Process Parameters Effect on Strip Electrode Electroslag Surfacing

Literature Overview

This paper by Zhang Liangcheng and Yin Heping, published in China Chemical Equipment (Vol. 4, No. 1, 2002, pp. 23-34), provides a comprehensive technical analysis of strip electrode electroslag surfacing (ESW) and its process parameters. The work is particularly significant for engineers working on large-scale surfacing applications, such as boiler tubes, pressure vessel linings, and large structural components where high deposition rates are required.

Electroslag Surfacing vs. Submerged Arc Surfacing

Fundamental Comparison

The paper provides a detailed comparison between strip electrode electroslag surfacing (ESW) and conventional strip electrode submerged arc surfacing (SAW):

Parameter	SAW (Submerged Arc)	ESW (Electroslag)
Arc voltage	25-35 V	30-45 V
Current density	20-40 A/mm²	80-150 A/mm²
Deposition rate	5-15 kg/h	15-50 kg/h
Penetration depth	3-8 mm	8-20 mm
Slag consumption	Moderate	High
Equipment complexity	Moderate	High
Surface quality	Good	Requires grinding
Dilution rate	15-30%	25-45%

The electroslag process operates at significantly higher current densities than the submerged arc process. In ESW, the arc is submerged under a thick layer of molten slag, and the heat source is primarily the resistance heating of the slag pool rather than the arc itself. This fundamental difference in heat generation mechanism results in a much wider, shallower weld pool with higher deposition rates.

Process Principle

In strip electrode ESW:

A strip electrode (typically 10-20 mm wide, 1-3 mm thick) is fed through a contact shoe into the molten slag pool.
The electrical resistance of the slag generates intense heat, melting the electrode tip and the base metal surface.
The molten metal is displaced by the rising slag and solidifies to form the weld.
The slag pool acts as a thermal shield, reducing heat loss to the atmosphere and maintaining a stable process.

The key process variables include:

Strip electrode width and thickness
Electrode feed speed
Welding current and voltage
Slag composition and thickness
Travel speed
Electrode stick-out length

Process Parameter Analysis

Current and Voltage

The welding current is the primary parameter governing heat input and deposition rate:

Current range: 800-3000 A for typical strip electrode ESW operations.
Voltage range: 30-45 V, depending on the slag composition and electrode configuration.

Increasing the current increases the heat input, resulting in deeper penetration and higher deposition rate. However, excessive current can cause:

Slag splash and process instability.
Excessive dilution of the base metal.
Widening of the weld bead beyond acceptable limits.
Increased risk of cracking due to higher residual stresses.

The optimal current is determined by the desired bead width, penetration depth, and the mechanical properties required of the overlay.

Travel Speed

Travel speed directly affects the heat input per unit length:

Low travel speed: High heat input, deep penetration, wide bead, high dilution.
High travel speed: Low heat input, shallow penetration, narrow bead, low dilution.

For surfacing applications, the travel speed must be selected to balance deposition rate against dilution control. A typical range is 200-600 mm/min, depending on the current level and desired overlay properties.

Slag Composition

The slag composition is critical for process stability and weld quality:

Slag Component	Function	Typical Range
SiO₂	Viscosity control	30-50%
CaF₂	Fluidity improvement	20-35%
Al₂O₃	Viscosity control	5-15%
CaO	Basicity control	10-25%
MnO	Deoxidation	5-15%

The slag must maintain appropriate viscosity to support the electrode while allowing smooth flow. Too viscous a slag causes process instability and bead irregularity; too fluid a slag causes slag inclusion and surface defects.

Electrode Stick-Out Length

The stick-out length (the distance from the contact shoe to the slag pool surface) affects:

Heat distribution in the electrode.
Current density at the electrode tip.
Arc stability.

A typical stick-out length is 5-15 mm. Too short a stick-out causes excessive heating of the contact shoe; too long a stick-out causes electrode drooping and process instability.

Process Window and Parameter Optimization

The paper provides guidance on establishing the process window for strip electrode ESW:

Start with a baseline parameter set based on the electrode width and thickness, and the desired bead geometry.
Vary one parameter at a time to establish its individual effect on bead width, penetration, and surface quality.
Identify the stable operating range for each parameter, avoiding the boundaries where process instability occurs.
Optimize for the specific application by balancing deposition rate, dilution control, and surface quality requirements.

For surfacing applications, the following optimization targets are typical:

Bead width: 20-50 mm (depending on component geometry).
Penetration: 3-8 mm (sufficient for fusion bond without excessive dilution).
Surface roughness: Ra < 25 μm (before grinding).
Dilution rate: < 35% (for property-controlled overlays).

Engineering Applications

Strip electrode ESW is particularly suited for:

Large-format surfacing of boiler tubes and pressure vessel linings.
Thick overlay build-up on heavy structural components.
Corrosion-resistant and wear-resistant surfacing on large areas.

The high deposition rate of ESW (15-50 kg/h) makes it significantly more productive than conventional arc surfacing methods for large-area applications. However, the process requires specialized equipment (strip electrode feed mechanism, slag control system, and travel mechanism) and is less flexible for small or irregular geometries.

Quality Control Considerations

For strip electrode ESW surfacing, the following quality control measures are recommended:

Process monitoring: Continuous monitoring of current, voltage, travel speed, and electrode feed speed to detect process deviations.
Visual inspection: Check for surface defects, slag inclusion, and bead uniformity.
Dimensional inspection: Verify overlay thickness and surface profile.
Hardness testing: Verify overlay hardness profile, particularly at the fusion line where dilution is highest.
Metallographic examination: Check for lack of fusion, cracks, and inclusions at the fusion boundary.

Key Reflections

This paper provides a thorough technical foundation for engineers considering strip electrode ESW for large-scale surfacing applications. The systematic approach to process parameter analysis—understanding the individual effect of each parameter and then optimizing the parameter combination for the specific application—is a methodology that can be applied to any welding process. The comparison between ESW and SAW is particularly valuable for process selection decisions, as it clearly demonstrates the trade-offs between deposition rate, dilution control, and equipment requirements. Engineers should note that while ESW offers superior productivity for large-format applications, the higher dilution rate must be carefully managed through parameter optimization and consumable selection to ensure the overlay meets the required property specifications.