Strain‐Induced Martensitic Transformation During Uniaxial Tension and Low Cycle Fatigue in a 0.4% C Direct Quenched and Partitioned Martensitic Steel

Strain‐induced martensitic transformation is studied in a 0.4% C martensitic steel that is hot rolled, direct quenched to the M s –M f temperature range and partitioned (DQP) to yield finely divided ~14.7% retained austenite (RA) in martensite, under tension and strain‐controlled low cycle fatigue (LCF). For comparison, the same steel under un‐partitioned condition (DQ) having 5.8% RA is studied. The martensitic transformation is tracked by interrupting the tensile and LCF tests, and evaluating the %RA contents by X‐ray diffraction. During tensile testing, the RA progressively transforms to martensite beyond the yield strength, transforming nearly completely at the ultimate tensile strength. Under LCF, phase transformation starts early and completes at fracture. Cyclic hardening, saturation and softening near the fracture are observed with hardening being more prominent in DQP condition. The total strain amplitude versus reversals‐to‐failure relation is comparable for both DQP and DQ conditions. Although higher plasticity in the DQP condition is expected to give higher fatigue life, the cooccurrence of phase transformation produces similar microstructures in both steels during the course of fatigue, thus giving similar fatigue lives. Importantly the phase transformation is controlled by the cumulative plastic strain energy absorbed during fatigue rather than stress experienced during fatigue.