Name: DANIELA BAHIENSE DE OLIVEIRA VARGAS
Publication date: 04/12/2023
Examining board:
Name |
Role |
|---|---|
| ADONIAS RIBEIRO FRANCO JÚNIOR | Examinador Externo |
| ANTONIO CESAR BOZZI | Presidente |
| JÚLIO CÉSAR GIUBILEI MILAN | Examinador Externo |
| MARCOS TADEU D AZEREDO ORLANDO | Examinador Interno |
| SIDNEY NICODEMOS DA SILVA | Examinador Externo |
Summary: Cobalt superalloys are widely used due to their ability to maintain their mechanical, physical and chemical properties when used at high temperatures and for long periods. However, concerns about the increasing demand for cobalt, unstable prices and few suppliers, make it essential to assess the safety of the resources used and the efficiency of the materials applied. In this context, the present work investigated the effect of heat treatments (solution and aging treatments) and thermochemical plasma carbonitriding on the tribological behavior of the Stellite 250 alloy. The solution treatment was carried out at 1200°C for 1,5 hours and aging at 850°C for 4,5 hours. Thermochemical treatments were carried out at 360°C and 380°C for 9 hours. For tribological characterization, scratch tests were carried out with a progressive load (2 to 20N) and cavitation erosions tests were performed during 15 hours of exposure. The cross-sectional area of the scratches was verified by 3D profilometry. The wear mechanisms of the tribological tests were analyzed by scanning electron microscopy (SEM). The result of the microstructural characterization showed that for the cast alloy, the structure is formed by coarse grains, consisting of the and phases. From the heat treatments, the structure remained mixed, but a higher fraction of hexagonal compact phase was observed for solution treated condition. Under all carbonitring conditions there was formation of a double layer of S-phase, composed of an N- enriched phase (SN) and another C-enriched layer (SC). The surface hardness values observed under carbonitrided conditions are in the order of 1100-1500 HV0.01. In scratch tests, the application of surface treatments resulted in lower friction coefficient and smaller scratch depths for all conditions comparing to the Stellite 250 alloy. The main mechanisms were tensile cracks in carbonitrided conditions; and in heat-treated conditions: formation of slip bands on the sides and cracks. In the cavitation erosion test, all conditions showed greater resistance to cavitation erosion wear than AISI 304. Wear mechanisms include premature damage with formation and coalescence of pits, crater formation and brittle behavior of the material due to fatigue.
Keywords: Stellite 250 alloy; heat treatment; thermochemical treatment; scratch test; cavitation erosion.
