Insight to the Microstructure Characterization of a Hp Austenitic Heat Resistant Steel After Long-Term Service Exposure

Abstract

<p>Heat-resistant steels of HP series (Fe-25Cr-35Ni) are used in high temperature structural applications. Their composition include Nb as strong carbide former. Electron Backscatter Diffraction (EBSD) investigations revealed that, in the as-cast condition, alloys exhibit austenitic matrix with intergranular primary carbides such as MC, M<sub>23</sub>C<sub>6</sub> and/or M<sub>7</sub>C<sub>3</sub>. During exposure at a high temperature, phase transformations occurred: chromium carbides of M<sub>7</sub>C<sub>3</sub> type transform into the more stable M<sub>23</sub>C<sub>6</sub> type, intergranular M<sub>23</sub>C<sub>6</sub> carbides precipitate, and Lave phase due to increase of Niobium activity with temperature increase, as thermodynamic simulation confirmed. Therefore, combination of EBSD-EDS technique with thermodynamic calculation is one of the novel and most accurate method to investigation of phase transformation, as the precipitations are identified on the basis of their crystal structure, chemical composition and their thermodynamic features. </p>

NbC TRANSFORMATION DURING AGING IN HP40-Nb HEAT RESISTANT ALLOY

“…In the as-cast condition, the microstructure of HP40-Nb alloys consists of an austenitic matrix with intergranular and interdendritic eutectic-like primary Cr-rich carbides of the M23C6 type and Nb-rich carbides of the MC type [6]. During aging, intragranular secondary M23C6 carbides precipitate within the matrix [16,17]. At operating temperatures, in Ni-Cr-Fe based alloys MC-type carbides are thermally unstable, since they would transform into a Ni-Nb silicide, known as G-phase [18][19][20].…”