A method and a mould for casting a half steam diaphragm comprising a plurality of aerofoil section blades (1) spaced around and aligned lengthways radially of the diaphragm and fixed at their inner and outer extremities by cast semicircular rings (6, 7), in which the mould comprises an outer fixed portion and an inner movable portion and where said inner portion is constructed by assembling said blades in a jig, disposing casting sand (5) within spaces in said inner portion left by said blades and said jig, disassembling and removing said jig, and then lifting said inner portion into said outer portion.