Secondary hardening
A phenomenon during the tempering of alloy steels where a temporary hardness increase contrary to the normal declining trend is achieved through the precipitation of fine special carbides.
Secondary hardening occurs in some steels due to the precipitation of fine special carbides. They make the structure harder but also more brittle. When welding, this means: if you weld such materials, the heat-affected zone can become so hard afterwards that it tends to crack. This is especially critical when the component is later subjected to load. So you have to be careful not to unnecessarily promote hardening. That means: don't forget preheating, keep an eye on the interpass temperature, and if necessary, perform a post-weld heat treatment. If the carbides become too coarse, that's also not good, then toughness suffers again. So: control it properly, don't just weld away.