Microstructural evolution of a modified HP alloy: experimental and complementary computational study

Abstract

We present results of the microstructural characterization of austenitic stainless steel (HP series) modified with Nb, aged at temperatures of 750, 800, 850, 900 and 950oC in air at different times. Microstructural changes were analyzed using optical microscopy, scanning electron microscopy (SEM) equipped with (EDS), x-ray diffraction and Vickers hardness. In the as-cast condition, the microstructure consists of an austenitic matrix and eutectic carbides network, rich carbides and other Cr, Nb-rich. The Cr-rich carbides are M7C3 type, whereas, those rich in Nb are NbC. During aging, there is a second precipitation in very fine needle form in the matrix of M23C6 carbides, which leads to an increase in hardness. Computational study shows that M23C6 is the lesser stable carbide presenting the biggest changes in the orbitals atomic populations.