Steel that has been cooled rapidly in the quench will have some parts cool faster than others, I'm sure many of you have seen this:
And with this twisting comes a build up of internal stresses that can either pull or push against each other, or tear the blade apart.
Once the blade has been quenched, tempered and brought to room temperature, we'd like to think that the steel is in a stable state. And for the most part, it is. But because we don't live at absolute zero, there is still a bit of heat around at room temperature that can continue to make small changes to the internal structure of the steel. Enough to sometimes make a knife change shape over time, if only by a little bit.
This phenomenon is well known to Japanese bladesmiths, who sometimes hold huge stocks of forged blanks for a few months before moving to the next stage of grinding them to shape, because they are waiting for the steel to 'settle' into it's final shape.
You could do that, or you could just temper the blade again. By increasing the temperature we accelerate any internal changes that want to naturally happen over an extended period of time.
