Reduction in wear of sewage pump through solution nitriding

Centrifugal pumps, with impellers and discs made of a stainless duplex steel, were tested for 11 weeks in a recirculation loop of a sewage plant. One pump was equipped with solution annealed (SA) components the other with solution nitrided (SN) ones. The surface of the latter was enriched with nitrogen in solid solution, which transformed the ferritic grains into austenitic ones and raised the hardness. The change of microstructure depending on the nitrogen profile of the case was studied in detail. The sewage contained mainly SO42− and Cl− ions in solution and organic as well as inorganic substances in suspension. Solution nitriding decreased the mass loss of the impeller by 40%. Erosion by fine quartz particles was responsible for the major part of the mass loss.