Calculate ferrite content in stainless steel & duplex stainless steel


Importance of Ferrite in weld

The control of the ferrite content in duplex steel welds is very important. A too high ferrite content in HAZ or weld metal may lead to low toughness properties, low ductility, and increased corrosion cracking susceptibility of the welded area.

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.

Measurement methods

The most common measurement methods to determine the ferrite content in weld metal are magnetic methods as ferritscope (results in F% or FN) or magne-gage (results in FN).

Note that they cannot be used for very localized measurements. Indeed, the metallographic grid method as per ASTM E562 with sufficient magnification is the only way to obtain a relevant result in HAZ. Based on general practice and experience, a magnification higher or equal than x 400 is recommended for ferrite measurement.

Ferrite measurement can be performed by other methods, for instance by image analysis according to ASTM E1245.

Range of Ferrite Content for Duplex Stainless-Steel Welding Qualification:

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.

Influence of Elements and Cooling rate on % Ferrite

Fast Cooling — High Ferrite

Slow Cooling — Low Ferrite

Effect of Ferrite & austenite in Duplex Stainless Steel

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