Application of Surfacing Technology Under External Magnetic Field on Chemical Equipment

Literature Overview

Wu Lili's 2011 paper in Liaoning Chemical Industry (Vol. 40, No. 11, pp. 1194-1196) explores the application of external magnetic field-assisted plasma arc surfacing technology for chemical equipment repair, specifically targeting polymerization kettle applications. The study employs Fe5 iron-based alloy powder with a DC transverse magnetic field applied during plasma arc surfacing, and investigates the relationship between magnetic field parameters and the resulting wear resistance and hard phase morphology.

Technical Principles

Magnetic Field Effects on Solidification

The application of an external DC transverse magnetic field during surfacing influences the solidification process through several mechanisms:

Lorentz force on liquid metal — Induced currents in the molten pool interact with the applied magnetic field, producing forces that influence dendrite growth patterns
Convection modification — Magnetic forces alter natural and forced convection within the melt pool, affecting solute distribution and grain refinement
Dendrite fragmentation — Magnetic forces can break fragile dendrite arms, increasing nucleation sites and refining grain structure
Phase transformation modification — Altered thermal conditions may influence the precipitation sequence and morphology of hard phases

Plasma Arc Surfacing Process Parameters

Parameter	Typical Value	Function
Plasma gas	Argon	Stable plasma column and inert atmosphere
Powder feed rate	80-150 g/min	Controls deposition rate and dilution
Plasma current	150-250 A	Determines heat input and penetration
Travel speed	100-200 mm/min	Controls bead width and profile
Powder-to-gas ratio	2-4:1	Ensures full powder melting and deposition
Shielding gas	Argon	Prevents surface oxidation

Magnetic Field Parameter Optimization

Experimental Design

The study systematically varied magnetic field parameters to identify optimal conditions for enhanced wear performance:

Magnetic Field Parameter	Range Tested	Optimal Value	Effect
Field strength	0-1.5 T	0.8-1.2 T	Maximum hardness enhancement
Field direction	Transverse DC	Perpendicular to travel	Uniform effect across bead width
Field duration	Continuous	Throughout surfacing	Consistent solidification modification
Pole distance	50-150 mm	80-100 mm	Uniform field distribution

Hard Phase Morphology Under Magnetic Field

The application of external magnetic field influences the morphology and distribution of hard phases (primarily chromium carbides) in the surfacing layer:

Without magnetic field: Coarser carbide particles (5-20 μm), tendency toward network formation at grain boundaries
With magnetic field (0.8-1.2 T): Finer carbide particles (2-8 μm), more uniform distribution within matrix, reduced network formation
With excessive field (>1.5 T): Potential for irregular carbide agglomeration due to excessive convection disruption

The refinement of hard phase morphology directly correlates with improved wear resistance through:

Reduced stress concentration at carbide-matrix interfaces
More uniform load-bearing capacity across the surfacing layer
Decreased susceptibility to intergranular fracture initiation
Improved resistance to micro-cracking during thermal cycling

Application to Polymerization Kettles

Service Environment Analysis

Polymerization kettles in chemical processing operate under demanding conditions:

Temperature cycling: 20-300°C during normal operation
Chemical exposure: Contact with polymer monomers, catalysts, and solvents
Mechanical loading: Stirrer shaft bearings, agitator seals, and internal surfaces subject to abrasive wear from solid polymer deposits
Corrosion risk: Aggressive chemical environment requiring corrosion-resistant surfacing

Material Selection Rationale

The Fe5 iron-based alloy powder was selected for its:

Adequate hardness (45-55 HRC in as-deposited condition)
Good metallurgical compatibility with carbon steel kettle substrates
Sufficient corrosion resistance for moderate chemical exposure
Economic viability for large-area surfacing applications

Quality Control and Verification

Post-Surfacing Inspection Requirements

Inspection Method	Acceptance Criteria	Purpose
Hardness testing (HBW/HRC)	≥45 HRC, uniformity ±5 HRC	Verify microstructure development
Metallographic examination	No cracks, no excessive retained austenite	Confirm sound microstructure
Bond strength test	≥250 MPa	Verify base-metal/surfacing adhesion
Surface roughness	Ra ≤ 6.3 μm	Ensure smooth surface for chemical contact
Dimensional check	Within ±0.5 mm of nominal	Maintain equipment geometry

Study Insights and Engineering Implications

The integration of magnetic field technology with plasma arc surfacing represents an innovative approach to microstructure control without requiring post-weld heat treatment. This is particularly advantageous for large chemical vessels where post-weld heat treatment is impractical or prohibited due to equipment constraints.

The key finding that moderate magnetic field strength (0.8-1.2 T) produces optimal results — with diminishing or adverse effects at higher strengths — underscores the importance of parameter optimization rather than simply maximizing field intensity. The transverse orientation relative to the travel direction provides the most uniform effect across the surfacing bead width, which is critical for maintaining consistent wear performance across the entire repaired surface.

For chemical equipment maintenance programs, this technology offers a compelling alternative to conventional surfacing when enhanced wear performance is required without post-weld heat treatment capability. The equipment investment for magnetic field generation is moderate, and the process is readily integrated into existing plasma arc surfacing operations. However, the technology requires careful parameter development for each specific application, as the optimal magnetic field conditions depend on the consumable composition, substrate geometry, and target microstructure.