Strip Electrode Electroslag Welding of Inconel 625 on 2.25Cr1MoV

Literature Overview

This study note examines the application of strip electrode electroslag welding (ESW) to deposit Inconel 625 overlay on 2.25Cr1MoV low-alloy steel substrates, as reported by Wang Qingguo, Yang Chunlai, Wen Fa, and Zhang Jinyu from China Aviation Liming Jinxi Chemical Machinery Group and its Inner Mongolia affiliate in 2016. The work addresses a critical engineering need in petrochemical and hydrogenation equipment manufacturing, where 2.25Cr1MoV forgings and rolled plates are widely used for pressure vessels operating at elevated temperatures (up to 550 °C) and pressures, while the internal surface requires corrosion resistance against hydrogen-containing environments and sour media. Inconel 625, a nickel-chromium-molybdenum superalloy, offers excellent resistance to hydrogen embrittlement, sulfuric acid corrosion, and chloride stress corrosion cracking, making it an ideal overlay material for such applications.

Core Technical Analysis

The combination of 2.25Cr1MoV substrate and Inconel 625 overlay presents several metallurgical challenges that must be carefully managed. The following table summarizes the key material properties and process considerations:

Property	2.25Cr1MoV (Substrate)	Inconel 625 (Overlay)
Carbon (%)	0.18–0.22	≤ 0.08
Chromium (%)	2.0–2.5	20.0–23.0
Molybdenum (%)	0.8–1.1	8.0–10.0
Nickel (%)	≤ 0.5	58.0–62.0
Thermal conductivity (W/m·K)	28–32	11–13
Thermal expansion (×10⁻⁶/K)	12–13	13–14
YS (MPa)	415–585	415–585

The significant difference in thermal conductivity between the substrate and overlay creates a thermal gradient that can induce high residual stresses at the interface. ESW is particularly well-suited for this application because the slag pool provides a stable, high-heat-input process with slow cooling rates, which minimizes the risk of cracking. The strip electrode configuration allows for high deposition rates (typically 30–80 kg/h), making it economically viable for thick overlays required in pressure vessel applications.

The typical ESW process parameters for Inconel 625 overlay on 2.25Cr1MoV include:

Parameter	Value
Strip electrode thickness	1.2–2.0 mm
Strip electrode width	30–50 mm
Slag	F5A or equivalent (low-hydrogen, high basicity)
Current	2500–4000 A
Voltage	35–45 V
Travel speed	80–200 mm/min
Preheat temperature	200–300 °C
Interpass temperature	150–300 °C
Post-weld heat treatment	620 °C × 2 h for substrate; overlay layer excluded

Interface Metallurgy and Defect Analysis

The weld interface between 2.25Cr1MoV and Inconel 625 is the most critical region from a metallurgical standpoint. The dilution ratio at the first pass is typically 20–35%, depending on process parameters. Excessive dilution can lead to formation of brittle intermetallic compounds (such as Ni₃Fe, Ni₃Mo) at the fusion boundary, which can compromise bond strength and corrosion resistance. Conversely, too little dilution results in a sharp compositional gradient that can promote cracking during thermal cycling.

Defect	Cause	Detection Method	Acceptance Criteria
Intermetallic phase formation	Excessive dilution, high heat input	Metallographic examination	≤ 5% volume fraction
Hot cracking	Low melting point eutectics at grain boundaries	RT or UT	No cracks > 1 mm
Cold cracking	Hydrogen embrittlement in martensitic substrate	MT after PWHT	No linear indications
Slag inclusion	Incomplete slag removal between passes	RT or UT	Area ≤ 25 mm²
Lack of fusion	Insufficient penetration, poor strip alignment	UT (PAUT)	No lack of fusion

The post-weld heat treatment strategy is particularly important. The 2.25Cr1MoV substrate requires stress relief at 620 °C to restore its temper strength and reduce residual stresses. However, the Inconel 625 overlay must not be exposed to temperatures above 620 °C for extended periods, as this can lead to formation of detrimental Laves phase (Ni₂Mo) and degradation of corrosion resistance. A careful PWHT schedule must be developed, often involving a two-step treatment: first stress relieving the substrate at 620 °C, then a separate overlay-specific anneal at 900–950 °C if required.

Engineering Practice Considerations

In hydrogenation reactor fabrication, the Inconel 625 overlay is typically applied to the entire internal surface of the vessel shell, heads, and tube sheets. The overlay thickness is usually specified as 3–6 mm, applied in 2–4 passes using ESW. The process must be qualified per NB/T 47014 or ASME IX, with qualification tests including bond strength peel tests, hardness surveys across the overlay, and intergranular corrosion testing per ASTM A923 Method 1B.

A practical challenge encountered in large-diameter vessel fabrication is the control of overlay flatness and thickness uniformity. The thermal distortion caused by the high heat input of ESW can lead to warping of thin-walled sections. This is mitigated by using back-of-plate cooling, fixture clamping, and sequential welding sequences that balance thermal input around the circumference.

Study Insights and Reflections

This 2016 study represents a mature application of ESW overlay technology in Chinese petrochemical equipment manufacturing. The authors' experience with 2.25Cr1MoV/Inconel 625 combinations reflects the growing demand for high-performance overlays in hydrogenation and refining equipment, driven by the increasing severity of operating conditions in modern refineries. The systematic approach to process qualification, interface metallurgy evaluation, and defect control demonstrates the level of engineering rigor now expected in this domain. The findings are directly applicable to current practice, and the process windows identified provide a reliable starting point for new ESW overlay qualification procedures.