Zhuojin Pipe Fitting Co., LtdStrip Electrode Electroslag Overlay Welding of Inconel 600 on 15CrMo Forgings
Literature Overview
This paper by Wang Qingguo from AVIC Liming Jinxi Chemical Machinery (published in Petrochemical Equipment Technology, Vol. 37, No. 6, 2016) presents a comprehensive study on the application of strip electrode electroslag overlay welding (SE-ESOW) to deposit Inconel 600 (UNS N06600) on 15CrMo forged components. The target application is the internal wall overlay of the transition section of the lower dished head of a coke oven tower—a critical component in delayed coking units that operates under extreme thermal cycling and corrosive conditions.
Core Technical Points
Process Fundamentals of SE-ESOW
Strip electrode electroslag overlay welding is a high-deposition-rate process that utilizes a continuous strip electrode and a consumable flux to create a slag pool that heats and melts the base metal and electrode simultaneously. The process offers deposition rates 3–5 times higher than conventional arc welding methods, making it economically attractive for thick overlay applications.
| Parameter | Typical Value for SE-ESOW |
|---|---|
| Deposition rate | 3–5 kg/h |
| Heat input | 80–150 kJ/mm |
| Preheat temperature | 250–350°C |
| Interpass temperature | 300–400°C |
| Strip electrode composition | Inconel 600 (Ni-16Cr-8Fe) |
| Flux composition | Calcium fluoride-based, low-moisture |
| Number of passes | 3–5 (for 6–12 mm overlay) |
| Post-weld heat treatment | Solution treatment at 1050–1100°C |
Metallurgical Challenges
The 15CrMo/Inconel 600 joint represents a dissimilar metal weld between a low-alloy martensitic steel and a nickel-base superalloy. This combination introduces several metallurgical challenges:
- High dilution: The large heat input of SE-ESOW results in significant base metal dilution (typically 15–30%), which can compromise the corrosion resistance of the Inconel 600 overlay.
- Microstructural incompatibility: The martensitic microstructure of 15CrMo is fundamentally different from the austenitic structure of Inconel 600, creating a steep gradient in thermal expansion coefficients across the interface.
- Stress concentration: Differential thermal expansion during cooling and subsequent thermal cycling can generate high interfacial stresses, potentially leading to cracking or delamination.
- Carbon migration: In the long term, carbon can migrate from the 15CrMo side into the Inconel 600 overlay, forming a chromium-depleted zone at the interface that is susceptible to intergranular corrosion.
Process Development and Validation
The author describes a systematic approach to process development:
- Qualification trials: Welding coupons with varying preheat temperatures, strip speeds, and pass numbers to optimize the dilution rate and microstructure.
- Metallographic examination: Cross-sectional analysis to assess the dilution zone, microstructure of each pass, and presence of defects.
- Mechanical testing: Hardness mapping, tensile testing of transverse welds, and impact testing of the HAZ.
- Corrosion testing: Electrochemical potentiodynamic polarization and salt spray testing to evaluate the corrosion resistance of the overlay.
- Service validation: Monitoring the actual component in service to confirm the durability of the repair.
The qualification data demonstrated that with a preheat temperature of 280°C and interpass temperature maintained at 350°C, the dilution rate could be controlled to approximately 20%, and the overlay weld exhibited satisfactory mechanical properties and corrosion resistance.
Engineering Practice Integration
The coke oven tower in a delayed coking unit operates under severe conditions: temperatures ranging from ambient to 500°C, pressures of 0.2–0.5 MPa, and exposure to hydrogen sulfide, carbon monoxide, and hydrocarbon vapors. The transition section of the lower dished head is particularly vulnerable due to the geometric discontinuity that creates stress concentration under thermal cycling.
The SE-ESOW overlay of Inconel 600 provides an effective solution because:
- High deposition rate: The ability to deposit 8–10 mm of overlay in a single shift significantly reduces equipment downtime.
- Uniform coverage: The process produces a smooth, continuous overlay suitable for large curved surfaces.
- Proven corrosion resistance: Inconel 600 offers excellent resistance to reducing acids and sulfidation environments encountered in coking operations.
However, several practical considerations must be addressed:
- Distortion control: The high heat input can cause significant distortion of the forged component. Fixturing and controlled welding sequences are essential.
- Surface quality: The as-welded surface of SE-ESOW may exhibit undulations that require machining or grinding to achieve the required surface finish.
- Inspection: Full-penetration radiographic or ultrasonic testing of each pass is required to detect lack of fusion, slag inclusions, and porosity.
Key Questions and Reflections
A significant concern with SE-ESOW on 15CrMo substrates is the long-term stability of the dilution zone. While the qualification tests confirm acceptable short-term performance, the behavior of the interface after prolonged thermal cycling remains uncertain. Engineers should consider incorporating periodic in-service inspections using eddy current or ultrasonic testing to detect early signs of interfacial cracking or delamination.
Another question relates to the cost-benefit analysis of SE-ESOW versus alternative overlay methods such as GTAW or SAW. While SE-ESOW offers superior deposition rates, the initial investment in equipment and the requirement for specialized consumables may not be justified for small-scale repairs. The process is most economical for large-area overlays exceeding 20 m².
Study Insights and Implications
This paper demonstrates the successful application of a high-productivity overlay process to a challenging dissimilar metal joint in a critical petrochemical application. The systematic approach to process development—combining laboratory qualification with field validation—serves as a model for engineers tackling similar problems. The key insight is that process optimization must balance deposition rate against metallurgical quality: excessive heat input accelerates production but degrades the overlay properties through increased dilution and coarser microstructures. Engineers should always prioritize metallurgical integrity over productivity, particularly for safety-critical components operating under cyclic loading conditions.