Joe nailed it. If you look at a graph of hardness vs tempering temperature for almost any martensitic stainless steel, you'll find a little bump in the hardness after a relatively flat line. That bump happens at about the same temperature and shows a slight increase (or large, depending on alloying) in hardness and a decrease in toughness. What is happening is the chromium still in solution is precipitating as carbides, typically around the grain boundaries. A lot of stuff just fits there better. Once the chromium is in a carbide, it can't help with corrosion resistance. So you have an area around the grain boundaries that will corrode while the interior of the grains is basically unchanged. We can't see that, but it shows up as general corrosion. If allowed to go long enough, there will be enough corrosion around the grains that they just fall out. This is called grain dropping and can be a problem in even 300 series stainless steels. On micrographs it looks cool, as there is a grain with nearly nothing visible holding it in place.
