Microstructure and nanoindentation studies of M₂₃C₆ carbides in Fe-18Cr-8Ni steel after long-term ageing at high temperature

ABSTRACT

Understanding the M₂₃C₆ carbides precipitation sequence aims to determine a correct heat treatment design in order to optimise the mechanical properties of the Fe-18Cr-8Ni steel. In this paper, the ageing influence at 800°C on the mechanical properties and microstructure of this steel has been studied. The aged material was examined with nanoindentation, macrohardness, scanning electron microscopy, light microscopy, and X-ray diffraction techniques. The hardness test results showed a continuous softening of the Fe-18Cr-8Ni steel when increasing the treatment time up to 6000 minutes. The nanoindentation tests revealed a decrease in the nanohardness of the austenite and M₂₃C₆ carbides as the ageing time increased. Before the ageing process, the solution treatment produced completely austenitic steel. After 30 and 60 minutes of ageing, the M₂₃C₆ carbides were located at the grain boundaries. The precipitation of the M₂₃C₆ carbides within the grains was observed from 300 to 6000 minutes of ageing. The coarsening of the M₂₃C₆ carbides occurred at high holding times.

KEYWORDS:

• Fe-18Cr-8Ni steel
• ageing
• nanoindentation
• microstructure
• M₂₃C₆ carbides
• precipitation
• coarsening

Disclosure statement

No potential conflict of interest was reported by the author(s).