INCONEL 690 Nickel-Based Alloy Tape Electrode Electroslag Cladding Process Development and Performance

Literature Overview

This paper by Zhang Maolong, Yan Changgen from Shanghai Boiler Works Co., Ltd., and You Junfu from Shanghai Institute of Testing Technology, published in Boiler Technology in 1997 (Vol. 28, No. 2, pp. 24-27), presents the development and evaluation of a complete electroslag cladding (ESC) process for depositing INCONEL 690 nickel-based alloy using tape electrode technology. This early work represents pioneering research in the application of electroslag cladding for high-nickel alloys on power boiler components, addressing the critical need for corrosion and oxidation resistance in supercritical and ultra-supercritical boiler water walls and heat transfer surfaces.

Technical Background and Process Development

Electroslag cladding is a specialized welding process that utilizes the high thermal efficiency of the electroslag welding (ESW) principle to deposit cladding layers. Unlike conventional arc cladding methods, ESC produces a stable molten slag pool that provides excellent heat input control, low dilution rates, and high deposition rates. The tape electrode configuration, using a continuous strip of alloy material as the consumable, further enhances deposition efficiency and compositional control compared to wire electrode variants.

Process Fundamentals

Process Parameter	Typical Range	Function
Welding current	400-800 A	Controls heat input and melting rate
Travel speed	100-400 mm/min	Controls bead geometry and dilution
Electrode feed speed	150-600 mm/min	Controls deposition rate and bead height
Slag flux composition	CaF₂-CaO-Al₂O₃ system	Controls slag properties and heat distribution
Shielding gas	Ar or Ar-CO₂ mixture	Prevents oxidation of molten pool
Preheat temperature	150-300 °C	Reduces cracking susceptibility
Interpass temperature	150-400 °C	Controls cooling rate and residual stress

Process Conditions and Quality Factors

The authors systematically addressed the basic conditions required for successful nickel-based alloy tape electrode electroslag cladding:

Substrate Requirements

Base material: Carbon steel or low-alloy steel boiler tube material
Surface preparation: Machined or ground to remove scale and contamination
Geometry: Flat or slightly curved surfaces for optimal electrode contact
Preheating: Required to prevent cold cracking at the fusion interface

Weld Material Specifications

The INCONEL 690 tape electrode composition is critical for achieving the target performance:

Element	Nominal Content (wt%)	Function
Ni	Balance (>68%)	Matrix element, corrosion resistance
Cr	29-31	Oxidation and corrosion resistance
Mo	12-14	Pitting resistance, strength
Fe	5-7	Cost control, dilution buffer
Nb	0.5-1.0	Stabilizer, precipitation strengthening
C	<0.01	Carbide suppression

Critical Quality Factors

The paper identifies several factors that significantly influence cladding layer quality:

Dilution rate: Must be minimized to preserve the Ni-Cr-Mo composition integrity. ESC typically achieves dilution rates of 3-10%, significantly lower than arc cladding methods.
Thermal cycle control: The high heat input of ESC creates slow cooling rates that can promote coarse grain formation and sensitization. Post-weld heat treatment may be required.
Slag management: The slag composition and properties directly affect heat distribution, electrode melting behavior, and surface quality of the cladding layer.
Electrode alignment: Tape electrode geometry and alignment affect bead uniformity and defect formation.

Cladding Layer Performance Analysis

Mechanical Properties

The cladding layer exhibits the following mechanical characteristics:

Property	Cladding Layer	Cast INCONEL 690	Base Metal
Tensile strength (MPa)	650-800	700-850	400-550
Yield strength (MPa)	300-400	350-450	250-350
Elongation (%)	30-40	35-45	20-25
Hardness (HV)	180-220	200-240	150-180

The mechanical properties of the cladding layer are slightly lower than cast INCONEL 690, which is expected due to the dilution effect and the differences in solidification conditions between casting and welding. However, the properties remain well above those of the base metal, providing significant improvement in both strength and ductility.

Metallographic Microstructure

The cladding layer microstructure consists of:

Primary phase: γ-Ni solid solution matrix with austenitic structure
Secondary phases: Minor amounts of δ-ferrite (if dilution is excessive) and NbC precipitates
Grain structure: Coarse equiaxed grains typical of high-heat-input welding processes
Interface structure: Clear fusion boundary with controlled dilution zone

Corrosion and Oxidation Resistance

The INCONEL 690 cladding layer provides excellent resistance to:

High-temperature oxidation in air and water vapor
Sulfidation in fuel gas environments
Erosion-corrosion in ash-laden flue gas
Stress corrosion cracking in chloride-containing environments

Engineering Application Context

The primary application context for this work is the protection of supercritical and ultra-supercritical power boiler water wall tubes and heat transfer surfaces. These components operate under extreme conditions:

Operating Parameter	Typical Value	Challenge
Steam temperature	560-620 °C	Oxidation and creep
Steam pressure	25-35 MPa	Creep rupture
Wall temperature	600-700 °C	Oxidation acceleration
Ash content in fuel	1-15%	Erosion-corrosion
SO₃ concentration	50-300 ppm	Sulfidation

The INCONEL 690 cladding provides a protective barrier that extends the service life of carbon steel boiler tubes by orders of magnitude in these aggressive environments.

Process Advantages for Boiler Applications

High deposition rate: Suitable for large surface areas typical of boiler water walls
Low dilution: Preserves alloy composition integrity
Good bonding: Metallurgical bond ensures long-term service reliability
Scalability: Process can be adapted to flat and curved geometries
Cost effectiveness: Lower cost per unit area compared to overlay welding with wire electrodes

Study Insights and Reflections

This 1997 paper represents a significant early contribution to the field of nickel-based alloy cladding for power generation equipment. The systematic approach to process development, covering substrate preparation, electrode selection, parameter optimization, and performance verification, provides a comprehensive framework that remains applicable to contemporary cladding operations. The identification of dilution control as the primary quality factor is consistent with subsequent research and industry practice. Engineers working on modern boiler cladding applications should recognize that the fundamental process challenges identified in this early work—thermal cycle management, dilution control, and interface integrity—remain the same, even as equipment capabilities and monitoring technologies have advanced.

The mechanical properties achieved, while slightly below cast INCONEL 690 values, are more than adequate for the intended application, where the primary function is corrosion and oxidation protection rather than structural load bearing. The ductility of the cladding layer (30-40% elongation) is particularly important for accommodating thermal cycling and differential expansion between the cladding and base metal during boiler start-up and shutdown transients. This early work laid the foundation for the extensive use of nickel-based alloy cladding in modern supercritical and ultra-supercritical power plants, where such protection is essential for achieving the design life and availability targets required by utility operators. The methodology demonstrated here—comprehensive process development followed by thorough performance characterization—remains the gold standard for welding process qualification in critical power generation applications.