EBSD Analysis of Microstructure in Strip Submerged Arc Cladding of EQ309L Stainless Steel

Literature Overview

This study employs Electron Backscatter Diffraction (EBSD) to characterize the microstructure of EQ309L stainless steel cladding layers produced by strip submerged arc welding (strip SAW). EQ309L is a hypermanganese stainless steel welding consumable (AWS A5.9 ER309L equivalent) designed for welding dissimilar metal joints between austenitic stainless steel and carbon or low-alloy steel. Strip SAW is a high-productivity cladding process that uses a continuous strip electrode rather than a wire electrode, enabling deposit rates of 5–10 kg/h compared to 1–3 kg/h for conventional wire SAW. The EBSD analysis provides detailed crystallographic orientation mapping, grain boundary character distribution, and texture analysis that conventional metallography cannot achieve, offering deeper insight into the relationship between welding parameters and microstructural evolution.

EBSD Methodology and Key Parameters

EBSD analysis was performed on cross-sections of the cladding layer, with samples prepared by standard metallographic techniques including grinding, polishing, and chemical etching. The EBSD data was collected using a field emission scanning electron microscope (FE-SEM) equipped with an EBSD detector, with an acceleration voltage of 20 kV, a working distance of 15–20 mm, and a step size of 0.2–0.5 µm for high-resolution mapping.

The EBSD analysis revealed several key microstructural features that are critical for understanding the cladding performance:

• Grain morphology: The cladding layer exhibits a columnar grain structure extending from the fusion boundary to the surface, with grain widths of 50–150 µm. The columnar grains are oriented preferentially along the thermal gradient direction, consistent with competitive grain growth during directional solidification.
• Grain boundary character: Approximately 65–70% of the grain boundaries are high-angle boundaries (misorientation greater than 15°), with the remaining 30–35% being low-angle boundaries. The high-angle boundary fraction is higher than typical for wrought austenitic stainless steels (which typically have 40–50% high-angle boundaries), indicating a more equiaxed-like grain structure that is beneficial for corrosion resistance.
• Texture analysis: The cladding layer exhibits a strong {111} fiber texture, with the crystallographic {111} planes preferentially aligned with the welding direction. This texture is characteristic of austenitic stainless steel welds and is associated with the low stacking fault energy of austenite, which promotes planar slip and directional solidification.

Microstructural Evolution and Welding Parameter Effects

The EBSD analysis reveals that the microstructure of the EQ309L strip SAW cladding layer is strongly influenced by welding parameters, particularly heat input and travel speed. Higher heat input (above 25 kJ/mm) promotes grain coarsening and reduces the fraction of columnar grains, leading to a more equiaxed grain structure near the fusion boundary. Conversely, lower heat input (below 15 kJ/mm) results in finer columnar grains but may increase the risk of hot cracking due to rapid solidification.

The strip SAW process offers unique advantages for cladding applications. The continuous strip electrode provides a more stable arc and more uniform heat input compared to wire SAW, resulting in a more homogeneous microstructure across the cladding layer. EBSD analysis confirms this, showing lower grain size variation and more consistent texture intensity across the cladding cross-section compared to conventional wire SAW cladding.

A particularly interesting finding from the EBSD analysis is the presence of a thin delta-ferrite network at the grain boundaries in the cladding layer. Although EQ309L is designed to produce an austenitic weld metal with minimal delta-ferrite (typically less than 5%), the EBSD analysis reveals that delta-ferrite content can reach 8–12% in strip SAW deposits, particularly near the fusion boundary where dilution with the carbon steel substrate is highest. This delta-ferrite is not visible in conventional metallographic examination but is clearly identified through crystallographic orientation analysis.

The delta-ferrite network has significant implications for corrosion resistance. While delta-ferrite itself is not inherently susceptible to pitting or intergranular corrosion, it can act as a pathway for crack propagation in the presence of chloride stress corrosion cracking (Cl-SCC). Furthermore, the delta-ferrite network can promote sensitization during post-weld heat treatment or service exposure at elevated temperatures, leading to chromium depletion at the ferrite-austenite interfaces.

Quality Control and Engineering Implications

The EBSD findings have direct implications for the quality control of EQ309L strip SAW cladding operations. Conventional non-destructive testing (NDT) methods such as radiographic testing (RT) and ultrasonic testing (UT) cannot detect delta-ferrite networks or grain boundary character variations. Therefore, EBSD or alternative crystallographic analysis techniques (such as neutron diffraction or X-ray diffraction) should be considered for critical applications where microstructural quality is paramount.

For pressure vessel applications involving austenitic stainless steel cladding, the following quality control measures are recommended based on the EBSD findings:

• Welding parameter control: Maintain heat input within the range of 15–25 kJ/mm to ensure adequate grain refinement without excessive grain coarsening.
• Dilution monitoring: Control the dilution rate between the EQ309L deposit and the carbon steel substrate to below 25% to minimize delta-ferrite formation. This can be achieved through careful control of travel speed, electrode stick-out, and preheating temperature.
• Post-weld inspection: In addition to conventional NDT, consider metallographic examination with EBSD or XRD for critical applications to verify grain boundary character and delta-ferrite content.
• Post-weld heat treatment: Solution heat treatment at 1050°C followed by rapid cooling can dissolve delta-ferrite and restore the austenitic microstructure, but must be carefully controlled to avoid excessive grain growth or sensitization.

Defect Analysis and Countermeasures

The EBSD analysis also provides insight into the formation and prevention of common defects in EQ309L strip SAW cladding:

Summary and Outlook

The EBSD analysis of EQ309L strip SAW cladding layers provides unprecedented insight into the microstructural evolution during the cladding process, revealing features that are invisible to conventional metallographic techniques. The key findings—columnar grain structure, high-angle boundary dominance, {111} fiber texture, and delta-ferrite network formation—have direct implications for the corrosion resistance, mechanical properties, and long-term reliability of the cladding layer. For engineering practice, the EBSD findings emphasize the importance of welding parameter control, dilution management, and post-weld heat treatment in achieving optimal cladding quality. The strip SAW process offers significant productivity advantages for cladding applications, but the EBSD analysis also highlights the need for careful process control to ensure that the microstructural quality meets the requirements of critical applications. Future work should explore the relationship between EBSD-identified microstructural features and long-term corrosion performance under actual service conditions, as well as the potential of advanced EBSD techniques for in-service inspection and remaining life assessment of cladded components.