Abstract: In accordance with this invention, a waste gas containing toxic NF.sub.3 gas is contacted with a honeycomb structure of a carbonaceous material to thereby convert NF.sub.3 into nontoxic CF.sub.4 and N.sub.2 gases with high efficiency.

Abstract: Upon forming a plating coat on steel works, the works are held in a fluorine- or fluoride-containing gas atmosphere in a heated condition to thereby remove oxidized scales such as SiOx, MnOx and the like on the surface layer of the work and to form a fluoride layer to prevent forming new oxidized scale. The fluoride layer is easily eliminated by an action of added flux to a plating bath or by heating the fluorinated steel works in an atmosphere containing hydrogen prior to dipping in the plating bath to activate the steel surface. The activated steel surface is improved in compatibility with the plating bath and the like to realize a good quality plating coat formation.

Abstract: In accordance with this invention, a waste gas containing toxic NF.sub.3 gas is contacted with a honeycomb structure of a carbonaceous material to thereby convert NF.sub.3 into nontoxic CF.sub.4 and H.sub.2 gases with high efficiency.

Abstract: Steel is nitrided first by treating the steel to be nitrided with NF.sub.3 at elevated temperatures to form a fluorinated layer on the steel, and then the steel is nitrided by heating in a nitriding atmosphere.

Abstract: Steel is nitrided first by treating the steel to be nitrided with NF.sub.3 at elevated temperature to form a fluorinated layer on the steel, and then the steel is nitrided by heating in a nitriding atmosphere.

Abstract: The primary object of the invention is to clean and activate the surface of metallic works prior to such thermal treatment as nitriding, thermal spraying or dip plating by removing oxidized and other passive layers and foreign matters from the metallic work surface.The method of pretreating metallic works comprises heating a metallic work in a furnace and introducing a fluorine- or fluoride-containing gas into the furnace in that state to thereby cause destruction and elimination of the foreign matters adhering to the metallic work surface and of the oxidized layer occurring on the metallic work surface and simultaneous formation of a fluorinated layer. Just prior to the main thermal treatment, for example nitriding, the fluorinated layer is decomposed and eliminated by introducing an appropriate gas, for example H.sub.2, into the furnace. In this way, the metallic work reveals its cleaned and activated surface.