Abstract:
The effect of Laves phase (Fe2Nb) formation on the Charpy impact toughness of the ferritic stainless steel
type AISI 441 was investigated. The steel exhibited good room temperature toughness after solution
treatment of 30 min at 850 ◦C, but above and below this treatment temperature the room temperature
impact toughness decreased sharply. In as received and already brittle specimens where different volume
fractions of Laves phase were introduced through isothermal equilibration at various temperatures below
850 ◦C, it was observed that a decrease in the Laves phase volume fraction with increasing annealing
temperature towards 850 ◦C (Thermo-Calc® predicted a Laves phase solvus temperature of 825 ◦C versus
a later measured solvus of 875 ◦C for this steel) corresponded to an increase in the impact toughness
of the steel. On the other hand, annealing at various temperatures above 900 ◦C where no Fe2Nb exists,
grain growth was found to also have a very negative influence on the steel’s room temperature impact
properties. Through deliberate prior grain growth and by varying the cooling rate after solution treatment,
it was found that where both a large grain size and Fe2Nb are present, it appears that the grain size of the
two is the dominant embrittling mechanism. It was possible to qualitatively relate the impact strength
results to current models on the effects of grain size and brittle grain boundary precipitates on the brittle
fracture of ferritic materials. Finally, both the presence of Fe2Nb and grain growth, therefore, have a
significant influence on the impact properties of the type AISI 441 stainless steels, which leads to a
relatively narrow processing “window” in the final hot rolling temperature followed by rapid cooling
rates in the manufacturing process.
