CLADDING TECHNOLOGY SHANXI CO., LTDDilution Rate Exceedance Assessment in Weld Overlay Layers
The Critical Role of Dilution Control
In weld overlay applications, the dilution rate—the proportion of base metal elements that mix into the overlay layer during welding—directly determines the chemical composition and microstructure of the deposited metal. When dilution exceeds the design tolerance, the overlay layer loses its intended corrosion resistance, mechanical properties, or both. This is particularly critical for stainless steel and nickel alloy overlays applied to carbon steel or low-alloy steel substrates, where the base metal contains carbon, manganese, and other elements that can severely degrade the overlay's performance if allowed to migrate in excessive quantities.
Detection Methods and Acceptance Criteria
The assessment of dilution rate is performed through chemical analysis of the overlay layer, typically using optical emission spectroscopy (OES) for bulk composition and energy-dispersive X-ray spectroscopy (EDS) for localized analysis at the interface region. The acceptance criteria vary by application but generally follow the principles outlined below:
| Overlay Material | Base Metal | Key Element Limit | Typical Dilution Tolerance |
|---|---|---|---|
| 308L (Cr-Ni austenitic SS) | Carbon steel | Cr ≥ 18%, Ni ≥ 8% | Dilution ≤ 15% |
| 316L (Mo-bearing SS) | Carbon steel | Cr ≥ 17%, Mo ≥ 2% | Dilution ≤ 12% |
| Inconel 625 | Carbon steel | Ni ≥ 55%, Mo ≥ 8% | Dilution ≤ 10% |
| Hastelloy C-276 | Carbon steel | Ni ≥ 55%, Mo ≥ 15% | Dilution ≤ 8% |
When the ferrite number (FN) of the overlay layer falls outside the specified window of 4 to 12, this indicates either excessive dilution (low FN, indicating a fully austenitic structure susceptible to cracking) or improper filler metal selection (high FN, indicating a ferrite-rich structure with reduced corrosion resistance). The FN measurement is performed using a ferrite gun and provides a rapid, non-destructive indicator of the overlay composition.
Root Causes of Excessive Dilution
Excessive dilution is typically caused by one or more of the following factors:
- High heat input per unit length, which melts more base metal into the weld pool
- Low travel speed relative to the current setting
- Large electrode or wire diameter relative to the weld geometry
- Insufficient preheating of the base metal, leading to rapid solidification that traps base metal inclusions in the overlay
- Inappropriate joint design that exposes excessive base metal to the arc
The transition layer—a deliberately deposited layer of a compositionally intermediate filler metal between the base metal and the final overlay—is the primary engineering countermeasure against excessive dilution. However, a transition layer can itself fail if the dilution from the base metal into the transition layer exceeds the design limit, effectively rendering the transition layer compositionally identical to the base metal and providing no protection to the final overlay.
Engineering Practice: Case Study on Hydrogenation Reactor Liner
In a hydrogenation reactor project requiring a 6 mm thick Inconel 625 overlay on a 25Cr-2Ni steel shell, the initial welding procedure specified a single-layer transition of 309L stainless steel followed by a single-layer Inconel 625 overlay. Chemical analysis of the completed overlay revealed chromium content of only 14.2% (below the 18% minimum) and ferrite number of 2.1 (well below the 4–12 window). Investigation revealed that the ESW overlay process produced a dilution rate of 22%, far exceeding the design assumption of 10%. The corrective action involved adding a second transition layer of 309L, reducing the ESW current by 15%, and increasing the travel speed by 20%. The revised procedure achieved a dilution rate of 9.5% and overlay composition within specification.
Key Insights and Recommendations
The dilution rate assessment is not merely a compliance exercise but a fundamental indicator of whether the overlay design has been properly executed. I recommend that all weld overlay procedures include a mandatory dilution rate verification step in the qualification testing phase, with chemical analysis of the overlay layer at multiple locations along the weld length. Furthermore, the specification should clearly define the acceptance limits for each critical element (Cr, Ni, Mo, C, Fe) rather than relying solely on ferrite number, because FN does not capture dilution of elements such as molybdenum or carbon that are equally important for corrosion resistance in many service environments.