The tensile and impact properties, the weldability, and the corrosion resistance of iron-chromium-nickel alloy castings may be influenced beneficially or detrimentally by the ratio of the amount of ferrite to the amount of austenite in the microstructure. The ferrite content may be limited by purchase order requirements or by the design construction codes governing the equipment in which the castings will be used. The quantity of ferrite in the structure is fundamentally a function of the chemical composition of the alloy and its thermal history. Because of segregation, the chemical composition, and, therefore, the ferrite content, may differ from point to point on a casting. Determination of the ferrite content by any of the procedures described in the following practice is subject to varying degrees of imprecision which must be recognized in setting realistic limits on the range of ferrite content specified. Sources of error include the following:

In Determinations from Chemical Composition-Deviations from the actual quantity of each element present in an alloy because of chemical analysis variance, although possibly minor in each case, can result in substantial difference in the ratio of total ferrite-promoting to total austenite-promoting elements. Therefore, the precision of the ferrite content estimated from chemical composition depends on the accuracy of the chemical analysis procedure.

In Determinations from Magnetic Response-Phases other than ferrite and austenite may be formed at certain temperatures and persist at room temperature. These may so alter the magnetic response of the alloy that the indicated ferrite content is quite different from that of the same chemical composition that has undergone different thermal treatment. Also, because the magnets or probes of the various measuring instruments are small, different degrees of surface roughness or surface curvature will vary the magnetic linkage with the material being measured.

In Determinations from Metallographic Examination-Metallographic point count estimates of ferrite percentage may vary with the etching technique used for identification of the ferrite phase and with the number of grid points chosen for the examination, as explained in Test Method E562.

The estimation of ferrite percent by chemical composition offers the most useful and most common method of ferrite control during melting of the metal.

For most accurate estimate of ferrite percent, a quantitative metallographic method should be used.