OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 24 No. 1 2022 Ta b l e 2 Cavitation test results Testing time, min Weight loss, mg AISI 316L E308L-17 60Cr8TiAl 0 0.00 0.00 0.00 5 0.67 0.47 0.10 10 1.10 0.75 0.31 20 1.65 0.90 0.66 40 2.02 1.03 0.87 60 2.90 1.13 0.99 90 5.04 1.57 1.24 120 7.74 2.43 1.48 180 15.44 4.72 1.76 240 22.13 8.07 2.06 300 28.65 12.13 2.49 A noticeable increase in the wear rate for AISI 316L and E308L-17 in comparison with 60Cr8TiAl is observed after 40 and 90 minutes, respectively. According to XRD, Fig. 4, it is noticed that, before cavitation tests, the share of the α-phase in the surface layer was 29.5 % in 60Cr8TiAl, 2 % in AISI 316L, and was not found in E308L-17. The presented combination of austenite and martensite in 60Cr8TiAl is due to the infl uence of alloying elements. Carbon is a strong austenitizer, and at a given C/Cr ratio, the initial martensitic transformation temperature (Ms) decreases. Calculations using predictive equations for the main chemical composition of 0.6 % C and 8 % Cr [35] showed that Ms is in the range of 170–220°C. Aluminum and titanium within the indicated limits induce γ → αʹ transformation, contributing to increasing the number of crystallization centers and obtaining a fi ne-grained structure [36]. As indicated by XRD analysis an increase in the amount of deformation martensite was observed in the surface layer of all samples during the cavitation tests, Fig. 5. In 60Cr8TiAl, the quantity of martensite increased to 73 %, which is much higher than that of E308L-17 and AISI 316L. This indicates a signifi cantly lower austenite stability in 60Cr8TiAl. The deformation martensite causes an increase in hardness, dissipation of the energy of external loading, and development of compressive stresses preventing the formation of microcracks. For 60Cr8TiAl and E308L-17, the slope of the curves for the dependence of the proportion of martensite on the duration of testing changes, which indicates the stabilization of austenite. Therefore, at latest test stage with a slight increase in the quantity of martensite, there is an additional strain hardening of the previously formed dispersed crystals of α’-martensite. For AISI 316L steel, during the fi rst 60 min of cavitation, no noticeable formation of αʹ-martensite was observed. This indicates a high stability of austenite, which is also confi rmed by other studies [26]. Only prolonged for 300 min cavitation exposure led to the formation of 25 % martensite on the metal surface. This means that the formation of α’-martensite occurs in the already hardened austenite of this steel. Comparing the results of cavitation tests and XRD data, it can be concluded that there is a correlation between the erosion resistance of austenitic steels and the intensity of the martensitic transformation. The latter one develops under the infl uence of cavitation, contributing to erosion resistance increase, Fig. 6.
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