Electroslag Welding Fabrication of Stainless Steel/Carbon Steel Clad Plates: Microstructure and Mechanical Properties

1. Definition and Fundamental Principles

Electroslag welding (ESW) for clad plate fabrication is a solid-state bonding process that employs a molten slag pool and an electric arc to achieve metallurgical and/or mechanical bonding between a corrosion-resistant stainless steel facing layer and a structural carbon steel backing layer. Unlike conventional fusion welding processes, electroslag welding operates on the principle of electrothermal heating: an electric current passes through a continuously supplied molten flux (slag), which generates intense resistive heating. This heat melts the base metals at the interface and the consumable filler wire, producing a weld pool that solidifies progressively as the torch assembly traverses the joint.

In the context of clad plate manufacturing, ESW serves as a hybrid bonding technique where the stainless steel cladding layer is deposited onto the carbon steel substrate through successive passes of electroslag welding. The process is particularly suited for thick-section clad plates (typically 12 mm and above) where the thermal input is managed to achieve sound metallurgical bonding without excessive dilution of the base metal or degradation of the cladding alloy composition.

The fundamental bonding mechanism depends on the degree of interfacial mixing:

• Metallurgical bonding — achieved when sufficient thermal input causes interfacial melting and interdiffusion between the stainless steel and carbon steel, creating a continuous microstructural transition zone with no discrete interface.
• Mechanical bonding — achieved when the interface remains partially unmelted but achieves intimate contact under pressure and subsequent cooling, forming a mechanically interlocked joint.

For production-grade clad plates used in pressure vessels and piping systems, metallurgical bonding is the preferred and typically required outcome, as it ensures superior resistance to peel-off and shear loading under cyclic and thermal stress conditions.

2. Category and Business Positioning

Within Cladding Technology Shanxi Co., Ltd.'s capability portfolio, electroslag welding for clad plate fabrication occupies a distinct and complementary position alongside the company's three primary technology routes: TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding. The following table delineates the positioning:

Technology Route	Typical Clad Thickness	Typical Substrate Thickness	Primary Bonding Mechanism	Key Advantage
TIG/MIG Weld Overlay	1–10 mm	5–100 mm	Full metallurgical	Low dilution, precise composition control
Hydraulic Explosive Bonding	1–8 mm	3–50 mm	Mechanical (jetting)	Minimal dilution, preserves alloy properties
Explosion Welding	1–15 mm	3–100 mm	Mechanical (jetting)	Large panel production, high production rate
Electroslag Weld Clad	3–25 mm	12–200 mm	Mixed metallurgical/mechanical	Thick-section capability, high deposition rate

Electroslag welding is particularly valuable for applications requiring thick cladding layers on heavy structural substrates—such as reactor internals, large-diameter pressure vessel heads, and thick-walled pipe spools—where TIG/MIG overlay would require excessive number of passes and hydraulic explosive bonding may face dimensional limitations. The high deposition rate of ESW (typically 5–15 kg/h) provides a significant productivity advantage over arc-based overlay methods for thick cladding requirements.

3. Technical Purpose and Value

3.1 Primary Technical Objectives

1. Interfacial bond integrity — Achieving a defect-free metallurgical bond at the clad-base interface with sufficient shear and peel strength to withstand service loading and thermal cycling.
2. Microstructural control — Managing the dilution ratio, grain morphology, and phase composition at the interface to prevent brittle phases (such as martensite in carbon steel or sigma phase in stainless steel) that compromise toughness and corrosion resistance.
3. Mechanical property optimization — Ensuring the cladding layer retains its corrosion resistance while the base metal maintains its structural strength, with a controlled transition zone that does not create a weak link.
4. Weld quality assurance — Eliminating internal defects (slag inclusions, porosity, hot cracks, lack of fusion) that would compromise the structural integrity of the clad assembly.

3.2 Value to Product Delivery and Customer Satisfaction

The research findings on microstructure and properties of ESW-fabricated clad plates directly contribute to:

• WPS (Welding Procedure Specification) qualification — Providing the metallurgical justification for process parameter ranges (current, voltage, travel speed, flux composition) that produce acceptable microstructures and mechanical properties.
• Non-conformance reduction — Understanding the relationship between process parameters and interfacial defects enables proactive control, reducing the rate of rejected material and rework.
• Customer confidence — Documented microstructural and mechanical property data provides customers with evidence of product quality, supporting certification submissions to regulatory bodies.
• Process innovation — Identifying optimal flux compositions, preheat temperatures, and interpass temperature ranges that improve weldability and reduce residual stress in thick-section clad plates.

4. Key Process and Implementation Points

4.1 Process Parameters

The following table summarizes typical electroslag welding parameters for stainless steel (e.g., 304L, 316L, 321) clad on carbon steel (e.g., Q345R, SA-516 Gr.70, 16MnR) substrates:

Parameter	Typical Range	Influence on Microstructure	Control Strategy
Welding Current	600–1200 A	Higher current increases dilution and grain coarsening	Step-wise optimization per clad layer thickness
Welding Voltage	35–45 V	Higher voltage increases slag pool volume and heat input	Maintain within ±2 V for uniform bead profile
Travel Speed	80–200 mm/min	Slower speed increases dilution and cooling time	Correlate with current for constant heat input
Preheat Temperature	100–250 °C	Reduces thermal gradients and residual stress	Adjust per substrate thickness and ambient conditions
Interpass Temperature	≤250 °C (typically 150–200 °C)	Higher interpass temp increases grain coarsening and dilution	Monitor with IR pyrometer; cool between passes
Flux Composition	CaO-SiO₂-Al₂O₃-MnO system	Affects slag viscosity, deoxidation, and dilution rate	Customize per clad-base material combination
Filler Wire	ER308L, ER316L, ER309L	Determines cladding alloy composition and dilution behavior	Match to target clad alloy; consider dilution in selection

4.2 Critical Process Sequence

1. Surface preparation — Both the stainless steel cladding strip and carbon steel substrate surfaces must be cleaned to remove oxide, scale, oil, and moisture. Methods include mechanical grinding to bare metal, chemical pickling, or shot blasting. Surface roughness should be controlled to Ra ≤ 12.5 μm for reliable bonding.
2. Assembly and fit-up — The cladding strip is positioned on the prepared substrate surface. For multi-layer cladding, successive strips are stacked and each layer is welded to the previously deposited layer. Gap and misalignment tolerances should be within ±0.5 mm.
3. Flux preheating and initiation — The flux is preheated to a molten state (typically 1000–1200 °C) before welding initiation. The arc is struck to create the initial slag pool, after which the electrode is advanced and the torch assembly begins traversal.
4. Multi-pass welding — For clad thicknesses exceeding 5 mm, multiple passes are required. Each subsequent pass is deposited with controlled interpass temperature to manage cumulative heat input and dilution. The number of passes depends on clad thickness and single-pass bead width.
5. Post-weld treatment — Depending on the application and material combination, post-weld heat treatment (PWHT) may be required. For austenitic stainless steel cladding on carbon steel, stress relief at 550–650 °C for 2–4 hours is typical. Solution annealing of the clad layer (1050–1100 °C followed by rapid cooling) may be applied when full corrosion resistance restoration is required.

4.3 Microstructural Analysis Key Findings

Systematic metallographic examination of ESW-fabricated clad plates reveals several critical microstructural features that govern performance:

• Interface zone morphology — The clad-base interface typically exhibits a transition zone 50–300 μm wide where elemental interdiffusion occurs. The width of this zone is directly proportional to heat input and interpass temperature. Excessive zone width (>500 μm) indicates excessive dilution and potential loss of corrosion resistance in the transition region.
• Dilution ratio — The dilution of base metal carbon into the cladding layer is a critical parameter. For 304L/316L cladding on carbon steel, dilution should be controlled to ≤10–15% to maintain adequate corrosion resistance. Dilution exceeding 20% can result in carbide precipitation and sensitization at grain boundaries.
• Grain structure — The clad layer typically exhibits columnar dendritic grains growing from the interface toward the surface. Columnar grain width increases with decreasing cooling rate (higher heat input). Fine equiaxed grains near the interface indicate rapid solidification, while coarse columnar grains in upper layers indicate slower cooling.
• Phase composition — In the transition zone, mixed phases of austenite (from clad), ferrite (from base), and martensite (from rapid cooling of dilution zone) may coexist. Martensitic phases in the transition zone are undesirable as they reduce toughness and may crack under thermal cycling.
• Carbide precipitation — Chromium carbides (Cr₂₃C₆, Cr₇C₃) may precipitate at grain boundaries in the heat-affected zone (HAZ) of the cladding layer if the material is held in the sensitization temperature range (450–850 °C) for extended periods. This is a primary concern for corrosion performance.

4.4 Mechanical Property Benchmarks

Test Property	Acceptance Criteria (Typical)	Test Method	Failure Indication
Shear Strength (Interface)	≥ 200 MPa (or ≥ 0.8 × UTS of base metal)	ASTM E290 / GB/T 2651	Delamination or interfacial fracture
Peel Strength	≥ 30 MPa (or no separation per visual inspection)	ASTM E290 / ISO 14230	Clad-base separation under peel load
Hardness (Clad Layer)	≤ 250 HV (for 304L/316L); ≤ 300 HV (for 321)	ASTM E92 / GB/T 231	Excessive hardness indicates martensite or sensitization
Hardness (Base Metal HAZ)	≤ 350 HV (per ASME Sec. IX)	ASTM E92 / GB/T 231	Excessive HAZ hardening indicates brittle microstructure
Tensile Strength (Clad)	≥ 515 MPa (304L); ≥ 515 MPa (316L)	ASTM A370 / GB/T 228	Below specification indicates excessive dilution
Impact Toughness (Clad)	≥ 27 J at −29 °C (if required)	ASTM E23 / GB/T 229	Low toughness indicates embrittlement or cracking

5. Applicable Standards and Acceptance Criteria

5.1 Clad Plate Product Standards

• GB/T 13401 — Steel composite plates (Chinese national standard for clad plate specifications, testing, and acceptance)
• NB/T 47057 — Composite plates for pressure vessels (Chinese industry standard specific to pressure equipment)
• ASTM A270 — Standard specification for clad steel plate (American standard for general clad plate)
• ASME SA-467 — Clad steel plate for pressure vessel and similar applications
• ISO 14230 — Bilateral steel plates — General technical delivery conditions
• EN 10444 — Bilateral steel plates — Technical delivery conditions

5.2 Welding Procedure and Qualification Standards

• ASME Section IX — Qualification of welding procedures, welders, and welding operators (WPS and WPQ requirements)
• ASME Section II Part D — Specifications for weld overlay materials
• ASME Section VIII Div. 1 & Div. 2 — Rules for construction of pressure vessels (clad vessel requirements)
• GB/T 985 — Welding procedure qualification rules (Chinese standard)
• NB/T 47014 — Rules for welding procedure qualification of pressure vessels and pressure components
• ISO 15614-1 — Qualification testing of welding procedures for metallic materials (arc welding)
• ISO 15614-9 — Qualification testing of welding procedures for metallic materials (electroslag welding)

5.3 Non-Destructive Testing Standards

• ASTM E164 — Standard practice for liquid penetrant examination of welds
• ASTM E1417 — Standard practice for liquid penetrant inspection
• ASTM E797 — Standard practice for magnetic particle examination of welds
• GB/T 19871 — Non-destructive testing of welds (magnetic particle method)
• NB/T 47013 — Non-destructive testing methods for pressure equipment (ultrasonic, radiographic, magnetic particle, penetrant)
• ASTM E165 — Standard practice for ultrasonic examination of welds (for clad-base bond verification)
• ASTM E1270 — Standard practice for ultrasonic pulse-echo examination of clad steel plates for bond quality

5.4 Acceptance Criteria Summary

For ESW-fabricated clad plates, the following acceptance criteria are typically applied:

1. Visual inspection (VT) — No surface cracks, undercut, excessive reinforcement (>3 mm), or slag inclusions visible on the clad surface. All weld beads must be uniform in profile and free of surface discontinuities per ASTM E165 / GB/T 3323.
2. Ultrasonic testing (UT) for bond quality — 100% UT coverage of the clad-base interface per ASTM E1270 or GB/T 19871. Acceptance: no indications exceeding the specified threshold for bond defects. The test must confirm continuous metallurgical bonding across the entire clad area.
3. Radiographic testing (RT) — Spot or 100% radiographic examination of weld zones per ASME Sec. V Article 2 or GB/T 3323. Acceptance: no slag inclusions, porosity, or lack of fusion exceeding the severity limits for the applicable quality level (typically UT or B level per ISO 17636-2).
4. Hardness testing — Hardness mapping across the clad layer, interface, and base metal HAZ. Acceptance per Section 4.4 above.
5. Shear/peel testing — Destructive bond strength testing per ASTM E290 on coupon specimens. Acceptance: ≥ 200 MPa shear strength or no interfacial separation in peel test.
6. Chemical analysis — Spectrometric analysis of clad layer composition to verify dilution is within specified limits and alloy content meets the target grade (e.g., 304L: Cr ≥ 17.5%, Ni ≥ 8.0%, C ≤ 0.03%).
7. Corrosion testing — For critical applications, pitting corrosion resistance testing (ASTM G48), intergranular corrosion testing (ASTM A262 Practice E), or salt spray testing (ASTM B117) may be required to verify the clad layer's corrosion performance is not compromised by dilution or sensitization.

6. Common Risks and Controls

Risk	Cause	Detection Method	Control Measure
Interfacial lack of fusion	Insufficient heat input, contaminated surfaces, excessive travel speed	UT (ASTM E1270), shear/peel test	Optimize current/voltage; ensure surface cleanliness; reduce travel speed
Excessive dilution	High current, slow travel speed, high interpass temperature	Chemical analysis, hardness mapping	Reduce heat input; lower interpass temperature; use lower-carbon filler wire
Slag inclusions	Inadequate slag pool coverage, improper flux composition, excessive travel speed	RT, UT	Ensure proper flux coverage; adjust flux composition; maintain constant travel speed
Hot cracking in clad layer	Low sulfur/phosphorus content, high dilution, high restraint	VT, RT	Control sulfur content (0.015–0.03%); reduce dilution; use preheat
Carbide precipitation / sensitization	Prolonged exposure to 450–850 °C during multi-pass welding or PWHT	Intergranular corrosion test (ASTM A262), metallography	Use low-carbon filler (304L, 316L); minimize interpass temperature; solution anneal if required
Excessive residual stress	High heat input, asymmetric cladding, high restraint	Strain gauge measurement, X-ray diffraction	Apply symmetric cladding; use preheat; apply PWHT (stress relief)
Porosity	Moisture in flux, contamination, insufficient deoxidation	RT, UT	Dry flux at 300 °C for 2 hours; clean surfaces; ensure adequate flux coverage
Excessive hardness in HAZ	High carbon content in base metal, rapid cooling	Hardness mapping (ASTM E92)	Apply preheat; control cooling rate; consider PWHT

6.1 Dilution Control Strategy

Dilution is the most critical metallurgical variable in ESW clad plate fabrication. The dilution ratio (D) can be estimated using the formula:

D = (W_base × V_base) / (W_base × V_base + W_wire × V_wire)

where W represents weight and V represents volume. For typical ESW parameters, dilution ranges from 10% to 25%. To minimize dilution:

• Reduce welding current and voltage to the minimum values that maintain slag pool stability
• Increase travel speed to reduce heat input per unit length
• Use a higher preheat temperature to reduce the thermal gradient at the interface
• Select filler wire with higher alloy content to compensate for dilution (e.g., use ER309L instead of ER308L when dilution is expected to be high)
• Apply a thin "cushion" layer of low-dilution alloy (e.g., 309L) before the final clad layers to act as a diffusion barrier

6.2 Residual Stress Management

Electroslag welding generates significant residual stresses due to the high heat input and asymmetric thermal expansion/contraction between the clad layer and substrate. These stresses can lead to distortion, cracking, or reduced fatigue life. Control measures include:

• Preheat — Apply 100–250 °C preheat to reduce thermal gradients and residual stress magnitude
• Interpass temperature control — Maintain interpass temperature ≤ 250 °C to limit cumulative heat input
• Multi-directional welding — Use a zigzag or multi-pass pattern to distribute heat input more uniformly
• Post-weld stress relief — Apply PWHT at 550–650 °C for 2–4 hours (for carbon steel base) or 1050–1100 °C solution anneal (for full clad layer restoration)
• Shot peening or vibration stress relief — Apply mechanical stress relief after PWHT if required

7. Application Scenarios Across Technology Routes

7.1 Complementarity with TIG/MIG Weld Overlay

Electroslag welding and TIG/MIG weld overlay serve different niches within the company's clad plate product range. TIG/MIG overlay is preferred for thin cladding layers (1–5 mm) where precise composition control and minimal dilution are critical, such as in nuclear-grade clad plates or food-grade piping. ESW is preferred for thick cladding layers (5–25 mm) on heavy structural substrates where deposition rate and productivity are paramount. The microstructural and mechanical property data obtained from ESW research directly informs the WPS qualification for both processes, as the fundamental metallurgical principles (dilution control, interfacial bonding, phase evolution) are transferable.

7.2 Complementarity with Hydraulic Explosive Bonding

Hydraulic explosive bonding produces clad plates with near-zero dilution and excellent preservation of both clad and base metal properties. However, it is limited in clad thickness (typically ≤ 8 mm) and substrate dimensions. For applications requiring thicker cladding (e.g., 10–25 mm) on large-format substrates, ESW provides a viable alternative. The company may employ a hybrid approach: hydraulic explosive bonding for the initial bond layer (ensuring metallurgical continuity) followed by ESW for thickening the clad layer. The microstructural research on ESW interfaces provides the data needed to qualify such hybrid processes.

7.3 Complementarity with Explosion Welding

Explosion welding is the primary method for producing large-area clad plates with high production rates and minimal dilution. However, explosion welding is limited by the aspect ratio of the cladding layer to substrate (typically 1:3 to 1:10), and very thick cladding layers (>15 mm) may be impractical. For applications requiring thick cladding on large substrates, ESW serves as a complementary process. Additionally, ESW can be used to repair or supplement explosion-welded clad plates where local defects (e.g., incomplete bonding in edge regions) require remediation.

7.4 Typical Application Scenarios

Application	Clad Material	Base Material	Typical Clad Thickness	Preferred Process	Key Requirement
Reactor vessel internals	316L / 321	SA-516 Gr.70 / 16MnR	6–15 mm	ESW or TIG overlay	Nuclear-grade qualification, low dilution
Pressure vessel heads	304L	Q345R / SA-516 Gr.70	3–8 mm	Explosion welding or ESW	Large diameter, high production rate
Thick-walled pipe spools	316L / Alloy 6	SA-106 Gr.B	5–20 mm	ESW	Thick cladding, high deposition rate
Heat exchanger tubesheets	316L	SA-516 Gr.70	2–5 mm	TIG overlay	Thin cladding, low dilution
Chemical reactor linings	904L / Alloy 20	Q345R	10–25 mm	ESW	Very thick cladding, high corrosion resistance
Large storage tank linings	304L	Q235B / SA-283 Gr.C	3–6 mm	Explosion welding	Large area, low cost

8. Contribution to Qualification Building and Customer Value

8.1 WPS Qualification and Certification

The research on microstructure and properties of ESW-fabricated clad plates is directly applicable to welding procedure qualification (WPS) under ASME Section IX, GB/T 985, and NB/T 47014. The following qualification elements are informed by the research:

• Essential variables — The research identifies the critical process parameters (current, voltage, travel speed, flux composition, preheat, interpass temperature) that must be controlled within qualified ranges to produce acceptable microstructures and mechanical properties.
• Qualification testing — The mechanical property benchmarks (shear strength, peel strength, hardness, tensile strength, impact toughness) define the acceptance criteria for qualification testing.
• Qualification records — The metallographic and mechanical test data form the basis of the qualification record, providing evidence of process capability to regulatory inspectors and customer auditors.

8.2 Product Delivery and Quality Assurance

The research findings enable the company to:

1. Define process control limits — Establish statistical process control (SPC) limits for welding parameters based on the relationship between parameters and microstructural/mechanical outcomes.
2. Develop inspection protocols — Create NDT procedures and acceptance criteria tailored to the specific defect modes identified in the research (e.g., interfacial lack of fusion, slag inclusions, porosity).
3. Reduce non-conformance — By understanding the root causes of common defects, implement preventive measures that reduce the rate of rejected material and rework, improving on-time delivery and cost competitiveness.
4. Support customer audits — Provide detailed technical documentation (microstructural analysis reports, mechanical property test certificates, NDT reports) that demonstrates compliance with applicable standards and customer specifications.

8.3 Customer Value Proposition

For customers in the petrochemical, nuclear, power generation, and marine industries, the company's ESW clad plate capability provides the following value propositions:

• Thick-section capability — Ability to produce clad plates with cladding thicknesses up to 25 mm, which is beyond the practical range of TIG/MIG overlay and explosion welding.
• High productivity — Deposition rates of 5–15 kg/h enable rapid production of thick-clad plates, reducing lead time and cost.
• Quality assurance — Documented microstructural and mechanical property data provides confidence in product performance, supporting regulatory certification and customer qualification.
• Customization — Ability to tailor clad composition, thickness, and mechanical properties to specific application requirements through process parameter optimization.
• Hybrid process options — Ability to combine ESW with explosion welding or hydraulic bonding for hybrid clad plate configurations that leverage the strengths of multiple technologies.

9. Conclusion and Recommendations

Electroslag welding for stainless steel/carbon steel clad plate fabrication is a mature and highly productive technology that occupies a unique position in the company's capability portfolio. The research on microstructure and properties provides the metallurgical foundation for process qualification, quality control, and product delivery. Key recommendations for continued development include:

1. Expand WPS qualification database — Qualify additional material combinations (e.g., 904L, Alloy 20, Alloy 6, Hastelloy C-276) on ESW to broaden the product range.
2. Develop hybrid process procedures — Qualify hybrid processes combining explosion welding or hydraulic bonding with ESW for thick cladding applications.
3. Implement advanced NDT — Adopt phased array ultrasonic testing (PAUT) for improved detection of interfacial defects and bond quality verification.
4. Develop predictive models — Create computational models (e.g., finite element analysis for thermal and residual stress prediction) to optimize process parameters and reduce trial-and-error qualification.
5. Establish industry partnerships — Collaborate with research institutions and regulatory bodies to develop industry standards for ESW clad plate fabrication and qualification.

By leveraging the metallurgical insights gained from ESW research, Cladding Technology Shanxi Co., Ltd. can continue to deliver high-quality, certified clad plate products that meet the demanding requirements of the petrochemical, nuclear, power, and marine industries, while maintaining a competitive advantage through productivity, quality, and customization capabilities.