Microstructural variation across inertia friction welded Aermet 100 (ultra-high strength secondary hardening steel) and SCMV (high strength low alloy Cr-Mo steel) has been studied. Retained austenite was mapped in two dimensions across the inertia friction welds using synchrotron-X ray diffraction. Microhardness testing and SEM were used to characterize the general microstructure variation across the weld line in the as-welded and post-weld-heat-treated (PWHT) conditions. Four distinct microstructural zones were observed for the as-welded condition in the heat-affected zone. Subsequent PWHT resulted in a reduction of the peak hardness on adjacent sides of the weld with all other areas increasing in hardness. These changes in hardness are explained based on tempering of the martensitic structure and precipitation reactions during PWHT. © 2007 Elsevier B.V. All rights reserved.