Microstructural evolution in 13Cr–8Ni–2.5Mo–2Al martensitic precipitation-hardened stainless steel

The microstructure of 13Cr–8Ni–2.5Mo–2Al martensitic precipitation-hardened (PH) stainless steel has been investigated using transmission electron microscopy, three-dimensional atom probe and small-angle X-ray scattering. A high number density (∼1023–25 m−3) of ultra-fine (1–6 nm) β-NiAl precipitates are formed during aging at 450–620 °C, which are spherical in shape and dispersed uniformly with perfect coherency with the matrix. As the annealing temperature increases, the size and concentration of the precipitates increase concurrently while the number density decreases. The Mo and Cr segregation to the precipitate–matrix interface has been detected and is suggested to suppress precipitate coarsening. In the sample aged for 500 h at 450 °C, the matrix decomposes into Cr-rich (α′) and Cr-poor (α) regions. The decrease in the strength at higher temperature (above 550 °C) is attributed to the formation of larger carbides and reverted austenite.