5.5.3 Diaphragm construction
HP and IP diaphragm nozzles may be individually machined or cast and the inner and outer blade up-stands fitted into grooves machined in half rings, which may be machined from plate or may also be cast. The diaphragm half rings are then built up by welding the nozzles to the inner and outer half rings on the axial faces of the half rings.
Castings in stainless steel suffer from the formation of regions of delta ferrite, particularly near the trailing edges of fixed blades, and cracking near changes in section unless cooling rates are very carefully controlled. Also, since the welds penetrate only part way into the inner and outer half rings, a circumferential crack exists in the middle of the rings. Radial cracks also exist between each nozzle section where they butt against one another. These cracks weaken the assembly and can provide initiation points for fatigue crack propagation. Investigations are being carried out to develop welded diaphragms, using full-penetration electron beam welding. This will largely avoid the problems of internal cracks or voids.
A more modern method of construction is to electrochemically machine each complete half ring out of a solid stainless steel blank, thus obviating the problems of welding and casting.
The first four LP diaphragms are usually a welded construction, similar to the HP and IP, with stainless steel blades, rims and centres. Later stages may have pre-machined blades cast into cast-iron inner and outer rings. Cast iron enables optimum shapes of steam passages to be formed. These diaphragms may also be cast as complete halves in stainless steel.
Very large modern last-stage LP diaphragms have massive sections for the rims and centres and can be extremely heavy if made from solid castings. The rims and centres are therefore sometimes fabricated from plate into semicircular hollow box sections. Welded constructions are always heat treated to remove locked-in stresses and allowance must be made for distortion.