Fusion Zone Characteristics of Ultra-Low Carbon Austenitic Stainless Steel Strip Electrode Electroslag Cladding Welds

Literature Overview

This 1999 study by Wang Jiachun, Zhang Huiwen, Sun Dunwu, Ma Xijin, and colleagues from the Harbin Welding Institute, in collaboration with Shanghai Power Plant Auxiliary Machinery Factory and Harbin Turbine Co., Ltd., investigates the fusion zone metallurgy of ultra-low carbon austenitic stainless steel cladding deposits produced by strip electrode electroslag welding (ESW). Published in the journal "Welding," this work addresses a critical engineering challenge in the fabrication of large-diameter pressure vessels and heat exchangers for power generation applications, where austenitic stainless steel cladding layers must be deposited on carbon or low-alloy steel substrates with excellent metallurgical bonding and minimal dilution.

Core Technical Content

Ultra-low carbon austenitic stainless steel, typically defined as having carbon content below 0.03 wt% (often specified as ≤0.02 wt% per ASTM A263 Type 316L or similar grades), is widely used for cladding in applications requiring resistance to intergranular corrosion in high-temperature, oxidizing, or reducing environments. The extremely low carbon content minimizes the formation of chromium carbide precipitates at grain boundaries during welding heat-affected zone (HAZ) exposure, thereby preserving the material's resistance to sensitization-induced intergranular corrosion.

Strip electrode electroslag welding is particularly suited for cladding applications involving thick sections (typically >25 mm) because of its high deposition rate, deep penetration, and ability to produce wide, single-pass welds. However, the high heat input and slow cooling rates associated with ESW create unique metallurgical challenges in the fusion zone, including excessive grain growth, delta ferrite formation, and potential sensitization of the base metal.

Typical Cladding Configuration and Process Parameters

Fusion Zone Metallurgical Characteristics

The fusion zone in ESW cladding of austenitic stainless steel on carbon steel substrates is characterized by a complex microstructure resulting from the dilution of the cladding alloy with base metal. The dilution ratio—the fraction of base metal in the weld metal—typically ranges from 15% to 40% for the first pass and decreases with subsequent passes. This dilution has profound effects on the weld metal chemistry and, consequently, on its microstructure and properties.

The primary microstructural features in the fusion zone include:

• Austenite matrix with delta ferrite: The dilution introduces additional manganese and silicon from the base metal, which stabilize austenite and can suppress delta ferrite formation. However, the high heat input of ESW can promote delta ferrite precipitation during solidification, particularly in the heat-affected zone of the base metal.
• Grain boundary precipitation: Despite the ultra-low carbon content, prolonged exposure to elevated temperatures during multi-pass welding can lead to the precipitation of chromium carbides and sigma phase at grain boundaries, particularly in the HAZ of the base metal and in the first-pass weld metal with higher dilution.
• Segregation and banding: The slow cooling rates in ESW can promote macrosegregation, leading to banding of carbide-forming elements and localized variations in composition that affect corrosion resistance.

Technical Points and Interpretation

The study systematically examined the microstructural evolution, chemical composition, and mechanical properties of the fusion zone using optical metallography, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and hardness profiling. The findings reveal that the fusion zone hardness typically ranges from 200 to 280 HV, with the highest values occurring at the interface between the base metal and the first cladding pass, where the dilution is greatest.

Dilution and Its Effects on Cladding Properties

The data clearly demonstrate that achieving acceptable corrosion resistance in the cladding layer requires a minimum of three to four passes, with the first pass serving primarily as a transition layer to reduce the dilution effect on subsequent passes. This has direct implications for cost estimation and quality planning in cladding fabrication.

Delta Ferrite Control

Delta ferrite content is a critical parameter in austenitic stainless steel welds, as it influences both ductility and corrosion resistance. Excessive delta ferrite (above 10–15%) can lead to reduced ductility and increased susceptibility to intergranular corrosion, while insufficient delta ferrite (below 3–5%) increases the risk of hot cracking. The study found that the strip electrode composition and welding parameters could be optimized to achieve delta ferrite content in the range of 5–10% in the first pass, which provides adequate resistance to hot cracking while maintaining acceptable corrosion resistance.

The ferrite number (FN) can be estimated using the DeLong equation or measured directly using a ferritecope. For the ultra-low carbon austenitic stainless steel strip electrode composition used in this study, the target FN range of 5–10 corresponds to a balance between crack resistance and corrosion resistance.

Standards and Quality Requirements

The cladding welds must satisfy the requirements of relevant standards, including GB/T 150 for pressure vessel design, NB/T 47014 for weld procedure qualification, and ASTM A263 for stainless steel clad plate specifications. Key quality requirements include:

• Bond strength: The cladding layer must be metallurgically bonded to the base metal with no interfacial defects. Bond strength is typically verified by macroetching, where the cladding layer and base metal are machined to expose the interface, and the interface is examined for any non-bonded areas.
• Corrosion resistance: The cladding layer must pass intergranular corrosion tests per ASTM A263 or GB/T 4334, with no evidence of intergranular attack after exposure to 65% boiling HNO3 for 48 hours.
• Mechanical properties: The cladding layer must meet minimum hardness and tensile strength requirements specified in the applicable material standard.
• Non-destructive testing: The cladding welds must be inspected by magnetic particle testing (MT) or dye penetrant testing (PT) for surface defects, and by ultrasonic testing (UT) for subsurface defects such as lack of fusion and porosity.

Integration with Engineering Practice

In the fabrication of large-diameter pressure vessels for power plants, such as hydrogenation reactors and heat exchangers, the ESW cladding process described in this study is routinely employed. The practical challenges include maintaining consistent welding parameters over long weld lengths, controlling interpass temperature to prevent sensitization, and ensuring adequate bond strength at the base metal/cladding interface.

A common practice is to apply a preheat of 150–200 °C to reduce residual stresses and minimize the risk of cold cracking in the base metal. The interpass temperature is maintained below 250 °C to prevent excessive grain growth and sensitization. Post-weld heat treatment (PWHT) is typically not performed on the cladding layer, as it can promote sigma phase formation; however, if PWHT is required for the base metal (e.g., for low-alloy steel vessels), it must be carefully controlled to avoid adverse effects on the cladding layer.

Common Defects and Countermeasures

Key Questions and Reflections

One question that arises from this study is the long-term performance of ESW-clad components in service. The fusion zone, with its complex microstructure and varying dilution levels, represents the most vulnerable region for corrosion initiation. In-service monitoring programs should include periodic inspection of the cladding layer, particularly at the fusion zone, to detect early signs of intergranular corrosion or stress corrosion cracking.

Another important consideration is the effect of welding sequence on the overall cladding quality. In large-diameter vessels, the welding sequence must be carefully planned to minimize residual stresses and distortions, which can affect the bond strength at the base metal/cladding interface. A well-designed welding sequence can reduce residual stresses by up to 30%, thereby improving the long-term reliability of the cladding.

Study Insights and Implications

This study provides valuable insights into the metallurgical behavior of ultra-low carbon austenitic stainless steel cladding deposits produced by ESW. The findings underscore the importance of controlling dilution through multi-pass welding, optimizing delta ferrite content for crack resistance, and maintaining strict interpass temperature control to prevent sensitization. For engineers involved in the fabrication of bimetallic pressure vessels, the study reinforces the principle that cladding quality is determined not only by the cladding alloy composition but also by the welding process parameters and sequence, which must be carefully controlled to achieve the desired metallurgical properties.

The practical significance of this work extends beyond the specific alloy system studied. The metallurgical principles governing fusion zone behavior—dilution control, delta ferrite management, and sensitization prevention—are universally applicable to all austenitic stainless steel cladding applications. Engineers should adopt these principles as foundational knowledge when developing welding procedures for cladding operations in any industrial setting.