If free ferrite is present, the free-ferrite layer’s maximum depth (often variable) plus the depth of partial decarburization to the unaffected core is measured. This total—FFD + PDD—is called maximum affected depth (MAD).These depths are not uniform and can vary substantially, leading to measurements of average FFD, PDD, and MAD, as well as maximum values for each. ASTM E1077 covers decarburization measurement.