EBSD Analysis of Strip Electrode Submerged Arc Surfacing EQ309L Stainless Steel on Q345R

Literature Overview

This paper 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 was published in the journal Rare Metal Materials and Engineering (稀有金属材料与工程), Volume 51, Issue 1, 2022, pages 98-105. The study employs electron backscatter diffraction (EBSD) to characterize the microstructural evolution across the interface between Q345R low-alloy steel substrate and EQ309L austenitic stainless steel deposited by strip electrode submerged arc welding (SAW). The work provides detailed crystallographic orientation mapping of the base metal, heat-affected zone (HAZ), transition zone, and surfacing layer, offering insights into grain structure, texture, and phase transformation behavior that are critical for understanding the metallurgical compatibility and service performance of the cladding.

Core Technical Analysis

Strip electrode submerged arc welding is a highly productive method for depositing thick surfacing layers, particularly for large components such as pressure vessel heads, pipe spools, and heavy plate assemblies. The use of EQ309L (an austenitic stainless steel equivalent to ER309L) on Q345R (a low-alloy structural steel) creates a dissimilar metal joint with significant differences in thermal expansion, magnetic properties, and microstructural evolution. The EBSD analysis reveals several important microstructural features:

Q345R Base Metal Side:

• The base metal exhibits severe internal distortion with pronounced banding of grains aligned along the original rolling direction.
• In the overheated coarse grain zone (CGHAZ), the average grain size is 30-40 μm, indicating moderate grain coarsening.
• In the fine grain zone (FGHAZ), the average grain size is 10-20 μm, reflecting the recrystallization and growth behavior under the thermal cycle.

Transition Zone:

• Extending 35-40 μm from the fusion line into the surfacing layer, the transition zone exhibits a body-centered cubic (BCC) structure.
• This zone represents a region of mixed metallurgy where the austenitic surfacing alloy and the ferritic-pearlitic base metal interact during solidification.
• The BCC structure indicates that this zone is dominated by ferritic phases, suggesting that the base metal composition has a significant influence on the solidification behavior near the fusion line.

EQ309L Surfacing Layer:

• The surfacing layer exhibits coarse columnar grains with a pronounced texture.
• The columnar grain structure grows perpendicular to the deposition direction, which is typical of rapid solidification in a highly directional thermal field.
• The texture characteristics indicate a strong crystallographic preferred orientation, which may affect the anisotropy of mechanical properties and corrosion resistance.

Implications for Engineering Practice

The EBSD findings have several important implications for the engineering application of strip electrode SAW surfacing of austenitic stainless steel on low-alloy steel:

Mechanical Compatibility:

The transition zone with BCC structure represents a region of metallurgical incompatibility between the austenitic surfacing alloy and the ferritic base metal. This zone may be susceptible to cracking under thermal cycling or mechanical loading because of the mismatch in thermal expansion coefficients and ductility between the BCC transition zone and the FCC surfacing layer. Engineers should be aware that the transition zone, though narrow, may be the critical location for crack initiation in service.

Corrosion Behavior:

The coarse columnar grain structure of the EQ309L surfacing layer may promote intergranular corrosion susceptibility, particularly in environments where chromium depletion at grain boundaries is a concern. The strong texture may also lead to anisotropic corrosion behavior, with some grain orientations being more susceptible to attack than others. For applications in aggressive environments such as sour gas service or chloride-containing solutions, the microstructural characteristics identified by EBSD should be considered in the selection of surfacing alloy and process parameters.

Welding Process Optimization:

The findings suggest that process parameters should be optimized to minimize the width of the transition zone and to promote a more uniform microstructure in the surfacing layer. Techniques such as pulsed current SAW, which can control the solidification rate and promote equiaxed grain formation, may be beneficial. Additionally, the use of a preheating and interpass temperature control strategy can help reduce the thermal gradient and minimize grain coarsening in the HAZ.

Key Questions and Reflections

A significant question raised by this study is whether the BCC transition zone is inherent to the EQ309L on Q345R combination or whether it can be mitigated through process modification. The formation of a BCC transition zone suggests that the local composition near the fusion line is influenced by dilution from the base metal, shifting the solidification behavior away from the fully austenitic structure expected for pure EQ309L. Understanding the extent of dilution and its effect on phase formation is critical for predicting the long-term performance of the cladding.

Another consideration is the effect of multiple surfacing passes on the microstructure evolution. The EBSD analysis likely represents a single-pass or limited-pass condition, but in practice, thick cladding layers require multiple passes. Each subsequent pass subjects the previous pass to a heat treatment effect, which can recrystallize and coarsen the microstructure. The final microstructure of a multilayer cladding may differ significantly from the as-deposited single-pass structure, and the EBSD findings should be validated for multilayer conditions.

Study Insights and Implications

This study demonstrates the power of EBSD as a tool for understanding the complex microstructural evolution in dissimilar metal surfacing welds. The detailed crystallographic information obtained through EBSD provides insights that are not accessible through conventional optical microscopy or XRD analysis. For engineers involved in the design and qualification of stainless steel cladding on carbon and low-alloy steel components, the key takeaways are:

• The transition zone between the austenitic surfacing and ferritic base metal is a critical region that requires careful attention in design and inspection.
• The coarse columnar grain structure of the surfacing layer may have implications for corrosion resistance and mechanical anisotropy that should be considered in service life assessment.
• EBSD analysis should be incorporated into the qualification program for critical surfacing applications to provide a deeper understanding of the microstructural evolution and its implications for performance.
• Process parameter optimization, including the use of advanced welding techniques such as pulsed SAW, may help achieve more favorable microstructural characteristics in the surfacing layer.

The integration of advanced characterization techniques such as EBSD into routine engineering practice represents a significant advancement in our ability to understand and control the metallurgical quality of surfacing welds, ultimately leading to more reliable and longer-lasting surface protection solutions for critical infrastructure components.