Aging 17-4 PH martensitic stainless steel prior to hardening: effects on martensitic transformation, microstructure and properties

Precipitation hardening martensitic stainless steel 17-4 PH is conventionally austenitized and air cooled to room temperature to form martensite. On aging the martensitic condition tiny Cu-rich precipitates are formed that provide high strength. In the present investigation, the steel was aged in austenitic condition prior to martensite formation. Dilatometry, transmission electron microscopy, atomic probe tomography, synchrotron X-Ray diffraction, electron backscatter diffraction, hardness tests and tensile tests were applied to study austenite aging and its effects on: (i) the subsequent transformation of austenite into martensite, (ii) the microstructure of the forming martensite and (iii) the mechanical properties of the material. Austenite aging favors early formation of Cu clusters followed by precipitation of Cu particles. The evolution of the Cu precipitate size with aging time follows traditional Ostwald ripening kinetics which is rate controlled by Cu diffusion in austenite. Austenite aging affects the kinetics of martensite formation and the substructure of the martensite laths; overall, it provides significant strengthening to the martensitic material. The contribution of precipitates to the strength of martensite is interpreted in terms of the Russell-Brown model for modulus strengthening. The data and the model are reconciled by re-evaluating the adjustable parameter used in the original work. Finally, the work reports the influence of austenite aging on the mechanical properties of the material when this is further aged to peak strength in martensitic condition.