Technical Sharing #1: WHY Do We Need to Measure the Amount of Ferrite for Austenitic Stainless Steel Weld? HOW to Measure?

The WHY:

• ?Fully austenitic alloys tend to be hot short, and welds in these materials show fine cracking or microfissuring, also known as liquation cracking, either in the weld metal itself or in the HAZ, while those that solidify with delta ferrite as the primary solidification mode are the least susceptible to hot cracking.
• Avoiding liquation cracking is the most important issue in fusion-welding of austenitic SS. In general, alloys that solidified with ferrite as the primary phase and containing 4%–11% by volume of ferrite in the as-welded condition were more resistant to cracking than alloys solidified as austenite.?
• In order to control microfissuring in stainless welding, the alloying elements are balanced in such a way as to attempt to produce a weld deposit containing ferrite. Following are the minimum recommended levels of weld metal ferrite content for various types of austenitic SS:
• Type 308/308L: 3 FN
• Type 309/309L: 5 FN
• Type 316/316L: 3 FN
• Type 347: 6 FN
• The FN is a magnetically determined scale of ferrite measurement. This provides sufficient control in most applications where minimum ferrite content or a ferrite range is specified.

The HOW:

4 Methods for the determination of ferrite at room temperature:

(1) Metallographic examination:

• The ferrite content is determined by microscopic analysis. This method provides a STATISTICAL determination of the amount of ferrite.?
• One of the common statistical methods of metallographic examination is by POINT-COUNT analysis, where evenly spaced points are superimposed on a field on the specimen and the points found on a constituent are counted. A large number of fields are sampled to ensure statistical accuracy.?
• The drawbacks associated with this method include the following: Examination is DESTRUCTIVE in nature, interpretation is subject to error, and due to variations in etching, lab-to-lab reproducibility is sometimes poor.

(2) Constitution diagrams:

• ?Schaeffler, DeLong & WRC 1992 diagram
• Used as GUIDANCE to choosing a particular consumable or for an approximation of ferrite content and NEVER as a final word regarding weld metal ferrite content. Where accurate, ferrite measurement is required, magnetic instruments should be used, and the consumable manufacturer’s recommendations followed.

(3) Magnetic measuring instruments:

• ?Ferrite is ferromagnetic, while austenite is not; Ferrite determinations by magnetic means using FERRITE GAUGES are reproducible, and this is a non-destructive test.
• Standard methods of ferrite determination by magnetic methods for industrial use are discussed in ANSI/AWS A4.2 and ISO 8249.?
• Portable ferrite gauges designed for on-site use. Various trade names for magnetic measuring instruments are Magne-Gage, Ferritescope, Severn gauge, Foerster gauge, and Elcometer.
• Methods of measuring ferrite as per magnetic properties of steels and their principles are given as follows:?
• a. Ferrite indicator: comparing the magnetic attraction between a standard ferrite percent insert and a test specimen
• b. Ferrite scope: measuring the change of magnetic induction affected by the ferrite content of a test specimen
• c. Magne-Gage: measuring the pull-off force necessary to detach a standard permanent magnet from a test specimen.

(4) X-rays:

• ?Relies on the fact that all materials consist of different crystal structures.
• ?The intensities of DIFFRACTION LINES of the different phases allow the determination and measurement of their presence.
• Only possible on the surface thickness (approximately 2 mils) and can only measure amounts of ferrite in excess of 3 FN.?
• Costly, complicated, and not always practical.

References:?

?1. AWS Standards & publications?

?2. API RP 577?

?3. API RP 582

4. Heat Exchanger Design Handbook-Kuppan Thulukkanam