Aging condition and trapped hydrogen effects on the mechanical behavior of a precipitation hardened martensitic stainless steel

PH13-8Mo is a precipitation hardened martensitic stainless steel, known for its high strength but also for its high sensitivity to hydrogen embrittlement. The aging condition of the alloy is known to play a key role in the mechanical behavior of the as-hydrogen-charged state. Yet the exact effects of residual trapped hydrogen on this behavior were not explored. In this study a novel simple methodology utilizing the small punch test (SPT) and thermal desorption analysis (TDA) is developed. It is used to investigate the effects of hydrogen trapping at different aging conditions on the mechanical properties and fracture behavior of PH13-8Mo steel. The effects of hydrogen were tested at the fully-aged (550 °C for 4 h) and over-aged (600 °C for 4 h) conditions. Samples were cathodically hydrogen charged and were tested in both the as-charged condition and also after holding at ambient condition for specific delay times. SPT load-displacement behavior and fractographic scanning electron microscopy (SEM) examinations were performed. Transmission electron microscopy (TEM) and x-ray diffraction (XRD) analyses were used as complementary characterization tools. It is shown that the over-aged condition (related specifically to the reverted austenite content), in contrast to the general notion, exhibits an embrittling effect on the mechanical behavior due to residual trapped hydrogen. The significance of these findings and their relation to the microstructure are discussed.