On the Strain Hardening and Tensile Properties of AISI 4140 Steel via Single-Step Quenching and Partitioning Treatments

The present study explores the application of single-step quenching and partitioning (QP) treatments to low-silicon commercial AISI 4140 steel. Fractions between 4.6% and 7.8% of retained austenite (RA) are stabilized at room temperature. Specimens quenched at 240 °C and partitioned for 10 min exhibit the highest tensile strength, surpassing 2000 MPa, with an elongation at break of up to 10%. This condition marks the effectiveness of austenite in improving tensile properties. Partitioning times exceeding 5 min lead to a second deformation stage, linked to an intense strain-induced transformation of austenite into martensite. Comparative quenching and tempering (QT) treatments are conducted to highlight the role of RA in enhancing ductility. The QP samples show improved ductility and strain hardening. Furthermore, an energetic approach is employed to highlight the stability of RA by analyzing the difference in the tensile curves observed between QP and QT conditions. A critical value for stability is reached between 5 and 10 min of partitioning. The work proves the viability of successfully applying QP on AISI 4140 achieving high mechanical properties and improved ductility.