A methodology that altered weld metal composition for the purpose of creating residual compressive stresses in plain-carbon steel structural welds that are fatigue-resistant was established. This study showed that systematic changes in composition in metal-cored filler wires can be used to design Mn-Cr-Si supermartensitic weld metals with good bead morphology and relatively low cost that would induce a compressive residual stress field along the weld toe. The resulting compressive stress field is inversely proportional to the Ms which was measured to be maximum at the toe surface. Primary austenite results for Creq/Nieq ratios less than 1.55 and primary ferrite forms at higher ratios. Within the range of compositions of this study, the final microstructures observed were adequately predicted by extending the Suutala line (3) to leaner compositions and with a modification for the manganese effect from 0.5Mn to 0.66Mn. These results were used to maximize the fraction of low-carbon martensite in weld metals by defining compositional limits, thereby maximizing the compressive residual stress to improve fatigue resistance.