Ferrite Percentage to Ferrite Number Calculator

Understanding Ferrite Content: How to Convert Ferrite Percentage to Ferrite Number

Ferrite content is a critical factor in stainless steel welding, influencing the weld’s resistance to corrosion, hot cracking, and embrittlement. Welders often need to measure or specify ferrite content, either as a ferrite percentage or a ferrite number (FN), depending on standards like AWS A4.2 or the WRC-1992 diagram. But what’s the difference between these two metrics, and how do you convert between them? In this blog post, we’ll explain the importance of ferrite content, the relationship between ferrite percentage and ferrite number, provide a step-by-step conversion guide, and offer practical insights for welders. Plus, we’ve included a Ferrite Percentage to Ferrite Number Calculator to simplify the process.

Why Ferrite Content Matters in Welding

Ferrite is a phase in stainless steel welds that affects their mechanical and corrosion properties. In austenitic stainless steels (e.g., 304, 316L), a small amount of ferrite in the weld metal helps prevent hot cracking during solidification. In duplex stainless steels (e.g., 2205), a balanced ferrite-austenite ratio ensures optimal corrosion resistance and toughness. However, too much ferrite can lead to embrittlement, especially at high temperatures, while too little can compromise weld integrity.

Ferrite content is typically expressed in two ways:

Ferrite Percentage (%): The volume fraction of ferrite in the weld, often measured using a feritscope or magnetic methods.
Ferrite Number (FN): A dimensionless value correlated to ferrite percentage, commonly used in standards like AWS A4.2 and the WRC-1992 diagram. FN is often preferred in welding specifications because it accounts for magnetic properties and is easier to standardize.

Converting between these two metrics is essential when comparing measurements, meeting project specs, or troubleshooting weld issues. For example, a specification might require an FN of 4–12 for duplex stainless steel, but your feritscope gives a percentage reading—knowing how to convert ensures you’re on the right track.

Ferrite Percentage vs. Ferrite Number: What’s the Difference?

The main difference lies in how they’re measured and used:

Ferrite Percentage: A direct measurement of ferrite volume, often obtained through metallographic analysis or a feritscope. It’s expressed as a percentage (0–100%). For example, a weld with 5% ferrite means 5% of its microstructure is ferritic, with the rest typically austenitic.
Ferrite Number (FN): A standardized value that correlates to ferrite percentage but is adjusted for magnetic properties and measurement consistency. FN is not a direct percentage but a scaled number (0–150+), where 1% ferrite might correspond to roughly 1.5 FN in lower ranges, though the relationship isn’t strictly linear.

The conversion between the two depends on the ferrite range, as the correlation varies at higher values. For instance, 5% ferrite might be 7.5 FN, but 50% ferrite might be closer to 60 FN due to scaling adjustments in standards like WRC-1992.

How to Convert Ferrite Percentage to Ferrite Number (and Vice Versa)

While precise conversions often require tools like the WRC-1992 diagram or a feritscope with built-in calibration, we can use approximate relationships for practical purposes. Here’s a simplified step-by-step guide:

Converting Ferrite Percentage to Ferrite Number

Measure Ferrite Percentage: Use a feritscope or other method to determine the ferrite content (e.g., 5%).
Apply the Conversion (approximate, based on typical correlations):
- For 0–10%: FN ≈ Percentage × 1.5.
  Example: 5% × 1.5 = 7.5 FN.
- For 10–20%: FN ≈ (Percentage × 1.25) + 2.5.
  Example: 15% → (15 × 1.25) + 2.5 = 21.25 FN.
- For 20–50%: FN ≈ (Percentage × 1.1) + 5.
  Example: 30% → (30 × 1.1) + 5 = 38 FN.
- For >50%: FN ≈ (Percentage × 0.9) + 15.
  Example: 60% → (60 × 0.9) + 15 = 69 FN.
Interpret the Result: Check if the FN meets your specification (e.g., FN 4–12 for duplex stainless steel).

Converting Ferrite Number to Ferrite Percentage

Obtain Ferrite Number: From a specification or measurement (e.g., 38 FN).
Apply the Reverse Conversion:
- For 0–15 FN: Percentage ≈ FN ÷ 1.5.
  Example: 7.5 FN ÷ 1.5 = 5%.
- For 15–30 FN: Percentage ≈ (FN – 2.5) ÷ 1.25.
  Example: 21.25 FN → (21.25 – 2.5) ÷ 1.25 = 15%.
- For 30–55 FN: Percentage ≈ (FN – 5) ÷ 1.1.
  Example: 38 FN → (38 – 5) ÷ 1.1 = 30%.
- For >55 FN: Percentage ≈ (FN – 15) ÷ 0.9.
  Example: 69 FN → (69 – 15) ÷ 0.9 = 60%.
Assess Weld Quality: Ensure the percentage aligns with your material’s requirements.

Practical Example

Scenario 1: Percentage to FN
You measure a 316L stainless steel weld at 5% ferrite using a feritscope. Convert to FN:
Since 5% is in the 0–10% range, FN = 5 × 1.5 = 7.5 FN.
This falls within the typical FN 4–8 range for austenitic stainless steel, reducing hot cracking risk.
Scenario 2: FN to Percentage
A duplex stainless steel specification requires 38 FN. Convert to percentage:
Since 38 FN is in the 30–55 FN range, Percentage = (38 – 5) ÷ 1.1 = 30%.
This is a balanced ferrite-austenite ratio, ideal for corrosion resistance in duplex alloys.

Practical Guidelines for Ferrite Content in Welding

Ferrite content affects weld performance, so here are some guidelines:

Austenitic Stainless Steels (e.g., 304, 316L):
- Target: 2–8% ferrite (FN 3–12).
- Why: Prevents hot cracking during solidification. Below 2%, the weld is fully austenitic and prone to cracking. Above 12 FN, corrosion resistance may decrease.
Duplex Stainless Steels (e.g., 2205):
- Target: 25–60% ferrite (FN 30–70), ideally FN 4–12 for critical applications.
- Why: Balances austenite and ferrite for optimal toughness and corrosion resistance. Too much ferrite (>70 FN) risks embrittlement.
High Ferrite Risks:
- Above 30% (FN >30): Ferrite transforms to brittle sigma phase at high temperatures, reducing toughness.
Low Ferrite Risks:
- Below 2% (FN <3): Fully austenitic welds are prone to hot cracking, especially in high-restraint joints.

Factors Affecting Ferrite Content

Ferrite content in a weld depends on several variables:

Base Material Composition:
- High nickel and nitrogen (austenite stabilizers) reduce ferrite. High chromium and molybdenum (ferrite stabilizers) increase it.
- Example: 316L (higher Mo) has more ferrite than 304.
Filler Metal:
- Choose fillers with balanced chemistry. For 316L, use 316LSi with controlled ferrite (FN 5–10).
Welding Process:
- GTAW (TIG) produces lower ferrite due to slower cooling. GMAW (MIG) increases ferrite with faster cooling.
Cooling Rate:
- Faster cooling (e.g., thin sections) retains more ferrite. Slower cooling (e.g., thick plates) promotes austenite formation.
Heat Input:
- High heat input (e.g., 2.5 kJ/mm) reduces ferrite by allowing more austenite formation. Low heat input (e.g., 1 kJ/mm) increases ferrite.

Measuring Ferrite Content

To measure ferrite content accurately:

Feritscope: A handheld device that measures ferrite percentage via magnetic induction. Many models also display FN.
WRC-1992 Diagram: Predicts FN based on alloy composition (Cr_eq and Ni_eq). Useful for planning but not for measuring welds.
Metallography: Microscopic analysis to calculate ferrite percentage, though it’s time-consuming.
Magnetic Testing: AWS A4.2 uses magnetic methods to determine FN, often with calibration standards.

Using the Ferrite Percentage to Ferrite Number Calculator

To make conversions easier, we’ve created an interactive Ferrite Percentage to Ferrite Number Calculator (available in other posts on our site). Simply select whether you’re converting from percentage to FN or FN to percentage, enter your value, and get instant results. The calculator also provides practical notes, such as warnings about low ferrite (hot cracking risk) or high FN (embrittlement risk), and logs your calculations in a history table for reference.

Example Using the Calculator

Input: 5% ferrite, converting to FN.
Result: “Ferrite Number: 7.5 FN, Notes: FN 4–12 ideal for duplex SS.”
Derivation: Approximate: 5% × 1.5 = 7.5 FN.
Input: 38 FN, converting to percentage.
Result: “Ferrite Percentage: 30%, Notes: High FN (>30) may cause embrittlement.”
Derivation: Approximate: (38 – 5) ÷ 1.1 = 30%.

Conclusion

Ferrite content is a key consideration in stainless steel welding, and understanding how to convert between ferrite percentage and ferrite number ensures you meet specifications and achieve quality welds. Whether you’re working with austenitic stainless steel (targeting FN 3–12) or duplex alloys (aiming for FN 30–70), the right ferrite level prevents issues like hot cracking and embrittlement. Use the approximate conversion methods in this guide for quick estimates, or try our Ferrite Percentage to Ferrite Number Calculator for instant results. For critical applications, always verify with a feritscope or consult the WRC-1992 diagram to ensure accuracy.