Intergranular Corrosion Resistance of Austenitic Stainless Steel Overlay Deposited by Strip Electroslag Welding

Overview of the Study

Strip electroslag welding (ESW) is one of the most productive processes for depositing thick overlay layers, with deposition rates reaching 5–15 kg/h. However, the high thermal input and prolonged residence time of the molten pool in the slag pool raise concerns about grain coarsening, carbide precipitation, and sensitization—particularly for austenitic stainless steel overlay alloys where intergranular corrosion (IGC) resistance is a primary design requirement. This study investigates the IGC performance of austenitic stainless steel overlay layers deposited by strip ESW on carbon steel and low-alloy steel substrates, examining the influence of welding parameters, interpass temperature, and post-weld heat treatment on sensitization behavior and corrosion resistance.

Sensitization Mechanisms and Their Consequences

Carbide Precipitation and Chromium Depletion

The fundamental mechanism of sensitization in austenitic stainless steels involves the precipitation of chromium-rich carbides (primarily Cr23C6) at grain boundaries during thermal exposure in the range of 450–850 °C. This precipitation depletes the adjacent matrix of chromium below the critical threshold of approximately 12% Cr, rendering the region susceptible to intergranular attack in corrosive environments. In the context of strip ESW overlay, the prolonged exposure of the molten pool and subsequent solidification zone to sensitization temperatures creates a unique challenge that does not exist in lower-heat-input processes such as TIG or SAW.

The strip ESW process, with its heat input typically ranging from 5 to 20 kJ/mm, exposes the overlay material to sensitization temperatures for durations significantly longer than those encountered in arc-based processes. The slag pool acts as an insulating blanket, maintaining the top surface of the weld at elevated temperatures for extended periods as the arc moves away. This creates a wide sensitized zone along the grain boundaries, particularly in the upper portions of each pass where cooling rates are lowest.

Influence of Welding Parameters

The study identifies several process parameters that directly influence the degree of sensitization:

• Strip composition: Using low-carbon (≤0.03% C) or stabilized (Ti, Nb) strips such as 321, 347, or 316L reduces carbide precipitation susceptibility. The study confirms that 316L strips produce overlay layers with IGC resistance equivalent to wrought 316L, while 304 strips exhibit significant sensitization under comparable thermal conditions.
• Welding current and voltage: Higher currents and voltages increase heat input and pool residence time, exacerbating sensitization. The optimal range identified is 400–600 A and 30–38 V, which balances productivity with acceptable sensitization levels.
• Travel speed: Higher travel speeds reduce heat input and pool residence time, but excessively high speeds (>150 mm/min) can lead to incomplete strip melting and poor bond quality. The recommended range is 80–120 mm/min for typical 6 mm strips.
• Slag composition: The slag chemistry affects heat transfer and solidification rate. Slags with higher alumina content increase viscosity and reduce heat loss, extending the sensitization exposure. Slags with higher silica and lime content promote faster solidification and reduce sensitization.

Interpass Temperature Control

Interpass temperature is arguably the most critical parameter for controlling sensitization in strip ESW overlay. The study demonstrates that maintaining interpass temperature below 200 °C significantly reduces sensitization compared to conventional practice of allowing interpass temperatures to reach 300–400 °C. However, enforcing low interpass temperatures in strip ESW is challenging due to the high thermal mass of the slag pool and the large heat input per pass.

The study recommends a multi-layer approach with strategic cooling between layers:

For overlay builds exceeding 5 mm in thickness, water cooling of the base plate or active gas cooling of the slag pool surface is recommended to prevent interpass temperature from exceeding 250 °C. This approach has been validated in the fabrication of large-diameter hydrogenation reactors and heat exchanger tubesheets where thick overlay layers are required.

Post-Weld Heat Treatment and Its Limitations

Solution heat treatment (SHT) at 1050–1100 °C followed by water quenching is the most effective method for reversing sensitization and restoring IGC resistance. However, SHT introduces several complications for thick overlay layers on dissimilar substrates:

• Thermal stresses: The differential thermal expansion between the austenitic overlay (α ≈ 17.5 × 10⁻⁶ /°C) and the ferritic base metal (α ≈ 12.0 × 12.5 × 10⁻⁶ /°C) generates significant residual stresses during SHT. For overlay layers thicker than 6 mm, these stresses can exceed the yield strength of the overlay material, leading to cracking or delamination.
• Ferrite formation: SHT temperatures above 1100 °C can promote delta ferrite formation in the overlay, particularly if the dilution rate is significant. While delta ferrite is generally acceptable in duplex stainless steels, it is undesirable in fully austenitic overlays as it reduces ductility and can itself be susceptible to corrosion.
• Base metal softening: SHT at 1050–1100 °C can soften low-alloy steel base plates (e.g., 15CrMo, 12Cr1MoV), reducing their mechanical properties and potentially violating the design requirements of the pressure vessel.

The study concludes that SHT is necessary for overlay layers deposited with sensitization-susceptible strips (e.g., 304) but is not required when using stabilized or low-carbon strips (e.g., 316L, 321, 347) with controlled interpass temperatures. For applications where SHT is not feasible due to base metal constraints, the use of stabilized strips with strict interpass temperature control is the preferred approach.

Testing and Evaluation

Intergranular Corrosion Testing

The study employs ASTM A263 Practice E (acid solution test) and ASTM G48 Practice B (oxalic acid test) to evaluate IGC resistance of the overlay layers. Results show that overlay layers deposited with 316L strips at interpass temperatures below 200 °C pass the acid solution test (ASTM A263, Practice E, 100% acid) without evidence of intergranular attack, while overlay layers deposited with 304 strips fail the test after sensitizing heat treatment at 815 °C for 1 hour.

Bond Strength and Microstructural Examination

Bond strength testing in accordance with NB/T 47014 and ASME IX confirms that the overlay-to-base bond strength exceeds the tensile strength of the overlay material itself, indicating that failure occurs within the overlay rather than at the interface. Metallographic examination reveals that the overlay microstructure consists of equiaxed austenite grains with grain sizes of 20–50 μm, coarser than those produced by arc-based processes due to the lower cooling rates inherent to ESW. The fusion boundary shows a narrow transition zone with minimal dilution of the base metal into the overlay, confirming the effectiveness of the ESW process for producing well-defined overlay layers.

Engineering Practice and Standards Compliance

The findings have direct implications for the qualification and application of strip ESW overlay in pressure vessel fabrication. Under ASME VIII Div.1 and NB/T 47002, overlay welding procedures must be qualified in accordance with ASME IX or NB/T 47014, which requires demonstration of acceptable mechanical properties, bond strength, and corrosion resistance. The study provides the technical basis for procedure qualification, identifying the parameter ranges that produce overlay layers meeting the corrosion resistance requirements of ASTM A263 and the mechanical property requirements of ASTM A270 or equivalent specifications.

For hydrogenation reactors and heat exchangers operating under NACE MR0175/ISO 15156 conditions, the overlay layer must also demonstrate resistance to sulfide stress corrosion cracking (SSC). The study notes that while IGC resistance is the primary concern for strip ESW overlay, SSC resistance is also affected by the overlay microstructure and residual stress state. Low interpass temperatures and, where feasible, post-weld stress relief at 425–450 °C (below the sensitization range) are recommended to mitigate SSC susceptibility.

Key Questions and Reflections

A significant question raised by this study is the scalability of strip ESW overlay for very thick overlay layers (>10 mm). While the process is highly productive for layers up to 6–8 mm, building thicker overlays requires multiple layers with careful interpass temperature management, which reduces the productivity advantage. For applications requiring overlay thicknesses exceeding 10 mm, alternative approaches such as explosive cladding or roll-bonded cladding followed by weld overlay may be more practical.

Another consideration is the effect of strip quality on overlay performance. The study assumes uniform strip composition and geometry, but in practice, strip manufacturing variations (composition segregation, surface contamination, dimensional tolerances) can affect overlay quality. Incoming inspection of strips, including spectroscopic analysis and dimensional verification, is essential for maintaining overlay quality in production environments.

Summary

This study provides comprehensive guidance on achieving acceptable intergranular corrosion resistance in austenitic stainless steel overlay layers deposited by strip electroslag welding. The key conclusions are that low-carbon or stabilized strips (316L, 321, 347) must be used, interpass temperatures must be maintained below 200 °C, and post-weld heat treatment should be avoided unless absolutely necessary. For engineering practice, these findings support the use of strip ESW for thick overlay applications where productivity is paramount, provided that rigorous interpass temperature control and strip quality assurance are implemented. The study serves as a valuable reference for procedure qualification and process optimization in the fabrication of bimetal pressure vessels and heat exchangers requiring austenitic stainless steel overlay protection.