Application Study of Wide Strip Single-Layer High-Speed Electroslag Cladding

Literature Overview

The study by Liu Baojian, Kong Fanhong, Wang Tianxian, and Lei Chenglong from Qingdao Lanchi Heavy Machinery Equipment Co., Ltd., published in Pressure Vessel Technology (2019), addresses the practical application of wide strip single-layer high-speed electroslag cladding for pressure vessel manufacturing. This work represents a direct bridge between academic research and industrial practice, focusing on the feasibility and quality assurance of a high-productivity cladding technique for large-diameter pressure vessels.

Process Description and Technical Parameters

Wide strip electroslag welding (ESW) uses a continuous strip of consumable metal as the electrode, fed into a molten slag pool where it melts and deposits as a weld. The "single-layer" designation indicates that the entire cladding thickness is deposited in one pass, which is only feasible for relatively thin cladding layers (typically 6-12 mm). The "high-speed" aspect refers to optimized travel speeds that maximize deposition rate while maintaining acceptable quality.

Parameter	Typical Value	Engineering Significance
Strip width	30-50 mm	Determines weld width and penetration
Strip thickness	1.5-3.0 mm	Affects feeding stability and heat input
Welding current	800-1500 A	Controls melting rate and penetration
Travel speed	200-400 mm/min	High-speed operation
Slag composition	CaF₂-CaO-Al₂O₃ system	Controls pool fluidity and protection
Cladding thickness	6-12 mm	Single-layer capability
Deposition rate	8-15 kg/h	Productivity advantage
Preheat temperature	100-200°C	Reduces cracking risk

Advantages of Wide Strip Single-Layer ESW Cladding

High deposition rate: The single-layer approach eliminates interpass cooling time and reduces total welding time by 50-70% compared to multi-layer wire-fed processes.
Uniform microstructure: A single solidification event produces a more homogeneous microstructure compared to multi-layer deposits where each layer experiences different thermal histories.
Reduced residual stress: The wider, flatter weld bead produces a more uniform stress distribution compared to narrow multi-pass welds.
Lower porosity: The thick slag layer provides excellent protection against atmospheric contamination, and the slower solidification rate allows gas bubbles to escape.
Reduced dilution: The high current density at the strip-substrate interface promotes good penetration while limiting lateral dilution.

Quality Control Challenges

Despite its advantages, wide strip single-layer ESW cladding presents several quality challenges that must be addressed:

Bond strength verification: The single-layer deposit must achieve adequate bonding with the base metal. Shear bond strength testing per ASTM A522 or equivalent is mandatory. Minimum requirements for pressure vessel applications are typically 150-200 MPa depending on the base metal grade.

Microstructural homogeneity: The single-layer deposit may exhibit columnar grain growth from the base metal interface to the top surface. This columnar structure can create anisotropic properties and potential cracking paths. The grain structure should be evaluated through metallographic examination at multiple locations across the deposit width.

Surface quality: The top surface of a single-layer ESW deposit may exhibit uneven topography due to the oscillation of the molten pool. Surface finishing (grinding or machining) is typically required to achieve the specified surface finish for pressure vessel internal surfaces.

Cracking susceptibility: The rapid solidification at the strip-substrate interface can create steep thermal gradients that promote cracking, particularly in high-strength base metals. The following countermeasures are recommended:

Preheating to 150-200°C for carbon steel base metals
Use of low-hydrogen slag composition
Controlled cooling rate through back-gas heating or insulating blankets
Post-weld heat treatment per applicable code requirements

Application to Pressure Vessel Fabrication

For pressure vessel applications governed by GB/T 150, NB/T 47002, or ASME VIII Div.1, the following qualification requirements apply:

Welder qualification: The ESW process must be qualified per NB/T 47014 or ASME IX Section IX. Qualification parameters include current range, travel speed range, strip dimensions, and preheat temperature.
Procedure qualification: The welding procedure specification (WPS) must define all essential and non-essential variables. For ESW, essential variables typically include electrode type, current range, travel speed range, and filler metal composition.
NDE requirements: Radiographic testing (RT) or ultrasonic testing (UT) is required for the cladding-to-base metal interface. Surface NDE (MT or PT) is required for the cladding surface.
Hydrostatic testing: The completed vessel must pass hydrostatic testing at 1.25-1.5 times the design pressure, which verifies the integrity of the cladding-to-base metal bond.

Engineering Case Analysis

In the fabrication of large-diameter hydrogenation reactors (typically DN2000-DN4000), wide strip single-layer ESW cladding with 316L or 321 stainless steel strip has been successfully applied for internal corrosion protection. The typical configuration involves:

Base metal: 16MnR or Q345R carbon steel, thickness 40-80 mm
Cladding material: 316L or 321 stainless steel strip, 1.5-2.0 mm thick
Cladding thickness: 8-10 mm (single layer)
Application: Internal surface of reactor shell and head

The productivity advantage is significant. For a reactor shell with an internal surface area of approximately 50 m², the single-layer ESW cladding can be completed in 2-3 days compared to 7-10 days for multi-layer SAW cladding. This reduction in welding time translates directly to cost savings and shorter project schedules.

Study Insights and Reflections

The practical application of wide strip single-layer ESW cladding demonstrates that productivity improvements in pressure vessel manufacturing do not necessarily require compromises in quality. The key to successful implementation lies in thorough process qualification and rigorous quality control.

From a PDCA (Plan-Do-Check-Act) perspective, the implementation of this technology requires:

Plan: Thorough process qualification, WPS development, and welder certification
Do: Production welding with real-time monitoring of current, voltage, and travel speed
Check: In-process NDE (UT for bond quality, MT for surface defects) and post-weld mechanical testing
Act: Continuous improvement through parameter optimization and defect analysis

A critical insight is that the single-layer approach, while faster, requires more careful attention to parameter control during welding. Multi-layer processes have inherent quality built-in through the re-melting of previous layers, which can heal minor defects. Single-layer deposits have no such self-healing capability, making parameter stability even more important.

The technology is particularly well-suited for applications where the cladding thickness requirement is moderate (6-12 mm) and the surface area is large. For thicker cladding requirements (>15 mm), a combination of ESW for the bulk deposit and a finishing pass with wire-fed SAW or laser cladding may be more appropriate.

This research validates the industrial applicability of wide strip single-layer high-speed ESW cladding for pressure vessel manufacturing. The technology offers a compelling balance of productivity, quality, and cost-effectiveness for specific application ranges, and its adoption should be considered for any large-diameter pressure vessel requiring corrosion-resistant cladding within the 6-12 mm thickness range.