Molybdenum has been found to influence the complex precipitation process in a martensitic precipitation hardening stainless steel during aging at 475 °C in several different ways. Three steels with different Mo content (0, 1.2 and 2.3 at.%) were investigated. Studies of the microstructure were performed with atom probe tomography and energy filtered transmission electron microscopy. It is shown that, at the initial stage of aging, a faster nucleation of Cu-rich clusters takes place with increasing Mo content. The Cu-clusters act as precipitation sites for other solute elements and promote the nucleation of Ni-rich phases. During further aging, a higher Mo content in the material instead slows down the growth and coarsening of the Ni-rich phases, because Mo segregates to the interface between precipitate and matrix. Additionally, Mo promotes decomposition of the matrix into α and α′ regions. After longer aging times (>40 h) quasicrystalline Mo-rich R′ phase forms (to a greater extent in the material having the highest Mo content). The observations serve to understand the hardness evolution during aging.
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