Clustering of Si–Ni in Cementite in 18MND5 Steel Weldment

In this work, a new phenomenon-clustering of Si-Ni in cementite was observed in a thermal aged 18MND5 weldment and the formation mechanism was briefly discussed. 18MND5 weldment mock-up was thermally aged at 400 degrees C for 3000 h and its microstructure and composition were investigated by SEM, TEM and 3DAP. Silicon-nickel clusters in cementite were observed in the heat-affected zone of 18MND5 base metal by atom probe tomography. The formation of Si-Ni cluster in cementite is closely linked to the welding, tempering and thermal aging processes. This clustering phenomenon might be explained by that in the process of cementite formation, an alloying element 'spike' generated ahead of the interface of cementite and alpha-Fe matrix based on the partition local equilibrium model and silicon atoms were trapped for Mn, Cr and Mo 'spikes' act as a barrier to Si diffusion. As a result of thermal aging, those trapped Si atoms were compelled to generate metastable clusters in cementite for lowering the free energy.