Automatic Welding Surfacing Process for Duplex Stainless Steel 2205

Literature Overview

This paper by Wang Li, Gao Junsong, and Wu Daowen, published in Welding Technology (Vol. 39, No. 2, 2010, pp. 61-63), reports on the development and qualification of an automatic welding surfacing process for duplex stainless steel 2205 on large-diameter 16Mn forged tube plates. The authors employed a two-layer surfacing approach: a transition layer deposited by submerged arc welding (SAW) using D309MoL weld strip, followed by a duplex stainless steel overlay layer deposited by MIG welding using ER2209 wire. The surfaced test plates were subjected to mechanical property testing and corrosion resistance testing to verify compliance with duplex steel 2205 service requirements. This study is particularly relevant to engineers working on pressure vessel and heat exchanger tube plate manufacturing, where the combination of carbon steel substrate and duplex stainless steel overlay is a common configuration for corrosion-resistant applications.

Core Technical Findings

The study successfully established a qualified surfacing process that achieves the required mechanical and corrosion performance for duplex steel 2205 applications. The two-layer approach of transition layer followed by overlay layer is a well-established strategy for managing the metallurgical compatibility between carbon steel substrates and austenitic or duplex stainless steel overlays.

Two-Layer Surfacing Strategy

The two-layer surfacing approach addresses the fundamental metallurgical challenge of depositing stainless steel on carbon steel substrates. The transition layer serves to dilute the carbon content at the interface, reducing the risk of intergranular corrosion and ensuring adequate toughness at the critical interface region. The overlay layer provides the full duplex stainless steel microstructure with its characteristic combination of austenite and ferrite phases, delivering the required corrosion resistance and mechanical properties.

The selection of D309MoL for the transition layer is significant because this consumable contains molybdenum, which enhances the corrosion resistance of the transition layer while maintaining adequate dilution of the carbon from the 16Mn substrate. The ER2209 wire for the overlay layer is a standard duplex stainless steel welding consumable designed to produce a weld metal with approximately 50% austenite and 50% ferrite microstructure, matching the base metal composition of 2205 duplex stainless steel.

Mechanical Property Results

The mechanical property testing of the surfaced test plates confirmed that the developed process produces deposits with properties meeting or exceeding the requirements for 2205 duplex stainless steel applications. The key mechanical properties assessed typically include tensile strength, yield strength, elongation, and hardness, all of which must be consistent with the duplex stainless steel specification.

Corrosion Resistance Verification

The corrosion resistance testing is critical for validating the suitability of the surfacing process for the intended service environment. Duplex stainless steel 2205 is valued for its excellent resistance to chloride-induced pitting and crevice corrosion, as well as its resistance to stress corrosion cracking (SCC) in chloride-containing environments. The testing protocol likely included pitting resistance assessment through critical pitting temperature (CPT) determination or equivalent standard tests, as well as assessment of intergranular corrosion resistance.

Process Qualification and Standards Compliance

The process qualification aspect of this study is particularly important from a quality assurance perspective. The authors conducted comprehensive mechanical and corrosion testing to demonstrate that the surfacing process produces deposits meeting the performance requirements for 2205 duplex stainless steel service. This qualification approach is consistent with the requirements of welding procedure qualification standards such as ASME Section IX, AWS D10.9, or equivalent national standards.

Process Parameter Considerations

The interpass temperature control is particularly critical for the overlay layer because the duplex microstructure is sensitive to thermal cycling. Excessive interpass temperatures can promote the formation of sigma phase or other intermetallic phases that degrade corrosion resistance and toughness. Conversely, excessively low interpass temperatures can result in excessive residual stress and cracking susceptibility.

Engineering Practice Integration

For engineers involved in the manufacture of heat exchanger tube plates, pressure vessel components, and other equipment requiring corrosion-resistant overlays on carbon steel substrates, this study provides a validated process approach. Several practical considerations emerge:

1. Substrate Preparation: The 16Mn forged tube plate must be properly prepared to ensure good bonding of the transition layer. Surface cleanliness, removal of scale and contaminants, and appropriate preheat are essential.
2. Layer Thickness Control: The thickness of the transition layer and overlay layer must be controlled to ensure adequate carbon dilution and sufficient duplex stainless steel coverage. Typical transition layer thicknesses range from 3 to 6 mm, with overlay layer thicknesses determined by the required corrosion allowance.
3. Inspection Requirements: Each layer must be inspected for soundness using appropriate non-destructive testing methods. The transition layer should be inspected for lack of fusion and porosity, while the overlay layer should be inspected for porosity, cracks, and lack of penetration.

Quality Control Plan

Study Insights and Reflections

This paper demonstrates a practical and effective approach to surfacing duplex stainless steel 2205 on carbon steel substrates using a combination of submerged arc welding for the transition layer and MIG welding for the overlay layer. The two-process approach leverages the strengths of each welding method: SAW provides excellent penetration and dilution control for the transition layer, while MIG provides good deposition rate and microstructural control for the overlay layer.

One significant insight from this study is the importance of the transition layer in managing the metallurgical compatibility between the carbon steel substrate and the duplex stainless steel overlay. Without an adequate transition layer, the high carbon content of the substrate can lead to the formation of brittle carbides at the interface, severely degrading the toughness and corrosion resistance of the surfaced component. The selection of D309MoL for the transition layer is particularly appropriate because the molybdenum content enhances the corrosion resistance of the transition layer while maintaining the necessary carbon dilution.

The study also highlights the importance of process qualification in surfacing operations. The comprehensive mechanical and corrosion testing performed on the surfaced test plates provides the necessary evidence to demonstrate that the process produces deposits meeting the required performance standards. This qualification data is essential for regulatory approval and customer acceptance in critical applications such as pressure vessels and heat exchangers.

In conclusion, this paper provides a validated and qualified surfacing process for duplex stainless steel 2205 on carbon steel substrates, addressing a common and important manufacturing requirement in the pressure vessel and heat exchanger industries. The two-layer approach combining SAW and MIG welding offers a practical and effective solution that balances metallurgical compatibility, mechanical performance, and corrosion resistance. Engineers involved in similar surfacing applications should consider this process approach as a benchmark for their own qualification efforts.