Abstract: There are mechanically mixed zones,micro-diffusion zones and locally transformed zones but no melted one found in the friction welded seam of 21-12N to 4Cr10Si2Mo steels.Under a strong plastic deformation,an ultra microcrystalline or fine-grained zone was formed close to the two sides of interface in the friction welded seam,and it was followed by recrystallization zone, recovery zone and deformation zone.However,the microstructural changes at each side of the interface are different after friction welding.