INCONEL 690 Nickel-Based Alloy Strip Electrode Electroslag Weld Overlay: Process Qualification and Cladding Performance Research

1. Definition and Fundamental Principles

Electroslag weld overlay (ESWO) using INCONEL 690 strip electrodes is an advanced thermal spray-adjacent process in which a continuous strip of nickel-based superalloy is fed as both filler metal and consumable electrode into an electroslag pool. The electroslag pool—maintained at temperatures between 1,200°C and 1,600°C—acts as a thermal reservoir that melts the strip electrode and the base substrate surface simultaneously. The resulting molten metal pool is confined beneath a thin slag layer, which provides intense heat input, slow cooling rates, and excellent metallurgical control.

INCONEL 690 (UNS N06690) is a precipitation-strengthened nickel-chromium-iron alloy containing approximately 62% Ni, 29.5% Cr, 3% Mo, and 1.15% Ti. Its exceptional resistance to carbide precipitation in austenitic stainless steels, superior resistance to chloride stress corrosion cracking (Cl-SCC), and outstanding pitting resistance in high-temperature aqueous environments make it the alloy of choice for nuclear steam generator tubes, nuclear reactor internals, and high-purity process equipment in the chemical and petroleum industries.

The electroslag process differs fundamentally from conventional TIG or MIG weld overlay in that the heat source is the resistance heating of the slag pool itself, rather than an arc. This results in significantly higher heat input (typically 15–30 kJ/mm compared to 5–15 kJ/mm for TIG), which produces a broader, more diluted weld profile but also enables single-pass deposition of thick cladding layers (up to 6–10 mm per pass) with minimal interpass temperature management.

2. Category and Business Positioning

Within Cladding Technology Shanxi Co., Ltd's portfolio, this technology entry occupies a critical position at the intersection of research development and process qualification. It belongs to the weld overlay technology route (TIG/MIG/ESWO), specifically in the advanced electroslag sub-category that addresses large-scale, thick-section cladding requirements where conventional arc methods are either impractical or insufficient.

The "learning心得" (learning and insight) designation indicates this is a research-driven capability, representing the company's investment in fundamental process understanding. This positions the company not merely as a fabricator but as a technology developer capable of:

3. Technical Purpose and Value

3.1 Primary Technical Objectives

3.2 Customer Value

For nuclear power plant operators, the INCONEL 690 electroslag overlay enables refurbishment and life extension of steam generator tube sheets, feedwater piping, and reactor internals without requiring complete replacement. For chemical and petroleum industries, it provides a cost-effective means of upgrading existing equipment to resist aggressive chloride-containing environments. The research-driven approach ensures customers receive process-validated solutions backed by comprehensive performance data.

4. Key Process and Implementation Points

4.1 Substrate Preparation

Proper substrate preparation is the foundation of successful electroslag weld overlay. The following requirements must be met:

4.2 Critical Process Parameters

Parameter Typical Range Optimization Notes
Travel Speed 60–150 mm/min Lower speeds increase dilution; higher speeds risk incomplete melting
Electrode Feed Rate 1.2–2.5 m/min Must maintain slag pool stability; too fast causes cold shuts
Slag Pool Temperature 1,200–1,600°C Maintain visual confirmation of proper slag fluidity
Weld Current 150–400 A (depending on strip width) Strip width 10–25 mm; wider strips require higher current
Interpass Temperature ≤ 300°C (max 400°C) Exceeding limits causes grain coarsening and reduced toughness
Strip Electrode Width 10, 15, 20, 25 mm Match to required overlay width and substrate geometry
Strip Electrode Thickness 1.5–3.0 mm Thicker strips for higher deposition rates; thinner for better conformability
Preheat Temperature 150–250°C Critical for thick sections and low-ductility substrates
Post-Weld Heat Treatment 1,050°C ± 10°C for 1–2 h, air cool Solution treatment to dissolve carbides and restore precipitation hardening capability

4.3 Multi-Pass Strategy

For cladding thicknesses exceeding 3 mm, a multi-pass approach is recommended:

  1. Pass 1 (Bonding Pass): Minimum thickness (1.5–2.0 mm) deposited at reduced parameters to ensure metallurgical bonding with the transition layer. Higher dilution is acceptable at this stage.
  2. Passes 2–N (Build-up): Standard parameters applied to build remaining thickness. Dilution decreases significantly after Pass 1.
  3. Final Pass: May be finished with TIG weld overlay of INCONEL 690 wire for surface quality improvement and final dilution control.

4.4 Performance Characterization Requirements

Property Test Method Acceptance Criteria
Hardness ASTM E18 (Rockwell C) or ASTM E92 (Vickers) 25–40 HRC (as-deposited); 30–45 HRC (after PWHT)
Tensile Strength ASTM E8/E8M ≥ 690 MPa (minimum yield); ≥ 860 MPa (UTS)
Impact Toughness ASTM E23 (Charpy V-Notch) ≥ 27 J at -46°C (nuclear applications)
Dilution Optical Emission Spectroscopy (OES) ≤ 10% base metal dilution in final cladding face
Corrosion Resistance ASTM G48 (pitting), ASTM G58 (crevice), ASTM G36 (CCT) No pitting at 6% Cl⁻, 60°C, 24 h; CCT ≥ 40°C
Microstructure OM/SEM with 10% Nital or electrolytic etch No Type I carbide network; fine Type II/III acceptable

5. Applicable Standards and Acceptance Criteria

5.1 Process and Material Standards

5.2 Non-Destructive Testing Standards

5.3 Acceptance Criteria Hierarchy

  1. Level 1 (Standard Industrial): No cracks, no slag inclusions > 0.5 mm, porosity per ASME Section IX Table 2
  2. Level 2 (Pressure Vessel): Zero tolerance for cracks; porosity and slag per ASME Section VIII Div. 1
  3. Level 3 (Nuclear Grade): Zero tolerance for all defects; 100% UT and PT coverage; impact testing at service temperature; full spectroscopic verification of dilution

6. Common Risks and Controls

6.1 Metallurgical Risks

Risk Root Cause Control Measures
Hot Cracking High S/P content in base metal; excessive dilution creating vulnerable solidification zone Limit dilution to ≤ 10%; use low-sulfur transition layer; control interpass temperature ≤ 300°C
Cold Cracking High carbon equivalent substrate; insufficient preheat; rapid cooling Preheat to 200°C minimum; use low-hydrogen consumables; control cooling rate with insulating blankets
Carbide Precipitation (Sensitization) Exposure to 450–850°C range without PWHT; excessive interpass temperature Mandatory solution heat treatment at 1,050°C; strict interpass temperature control; minimize time in sensitization range
Intergranular Corrosion Chromium carbide network at grain boundaries (Type I) Post-weld solution treatment; verify via ASTM A262 Practice A (AST) or Practice E (5% CuSO₄)
Excessive Dilution Too high travel speed; too thin bonding pass; substrate geometry effects Multi-pass strategy with thick first pass; verify by OES after each pass; adjust parameters based on dilution feedback

6.2 Process Risks

6.3 Inspection Risks

7. Application Scenarios Across Technology Routes

7.1 TIG/MIG Weld Overlay Route (Primary Integration)

The INCONEL 690 electroslag research directly supports and enhances the company's TIG/MIG weld overlay capabilities:

7.2 Hydraulic Explosive Bonding Route

While electroslag is a fusion-based process and hydraulic explosive bonding is a solid-state process, the research contributes indirectly:

7.3 Explosion Welding Route

The research supports explosion welding applications through:

8. Qualification Building and Strategic Impact

8.1 WPS/PQR Development Framework

This research entry represents a foundational step in building a comprehensive WPS qualification portfolio. The progression is:

  1. Stage 1 – Research: Parameter exploration and performance characterization (current stage per this entry)
  2. Stage 2 – WPS Development: Formalization of optimized parameters into documented Welding Procedure Specifications per ASME Section IX or NB/T 20305
  3. Stage 3 – PQR Execution: Fabrication of qualification coupons with full mechanical, metallurgical, and corrosion testing
  4. Stage 4 – WPQ Issuance: Formal Welding Procedure Qualification documentation suitable for customer and regulatory submission
  5. Stage 5 – Production Application: Deployment of qualified procedures in commercial fabrication with ongoing monitoring and improvement

8.2 Customer Value Enhancement

8.3 Intellectual Property and Competitive Advantage

The research-driven approach to INCONEL 690 electroslag overlay creates multiple competitive advantages:

9. Implementation Recommendations

  1. Complete the PQR cycle for at least three substrate combinations (A105 carbon steel, 304L austenitic stainless steel, 2205 duplex stainless steel) to establish a comprehensive qualification matrix
  2. Develop a dilution control model correlating travel speed, electrode feed rate, and substrate geometry to predicted dilution levels, enabling real-time parameter adjustment during production
  3. Establish a corrosion testing protocol including ASTM G48, ASTM G58, ASTM G36, and autoclave testing per ASTM G20 for nuclear applications
  4. Create a defect database documenting all observed defects with root cause analysis and corrective actions, building institutional knowledge for continuous improvement
  5. Train qualified welders per ISO 9606-1 with specific electroslag overlay qualifications, including written examination on INCONEL 690 metallurgy and performance requirements
  6. Develop a hybrid process specification combining electroslag bulk deposition with TIG finishing, documented as a single qualified WPS with clear pass-by-pass requirements
Key Takeaway: The INCONEL 690 strip electrode electroslag weld overlay research represents a strategic capability investment that bridges fundamental metallurgical understanding with commercial process qualification. When fully developed into qualified WPS/PQR documentation, this research enables Cladding Technology Shanxi Co., Ltd to address the highest-value cladding applications in nuclear power, chemical processing, and petroleum industries—applications where performance documentation and regulatory compliance are non-negotiable prerequisites for contract award.