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

Abstract

17-4PH martensitic steel is usually used as valve stems in nuclear power plants and it suffers from thermal aging embrittlement due to long-time service in a high-temperature and high-pressure environment. Here, we characterized the evolution of microstructures at the nano-scale in 17-4PH steel by in situ small-angle neutron scattering (SANS) with a thermo-mechanically coupled loading device. The device could set different temperatures and tensile so that an in situ SANS experiment could dynamically characterize the process of nanoscale structural changes. The results showed that with increasing thermal aging time, the ε-Cu phase precipitates and grows as the temperature is 475 °C and 590 °C, and the ε-Cu phase is spherical at 475 °C but became elongated cylinders at 590 °C. Moreover, the loading stress could aid in the growth of the ε-Cu phase at 475 °C.

Influence of Heat Treatment Parameters on the Microstructure of 17-4 PH Single Tracks Fabricated by Direct Energy Deposition

“…Conversely, with an increase in the aging time (corresponding to the HT4 and HT5 treatments), a decrease in the mean microhardness was observed. As already mentioned in [52], with long-term aging at 480 • C, the CRPs start to coarsen, leading to an increase in their size and a decrease in their spatial density. However, it should be highlighted that, for longer aging times, small Nb carbides start to precipitate, thus contributing, to a minor extent, to the strengthening of the material.…”

“…The subsequent aging treatment performed at 480 • C after rapid quenching was expected to promote the precipitation of nanometric Cu-rich particles and at least Nb carbides [48]. It is worth highlighting that, during the first 30-60 min of the aging treatment at 480 • C, the steel was expected to achieve its maximum hardness due to the precipitation of Cu clusters, which nucleate and grow coherently with the matrix until reaching the optimized size and density [48,52]. The TEM image shown in Figure 7a, obtained in the STEM-HAADF imaging mode to promote the compositional contrast, shows an example of a Nb carbide particle.…”