Comparison of 304L Stainless Steel Overlay Welding in Air and Underwater Environments

Literature Overview

This study by Zhao Youliang and colleagues from Beijing University of Chemical Engineering, published in China Surface Engineering (2024, Vol. 37, No. 4, pp. 280-290), addresses a critical yet underexplored engineering challenge: the overlay welding repair of 304L stainless steel in underwater environments. The research is funded by the National Natural Science Foundation of China (U22B20127), the Beijing Science and Technology Program (KZ202210017023), and the Beijing Municipal University Classification Development Project. The work is motivated by the real-world problem of spent fuel pool corrosion in nuclear power plants, where long-term service leads to cracking and pitting defects that compromise safety, yet underwater welding repair remains poorly studied in the literature.

Core Technical Approach

The authors employed ER-2209 duplex stainless steel solid wire to deposit overlay layers on 304L stainless steel substrates under two distinct conditions: ambient air and underwater environments. The choice of ER-2209 is technically significant, as the duplex microstructure (approximately 50% austenite and 50% ferrite) offers superior mechanical properties and enhanced resistance to chloride-induced stress corrosion cracking compared to conventional austenitic overlay materials. The comparison methodology is well-structured, examining macro morphology, microstructure, phase composition, mechanical properties, microhardness, and corrosion resistance systematically.

Key Findings on Microstructure

The overlay layers in both environments exhibited continuous and uniform surface formation without cracks, slag inclusions, weld spatter, or undercutting defects. This is a notable achievement, particularly for underwater welding, where arc stability and shielding gas control are inherently more challenging.

The microstructural analysis revealed a dual-phase distribution within the overlay layers, containing both duplex stainless steel microstructure and austenitic stainless steel microstructure in a regionally distributed pattern. This segregation is expected given the dilution effects at the interface between the ER-2209 deposit and the 304L substrate. The critical finding lies in how the underwater environment's rapid cooling effect differentially influences these two microstructural regions.

In the duplex stainless steel microstructure, the rapid cooling in the underwater environment suppressed the transformation of delta ferrite grains toward gamma austenite, resulting in the retention of larger delta ferrite grains. Since delta ferrite possesses a higher stacking fault energy, this retention directly contributed to increased overlay layer hardness. In the austenitic stainless steel microstructure, the rapid cooling promoted the formation of lenticular ferrite distributed along austenite grain boundaries. The regional distribution of this lenticular ferrite further elevated the hardness of the austenitic microstructure regions.

Mechanical Properties Analysis

The transverse tensile test results provide particularly instructive insights. The stress-strain curves of the overlay layer specimens demonstrated tensile strength exceeding that of the base material. During deformation, strain-induced martensitic transformation was triggered, which enhanced the strength in the later stages of the base material's stress-strain curve. Overall, the elongation of the base material exceeded that of the overlay layer, which is consistent with the higher hardness and lower ductility expected from the duplex overlay.

At the fusion zone between weld passes, the crystallographic orientation of austenite grains deviated, and the texture intensity of the weld pass increased. The top transverse tensile specimens from both environments exhibited higher tensile strength but lower elongation, which aligns with the texture strengthening mechanism observed at inter-pass fusion boundaries.

Corrosion Resistance Assessment

Polarization curve analysis confirmed that the overlay layers exhibited a distinct passivation region, indicating good corrosion resistance. This is a critical requirement for nuclear service applications where the overlay must withstand prolonged exposure to corrosive coolant environments. The ER-2209 duplex overlay's corrosion performance is attributed to the synergistic effect of the Cr-Mo-Ni alloying system and the balanced austenite-ferrite microstructure.

Engineering Practice Implications

From a practical standpoint, this study provides valuable guidance for nuclear facility maintenance engineers who must perform in-situ repairs of spent fuel pool structures. The finding that underwater welding with ER-2209 wire can produce defect-free overlay layers with adequate mechanical and corrosion properties is encouraging. However, the increased hardness and reduced ductility in the underwater condition must be carefully considered when specifying repair procedures for components subject to cyclic loading or seismic events.

Critical Reflections

One aspect that warrants further consideration is the long-term stability of the retained delta ferrite in the underwater-deposited overlay. While the increased hardness is beneficial for wear resistance, the larger delta ferrite grains could potentially serve as initiation sites for intergranular corrosion under prolonged chloride exposure. The study does not address aging or thermal cycling effects, which are relevant for nuclear applications where temperature fluctuations are expected.

Additionally, the texture strengthening observed at inter-pass fusion boundaries raises questions about the optimal inter-pass temperature control in underwater welding. In air welding, inter-pass temperature management is well-established, but the underwater environment introduces additional variables such as water flow rate, ambient temperature, and hydrostatic pressure that could influence the thermal cycle and, consequently, the texture development.

Study Insights and Conclusions

This research makes a meaningful contribution to the limited body of knowledge on underwater overlay welding for stainless steel components. The systematic comparison between air and underwater conditions, combined with detailed microstructural characterization and mechanical testing, provides engineers with actionable data for repair procedure development. The use of ER-2209 duplex wire as an overlay material for 304L substrate repair is technically sound and demonstrates that acceptable results can be achieved even under the challenging conditions of underwater welding. For nuclear industry practitioners, this work supports the feasibility of underwater repair strategies for spent fuel pool integrity maintenance, provided that the slightly reduced ductility of underwater-deposited overlays is accounted for in the repair design criteria.