Ferrite Number Calculation Formula

Calculate the ferrite number of stainless steel welds to predict microstructure and prevent cracking. Based on the WRC-1992 diagram and DeLong constitution diagram.

Introduction & Importance of Ferrite Number Calculation

The ferrite number (FN) is a critical metallurgical parameter used to predict the microstructure of austenitic and duplex stainless steel welds. Developed by the Welding Research Council (WRC) in 1992, the FN system replaced the older ferrite content percentage measurements to provide more accurate predictions of weld metal properties.

Ferrite in stainless steel welds serves several crucial functions:

• Crack Prevention: Ferrite helps prevent hot cracking (solidification cracking) in austenitic stainless steel welds by providing a secondary phase that can accommodate impurities.
• Mechanical Properties: The amount of ferrite influences the strength, toughness, and corrosion resistance of the weld metal.
• Microstructure Control: FN values help metallurgists predict the balance between austenite and ferrite phases in the weld microstructure.
• Weldability Assessment: Proper FN ranges ensure good weldability and reduce the risk of defects like fissuring or sigma phase formation.

Industries that rely on accurate FN calculations include:

• Petrochemical processing (pipelines, pressure vessels)
• Nuclear power plant construction
• Aerospace component manufacturing
• Food processing equipment fabrication
• Pharmaceutical manufacturing equipment

The most widely used method for calculating FN is the WRC-1992 diagram, which builds upon the earlier DeLong constitution diagram. This calculator implements the precise mathematical formulas from these diagrams to provide accurate FN predictions for stainless steel weld metal compositions.

How to Use This Ferrite Number Calculator

Follow these step-by-step instructions to accurately calculate the ferrite number for your stainless steel weld:

1. Gather Composition Data: Obtain the chemical composition of your weld metal, either from the filler metal certification or through chemical analysis. You’ll need percentages for Cr, Ni, Mn, Mo, Cu, Si, C, and N (in ppm).
2. Input Values:
   • Enter chromium (Cr) percentage (typically 16-30% for stainless steels)
   • Enter nickel (Ni) percentage (typically 6-14% for austenitic stainless)
   • Enter manganese (Mn) percentage (usually 0.5-2.0%)
   • Enter molybdenum (Mo) percentage (0-4% for common alloys)
   • Enter copper (Cu) percentage (0-1% in most cases)
   • Enter silicon (Si) percentage (typically 0.3-1.0%)
   • Enter carbon (C) percentage (usually 0.02-0.08%)
   • Enter nitrogen (N) in parts per million (ppm) (typically 100-800 ppm)
3. Calculate: Click the “Calculate Ferrite Number” button. The calculator will:
   • Compute chromium and nickel equivalents
   • Apply the WRC-1992 formula
   • Display the ferrite number (FN)
   • Provide interpretation of the result
   • Generate a visual representation of where your composition falls on the WRC diagram
4. Interpret Results:
   • FN 0-4: Fully austenitic (high risk of hot cracking)
   • FN 4-8: Austenitic with trace ferrite (acceptable for many applications)
   • FN 8-12: Optimal range for most austenitic stainless steel welds
   • FN 12-20: Higher ferrite content (good for duplex stainless steels)
   • FN >20: Excessive ferrite (may reduce toughness and corrosion resistance)
5. Adjust Composition (if needed): If your FN falls outside the desired range, consider:
   • Increasing chromium or molybdenum to raise FN
   • Increasing nickel or manganese to lower FN
   • Adjusting welding parameters or using different filler metal

Pro Tip:

For duplex stainless steels (like 2205), target FN values between 35-65. These alloys require higher ferrite content for proper phase balance and corrosion resistance.

Ferrite Number Formula & Methodology

The ferrite number calculation is based on the WRC-1992 diagram, which uses chromium equivalent (Cr_eq) and nickel equivalent (Ni_eq) to predict the weld metal microstructure. Here’s the detailed methodology:

1. Chromium Equivalent (Cr_eq) Calculation:

The chromium equivalent represents the ferrite-stabilizing elements in the alloy. The formula is:

Cr_eq = %Cr + %Mo + 1.5 × %Si + 0.5 × %Nb

Where:

• %Cr = Chromium percentage
• %Mo = Molybdenum percentage
• %Si = Silicon percentage
• %Nb = Niobium percentage (not included in our simplified calculator)

2. Nickel Equivalent (Ni_eq) Calculation:

The nickel equivalent represents the austenite-stabilizing elements. The formula is:

Ni_eq = %Ni + 30 × %C + 0.5 × %Mn + 30 × (%N/100)

Where:

• %Ni = Nickel percentage
• %C = Carbon percentage
• %Mn = Manganese percentage
• %N = Nitrogen in percent (ppm/10000)

3. Ferrite Number (FN) Determination:

Once Cr_eq and Ni_eq are calculated, the ferrite number is determined by plotting these values on the WRC-1992 diagram. The diagram is divided into regions corresponding to different FN values.

Our calculator uses the following mathematical approximation of the WRC-1992 diagram:

FN = 3 × (Cr_eq – 1.37 × Ni_eq – 9.2)

This formula provides a good approximation for FN values between 0 and 20. For higher FN values (common in duplex stainless steels), a more complex polynomial equation is used.

4. Diagram Interpretation:

The WRC-1992 diagram consists of:

• A horizontal axis representing Ni_eq
• A vertical axis representing Cr_eq
• Curved lines representing constant FN values
• Regions indicating fully austenitic, austenitic-ferritic, and fully ferritic microstructures

The diagram was developed through extensive research by the Welding Research Council, correlating actual ferrite measurements (using magnetic or metallographic methods) with calculated Cr_eq and Ni_eq values.

Important Note:

While the WRC-1992 diagram is highly accurate for most stainless steels, it has limitations:

• Doesn’t account for nitrogen as effectively as newer diagrams
• May be less accurate for highly alloyed super austenitic or super duplex stainless steels
• Assumes equilibrium cooling conditions (actual welds may cool differently)

For critical applications, consider using the WRC-2017 diagram or actual ferrite measurements.

Real-World Examples & Case Studies

Case Study 1: Type 304L Stainless Steel Weld

Scenario: Welding 304L stainless steel plates with ER308L filler metal in a petrochemical pipeline application.

Composition:

• Cr: 19.5%
• Ni: 9.8%
• Mn: 1.5%
• Mo: 0.2%
• Cu: 0.1%
• Si: 0.5%
• C: 0.02%
• N: 400 ppm

Calculation:

Cr_eq = 19.5 + 0.2 + 1.5 × 0.5 + 0.5 × 0 = 20.475

Ni_eq = 9.8 + 30 × 0.02 + 0.5 × 1.5 + 30 × (0.04) = 11.35

FN ≈ 3 × (20.475 – 1.37 × 11.35 – 9.2) ≈ 5.8

Result: FN 5.8 – Excellent weldability with sufficient ferrite to prevent hot cracking while maintaining good corrosion resistance.

Case Study 2: Type 316L Stainless Steel for Pharmaceutical Equipment

Scenario: Fabricating a pharmaceutical mixing tank from 316L stainless steel using ER316L filler metal.

Composition:

• Cr: 17.2%
• Ni: 12.1%
• Mn: 1.8%
• Mo: 2.3%
• Cu: 0.3%
• Si: 0.6%
• C: 0.018%
• N: 350 ppm

Calculation:

Cr_eq = 17.2 + 2.3 + 1.5 × 0.6 + 0.5 × 0 = 20.4

Ni_eq = 12.1 + 30 × 0.018 + 0.5 × 1.8 + 30 × (0.035) = 13.86

FN ≈ 3 × (20.4 – 1.37 × 13.86 – 9.2) ≈ 0.5

Result: FN 0.5 – Very low ferrite content. While acceptable for some applications, this weld may be susceptible to hot cracking. Consider using ER316LSi filler which typically produces FN 3-6.

Case Study 3: Duplex 2205 Stainless Steel for Offshore Platform

Scenario: Welding duplex 2205 stainless steel for an offshore oil platform using ER2209 filler metal.

Composition:

• Cr: 22.5%
• Ni: 8.5%
• Mn: 1.2%
• Mo: 3.2%
• Cu: 0.1%
• Si: 0.4%
• C: 0.02%
• N: 600 ppm

Calculation:

Cr_eq = 22.5 + 3.2 + 1.5 × 0.4 + 0.5 × 0 = 26.3

Ni_eq = 8.5 + 30 × 0.02 + 0.5 × 1.2 + 30 × (0.06) = 11.06

FN ≈ Special duplex formula: FN = 3 × (Cr_eq – 1.37 × Ni_eq – 8.1) ≈ 35

Result: FN 35 – Ideal for duplex stainless steel, providing excellent balance between ferrite and austenite phases for optimal strength and corrosion resistance.

Ferrite Number Data & Comparative Analysis

Comparison of Common Stainless Steel Filler Metals

Filler Metal	AWS Classification	Typical FN Range	Primary Applications	Cr Eq.	Ni Eq.
ER308L	A5.9 ER308L	3-8	General purpose 304/304L	19-21	9-11
ER308LSi	A5.9 ER308LSi	5-10	Improved wettability for 304/304L	19-21	9-11
ER316L	A5.9 ER316L	0-5	316/316L for corrosion resistance	18-20	11-13
ER316LSi	A5.9 ER316LSi	3-8	Improved wettability for 316/316L	18-20	11-13
ER309L	A5.9 ER309L	8-15	Dissimilar metals, clad overlays	22-24	12-14
ER2209	A5.9 ER2209	35-55	Duplex 2205 stainless steel	25-27	8-10
ER2553	A5.9 ER2553	45-65	Super duplex 2507	27-29	9-11

Effect of Ferrite Number on Weld Properties

Ferrite Number Range	Microstructure	Hot Cracking Resistance	Toughness	Corrosion Resistance	Typical Applications
0-3	Fully austenitic	Poor	Excellent	Good (but susceptible to SCC)	Specialized applications with controlled welding
3-8	Austenitic with trace ferrite	Good	Very good	Very good	Most common for 300-series stainless
8-15	Austenitic-ferritic	Excellent	Good	Excellent	Heavy sections, high restraint welds
15-30	Ferritic-austenitic	Excellent	Fair	Very good	Duplex stainless steels
30-60	Balanced duplex	Excellent	Good	Excellent	Duplex and super duplex stainless
>60	Ferrite dominant	Excellent	Poor	Reduced	Avoid – risk of sigma phase formation

Data sources: American Welding Society, NIST Materials Data, and TWI Welding Research.

Expert Tips for Ferrite Number Control

Pre-Weld Considerations:

1. Material Selection:
   • For austenitic stainless steels, choose filler metals with FN 4-12
   • For duplex stainless steels, target FN 35-55
   • Consider the base metal dilution effect (typically 20-40%)
2. Joint Design:
   • Use joint designs that minimize restraint to reduce cracking risk
   • Consider groove angles that allow proper weld pool formation
   • Avoid excessive root gaps that can lead to high dilution
3. Preheat Requirements:
   • Most austenitic stainless steels don’t require preheat
   • Duplex stainless may benefit from 50-150°C preheat to balance ferrite/austenite
   • Avoid preheat >200°C which can destabilize the microstructure

During Welding:

4. Heat Input Control:
   • Maintain heat input between 0.5-2.5 kJ/mm for most stainless steels
   • Higher heat input increases ferrite content in austenitic welds
   • Lower heat input (with higher cooling rates) favors austenite formation
5. Interpass Temperature:
   • Keep interpass temperature below 150°C for austenitic stainless
   • Duplex stainless may allow up to 200°C interpass
   • Monitor with temperature sticks or infrared thermometers
6. Welding Process Selection:
   • GTAW (TIG) produces the cleanest welds with most predictable FN
   • GMAW (MIG) can be used with proper gas shielding (Ar+2%O₂ or tri-mix)
   • Avoid SMAW with high-hydrogen electrodes for critical applications

Post-Weld Operations:

7. Ferrite Verification:
   • Use a feritscope for non-destructive FN measurement
   • Metallographic examination provides most accurate results
   • Magnetic testing can give qualitative ferrite content estimates
8. Heat Treatment:
   • Austenitic stainless typically doesn’t require post-weld heat treatment
   • Duplex stainless may need solution annealing (1020-1100°C) to restore phase balance
   • Avoid stress relief temperatures (400-900°C) that can form sigma phase
9. Corrosion Testing:
   • Perform ASTM A262 Practice E for duplex stainless steels
   • Conduct salt spray testing for critical applications
   • Check for intergranular corrosion susceptibility

Troubleshooting:

10. Low Ferrite Number Issues:
    • Increase chromium or molybdenum content
    • Reduce nickel or manganese
    • Switch to a filler metal with higher ferrite potential
    • Increase heat input slightly
11. High Ferrite Number Issues:
    • Increase nickel content in filler metal
    • Use a more austenitic filler metal
    • Reduce heat input
    • Consider nitrogen additions (for duplex steels)

Advanced Tip:

For critical applications, consider using the WRC-2017 diagram which:

• Better accounts for nitrogen’s austenite-stabilizing effect
• Includes adjustments for copper and cobalt
• Provides more accurate predictions for highly alloyed stainless steels
• Offers improved correlation with actual ferrite measurements

The WRC-2017 diagram is particularly valuable for:

• Super austenitic stainless steels (6% Mo alloys)
• Super duplex stainless steels (PREN > 40)
• High-nitrogen stainless steels
• Welds with significant copper additions

Interactive Ferrite Number FAQ

What is the difference between ferrite number (FN) and ferrite content percentage?

The ferrite number (FN) and ferrite content percentage are related but distinct measurements:

• Ferrite Content (%): Represents the actual volume percentage of ferrite in the microstructure, typically measured using metallographic techniques or magnetic methods.
• Ferrite Number (FN): A standardized measurement system developed by the Welding Research Council that correlates with but isn’t identical to ferrite content. FN provides a more consistent way to specify and control ferrite levels.

The relationship between FN and ferrite content is approximately:

• FN 0 ≈ 0% ferrite
• FN 4 ≈ 2-3% ferrite
• FN 8 ≈ 5-6% ferrite
• FN 12 ≈ 8-10% ferrite
• FN 20 ≈ 15-20% ferrite

FN was introduced because ferrite content measurements can vary based on the measurement method, while FN provides a more consistent reference for welding procedures and specifications.

How does nitrogen affect ferrite number calculations?

Nitrogen has a significant impact on ferrite number calculations because it’s a strong austenite stabilizer. In the WRC-1992 diagram:

• Nitrogen is accounted for in the nickel equivalent (Ni_eq) calculation
• Each 0.1% nitrogen (1000 ppm) increases Ni_eq by about 3 points
• This strong effect means small changes in nitrogen content can significantly lower the FN

For example, increasing nitrogen from 0.05% to 0.15% (500 ppm to 1500 ppm) would:

• Increase Ni_eq by about 3 points (30 × 0.01 = 0.3)
• Potentially reduce FN by 4-6 points depending on the alloy composition

Modern stainless steels often use nitrogen additions to:

• Stabilize austenite without adding expensive nickel
• Improve strength through interstitial solid solution strengthening
• Enhance corrosion resistance, particularly to pitting and crevice corrosion

For duplex stainless steels, nitrogen is particularly important as it helps maintain the proper austenite/ferrite balance during welding and service.

What are the limitations of the WRC-1992 diagram?

While the WRC-1992 diagram is widely used and generally accurate, it has several important limitations:

1. Nitrogen Limitations:
   • Doesn’t fully account for nitrogen’s strong austenite-stabilizing effect
   • Underestimates FN for high-nitrogen stainless steels
2. Alloy Range:
   • Most accurate for Cr_eq 18-28 and Ni_eq 8-20
   • Less reliable for highly alloyed super austenitic or super duplex stainless steels
3. Cooling Rate Assumptions:
   • Assumes equilibrium cooling conditions
   • Actual welds may cool at different rates affecting phase balance
4. Element Limitations:
   • Doesn’t account for copper, cobalt, or tungsten
   • Simplifies the effect of niobium and titanium
5. Magnetic Response:
   • FN correlates with but isn’t identical to magnetic ferrite measurements
   • Some non-ferritic phases (like sigma) can affect magnetic readings
6. Temperature Effects:
   • FN is typically measured at room temperature
   • Phase balance may change at service temperatures
7. Measurement Methods:
   • Different ferrite measurement techniques (magnetic, metallographic) can give varying results
   • FN provides a standardized reference but may not match all measurement methods exactly

For applications where these limitations are critical, consider:

• Using the newer WRC-2017 diagram
• Performing actual ferrite measurements on production welds
• Conducting weld procedure qualification tests
• Using more advanced prediction models for highly alloyed materials

How does ferrite number affect corrosion resistance?

The ferrite number significantly influences the corrosion resistance of stainless steel welds through several mechanisms:

Positive Effects of Ferrite:

• Chromium Distribution: Ferrite helps distribute chromium more evenly, reducing the risk of chromium carbide precipitation and sensitization.
• Phase Balance: In duplex stainless steels, the proper ferrite/austenite balance (FN 35-55) provides excellent resistance to stress corrosion cracking and pitting.
• Sigma Phase Prevention: Appropriate ferrite levels help prevent the formation of sigma phase during service, which can severely reduce corrosion resistance.
• Molybdenum Utilization: Ferrite helps utilize molybdenum more effectively for pitting resistance, especially in duplex stainless steels.

Negative Effects of Excessive Ferrite:

• Galvanic Coupling: High ferrite content (FN > 20) can create galvanic cells between ferrite and austenite, accelerating corrosion in some environments.
• Selective Attack: In some aggressive environments, ferrite may be preferentially attacked, leading to selective corrosion.
• Reduced PREN: Very high ferrite content can reduce the Pitting Resistance Equivalent Number (PREN) by diluting chromium and molybdenum in the austenite phase.

Optimal FN Ranges for Corrosion Resistance:

• Austenitic Stainless Steels (300 series): FN 4-12 provides the best balance of cracking resistance and corrosion performance.
• Duplex Stainless Steels (2205): FN 35-55 offers optimal resistance to pitting, crevice corrosion, and stress corrosion cracking.
• Super Duplex Stainless Steels (2507): FN 45-65 maintains the proper phase balance for maximum corrosion resistance.
• Super Austenitic Stainless Steels (6% Mo): FN 0-3 is typically acceptable due to their high alloy content providing inherent corrosion resistance.

For critical corrosion applications, consider:

• Performing ASTM G48 (pitting resistance) tests on weld samples
• Evaluating the PREN (Pitting Resistance Equivalent Number) of the weld metal
• Conducting long-term exposure tests in the actual service environment
• Using electrochemical testing methods like cyclic potentiodynamic polarization

What welding processes produce the most consistent ferrite numbers?

The consistency of ferrite numbers in welds depends significantly on the welding process used. Here’s a comparison of common processes:

Ranked from Most to Least Consistent FN:

1. Gas Tungsten Arc Welding (GTAW/TIG):
   • Produces the most consistent and predictable FN
   • Precise control of heat input and shielding
   • Minimal spatter and slag inclusions
   • Best for critical applications where FN control is essential
2. Gas Metal Arc Welding (GMAW/MIG) with Pulsed Transfer:
   • Good FN consistency with proper parameter control
   • Pulsed transfer minimizes heat input variation
   • Requires careful gas selection (typically Ar+2%O₂ or tri-mix)
   • Suitable for most production welding of stainless steels
3. Flux-Cored Arc Welding (FCAW):
   • Can produce consistent FN with proper filler metal selection
   • More sensitive to heat input variations than GTAW/GMAW
   • Slower cooling rates may affect phase balance
   • Good for out-of-position welding where GMAW isn’t practical
4. Shielded Metal Arc Welding (SMAW/Stick):
   • FN consistency depends heavily on electrode selection and welder skill
   • Higher risk of hydrogen-induced cracking with some electrodes
   • Slower deposition rates can lead to more heat input variation
   • Best for maintenance/repair where other processes aren’t available
5. Submerged Arc Welding (SAW):
   • Can produce consistent FN but requires careful parameter control
   • High heat input can significantly increase ferrite content
   • Flux composition can affect alloy pickup and FN
   • Best for thick sections where high deposition rates are needed

Factors Affecting FN Consistency Across All Processes:

• Heat Input Control: Consistent travel speed and voltage/amperage settings are crucial
• Interpass Temperature: Maintaining consistent interpass temperatures helps stabilize FN
• Shielding Gas: Proper gas composition and flow rates prevent oxidation that can affect FN
• Filler Metal Storage: Proper handling prevents contamination that could alter alloy composition
• Joint Preparation: Consistent joint fit-up ensures uniform dilution and heat input
• Welder Technique: Skilled welders produce more consistent weld beads and heat input

For processes with inherently lower consistency (like SMAW or SAW), consider:

• Using filler metals with a wider acceptable FN range
• Performing more frequent FN verification during production
• Implementing stricter process controls and welder qualification
• Using pre-qualified welding procedures with documented FN results

Can ferrite number be measured after welding? If so, how?

Yes, ferrite number can be measured after welding using several methods, each with different advantages and limitations:

1. Magnetic Measurement (Feritscope):

• Principle: Measures the magnetic response of the weld, which correlates with ferrite content
• Equipment: Portable feritscopes (e.g., Fischer Feritscope, Elcometer Ferrite Content Meter)
• Advantages:
  • Non-destructive
  • Quick and portable
  • Can measure in the field
  • Provides immediate results
• Limitations:
  • Calibration required for specific alloy systems
  • Surface condition affects readings
  • Can be influenced by residual stresses
  • Less accurate for very low or very high FN values
• Typical Use: Production quality control, field inspections

2. Metallographic Examination:

• Principle: Polished and etched cross-sections are examined under a microscope to determine phase balance
• Equipment: Optical or electron microscope with image analysis software
• Advantages:
  • Most accurate method
  • Can distinguish between ferrite and other phases
  • Provides visual confirmation of microstructure
  • Can detect harmful phases like sigma or chi
• Limitations:
  • Destructive (requires sample preparation)
  • Time-consuming
  • Requires skilled metallurgist
  • Not suitable for field use
• Typical Use: Research, failure analysis, procedure qualification

3. X-Ray Diffraction (XRD):

• Principle: Measures the crystal structure of phases present to quantify ferrite/austenite balance
• Equipment: X-ray diffractometer
• Advantages:
  • Very accurate phase identification
  • Can detect multiple phases simultaneously
  • Non-destructive for surface measurements
• Limitations:
  • Expensive equipment
  • Requires sample preparation for bulk measurements
  • Complex data interpretation
  • Not portable
• Typical Use: Research, complex alloy development

4. Electrochemical Methods:

• Principle: Different phases have different electrochemical potentials that can be measured
• Equipment: Potentiostat with specialized electrodes
• Advantages:
  • Can be non-destructive
  • Sensitive to small phase changes
  • Can correlate with corrosion behavior
• Limitations:
  • Complex setup and interpretation
  • Requires electrolyte contact
  • Surface condition critical
  • Not commonly used for routine FN measurement
• Typical Use: Research, corrosion studies

Comparison of Measurement Methods:

Method	Accuracy	Speed	Cost	Portability	Destructive?	Best For
Feritscope	Good	Very Fast	Low	Excellent	No	Production QC, field inspections
Metallography	Excellent	Slow	Moderate	No	Yes	Research, failure analysis
XRD	Excellent	Moderate	High	No	Sometimes	Research, phase identification
Electrochemical	Good	Moderate	High	Limited	No	Research, corrosion studies

For most industrial applications, the feritscope provides the best balance of accuracy, speed, and practicality. Metallographic examination should be used when precise phase identification is required or when investigating welding problems.

How does ferrite number relate to the Schaeffler and DeLong diagrams?

The ferrite number (FN) system is closely related to but distinct from the Schaeffler and DeLong constitution diagrams. Here’s how they compare:

1. Schaeffler Diagram (1949):

• Purpose: The first constitution diagram for predicting stainless steel weld microstructure
• Axes:
  • Chromium equivalent (Cr_eq) = %Cr + %Mo + 1.5×%Si + 0.5×%Nb
  • Nickel equivalent (Ni_eq) = %Ni + 30×%C + 0.5×%Mn
• Output: Predicts microstructure zones (fully austenitic, austenitic-ferritic, martensitic, etc.)
• Limitations:
  • No quantitative ferrite measurement
  • Doesn’t account for nitrogen
  • Less accurate for modern high-alloy stainless steels
• Relation to FN: The Schaeffler diagram provides qualitative predictions that correlate with FN ranges but isn’t directly convertible to FN values.

2. DeLong Diagram (1974):

• Purpose: Improved constitution diagram that introduced the concept of ferrite number
• Axes:
  • Cr_eq similar to Schaeffler but with refined coefficients
  • Ni_eq = %Ni + 30×%C + 0.5×%Mn + 30×%N
• Output:
  • Introduced ferrite number (FN) as a quantitative measure
  • FN contours overlaid on the diagram
  • More accurate for modern stainless steels
• Relation to FN: The DeLong diagram was the first to directly incorporate FN values, though the scale was later refined in the WRC-1992 diagram.

3. WRC-1992 Diagram:

• Purpose: Current standard for FN prediction, replacing the DeLong diagram
• Axes:
  • Cr_eq = %Cr + %Mo + 1.5×%Si + 0.5×%Nb
  • Ni_eq = %Ni + 30×%C + 0.5×%Mn + 30×%N
• Output:
  • Detailed FN contours (0, 4, 8, 12, 20, etc.)
  • More accurate for FN > 20 (important for duplex stainless)
  • Better correlation with actual ferrite measurements
• Relation to FN: This is the current standard diagram for FN prediction, used by our calculator.

Key Differences and Evolution:

Feature	Schaeffler (1949)	DeLong (1974)	WRC-1992
Ferrite Measurement	Qualitative (microstructure zones)	Introduced FN concept	Refined FN contours
Nitrogen Consideration	No	Yes (in Nieq)	Yes (improved)
Accuracy for FN > 20	Poor	Moderate	Good
Modern Alloy Applicability	Poor	Moderate	Good
Duplex Stainless Steels	Not suitable	Marginal	Good
Current Usage	Historical reference	Occasionally used	Industry standard

The evolution from Schaeffler to DeLong to WRC-1992 reflects:

• Improved understanding of stainless steel metallurgy
• Development of new alloy systems (especially duplex stainless steels)
• Better measurement techniques for ferrite content
• Increased need for quantitative predictions in industrial applications

For most modern applications, the WRC-1992 diagram is preferred. However, understanding the historical development helps in interpreting older welding procedures and research data that may reference the Schaeffler or DeLong diagrams.