Surfacing of Cobalt-Based Hardfacing Alloys for Pressure Vessel Applications

Literature Overview

This paper by You Guangwei and Dong Anxia from Nanjing Chemical Industrial Company, Sinopec Group, documents practical experience with cobalt-based hardfacing alloy surfacing on pressure vessels in the coal chemical industry. Published in "Pressure Vessel" journal in 2007, the paper provides valuable field-level insights into the weldability and process control of cobalt-based hardfacing alloys, particularly Stellite 6, in the context of critical pressure-containing equipment.

Technical Background

The coal chemical industry imposes demanding requirements on pressure vessels, combining:

• Corrosion resistance to hydrogen sulfide, carbon dioxide, and other acid gases
• High-pressure resistance (typically 20-80 MPa operating pressure)
• Fatigue resistance under cyclic thermal and pressure loading
• Impact and wear resistance for components exposed to slurry or particulate flow

Cobalt-based hardfacing alloys, particularly Stellite 6 (UNS J91100 / CoCr16Mo6W6), offer an excellent combination of these properties due to:

Weldability Challenges

Cobalt-based hardfacing alloys present unique welding challenges:

Metallurgical Challenges

• High melting point: Stellite 6 melts at approximately 1340-1400°C, requiring high heat input
• Limited ductility: As-cast Stellite 6 has low elongation, making it susceptible to cracking
• Carbide formation: Chromium carbides (Cr7C3, Cr23C6) form at grain boundaries, reducing toughness
• Dilution sensitivity: Excessive base metal dilution can degrade the alloy properties
• Residual stress: High thermal contraction coefficient generates significant residual stresses

Process Challenges

• Preheating requirements: Typically 300-500°C preheat required to reduce cooling rate
• Interpass temperature control: Must be maintained above 200°C to prevent cold cracking
• Post-weld heat treatment: Stress relief at 800-900°C may be required for critical applications
• Welding position limitations: Gravity-fed molten metal limits vertical and overhead applications

Process Parameters and Control

Based on the paper and engineering practice, typical welding parameters for Stellite 6 surfacing include:

Welding Procedure Development

A systematic approach to Stellite 6 surfacing procedure development:

1. Base metal preparation: Remove paint, rust, and contaminants; bevel if thick overlay required
2. Preheating: Uniform preheat to 300-500°C; verify with temperature measurement
3. First pass: Use transition alloy (e.g., Ni-Fe or Ni-Cr) to reduce dilution cracking
4. Subsequent passes: Apply Stellite 6 consumables with controlled heat input
5. Interpass monitoring: Maintain temperature above 200°C throughout welding
6. Post-weld treatment: Stress relief if required by design code
7. Inspection: Visual, PT, and dimensional verification

Engineering Practice Considerations

Pressure Vessel Code Compliance

For pressure vessels governed by ASME Section VIII or GB/T 150, surfacing operations must comply with:

• WPS/PQR requirements: Welding procedure qualification per applicable code
• NDE requirements: Typically PT for surface defects; UT or RT for subsurface defects
• Welder qualification: Special qualification for hardfacing deposits
• Documentation: Complete records of procedure, parameters, and inspections

Quality Control Protocol

Key Questions and Reflections

The paper mentions the development of two pressure vessels with hardfacing, but does not provide detailed information on the specific service conditions or the long-term performance of the surfacing. In coal chemical applications, the primary wear mechanisms are typically:

• Erosive wear: From high-velocity slurry flow containing solid particles
• Corrosive wear: Combined chemical attack and mechanical abrasion
• Thermal shock: From rapid temperature changes during startup and shutdown

The critical question is whether the Stellite 6 overlay maintains its integrity under these combined loading conditions. Field experience suggests that Stellite 6 performs well in erosive environments up to approximately 600°C, but above this temperature, the carbide structure may begin to coarsen, reducing wear resistance.

Another important consideration is the interaction between the hardfacing overlay and the base metal during thermal cycling. The coefficient of thermal expansion mismatch between Stellite 6 (~13 × 10⁻⁶/°C) and typical carbon steel base metals (~12 × 10⁻⁶/°C) is relatively small, which is advantageous. However, the residual stresses from the surfacing process can be significant, and cyclic loading may eventually lead to fatigue cracking at the overlay-base metal interface.

Study Insights and Implications

This paper provides valuable practical experience with cobalt-based hardfacing in the demanding environment of coal chemical pressure vessels. The key insight is that successful hardfacing of critical pressure-containing equipment requires not only appropriate consumable selection but also rigorous process control, particularly regarding preheating and interpass temperature management. The experience documented here underscores the importance of treating hardfacing as a critical welding operation subject to the same quality control rigor as structural welding. For engineers specifying hardfacing on pressure vessels, the recommendations are: always use a transition layer, maintain strict thermal control, perform comprehensive NDE, and consider post-weld stress relief for critical applications.