Laves phase embrittlement of the ferritic stainless steel type AISI 441

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.