Strip-Electrode Electroslag Welding Overlay and Magnetic Control Device Application in Hydrogenation Reactor Cladding

Literature Overview

This paper by Gao Yan, published in Welding Technology in 2010 and authored by an engineer from Daqing Petrochemical Machinery Manufacturing Company, focuses on the application of strip-electrode electroslag welding (ESW) overlay technology with a magnetic control device in hydrogenation reactor cladding. Hydrogenation reactors are among the most demanding pressure vessels in the petrochemical industry, operating under high temperature, high pressure, and hydrogen-rich environments that require corrosion-resistant overlay layers on carbon or low-alloy steel shells.

Process Description and Technical Parameters

Strip-electrode ESW overlay involves feeding a continuous strip electrode into an electrically conductive slag pool, where the strip melts and forms the overlay layer. The magnetic control device is employed to stabilize the molten pool, improve heat distribution, and enhance the quality of the overlay. This technique offers high deposition rates, excellent metallurgical bonding, and uniform layer thickness, making it particularly suitable for large-diameter reactor shells.

Process Parameter	Typical Range	Function
Welding current	8000–12000 A	Controls melting rate
Travel speed	80–150 mm/min	Controls layer thickness
Strip thickness	3.0–4.0 mm	Determines pass thickness
Strip width	40–60 mm	Controls bead overlap
Preheat temperature	200–300 °C	Reduces residual stress
Interpass temperature	< 400 °C	Controls cooling rate

Magnetic Control Device Function

The magnetic control device generates a controlled magnetic field around the welding zone, which serves several purposes. First, it stabilizes the slag pool and prevents fluctuations in the molten metal that could lead to uneven deposition. Second, it influences the solidification pattern, promoting columnar to equiaxed grain transition and reducing the likelihood of hot cracking. Third, it can help manage the directional solidification of the overlay, which is particularly important for thick multi-pass cladding on large reactor shells.

Application in Hydrogenation Reactors

Hydrogenation reactors typically use 2.25Cr-1Mo or 1.25Cr-0.5Mo steel shells with nickel-based alloy overlays such as Inconel 625 or Monel 400. The ESW overlay process is preferred for large-diameter vessels due to its high deposition efficiency and deep penetration, which ensures reliable bonding between the base material and the overlay. The magnetic control device addresses the challenge of maintaining consistent overlay quality over long welding runs typical of large vessel fabrication.

Quality Control and Defect Prevention

Defect Type	Cause	Countermeasure
Slag inclusion	Poor slag fluidity	Optimize slag composition and temperature
Lack of fusion	Insufficient heat input	Increase current or reduce travel speed
Hot cracks	High carbon dilution	Use low-carbon transition layer
Undercut	Excessive arc force	Adjust magnetic control parameters
Excessive dilution	Too high heat input	Reduce current, increase travel speed

Engineering Practice Insights

The use of magnetic control in ESW overlay represents an advanced technique for improving process stability. In hydrogenation reactor fabrication, the overlay layer must withstand hydrogen attack, which requires careful control of dilution and microstructure. Post-weld heat treatment (PWHT) is mandatory to relieve residual stresses and stabilize the overlay microstructure. Non-destructive testing typically includes ultrasonic testing (UT) for bond integrity and magnetic particle testing (MT) for surface defects.

Summary

This work demonstrates that the integration of magnetic control technology with strip-electrode ESW overlay significantly enhances the quality and reliability of cladding on hydrogenation reactors. The magnetic stabilization of the slag pool reduces process variability, while the high deposition rate of ESW makes it economically viable for large-scale reactor fabrication. Engineers working on similar applications should pay close attention to the interplay between magnetic field parameters, welding current, and travel speed to achieve optimal overlay quality.