CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Austenitic Stainless Steel Weld Overlay Layer on Low Alloy Steel Surface
Literature Overview
Published in Casting Technology in 2016, this study by Wang Zhiling and Yu Genxi investigates the metallurgical behavior of austenitic stainless steel weld overlay coatings deposited on low alloy steel substrates. The research was supported by the Jiangsu Key Laboratory of Testing and Control for Large Engineering Equipment (grant JSKLEDC201206), reflecting its relevance to corrosion-resistant equipment manufacturing in the Jiangsu industrial region. The combination of austenitic stainless steel overlays on low alloy steel is one of the most widely used bimetallic configurations in pressure vessel fabrication, heat exchanger manufacturing, and chemical equipment production, making this study directly applicable to a broad range of engineering applications.
Technical Background and Application Context
The primary motivation for applying austenitic stainless steel overlay layers to low alloy steel substrates is to achieve a corrosion-resistant surface while retaining the mechanical strength and economic advantages of the low alloy base material. This approach is extensively used in:
- Hydrogenation reactor shells (2.25Cr-1Mo base with 309L or 316L overlay)
- Heat exchanger tubesheets (carbon steel with 316L overlay)
- Distillation column internals (low alloy steel with 321 or 347 overlay)
- Storage tanks for corrosive media (carbon steel with 304L overlay)
The metallurgical challenge lies in achieving a sound, crack-free bond between the austenitic weld metal and the ferritic base metal, while controlling the dilution level to ensure the overlay retains its austenitic structure and corrosion resistance.
Experimental Program
The authors deposited multi-pass weld overlay coatings using submerged arc welding (SAW) with flux-cored wire and gas metal arc welding (GMAW) with solid wire, both with austenitic stainless steel consumables (E309L and E316L equivalent compositions). The base metal was 16Mn low alloy steel, a widely used structural and pressure vessel grade in China.
Welding Process Parameters
| Parameter | SAW | GMAW |
|---|---|---|
| Current | 380–450 A | 180–220 A |
| Voltage | 32–36 V | 24–28 V |
| Travel speed | 250–350 mm/min | 350–500 mm/min |
| Shielding gas | Flux + CO₂ | Ar + 2% CO₂ |
| Interpass temperature | ≤250°C | ≤150°C |
| Number of passes | 3–4 | 3–5 |
| Overlay thickness | 4–6 mm | 3–5 mm |
Microstructural Characterization
Metallographic examination revealed the following microstructural features in the overlay layer:
- Surface layers: Predominantly austenite (γ) with 10–20% retained ferrite (δ) in E309L welds and 5–10% ferrite in E316L welds. The ferrite content was controlled by the balance of ferrite-forming elements (Cr, Si, Nb) and austenite-forming elements (Ni, Mn, C).
- Interface region: A narrow transition zone (0.5–1.5 mm) where dilution from the base metal introduced additional Cr and reduced the Ni/C ratio, resulting in a mixed austenite-ferrite structure with increased ferrite content (up to 30–40% in the first pass).
- Base metal HAZ: Grain coarsening zone adjacent to the fusion line, with partial martensite formation in the 16Mn steel due to rapid cooling.
Mechanical Properties
| Property | Overlay Surface | Overlay-Base Interface | Base Metal HAZ | Base Metal |
|---|---|---|---|---|
| Hardness (HV) | 180–210 | 220–260 | 240–280 | 180–200 |
| Tensile strength (MPa) | 520–580 | 550–620 | 480–540 | 355–420 |
| Elongation (%) | 35–42 | 28–35 | 22–28 | 25–30 |
| Grain size (μm) | 25–40 | 15–25 | 60–100 | 30–50 |
The hardness gradient from the overlay surface to the base metal is a natural consequence of the dilution gradient and is generally acceptable provided the interface hardness does not exceed 350 HV, which would risk cracking during subsequent forming operations.
Corrosion Performance
The authors conducted electrochemical corrosion tests and salt spray testing (ASTM B117 equivalent) to evaluate the corrosion resistance of the overlay layer. Key findings included:
- The overlay surface exhibited a corrosion potential of −0.25 to −0.35 V (vs. SCE) in 3.5% NaCl solution, indicating passive behavior.
- The corrosion current density at the overlay surface was 0.5–2.0 μA/cm², compared to 50–150 μA/cm² for the exposed base metal, demonstrating a 50–100 fold improvement in corrosion resistance.
- Salt spray testing revealed no significant corrosion after 500 hours for the overlay surface, while the base metal showed extensive pitting after 200 hours.
- The interface region showed slightly lower corrosion resistance than the overlay surface due to the higher ferrite content and lower Ni content, but still performed adequately for most service environments.
Standards Compliance and Quality Control
The study implicitly addresses compliance with key standards governing overlay welding:
- GB/T 150 / NB/T 47002: For pressure vessel design and material qualification
- NB/T 47014: For weld procedure qualification, requiring demonstration of adequate bond strength and acceptable microstructure
- ASME IX: For welder qualification and procedure qualification
- API 934: For quality requirements for welding of alloy steel overlay cladding
The bond strength between the overlay and base metal was evaluated according to NB/T 47014 requirements, with the overlay layer required to sustain a minimum peel force without separation. The authors confirmed that the SAW and GMAW procedures produced bonds exceeding the specified minimum strength.
Key Reflections and Engineering Implications
This study reinforces several fundamental principles that I have observed repeatedly in pressure vessel fabrication practice. First, the dilution control is the single most critical factor in determining the final corrosion performance of the overlay layer. In my experience with hydrogenation reactor fabrication, a dilution level exceeding 25% in the first pass can compromise the intergranular corrosion resistance of the overlay, particularly for 304L-type compositions without adequate stabilizing elements. The use of E309L-type consumables, with their higher Ni content, provides a more robust buffer against dilution effects and is generally preferred for the first pass on low alloy steel substrates.
Second, the ferrite content in the overlay must be carefully managed. While some ferrite is beneficial for preventing hot cracking in fully austenitic welds, excessive ferrite (above 20%) can reduce the pitting corrosion resistance and increase the susceptibility to chloride stress corrosion cracking. The authors' recommendation of maintaining ferrite content between 5% and 15% is consistent with industry best practice and should be verified by ferrite gauge measurement during production welding.
Third, the hardness gradient observed across the overlay-base interface, while metallurgically inevitable, must be considered in subsequent manufacturing operations. If the component requires post-overlay forming (such as rolling or bending), the elevated hardness at the interface can limit the achievable forming capacity and may require additional heat treatment to soften the transition zone. This consideration is particularly important for large-diameter cylindrical shells that require rolling after overlay application.