Extending Water Droplet Erosion Incubation by Austenite-to-Martensite Transformation in-situ Hardening

Water droplet erosion (WDE) poses a threat to the structural integrity of steam and gas turbine blades. Conventional mitigation strategies prioritize maximizing surface hardness through surface treatments and coatings. However, this strategy remains limited because higher initial hardness often compromises ductility and reduces the surface’s ability to sustain hardening under repeated droplet impacts. This study shows that in-situ hardening due to phase transformation is a promising route for extending the WDE incubation period. To assess this, a metastable austenitic stainless steel (MASS), CaviTec®, was tested and compared to a phase-stable 12% Cr stainless steel extracted from an ex-service steam turbine blade. CaviTec® and 12% Cr stainless steel had similar initial Vickers microhardness values of 259 ± 3 and 261 ± 3, and were subjected to identical WDE conditions. Results show the incubation period for CaviTec® is three to four times longer than that of the 12% Cr stainless steel at impact speeds of 250, 300 and 350 m/s. At 250 m/s, 12% Cr stainless steel showed limited in-situ hardening (9%), whereas CaviTec® activated an additional phase transformation-mediated energy-absorption route, producing an in-situ hardening response 5.9 times that of 12% Cr stainless steel (53% vs. 9%). A hardening-mechanism partitioning analysis showed that, during the incubation stage of CaviTec®, phase transformation accounted for the dominant share of total impact-induced in-situ hardening (57%), exceeding the contributions from twinning and residual hardening (32%) and dislocation slip (11%). Microstructural analysis confirmed a sequential impact-induced phase evolution in the affected areas (FCC-austenite → HCP-martensite → BCT-martensite).