Friction stir lap processing of 1.1 mm thick TRIP steel sheets was performed by varying the pin length of a Si3N4 tool so as to stir or not a 0.1 mm thick Ni interlayer. Numerous complementary techniques were used to investigate the joints microstructure at its various length scales in order to deal on the one hand with the bond soundness, and on the other hand with the material flow, recrystallization, and phase transformations during processing. With short pin lengths, without Ni stirring, a poor diffusion bonding only occurs at the interlayer interfaces. With the longest probe, the reduced malleability of nickel, which played the role of a material flow marker, was inadequate for obtaining sound welds. Irrespective of the pin length, the nuggets microstructure was essentially α′ martensitic. The Ni stirring by the two longest pins gave rise to areas locally graded in both chemical composition and thus phase nature inside the nugget. A nickel gradation was even faced in some grains. A marked diffusion of Ni in the nugget stabilized austenite with a few micrometers grain size. At last, a thorough study of the fracture pattern of tensile shear samples explains the mechanical resistance of the various joints by their microstructural soundness. Some ways of optimization are finally proposed.