Development of Duplex Stainless Steel Strip Electrode for Electroslag Overlay Welding

Literature Overview

This study addresses the development of duplex stainless steel (DSS) strip electrodes for electroslag welding (ESW) overlay applications. Duplex stainless steels, with their balanced ferrite-austenite microstructure, offer superior mechanical properties and corrosion resistance compared to conventional austenitic stainless steels. However, the application of DSS in electroslag overlay welding presents unique metallurgical challenges due to the sensitivity of the duplex phase balance to thermal cycles. The research focuses on consumable design, process parameter optimization, and the control of microstructural evolution during multi-pass ESW overlay.

Metallurgical Challenges in DSS Electroslag Overlay

The primary metallurgical challenge in ESW overlay of duplex stainless steel is maintaining the target ferrite content (typically 40-60 percent by volume) throughout the multi-pass welding sequence. Electroslag welding involves repeated reheating of previously deposited material, which can cause phase transformations that shift the ferrite-austenite balance. Each successive pass in an ESW build-up exposes the previously deposited material to a thermal cycle that can promote ferrite dissolution or austenite formation, depending on the peak temperature and cooling rate.

The equilibrium phase diagram for Fe-Cr-Ni-Mo-N systems shows that the ferrite-austenite boundary is sensitive to temperature and composition. At welding temperatures above 1400 degrees Celsius, the equilibrium phase is predominantly austenite, while at temperatures below 900 degrees Celsius, ferrite is the stable phase. The cooling rate from the weld pool determines the non-equilibrium phase distribution, with faster cooling rates generally preserving more ferrite.

Consumable Design and Composition Optimization

The development of the DSS strip electrode requires careful selection of the alloy composition to achieve the target ferrite content after accounting for dilution from the substrate and previous passes. The base composition is typically based on UNS S31803 or S32750, with chromium content in the range of 22-25 percent and molybdenum content of 3-4 percent. Nitrogen content of 0.15-0.25 percent is critical for promoting austenite formation and stabilizing the duplex structure.

The strip electrode dimensions are typically 25-50 mm wide and 3-6 mm thick, with a length of 3-6 meters. The strip composition must be slightly enriched in ferrite-promoting elements (Cr, Mo, Si) relative to the target deposit composition to compensate for the dilution effect. The research shows that a strip composition with a Ferrite Number (FN) of 55-65 yields a final deposit with FN of 45-55 after accounting for typical ESW dilution rates of 25-35 percent.

The flux composition is also critical for ESW overlay. The flux must provide adequate slag viscosity to maintain a stable weld pool, sufficient deoxidation to prevent porosity, and appropriate alloying element addition to refine the grain structure. Fluxes based on CaF2, CaO, and SiO2 systems are commonly used, with the addition of Fe2O3 or Fe3O4 to promote deoxidation and grain refinement.

Process Parameter Optimization

The ESW process parameters must be optimized to achieve a stable welding process, good weld geometry, and the target microstructure. The current density should be in the range of 15-25 A/mm^2 for strip electrodes, providing sufficient heat input to maintain a liquid slag pool without excessive dilution. The travel speed should be adjusted to maintain a consistent slag pool depth of 5-10 mm, which provides good protection of the weld pool from atmospheric contamination.

The interpass temperature control is particularly important for DSS ESW overlay. Maintaining an interpass temperature between 200 and 300 degrees Celsius prevents excessive heat accumulation while allowing sufficient cooling to avoid grain coarsening. Exceeding 400 degrees Celsius interpass temperature can lead to excessive grain growth and a shift in the phase balance toward austenite.

The welding sequence for multi-pass ESW overlay should follow a balanced pattern to minimize residual stresses and phase segregation. A recommended sequence involves welding from the center outward, alternating between the left and right sides of the plate, which distributes the thermal load more evenly and reduces the risk of warping.

Microstructural Characterization and Properties

Microstructural analysis of the ESW overlay deposits reveals a typical duplex structure with equiaxed ferrite and austenite grains. The grain size is generally in the range of 50-150 micrometers, depending on the cooling rate and the grain refining elements in the consumable. The ferrite phase is typically continuous, with austenite precipitating along the ferrite grain boundaries during solidification.

The mechanical properties of the DSS ESW overlay are superior to those of conventional austenitic stainless steel overlays. The yield strength is typically 550-700 MPa, approximately 50 percent higher than 316L austenitic steel. The tensile strength is 650-800 MPa, and the elongation at break is 25-35 percent, indicating good ductility. The hardness is in the range of 230-280 HV, which is higher than austenitic stainless steel but lower than martensitic grades.

Corrosion resistance testing demonstrates that the DSS overlay provides excellent resistance to pitting and crevice corrosion in chloride-containing environments. The pitting resistance equivalent number (PREN = %Cr + 3.3 x %Mo + 16 x %N) is typically 34-38 for the optimized consumable, providing superior resistance compared to 316L (PREN = 24-26) and comparable to 6Mo super austenitic stainless steels.

Engineering Applications

The DSS ESW overlay technology finds applications in chemical processing equipment, offshore oil and gas structures, and marine engineering components. In the chemical industry, DSS overlay is used on carbon steel pressure vessels and heat exchangers exposed to chloride-containing process streams. In the offshore sector, DSS overlay protects structural steel components from chloride-induced stress corrosion cracking.

The ESW process is particularly advantageous for large-area overlay applications due to its high deposition rate, which can reach 10-20 kg/h compared to 2-5 kg/h for GMAW. This makes ESW economically attractive for overlaying large components such as vessel shells, heat exchanger tubesheets, and structural plates.

Summary and Reflections

The development of duplex stainless steel strip electrodes for electroslag overlay welding represents a significant advancement in the ability to apply high-performance corrosion-resistant overlays to large carbon steel components. The key challenge lies in maintaining the duplex phase balance through the multi-pass welding sequence, which requires careful consumable design, process parameter optimization, and interpass temperature control. The resulting overlay deposits offer a superior combination of mechanical strength, corrosion resistance, and toughness compared to conventional austenitic stainless steel overlays. Engineers should consider DSS ESW overlay as a viable alternative to full austenitic stainless steel construction when the substrate is carbon steel and the service environment demands high corrosion resistance and mechanical strength. The technology is particularly well-suited for retrofitting existing equipment to extend service life in aggressive environments.