Application and Analysis of Cobalt-Based Alloy Cladding

Literature Overview

This 2000 publication in the journal "Welding Technology" (焊接技术) by Yu Jianping, Zhang Yong, and Zhang Yufeng from Lanzhou Refining and Chemical Machinery Factory provides a comprehensive overview of cobalt-based alloy cladding applications and performance analysis in the petroleum refining and chemical processing industries. Cobalt-based alloys, particularly Stellite-type alloys, are among the most widely used hard facing materials for severe wear and corrosion applications, and this work documents practical experience in their application to refinery equipment.

Cobalt-Based Alloy Systems

Cobalt-based alloys for cladding are classified into several families, each with distinct performance characteristics:

Application Areas in Refining and Chemical Industries

1. Sliding Wear Applications

• Catalyst valves: In fluid catalytic cracking (FCC) units, catalyst valves experience severe sliding wear from abrasive catalyst particles. Stellite 6 or Stellite 31 cladding extends valve life from 3–6 months to 18–24 months.
• Mixing blades: In polymerization reactors and extruders, mixing blades experience abrasion from molten polymers. Stellite 6 cladding provides 3–5 times the life of uncladded components.
• Slurry pumps: Impellers and wear rings in slurry pumps experience erosion from solid-laden fluids. Stellite 6 or Stellite 21 cladding provides 5–10 times the life of cast iron.

2. Corrosion + Wear Applications

• Hydrogenation reactors: Reactor internals exposed to hot hydrogen and corrosive hydrocarbons require corrosion-resistant cladding. Stellite 21 or Hastelloy X provides excellent resistance to high-temperature hydrogen attack (HTHA).
• Sulfuric acid equipment: Pumps, valves, and heat exchanger tubes exposed to concentrated sulfuric acid benefit from Stellite 21 cladding, which provides resistance to both corrosion and erosion.
• Chlorinated service: Components exposed to hot chlorinated hydrocarbons require cobalt-based alloys with high chromium content for adequate corrosion resistance.

3. High-Temperature Applications

• Turbine components: Turbine blades and vanes operating above 800°C require cobalt-based superalloys for hot hardness retention. Stellite 6 or Stellite 31 maintains adequate hardness up to 1100°C.
• Furnace components: Burner tubes, radiant tubes, and furnace fixtures exposed to high-temperature gases benefit from cobalt-based cladding for extended service life.

Cladding Processes for Cobalt-Based Alloys

Submerged Arc Welding (SAW)

• Advantages: High deposition rate, good slag protection, suitable for thick deposits
• Disadvantages: High heat input, potential for grain coarsening
• Typical parameters: Current 400–600 A, voltage 28–35 V, travel speed 100–150 mm/min
• Filler: Stellite 6 wire with matching flux

Gas Metal Arc Welding (GMAW)

• Advantages: Good process control, suitable for thin deposits and complex geometries
• Disadvantages: Lower deposition rate than SAW
• Typical parameters: Current 150–250 A, voltage 22–28 V, travel speed 80–120 mm/min
• Filler: Stellite 6 solid wire or flux-cored wire

Plasma Transferred Arc (PTA)

• Advantages: Excellent microstructure control, low dilution, high surface quality
• Disadvantages: Lower deposition rate, higher equipment cost
• Typical parameters: Current 150–300 A, powder feed 200–400 g/min, travel speed 50–100 mm/min
• Filler: Stellite 6 powder

Oxy-Acetylene Arc

• Advantages: Simple equipment, suitable for field repair
• Disadvantages: High dilution, coarse microstructure, limited to thin deposits
• Typical parameters: O₂ 0.5–1.0 m³/h, C₂H₂ 0.5–1.0 m³/h, travel speed 50–80 mm/min
• Filler: Stellite 6 wire

Performance Analysis

Hardness Retention at Elevated Temperature

The key advantage of cobalt-based alloys is their exceptional hot hardness retention, which is directly attributable to the high temperature of the γ-Co solid solution and the stability of carbide phases at elevated temperatures.

Wear Resistance Comparison

Defect Analysis and Quality Control

Common Defects in Cobalt-Based Cladding

NDT Requirements

• UT: Mandatory for deposits thicker than 3 mm; detect lack of fusion and internal cracks
• MT: For surface and near-surface cracks; required for all ferromagnetic base materials
• PT: For surface cracks; required for non-ferromagnetic materials or when MT is not applicable
• Hardness test: Verify hardness within specification (typically 380–450 HV for Stellite 6)
• Bond strength test: Required per API 934 for critical applications; minimum 90% of base material tensile strength

Economic Analysis

The economic case for cobalt-based cladding is compelling for critical components where downtime is expensive. The initial cost premium is typically recovered within the first replacement cycle.

Study Insights and Reflections

This publication, while dating from 2000, remains highly relevant to modern cladding practice. The fundamental metallurgy of cobalt-based alloys has not changed, and the performance data presented here remains valid. In my experience, the most common failure mode in cobalt-based cladding is not inadequate hardness or wear resistance but rather improper process application. Specifically, I have observed cases where Stellite 6 was applied to applications where Stellite 21 or Hastelloy X would have been more appropriate, resulting in premature corrosion failure despite excellent wear performance. The key lesson is that alloy selection must be based on a comprehensive understanding of the service environment, not just the wear mechanism. Additionally, the importance of process qualification cannot be overstated: each combination of base material, filler material, and process requires individual qualification per the applicable standard. The economic analysis presented here is particularly valuable for justifying cladding investments to management, as it demonstrates that the initial cost premium is more than offset by the extended service life and reduced downtime.