Microstructural changes during the annealing of ductile iron alloyed with vanadium

Abstract

Ductile irons have been microalloyed with up to 0.15% V and heat treated to produce precipitation hardening in a fully ferritic matrix. This has been achieved by two stage annealing, consisting of austenitising in the range 900–1050°C, followed by cooling at different rates to a second holding temperature and subsequent cooling to room temperature. Dilatometry has been used to monitor the precipitation of vanadium carbide during heat treatment and showed that the onset of precipitation in austenite occurred at about 860 °C. Electron microscopy has revealed three types of carbide morphologies and analysis showed them to be VC_0.75. The precipitate size has been measured and related to the cooling rate and holding temperature. Vanadium has also been shown to have a grain refining effect, although this became less marked as the austenitising temperature and/or time were increased.

Keywords:

• ductile iron
• annealing
• precipitation hardening
• grain size refinement