Eventos Anais de eventos
COBEM 2023
27th International Congress of Mechanical Engineering
INVESTIGATION OF THE CAVITATION PHENOMENON IN THE MECHANICAL AND METALLURGICAL PROPERTIES IN THE ROTOR OF THE FRANCIS TYPE TURBINE MADE OF STAINLESS STEEL ASTM A743 GR CF20
Submission Author:
Cristian Meinhardt , RS , Brazil
Co-Authors:
Carlos Eduardo da Silva Moreira, Cristian Meinhardt, Lecino Caldeira, Matheus José Cunha De Oliveira, Paula Silvestre
Presenter: Carlos Eduardo da Silva Moreira
doi://10.26678/ABCM.COBEM2023.COB2023-2115
Abstract
Industrial equipment such as hydraulic turbines are susceptible to cavitation wear. This generates a progressive loss of material on the rotor surface, reducing the useful life of the turbine. This loss is caused by the collapse of vapor bubbles formed in a liquid at critical pressure and temperature. It is known that knowing the condition of the internal structure of the material, that is, its microstructure, is of fundamental importance to verify the existence of some correlation between the microstructure and the phenomenon of cavitation. For research purposes, the focus of this work is a Francis model rotor, with a diameter of 740 mm, widely used in Hydroelectric Power Plants (HPPs) to generate up to 750 MW. Material of construction is ASTM A743 GR CF20 austenitic stainless steel, a foundry alloy containing iron-chromium-nickel for general applications requiring corrosion and cavitation resistance. The characterization of the sample taken in the region of the rotor affected by cavitation was performed through microstructural analysis, macrography, energy dispersive Xray spectroscopy (EDX) test and hardness test. According to the results obtained, it was verified that the analyzed material presents porosities and cracks under the influence of tensile stresses, which can facilitate the occurrence of the phenomenon of cavitation on the surface of the rotor. In microscopy, dendritic structures were observed in the region of the Fused Zone (FZ), grain growth and intergranular corrosion (sensitization) in the region of the Heat Affected Zone (HAZ), characterized by the precipitation of chromium carbide in the grain boundaries. The analysis highlights the formation of cracks from the FZ towards the HAZ and the formation of cavities in the Base Metal (BM) region. The analyzed crack presents intragranular aspects in the HAZ region, and are attributed to sensitization and to the state of tensile stress, characteristic of the rotor in operation. The sample also presents singularities (circular shape) in the BM region. These singularities were formed by the cavitation phenomenon, and can assist the formation of cracks, which were also observed in the MB region, taking into account the effect of stress concentration caused by the singularities and adding to the state of tensile stress, characteristic of the operation. In the EDX test, percentages of chromium and nickel were found within the range of the chemical composition of austenitic stainless steel CF20, 18% to 21% of chromium and 8% to 11% of nickel, in accordance with the ASTM A743 standards.
Keywords
Cavitation, rotor, Materials Characterization, stainless steels
