CLADDING TECHNOLOGY SHANXI CO., LTDStrip Electroslag Welding of Inconel 625 Overlay on 2.25Cr1MoV Base Metal
Technical Background and Industrial Significance
Published in Chemical Equipment and Piping in 2016, this study by Wang Qingguo, Yang Chunlai, Wen Fa, and Zhang Jinyu from AVIC Liming Jixi Chemical Machinery Group and AVIC Liming Jihua Chemical Equipment (Inner Mongolia) addresses a critical engineering challenge: the application of Inconel 625 overlay on 2.25Cr1MoV low-alloy steel base metal using strip electroslag welding (ESW). This combination is highly relevant to hydrogenation reactors, high-pressure hydrogen systems, and chemical process equipment operating at elevated temperatures and pressures in corrosive environments.
2.25Cr1MoV is a widely used low-alloy steel for high-pressure vessels and heat exchangers due to its excellent creep strength at temperatures up to 500°C. However, it is susceptible to corrosion in hydrogen-containing environments, particularly at temperatures above 200°C. Inconel 625, a nickel-chromium-molybdenum-titanium alloy, provides outstanding resistance to hydrogen attack, pitting, and crevice corrosion. The overlay of Inconel 625 on 2.25Cr1MoV creates a bimetallic structure that combines the cost-effective strength of the low-alloy steel with the corrosion resistance of the nickel-based alloy.
Process Parameters and Welding Metallurgy
Strip electroslag welding is particularly well-suited for thick-section overlay applications because it offers high deposition rates, deep penetration, and uniform weld bead geometry. The process uses a continuously fed strip electrode that melts into the slag pool, creating a stable arc and consistent heat input.
| Process Parameter | Typical Value | Rationale |
|---|---|---|
| Strip electrode | Inconel 625 (UNS N06625) | Corrosion-resistant overlay material |
| Base metal | 2.25Cr1MoV (SA-387 Gr.22 Cl.2) | High-temperature structural material |
| Preheat temperature | 200-250°C | Reduce cracking susceptibility |
| Interpass temperature | 250-350°C | Control cooling rate |
| Travel speed | 200-400 mm/min | Balance penetration and dilution |
| Slag flux | Low-hydrogen ESW flux | Control hydrogen levels |
| Post-weld heat treatment | 720-760°C, 2 h/mm | Stress relief and microstructure optimization |
The metallurgical challenge in this application lies in the large difference in thermal expansion coefficients between the base metal and the overlay. 2.25Cr1MoV has a thermal expansion coefficient of approximately 12.5 x 10^-6 /°C, while Inconel 625 has a coefficient of about 13.0 x 10^-6 /°C. This mismatch generates residual stresses at the overlay-base metal interface during cooling, which can lead to cracking if not properly managed.
The dilution between the Inconel 625 overlay and the 2.25Cr1MoV base metal is a critical factor. Excessive dilution introduces carbon and sulfur from the base metal into the overlay, which can form chromium carbides and reduce the corrosion resistance of the Inconel 625 layer. The first pass of the overlay typically exhibits the highest dilution, and subsequent passes progressively reduce the dilution as the Inconel 625 composition dominates. A minimum of 3-5 passes is recommended to achieve acceptable dilution levels below 5-10%.
Quality Control and Inspection Requirements
The quality of the Inconel 625 overlay is critical for the long-term performance of the pressure vessel. The following inspection requirements apply:
| Inspection Method | Requirement | Purpose |
|---|---|---|
| Visual testing (VT) | 100% | Surface defects, undercut, porosity |
| Magnetic particle testing (MT) | 100% of overlay surface | Surface and near-surface cracks |
| Radiographic testing (RT) | 10-20% or per code | Internal porosity, lack of fusion |
| Ultrasonic testing (UT) | 100% of overlay-base interface | Bonding defects, delamination |
| Bond strength test | Tensile test per ASTM A263 | Verify overlay-base metal bonding |
| Hardness test | 3-5 points per 100 mm | Confirm overlay composition |
| Chemical analysis | Overlay surface | Verify Inconel 625 composition |
Post-weld heat treatment (PWHT) is essential to relieve residual stresses and optimize the microstructure of both the overlay and the base metal heat-affected zone. The PWHT temperature must be carefully controlled to avoid sensitization of the Inconel 625 overlay, which can occur at temperatures above 650°C if held for extended periods. A typical PWHT cycle of 720-760°C for 2 hours per inch of thickness, followed by furnace cooling, provides adequate stress relief without compromising the overlay's corrosion resistance.
Engineering Practice Reflections
This study demonstrates the practical viability of strip ESW for Inconel 625 overlay on 2.25Cr1MoV in industrial production. The process offers significant advantages over alternative methods such as submerged arc welding (SAW) or gas metal arc welding (GMAW) for thick-section overlay applications. The high deposition rate of strip ESW (up to 5-8 kg/h) makes it economically attractive for large vessels where the overlay area is extensive.
However, engineers must be aware of the limitations of the process. Strip ESW produces a relatively coarse microstructure in the overlay due to the high heat input, which may reduce the corrosion resistance compared to laser cladding or plasma transferred arc (PTA) methods. For applications requiring the highest level of corrosion resistance, a hybrid approach may be considered: strip ESW for the bulk of the overlay followed by a final laser cladding or PTA pass to refine the surface microstructure.
The study also highlights the importance of procedure qualification under NB/T 47014 or ASME IX Section IX, which requires demonstration of the welding procedure's ability to produce acceptable welds across the range of variables specified. For Inconel 625 overlay on 2.25Cr1MoV, the qualification procedure must address the specific challenges of dissimilar metal welding, including dilution control, residual stress management, and post-weld heat treatment requirements.