Epsilon carbide
A metastable transition carbide with a hexagonal crystal structure that forms during the initial stage of tempering martensitic steels at temperatures below 200 degrees Celsius. Its precipitation causes an initial increase in hardness during the tempering process.
Epsilon carbide is an intermediate stage that forms when tempering martensitic steels in the lower temperature range. It appears early in the tempering process and can briefly increase hardness before the structure transitions to a more stable state. When welding, you don't need to worry much about it. If you temper the weld normally to relieve stress, epsilon carbide is just a transient phase. It disappears again once the temperature is high enough and the tempering time is sufficient. The important thing is not to stop tempering too early or keep the temperature too low. Otherwise, the structure remains in an unstable, hard state, which tends to crack under load. So: let it heat through properly, then it'll be fine.