Intergranular Corrosion Resistance of Austenitic Stainless Steel Overlay Deposited by Strip Electrode Electroslag Welding

Literature Overview

This study, published in the Welding Journal of China (Hanjie Xuebao) in 2016 (Volume 37, Issue 6, pages 65-68), compares two strip electrode welding processes — strip electrode electroslag welding (SE-ESW) and strip electrode submerged arc welding (SE-SAW) — for overlaying Cr-Ni austenitic stainless steel. The authors from Hohai University and Changzhou Baoling Heavy Machinery Co., Ltd. investigated how welding speed and process selection affect the microstructure and intergranular corrosion (IGC) resistance of the overlay deposits. The work was funded by the National Natural Science Foundation of China (51101050) and the Jiangsu Provincial Natural Science Foundation (BK20141156). Keywords include strip electrode electroslag welding, overlay metal, intergranular corrosion, and EPR cyclic voltammetry curve, indicating a combined metallurgical and electrochemical investigation approach.

Core Technical Findings

Microstructural Comparison

Both SE-ESW and SE-SAW produce overlay deposits with a microstructure consisting of austenite plus delta ferrite. However, the delta ferrite content and its sensitivity to welding speed differ significantly between the two processes.

The delta ferrite in SE-ESW deposits increases from 6.8 percent to 20.4 percent as welding speed rises, reflecting the thermal dynamics of the electroslag process where the slag pool maintains a stable, high-temperature environment that promotes ferrite dissolution at lower speeds and ferrite retention at higher speeds. In contrast, SE-SAW deposits exhibit a consistently higher delta ferrite content of up to 23.6 percent, likely due to the different heat input distribution and cooling rate characteristics of the submerged arc process.

Electrochemical Evaluation: EPR Method

The study employs the Electrochemical Potentiokinetic Recrystallization (EPR) method to quantitatively assess intergranular corrosion susceptibility. The recrystallization rate (RR) serves as the key metric, where a lower RR indicates better IGC resistance.

The optimal IGC resistance is achieved at a welding speed of 8 meters per hour using SE-ESW, with a recrystallization rate of only 3.22 percent. Deviation from this optimal speed in either direction — slower or faster — increases the recrystallization rate and degrades IGC resistance. This suggests that 8 m/h represents a thermal balance point where the cooling rate is sufficient to prevent excessive chromium carbide precipitation at grain boundaries while maintaining adequate heat input for proper austenite formation.

Correlation with Oxalic Acid Electrolytic Etching

The 10 percent oxalic acid electrolytic etching test results corroborate the EPR findings, providing a visual confirmation of the electrochemical data. The deposits with lower recrystallization rates exhibit less intergranular attack under the etching conditions, confirming the consistency of the two evaluation methods.

Process Analysis and Engineering Implications

The superior IGC performance of SE-ESW at 8 m/h can be attributed to the unique thermal characteristics of the electroslag process. The slag pool in ESW acts as a thermal buffer, providing a more uniform and controllable heat input compared to SAW. This results in a more homogeneous microstructure with reduced chromium carbide precipitation at grain boundaries, which is the primary mechanism of sensitization and subsequent intergranular corrosion.

For engineers specifying overlay welding procedures for corrosion-resistant applications:

1. Process selection: SE-ESW is preferred over SE-SAW when intergranular corrosion resistance is a critical requirement, as the electroslag process offers better control over delta ferrite content and grain boundary chemistry.
2. Welding speed optimization: The welding speed must be carefully controlled within the optimal window. For SE-ESW overlay of austenitic stainless steel, 8 m/h represents the best balance between heat input and cooling rate for maximizing IGC resistance.
3. Dual-method verification: Combining EPR electrochemical testing with oxalic acid etching provides a robust qualification approach for overlay deposits in service-critical applications.
4. Delta ferrite management: While delta ferrite is generally beneficial for cracking resistance, excessive amounts (above 20 percent) can promote sensitization. The SE-ESW process at 8 m/h achieves a favorable balance.

Study Insights and Reflections

This paper demonstrates the value of combining metallurgical characterization with electrochemical testing to establish a comprehensive understanding of overlay performance. The finding that welding speed has a pronounced effect on IGC resistance — with an optimal point rather than a simple monotonic trend — is particularly important for procedure qualification. Engineers must recognize that overlay welding is not merely about achieving the correct composition but also about controlling the thermal history that determines the microstructural state. The SE-ESW process, with its inherent thermal stability from the slag pool, appears to offer superior control for this purpose compared to SE-SAW. However, the practical applicability of SE-ESW may be limited by equipment availability and the requirement for a horizontal or near-horizontal welding position, which constrains its use in certain field applications.