CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Austenitic Stainless Steel Cladding Layers Produced by Dual-Tungsten TIG Welding
Overview of the Literature
This study investigates the microstructure, mechanical properties, and corrosion resistance of austenitic stainless steel cladding layers produced by the dual-tungsten electrode TIG (GTAW) process. The research focuses on understanding the relationship between welding parameters, microstructural evolution, and the resulting performance of the clad deposit. The cladding layers were produced on carbon steel (SA-516 Gr.70) and low-alloy steel (SA-387 Gr.11) base metals using ER308L and ER316L filler wires, with the dual-electrode TIG process parameters optimized for minimum dilution and maximum metallurgical quality.
Core Technical Content
The microstructural evolution of the clad deposit was analyzed using optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). The results revealed a complex microstructure consisting of columnar dendrites growing perpendicular to the fusion boundary, with secondary dendrite arm spacing (SDAS) of 15–30 μm in the clad deposit. The grain structure was finer in the dual-electrode TIG deposits compared to single-electrode TIG deposits, with an average grain size of 25–35 μm versus 40–55 μm, respectively.
| Microstructural Feature | Dual-Tungsten TIG | Single-Electrode TIG | Conventional SAW |
|---|---|---|---|
| Grain size (μm) | 25–35 | 40–55 | 60–90 |
| SDAS (μm) | 15–30 | 25–45 | 40–70 |
| Sigma phase | Not detected | Trace (<0.5%) | Possible |
| Delta ferrite (%) | 1–3% | 2–5% | 3–8% |
| Grain orientation | Columnar | Columnar | Columnar |
| Solidification mode | Dendritic | Dendritic | Dendritic |
| Melt pool depth (mm) | 2–3 | 1.5–2.5 | 3–5 |
| Cooling rate (°C/s) | 50–150 | 80–200 | 30–80 |
Mechanical Properties of the Clad Deposit
The mechanical properties of the clad deposit were evaluated through tensile testing, hardness profiling, and impact testing. The tensile properties of the 308L clad deposit were found to be excellent, with a yield strength of 280–320 MPa, ultimate tensile strength of 550–620 MPa, and elongation of 38–45%. The 316L clad deposit exhibited slightly lower tensile strength (520–580 MPa) but comparable elongation (40–48%).
The hardness profile across the clad deposit showed a characteristic pattern: the base metal hardness was 150–180 HV, the dilution zone hardness was 200–260 HV, and the pure clad metal hardness was 180–220 HV. The hardness variation was smooth and gradual, with no sharp transitions that could promote crack initiation. The dual-electrode TIG process produced a narrower dilution zone (2–3 mm) compared to single-electrode TIG (3–5 mm) and conventional SAW (5–8 mm), which is beneficial for maintaining the corrosion resistance of the clad layer.
Corrosion Resistance Evaluation
The corrosion resistance of the clad deposit was evaluated through electrochemical testing (potentiodynamic polarization, electrochemical impedance spectroscopy) and immersion testing in various corrosive media. The results demonstrated that the dual-electrode TIG clad deposit exhibited excellent corrosion resistance in sulfuric acid (5%, 60°C), hydrochloric acid (3%, 25°C), and simulated wastewater solutions. The corrosion rate of the 316L clad deposit was 0.02–0.05 mm/year in 5% sulfuric acid, which is comparable to cast 316L stainless steel (0.015–0.04 mm/year).
The intergranular corrosion (IGC) resistance was evaluated using the ASTM A262 Practice E test (65°C sulfuric acid-copper sulfate solution). The dual-electrode TIG clad deposit showed no evidence of intergranular corrosion after 72 hours of immersion, indicating excellent resistance to sensitization. This is attributed to the low carbon content of the ER308L and ER316L wires (C ≤ 0.03%) and the rapid cooling rates achieved with the dual-electrode process, which minimize the formation of chromium carbides at the grain boundaries.
Engineering Practice Implications
The metallurgical quality of the dual-electrode TIG cladding deposit is directly related to the welding parameters and the base metal condition. The study recommends the following practices to ensure optimal clad quality: (1) maintain the inter-pass temperature below 150°C to prevent excessive grain growth and sensitization, (2) use low-carbon filler wires (ER308L, ER316L) to minimize the risk of intergranular corrosion, (3) perform post-weld solution heat treatment (1050–1100°C water quench) for critical applications requiring maximum corrosion resistance, and (4) verify the dilution rate through chemical analysis of the clad deposit to ensure it remains below the acceptable limit (typically < 30% for 308L/316L on carbon steel).
The mechanical properties of the clad deposit are also influenced by the welding sequence and the number of passes. A multi-pass cladding sequence with alternating directions was found to produce a more uniform microstructure and mechanical properties compared to a single-direction sequence. The study recommends a maximum of 4–6 passes for each cladding layer, with each pass overlapping the previous pass by 50–60% to ensure complete fusion and minimize the risk of lack of fusion defects.
Key Questions and Reflections
The study identified a critical relationship between the dilution rate and the corrosion resistance of the clad deposit. The dilution rate increases with each subsequent pass, and the corrosion resistance decreases as the dilution rate increases. The study recommends that the dilution rate be monitored throughout the cladding process and that the welding parameters be adjusted to maintain the dilution rate below 25% for the first two passes and below 35% for subsequent passes. This requires careful control of the heat input, travel speed, and wire feed rate, as well as periodic chemical analysis of the clad deposit.
Another important finding is that the delta ferrite content in the clad deposit plays a critical role in preventing hot cracking and controlling the mechanical properties. The dual-electrode TIG process produces a delta ferrite content of 1–3%, which is within the optimal range (1–10%) for preventing hot cracking while maintaining adequate ductility and corrosion resistance. The delta ferrite content can be controlled by adjusting the welding parameters and the filler wire composition, and it should be verified through magnetic ferrite gauge measurements during production welding.
Study Insights and Recommendations
The dual-tungsten electrode TIG cladding process produces austenitic stainless steel clad deposits with excellent metallurgical quality, mechanical properties, and corrosion resistance. The key factors influencing the clad quality are the welding parameters, the base metal condition, and the post-weld treatment. The recommended engineering practices include: (1) develop a comprehensive WPS that specifies all critical welding parameters and their acceptable ranges, (2) implement a rigorous quality control program that includes visual inspection, NDT, and mechanical testing of each clad deposit, (3) perform periodic metallurgical evaluations to verify the microstructure and properties of the clad deposit, and (4) maintain detailed records of all welding parameters and quality data for traceability and continuous improvement. The successful application of this technology requires a deep understanding of the metallurgy of austenitic stainless steels and the welding process, as well as a commitment to quality and continuous improvement.