Abstract
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.
Highlights
Precipitation hardening (PH) is the basis for high strength and toughness in martensitic PH stainless steels [1,2]
When the material is subjected to an additional heat treatment at around 450–550 ◦ C, intermetallic phases are precipitated in the matrix, which results in highly increased strength
In solution annealed of the three alloys as a function of aging time at the three alloys as a function of aging time at 475 °C is presented in Figure In materials the hardness increases with Mo content
Summary
Precipitation hardening (PH) is the basis for high strength and toughness in martensitic PH stainless steels ( called maraging steels) [1,2]. After solution annealing at 1050–1150 ◦ C and cooling, the matrix becomes supersaturated with alloying elements like Ni, Ti, Al and Cu. When the material is subjected to an additional heat treatment (aging) at around 450–550 ◦ C, intermetallic phases are precipitated in the matrix, which results in highly increased strength. As the precipitates are just a few nanometers in size and the number density is high, atom probe tomography (APT) has been an important tool in many investigations [3,4,5,6,7,8]. Due to a very complex precipitation process, the ultimate tensile strength of Nanoflex® reaches 3 GPa with retained high ductility [9]. After aging for a few hours, two sets of Ni-rich precipitates, η-Ni3 (Ti, Al) and γ0 -Ni3 (Ti, Al, Si), are observed [12]
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