CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructural Analysis of Austenitic Stainless Steel Strip Electroslag Weld Overlay Cladding
Literature Overview and Technical Context
This study focuses on the microstructural characterization of overlay layers produced by strip electroslag welding (ESW) using austenitic stainless steel strip electrodes, typically grades such as 308L, 309L, or 316L equivalent. Strip ESW overlay is a widely used industrial technique for applying corrosion-resistant or wear-resistant cladding layers to carbon steel and low-alloy steel substrates, particularly for large-scale pressure vessels, heat exchanger tubesheets, and pipe spools. The process is characterized by high deposition rates (5–15 kg/h), deep heat penetration, and the ability to produce thick overlay layers (up to 25–50 mm) in a single setup.
The microstructural analysis is critical because the metallurgical quality of the ESW overlay layer directly determines its corrosion resistance, mechanical properties, and long-term service reliability. Unlike fusion welding of homogeneous materials, overlay welding involves the interaction of two dissimilar materials—the austenitic stainless steel strip and the ferrous substrate—which creates complex solidification and solid-state transformation phenomena at the interface and within the overlay layer itself.
Core Technical Findings
Solidification Microstructure of the Overlay Layer
The overlay layer produced by strip ESW exhibits a characteristic columnar dendrite microstructure that grows epitaxially from the substrate interface. The columnar grains are elongated in the direction of heat extraction (perpendicular to the fusion boundary), with primary dendrite arm spacing (PDAS) typically ranging from 150–400 μm depending on the solidification rate. The wide thermal cycle associated with ESW—characterized by high preheat temperatures (200–400°C) and slow cooling rates (0.5–5°C/s)—promotes coarse dendritic growth and significant segregation of alloying elements between the dendrite arms and the interdendritic regions.
Near the fusion boundary, the solidification microstructure transitions from columnar to a mixed columnar-equiaxed morphology within the first 2–3 mm of the overlay. This transition is attributed to constitutional undercooling that increases as the solidification front moves away from the preheated substrate. The equiaxed region in the upper portion of the overlay layer contains randomly oriented grains with sizes of 200–500 μm, indicating a more homogeneous microstructure compared to the columnar region near the interface.
| Zone | Distance from Interface | Microstructure | Grain Size | PDAS (μm) |
|---|---|---|---|---|
| Fusion boundary | 0–1 mm | Fine columnar | 50–150 μm | 80–150 |
| Columnar region | 1–8 mm | Coarse columnar | 200–400 μm | 200–350 |
| Equiaxed region | 8–20 mm | Equiaxed | 200–500 μm | N/A |
| Top surface | >20 mm | Coarse equiaxed | 300–600 μm | N/A |
Phase Composition and Segregation
Chemical microanalysis (EDS) reveals significant microsegregation of chromium and nickel between the dendrite cores and interdendritic regions. The dendrite cores are enriched in chromium and nickel relative to the interdendritic regions, which are depleted due to the rejection of these elements during solidification. In 308L-type overlay material, the interdendritic regions may contain chromium-depleted zones with Cr content as low as 16–17 wt% (compared to the nominal 19–21 wt%), creating localized susceptibility to intergranular corrosion.
The presence of δ-ferrite (delta ferrite) in the interdendritic regions is a common observation in austenitic stainless steel ESW overlays, particularly when the alloying balance favors ferrite formation. The ferrite content typically ranges from 5–20% depending on the strip electrode composition and the dilution from the substrate. While moderate δ-ferrite content (5–15%) is beneficial for preventing hot cracking and controlling solidification segregation, excessive ferrite (>20%) can impair corrosion resistance and ductility.
The study also examined the presence of secondary phases such as chromium carbides (Cr₂₃C₆, Cr₇C₃) and chromium oxides (Cr₂O₃) at grain boundaries and within the interdendritic regions. These phases form during solidification due to the precipitation of carbon and oxygen from the melt. Their morphology and distribution significantly influence the pitting and intergranular corrosion resistance of the overlay layer.
Interface Microstructure and Bonding Quality
The overlay-substrate interface is a critical region for evaluating the metallurgical soundness of the ESW cladding. The study identified a diffusion zone approximately 20–50 μm thick at the interface where there is gradual compositional transition between the austenitic overlay and the ferritic-pearlitic substrate. This diffusion zone is characterized by the presence of mixed phases including austenite, ferrite, and carbides, formed by the mutual diffusion of alloying elements during the prolonged thermal exposure inherent to ESW.
No macroscopic defects such as lack of fusion, cracks, or inclusions were observed at the interface in the qualified specimens. However, microcracks parallel to the interface were identified in specimens with high residual stress levels, particularly in the substrate HAZ where the carbon and alloy content promote martensite formation during cooling. These microcracks, while not immediately detectable by conventional NDT methods, can serve as initiation sites for stress corrosion cracking (SCC) in chloride-containing environments.
Effect of Preheat and Interpass Temperature
The study systematically varied the preheat temperature from 150°C to 450°C and observed its influence on the overlay microstructure. Higher preheat temperatures (350–450°C) reduced the cooling rate below 1°C/s, promoting coarse grain growth, increased δ-ferrite content, and enhanced carbide precipitation. Conversely, lower preheat temperatures (150–200°C) maintained cooling rates above 2°C/s, resulting in finer grains, reduced ferrite content, and improved corrosion resistance. The recommended preheat range for 308L strip ESW overlay on carbon steel substrates was identified as 200–300°C, balancing the need for crack-free deposition with acceptable microstructural quality.
Engineering Practice Integration and Key Reflections
The microstructural findings from this study have direct implications for the qualification and control of ESW overlay procedures in pressure vessel and piping fabrication. The coarse columnar microstructure and significant microsegregation observed in ESW overlays represent inherent challenges that must be addressed through careful process parameter selection and, where necessary, post-weld heat treatment.
For pressure vessel applications governed by standards such as NB/T 47002 or ASME VIII Div.1, the corrosion resistance of the overlay layer is paramount. The study's observations regarding chromium-depleted interdendritic regions underscore the importance of performing intergranular corrosion tests (ASTM A263 Practice E or ASTM G48) on production welds, not just qualification coupons. The results of these tests should be documented in the welding procedure specification (WPS) and used as acceptance criteria.
The δ-ferrite content is another critical parameter that should be monitored during production. Ferrite measurements can be performed using a ferritecope (per ASTM A913) or through metallographic examination with specific etchants. The acceptable ferrite range for 308L-type overlays is typically 5–15% ferrite number (FN), and deviations from this range should trigger a review of the strip electrode chemistry and welding parameters.
A practical consideration highlighted by this study is the effect of substrate carbon content on the overlay-substrate interface. High-carbon substrates (above 0.25% C) can promote the formation of hard, brittle carbide layers at the interface, which may reduce the peel strength of the overlay and increase the risk of interfacial cracking during PWHT or service. For such substrates, the study recommends using a transition layer of 309L or 310L strip to buffer the dilution and reduce the carbon activity at the interface.
In summary, this literature provides a comprehensive microstructural analysis of austenitic stainless steel strip ESW overlay that highlights the complex interplay between solidification conditions, microsegregation, phase formation, and final material properties. The coarse columnar microstructure and interdendritic chromium depletion are inherent features of the ESW process that cannot be entirely eliminated but can be mitigated through optimized preheat, controlled interpass temperature, and appropriate strip electrode selection. Engineers involved in ESW overlay fabrication should treat microstructural evaluation as an integral part of quality assurance, not merely a qualification exercise, to ensure the long-term reliability of cladded pressure vessels and heat exchangers in corrosive service environments.