04/12/2025
Ferrite Testing is a process used to determine the ferrite content (delta ferrite) in Duplex stainless steel (e.g. 2205 grade) and austenitic stainless steels (e.g. 316, 321 Grade).
Ferrite testing is important because it can be used to predict the behavior of austenitic steels and duplex stainless steels during welding. For example, steels with high ferrite content are more likely to have reduced corrosion resistance than those with low ferrite content.
A too low ferrite content makes material or weldment highly susceptible for hot cracking or also called solidification Cracking.
High ferrite content also decreases the mechanical properties (mainly tensile, toughness & hardness) of the material.
The determination of the ferrite content in austenitic steels can be made either metallographically ( E.g. planimetry, lineal analysis, or differential point counting) by using micro-sections (destructive methods) or magneto-inductively ( for example using Ferritoscope) (nondestructive methods) directly at the weld.
Mostly, measurement using image analysis from the manual measurements using Ferritoscope are widely used in the industries due to their ease to use and quicker operations.
Ferritoscopes are handheld instrument works on the principle of magneto-inductivity. It relies on the technique of magnetic induction to make the ferrite measurement.
During testing, ferritoscope measures all magnetic areas and leaves all non-magnetic microstructure, thus producing an overall ferrite content in the austenitic phase.
Equipment needs to be calibrated before use using the calibration blocks otherwise the results can be misrepresented.
Ferrite number to ferrite percentage conversion easily convert ferrite number (FN) to ferrite percentage using the formula. Up to 10 FN Ferrite is equal to the percent ferrite (FN%). So up to FN 10, it will be the same Ferrite percentage value.
To convert from ferrite % to FN, the following can be used:
For 22 % Cr duplex stainless steel, ferrite % = 0.7 × FN;
For 25 % Cr duplex and super duplex stainless steels, ferrite % = 0.65 × FN.
Ferrite Number (FN) = (ferrite%) x [-0.025813 (Iron)2 + 5.408679 (Iron) – 102.3902]/100
Another method to convert FN to Ferrite percentage is using the Delong diagram.
Difference between Ferrite Number & Ferrite Percentage
Ferrite Number is known as (FN) and Ferrite percentage is written as F% where F is a numeric value.
During in field testing and following the standard calibration methods, is it expected to have some variation in ferrite measurements about ±5 percent, or at the most, ±10 percent of the measured ferrite value and this need to considered in the acceptance criteria.
Ferrite number to ferrite percentage conversion is an amount of ferrite in an alloy is measured by its ferrite number. This number can be converted to a percentage, which will tell you how much of the alloy is made up of this essential element.
Converting ferrite numbers to percentages can be done by using a simple formula. To convert from ferrite % to FN, the following can be used:
See also ER316L Filler Wire
-For 22 % Cr duplex stainless steel,
Ferrite % = 0.7 × FN;
For 25 % Cr duplex and super duplex stainless steels,
Ferrite % = 0.65 × FN.
What is the roles of Ferrite in Welds?
In welding ferrite is beneficial and detrimental both- but this is a property that is reviewed & considered during design.
Ferrite is not good when we need material for cryogenic temperature application but in welding ferrite is very good as it minimize the chances of hot cracking or fissures (micro crack visible at minimum 50X magnification) during welding.
It is good to have some amount of ferrite in austenitic stainless steel welds to resists the weld cracking that might occurs during solidification.
Having ferrite in welds avoid micro cracks which could otherwise propagate during product service and can be serious damage. Ferrite improve the strength in steel and maintain lower hardness.
Ferrite Test Acceptance Criteria ASME
Ferrite Acceptance for Duplex Stainless Steel
For duplex stainless steel, the minimum and maximum range of ferrite number in the welding, HAZ and base metal shall be as following (Ref: API 582):
Weld metal—30 % to 65 %,
HAZ—40 % to 65 %,
Base metal—40 % to 60 %.
NOTE 1 Ferrite content down to 25 % may be acceptable for the weld metal if corrosion or other tests are satisfactory to the purchaser.
Nickel being a strong austenitic stabilizer eliminate the need to check ferrite content for weld which are welded using a nickel base consumables such ERNiCrMo-3 or ENiFeCr-2 etc.
To convert from ferrite % to FN, the following can be used: For 22 % Cr duplex stainless steel, ferrite % = 0.7 × FN; For 25 % Cr duplex and super duplex stainless steels, ferrite % = 0.65 × FN.
Ferrite Acceptance for Austenitic Stainless Steel
Ferrite content or FN number for austenitic welds are usually restricted based on the welding consumable used.
Most of welding consumables in the E3### or ER3### classification as per specification AWS A5.4 and 5.9 are fully austenitic type.
Examples of such rods are: E310 types, E316LMn, E320, E320LR, E330, E383, and E385 etc. Welds made with ER316 or E316 usually may have little ferrite as it gives better corrosion protection.
The ferrite number in above electrodes are kept above FN4 to have better crack resistance properties.
Welding rods such as ER308LMo and E309L, E309LMo can have ferrite (FN) more than FN15. E16-8-2, a special welding rod with low ferrite number is used for welding of sensitized austenitic stainless steel welds are made with very low ferrite content.
Welding consumables for duplex stainless steels such as ER2594, E2595, E2209 are made with higher ferrite, generally more than 20FN.
