Zhuojin Pipe Fitting Co., LtdIntergranular Corrosion Resistance of Austenitic Stainless Steel Overlay Deposited by Strip Electrode Electroslag Surfacing
Literature Overview
This study by Bao Yefeng et al. (2016, Welding Journal, Vol. 37, No. 6) compares the intergranular corrosion (IGC) resistance of austenitic stainless steel overlay welds produced by two strip electrode processes: strip electrode electroslag surfacing (SES) and strip electrode submerged arc surfacing (SSA). The research investigates the effect of welding speed on microstructure and IGC resistance, employing EPR (electrochemical potential reactivation) cyclic voltammetry and 10% oxalic acid electrolytic etching. The work is supported by the National Natural Science Foundation of China (51101050) and the Jiangsu Provincial Natural Science Foundation (BK20141156).
Core Findings and Process Comparison
Microstructural Comparison
| Process | Welding Speed | Delta Ferrite Content | Microstructure | Reactivation Rate | IGC Resistance |
|---|---|---|---|---|---|
| SES | 8 m/h (optimal) | 6.8-20.4% (increases with speed) | Austenite + delta ferrite | 3.22% | Best |
| SES | Higher speeds | ~20.4% | Austenite + delta ferrite | Higher | Reduced |
| SSA | Standard speed | 23.6% | Austenite + delta ferrite | Higher | Lower |
The key finding is that the SES process at a welding speed of 8 m/h produces the lowest reactivation rate of 3.22%, indicating the best intergranular corrosion resistance. Both processes produce a dual-phase microstructure of austenite plus delta ferrite, but the SES process achieves a lower delta ferrite content at optimal conditions compared to SSA.
Interpretation of Technical Points
Role of Delta Ferrite in Intergranular Corrosion
The delta ferrite phase plays a dual role in austenitic stainless steel overlay welds. On one hand, a moderate amount of delta ferrite (typically 3-10% by area fraction) is beneficial for preventing hot cracking and reducing distortion. On the other hand, delta ferrite is a preferential site for chromium carbide precipitation during sensitization, which depletes the adjacent austenite matrix of Cr and renders it susceptible to intergranular corrosion. The SES process at 8 m/h achieves a delta ferrite content of approximately 6.8%, which falls within the beneficial range while minimizing the sensitization risk.
Effect of Welding Speed on Solidification Microstructure
In the SES process, increasing the welding speed from the optimal 8 m/h leads to a higher delta ferrite content (up to 20.4%). This counterintuitive result can be explained by the thermal dynamics of the electroslag process: at higher speeds, the heat input per unit length decreases, but the slag pool geometry changes, affecting the solidification rate and the ferrite-austenite phase equilibrium. The faster cooling rate at higher speeds may promote delta ferrite retention by limiting the time available for the delta-to-austenite transformation during cooling.
EPR Method Validation
The electrochemical potential reactivation (EPR) method is a well-established technique for assessing IGC susceptibility. The reactivation rate (the ratio of reactivation current density to passivation current density) is inversely proportional to IGC resistance. The study validates the EPR results with 10% oxalic acid electrolytic etching, which provides a visual confirmation of intergranular attack. The consistency between the two methods strengthens the reliability of the findings.
Engineering Practice Implications
For engineers designing overlay welds for corrosive service environments, such as chemical processing equipment, marine applications, or nuclear components, this study provides several critical guidelines:
- Process selection: SES is preferred over SSA for austenitic stainless steel overlays where IGC resistance is critical, as SES can achieve lower delta ferrite content and better corrosion resistance at optimal parameters.
- Welding speed optimization: The optimal welding speed of 8 m/h for SES is a critical process window parameter. Deviations from this speed, either higher or lower, degrade IGC resistance.
- Post-weld heat treatment: A solution treatment at 1050-1100 degrees Celsius followed by rapid quenching can dissolve any precipitated chromium carbides and restore full IGC resistance, though this must be evaluated for its effect on residual stress and distortion.
- Quality verification: Both EPR and oxalic acid etching should be included in the qualification testing protocol for overlay welds in corrosive service.
Key Reflections and Study Insights
This study elegantly demonstrates that the welding process parameters have a profound and sometimes counterintuitive influence on the corrosion performance of overlay welds. The finding that increasing welding speed increases delta ferrite content in the SES process challenges the conventional assumption that faster cooling always reduces delta ferrite. This nuance is directly related to the unique thermal and metallurgical conditions of the electroslag process, where the slag pool acts as a heat reservoir and the solidification occurs under a thick slag layer with limited gas shielding. For engineers in the nuclear and chemical industries, where IGC resistance of stainless steel overlays is a safety-critical requirement, understanding these process-microstructure-property relationships is essential for developing robust welding procedures and avoiding premature corrosion failures in service.