| Abstract: | A study has been carried out on the effects of isothermal heat treatment at 475 and 550‡C and of continuous cooling at different
rates from 850°C on the brittleness (as assessed by the ductile-brittle impact transition temperature) of a vacuum melted
Fe-25 Cr alloy. The ductile-brittle transition temperature was found to be the lowest for the water quenched condition and
highest for the furnace cooled condition and for material aged at 475‡C for long times (~500 h). An increase of brittleness
with decreased cooling rate in the continuously cooled samples is attributed to the formation of more continuous and larger
amounts of chromium nitrides and carbonitrides at the grain boundaries. Very little or no body centered cubic chromium-rich
phase (alpha prime), associated with 475°C embrittlement, was observed. On aging at 550°C, the increased brittleness with
time is also accounted for by the formation of grain boundary nitrides and carbonitrides. Although a similar effect takes
place in the alloy heat treated at 475°C, the precipitation of alpha prime after long aging times enhances the brittleness.
The tendency towards a more brittle condition with aging treatment and slower cooling rate is explained in terms of the Cottrell
theory for brittle fracture. |
