CLADDING TECHNOLOGY SHANXI CO., LTDFerrite Number Determination for Austenitic Weld Overlay Cracking Control
Introduction and Metallurgical Rationale
Ferrite number (FN) determination is a critical quality control parameter for austenitic weld overlay cladding, particularly for deposits made with 316L, 304L, and similar stainless steel consumables. The ferrite number quantifies the volume fraction of delta ferrite present in the as-welded austenitic deposit, expressed as an equivalent percentage on a scale of 0 to 100. The control of ferrite content within the range of 4 to 12 FN is essential for preventing hot cracking while maintaining adequate corrosion resistance and mechanical properties.
Delta ferrite forms during solidification of austenitic stainless steel welds due to the thermodynamic stability of the ferrite phase at high temperatures. While excessive ferrite can reduce corrosion resistance and ductility, a controlled amount of ferrite is beneficial for preventing hot cracking by disrupting the continuous columnar grain structure and providing a liquid channel for impurity segregation. The balance between these competing requirements is achieved by maintaining the ferrite number within the recommended range of 4 to 12 FN for most austenitic overlay applications.
Measurement Methodology
The ferrite number is measured using the magnetic induction method, which exploits the ferromagnetic properties of delta ferrite relative to the paramagnetic austenite matrix. The measurement is performed on the as-welded deposit surface using a portable ferrite gauge that applies a magnetic field and measures the induced magnetization.
| Parameter | Specification |
|---|---|
| Measurement method | Magnetic induction per ASTM E490 / ISO 8044 |
| Gauge type | Portable electromagnetic ferrite meter |
| Calibration standard | 100% austenite (0 FN) and 100% ferrite (100 FN) reference blocks |
| Measurement area | Deposit surface, perpendicular to weld axis |
| Sample preparation | Light grinding to remove surface oxide, no polishing required |
| Measurement spacing | Every 25–50 mm along weld length for production |
| Data recording | Average of 3 readings per measurement point |
The measurement must be performed on the as-welded deposit before any post-weld heat treatment, as solution annealing will dissolve the delta ferrite and render the measurement meaningless. The deposit surface should be clean and free of oxide scale, which can affect the magnetic measurement. For multi-pass welds, the ferrite number should be measured on the final cap pass, which represents the final composition of the overlay layer.
Ferrite Number and Weldability Relationship
The ferrite number directly influences the susceptibility of the weld deposit to hot cracking, which is a solidification cracking phenomenon occurring in the last-liquid regions of the weld. Hot cracking is promoted by the segregation of low-melting-point impurities (sulfur, phosphorus) to grain boundaries during solidification, combined with the restraint stresses developed during cooling.
| FN Range | Cracking Susceptibility | Corrosion Resistance | Mechanical Properties | Application Suitability |
|---|---|---|---|---|
| 0–3 | High risk of hot cracking | Excellent | High ductility | Not recommended |
| 4–8 | Low risk | Good | Good | General service overlay |
| 8–12 | Very low risk | Good | Good | Critical service overlay |
| 12–15 | Very low risk | Moderate | Moderate | Limited applications |
| >15 | Very low risk | Poor | Reduced ductility | Not recommended for corrosion service |
The optimal ferrite number range of 4 to 12 FN provides a balance between hot cracking resistance and corrosion performance. Below 4 FN, the deposit is susceptible to hot cracking, particularly in thick-section welds with high restraint. Above 12 FN, the ferrite content begins to adversely affect corrosion resistance, particularly pitting and intergranular corrosion resistance, and may reduce ductility.
Factors Influencing Ferrite Number
Several factors influence the ferrite number of austenitic weld deposits, including consumable composition, welding parameters, and dilution from the base metal. Understanding these factors enables engineers to predict and control the ferrite number during procedure development.
| Factor | Effect on FN | Control Strategy |
|---|---|---|
| Nickel content in consumable | Higher Ni reduces FN | Use higher Ni wire for low-FN applications |
| Chromium content in consumable | Higher Cr increases FN | Balance Cr and Ni for target FN |
| Molybdenum content | Slightly increases FN | Minor effect, usually not controlling |
| Carbon content | Increases FN | Low-carbon wires (L grades) give lower FN |
| Heat input | Higher heat input slightly reduces FN | Control heat input per pass |
| Dilution from base metal | Carbon steel dilution increases FN | Multi-pass overlay to reduce dilution |
| Weld geometry | Deeper penetration increases dilution | Optimize weld preparation |
For ER316L wire overlay on carbon steel, the first pass typically exhibits a higher ferrite number due to dilution from the carbon steel substrate. The dilution introduces carbon and reduces the effective nickel content, shifting the solidification mode toward more ferrite formation. Subsequent passes show progressively lower ferrite numbers as the preceding overlay metal becomes the substrate. This dilution effect underscores the importance of multi-pass overlay to achieve a uniform and controlled ferrite number throughout the overlay thickness.
Procedure Qualification and Production Control
During welding procedure qualification per NB/T 47014 or ASME IX, the ferrite number must be measured and recorded as part of the procedure qualification record (PQR). The measured FN must fall within the specified acceptance range (typically 4–12 FN) for the procedure to be qualified. The essential variables that affect ferrite number, including consumable type and heat input, must be controlled within the qualified range during production.
In production, ferrite number measurement is performed at defined intervals as specified in the welding procedure specification (WPS). Typical requirements include measurement of every weld or at intervals not exceeding 250 mm of weld length. The results must be documented and reviewed, with any out-of-specification results triggering investigation and corrective action.
Defect Analysis and Corrective Actions
When ferrite number measurements fall outside the acceptable range, systematic investigation is required to identify the root cause and implement corrective actions. High ferrite numbers (above 12 FN) may indicate excessive dilution, improper consumable selection, or consumable composition outside specification. Low ferrite numbers (below 4 FN) may indicate hot cracking risk and require evaluation of the deposit for cracking.
| Finding | Investigation Steps | Corrective Actions |
|---|---|---|
| FN > 12 | Check consumable composition, dilution, heat input | Switch to higher Ni wire, reduce heat input, add overlay passes |
| FN < 4 | Check consumable composition, dilution, heat input | Switch to lower Ni wire, increase heat input, verify consumable lot |
| Variable FN across weld | Check travel speed uniformity |