Study Note on Fusion Zone Characteristics of Ultra-Low Carbon Austenitic Stainless Steel Strip Electroslag Overlay Weld Joints

Literature Overview and Technical Context

This paper, published in 1999 in the journal "Welding" (焊接), was authored by Wang Jiachun, Zhang Huiwen, Sun Dunwu, and Ma Xijin from the Harbin Welding Research Institute, in collaboration with engineers from Shanghai Power Plant Auxiliary Machinery Factory and Harbin Turbine Co., Ltd. (Wu Jiazhen, Wang Guoqi, Wu Changjiang, Niu Lijun, and Wei Yonggang). The study investigates the fusion zone characteristics of ultra-low carbon austenitic stainless steel overlay welds produced by strip electroslag welding (ESW). This is a highly specialized topic with direct relevance to the fabrication of large-scale pressure vessels, heat exchangers, and turbine components used in power generation and petrochemical industries, where corrosion resistance in the fusion zone is a critical design requirement.

The strip ESW overlay process is particularly suited for producing thick, uniform overlay layers on large components. The process uses a continuous strip of stainless steel as the electrode, fed through a slag pool, with the molten metal deposited in successive passes. The high heat input and slow cooling rate characteristic of ESW create unique metallurgical conditions in the fusion zone that differ significantly from those produced by lower-energy processes such as GTAW or GMAW.

Core Technical Content and Fusion Zone Metallurgy

Ultra-low carbon austenitic stainless steels (typically 304L, 316L, or equivalent grades with carbon content below 0.03%) are selected for overlay applications where both corrosion resistance and resistance to sensitization-induced intergranular corrosion are required. The "L" designation denotes ultra-low carbon content, which suppresses the formation of chromium carbide precipitates (Cr23C6) at grain boundaries during welding and service exposure in the sensitization temperature range (450–850°C).

However, even in ultra-low carbon grades, the fusion zone represents a critical area of concern. The fusion zone microstructure is a mixture of base metal and deposited material, with the exact composition depending on the dilution ratio. In strip ESW overlay, the dilution rate is typically 10–25% for the first pass, decreasing to 5–10% in subsequent passes. The fusion zone composition, microstructure, and properties are therefore intermediate between the base metal and the overlay material.

The following table summarizes the key metallurgical features of the fusion zone:

The authors conducted detailed metallographic examination, microhardness mapping, and intergranular corrosion testing (ASTM A262 Practice E) on the fusion zone. The results revealed that the fusion zone exhibited a columnar dendritic microstructure with grain sizes significantly larger than those in the overlay deposit (typically 0.5–2.0 mm grain width versus 0.1–0.3 mm in the overlay). The microhardness in the fusion zone ranged from 220 to 280 HV, intermediate between the base carbon steel (180–200 HV) and the overlay deposit (200–250 HV for 304L-grade material).

A critical finding was the observation of chromium-depleted zones along grain boundaries in the fusion zone, even in ultra-low carbon grades. This phenomenon, known as sensitization, occurred due to the slow cooling rate of ESW, which allowed sufficient time for carbon to diffuse to grain boundaries and precipitate as chromium carbides. Although the carbon content was below 0.03%, the extended time in the sensitization temperature range during ESW cooling was sufficient to cause measurable chromium depletion, resulting in localized intergranular corrosion susceptibility.

Process Analysis and Parameter Optimization

The strip ESW process parameters directly influence the fusion zone metallurgy. The following table presents the typical process parameters used in the study:

The authors identified several key process optimization strategies to minimize sensitization in the fusion zone:

1. Increase travel speed to reduce heat input and cooling time in the sensitization range. However, excessive travel speed may reduce the quality of slag-metal interaction and increase porosity.
2. Reduce welding current to decrease dilution and limit the volume of base metal melted into the fusion zone.
3. Use a multi-pass strategy with the first pass applied at reduced heat input to establish a transition layer, followed by subsequent passes at optimized parameters.
4. Apply a post-weld heat treatment (solution treatment at 1050–1100°C followed by rapid quenching) to dissolve any chromium carbide precipitates and restore chromium homogeneity at grain boundaries.

Defect Analysis and Quality Control

The fusion zone of strip ESW overlay welds is susceptible to several defects, each with specific metallurgical origins:

The study emphasized that the combination of slow cooling and ultra-low carbon content does not guarantee immunity from sensitization. The time-temperature exposure in the sensitization range (450–850°C) during ESW cooling can be several minutes, which is sufficient for chromium carbide precipitation even in 304L-grade material. This is in contrast to processes such as GTAW or GMAW, where cooling rates are typically 10–100°C/s, and the time in the sensitization range is only seconds.

Integration with Engineering Practice

Strip ESW overlay is predominantly used in the fabrication of large-scale equipment such as:

• Hydrogenation reactor pressure vessels (clad with 304L or 316L on carbon steel shells)
• Large heat exchanger channels and tubesheets
• Power plant turbine casing components
• Petrochemical reactor internals

A representative application case involved the overlay of a 316L stainless steel layer on a 16 mm thick 16MnR carbon steel shell for a hydrogenation reactor operating at 350°C and 25 MPa. The overlay was 12 mm thick, applied in four passes using 30 mm wide strip ESW. The first pass was applied at reduced current (800 A) and slower travel speed (250 mm/min) to establish a transition layer with controlled dilution (approximately 25%). Subsequent passes were applied at 1000 A and 350 mm/min to achieve dilution below 10%. Post-weld solution heat treatment at 1050°C for 2 hours followed by water quenching was applied to dissolve any chromium carbide precipitates. Intergranular corrosion testing (ASTM A262 Practice E, 65% boiling H2SO4, 1 hour) confirmed that the fusion zone met the acceptance criteria with less than 1% mass loss.

The economic consideration of post-weld heat treatment is significant. For large components, solution heat treatment requires heating the entire component to 1050°C, which may be impractical due to size constraints, distortion concerns, or the presence of embedded components. In such cases, the process parameters must be optimized to achieve a sensitization-free fusion zone without post-weld treatment. This requires careful control of heat input and cooling rate, which is challenging in ESW due to the inherently high heat input of the process.

Key Questions and Reflections

The study raises an important question about the adequacy of ultra-low carbon grades for ESW overlay applications. While 304L and 316L are the standard choices for corrosion-resistant overlay, the slow cooling rates of ESW may require even lower carbon content or the use of stabilized grades (321 with Ti, 347 with Nb) to prevent sensitization. The authors noted that stabilized grades (321L, 347L) showed improved resistance to sensitization in the fusion zone but at the cost of reduced weldability and increased susceptibility to hot cracking.

Another reflection concerns the long-term performance of the fusion zone under service conditions. The intergranular corrosion resistance measured at room temperature may not be representative of performance at elevated temperatures (300–400°C) where the component operates. At elevated temperatures, the diffusion rates of carbon and chromium are higher, and the equilibrium solubility of carbon in austenite is greater. This means that the sensitization behavior observed at room temperature may be different from that experienced during service. Long-term aging studies at service temperatures are essential to validate the durability of the fusion zone.

Study Insights and Implications

This study provides valuable insights into the metallurgical challenges of strip ESW overlay welding, particularly regarding fusion zone sensitization in ultra-low carbon austenitic stainless steels. The finding that even ultra-low carbon grades are susceptible to sensitization under ESW cooling conditions is a critical design consideration that must be addressed through process optimization and/or post-weld heat treatment. Engineers specifying ESW overlay for critical pressure vessel applications should ensure that the welding procedure specification includes provisions for sensitization control, including appropriate process parameters, dilution management, and post-weld heat treatment where feasible. The collaborative approach between welding research institutions and equipment manufacturers demonstrated in this study is a model for effective technology transfer in the welding and fabrication industry.