Plasma process for cleaning and thermochemically treating metallic pieces, which process comprises using a plasma reactor comprising an outer cover, a metallic casing provided internal to the outer cover with a distance from said outer cover, and defining a reaction chamber provided with: a support; an anode-cathode system associated with an electric power source; a ionizable gaseous charge inlet; and a gaseous charge exhaustion outlet connected to a vacuum system, wherein the process comprises the following cleaning steps: a) connecting the support to the grounded anode and the cathode to a negative potential of the electric power source; b) positioning the metallic pieces on the support in the interior of the reaction chamber; c) surrounding the support and the metallic pieces with a ionizable gaseous charge fed into the reaction chamber; d) heating the interior of the reaction chamber, from the outer side of the metallic casing and from within the outer cover, to vaporization temperatures of the contaminants to be dissociated from the metallic pieces being treated in the interior of the reaction chamber; e) applying an electrical discharge to the cathode, in order to provoke the formation of a gaseous plasma of ions with high kinetic energy surrounding the metallic pieces and the support, and a bombardment of electrons on the metallic pieces for molecular dissociation of the contaminants; and f) providing, by the vacuum system, the exhaustion of the gaseous charge and of the contaminants maintained in gaseous state, from the interior of the reaction chamber, whereby the process comprises the additional steps of thermochemically treating the surface of the metallic pieces in the same plasma reactor, said steps comprising: g) inverting the energization polarity of the anode-cathode system so that the support, with the metallic pieces, defines the cathode; h) surrounding the support and the metallic pieces with a new ionizable gaseous charge that is fed into the reaction chamber; i) maintaining the interior of the reaction chamber heated from the outer side of the plasma reactor and conducting the temperature therein to the levels required in the desired surface thermochemical treatment; j) applying an electrical discharge to the cathode, in order to provoke the formation of a gaseous plasma of ions surrounding the metallic pieces and the support, as well as an ionic bombardment on the metallic pieces; and k) providing, by the vacuum system, the exhaustion of the gaseous charge from the interior of the reaction chamber. a) connecting the support to the grounded anode and the cathode to a negative potential of the electric power source; b) positioning the metallic pieces on the support in the interior of the reaction chamber; c) surrounding the support and the metallic pieces with a ionizable gaseous charge fed into the reaction chamber; d) heating the interior of the reaction chamber, from the outer side of the metallic casing and from within the outer cover, to vaporization temperatures of the contaminants to be dissociated from the metallic pieces being treated in the interior of the reaction chamber; e) applying an electrical discharge to the cathode, in order to provoke the formation of a gaseous plasma of ions with high kinetic energy surrounding the metallic pieces and the support, and a bombardment of electrons on the metallic pieces for molecular dissociation of the contaminants; and f) providing, by the vacuum system, the exhaustion of the gaseous charge and of the contaminants maintained in gaseous state, from the interior of the reaction chamber, whereby the process comprises the additional steps of thermochemically treating the surface of the metallic pieces in the same plasma reactor, said steps comprising: g) inverting the energization polarity of the anode-cathode system so that the support, with the metallic pieces, defines the cathode; h) surrounding the support and the metallic pieces with a new ionizable gaseous charge that is fed into the reaction chamber; i) maintaining the interior of the reaction chamber heated from the outer side of the plasma reactor and conducting the temperature therein to the levels required in the desired surface thermochemical treatment; j) applying an electrical discharge to the cathode, in order to provoke the formation of a gaseous plasma of ions surrounding the metallic pieces and the support, as well as an ionic bombardment on the metallic pieces; and k) providing, by the vacuum system, the exhaustion of the gaseous charge from the interior of the reaction chamber.