CLADDING TECHNOLOGY SHANXI CO., LTDCurrent State of Electric Spark Deposition and Weld Overlay Technology Worldwide
Literature Overview and Motivation
The study of electric spark deposition (ESD), sometimes referred to as spark cladding or electric discharge welding overlay, has attracted growing attention in recent years as a promising alternative to conventional thermal overlay methods. This literature review summarizes the domestic and international application status of ESD technology, comparing its capabilities against established processes such as submerged arc welding (SAW), gas tungsten arc welding (GTAW), plasma transferred arc (PTA), and laser cladding. The motivation for studying this topic stems from the increasing demand for high-performance surface modifications in power generation, petrochemical, nuclear, and aerospace industries, where conventional overlay methods may introduce excessive heat input or produce undesirable metallurgical dilution.
Electric spark deposition operates by generating localized micro-arcs between a consumable electrode and the substrate, depositing molten metal droplets onto the base material in a layer-by-layer fashion. The process typically operates at relatively low heat input compared to arc welding methods, which is advantageous for reducing thermal distortion and minimizing dilution of the overlay layer with the base material. This characteristic makes ESD particularly suitable for overlaying dissimilar metals where metallurgical compatibility is a concern.
Core Technical Principles and Process Parameters
The fundamental mechanism of electric spark deposition involves the formation of a controlled electric arc between the electrode tip and the workpiece surface. Each discharge event melts a small volume of electrode material, which is then transferred to the substrate. The process parameters that govern the quality of the deposited layer include arc current, electrode feed rate, travel speed, electrode diameter, and inter-pass temperature.
| Parameter | Typical Range | Effect on Overlay Quality |
|---|---|---|
| Arc Current | 20–80 A | Controls droplet size and penetration depth |
| Electrode Feed Rate | 50–300 mm/min | Affects layer thickness and deposition rate |
| Travel Speed | 50–500 mm/min | Influences bead width and overlap uniformity |
| Electrode Diameter | 1.0–6.0 mm | Determines deposition efficiency and surface finish |
| Inter-pass Temperature | <250 °C (for most applications) | Prevents excessive thermal cycling and cracking |
One of the distinguishing features of ESD is its ability to operate with minimal shielding gas, as the molten droplets are transferred rapidly enough to reduce atmospheric contamination. However, in applications requiring high purity overlay layers, such as nickel-based alloy coatings for corrosion resistance, inert gas shielding (argon or helium) is often employed to protect the molten pool.
The dilution rate in ESD is typically lower than in conventional arc welding overlay processes. For nickel-based overlay on carbon steel, dilution rates of 5–15% have been reported, compared to 20–35% in SAW overlay. This lower dilution is attributed to the short arc duration and rapid solidification, which limit the time available for base metal dissolution into the molten pool.
International Application Status
Internationally, electric spark deposition technology has been developed and commercialized by several companies, primarily in Europe and Asia. In Japan, the process has been applied to repair and overlay of turbine blades, pump impellers, and hydraulic components. In Germany, research institutions have investigated ESD for depositing wear-resistant high-chromium cast irons on steel substrates used in mining and material handling equipment.
The United States has seen limited commercial adoption of ESD, with most surface engineering efforts concentrated on laser cladding and PTA. However, academic research at institutions such as the University of Michigan and the National Institute of Standards and Technology (NIST) has explored the fundamental aspects of electric spark deposition, particularly its potential for additive manufacturing of functionally graded materials.
In Europe, the European Materials Research Society (Euromat) has featured several presentations on ESD, highlighting its advantages in depositing titanium alloys on steel substrates. The process has been evaluated for producing titanium/steel bimetallic layers with bond strengths exceeding 100 MPa in shear test configurations, meeting the requirements of EN 10028-7 for explosion-clad plate equivalents.
Domestic Application Status
In China, the development of electric spark deposition technology has been driven primarily by national research institutes and large engineering enterprises. The China First Heavy Industries Corporation (CFHI) and the China National Nuclear Corporation (CNNC) have investigated ESD for overlaying nuclear-grade stainless steel on carbon steel pressure vessel components. The process has been evaluated for depositing 316L stainless steel overlay layers on SA-516 Gr.70 base plates, with overlay thicknesses ranging from 1.0 to 3.0 mm.
Key domestic research findings include:
- The ability of ESD to deposit multi-layer overlays with uniform microstructure and minimal interpass cracking.
- The production of overlay layers with hardness values of 200–250 HV for 316L deposits, consistent with the requirements of GB/T 150 for stainless steel cladding layers.
- Successful application of ESD for local repair of erosion-corrosion damage in power plant condensers and heat exchanger tubesheets.
The Chinese standard GB/T 32389-2015, which addresses electric spark welding for surface engineering, provides guidelines for process qualification and quality assessment. However, the standard remains less comprehensive than ASME IX for conventional welding processes, and there is a recognized need for further standardization of ESD in pressure vessel fabrication.
Comparison with Conventional Overlay Processes
| Feature | ESD | SAW Overlay | PTA | Laser Cladding |
|---|---|---|---|---|
| Heat Input | Low | High | Medium | Low |
| Dilution Rate | 5–15% | 20–35% | 10–20% | 3–10% |
| Deposition Rate | Medium | High | Medium | Low |
| Equipment Cost | Medium | Low | High | High |
| Operator Skill | Medium | Low | High | High |
| Layer Thickness | 0.5–5.0 mm | 1.0–10.0 mm | 0.5–3.0 mm | 0.1–2.0 mm |
| Applicable Substrates | Wide range | Thick sections | Thin sections | Wide range |
| Surface Finish | Good | Fair | Excellent | Excellent |
The comparison highlights that ESD occupies a middle ground between the high-deposition-rate but high-heat-input SAW process and the low-heat-input but low-rate laser cladding process. This positioning makes ESD attractive for applications where moderate deposition rates are acceptable but thermal distortion must be controlled, such as overlay of thin-walled components or large diameter tubesheets.
Key Defects and Countermeasures
Despite its advantages, electric spark deposition is not without challenges. The following table summarizes common defects observed in ESD overlays and the corresponding countermeasures:
| Defect | Cause | Countermeasure |
|---|---|---|
| Porosity | Incomplete fusion between passes; atmospheric contamination | Increase overlap; apply inert gas shielding; preheat substrate |
| Cracking | Residual stress accumulation; high carbon content in base metal | Reduce interpass temperature; use low-carbon electrode; apply post-weld heat treatment |
| Excessive Dilution | Excessive arc current; excessive electrode feed rate | Reduce arc current; increase travel speed; use smaller electrode diameter |
| Surface Roughness | Irregular droplet transfer; unstable arc | Optimize electrode feed rate; ensure stable electrode alignment |
| Bond Strength Deficiency | Insufficient penetration; contamination at interface | Increase arc current slightly; clean substrate thoroughly; preheat to 150–200 °C |
In practice, the most challenging defect to control is cracking in the overlay layer, particularly when depositing austenitic stainless steel on high-carbon steel substrates. The carbon content in the base metal can migrate into the overlay during deposition, forming hard martensitic phases at the weld interface. This phenomenon is analogous to the cracking issues encountered in SAW overlay of stainless steel on carbon steel, and similar countermeasures apply, including the use of a low-carbon transition layer and post-weld heat treatment at 650–750 °C.
Engineering Practice Cases
A notable engineering application of ESD is the repair of tubesheets in large heat exchangers used in refinery units. A 1.5-meter diameter tubesheet fabricated from SA-285 C steel was found to have localized erosion-corrosion damage at the tube holes after 8 years of service. Conventional repair by drilling out the damaged area and welding in a new plug was deemed too disruptive. Instead, ESD was used to deposit a 2.0 mm thick layer of 316L stainless steel over the damaged region. The overlay was performed with the following parameters: arc current 45 A, electrode feed rate 120 mm/min, travel speed 200 mm/min, and electrode diameter 3.2 mm. Post-overlay inspection by magnetic particle testing (MT) revealed no surface cracks, and a bond strength test coupon deposited on a similar substrate showed a shear strength of 145 MPa, well above the 100 MPa minimum specified in NB/T 47014.
Another application involved the overlay of Inconel 625 on a Hastelloy C276 base plate for a hydrogenation reactor internals component. The ESD process produced an overlay layer with a dilution rate of approximately 8%, as confirmed by optical emission spectroscopy (OES) analysis of a cross-section. The microstructure of the overlay consisted of columnar austenite grains with a small amount of delta ferrite, consistent with the expected solidification behavior of Inconel 625. No intergranular cracking was observed, and the overlay passed a 6-hour intergranular corrosion test in 65% boiling HNO3 according to ASTM A263.
Reflections and Study Insights
The study of electric spark deposition technology reveals several important insights for engineering practice. First, the process offers a viable alternative to conventional arc welding overlay in applications where thermal distortion is a critical concern, such as thin-walled components and large flat surfaces. Second, the lower dilution rate of ESD compared to SAW is a significant advantage for depositing corrosion-resistant alloys, as it preserves the alloying elements in the overlay layer and reduces the risk of forming brittle intermetallic phases at the interface. Third, the process is relatively easy to automate, which is beneficial for large-scale production of bimetallic components.
However, the technology also has limitations. The deposition rate of ESD is generally lower than that of SAW, which can be a disadvantage for thick overlay layers or large surface areas. Additionally, the process requires precise control of electrode alignment and feed rate, which demands operator skill or a well-calibrated automated system. The standardization of ESD for pressure vessel applications remains incomplete, and engineers must exercise caution when applying the process to critical components governed by ASME or GB/T standards.
In conclusion, electric spark deposition represents a promising surface engineering technology with demonstrated capabilities in both laboratory and industrial settings. Its low heat input, moderate dilution rate, and adaptability to various substrate-overlay combinations make it a valuable tool in the cladding engineer's arsenal. As standardization efforts progress and process qualification databases expand, ESD is likely to find broader acceptance in pressure vessel fabrication, particularly for applications requiring precision overlay of dissimilar metals with minimal thermal impact.