Study Note on 2.25Cr1MoV Strip Electrode Electroslag Cladding with Inconel 625

Technical Background and Significance

2.25Cr1MoV (also designated as P91 or 9Cr-1Mo-V) is a high-strength, creep-resistant ferritic steel widely used in power generation, petrochemical, and hydrogenation applications where temperatures up to 650 °C are encountered. However, in reducing and oxidizing environments, 2.25Cr1MoV exhibits limited corrosion resistance, particularly in the presence of sulfur compounds and chlorides. To address this limitation, weld overlay cladding with a nickel-based alloy such as Inconel 625 is employed to provide a corrosion-resistant barrier while maintaining the structural integrity of the base material.

The literature under study investigates the application of strip electrode electroslag welding (ESW) for depositing Inconel 625 cladding on 2.25Cr1MoV base plates. Electroslag welding is chosen for its high deposition rate, low dilution rate, and excellent arc stability, making it particularly suitable for thick cladding layers on thick base materials.

Process Design and Parameters

The ESW cladding process uses a flat strip electrode of Inconel 625 composition (approximately 58% Ni, 22% Cr, 9% Mo, 3% Nb, 3% Ti) as the filler metal. The flux is a specialized high-silica, high-alumina flux designed to maintain a stable slag pool and to prevent excessive dilution of the nickel-based cladding. The flux composition includes SiO₂, Al₂O₃, CaF₂, and FeO as the main constituents.

The following table presents the optimized welding parameters:

The preheat temperature is critical to prevent cold cracking in the 2.25Cr1MoV base metal, which has a carbon equivalent of approximately 0.45. The interpass temperature is limited to prevent excessive grain growth in the heat-affected zone.

Metallurgical Analysis

The study examines the microstructure of the cladding layer, the dilution zone, and the heat-affected zone (HAZ) using optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD).

The Inconel 625 cladding layer consists of an equiaxed austenitic matrix with dispersed Laves phase (Ni₃Nb) precipitates. The Laves phase content is approximately 5–10 vol%, which provides solid-solution strengthening and contributes to the high-temperature strength of the cladding layer. The hardness of the cladding layer is in the range of 280–320 HV.

The dilution zone shows a gradual transition from the austenitic Inconel 625 microstructure to the ferritic 2.25Cr1MoV microstructure. The width of the dilution zone is approximately 0.3–0.5 mm, depending on the welding parameters. The hardness in the dilution zone ranges from 320 HV at the cladding side to 380 HV at the base metal side.

The HAZ of the 2.25Cr1MoV base metal shows a tempered martensite microstructure with a hardness of 350–400 HV, which is consistent with the base metal properties. No cracking or microstructural degradation was observed in the HAZ.

Dilution Control and Corrosion Performance

The dilution rate is a critical parameter that directly affects the corrosion resistance of the cladding layer. The study reports dilution rates of 8–15% under the optimized process conditions. With a dilution rate below 12%, the cladding layer achieves a chromium content of 20–22% and a nickel content of 50–55%, which meets the requirements of ASTM A263 for corrosion-resistant cladding.

Corrosion testing was performed using potentiodynamic polarization in a 3.5% NaCl solution at 60 °C and in a simulated flue gas environment containing SO₂ and HCl. The cladding layer exhibits a passive current density of less than 1 μA/cm² in the NaCl solution, indicating excellent corrosion resistance. In the flue gas environment, the cladding layer shows no signs of intergranular corrosion or pitting after 500 hours of exposure.

Application in Hydrogenation Reactors

The study documents the application of this cladding process in a hydrogenation reactor operating at 380 °C and 20 MPa hydrogen partial pressure. The reactor internals were clad with a 6 mm Inconel 625 layer using the strip electrode ESW process. The cladding layer was inspected by 100% magnetic particle testing and 20% ultrasonic testing for bond strength verification. The reactor has been in continuous service for over 3 years with no evidence of cladding failure or corrosion.

The study also reports the results of a hydrogen-induced cracking (HIC) test performed on the cladding layer according to NACE TM0177. The test specimen was exposed to a hydrogen-containing solution at 25 °C and 2 MPa for 750 hours. No HIC was observed, confirming the suitability of the Inconel 625 cladding for hydrogen service.

Quality Control and Defect Prevention

The study emphasizes the importance of quality control in ESW cladding. Common defects include slag inclusions, lack of fusion at the cladding-base metal interface, and porosity. The following table summarizes the defect prevention measures:

Summary

This study demonstrates that strip electrode electroslag welding is an effective process for depositing Inconel 625 cladding on 2.25Cr1MoV base materials. The process achieves low dilution rates, excellent corrosion resistance, and good metallurgical compatibility between the cladding layer and the base metal. The application in a hydrogenation reactor confirms the practical viability of this cladding process for severe service conditions. Engineers considering this process should pay close attention to process parameter optimization, dilution control, and quality control to ensure reliable performance in service.