Microstructure and Properties of GH2132 Alloy Flat Plate Weld Overlay Layer

Literature Overview

This 2022 publication by Liu Aisheng from Jiangsu Yangzhou Technician College, appearing in the journal Metal World, examines the microstructure and mechanical properties of a weld overlay layer deposited on flat plates using GH2132 alloy consumables. GH2132 is a nickel-chromium-iron superalloy with a nominal composition of approximately 60% Ni, 25% Cr, 10% Fe, and minor additions of molybdenum, titanium, and aluminum. This alloy is extensively used in high-temperature applications including gas turbine components, heat exchanger tubes, and furnace fixtures. The study addresses the deposition of GH2132 as an overlay layer, which is a common practice for protecting carbon steel or stainless steel substrates in high-temperature oxidizing or carburizing environments.

Core Technical Points

GH2132 is a wrought nickel-based superalloy with a fully austenitic (face-centered cubic) microstructure stabilized by the high nickel and chromium content. When deposited as a weld overlay layer, the solidification behavior differs significantly from wrought processing. The rapid solidification rates encountered in welding (typically 10–100 K/s) promote the formation of columnar dendritic structures with interdendritic segregation of low-melting-point phases. The primary solidification sequence in GH2132 weld deposits is typically austenite (γ) followed by eutectic γ + δ (ferrite) at the interdendritic regions.

The presence of δ-ferrite in the overlay layer is a significant concern. While small amounts of δ-ferrite (less than 5%) can contribute to creep strength through Orowan strengthening, excessive δ-ferrite (>15%) leads to reduced ductility and increased susceptibility to intergranular cracking. The formation of δ-ferrite is influenced by the welding process parameters, particularly the heat input and cooling rate. Lower heat input processes such as GTAW (TIG) tend to produce higher δ-ferrite fractions compared to higher heat input processes such as SAW or submerged arc welding.

Microstructural Characterization

The as-welded microstructure of GH2132 overlay layers typically exhibits a columnar dendritic structure with primary γ dendrites and interdendritic γ + δ eutectic. The δ-ferrite appears as irregular, interconnected phases at the dendrite boundaries. Upon aging at 950–1050 °C for 2–4 hours, the δ-ferrite partially dissolves and transforms back to γ, resulting in a more homogeneous austenitic structure. During this aging treatment, fine γ' (Ni3(Al,Ti)) and γ'' (Ni3Ti) precipitates form within the γ matrix, contributing to age-hardening.

The grain structure of the overlay is strongly influenced by the substrate grain orientation. When deposited on a carbon steel substrate with randomly oriented grains, the overlay grains tend to grow epitaxially, resulting in a columnar structure that can extend through multiple passes. This is advantageous for corrosion resistance as the columnar grains provide a direct diffusion path for any corrosion product to escape, but it can be detrimental for fatigue performance due to the preferential intergranular crack propagation along the columnar boundaries.

Engineering Practice and Application Considerations

In pressure vessel fabrication, GH2132 overlay layers are commonly applied to carbon steel or 304/316 stainless steel substrates using processes such as GTAW, SAW, or plasma transferred arc (PTA) powder cladding. The selection of process depends on the required overlay thickness, surface finish, and dilution control. For thin overlays (1–3 mm), GTAW with a GH2132 electrode is preferred due to its low dilution and excellent surface quality. For thicker overlays (3–10 mm), SAW with a GH2132 powder or strip electrode is more economical. PTA powder cladding offers the best control of dilution and composition uniformity for critical applications.

The dilution rate is a critical parameter. For GH2132 overlays on carbon steel substrates, the dilution should be controlled below 30% to maintain the required nickel and chromium content in the overlay. Excessive dilution reduces the corrosion resistance and high-temperature strength of the overlay. The use of a transition layer, such as a nickel-based or austenitic stainless steel underlay, can be employed to reduce dilution when the overlay must be deposited on heavily diluted substrates.

Quality assurance for GH2132 overlay layers includes visual inspection for surface defects, ultrasonic testing for subsurface cracks and lack of fusion, and metallographic examination for δ-ferrite content and grain structure. The overlay must also pass a bond strength test, typically a peel test per ASTM E2358 or a shear test, to verify adequate adhesion to the substrate.

This study provides valuable data on the microstructural evolution and mechanical properties of GH2132 overlay layers deposited on flat plates. The findings are directly applicable to the qualification of overlay processes for high-temperature pressure vessel components, where the overlay layer must withstand prolonged exposure to elevated temperatures in oxidizing or carburizing environments. Engineers should note that the flat plate geometry studied here may not fully represent the geometric complexities of curved pressure vessel surfaces, where residual stress distributions and thermal gradients differ significantly.