Electroslag Cladding of Heat Exchanger Flanges Using a Consumable Electrode

Literature Overview

This paper, published in 2006 in Welding Technology, was authored by Ye Yufen of the Production Technology Department at Zhenhai Refined Chemical Inspection and Safety Company, Zhejiang Province. The study addresses the application of electroslag welding (ESW) with a consumable electrode technique for cladding heat exchanger flanges, a critical component in petrochemical equipment where corrosion resistance and mechanical integrity are paramount.

Technical Background and Significance

Heat exchanger flanges in petrochemical service are subjected to aggressive chemical environments, high temperatures, and cyclic thermal loading. Carbon steel flanges are often clad with stainless steel or nickel-based alloys to provide corrosion resistance while maintaining the mechanical strength and economic viability of a carbon steel substrate. The electroslag welding process is particularly attractive for flange cladding due to its high deposition rate, low dilution, and ability to produce sound, defect-free welds in thick sections.

Why Electroslag Welding for Flange Cladding

Electroslag welding offers several distinct advantages for flange cladding applications:

High deposition rate: ESW can deposit 10–25 kg/h of metal, significantly faster than conventional arc welding methods. This is critical for large-diameter flanges where cladding thickness may range from 6 to 25 mm.
Low dilution: The slag pool in ESW acts as a thermal barrier, limiting the penetration into the base metal. Dilution ratios of 5–10% are typical, preserving the alloying integrity of the overlay.
Deep, uniform welds: The process produces welds with consistent geometry and minimal undercut, which is important for the machined sealing faces of flanges.
Low hydrogen content: The slag pool effectively excludes atmospheric contamination and reduces hydrogen pickup, minimizing the risk of hydrogen-induced cracking.

Process Configuration for Flange Cladding

The ESW cladding of flanges typically employs a vertical or horizontal process configuration depending on the flange orientation. For large flanges, the horizontal ESW process is often used, where the electrode moves around the circumference of the flange face. The process involves:

Wing electrodes: Two wing electrodes create a molten slag pool that fills the gap between the cladding layer and the base flange, ensuring complete fusion and preventing slag inclusion.
Main electrode: A consumable electrode of the desired cladding alloy composition (e.g., 304, 316L, or Inconel 625) is fed through the center of the wing electrode assembly.
Flux: A proprietary flux is used to generate the molten slag pool, control solidification rate, and protect the weld from atmospheric contamination.

Process Parameter	Typical Value for Flange Cladding	Purpose
Welding current	400–800 A	Controls deposition rate and penetration
Travel speed	100–300 mm/min	Determines bead width and height
Electrode feed speed	10–20 m/min	Maintains arc stability and deposit rate
Slag pool depth	20–40 mm	Controls heat input and solidification
Preheat temperature	150–250 °C	Reduces thermal gradient and cracking risk
Interpass temperature	250–400 °C	Prevents excessive cooling between passes

Microstructural Considerations

The metallurgical quality of ESW-clad flanges is determined by the solidification behavior of the overlay material and the thermal history of the joint. Key microstructural features include:

Columnar dendrites: The primary solidification structure in ESW welds, aligned with the heat extraction direction. For stainless steel cladding, these dendrites should exhibit a ferrite-austenite balance consistent with the base alloy composition.
Ferrite content: In austenitic stainless steel cladding (e.g., 304, 316L), the delta ferrite content should be maintained between 5–20% to prevent hot cracking. ESW processes typically produce slightly higher ferrite content than GTAW or GMAW due to the higher heat input and slower cooling rates.
Bond line quality: The interface between the carbon steel flange and the stainless steel overlay should show complete fusion with a clean, continuous bond. Any lack of fusion or unmelted particles at this interface represents a critical defect.

Dilution and Its Effects

The dilution ratio in ESW cladding is typically 5–15%, which is lower than most arc welding processes but still significant enough to affect the overlay composition. For a 316L stainless steel cladding layer, the dilution from carbon steel can increase the carbon equivalent and potentially promote martensite formation in the heat-affected zone. The following table summarizes the effect of dilution on overlay properties:

Dilution (%)	Carbon Content in Overlay	Ferrite Content	Hardness (HV)	Susceptibility to SCC
5	Slightly elevated	10–15%	180–220	Low
10	Moderately elevated	15–20%	200–250	Moderate
15	Significantly elevated	20–25%	220–280	Higher
20	High	25–30%	250–320	High

Quality Control and Inspection

The quality of ESW-clad flanges is verified through a combination of non-destructive and destructive testing methods:

Visual inspection: Each pass is examined for surface quality, undercut, and weld geometry.
Radiographic testing (RT): Used to detect internal defects such as slag inclusions, porosity, and lack of fusion. ESW welds generally produce cleaner internal quality than manual arc processes.
Ultrasonic testing (UT): Applied to the bond line to verify full fusion between the overlay and base metal.
Hardness testing: The overlay hardness should meet the specified range for the cladding alloy. For 304 stainless steel, the hardness should be below 200 HV to ensure adequate ductility and corrosion resistance.
Intergranular corrosion testing: Required for sensitized stainless steel overlays, particularly in chloride-containing environments.

Engineering Practice and Challenges

The practical implementation of ESW cladding on heat exchanger flanges presents several challenges that must be addressed during fabrication:

Positional welding: Flanges may need to be cladded in fixed positions, requiring process parameters adapted to gravity effects on the molten slag pool.
Thermal distortion: The high heat input of ESW can cause significant thermal distortion of thin flange sections. Pre-deformation or clamping fixtures may be necessary.
Machining of the overlay: The cladding layer is typically applied with an allowance of 2–4 mm above the final machined dimension. Post-weld machining must be performed carefully to avoid introducing residual stresses.
Reheat cracking: Low-alloy steel flanges clad with austenitic stainless steel are susceptible to reheat cracking in the heat-affected zone. Preheating and controlled cooling rates are essential to prevent this defect.

Study Insights and Implications

This study from 2006 represents a practical application of electroslag welding technology in the petrochemical industry, where the reliability of heat exchanger flanges is critical for plant safety. The authors' focus on process optimization and quality assurance reflects the engineering reality that cladding procedures must be qualified and controlled to meet the stringent requirements of pressure vessel codes.

A particularly noteworthy aspect of this work is the emphasis on the consumable electrode technique, which offers better control over the cladding composition compared to inert electrode ESW. The consumable electrode ensures that the deposited metal has the desired alloy content, which is essential for achieving the required corrosion resistance in aggressive service environments.

Modern practitioners should note that while ESW remains a viable option for flange cladding, alternative processes such as submerged arc welding (SAW) overlay and plasma transferred arc (PTA) cladding have gained popularity due to their greater flexibility and lower equipment costs. However, ESW's superior deposition rate makes it particularly attractive for large-scale production applications where throughput is a critical consideration.