Three-Dimensional Microstructural Characteristics of Nickel-Based Alloy Strip Electrode Overlay Metal

Literature Overview

Published in 2023 in the journal Welding (焊接), this paper by Cao Yukun, Guo Xiao, Xu Kai, Lü Xiaochun, and Wei Chao from the Harbin Welding Research Institute (part of China Mechanical Science and Technology Group) investigates the three-dimensional microstructural characteristics of a novel nickel-based alloy EQNiCrFe-13 strip electrode submerged arc overlay metal. The study employs optical microscopy and Electron Backscatter Diffraction (EBSD) to characterize the microstructure along three orthogonal directions (X, Y, Z) and analyzes grain size, grain orientation, and texture characteristics. The work was funded by the National Science and Technology Major Project (2018ZX06004001), reflecting the strategic importance of nickel-based alloy cladding for critical energy infrastructure.

Core Technical Content

Nickel-based alloys are widely used for corrosion-resistant cladding in chemical processing, oil and gas, power generation, and nuclear applications due to their excellent resistance to a wide range of aggressive environments. The EQNiCrFe-13 alloy represents a novel composition within the Ni-Cr-Fe system, designed to combine the corrosion resistance of nickel-based alloys with improved mechanical properties and cost efficiency through iron addition.

Three-Dimensional Microstructural Parameters

The Z-direction grain size is significantly smaller than the X and Y directions, reflecting the constrained solidification in the through-thickness direction where the solidification front advances perpendicular to the fusion boundary. The X and Y directions exhibit columnar grain structures with larger average grain sizes, consistent with the directional solidification pattern typical of overlay welding.

EBSD Analysis and Crystallographic Texture

The EBSD analysis provides detailed information about grain orientation and texture that is not accessible through conventional metallographic techniques. The Euler angle distributions across the three observation directions reveal important characteristics of the microstructure:

• Consistent Euler angle distributions: The similarity of Euler angle distributions across X, Y, and Z directions indicates a coherent solidification pattern with minimal variation in the local thermal gradient direction
• {001} and {011} preferred orientations: These orientations are characteristic of cubic crystal structures (austenitic or ferritic) and indicate strong crystallographic texture development during solidification
• Cubic texture and Gaussian texture: The presence of both cubic texture and Gaussian texture indicates complex texture evolution during solidification, influenced by the thermal gradient, solidification rate, and alloy composition

The strong preferred orientation (texture) has significant implications for the mechanical and corrosion properties of the overlay metal. Texture affects the anisotropy of mechanical properties, the directionality of corrosion attack, and the response to cyclic loading.

Solidification Pattern and Grain Growth Mechanism

The microstructural observations confirm that the crystallization direction is perpendicular to the fusion boundary, with grains exhibiting typical columnar growth characteristics. This is consistent with the expected solidification pattern in overlay welding, where the heat extraction direction is primarily through the fusion boundary into the base metal.

The columnar grain structure is a result of competitive grain growth during directional solidification. Grains with favorable crystallographic orientations relative to the thermal gradient direction outgrow less favorably oriented grains, leading to the development of strong texture. The Z-direction equiaxed morphology with smaller grain size suggests that the through-thickness solidification is more constrained, possibly due to the interaction between successive weld passes or the influence of the base metal on the initial solidification.

Engineering Implications and Property Correlations

The three-dimensional microstructural characterization has direct implications for the performance of nickel-based alloy cladding:

• Corrosion resistance: Columnar grain structures with strong texture may exhibit anisotropic corrosion behavior, with preferential attack along grain boundaries oriented parallel to the corrosion surface. The {001} and {011} orientations may influence the passive film stability and dissolution rate
• Mechanical properties: The columnar grain structure in the X and Y directions may result in lower transverse ductility compared to the longitudinal direction. The Z-direction equiaxed structure with smaller grain size may provide better through-thickness toughness
• Cyclic fatigue: Texture and grain orientation influence the initiation and propagation of fatigue cracks. The columnar grain structure may facilitate crack propagation along the columnar grain boundaries
• Creep resistance: At elevated temperatures, the grain size and orientation distribution influence creep deformation mechanisms, particularly dislocation creep and grain boundary sliding

Comparison with Conventional Nickel-Based Overlay Alloys

The EQNiCrFe-13 alloy represents an evolution from conventional nickel-based overlay alloys such as Hastelloy C-276, Inconel 625, or Stellite 6. The addition of iron to the Ni-Cr system reduces material cost while potentially maintaining or improving corrosion resistance in specific service environments. The microstructural characteristics of the EQNiCrFe-13 overlay metal—particularly the grain size and texture—should be compared with those of conventional alloys to assess the performance implications of the compositional change.

The grain sizes reported (180–310 μm) are relatively large compared to some wrought nickel-based alloys, which is typical of weld overlays where solidification rates are lower than in casting or hot-working processes. Grain refinement strategies, such as the addition of grain refiners or the use of thermal management techniques during welding, could potentially reduce grain size and improve properties.

Key Questions and Reflections

The EBSD analysis provides valuable microstructural data, but several questions remain for practical application. First, the relationship between the observed texture and the corrosion performance in specific service environments requires experimental validation. The {001} and {011} orientations may have different passive film characteristics, and the texture may influence the uniformity of corrosion attack across the overlay surface.

Second, the effect of multi-pass welding on the microstructure was not extensively characterized. In practical cladding applications, multiple overlay passes are required to achieve the desired thickness, and each subsequent pass may alter the microstructure of previously deposited layers through thermal cycling. The final microstructure after multi-pass welding may differ from the single-pass microstructure characterized in this study.

Third, the mechanical property data—particularly the cyclic fatigue and creep properties—should be correlated with the microstructural characteristics to establish property-microstructure relationships that can guide process optimization.

Summary

This paper provides a comprehensive three-dimensional microstructural characterization of a novel nickel-based alloy EQNiCrFe-13 strip electrode overlay metal using advanced EBSD techniques. The findings reveal a columnar grain structure with strong {001} and {011} preferred orientations, a Z-direction equiaxed morphology with smaller grain size, and significant crystallographic texture development. These microstructural characteristics have direct implications for the corrosion resistance, mechanical properties, and long-term performance of nickel-based alloy cladding in critical applications. The work contributes to the fundamental understanding of solidification behavior in advanced nickel-based overlay alloys and provides a microstructural baseline for future process optimization and property enhancement efforts.