CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructural Analysis of Austenitic Stainless Steel Strip Electroslag Welding Overlay Layer
Literature Overview
This research presents a detailed microstructural analysis of austenitic stainless steel overlay layers produced by strip electroslag welding (ESW) cladding. Strip ESW is a widely used industrial method for producing thick overlay layers on carbon steel and low-alloy steel substrates, particularly for corrosion-resistant linings in pressure vessels, heat exchangers, and chemical processing equipment. The study focuses on the metallurgical characteristics of the overlay layer including grain structure, phase composition, segregation patterns, and their implications for corrosion resistance and mechanical performance.
Process Characteristics and Microstructure Formation
Strip electroslag welding is characterized by its high heat input, slow cooling rate, and excellent deoxidation capability due to the slag flux environment. These characteristics result in a microstructure distinctly different from that produced by arc welding processes. The typical heat input ranges from 100 to 250 kJ/mm, significantly higher than SAW or GTAW processes.
The overlay layer microstructure exhibits a coarse columnar grain structure with grain sizes of 150–300 micrometers in the as-welded condition. The columnar grains grow perpendicular to the fusion boundary, with the grain boundaries parallel to the welding direction. This coarse structure is a direct consequence of the high heat input and slow solidification rate inherent to the ESW process.
| Parameter | Value / Range | Effect on Microstructure |
|---|---|---|
| Heat input | 100–250 kJ/mm | Coarse grain, slow solidification |
| Cooling rate | 0.5–2.0 °C/s | Coarse dendrite spacing |
| Grain size (as-welded) | 150–300 μm | Coarse columnar structure |
| Dendrite arm spacing | 80–150 μm | Wide interdendritic regions |
| Solidification rate | 0.3–1.0 mm/min | Low undercooling, planar growth |
The phase composition of the overlay layer consists primarily of austenite (γ) with minor delta ferrite (δ) content of 3–8% depending on the specific alloy composition and welding parameters. The delta ferrite forms in the interdendritic regions and plays a critical role in preventing hot cracking during solidification.
Phase Distribution and Segregation Analysis
Elemental segregation is a significant concern in ESW overlay layers due to the slow solidification rate allowing extensive solute redistribution. The study identifies chromium and molybdenum enrichment at grain boundaries, while carbon and nickel tend to segregate to interdendritic regions. This segregation pattern has direct implications for localized corrosion susceptibility.
The delta ferrite content is particularly important for crack resistance. The study finds that maintaining delta ferrite content in the range of 5–8% provides optimal hot crack resistance during welding while not significantly compromising corrosion resistance. Below 3% delta ferrite, hot cracking susceptibility increases markedly, while above 10% the corrosion resistance is degraded due to preferential dissolution of the ferrite phase.
The grain boundary segregation of chromium creates localized chromium-depleted zones that are susceptible to intergranular corrosion. The study recommends solution heat treatment at 1050–1100°C followed by water quenching to dissolve chromium carbides and restore chromium levels at grain boundaries.
Mechanical Properties and Corrosion Performance
The mechanical properties of the as-welded ESW overlay layer show tensile strength of 520–580 MPa, yield strength of 280–320 MPa, and elongation of 35–42%. These properties are adequate for most corrosion-resistant lining applications but are lower than those achievable with arc welding processes due to the coarse grain structure.
The intergranular corrosion resistance, tested by ASTM A262 Practice E (65% acetic acid-10% oxalic acid), shows moderate susceptibility in the as-welded condition. After solution heat treatment, the overlay layer passes the test without intergranular corrosion attack, demonstrating the effectiveness of post-weld treatment.
| Property | As-Welded | After Solution Treatment |
|---|---|---|
| Tensile strength (MPa) | 520–580 | 540–600 |
| Yield strength (MPa) | 280–320 | 300–340 |
| Elongation (%) | 35–42 | 38–45 |
| Delta ferrite (%) | 5–8 | 3–5 |
| IGC resistance | Marginal | Excellent |
| Grain size (μm) | 150–300 | 80–150 |
Engineering Implications and Study Insights
The study emphasizes that strip ESW overlay layers require careful post-weld treatment to achieve acceptable corrosion resistance. The coarse microstructure inherent to the process is not inherently detrimental for corrosion service if properly stabilized through heat treatment. The key engineering consideration is the selection of appropriate PWHT parameters based on the specific service environment.
For pressure vessel applications governed by ASME VIII Div.1 or NB/T 47002, the ESW overlay process is accepted with specific qualification requirements. The study notes that weld procedure qualification per ASME IX or NB/T 47014 must demonstrate adequate bond strength and corrosion resistance, with destructive testing including shear tests and intergranular corrosion testing being mandatory.
This literature provides essential metallurgical understanding for engineers specifying ESW overlay cladding in corrosion-resistant applications, highlighting the critical importance of post-weld heat treatment and delta ferrite control.