CLADDING TECHNOLOGY SHANXI CO., LTDLaser Vertical Welding of S32101 Stainless Steel in Air and Underwater Environments
Literature Overview
This study compares the weldability and weld joint properties of S32101 (321) austenitic stainless steel when subjected to laser vertical welding under two distinct environmental conditions: atmospheric (air) and underwater. S32101, stabilized with titanium (Ti), is a widely used austenitic stainless steel in heat exchangers, boiler tubes, and chemical processing equipment where resistance to intergranular corrosion at elevated temperatures is required. The research addresses a practical challenge in offshore and subsea construction, where underwater welding is often unavoidable, and provides comparative data on weld geometry, microstructure, mechanical properties, and corrosion resistance between the two environments.
Process Parameters and Weld Geometry Comparison
Laser welding was performed using a fiber laser with a power of 2-4 kW, a focal spot diameter of 0.2-0.3 mm, and a welding speed of 0.5-2.0 m/min. The vertical welding position (F-position or G-position) presents unique challenges due to gravity-induced weld pool instability. The study found that underwater welding required higher laser power (typically 15-25% more than air welding) to achieve equivalent penetration depth, owing to the heat absorption and scattering effects of the water medium.
| Parameter | Air Welding | Underwater Welding |
|---|---|---|
| Laser Power (kW) | 2.5-3.0 | 3.0-3.5 |
| Welding Speed (m/min) | 1.0-1.5 | 0.8-1.2 |
| Penetration Depth (mm) | 3.5-4.5 | 3.0-4.0 |
| Weld Width (mm) | 2.0-2.5 | 2.5-3.0 |
| Aspect Ratio | 1.5-1.8 | 1.2-1.5 |
| Shielding Gas | N₂ or Ar (external) | None (water acts as shield) |
The weld geometry in air conditions is characterized by a narrower, deeper weld with a higher aspect ratio, while underwater welding produces a slightly wider and shallower weld profile. This difference is attributed to the thermal conduction and convection effects of the surrounding water, which acts as both a heat sink and a shielding medium.
Microstructural Analysis
Metallographic examination reveals that both air-welded and underwater-welded joints exhibit a fully austenitic microstructure with some grain boundary Ti(C,N) precipitates. However, the grain morphology differs significantly between the two environments. In air welding, the weld metal shows elongated columnar grains growing from the fusion boundary toward the weld centerline, with grain sizes in the range of 20-50 μm. In underwater welding, the columnar grains are shorter and more equiaxed, with grain sizes of 15-35 μm, reflecting the higher cooling rate imposed by the water medium.
The HAZ in both conditions shows a sensitized zone where Ti(C,N) precipitation at grain boundaries can deplete the local chromium content, potentially reducing intergranular corrosion resistance. The study found that the sensitized zone width is slightly larger in underwater welding (approximately 0.3-0.5 mm) compared to air welding (0.2-0.4 mm), likely due to the different thermal cycle characteristics.
| Zone | Air Welding Grain Size (μm) | Underwater Welding Grain Size (μm) | Ti(C,N) Precipitation Level |
|---|---|---|---|
| Weld Center | 25-45 | 18-35 | Low |
| Weld Edge | 30-50 | 20-40 | Low-Moderate |
| Narrow HAZ | 40-70 | 30-55 | Moderate-High |
| Wide HAZ | 60-100 | 50-80 | Low |
Mechanical Property Comparison
Tensile testing of transverse specimens shows that both conditions produce weld joints with UTS values in the range of 520-580 MPa, meeting the minimum requirement of 515 MPa specified in ASTM A240 for S32101 plate. The elongation values are 30-35% for air welding and 28-33% for underwater welding, indicating slightly reduced ductility in the underwater condition. Hardness testing reveals values of 180-220 HV30 for both conditions, with no significant difference in the overall hardness profile across the joint.
Impact testing (Charpy V-notch at room temperature) shows that both conditions produce acceptable impact energy values exceeding 100 J, well above the minimum requirements for most pressure vessel applications. However, the study notes that the underwater welds exhibit slightly more scatter in impact energy values, which may be related to the less consistent thermal cycle imposed by the water medium.
Corrosion Resistance Assessment
The critical comparison lies in corrosion resistance, particularly intergranular corrosion (IGC). The ASTM A262 Practice E test (acid solution exposure) reveals that air-welded joints show no measurable intergranular corrosion after 65 hours of exposure, while underwater-welded joints exhibit slight intergranular attack in the narrow HAZ after 48 hours. This difference is attributed to the slightly wider sensitized zone and the higher cooling rate in underwater welding, which promotes finer Ti(C,N) precipitation at grain boundaries.
Salt spray testing (ASTM B117, 5% NaCl) shows that both conditions produce joints with acceptable pitting corrosion resistance, with no pitting observed after 500 hours. However, the underwater-welded joints show slightly more surface roughness and oxide inclusion defects, which could serve as pit initiation sites under prolonged exposure.
Engineering Practice Implications
For offshore and subsea fabrication, this study provides important guidance on the feasibility and limitations of underwater laser welding for S32101 stainless steel. The key findings are:
- Underwater laser welding is technically feasible for S32101 stainless steel, producing welds with acceptable mechanical properties and corrosion resistance for most applications.
- The water medium acts as a natural shielding gas, eliminating the need for external shielding, which simplifies the welding setup but introduces variability in the thermal cycle.
- The slightly reduced corrosion resistance in the underwater condition requires careful consideration for applications in aggressive chloride environments.
- For critical applications, post-weld annealing (1050-1100°C, 1 hour, rapid water quench) is recommended to restore the microstructure and dissolve any Ti(C,N) precipitates.
Key Reflections and Study Insights
This study highlights an important practical consideration that is often overlooked in welding procedure qualification: the welding environment itself can significantly influence weld quality. While the mechanical properties show only marginal differences between air and underwater conditions, the corrosion resistance differences, though small, can be critical for long-term service life in marine environments. The study also demonstrates that laser welding, with its high energy density and narrow heat-affected zone, is well-suited for stainless steel welding in both atmospheric and underwater conditions, provided that the process parameters are properly adjusted for each environment.
In summary, this comparative study provides valuable data for engineers selecting welding methods for S32101 stainless steel in offshore and subsea applications. The findings suggest that underwater laser welding is a viable alternative to air welding for most service conditions, but that additional attention to post-weld treatment and corrosion monitoring is warranted for critical applications. The study also underscores the importance of considering environmental factors in welding procedure development, not just material and process parameters.