Large-Area UNS N06625 Electroslag Strip Cladding Technology

Overview and Technical Significance

The paper by Wang Jinguang from China Petrochemical Engineering Corporation, published in "Petrochemical Equipment" in 2011, addresses the application of electroslag welding (ESW) strip cladding for large-area UNS N06625 (Inconel 625) overlay deposition. This research is of considerable practical importance in the petrochemical and chemical processing industries, where large-diameter vessels, heat exchanger shells, and reactor internals require extensive corrosion-resistant cladding over carbon steel or low-alloy steel substrates.

UNS N06625 is a nickel-chromium-molybdenum alloy with excellent resistance to a wide range of corrosive environments, including reducing acids, oxidizing acids, and chloride-containing solutions. Its application as a cladding material on large structural components provides an economical alternative to full-alloy construction while maintaining the necessary corrosion resistance at the fluid-contact surface.

Electroslag Strip Cladding Process Fundamentals

Process Description

Electroslag strip cladding, also known as electroslag surfacing, utilizes the high heat input and stable slag pool of the ESW process to deposit a continuous strip of cladding material onto the substrate surface. The process involves:

A steel strip electrode is fed continuously into the molten slag pool
An electric current passes through the slag, generating resistive heating
The strip melts at the bottom of the slag pool and fuses with the substrate
The slag pool shields the molten metal from atmospheric contamination
A water-cooled copper backing (or chill) solidifies the back of the deposit

The key advantage of ESW strip cladding over other processes is its extremely high deposition rate (typically 20–50 kg/h compared to 3–10 kg/h for GMAW), which makes it economically viable for large-area applications.

Process Parameters

The following table summarizes the typical process parameters for UNS N06625 ESW strip cladding:

Parameter	Typical Range	Notes
Current (DC)	400–700 A	Depends on strip width and thickness
Voltage	25–35 V	Includes slag pool voltage
Strip feed speed	100–300 mm/min	Controls deposition rate
Travel speed	100–300 mm/min	Matches feed speed for uniform thickness
Slag composition	CaF₂-CaO-Al₂O₃-SiO₂	Flux-cored or self-shielded strip
Strip composition	UNS N06625 equivalent	Ni-21Cr-9Mo-3Ti
Preheat temperature	150–250 °C	For carbon steel substrate
Interpass temperature	150–250 °C	Controlled to prevent cracking
Backing material	Water-cooled copper chill	Ensures sound back surface

Microstructural Characteristics of the Cladding Layer

Solidification Microstructure

The ESW process produces a coarse columnar dendritic microstructure in the cladding layer due to the high heat input and directional solidification. The columnar grains grow perpendicular to the substrate surface, with grain widths of 0.5–2.0 mm. This microstructure is characterized by:

Primary austenite (γ) dendrites
Secondary Laves phase (Ni₃Nb, Ni₃Mo) precipitates in the interdendritic regions
Possible δ-ferrite formation in regions of high Mo and Nb content

The presence of Laves phase is a concern because it is brittle and can reduce the ductility and corrosion resistance of the overlay. The formation of Laves phase can be mitigated by:

Adding Ti to the strip composition (N06625 contains 0.4–1.0% Ti to tie up Nb and prevent Laves phase)
Controlling the cooling rate through process parameter optimization
Post-weld solution heat treatment at 1050–1150 °C followed by rapid quenching

Dilution and Compositional Variation

In ESW strip cladding, the dilution ratio is typically lower than in conventional arc welding processes due to the stable slag pool and the geometry of the process. The dilution ratio for the first cladding pass is typically 5–15%, decreasing to less than 5% for subsequent passes. This low dilution is advantageous because it preserves the corrosion resistance of the N06625 alloy.

However, the high heat input of ESW can lead to compositional segregation within the cladding layer. The interdendritic regions may be enriched in Mo, Nb, and Ti, which can lead to localized formation of brittle phases. This is why post-weld heat treatment is often recommended for critical applications.

Engineering Application in Petrochemical Equipment

Typical Applications

UNS N06625 ESW strip cladding is widely applied in the following petrochemical equipment:

Equipment Type	Application Area	Corrosive Environment	Cladding Thickness
Hydrogenation reactors	Internal shell, head	H₂, H₂S, NH₃ at high T/P	6–12 mm
Crude oil fractionators	Top section, reflux drum	H₂S, CO₂, organic acids	3–6 mm
Ammonia synthesis loops	Reactor internals	High-temperature H₂/N₂	6–10 mm
Sulfur recovery units	Absorber, stripper	H₂S, SO₂, water	3–6 mm
Heat exchanger shells	Tube-side, shell-side	Sour crude, sour water	3–6 mm

Case Study: Hydrogenation Reactor Cladding

A representative case involves the cladding of a hydrogenation reactor with a shell diameter of 3200 mm and a wall thickness of 120 mm. The base material is 16MnR (a low-alloy pressure vessel steel), and the cladding is UNS N06625 with a total thickness of 10 mm deposited in three passes.

The fabrication procedure included:

Surface preparation of the base by shot blasting to Sa 2.5 cleanliness
Preheating to 200 °C
First pass: 4 mm ESW strip cladding with flux-cored strip
Second pass: 3 mm ESW strip cladding with flux-cored strip
Third pass: 3 mm ESW strip cladding with flux-cored strip
Post-weld heat treatment at 620 °C for stress relief
Solution heat treatment of the cladding at 1050 °C for 1 hour followed by water quenching
Non-destructive testing (UT for bond strength, PT for surface defects)

The resulting cladding layer exhibited a hardness of 190–220 HV, a tensile strength of 620–680 MPa, and excellent resistance to hydrogen attack and sulfide stress corrosion cracking (SSC).

Quality Control and Inspection

Non-Destructive Testing Requirements

The inspection of ESW strip cladding is governed by standards such as NB/T 47014, ASME IX, and API 934. The following NDT methods are typically required:

NDT Method	Purpose	Acceptance Criteria
Visual Testing (VT)	Surface defects, porosity, cracks	No cracks, no porosity > 1 mm
Dye Penetrant Testing (PT)	Surface cracks, lack of fusion	No linear indications
Ultrasonic Testing (UT)	Bond strength, subsurface defects	Full bond, no delamination
Radiographic Testing (RT)	Internal porosity, inclusions	Per ASME V, T-274
Hardness Testing	Verify overlay composition	190–250 HV

Mechanical Property Verification

The mechanical properties of the UNS N06625 cladding layer should be verified through:

Tensile testing of coupon samples (per ASME II Part A)
Impact testing at service temperature (per ASME II Part A)
Intergranular corrosion testing (per ASTM A240)
Sulfide stress corrosion cracking testing (per NACE MR0175)
Hydrogen-induced cracking testing (per NACE MR0175)

Key Technical Challenges and Solutions

Challenge 1: Cracking Sensitivity

UNS N06625 is susceptible to solidification cracking due to its narrow solidification range and the presence of sulfur and phosphorus impurities. The high heat input of ESW exacerbates this tendency. Solutions include:

Using low-sulfur, low-phosphorus strip material (S < 0.01%, P < 0.01%)
Adding Ti to the strip composition to refine the microstructure
Controlling the heat input to minimize the solidification range
Post-weld solution heat treatment to dissolve segregated phases

Challenge 2: Bond Strength

The bond strength between the N06625 cladding and the carbon steel substrate is critical for structural integrity. Poor bonding can result from:

Inadequate surface preparation (contamination, oxide)
Insufficient preheat leading to cold cracking
Excessive dilution leading to a brittle fusion zone

The recommended approach is to ensure a clean, oxide-free surface, maintain adequate preheat temperatures, and verify bond strength through UT testing before proceeding to subsequent passes.

Challenge 3: Distortion Control

The high heat input of ESW can cause significant thermal distortion of the base component. For large vessels, distortion control requires:

Symmetric welding sequences to balance thermal stresses
Use of backing plates and temporary stiffeners
Controlled preheat and interpass temperatures
Post-weld stress relief at 620 °C

Study Insights and Engineering Implications

The research by Wang Jinguang demonstrates that ESW strip cladding is a technically mature and economically viable process for large-area UNS N06625 overlay in petrochemical applications. The key to successful implementation lies in the careful selection of strip composition, process parameter optimization, and rigorous quality control.

For the practicing engineer, the most important considerations are:

The dilution ratio must be controlled to preserve the corrosion resistance of the N06625 alloy
The Laves phase formation must be monitored and controlled through composition and heat treatment
The bond strength must be verified through NDT before proceeding with fabrication
Post-weld heat treatment is essential for both stress relief and microstructural optimization

The future of ESW strip cladding lies in the development of advanced strip compositions with improved crack resistance and the integration of automated process control systems for consistent quality. As the petrochemical industry continues to demand more efficient and corrosion-resistant equipment, ESW strip cladding will remain a cornerstone technology for large-area overlay applications.

In conclusion, the ESW strip cladding of UNS N06625 represents a well-established and highly effective technology for the fabrication of corrosion-resistant petrochemical equipment, and its successful application requires a comprehensive understanding of metallurgy, process engineering, and quality assurance principles.