An experiment was developed to simulate the interface between liquid high manganese carbon steel and solid ultra-low alloyed steel in a thermal gradient, corresponding to the nugget/sheet interface of resistance spot welds. Despite the large difference in the interaction time between the experiment and the welding process, identical microstructures and phases were observed in both. The time and temperature control of the experimental set-up indicates that the solute diffusion in the solid and liquid phase together with the interface temperature when cooling determines the microstructure formation and morphology. The presence of epsilon-martensite requires a gamma/liquid interface with a manganese-driven dissolution. A criterion based on manganese super saturation is proposed and shows that with the studied steels the nucleation and growth of epsilon-martensite is almost unavoidable when using resistance spot welding. The cellular patterns observed during solutal melting at low interface temperature increase the interface surface and the amount of martensite. The origin of the cells is the carbon enrichment in the manganese diffusion zone of the gamma phase that provokes a solidus temperature gradient in the thermal gradient.