Microstructure evolution at high temperature of chromium-rich iron-based alloys containing hafnium carbides

Abstract

Abstract The high temperature behavior of three iron-based alloys rich in chromium and containing HfC carbides was studied in comparison with two ternary Fe – Cr – C alloys. All these alloys, contain 25 wt.% Cr, 0.25 or 0.50 wt.% C, and additionally 3.7 to 5.6 wt.% Hf for three of them. All alloys were produced in a foundry. They were exposed to air at 1 200 °C for 46 h. Their microstructures in the as-cast state and after exposure to high temperature were characterized using X-ray diffraction, electron microscopy and image analysis. The alloys were also subjected to indentation tests in their as-cast states and in their aged states. The obtained microstructures are composed of a dendritic bcc iron-based matrix and of HfC carbides. These carbides are of two types: script-like eutectic carbides mixed with matrix, and compact pre-eutectic carbides. Some chromium carbides were additionally obtained in two of the three Hf-containing alloys. These microstructures have roughly evolved during the high temperature exposure: variations of the surface fraction of the chromium carbides and increase in the HfC surface fractions. Consequently, the hardness was more (ternary alloys) or less (Hf-containing alloys) decreased. The morphologies of the carbides evolved: coalescence of the chromium carbides, coalescence and fragmentation of the HfC carbides. The behavior of these alloys in oxidation at high temperature is perfectible. It is to be improved to allow the use of such alloys at high temperature.