How many methods are there for measuring ferrite content in stainless steel

Ferrite content in stainless steel, particularly δ-ferrite, plays a critical role in determining the material’s mechanical properties, corrosion resistance, and weldability. Accurate measurement of ferrite content is essential, especially for castings, welds, and components used in demanding environments such as chemical plants and offshore structures.

There are three commonly used methods to measure δ-ferrite content in stainless steel: magnetic measurement, metallographic examination, and calculation based on chemical composition.

Measuring Ferrite Content in Stainless Steel

1.Magnetic Measurement Method

The magnetic method is a direct and widely accepted technique for measuring δ-ferrite content in austenitic stainless steels and their welds.

Since δ-ferrite is ferromagnetic and austenite is non-magnetic, the magnetic response of the material is proportional to the amount of δ-ferrite present.

Using a specialized magnetic measuring device, such as the Fischer Feritscope FMP30 from Germany, ferrite content can be determined quickly and accurately.

This method is non-destructive and ideal for quality control during production and inspection stages.

2.Metallographic Examination

The metallographic method involves analyzing the microstructure of stainless steel under a microscope.

δ-ferrite appears as small, discontinuous “pits” uniformly distributed within the austenitic matrix.

By observing these features under a metallographic microscope and comparing the observed ferrite area fraction with standard reference micrographs defined in national or industry standards, the δ-ferrite content can be evaluated.

This method provides a visual confirmation of ferrite distribution and is especially useful for detecting inhomogeneities.

3.Calculation Method

The calculation method estimates ferrite content based on the chemical composition of the material.

By using specified formulas to calculate the chromium equivalent (Creq) and nickel equivalent (Nieq), the ferrite content can be determined by locating the coordinates on a stainless steel constitution diagram (such as the Schaeffler, Delong, or WRC-1992 diagrams).

This method is convenient and does not require specialized equipment; as long as the chemical composition report is available, ferrite content can be estimated quickly.

The accuracy of the calculation depends on selecting the appropriate constitution diagram and correct Creq and Nieq formulas.

Importance of Ferrite Content Measurement

Measuring δ-ferrite content accurately ensures the mechanical integrity, corrosion resistance, and service performance of stainless steel components.

Improper ferrite content can lead to reduced toughness, increased risk of cracking, or compromised corrosion resistance, especially in critical applications like petrochemical plants, nuclear reactors, and marine environments.