Zhuojin Pipe Fitting Co., LtdEBSD Analysis of Strip Electrode Submerged Arc Overlay EQ309L Stainless Steel on Q345R
Literature Overview
This study by Zhao Fei, Liu Zijing, Ma Lifeng, and Zhao Guanghui from Taiyuan University of Science and Technology and the Taiyuan Heavy Machinery Engineering Research Center investigates the microstructural characteristics of EQ309L austenitic stainless steel strip electrode submerged arc welding (SAW) overlay on Q345R pressure vessel steel. Funded by multiple Shanxi Provincial research programs, the work is published in Rare Metal Materials and Engineering, 2022, Vol. 51, Issue 1, pages 98-105. The study employs Electron Backscatter Diffraction (EBSD) for detailed crystallographic and microstructural characterization of the overlay weld, heat-affected zone, and interface region.
Core Technical Findings
Microstructural Characterization by EBSD
The EBSD analysis reveals distinct microstructural features across the weld cross-section:
| Region | Crystal Structure | Grain Size | Morphology | Notable Features |
|---|---|---|---|---|
| Q345R base metal (rolled) | BCC | Variable | Banded (rolling direction) | Severe internal distortion |
| Q345R HAZ (overheated coarse grain) | BCC | 30-40 μm | Equiaxed | Moderate grain coarsening |
| Q345R HAZ (fine grain) | BCC | 10-20 μm | Equiaxed | Recrystallized |
| Transition zone (35-40 μm from fusion line) | BCC | Fine | Mixed | Diffusion-controlled transition |
| EQ309L overlay | FCC | Large | Columnar | Strong texture |
Interface Region Analysis
The interface between Q345R and EQ309L is characterized by a transition zone extending 35-40 μm from the fusion line into the EQ309L side. This zone retains a BCC structure, indicating that it represents a region of dilution and solidification influenced by the ferritic base metal. The transition zone is critical for understanding the mechanical behavior of the joint, as it represents the region of composition and structure gradient between the two dissimilar materials.
Texture and Crystallographic Orientation
The EQ309L overlay exhibits:
- Large columnar grains growing from the fusion boundary toward the weld surface.
- Strong texture characteristics indicating preferred crystallographic orientation development during solidification.
- Limited grain refinement due to the high heat input of strip electrode SAW and the low cooling rate associated with this process.
The Q345R base metal shows severe internal grain distortion with obvious banded grain structures aligned along the original rolling direction. In the overheated coarse grain zone, grain coarsening is moderate (30-40 μm average), which is acceptable for pressure vessel applications. The fine grain zone (10-20 μm) represents a beneficial recrystallization region.
Engineering Practice Implications
Implications for Pressure Vessel Overlay Welding
The findings have direct relevance to pressure vessel repair and upgrade operations:
- HAZ grain size control: The moderate grain coarsening (30-40 μm) in the Q345R HAZ is within acceptable limits for pressure vessel codes (ASME VIII Div. 1, GB/T 150), but monitoring is essential for repeated thermal cycling.
- Interface integrity: The 35-40 μm transition zone represents a potential weakness under thermal cycling or corrosive environments. Engineers should consider this zone when evaluating fatigue life and corrosion resistance.
- Overlay grain structure: The large columnar grains in the EQ309L overlay may be susceptible to transverse cracking under certain loading conditions. Post-weld heat treatment may be beneficial for grain refinement.
Process Optimization Recommendations
| Parameter | Current Practice | Recommended Improvement |
|---|---|---|
| Heat input | High (strip electrode) | Consider reducing to refine grains |
| Preheat temperature | Standard | May need adjustment for thick sections |
| Interpass temperature | Not specified | Control to limit HAZ coarsening |
| Post-weld treatment | Not specified | Consider stress relief or solution treatment |
Study Insights and Reflections
The application of EBSD to overlay weld microstructural analysis provides crystallographic information that is inaccessible through conventional metallographic examination. The identification of the BCC transition zone extending into the austenitic overlay is particularly significant, as it reveals a region of potential mechanical and corrosion performance compromise that would not be apparent from optical microscopy alone. For engineers involved in pressure vessel overlay welding, this study underscores the importance of understanding the full microstructural gradient across the joint, not merely the weld metal and base metal properties. The strong texture in the EQ309L overlay suggests that the mechanical properties may exhibit anisotropy, which should be considered in the design of overlay-repaired components subjected to directional loading. This work exemplifies how advanced characterization techniques can provide actionable insights for process optimization and quality assurance in critical industrial applications.