CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Austenitic Stainless Steel Weld Overlay on Low Alloy Steel Substrate
Literature Overview
This research focuses on the microstructural characteristics and mechanical properties of austenitic stainless steel weld overlay layers deposited on low alloy steel substrates. Austenitic stainless steel overlay is one of the most widely applied corrosion-resistant cladding solutions in pressure vessel fabrication, chemical processing equipment, and heat exchanger manufacturing. The study systematically examines the dilution effects, phase composition, corrosion resistance, and mechanical behavior of the overlay layer under different welding conditions and substrate compositions.
Core Technical Findings
The fundamental challenge in austenitic stainless steel overlay on low alloy steel is the dilution of alloying elements from the substrate into the weld metal. Carbon and manganese from the low alloy steel dilute into the overlay, potentially destabilizing the austenitic structure and promoting ferrite formation. The study quantified dilution rates and their effects on microstructure:
| Substrate Grade | Overlay Wire Grade | Dilution Rate (%) | Ferrite Content (%) | Hardness (HV) | Corrosion Resistance |
|---|---|---|---|---|---|
| 16Mn | 308L | 18–25 | 5–12 | 180–220 | Acceptable |
| 15CrMo | 309L | 22–30 | 8–18 | 190–240 | Marginal |
| Q345R | 309MoL | 20–28 | 6–15 | 185–230 | Good |
| 12Cr2Mo | 310L | 15–22 | 3–8 | 170–210 | Excellent |
The dilution rate was found to be strongly influenced by welding parameters, substrate thickness, and the number of overlay passes. Higher heat input and thinner substrates resulted in greater dilution, which in turn increased ferrite content in the overlay.
Phase Composition and Ferrite Control
Ferrite formation in austenitic stainless steel overlay is governed by the balance of austenite formers (Ni, C, N, Mn) and ferrite formers (Cr, Mo, Si, P, S) as described by the Schaeffler diagram and the DeLong diagram. When dilution introduces additional carbon and manganese from the substrate, the effective composition shifts, but the net effect depends on the specific alloying content of both the substrate and the filler. In practice, the study found that using 310L filler (with higher Ni content) effectively compensated for dilution from high-chromium substrates, maintaining ferrite content below 8% and ensuring excellent corrosion resistance.
Corrosion Resistance Evaluation
Intergranular corrosion testing following ASTM A263 requirements revealed that overlay layers with ferrite content above 15% exhibited measurable grain boundary attack after stabilization at 870 °C for 100 hours. The sensitivity to intergranular corrosion was directly correlated with chromium carbide precipitation at grain boundaries, which occurred preferentially in regions of higher dilution where carbon content was elevated. Overlay layers deposited with low-carbon fillers (308L, 309L) on substrates with carbon content below 0.20% showed no intergranular corrosion susceptibility, confirming the importance of carbon control in the overall dilution calculation.
Engineering Practice Implications
For pressure vessel fabrication involving austenitic stainless steel overlay on low alloy steel, the following engineering practices are recommended based on the study findings:
- Always perform dilution calculations before welding, accounting for the substrate composition, expected weld geometry, and number of passes, as required by NB/T 47014 qualification procedures.
- Select filler metals with sufficient Ni content (309L or 310L) when overlaying on high-carbon or high-chromium substrates to maintain ferrite content within acceptable limits.
- Limit heat input per pass to reduce dilution, particularly for the first pass which typically experiences the highest dilution rate.
- Conduct ferrite content measurement on production welds using a ferrite gauge, targeting values between 3–10% for optimal resistance to hot cracking and adequate corrosion resistance.
- Perform intergranular corrosion testing on qualification coupons when the substrate carbon content exceeds 0.20% or when dilution calculations predict carbon content in the overlay above 0.06%.
Key Questions and Reflections
The study raises an important question about the long-term stability of the overlay microstructure under thermal cycling conditions. Many pressure vessels operate at elevated temperatures where sensitization can occur in service, particularly in regions near the overlay-substrate interface where dilution is highest. The study's corrosion testing was conducted on laboratory specimens under controlled conditions, and the practical implications for vessels operating for decades under varying thermal conditions require further investigation.
Additionally, the mechanical compatibility between the austenitic overlay and the ferritic substrate under thermal cycling deserves attention. The coefficient of thermal expansion of austenitic stainless steel (approximately 17.3 × 10⁻⁶/°C) is significantly higher than that of low alloy steel (approximately 12.0 × 10⁻⁶/°C), creating thermal mismatch stresses during temperature transients. These stresses can accumulate over repeated cycles and potentially lead to overlay cracking or delamination, particularly at the interface.
Summary and Study Insights
The research comprehensively demonstrates that the performance of austenitic stainless steel overlay on low alloy steel is fundamentally governed by dilution control, which affects phase composition, corrosion resistance, and mechanical properties. The selection of appropriate filler metal grade, welding parameter optimization, and rigorous dilution calculation are essential engineering practices. Engineers should integrate dilution analysis into the welding procedure qualification process and adopt conservative filler metal selection for high-dilution applications to ensure long-term corrosion resistance and structural integrity of the overlay layer.