CLADDING TECHNOLOGY SHANXI CO., LTDNitrogen-Assisted 316L Stainless Steel Laser-MIG Hybrid Welding Microstructure and Corrosion Resistance
Literature Overview
This 2021 study published in the Transactions of the China Welding Institute investigates the microstructural evolution and corrosion resistance of 316L stainless steel welds produced via laser-MIG hybrid welding with nitrogen shielding assistance. The research was conducted at Huazhong University of Science and Technology under the State Key Laboratory of Material Forming and Die & Mould Technology, in collaboration with Eisenvell (Wuhan) Industrial Technology Co., Ltd. The work was supported by the National Key R&D Program of China under grant numbers 2018YFB1106501 and 2018YFB1106505. This topic is particularly relevant to the fabrication of stainless steel cladding layers and bimetal pressure vessels where the overlay layer must maintain excellent corrosion resistance.
Core Technical Content
The fundamental innovation of this study is the introduction of nitrogen as a supplementary shielding gas in the laser-MIG hybrid welding process. Nitrogen, being a reactive gas in the context of stainless steel welding, is typically avoided in conventional arc welding because it can cause nitrogen pickup in the weld metal, leading to increased hardness, potential nitride precipitation, and altered corrosion behaviour. However, this study demonstrates that under carefully controlled conditions, nitrogen can serve as a beneficial process variable that enhances the dilution control and microstructural refinement of the hybrid weld.
Process Configuration and Parameters
| Parameter | Value | Rationale |
|---|---|---|
| Laser power | 3-6 kW | Provides deep penetration and narrow weld profile |
| MIG arc current | 180-240 A | Fills the laser-generated keyhole and provides filler metal |
| MIG wire speed | 4.5-6.5 m/min | Controls deposit rate and dilution ratio |
| Travel speed | 0.4-0.8 m/min | Balances penetration depth and bead width |
| Laser-arc offset | 0-1 mm | Optimises interaction zone geometry |
| Nitrogen flow rate | 2-8 L/min | Modulates arc stability and heat input distribution |
| Argon flow rate | 12-18 L/min | Primary shielding to prevent oxidation |
| Filler wire | ER316L | Matches base metal composition |
The researchers found that nitrogen-assisted hybrid welding produced a weld microstructure characterised by finer grain size and a higher proportion of acicular ferrite compared to conventional argon-only shielding. The nitrogen introduced a controlled level of arc constriction and increased arc pressure, which promoted better keyhole stability and reduced porosity formation. The resulting weld metal exhibited a refined dendritic structure with reduced grain boundary segregation of chromium and molybdenum, which are the key elements governing the pitting resistance of 316L stainless steel.
Corrosion Performance Evaluation
The corrosion resistance was evaluated through multiple methods including potentiodynamic polarization testing in 3.5 wt% NaCl solution, pitting potential measurement, and salt spray testing per ASTM B117. The nitrogen-assisted welds demonstrated a pitting potential that was 20-40 millivolts higher than the base metal, indicating improved resistance to localized corrosion initiation. The linear polarization resistance values were also consistently higher, suggesting better general corrosion resistance.
| Corrosion Test Method | Nitrogen-Assisted Weld | Conventional Argon-Only Weld | Base Metal 316L |
|---|---|---|---|
| Pitting Potential (mV vs SCE) | +720 to +760 | +680 to +710 | +700 to +730 |
| Corrosion Current Density (uA/cm2) | 0.8-1.2 | 1.5-2.1 | 1.0-1.4 |
| Salt Spray Test (hours to pitting) | >1000 | 600-800 | >1000 |
| pitting resistance equivalent number (PREN) | 26-28 | 24-26 | 25-27 |
Standards and Engineering Practice Integration
This research has direct relevance to the fabrication of clad stainless steel pressure vessels governed by GB/T 150 and ASME Section VIII Division 1. The overlay layer in such vessels must maintain corrosion resistance comparable to the base clad material, and the hybrid welding process with nitrogen assistance offers a pathway to achieve this while maintaining high productivity. The work also aligns with the requirements of NB/T 47014 for qualification of welding procedures and the corrosion testing protocols specified in GB/T 4334 for stainless steel intergranular corrosion testing.
Process Control and Quality Assurance Considerations
The use of nitrogen introduces additional variables that must be controlled during production. The nitrogen-to-argon ratio must be maintained within a narrow window, as excessive nitrogen leads to harmful nitrogen pickup that can form chromium nitride precipitates and deplete chromium from the solid solution. The recommended nitrogen fraction should not exceed 15 percent of the total shielding gas mixture. Additionally, the process requires precise control of the laser-arc interaction zone to prevent nitrogen-induced arc instability at the keyhole boundary.
Study Insights and Engineering Implications
The most significant finding of this research is that nitrogen, traditionally considered detrimental in stainless steel welding, can be beneficial when used as a controlled process variable in hybrid welding. This challenges the conventional wisdom that all reactive gases must be excluded from stainless steel welding environments. For engineers involved in the fabrication of bimetal pressure vessels and clad equipment, this finding opens new possibilities for process optimization. The nitrogen-assisted hybrid welding process can potentially reduce the number of passes required for overlay welding, decrease overall heat input, and minimize the risk of sensitization in the heat-affected zone. However, careful qualification testing per NB/T 47014 is essential before implementing this process in production, particularly for critical applications involving sour service or high-temperature corrosion environments.