Zhuojin Pipe Fitting Co., LtdEffect of Three Stress Relief Methods on Microstructure and Properties of S32205 Duplex Stainless Steel FCAW Welds
Literature Overview
This paper, published in Transactions of Materials and Heat Treatment (Vol. 41, No. 8, 2020, pp. 182–189), investigates three distinct stress relief methods—chipping (air chisel), ultrasonic treatment, and post-weld heat treatment (PWHT)—applied to flux-cored MIG (FCAW) welds of S32205 duplex stainless steel. The authors employed optical microscopy, transmission electron microscopy (TEM), X-ray stress measurement, tensile testing, pitting corrosion and intergranular corrosion testing, and electrochemical workstation analysis. The study is supported by the National Key R&D Program of China (Project No. 2016YFC0401204), which signals its relevance to marine and offshore structural applications where duplex stainless steels are increasingly specified.
Core Technical Findings
The fundamental challenge addressed here is residual stress management in duplex stainless steel welds, where the austenite-ferrite phase balance is critical to both mechanical integrity and corrosion resistance. The authors' findings reveal nuanced differences among the three methods:
| Treatment Method | Residual Stress State (X/Y direction) | Phase Balance Change | Surface Grain Refinement | Hardness Change | Pitting Corrosion Rate |
|---|---|---|---|---|---|
| Chipping (Air Chisel) | Compressive stress in both X and Y | Increased austenite, decreased ferrite | Not reported | Moderate increase | Second lowest |
| Ultrasonic Treatment | Compressive stress in both X and Y | Increased austenite, decreased ferrite | 50–100 μm refined grain layer | Increased (surface) | Lowest |
| PWHT (250 °C × 24 h) | Tensile stress in both X and Y | No phase transformation observed | Not reported | Significant decrease | Highest among treated samples |
Interpretation of Stress State Outcomes
The most striking result is that PWHT at 250 °C × 24 h failed to eliminate tensile residual stresses; instead, it produced a stress redistribution that left tensile stresses in both the X and Y directions. This outcome is counterintuitive and deserves careful engineering interpretation. At 250 °C, the temperature is well below the typical stress relief range for austenitic or duplex stainless steels (usually 300–400 °C minimum), meaning the material has not reached a temperature sufficient for significant creep relaxation. Furthermore, the absence of phase transformation at this temperature means no α→γ or γ→α transition occurred to accommodate strain relief. The hardness reduction observed after PWHT likely stems from precipitation dissolution or softening of the δ-ferrite phase rather than true stress relief.
Chipping and ultrasonic treatment, by contrast, achieved compressive residual stresses through plastic deformation mechanisms. The chipping process introduces localized plastic deformation that redistributes stress from tensile to compressive states near the surface. Ultrasonic treatment operates through high-frequency mechanical vibration, which induces cyclic plastic deformation and dislocation rearrangement. The formation of a 50–100 μm grain refinement layer on the ultrasonically treated surface is a direct consequence of dynamic recrystallization driven by the intense shear strains imparted by the ultrasonic waves.
Phase Balance and Corrosion Performance
In duplex stainless steels, maintaining a balanced austenite-ferrite ratio (ideally 40:60 to 50:50) is paramount for corrosion resistance, particularly pitting and crevice corrosion resistance. The chipping and ultrasonic treatments both shifted the phase balance toward higher austenite content. This shift is attributed to the severe plastic deformation inducing dislocation structures that thermodynamically favor the austenite phase, potentially through strain-induced martensite transformation or preferential ferrite dissolution.
The pitting corrosion results confirm that all three treatments improved corrosion performance relative to the untreated baseline, but the ranking follows a clear hierarchy: ultrasonic treatment (best) > chipping > PWHT. The superior pitting resistance of ultrasonically treated welds can be attributed to the combined effects of compressive residual stress (which inhibits crack initiation and propagation in the passive film), refined grain structure (which enhances passive film formation kinetics), and the favorable phase balance shift.
Engineering Practice Implications
For offshore platforms, ship hulls, and marine structural components fabricated from S32205 duplex stainless steel, the selection of stress relief method has direct implications for service life and maintenance costs. The following engineering recommendations emerge from this study:
- Avoid low-temperature PWHT as a standalone stress relief method. A 250 °C × 24 h treatment is insufficient for effective stress relief in duplex stainless steels. If PWHT is required for other reasons (e.g., intergranular corrosion sensitivity mitigation), the temperature should be raised to at least 300–350 °C, with careful consideration of σ-phase precipitation kinetics above 450 °C.
- Prioritize mechanical stress relief methods for weld repair and field applications. Ultrasonic treatment and chipping are portable, require no thermal cycling, and produce beneficial compressive stresses. Ultrasonic treatment is particularly attractive for precision applications where surface integrity is critical.
- Combine methods for optimal results. A practical approach might involve initial PWHT at an appropriate temperature (350–400 °C) followed by ultrasonic peening to introduce compressive surface stresses. This hybrid approach leverages the benefits of both thermal and mechanical stress relief.
Key Questions and Reflections
Several questions arise from this study that merit further investigation:
- The long-term stability of compressive stresses introduced by chipping and ultrasonic treatment under cyclic loading and thermal cycling conditions in marine environments is not addressed. Fatigue life implications should be evaluated in follow-up studies.
- The study does not report chloride ion concentration or temperature in the pitting corrosion tests, limiting direct comparison with other published work. Standardization of test conditions (e.g., ASTM G48 or ISO 9650) would enhance comparability.
- The absence of σ-phase formation after PWHT at 250 °C is expected, but the long-term stability of the phase balance under service conditions above 300 °C remains a concern for duplex stainless steels.
Study Insights and Implications
This paper contributes valuable practical guidance for weld engineers working with duplex stainless steels in marine and offshore applications. The finding that mechanical stress relief methods outperform low-temperature PWHT in both residual stress elimination and corrosion resistance improvement is a significant departure from conventional practice, which often defaults to thermal stress relief as the primary approach. The study reinforces the principle that the selection of stress relief method should be driven by the specific performance requirements of the application rather than by tradition or convenience. For critical marine structures where pitting and crevice corrosion are the dominant degradation mechanisms, the adoption of ultrasonic treatment or chipping as post-weld stress relief methods represents a technically sound and cost-effective strategy that warrants wider implementation in industry.