temper brittleness of steel

hi, everyone,
i'm a material engineering student and i'm working on a project in which i'm supposed to study the heat treatments of the 51CrV4 steel (spring steel) and i noticed a drop of resilience K, in temperature between 300°C and 450°C, i need to know the transformation(s) that is(are) responsible for this, but i don't have access to a lab to identify it myself, any help in this topic would be much appreciated and thanks.