CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Welding Speed on 18-8 Stainless Steel Overlay Microstructure
Introduction and Technical Significance
The 18-8 stainless steel designation refers to austenitic stainless steels containing approximately 18 percent chromium and 8 percent nickel, with 304 and 316 being the most common commercial grades. These materials are widely used as overlay layers on carbon steel and low-alloy steel substrates to provide corrosion resistance in aggressive chemical environments. The welding speed during overlay deposition is a critical process parameter that directly influences the cooling rate, solidification morphology, phase composition, and ultimately the corrosion resistance and mechanical properties of the overlay layer.
Understanding the relationship between welding speed and microstructure is essential for optimizing overlay performance because the microstructure determines the material's susceptibility to intergranular corrosion, sensitization, and stress corrosion cracking. This literature study provides a detailed analysis of how varying welding speeds affect the solidification behavior and final microstructure of 18-8 stainless steel overlay welds, offering practical guidance for welding procedure optimization.
Solidification Behavior and Microstructural Evolution
The solidification of austenitic stainless steel weld metal is governed by the competition between austenite (gamma) and delta ferrite formation during cooling. The solidification mode—whether fully austenitic (A), austenitic with some delta ferrite (A+F), or fully ferritic with subsequent transformation (F+A)—is primarily determined by the chemical composition (expressed as the Ferrite Number or FN) and the cooling rate.
Welding speed directly controls the cooling rate through its effect on heat input. Higher welding speeds result in lower heat input and consequently higher cooling rates, which promote finer grain structures and can influence the balance between austenite and delta ferrite. The following table summarizes the typical effects of welding speed variations:
| Welding Speed (mm/min) | Heat Input (kJ/mm) | Cooling Rate t8/5 (s/mm) | Delta Ferrite (%) | Grain Size | Primary Phase |
|---|---|---|---|---|---|
| 100 | 1.8–2.5 | 5–12 | 2–5 | Coarse | A + F |
| 200 | 0.9–1.2 | 8–20 | 3–8 | Medium | A + F |
| 300 | 0.6–0.8 | 15–35 | 5–12 | Fine | A + F |
| 400 | 0.4–0.6 | 25–50 | 8–15 | Very fine | A + F |
| 500 | 0.3–0.5 | 40–80 | 10–20 | Very fine | A + F |
Low Welding Speed Regime
At low welding speeds (below 150 mm/min), the heat input is relatively high, resulting in slow cooling rates and coarse dendritic grain structures. The microstructure typically shows a predominantly austenitic matrix with 2 to 5 percent delta ferrite distributed along the grain boundaries and interdendritic regions. The coarse grain structure may be acceptable from a corrosion resistance standpoint if the delta ferrite content is sufficient to tie up carbon and prevent chromium carbide precipitation at grain boundaries, but it may be detrimental for applications requiring high fatigue strength or where thermal cycling is present.
High Welding Speed Regime
At high welding speeds (above 350 mm/min), the cooling rates become very rapid, leading to extremely fine grain structures with significantly increased delta ferrite content. The high cooling rates suppress the austenite-ferrite transformation equilibrium, trapping more delta ferrite in the final microstructure. While the fine grain structure offers improved mechanical properties, excessive delta ferrite (above 10-15 percent) can negatively impact corrosion resistance, particularly in chloride-containing environments, because delta ferrite is more susceptible to pitting and intergranular corrosion than austenite.
Phase Composition and Corrosion Implications
The phase composition of 18-8 stainless steel overlay welds is critical for determining their corrosion performance. The ideal microstructure for maximum corrosion resistance in most applications consists of a fully austenitic matrix with minimal delta ferrite, or alternatively, an austenitic matrix with 3 to 8 percent finely dispersed delta ferrite that provides adequate carbon scavenging without creating corrosion-prone phases.
The Schaeffler diagram or DeLong diagram is commonly used to predict the phase balance based on the equivalent chromium and nickel content of the weld metal. For 18-8 stainless steels, the target Ferrite Number is typically 5 to 10, corresponding to approximately 3 to 8 percent delta ferrite. The welding speed influences the achieved ferrite content by affecting the cooling rate and the subsequent phase transformation kinetics.
Intergranular Corrosion Sensitivity
One of the most critical concerns for 18-8 stainless steel overlays is susceptibility to intergranular corrosion (IGC) caused by chromium carbide (Cr23C6) precipitation at grain boundaries during exposure to temperatures in the sensitization range of 450 to 850 degrees Celsius. The welding speed influences IGC susceptibility through its effect on grain size and carbon distribution:
- Low welding speeds produce coarse grains with longer grain boundaries, potentially increasing the total area available for carbide precipitation.
- High welding speeds produce fine grains but may result in higher carbon supersaturation within the austenite matrix due to rapid solidification.
- The presence of delta ferrite acts as a carbon sink, reducing the carbon activity in austenite and thereby decreasing IGC susceptibility.
The optimal welding speed for IGC resistance is typically in the intermediate range of 200 to 350 mm/min, where the balance between grain refinement and adequate delta ferrite content provides the best overall corrosion performance.
Mechanical Properties and Welding Speed
The mechanical properties of 18-8 stainless steel overlay welds are significantly affected by welding speed through its influence on microstructure. Tensile strength, yield strength, elongation, and hardness all show systematic variations with welding speed:
| Property | Low Speed (100 mm/min) | Medium Speed (250 mm/min) | High Speed (450 mm/min) |
|---|---|---|---|
| Tensile Strength (MPa) | 550–620 | 580–680 | 620–750 |
| Yield Strength (MPa) | 220–280 | 260–340 | 300–400 |
| Elongation (%) | 40–50 | 35–45 | 25–35 |
| Hardness (HV) | 180–220 | 200–250 | 230–290 |
The general trend is that increasing welding speed increases strength and hardness while decreasing ductility, consistent with the Hall-Petch relationship governing grain refinement strengthening. However, the increase in delta ferrite content at very high speeds may also contribute to the hardness increase, as delta ferrite is inherently harder than austenite.
Process Optimization Recommendations
Based on the analysis of welding speed effects on microstructure, the following optimization guidelines are recommended for 18-8 stainless steel overlay applications:
- For maximum corrosion resistance: Use medium to high welding speeds (250–400 mm/min) to achieve fine grains with 5–10 percent delta ferrite, which provides adequate carbon scavenging while maintaining predominantly austenitic structure.
- For maximum mechanical strength: Use higher welding speeds (350–500 mm/min) to maximize grain refinement and delta ferrite content, accepting the trade-off of reduced ductility.
- For maximum ductility and formability: Use lower welding speeds (100–200 mm/min) to produce predominantly austenitic structures with minimal delta ferrite, accepting the potential for coarser grains.
- For applications requiring resistance to stress corrosion cracking: Ensure delta ferrite content is maintained between 3 and 8 percent through careful control of welding speed and filler metal selection.
Engineering Practice Considerations
In practical overlay welding operations, the selection of welding speed must be balanced against other process requirements including deposition rate, productivity, and the ability to maintain consistent bead geometry. Very high welding speeds may result in incomplete fusion, undercut, or excessive spatter, which compromise overlay integrity. Conversely, very low welding speeds may lead to excessive dilution, distortion, and potential sensitization of the heat-affected zone.
The use of automated or semi-automated welding systems is strongly recommended for 18-8 stainless steel overlay applications because they provide consistent welding speed control, which is essential for achieving uniform microstructure and consistent corrosion performance across the entire overlay surface. Manual welding introduces speed variations that result in microstructural heterogeneity, potentially creating localized areas of inferior corrosion resistance.
Summary and Conclusions
The welding speed is a fundamental process parameter that governs the microstructural evolution of 18-8 stainless steel overlay welds through its direct influence on cooling rate and solidification kinetics. The key findings from this study are that welding speed systematically affects delta ferrite content, grain size, mechanical properties, and corrosion resistance, with optimal values depending on the specific performance requirements of the application. Engineers designing overlay welding procedures for austenitic stainless steel applications should carefully select welding speeds within the recommended ranges, validate the resulting microstructure through metallographic examination and corrosion testing, and maintain tight process control to ensure consistent performance across the entire overlay surface. The interplay between welding speed, composition, and microstructure underscores the importance of integrated process design rather than isolated parameter optimization.