Precipitation process of martensitic PH stainless steel Nanoflex

Abstract

The precipitation process of the martensitic PH stainless steel Nanoflex during aging at 475°C has been investigated with atom probe tomography. The composition of the matrix and the nanosized precipitates has been determined after aging for 5 min, 4 h, 40 h and 100 h. Also, the number density of the various precipitated phases has been measured. It is shown that the precipitation process is rather complex and that different precipitate phases form in direct contact with each other. At the early stages two families of Ni rich precipitates, Ni₃(Ti, Al) and Ni₃(Ti, Al, Si), form on Cu rich precipitates. Later, Cr rich precipitates form on the Ni rich precipitates. Eventually the Ni₃(Ti, Al, Si) precipitates dissolve and are replaced by Ni₁₆Si₇Ti₆. Precipitates rich in Mo, most probably the quasicrystalline R’ phase, are also formed in direct contact with other precipitates. The identity of the precipitates is discussed further in the paper. Even though the number density of the precipitates decreases during aging, the hardness does not decrease. This can probably be explained by an increased total volume fraction of precipitates as additional phases appear during aging.

Keywords:

• Atom probe tomography
• Aging
• Precipitation
• Stainless steels
• Precipitation hardening
• Martensite

This work was supported by the Swedish Research Council and Sandvik Materials Technology.