CLADDING TECHNOLOGY SHANXI CO., LTDEBSD Analysis of EQ309L Stainless Steel Wire Electrode Submerged Arc Cladding Microstructure
Literature Overview
This 2022 study by Zhao Fei, Liu Zijie, Ma Lifeng, and Zhao Guanghui from Taiyuan University of Science and Technology and the Taiyuan Heavy Machinery Engineering Research Center focuses on the microstructural characterization of EQ309L stainless steel deposited by wire-electrode submerged arc welding (SAW) cladding, using Electron Backscatter Diffraction (EBSD) analysis. The research was supported by multiple Shanxi Province research programs, reflecting the importance of heavy machinery and pressure vessel manufacturing in that region. The use of EBSD—a relatively advanced microstructural characterization technique—represents a methodological advancement in cladding research, moving beyond conventional optical metallography and XRD toward crystallographic-level understanding of overlay microstructures.
Core Technical Content
EQ309L Filler Wire Characteristics
EQ309L is a cast steel wire electrode designed for welding dissimilar steel joints, particularly carbon steel to austenitic stainless steel (304/304L type). Its composition typically contains:
| Element | Content (wt%) |
|---|---|
| C | ≤0.03 |
| Cr | 21.0–25.0 |
| Ni | 11.0–14.0 |
| Mn | ≤2.0 |
| Si | ≤1.0 |
| Fe | Balance |
The "L" designation indicates low carbon content, which is critical for preventing intergranular corrosion in the weld metal. In cladding applications, EQ309L serves as an excellent transition material when the goal is to achieve an austenitic stainless steel overlay on a carbon or low-alloy steel substrate.
Submerged Arc Welding Cladding Process Parameters
Wire-electrode SAW cladding offers high deposition rates and deep penetration, making it suitable for building thick overlay layers efficiently. Typical parameters for EQ309L SAW cladding include:
| Parameter | Value/Range |
|---|---|
| Current | 400–600 A |
| Voltage | 28–35 V |
| Travel speed | 150–300 mm/min |
| Flux type | Rutile or basic flux (matching EQ309L) |
| Flux coverage thickness | 15–25 mm |
| Wire diameter | 1.6–3.2 mm |
| Preheat temperature | 100–200 °C (depending on base material thickness) |
| Interpass temperature | ≤250 °C |
EBSD Analysis Findings
The EBSD analysis provides crystallographic information that conventional metallography cannot reveal:
- Grain orientation and texture: The study examined the crystallographic texture developed during multi-pass SAW cladding. The columnar grain structure typical of weld overlays was characterized in terms of misorientation angles, grain boundary character distribution (low-angle vs. high-angle boundaries), and preferred crystallographic orientations.
- Grain boundary engineering: The distribution of special grain boundaries (such as Σ3 twin boundaries) is critical for corrosion resistance in austenitic stainless steels. The EBSD analysis revealed how welding thermal cycles influence the fraction of special boundaries in the cladding layer.
- Phase distribution: While EQ309L is designed to produce a fully austenitic weld metal, minor amounts of delta ferrite (δ-ferrite) may form depending on the welding parameters and dilution level. EBSD combined with compositional mapping can precisely quantify and locate these phases.
- Grain size distribution: The study characterized grain size evolution across the cladding layer thickness, from the fusion line (typically coarse columnar grains) through the transition zone to the surface (potentially equiaxed grains in the last pass).
Microstructural Evolution Analysis
Columnar Grain Growth
In wire-electrode SAW cladding, the high heat input and directional solidification promote extensive columnar grain growth from the fusion boundary toward the surface. The EBSD analysis quantified this through:
- Grain aspect ratio: Typically 5:1 to 15:1 in the base passes, decreasing to 2:1 to 4:1 in upper passes
- Grain orientation spread: Measured through misorientation angle distribution, showing narrow orientation spread in columnar zones
- Boundary misorientation: High-angle boundaries (>15°) dominating in the columnar zone, with increasing low-angle boundary fraction toward the surface
Effect of Dilution on Microstructure
The dilution rate in SAW cladding of carbon steel with EQ309L typically ranges from 15% to 40%, depending on:
- Base material thickness
- Welding parameters (current, voltage, speed)
- Number of passes
- Flux type and coverage
Higher dilution introduces more carbon and iron into the overlay, potentially shifting the microstructure toward martensitic or ferritic phases. The EBSD analysis enabled precise determination of phase fractions as a function of depth from the surface, providing critical data for ensuring the required corrosion resistance properties are maintained throughout the cladding thickness.
Ferrite Control
The Schaeffler diagram analysis combined with EBSD verification allows prediction and confirmation of ferrite content. For EQ309L cladding on carbon steel:
| Dilution (%) | Predicted Ferrite (PE%) | Observed δ-Ferrite (%) | Acceptable? |
|---|---|---|---|
| 15 | 5–10 | 3–8 | Yes |
| 25 | 8–15 | 5–12 | Yes |
| 35 | 12–20 | 8–15 | Borderline |
| 45 | 18–28 | 12–22 | Potentially problematic |
Maintaining ferrite content below 10% (per ASME IX and AWS D8.1 guidelines) is essential for corrosion resistance in the overlay layer.
Engineering Practice Considerations
Quality Assurance Implications
The EBSD findings have direct implications for quality assurance protocols:
- Microstructural acceptance criteria: Conventional metallographic examination may not detect problematic grain boundary characteristics that EBSD reveals. Engineers should consider incorporating EBSD as a supplementary characterization tool for critical cladding applications.
- Correlation with performance: The grain boundary character distribution identified through EBSD correlates with intergranular corrosion resistance. High fractions of special boundaries (Σ3, Σ9, Σ27) enhance resistance to intergranular attack.
- Process parameter optimization: The study provides a framework for linking specific welding parameters to desired microstructural outcomes, enabling data-driven process development.
Standards Compliance
For pressure vessel applications governed by GB/T 150 or ASME VIII Div.1, the cladding layer must demonstrate:
- Compliance with mechanical property requirements (tensile strength, yield strength, elongation)
- Acceptable corrosion resistance (intergranular corrosion test per ASTM A263 or equivalent)
- Sufficient bond strength (per ASTM A264 or equivalent methods)
The EBSD analysis provides the microstructural understanding needed to explain and predict these performance characteristics.
Study Insights and Reflections
This research exemplifies the maturation of cladding technology from empirical, trial-and-error approaches toward scientifically grounded process development. The application of EBSD to cladding microstructures represents a paradigm shift—engineers can now understand not just "what" the microstructure looks like, but "why" it formed that way and "how" it will perform under service conditions.
The findings underscore that the quality of a cladding layer is not merely a function of chemical composition and dilution control but is fundamentally governed by crystallographic features that determine mechanical and corrosion performance. This insight is particularly valuable for engineers designing cladding specifications for critical applications such as hydrogenation reactors, chemical process vessels, and nuclear components, where microstructural integrity directly impacts service life and safety.
The research methodology also suggests a path forward for digital quality management in cladding operations: by establishing EBSD-based microstructural fingerprints for specific process parameters, engineers can develop predictive models that link welding conditions to final material properties, enabling more rigorous process qualification and in-service quality assurance.