Strip Electrode Electroslag Surfacing with Magnetic Control for Hydrogenation Reactor Cladding

Literature Overview

This technical paper by Gao Yan, published in Welding Technology (2010, Vol. 39, No. S1, pp. 40-41), documents the application of strip electrode electroslag surfacing (SE-ESS) technology for the internal cladding of a large hydrogenation reactor. The reactor, manufactured in 2009 for Daqing Petrochemical Company's refinery, is constructed from 2.25Cr-1Mo heat-resistant steel (90 mm wall thickness, 3010 mm internal diameter, 23788 mm total length, designed for 8.61 MPa at 425°C) and requires internal corrosion-resistant cladding of TP309 (transition layer, 3 mm) plus TP347 (overlay layer, 3.5 mm) against a harsh operating environment containing oil, H2, H2S, and NH3.

Process Selection Rationale

The selection of SE-ESS over conventional submerged arc welding (SAW) was driven by several technical requirements:

Criterion	SAW Surfacing	Strip Electrode Electroslag Surfacing (SE-ESS)
Dilution rate	25-35%	5-10%
Penetration depth	Deep (10-15 mm)	Shallow (3-5 mm)
Travel speed	200-400 mm/min	400-800 mm/min
Layer quality	Porosity, uneven profile	Uniform, dense, low defects
Overlap groove	Deep, difficult to fill	Shallow, easy to bridge
Productivity (large vessels)	Moderate	High
Suitability for thick walls	Limited by dilution	Excellent for thick walls

For a 90 mm wall thickness reactor requiring only 6.5 mm total cladding, dilution control is paramount. SAW would produce excessive base metal dilution into the stainless steel overlay, degrading the corrosion resistance of the TP347 layer. SE-ESS, with its shallow penetration and low dilution, ensures that the overlay composition remains close to the nominal TP347 specification.

Magnetic Control Device Application

The magnetic control device is a critical innovation for this application. In conventional SE-ESS, the electroslag pool is unstable and tends to drift laterally, especially on curved surfaces of large-diameter vessels. The magnetic control system employs electromagnetic coils positioned around the welding zone to generate a controlled magnetic field that:

Stabilizes the slag pool: The Lorentz force acts on the molten slag and metal, confining the pool within the desired travel path.
Controls penetration profile: By adjusting the magnetic field intensity, the heat input distribution can be modified to achieve uniform layer thickness.
Reduces angular misalignment: On curved reactor walls, the magnetic force compensates for the geometric effects that would otherwise cause the strip electrode to drift.
Improves layer uniformity: The controlled electromagnetic environment produces consistent bead geometry along the entire circumference.

The magnetic field parameters were optimized through trial welding, with typical settings of 0.5-1.5 T magnetic flux density and 50 Hz frequency for the stabilization coils.

Quality Control Considerations

For hydrogenation reactor cladding, the quality requirements are exceptionally stringent due to the following service conditions:

Hydrogen attack resistance: The 2.25Cr-1Mo base material is susceptible to hydrogen blistering and cracking (HIC/HTHA) above 400°C. The TP309 transition layer provides a diffusion barrier, while TP347 offers superior resistance to sulfide stress cracking.
H2S corrosion: The overlay must maintain a minimum Cr content of 19% and Mo content of 2-3% to resist sulfide stress corrosion cracking (SSC) per NACE MR0175 requirements.
Residual stress control: Post-weld heat treatment (PWHT) at 620-660°C for 6-8 hours is mandatory to relieve welding residual stresses and prevent delayed hydrogen cracking.
Non-destructive testing: Full coverage ultrasonic testing (UT) per ASME Section V Article 4, plus dye penetrant testing (PT) of the surface, is required to detect lack of fusion, cracks, and porosity.

Engineering Practice Insights

This case study illustrates several principles that are broadly applicable to large vessel cladding:

Process selection must consider dilution: For thin cladding layers on thick walls, processes with inherently low dilution (SE-ESS, plasma arc surfacing, laser cladding) should be preferred over deep-penetration processes.
Magnetic control is underutilized: Despite its proven effectiveness in stabilizing electroslag pools, magnetic control remains rare in industrial practice. Its application could significantly improve quality consistency in large-diameter vessel cladding.
Transition layer design is critical: The TP309 layer serves as both a metallurgical buffer (matching thermal expansion between base and overlay) and a diffusion barrier. Its thickness of 3 mm is adequate for 90 mm wall thickness but should be increased proportionally for thicker sections.
Travel speed optimization: The high productivity of SE-ESS (400-800 mm/min) must be balanced against layer quality. Excessive travel speed can cause incomplete melting of the strip electrode and poor bonding.

The successful application of SE-ESS with magnetic control for this large reactor demonstrates that advanced surfacing technologies can meet the demanding requirements of modern refinery equipment, provided that process parameters are carefully optimized and quality control is rigorously implemented.