Secondary Carbide Precipitation
The formation of extremely fine carbide particles within the crystallites during tempering or hot working of alloy steels. It causes an increase in elevated-temperature strength by hindering dislocation movement.
Secondary carbides precipitate during tempering when the carbon in the structure reacts with alloying elements. This makes the steel harder and stronger, but also more brittle. When welding, you have to watch that you keep the heat input under control and that the cooling isn't too slow, otherwise you'll get too much carbide formation and the part will be prone to cracking. If you temper the steel after welding, go with a moderate temperature and keep the time short, then you'll set the desired hardness without ruining the structure.