The present invention relates to a welding mould, intended to the butt-jointing of two metal parts by casting a molten filler metal in a print defined by a mould. At least one of the mould elements (12, 13) comprises a keying member (17, 18) which is conformed and sized in such a way that it can be engaged in a space (8) between the two parts (1, 2) only when the latter are mutually spaced apart with a mutual spacing at least equal to a predetermined value so that, only in these conditions, the mutual assembly of the elements (12, 13) of the mould (10) around the parts (1, 2) to be welded is possible. Application particularly to weldings of railway rails, concrete reinforcement elements and metal sections.