The heat-affected zone (HAZ) of the low-alloy steel in dissimilar metal welds has been investigated by electron backscatter diffraction, scanning electron microscopy and atom probe tomography. The welding process generates a temperature gradient, creating two zones in the HAZ: a carbon depleted zone and a carbon non-depleted zone (NDZ). The evolution of the microstructure in the NDZ and the atomic distribution at the ferrite/ferrite and ferrite/cementite interfaces was investigated for different long-term thermal ageing conditions. Many alloying elements segregate to grain boundaries (ferrite/ferrite interfaces) with a different amount in relation to the misorientation of the grain boundaries, but only phosphorus (P) segregates to the ferrite/cementite interface. In addition, the amount of P segregation increases with the increase in temperature and ageing time and depends also on the structural parameters of the interface. The partitioning between cementite and ferrite phases presents a slight evolution with long-term thermal ageing, related to kinetic limitation.