Fretting wear of alloy steels at the blade tip of steam turbines

In order to reduce blade resonant vibration amplitude in turbomachinery, blades are assembled with a mutual interlocking at the tip. The aim of this study is to investigate the wear mechanism at the contact interface of the blade shroud in steam turbines. Experimental data are available concerning the wear mechanism at interfaces of aircraft engines blades, while the literature regarding the same effect on steam turbines is less rich. Moreover, the transposition of the results from the aero-engine to the steam turbine is difficult, because materials and working conditions are different. To overcome this lack of knowledge an experimental campaign was set up to investigate this wear mechanism under the specific conditions and with the distinctive materials used in steam turbines. Two base materials (alloy steels) were tested under different conditions: surface treatment (with and without laser quenching), temperature and normal load. Dissipated energies were determined from the hysteresis loops measured during the tests and were correlated to the test conditions. Profiles of worn surfaces were measured, and volume losses were accurately computed with a procedure that takes into account the roughness of the surfaces. Experiments were conducted both at room and low temperature (150 °C). At room temperature the surface temperature increased to 60-70 °C, due to the heat generated in the wear process. Comparison of volume losses at room and low temperature showed that at 150°C the volume losses decreased dramatically. This behavior was explained with a brittle-ductile transition. In other words, the same wear mechanism, adhesion and abrasion respectively in stick and gross slip condition, give very different results for a small softening effect of the material. Moreover, experimental results showed much more sensitive wear rates to the heat treatment than to the steel type.