SPINODAL DECOMPOSITION OF PRECIPITATION HARDENING Fe-17Cr-4Ni-4Cu STAINLESS STEEL AT 475 °C

Abstract

Microstructure evolution and mechanical properties in an Fe-17Cr-4Ni-4Cu alloy aged at 475 °C after different aging times were studied. Conventional transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM) studies revealed the formation of 9R-structure Cu-rich precipitates and Cr-rich α’ phase by spinodal decomposition in the samples aged at 475 °C after 100–1000 h. The fine Cu-rich precipitates and Cr-rich α’ phase by spinodal decomposition lead to a significant increase in the hardness, together in the early stages (100 h). Continued aging to 500 h leads to increased precipitation of the Cr-rich α’, which provides significant strengthening, reaching maximum hardening, despite the continued loss of hardening by weakening by the Ostwald ripening of the Cu-rich precipitates. Extending the aging time to 1000 h leads to substantial reversed austenite transformation and a large number of ripening -copper precipitates that causes softening. The results of the impact tests showed that the major fracture mode was cleavage and/or quasi-cleavage.

In Situ Characterization of 17-4PH Stainless Steel by Small-Angle Neutron Scattering

“…To quantitatively describe changes in the nanoscale structure, the SANS data needed to be fitted. According to previous reports, the ε-Cu phase of 17-4PH martensitic stainless steel began to precipitate after 480 °C aging, and was mostly spherical [ 25 , 26 ]. Our TEM experiment can also confirm this result.…”

“…The signal of the sample in the low Q region (Q < 0.2 nm −1 ) originates from the contribution of the relatively large carbides and grain boundaries inside the material that is larger than 30 nm; the signal in the high-Q region (Q < 0.2 nm −1 ) originates from the inhomogeneity of the small nano-precipitated phase. Studies have shown that spinodal decomposition occurs only under long-term thermal aging (>100 h) [ 25 ]. Therefore, the contribution of the scattered signal here mainly comes from the nano-precipitated phase.…”