It is a general characteristic of hardened (martensitic) stainless that there is a drop in toughness and a slight increase in hardness in that temperature range. Sensitization comes from precipitation of chromium carbides around the grain boundaries. These are the large Cr23 C6 carbides. Thats right, 23 atoms of Cr to 6 atoms of carbon. This is even a problem in 410 and 420, since very little carbon is needed to tie up a lot of Cr.
When Cr is tied up in carbides, it doesn't contribute to corrosion resistance. Now you have corrosion prone areas lining the grain boundaries. Corrosion can be so bad in these areas that the grains just fall out after the boundary is dissolved.
The decrease in toughness partially comes from these carbides around the grain boundaries. This increase/decrease of carbides/toughness and corrosion resistance is general for stainless steels. Specific grades can moderate or even eliminate the sensitization problem. It can get complicated pretty quickly. I would expect S30V to be better at corrosion resistance after exposure to this temperature range, since it contains a lot of vanadium, which will tie up the carbon before the Cr gets a chance. Thus, the Cr is left free to resist corrosion. The brittleness is a bit more involved and depends on where the carbides end up. Cr23C6 typically shows up in grain boundaries. Vanadium carbide may not, but I don't know. If not, it would be in the interior of the grains, and less likely to provide a continuous brittle carbide path for a crack to follow.
