Tertiary cementite
Very fine cementite that precipitates from ferrite at lower temperatures because ferrite can dissolve virtually no carbon at room temperature.
Tertiary cementite is a microstructural constituent that precipitates in ferrite during slow cooling. Normally, it's not a big deal, but you need to know that it can make the structure harder and more brittle if it forms unfavorably at grain boundaries. During welding, it usually forms due to too slow cooling in the heat-affected zone, especially in higher-carbon steels. If you want to keep the structure soft and tough, you have to control cooling so that the tertiary cementite remains finely dispersed and doesn't grow into coarse lamellae or networks. When in doubt, better do a post-weld heat treatment than rely on natural cooling.