverage for surface and near-surface defects
3. Ultrasonic testing (UT): For subsurface defects and delamination detection
4. Hardness testing: Systematic measurements across the overlay thickness
5. Dimensional verification: Confirmation of restored geometry
6. Pressure testing: Hydrostatic or pneumatic test to verify pressure integrity
7. Service life monitoring: Tracking performance during subsequent operation

Study Insights and Reflections

This paper highlights several important principles for overlay welding applications in severe service environments:

1. Understanding the failure mechanism is essential: The authors' identification of surface cracking as the primary failure mode guided the selection of overlay material and process parameters. Without this understanding, the repair might not address the root cause.
2. Multi-factor degradation: In severe service environments, components rarely fail from a single mechanism. The combination of thermal cycling, erosion, chemical attack, and mechanical stress creates complex degradation patterns that require comprehensive solutions.
3. Overlay welding as a repair strategy: For components operating in extreme conditions, overlay welding can extend service life significantly, but only if the overlay material and process are properly matched to the service environment.
4. Economic considerations: The cost of overlay repair is typically a small fraction of the cost of complete component replacement, making it an attractive maintenance strategy for expensive, critical components.

Modern Context

Contemporary overlay welding for severe service applications benefits from:

• Advanced nickel-based and cobalt-based overlay alloys with superior high-temperature performance
• Multi-layer overlay strategies with graded compositions for optimal property transitions
• Advanced process monitoring for real-time parameter control
• Metallurgical modeling for predicting microstructure and properties
• Advanced NDE techniques for comprehensive quality verification
• Thermographic monitoring for detecting early-stage degradation during service

Application to Pipe Manufacturing

The principles from this study are applicable to several pipe manufacturing challenges:

• Burner nozzle repair: Gas and oxygen burner nozzles in pipe heating furnaces experience similar thermal and erosive conditions
• Welding torch tip restoration: Tips used in pipe welding operations require periodic restoration due to wear
• Furnace tube repair: Tubes in pipe annealing and heat treatment furnaces require overlay welding for corrosion and erosion protection
• Cooling nozzle restoration: Water cooling nozzles in pipe manufacturing experience erosion from high-velocity water flow

Summary

This study demonstrates the successful overlay welding repair of pulverized coal slurry gasifier nozzles that developed surface cracking under extreme thermal, erosive, and pressure conditions. The technical approach—identifying the failure mechanism, selecting appropriate overlay material, and implementing a controlled welding process—provides valuable guidance for similar overlay welding challenges in the steel pipe manufacturing industry, particularly for components operating under combined thermal, erosive, and mechanical stress conditions.