Automatic Surfacing Process Development for Duplex Stainless Steel 2205 on 16Mn Forged Tube Sheets

Literature Overview

The paper by Wang Li, Gao Junsong, and Wu Daowen, published in Welding Technology (Vol. 39, Issue 2, 2010, pp. 61-63), describes the development and qualification of an automatic surfacing process for duplex stainless steel 2205 on large-diameter 16Mn forged tube sheets. The research, conducted at China Shipbuilding Industry Corporation No. 725 Research Institute, addresses a specific manufacturing challenge in shipbuilding and pressure vessel fabrication where corrosion-resistant duplex stainless steel overlays are required on carbon steel substrates.

Core Technical Findings

The study establishes a two-layer surfacing strategy: a transition layer deposited using submerged arc welding (SAW) with D309MoL welding strip, followed by a duplex stainless steel overlay deposited using MIG welding with ER2209 wire. This two-layer approach addresses the fundamental metallurgical challenge of joining dissimilar materials with significantly different thermal expansion coefficients and corrosion resistance requirements.

Process Development Rationale

The selection of D309MoL for the transition layer is based on the principle of gradual composition transition. D309MoL provides a moderate alloy content that bridges the gap between the low-alloy 16Mn substrate and the high-alloy 2205 duplex stainless steel overlay. The addition of molybdenum in the transition layer improves corrosion resistance while maintaining adequate weldability with the carbon steel substrate.

The choice of ER2209 wire for the duplex overlay is critical, as it is specifically designed to maintain the ferrite-austenite balance characteristic of duplex stainless steels. The "9" designation indicates a composition tailored for welding applications, with appropriate chromium, nickel, and molybdenum content to achieve the target duplex microstructure in the weld metal.

The two-layer approach is necessary because direct welding of 2205 duplex stainless steel onto 16Mn carbon steel would result in excessive dilution, loss of duplex character, and potential cracking due to the large thermal expansion mismatch. The transition layer absorbs much of the thermal mismatch and provides a metallurgically compatible interface.

Mechanical and Corrosion Testing Results

The study reports that mechanical property tests and corrosion resistance tests on the surfacing test plates meet the usage requirements for duplex stainless steel 2205. This indicates that the surfacing process successfully achieves the target performance characteristics despite the dilution from the carbon steel substrate.

The mechanical properties likely include tensile strength, yield strength, elongation, and hardness measurements on both the transition and overlay layers. The corrosion resistance testing probably includes salt spray testing, intergranular corrosion testing, and possibly crevice corrosion testing, all of which are relevant for the intended marine or pressure vessel applications.

Engineering Practice Implications

For shipbuilding and pressure vessel manufacturers, this study provides a proven process for duplex stainless steel surfacing on carbon steel substrates. The key practical considerations include:

1. The transition layer thickness should be sufficient to prevent excessive dilution of the duplex overlay, typically 3-5 mm is recommended.
2. The SAW process for the transition layer provides high deposition rates and good penetration, making it suitable for thick transition layers.
3. The MIG process for the overlay layer provides good bead control and is well-suited for automated deposition on large flat surfaces such as tube sheets.
4. Interpass temperature control is critical to maintain the duplex microstructure and prevent sigma phase formation.

The large diameter of the tube sheet presents additional challenges related to distortion control and stress management. The automatic welding process helps maintain consistency and reduce operator variability, which is essential for maintaining the required duplex microstructure throughout the surfacing area.

Key Questions and Reflections

The study raises important questions about the long-term performance of the surfacing system under thermal cycling and mechanical loading conditions. The thermal expansion mismatch between 16Mn and 2205 duplex stainless steel creates residual stresses that may promote cracking under cyclic loading. Engineers should consider the effect of thermal cycling on the interface between the transition layer and the substrate, as this is the most vulnerable location for interfacial cracking.

The corrosion resistance of the surfacing system depends on the quality of the transition layer, as any porosity or lack of fusion in the transition layer can provide a pathway for corrosive media to reach the carbon steel substrate. Thorough non-destructive testing (NDT) of the transition layer is essential to ensure complete protection of the substrate.

Study Insights and Reference Value

This study provides a practical and proven approach for duplex stainless steel surfacing on carbon steel substrates in shipbuilding and pressure vessel applications. The two-layer strategy with SAW transition and MIG overlay is a robust solution that addresses the metallurgical challenges of dissimilar material joining. The successful qualification of the process demonstrates that duplex stainless steel performance can be achieved on carbon steel substrates through appropriate process design.

For the steel pipe and fitting industry, the principles of this study are directly applicable to surfacing applications on carbon steel pipe spools, flanges, and tube sheets in marine and chemical processing environments. The emphasis on process qualification through mechanical and corrosion testing reflects the rigorous approach required for critical applications where failure could result in significant safety and economic consequences.