Cavitation and strain-induced transformation: the austenite phase behavior in a soft martensitic and an austenitic stainless steel ^*

Abstract

In this work the behavior of the austenite phase at the surface of a low-carbon martensitic stainless steel and an austenitic stainless steel subjected to cavitation through ultrasonic vibration tests in liquid phase was studied. Emphasis was given to the behavior of two distinct types of austenite, in the case the reversed austenite of the as-tempered ASTM CA-6NM martensitic stainless steel, and the thermally-stable austenite of the solution-treated AISI 304 austenitic stainless steel. The evolution of phases fraction at the studied surfaces by XRD technique was characterized by two ways. Firstly, cavitation test intercalated with XRD measurements as a function of the test time was carried out. In the sequence, an indirect measurement technique comprising the use of Vickers indentation and controlled material removal by polishing, also intercalated with XRD characterization along the incubation period of both steels was used. This procedure was strong enough to determine the mechanism that precedes the erosive wear defining the incubation-acceleration stages transition and the cavitation-affected depth presenting γ(austenite)→α'(martensite) strain-induced transformation in the material microstructure, measured from the surface of the tested region into the interior of the substrate bulk of each studied steel. Such mechanism covers deformation of the steel matrix and strain-induced transformation of the austenite phase at the surfaces subjected to cavitation, strongly influencing the beginning of the significant mass loss process for both low-carbon stainless steels studied here.