Secondary cementite
Cementite that precipitates from austenite along its grain boundaries during cooling below solidification temperatures due to decreasing carbon solubility in austenite.
If the steel has enough carbon, the excess carbon precipitates during cooling as hard and brittle carbide at the grain boundaries. That stuff then forms a network that no longer deforms and makes the weld extremely prone to cracking. This is especially a problem in the heat-affected zone, where the grain grows strongly due to the heat. You have to keep an eye on the carbon content. If you have to weld such steel, then preheat and keep the heat in during welding. Under no circumstances let it cool quickly, otherwise the part will crack. Best to let it cool slowly and then anneal it again so that the brittle carbide network transforms back into a non-critical form. After that, check the weld for cracks.