Strip Electrode Electroslag Overlay Welding of Inconel 625 on 2.25Cr1MoV

Literature Overview

The paper by Wang Qingguo et al. (2016, Chemical Engineering Equipment and Piping, Vol. 43, Issue 3, pp. 40–42) documents the development and industrial application of strip electrode electroslag overlay welding (SE-ESWO) for depositing Inconel 625 on 2.25Cr1MoV steel substrates. This work addresses a critical challenge in chemical equipment manufacturing: the need to combine the high-temperature and high-pressure resistance of 2.25Cr1MoV steel with the superior corrosion resistance of Inconel 625 in a single component. The study describes the qualification testing process, the resulting welding procedure specification (WPS), and the field performance of the overlay-welded reactor.

Technical Background and Process Characteristics

Strip electrode electroslag overlay welding is a specialized variant of electroslag welding that uses a continuous strip electrode rather than a solid wire electrode. This process is particularly well-suited for overlay welding applications because it provides:

High deposition rates (typically 3–5 kg/h, compared to 0.5–1.5 kg/h for conventional arc welding)
Excellent control over dilution through slag composition and process parameters
Ability to deposit thick overlay layers (10–20 mm or more) in a single operation
Uniform and consistent overlay quality due to the stable molten slag pool

Process Parameter	Typical Range	Purpose
Welding current	300–500 A	Controls heat input and deposition rate
Slag voltage	28–35 V	Maintains stable slag pool
Travel speed	100–200 mm/min	Controls dilution and overlay thickness
Strip electrode width	15–25 mm	Determines bead width and overlap
Preheating temperature	200–300 °C	Reduces cracking risk in 2.25Cr1MoV
Interpass temperature	200–350 °C	Controls cooling rate and residual stress
Post-weld heat treatment	700–750 °C, 2–4 h	Stress relief and microstructure stabilization

The dilution rate in SE-ESWO is typically lower than in conventional arc welding processes because the molten slag pool acts as a barrier between the base metal and the deposited metal. For Inconel 625 on 2.25Cr1MoV, a dilution rate of 5–15% is generally acceptable, depending on the specific corrosion resistance requirements of the application. Higher dilution rates can introduce chromium, molybdenum, and vanadium from the base material into the overlay, potentially forming brittle intermetallic phases that reduce corrosion resistance and ductility.

Metallurgical Considerations

The 2.25Cr1MoV steel substrate presents several metallurgical challenges for overlay welding. The base material has a relatively high carbon equivalent (CE ≈ 0.45–0.55), which increases the susceptibility to cold cracking. The 2.25Cr1MoV steel is typically supplied in the normalized or quenched and tempered condition, with a ferrite-pearlite or tempered martensite microstructure. During overlay welding, the heat-affected zone (HAZ) of the base material may experience significant microstructural changes, including grain coarsening and the formation of untempered martensite in the coarse-grained HAZ.

Inconel 625 is a nickel-based superalloy with a balanced composition of chromium (20–22 wt%), molybdenum (8–10 wt%), niobium (0.4–1.0 wt%), and iron (≤5 wt%). Its excellent corrosion resistance is attributed to the formation of a stable chromium oxide passive film and the absence of sensitizing carbide precipitation due to the niobium content. However, Inconel 625 is susceptible to solidification cracking due to its narrow freezing range and the formation of brittle Nb-rich phases at grain boundaries. The dilution from the 2.25Cr1MoV substrate can exacerbate this cracking tendency by introducing carbon, which promotes the formation of NbC and other brittle phases.

Qualification Testing and Quality Assessment

The qualification testing program described in the paper follows a systematic approach to validate the welding procedure. The testing sequence typically includes:

Macroscopic examination: Visual inspection and sectioning to verify overlay thickness, uniformity, and absence of gross defects.
Microstructural analysis: Metallographic examination of the overlay, interface, and HAZ to assess grain structure, phase composition, and presence of brittle phases.
Mechanical property testing: Hardness profiling across the overlay and HAZ, tensile testing of overlay coupons, and bend testing of interface coupons.
Non-destructive testing (NDT): Radiographic testing (RT) or ultrasonic testing (UT) to detect internal defects such as porosity, lack of fusion, and cracking.
Corrosion testing: Immersion testing, electrochemical testing, or field exposure testing to verify the corrosion resistance of the overlay.

The paper reports that all qualification tests were passed, with the overlay exhibiting hardness values of 200–250 HV (consistent with Inconel 625 properties), no detectable cracking or porosity, and acceptable corrosion resistance in simulated service environments. The field performance feedback from the reactor in service further confirmed the reliability of the welding procedure.

Engineering Practice and Application

The application of SE-ESWO for Inconel 625 overlay on 2.25Cr1MoV reactor components represents a cost-effective alternative to fabricating the entire component from Inconel 625. The base material provides the required mechanical strength and high-temperature creep resistance, while the Inconel 625 overlay provides the necessary corrosion resistance in aggressive chemical environments. This approach is particularly advantageous for large-diameter reactor shells and heads, where the cost of full Inconel 625 fabrication would be prohibitive.

Several practical considerations must be addressed in the implementation of this process. The preheating and interpass temperature control are critical to prevent cold cracking in the 2.25Cr1MoV substrate. The slag composition must be carefully controlled to ensure stable process operation and to minimize dilution. The strip electrode must be of consistent quality and composition to ensure uniform overlay properties. The welding fixture must provide adequate support and alignment to prevent distortion and to maintain the required gap between the strip electrode and the substrate.

Key Reflections and Study Insights

This paper demonstrates the practical viability of SE-ESWO as a production technology for overlay welding nickel-based alloys on low-alloy steel substrates. The systematic qualification approach described in the paper is a model for welding procedure development in chemical equipment manufacturing. The emphasis on field performance feedback as validation of the qualification testing is particularly commendable, as it bridges the gap between laboratory testing and actual service performance.

One important observation is that the paper does not extensively discuss the long-term stability of the Inconel 625 overlay under cyclic thermal loading conditions. In chemical reactor service, thermal cycling can cause fatigue cracking at the overlay-substrate interface due to the mismatch in thermal expansion coefficients between Inconel 625 (13.5 × 10⁻⁶ /°C) and 2.25Cr1MoV (12.5 × 10⁻⁶ /°C). While the mismatch is relatively small, it can still be significant under repeated thermal cycling. Future work should include thermal cycling fatigue testing to evaluate the long-term durability of the overlay under realistic service conditions.

Additionally, the paper could benefit from a more detailed discussion of the dilution effects on the overlay composition and properties. The dilution rate directly affects the chromium, molybdenum, and niobium content of the overlay, which in turn affects the corrosion resistance and mechanical properties. A dilution rate analysis, including the composition of the overlay at different depths, would provide valuable guidance for optimizing the process parameters.

In summary, this paper provides a valuable case study in the application of SE-ESWO for overlay welding Inconel 625 on 2.25Cr1MoV steel, demonstrating the process's capability to produce high-quality overlay welds suitable for demanding chemical equipment applications. The systematic qualification approach and field validation provide confidence in the reliability of the welding procedure.