Tailoring material properties for enhancing cavitation erosion resistance of metastable aisi 304l stainless steel by nitriding

Abstract

Abstract Cavitation erosion is a significant problem in components subjected to fluid-structure interaction. The formation, growth, and subsequent collapse of numerous small bubbles in a liquid cause dynamic loads on exposed materials, leading to plastic deformation, cracking, and eventual material failure. Most researchers simulate cavitation erosion in laboratory settings using an indirect/alternative vibratory cavitation apparatus (ASTM G32). However, maintaining proper alignment between the ultrasonic horn and the tested sample is often overlooked in the literature, resulting in uneven cavitation marks and, consequently, variations in the mass loss of the material. In this study, a fixture is designed to mitigate this issue, providing a simple and effective solution for conducting long-duration cavitation tests using an indirect vibratory apparatus. Austenitic stainless steels, particularly AISI 304L, are commonly used in structural components exposed to cavitation erosion due to their excellent weldability, corrosion resistance, and mechanical properties. However, their metastable nature, characterized by deformation-induced martensite (DIM) formation upon cavitation exposure, has been widely reported. While there is debate over whether this transformation improves or impairs cavitation resistance, it is well-documented that martensite formation can negatively affect the material's corrosion resistance. Furthermore, the cavitation erosion resistance of austenitic stainless steels with respect to martensitic transformation is greatly influenced by testing conditions and material composition. Previous studies had reported varying incubation periods due to martensitic transformation. However, all the studies reported a significant mass loss after the incubation period, even when the same material was used. This study investigates the cavitation erosion behavior of AISI 304L under indirect cavitation erosion testing. The results reveal that martensitic transformation begins as soon as the cavitation test is initiated. The formation of DIM leads to substantial mass loss after 15 hours of cavitation exposure. DIM forms at slip lines/slip bands, causing material pile-ups that act as preferential sites for material removal. To address this issue, the stabilization of austenite by performing nitriding/nitrocarburizing was targeted, a strategy not explored previously in the literature. Salt bath nitrocarburizing was performed on AISI 304L at two temperatures, 470 °C and 500 °C, keeping the treatment time constant at 6 hours. Treatment at 470 °C produced a nitrocarburized layer of approximately 6 µm thickness, whereas treatment at 500 °C resulted in a 26 µm thick layer. The thicker layer, obtained at 500 °C, consisted of an oxide layer (~2 µm), a compound layer (~17.5 µm), and expanded austenite (~6.5 µm). The thinner layer, obtained at 470 °C, had an oxide layer (~2 µm) and expanded austenite (~4 µm). Nitrocarburized specimens exhibited better cavitation erosion resistance than the untreated base material, showing a decrease in mass loss by up to 53% for the sample treated at 500 °C and 90% for the sample treated at 470 °C. The sample treated at 500 °C showed poor cavitation erosion resistance in the initial stages (~2 hours of testing) due to the removal of the brittle oxide and compound layers. On the contrary, although the sample treated at 470 °C shows a relatively poor initial erosion resistance with respect to the rest of the period due to the removal of brittle oxide layer, an excellent erosion resistance can be noticed with respect to the untreated and the sample treated at 500 °C. After 10 hours of testing, the erosion rates of both samples became similar. The improved cavitation erosion resistance was attributed to the high yield strength of the stabilized austenite, with no evidence of martensitic transformation observed in the expanded austenite. The formation of subsurface DIM was found to depend on the thickness of the nitrocarburized layer. For the sample treated at 470 °C, the presence of deformation-induced martensite at the subsurface led to the delamination of the nitrocarburized layer. During salt bath nitrocarburizing, a thicker layer produced at 500 °C can negatively impact the cavitation erosion as well as corrosion resistance by depleting chromium from the steel and precipitating it as CrN. On the other hand, the thin nitrocarburized layer formed at 470 °C tends to delaminate after longer exposure times. Therefore, a thick nitrogen diffusion zone is essential to both eliminate DIM and maintain the material’s corrosion resistance. To address this, High-Temperature Solution Nitriding (HTSN) was performed on AISI 304L, resulting in a nitrogen diffusion zone of approximately 500 µm thickness. The treatment led to a significant improvement in cavitation erosion resistance (CER), with mass loss reduced by up to 9.3 times compared to the untreated base material. Remarkably, no DIM was observed in the specimens even after 15 hours of cavitation erosion testing. The protruded grain boundaries and annealing twins formed during the HTSN treatment act as material removal sites due to high strain accumulation during cavitation attack. However, the overall mass loss is significantly lower than that of the base material. These findings suggest that nitriding treatment effectively mitigates the detrimental effects of DIM, significantly enhancing the cavitation erosion resistance.