CLADDING TECHNOLOGY SHANXI CO., LTDLarge-Area UNS N06625 Electroslag Strip Electrode Weld Overlay Technology
Literature Overview
This 2011 study by Wang Jinguang of Sinopec Engineering Corporation addresses the challenges of applying large-area weld overlay cladding using UNS N06625 (Inconel 625) via the electroslag welding (ESW) strip electrode process. The work is situated within the petrochemical equipment domain, where high-performance nickel-based alloy overlays are required on carbon steel or low-alloy steel pressure vessels and heat exchangers to resist severe corrosion environments involving hydrogen sulfide, chlorides, and high-temperature oxidizing media.
Core Technical Content
UNS N06625 is a nickel-chromium-molybdenum alloy with approximately 62% Ni, 22% Cr, and 3.2% Mo, offering outstanding resistance to pitting, crevice corrosion, and stress corrosion cracking across a wide range of aggressive chemical environments. When applied as a weld overlay on carbon steel substrates, it creates a bimetallic structure that combines the corrosion resistance of the overlay with the mechanical strength and cost-effectiveness of the base material.
Process Parameters for ESW Strip Electrode Overlay
| Parameter | Typical Value | Significance |
|---|---|---|
| Strip electrode thickness | 0.5–1.0 mm | Affects deposition rate and dilution control |
| Strip electrode width | 40–100 mm | Determines single-pass coverage width |
| Travel speed | 50–150 mm/min | Controls heat input and dilution |
| Current | 4000–8000 A | Electroslag pool stability and deposition rate |
| Flux composition | CaF2-CaO-Al2O3-SiO2 system | Stabilizes slag pool and protects molten metal |
| Preheat temperature | 150–250°C | Reduces hydrogen cracking susceptibility in base metal |
| Interpass temperature | 200–350°C | Controls thermal cycle and residual stress |
| Post-weld heat treatment | 870–900°C, 1–2 h + air cool | Relieves residual stress and stabilizes microstructure |
Dilution Control
The primary technical challenge in ESW overlay of N06625 on carbon steel is controlling the dilution of the base metal into the overlay. Excessive dilution introduces iron into the overlay, which can form brittle intermetallic phases (such as μ-phase) at chromium and molybdenum concentrations above certain thresholds. The μ-phase, with the approximate composition Cr3Mo3W, is extremely hard and brittle and significantly degrades the corrosion resistance and ductility of the overlay.
The electroslag process inherently produces higher dilution rates compared to processes such as plasma transferred arc (PTA) or laser cladding, typically in the range of 15–30% for a single pass. To mitigate this, multi-pass welding is employed, with the first pass accepting higher dilution and subsequent passes progressively reducing the dilution as the overlay builds up. The final overlay composition should maintain Cr above 20% and Mo above 2.5% to ensure adequate corrosion resistance.
Engineering Practice in Petrochemical Applications
In hydrogenation reactors, sulfuric acid concentration equipment, and heat exchangers operating in the presence of hydrogen sulfide and organic acids, N06625 overlays provide a reliable corrosion barrier. The large-area capability of the ESW strip electrode process makes it economically viable for covering extensive vessel surfaces, including the interior of large-diameter columns and reactors where the overlay area may exceed 50 m².
Quality Control Requirements
| Inspection Method | Acceptance Criteria | Standard Reference |
|---|---|---|
| Dye penetrant testing (PT) | No linear indications | NB/T 47013.5 / ASTM E165 |
| Ultrasonic testing (UT) | No bond loss or delamination | NB/T 47013.3 / ASTM E165 |
| Hardness testing | 180–260 HV per ASME IX | ASME IX QW-452 |
| Dilution analysis | Fe content < 15–20% in final pass | ASTM E1085 / ICP-OES |
| Impact testing | Meets specified temperature and energy per ASME IX | ASTM A370 |
| Intergranular corrosion | Pass per ASTM A240 | ASTM A240 Appendix G |
Process Optimization Strategies
- Multi-pass layering: Employ a build-up strategy where the first 1–2 passes serve as a transition layer, with subsequent passes using pure N06625 strip to progressively reduce dilution.
- Slag composition optimization: Adjust the basicity of the flux to control the wetting angle and slag fluidity, which directly affects the uniformity of the overlay surface.
- Travel speed control: Maintain consistent travel speed to prevent local overheating or insufficient fusion, both of which can lead to dilution variation and surface defects.
- Post-weld stress relief: Apply PWHT at 870–900°C for 1–2 hours to relieve welding residual stresses, which can otherwise cause delayed cracking or distortion.
Study Insights and Reflections
The ESW strip electrode process represents a mature and cost-effective solution for large-area overlay applications in the petrochemical industry. Its high deposition rate, typically 5–10 kg/h, makes it significantly more economical than PTA or laser cladding for covering large vessel surfaces. However, the trade-off is reduced control over dilution and microstructure refinement compared to thermal spray or laser-based methods.
From a design perspective, engineers must carefully evaluate the dilution profile across the overlay thickness. A common approach is to design for a total overlay thickness of 3–6 mm, with the understanding that the first 1–2 mm will have higher iron content and may not meet the full corrosion resistance specification. The design corrosion allowance should account for this gradient, and the overlay thickness should be sufficient to provide the required service life with adequate remaining thickness.
The study also highlights the importance of weld procedure qualification per ASME IX or NB/T 47014. The essential variables for ESW overlay include strip electrode composition, flux type, current range, voltage range, and travel speed. Failure to properly qualify the procedure can result in non-conforming overlays that fail to meet corrosion resistance or mechanical property requirements.