CLADDING TECHNOLOGY SHANXI CO., LTDPulsed MIG Welding Process Research on Austenitic Stainless Steel
Literature Overview
This study, conducted by Wan Chunfeng and Dai Xinlei from Shaanxi National Defense Industry Vocational and Technical College and Shaanxi Transportation Vocational and Technical College respectively, was published in 2016 in Shandong Chemical Industry. The research was supported by the Shaanxi National Defense Industry Vocational and Technical College 2014 Research Plan Project (Project No. GFY14-11). The paper investigates the pulsed Metal Inert Gas (pulsed MIG) welding process specifically applied to austenitic stainless steel, addressing weld quality, microstructure, and process parameter optimization.
Core Technical Content
Pulsed MIG welding is distinguished from conventional continuous MIG welding by its use of a modulated current waveform that alternates between a high current pulse and a low current background level. This modulation allows precise control over the metal transfer mechanism, typically achieving a one-drop-per-pulse transfer mode. For austenitic stainless steel, which is susceptible to hot cracking, excessive grain growth, and sensitization, the pulsed MIG process offers significant advantages over continuous current welding.
The key technical parameters investigated include pulse current amplitude, background current, pulse frequency, pulse duration, wire feed speed, travel speed, and shielding gas composition. The study examines how these parameters influence weld bead geometry, microstructure, and mechanical properties of the weld metal and heat-affected zone (HAZ).
| Parameter | Typical Range for Pulsed MIG on Austenitic SS | Effect on Weld Quality |
|---|---|---|
| Pulse current | 180–350 A | Controls droplet detachment force |
| Background current | 30–80 A | Maintains arc stability between pulses |
| Pulse frequency | 80–200 Hz | Determines metal transfer rate |
| Pulse duration | 3–8 ms | Affects droplet size and transfer timing |
| Wire feed speed | 4–10 m/min | Controls deposition rate |
| Travel speed | 200–500 mm/min | Influences heat input and bead profile |
| Shielding gas | Ar + 2–5% CO2 or pure Ar | Affects arc stability and penetration |
| Wire diameter | 0.8–1.2 mm | Influences current density and transfer mode |
Process Analysis
Austenitic stainless steels such as 304, 316, 321, and 347 are widely used in pressure vessel fabrication due to their excellent corrosion resistance and formability. However, welding these materials presents several challenges. The high thermal expansion coefficient (approximately 17.3 × 10⁻⁶/°C) leads to significant residual stresses. The low thermal conductivity (approximately 16 W/(m·K) for 304 SS) causes heat to concentrate in the weld region, promoting excessive grain growth in the HAZ. Additionally, the formation of chromium carbides (Cr23C6) in the temperature range of 500–800°C can lead to sensitization and intergranular corrosion.
The pulsed MIG process addresses these challenges by providing lower overall heat input compared to continuous MIG welding. The intermittent current delivery allows the weld pool to partially solidify between pulses, reducing the maximum temperature and the time spent in the sensitization range. The controlled droplet transfer also results in a more uniform weld bead profile with reduced spatter and improved surface finish.
Microstructural Considerations
The microstructure of the weld metal in pulsed MIG welds of austenitic stainless steel is typically fully austenitic with a fine grain structure. The lower heat input promotes a higher cooling rate, which refines the grain structure and reduces the risk of sensitization. However, if the heat input is too low, the cooling rate may become excessively high, potentially leading to martensitic transformation in the weld metal, which is undesirable in terms of ductility and corrosion resistance.
The HAZ microstructure is influenced by the thermal cycling history. In the coarse grain HAZ, the prior austenite grain boundaries are susceptible to intergranular corrosion if sensitized. The pulsed MIG process, by reducing the peak temperature and the time above 500°C, helps to minimize this risk. For welds subject to NACE MR0175/ISO 15156 requirements, the microstructural control provided by pulsed MIG welding is particularly valuable.
Engineering Practice and Standards Compliance
In pressure vessel fabrication, welds on austenitic stainless steel components must comply with applicable standards such as ASME VIII Division 1/2, GB/T 150, and NB/T 47014. The pulsed MIG process can be qualified under these standards, with the welding procedure specification (WPS) defining the pulse parameters within specified ranges. The welder performance qualification (WPQ) must demonstrate the ability to produce sound welds within the established parameter window.
For bimetallic pressure vessels where austenitic stainless steel cladding is welded to carbon steel or low-alloy steel base material, the pulsed MIG process offers a controlled approach to achieving low dilution and maintaining the integrity of the cladding layer. The dilution rate in such welds should typically be controlled below 20–30% to ensure the overlay retains its corrosion resistance properties.
Key Insights and Practical Recommendations
The research by Wan and Dai demonstrates that pulsed MIG welding is a viable and advantageous process for austenitic stainless steel applications. The process provides superior control over heat input, metal transfer, and weld geometry compared to continuous MIG welding. For engineers involved in pressure vessel fabrication, the adoption of pulsed MIG welding for austenitic stainless steel welds can lead to improved weld quality, reduced post-weld treatment requirements, and enhanced long-term service reliability. The study also highlights the importance of parameter optimization through systematic experimental investigation, which remains essential even in the era of advanced simulation tools.