Development of diffusion - bonded pump impellers made from a corrosion - fatigue resistant PM' steel

The use of the casting technology, especially the application of the lost wax method, would be too expensive and time consuming for manufacturing of models and moulds for a small number of corrosion resistant impellers. Therefore, this project aimed the design and production of a complex prototype pump impeller consisting of two parts to be assembled by sinter-joining. As this impeller has to operate in corrosive environment the basic material was the prealloyed austenitic PM steel Fe-18.0 per cent Cr-10.0 per cent Ni-2.0 per cent Mo-0.03 per cent C-0.8 per cent Si (similar to AISI 3161). By admixing of 0.2 per cent Cu with 0.3 and 0.7 per cent P, respectively, the material was modified in order to control the shrinkage of the upper and lower disks of the impeller during sinter-joining. On one hand the feasibility of sinter-joining by these material compositions was proven first by manufacturing of upper and lower disks with interfering nozzles. On the other hand fatigue test under art ificial seawater were carried out with unnotched and notched specimen. These tests proved good corrosion fatigue resistance of the materials developed. The calculated stresses in the critical areas of the impellers had a high safety margin compared to the fatigue test results. Also the stress corrosion results showed that calculated maximum operational stresses of the impeller were far below critical values. The prototype impellers were machined from isostatically pressed disks after presintering. Finally the machined upper and lower disks were assembled and sinter-joined. The shrinkage and diffusion-bonding occured in the contact areas of the milled nozzles tightly fitted into the grooves of the opposite disk. The operational safety and hydraulic performance of the new impeller was proved in a closed pump test loop in tap and artificial seawater under normal conditions (1500 rpm) and overspeed (3600 rpm). The impeller survived a strong operational service simulation without any signs of corrosion fatigue and cracks. Using the PM route a new type of pump impeller with good hydraulic performance for applications in corrosive environment was developed.