CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure Analysis of Austenitic Stainless Steel Strip Electrode Electroslag Cladding Layer
Literature Overview
This research by Yang Qin, Yang Ke, and Bao Yefeng from Hohai University, published in 2014 in the journal Electric Welding Machine, focuses on the microstructural characteristics of austenitic stainless steel cladding layers deposited using the strip electrode electroslag welding (ESW) process. Strip electrode ESW is a high-productivity cladding method widely used in the fabrication of large-scale bimetallic products such as pressure vessels, heat exchangers, and storage tanks where a corrosion-resistant lining is required on a carbon steel or low-alloy steel substrate. The study provides detailed metallographic analysis of the overlay microstructure, including grain morphology, phase distribution, and solidification patterns, which are essential for predicting the corrosion resistance and mechanical performance of the final cladding layer.
Process Characteristics of Strip Electrode ESW
Strip electrode ESW cladding operates by feeding a continuous stainless steel strip through a slag pool formed by an electric arc between the strip and the workpiece. The slag pool provides thermal insulation and chemical protection, resulting in a slow cooling rate that promotes the formation of coarse columnar grains. The process is characterized by high deposition rates (typically 3–8 kg/h), deep penetration, and good control over dilution. However, the slow cooling rate and high heat input also create challenges in terms of grain coarsening, potential intergranular corrosion susceptibility, and segregation of alloying elements.
| Parameter | Typical Value for Strip Electrode ESW |
|---|---|
| Current | 300–600 A |
| Voltage | 25–35 V |
| Travel Speed | 50–150 mm/min |
| Slag Ratio | 1.5–2.5 |
| Deposition Rate | 3–8 kg/h |
| Dilution Rate | 5–15% |
| Cooling Rate | Low (1–10 K/s) |
Microstructural Features
The microstructure of the austenitic stainless steel ESW cladding layer exhibits several distinctive features. The solidification pattern typically shows columnar grains growing from the fusion boundary upward, with a possible equiaxed grain region near the top surface. The grain size is significantly coarser than in conventional arc welding processes due to the low cooling rate, with grain widths often exceeding 200 micrometers. The phase composition is predominantly austenite with minor delta ferrite, the amount of which depends on the Cr/Ni ratio of the strip electrode material and the dilution from the base metal.
The study identifies that the delta ferrite content is a critical microstructural parameter. In austenitic stainless steels such as 304 or 316, the solidification sequence is typically austenite followed by delta ferrite, and the final ferrite content is governed by the Schaeffler diagram position. Excessive delta ferrite (above 10%) can lead to reduced corrosion resistance in chloride-containing environments, while insufficient ferrite (below 3%) increases susceptibility to hot cracking during welding. The optimal range is generally 3–8% delta ferrite for most applications.
Segregation and Inclusion Analysis
Elemental segregation is a notable feature of ESW cladding layers. Due to the slow solidification rate, there is significant time for diffusion, but the directional solidification pattern still results in macrosegregation of alloying elements. Chromium and molybdenum tend to segregate to the interdendritic regions, while nickel may concentrate in the austenite phase. This segregation can create local variations in corrosion resistance, with interdendritic regions being more susceptible to intergranular attack. Sulfide inclusions, particularly manganese sulfides, may form along grain boundaries and serve as initiation sites for corrosion.
The study also examines the heat-affected zone (HAZ) of the base metal, which is exposed to thermal cycles during the ESW process. For carbon steel substrates, the HAZ may exhibit coarse grain growth, martensite formation, or softening depending on the base metal composition and the thermal cycle severity. The dilution at the interface between the cladding layer and the base metal is another critical area, as the composition gradient affects both mechanical properties and corrosion behavior at the bond line.
Corrosion Resistance Implications
The microstructural features directly influence the corrosion resistance of the ESW cladding layer. Coarse grains provide fewer grain boundary barriers to corrosion propagation, potentially accelerating intergranular corrosion. The presence of delta ferrite, while beneficial for hot cracking resistance, can act as a preferential corrosion site in certain environments. The study suggests that solution heat treatment after cladding can dissolve carbides and homogenize the microstructure, significantly improving corrosion resistance. For applications requiring high corrosion resistance, a post-weld solution treatment at 1050–1100 °C followed by rapid water quenching is recommended to dissolve chromium carbides and restore full passivity.
Engineering Practice Considerations
In engineering practice, strip electrode ESW cladding is commonly used for large-diameter vessels and heat exchanger shells where productivity is critical. The key process controls include maintaining a stable slag pool, controlling the travel speed to achieve the desired dilution rate, and ensuring proper preheating to minimize thermal cracking of the base metal. Engineers should also consider the number of cladding layers and the thickness of each layer, as multiple thin layers generally produce a finer and more homogeneous microstructure than a single thick deposit.
| Cladding Layer Configuration | Microstructural Quality | Corrosion Resistance |
|---|---|---|
| Single thick layer (10–15 mm) | Coarse grains, high segregation | Lower |
| Two layers (5–8 mm each) | Moderate grains, reduced segregation | Moderate |
| Three or more layers (3–5 mm each) | Fine grains, low segregation | Higher |
Key Questions and Reflections
A significant question for engineers is how to balance productivity with microstructural quality. Strip electrode ESW offers excellent deposition rates, but the resulting coarse microstructure may not meet the corrosion resistance requirements of aggressive environments. One approach is to use a hybrid strategy: deposit a thick base layer by ESW for economy, followed by one or two thin finishing layers by a lower-heat-input process such as submerged arc welding or gas metal arc welding. This combination achieves both productivity and microstructural quality. Another consideration is the effect of welding sequence on residual stress and distortion, which becomes critical for large vessels where dimensional tolerance is tight.
Study Insights and Conclusions
This literature provides a comprehensive microstructural analysis of austenitic stainless steel ESW cladding layers, highlighting the interplay between process parameters, solidification behavior, and final properties. The key insight is that the slow cooling rate inherent to ESW produces a coarse-grained microstructure with potential segregation issues, which must be addressed through process optimization or post-weld heat treatment. Engineers should view ESW cladding not as a single-step operation but as part of an integrated process that includes material selection, process parameter control, and post-weld treatment. The microstructural understanding gained from this study enables more informed decisions about layer configuration, welding parameters, and heat treatment schedules, ultimately leading to higher-quality cladding layers with improved corrosion resistance and longer service life.