# The Evaluation of Thermal Stability, Electric Conductivity and Carbide Morphology of Austenitic Ductile Iron Castings

## TL;DR

This study examines the properties of Ni-Resist ductile iron, focusing on its stability at low temperatures and electrical conductivity.

## Contribution

The study identifies the minimum nickel content needed for cryogenic stability and characterizes microstructural features in Ni-Resist ductile iron.

## Key findings

- A minimum of 25 wt.% nickel ensures structural stability at cryogenic temperatures down to 25 K.
- Chromium-alloyed ductile irons show Cr enrichment near M7C3-type carbides.
- Electrical conductivity was measured for high-nickel austenitic cast irons.

## Abstract

The Ni-Resist ductile iron, with a nickel content ranging from 18% to 36%, is a material designed for service under extreme conditions. One of the main objectives of this study was to determine the minimum nickel content required to stabilize the austenitic structure at cryogenic temperatures. Additional aims included investigating structural features related to the solidification of austenite dendrites, graphite nodules, and eutectic carbides. Moreover, the electrical conductivity, which is critical for certain applications of Ni-Resist ductile irons, was also examined. To this end, castings with varying nickel content (21%, 25%, 28%, and 35%) and with or without chromium additions were prepared. Microstructural characterization was performed using optical, scanning, and transmission electron microscopy, X-ray diffraction (XRD), and electrical conductivity measurements. The results showed that a highly branched dendritic microstructure predominates, with graphite nodules located in interdendritic regions and along austenite grain boundaries. In chromium-alloyed ductile irons, the austenitic matrix contains Cr = 1.7 ± 0.3 wt.% in the vicinity of M7C3-type eutectic carbides. Furthermore, thermal stability analysis indicated that a minimum nickel content of 25 wt.% is sufficient to ensure structural stability at cryogenic temperatures down to 25 K. Finally, complementing the above-mentioned investigations, the electrical conductivity characteristics of the studied high-nickel austenitic cast irons were determined.