how to check ferrite in stainless welds


Ferrite measurements methods or techniques

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. Other methods such as using electron diffraction such as X-ray diffractometry (XRD) or electron backscatter diffraction (EBSD) are also used but they are quite expensive.

Mostly, measurement using image analysis from the microscope and manual measurements using Ferritoscope are widely used in the industries due to their ease to use and quicker operations.

However, in magneto-inductive testing, the reading is a function of the chemical composition of the alloy. Therefore, when dealing with high ferrite content the indicated ferrite content does not correspond to the actual content in the material, since high-alloyed ferrite tends to have a weaker magnetic reaction than the same ferrite content does with a lower alloy potential. For these reasons, the Ferrite number (FN) was introduced for indication of the content of ferrite (EN ISO 8249 and EN ISO 17 655).

Calculate ferrite content in stainless steel & duplex stainless steel

Ferrite measurement using Image analysis

The image analysis method for the ferrite measurement requires polished macro samples that are analyzed under high magnification. The Light optical microscope is used with special software to check the small areas of each phases presents. The method is most reliable along with point counting, although the operator skills are very important. The accuracy of the method depends on the macro sample preparation, etching, and polishing quality as they can highly affect the outcome. The technique follows the guidelines of ASTM E 562 (Standard Test Method for Determining Volume Fraction by Systematic Manual Point Count) and requires that the magnification shall be adequate to allow clear resolution of the microstructure. For stainless steel, duplex stainless steel, and other nickel base alloy, usually, magnification in the range of 500X to 1000X is used. Here, as the magnification is increased, the area of the field will decrease. So, a number of measurements are required.

The process of the image analysis method is having a different number of steps such as cutting the specimen, preparing the micro specimen, polishing, and etching followed by final measurement and then reporting the results. All these steps make this technique time-consuming.

Ferrite measurement using Ferrioscope

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.

Ferritscope is a non-destructive and fast result-producing technique for ferrite measurement in the field. The accuracy however depends upon the equipment calibration, operator skills, and surface conditions. Equipment needs to be calibrated before use using the calibration blocks otherwise the results can be misrepresented.

Simple Ferrite Calculation methods

The ferrite content in the weld metal can be predicted from its chemical composition using the ESPY diagram (results in ferrite percent F%) or the WRC-1992 diagram (results in FN). The other methods (SCHAEFFLER or DELONG) which do not consider nitrogen and copper contents cannot be used for duplex welds.

ferrite number to ferrite percentage conversion

You can easily convert ferrite number (FN) to ferrite percentage using the formula. The Ferrite Number, up to 10 FN, is to be considered equal to the percent ferrite term previously used. So up to FN 10, it will be the same Ferrite percentage value.

To convert from ferrite % to FN, the following can be used:

  1. For 22 % Cr duplex stainless steel, ferrite % = 0.7 × FN;
  2. For 25 % Cr duplex and super duplex stainless steels, ferrite % = 0.65 × FN.
  3. FN = (vol% ferrite) x [-0.025813 (Fe)2 + 5.408679 (Fe) – 102.3902]/100 (Reference:  Welding Journal, 76(1): 24-s to 37-s, 1997)

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. As stated earlier, up to FN 10 it is equal to F% but with the higher value, these both terms do not correspond to each other and vary.

The WRC (Welding Research Centre, America) Subcommittee has adopted the term Ferrite Number (FN) to be used in place of percent ferrite, to clearly indicate that the measuring instrument was calibrated to the WRC procedure. The Ferrite Number, up to 10 FN, is to be considered equal to the percent ferrite term previously used. It represents a good average of commercial U.S. and world practice on the percent ferrite. Through the use of standard calibration procedures, differences in readings due to instrument calibration are expected to be reduced to about ±5 percent, or at the most, ±10 percent of the measured ferrite value.

Roles of Ferrite in Welds

Ferrite is known to be very beneficial in reducing the tendency for cracking or fissuring in weld metals; however, it is not essential. Millions of pounds of fully austenitic weld metal have been used for years and provided satisfactory
service performance. Generally, ferrite is helpful when the welds are restrained, the joints are large, and when cracks or
fissures adversely affect service performance. Ferrite increases the weld strength level. Ferrite may have a detrimental
effect on corrosion resistance in some environments. It is also generally regarded as detrimental to toughness in cryogenic service, and in high-temperature service where it can transform into the brittle sigma phase.

ferrite test acceptance criteria

Ferrite Acceptance for Duplex Stainless Steel

Ferrite content shall be within the following ranges:

  1. a) Weld metal—30 % to 65 %,
  2. b) HAZ—40 % to 65 %,
  3. c) 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. When a nickel alloy consumable is used, the ferrite content in the weld deposit is not required to be tested.

NOTE 2 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.

In the E3XX classifications, many types such as E310, E310Mo, E310Nb, E316LMn, E320, E320LR, E330, E383, E385, and E3155, and E31-33 are fully austenitic. The E316 group can be made with little or no ferrite and generally is used in that form because it has better corrosion resistance in certain media. It also can be obtained in a higher ferrite form, usually over 4 FN, if desired. Many of the other E3XX classifications can be made in low-ferrite versions, but commercial practice usually involves ferrite control above 4 FN. Because of composition limits covering these grades and various manufacturing limits, most lots will be under 10 FN and they are unlikely to go over 15 FN, E308LMo and E309L can have ferrite levels in excess of 15 FN. E16-8-2 generally is controlled at a low-ferrite level, under 5 FN; E309LMo, E312, E2209, E2307, E2553, E2593, and E2594, and E2595 generally are quite high in ferrite, usually over 20 FN.

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