The direct manufacture of metal parts by rapid prototyping often involves building a porous skeleton from a metal powder. In this work, a method termed Homogeneous Steel Infiltration has been developed for infiltrating steel skeletons to make conventional tool steel alloys. The method uses a gated infiltration route that uses as the infiltrant a steel alloy with a lower melting point than the base powder. The infiltrant liquid may use carbon and/or silicon as a melting point depressant. Premature freeze-off of the steel infiltrant is avoided by operating at a temperature where some liquid is stable at chemical equilibrium. The compositions of the skeleton and infiltrant and the infiltration temperature are selected by using computational thermodynamics. Examples of successful infiltrations using D2 and A3 tool steels as target compositions are shown. The thermodynamic design method enables suitable parameters to make other tool steels, some stainless steels and manganese steels.