The present work is original since for the first time it deals with the lap friction stir welding of a steel to a cermet. A Si3N4 sintered tool was used. The paper more particularly focuses on the effect of the load exerted on the tool inserted in steel on (1) the tool wear and contamination, (2) the joint interface features and (3) the steel microstructure. The phase transformations in the different weld zones of the steel are explained by the thermal cycles. The hardness and shear lap tensile properties of the different joints are finally correlated to their microstructure.