CLADDING TECHNOLOGY SHANXI CO., LTDNickel-Based Alloy Strip Cladding by Electroslag Welding for Thick Sections
Literature Overview
This research, published in the Journal of Heilongjiang University of Science and Technology and supported by a consortium of China National Machinery Industry Corporation, Harbin Vocational and Technical University, Harbin Weil Welding Co., Ltd., and Hawell Huatong (Changzhou) Welding Industry Co., Ltd., addresses the fabrication technology for thick-section nickel-based alloy strip cladding using electroslag welding (ESW). The study is particularly significant for the energy, petrochemical, and nuclear industries where large-diameter pressure vessels and heat exchangers require extensive corrosion-resistant overlay layers on carbon steel or low-alloy steel substrates.
Process Fundamentals and Technical Parameters
Electroslag welding for strip cladding offers distinct advantages over conventional arc welding methods for thick-section applications, including higher deposition rates, lower hydrogen content, reduced dilution, and the ability to achieve uniform overlay thickness over large areas. The process utilizes a strip electrode and a consumable steel backing strip, with flux forming a molten slag pool that maintains the arc and shields the weld metal.
Recommended Process Parameters for Nickel-Based Strip Cladding
| Parameter | Typical Value | Notes |
|---|---|---|
| Substrate thickness | 40–200 mm | Carbon steel or low-alloy steel |
| Strip electrode composition | Inconel 625 / 626 / Hastelloy C-276 | Nickel-based alloy |
| Backing strip thickness | 3–6 mm | Consumable steel or nickel alloy |
| Current (A) | 1200–2500 | Depends on strip width and thickness |
| Voltage (V) | 30–45 | Electrode voltage |
| Travel speed (mm/min) | 100–300 | Controlled by mechanical feed |
| Flux composition | CaF2–Al2O3–SiO2 system | Specific flux for nickel alloys |
| Number of passes | 1–3 | For overlay thickness up to 6–12 mm |
| Preheat temperature (°C) | 150–300 | Reduces cracking tendency |
| Interpass temperature (°C) | 100–200 | Maintains thermal balance |
Microstructure and Mechanical Properties
The microstructure of nickel-based alloy strip cladding deposited by ESW is characterized by a columnar dendritic structure growing from the fusion boundary, with grain size influenced by the heat input and cooling rate. The following table presents typical properties of the deposited nickel-based overlay:
| Property | Inconel 625 Overlay | Hastelloy C-276 Overlay | Monel 400 Overlay |
|---|---|---|---|
| Tensile strength (MPa) | 700–900 | 600–800 | 550–700 |
| Yield strength (MPa) | 350–500 | 300–450 | 280–400 |
| Elongation (%) | 30–45 | 25–40 | 30–45 |
| Hardness (HV) | 220–280 | 200–260 | 180–240 |
| Dilution (%) | 3–8 | 4–10 | 3–8 |
The dilution rate in ESW strip cladding is typically lower than in conventional arc welding methods because the process geometry provides better shielding of the molten pool from the substrate. This is particularly important for maintaining the corrosion resistance of the nickel-based overlay, as excessive dilution from carbon steel can introduce carbon and manganese that form deleterious carbides in the nickel matrix.
Common Defects and Countermeasures
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Hot cracking | High sulfur/phosphorus segregation at dendrite boundaries | Use ultra-low sulfur flux; control cooling rate |
| Cold cracking | Hydrogen embrittlement in high-strength weld metal | Preheat to 200–300°C; use low-hydrogen consumables |
| Excessive dilution | High heat input or poor backing strip fit-up | Reduce current; ensure proper backing strip alignment |
| Incomplete fusion | Insufficient penetration or poor flux coverage | Increase current; verify flux distribution |
| Porosity | Moisture in flux or contamination on substrate | Dry flux at 300°C for 2 hours; clean substrate surface |
| Cracking at fusion boundary | Thermal mismatch between substrate and overlay | Apply intermediate transition layer; control interpass temperature |
Engineering Practice and Standards Compliance
For pressure vessel applications governed by ASME VIII Division 1 or GB/T 150, the weld procedure qualification (WPQ) and welder qualification (WQQ) must comply with ASME IX or NB/T 47014 respectively. The ESW strip cladding process requires special qualification considerations due to its unique process parameters and the large scale of deposition. Non-destructive testing (NDT) requirements typically include:
- Surface: Magnetic particle testing (MT) or liquid penetrant testing (PT) for 100% coverage
- Bond line: Ultrasonic testing (UT) or phased array UT (PAUT) for 100% coverage
- Volumetric: Radiographic testing (RT) for 100% of overlay thickness
- Dimensional: Visual inspection and thickness measurement at specified intervals
The collaboration between academic institutions and industrial partners in this research highlights the practical challenges of scaling ESW strip cladding from laboratory conditions to production environments. Issues such as flux supply continuity, electrode alignment automation, and thermal management of large-diameter vessels must be addressed for successful industrial implementation.
Study Insights and Implications
The ESW strip cladding process represents a cost-effective solution for thick-section nickel-based alloy overlay applications, offering deposition rates 3–5 times higher than conventional SAW or GMAW methods. However, the process requires careful control of flux composition, travel speed, and thermal balance to achieve the required microstructural quality and mechanical properties. The key insight from this research is that the combination of process parameter optimization and consumable selection can achieve overlay layers with dilution below 5%, ensuring that the corrosion resistance of the nickel-based alloy is preserved throughout the overlay thickness. This is particularly critical for applications in nuclear power plants, where the integrity of the corrosion-resistant barrier is essential for long-term service reliability.