Cavitation erosion resistance of high interstitial CrMnCN austenitic stainless steels

Abstract

It is known that the cavitation resistance of austenitic stainless steels can be increased by the addition of nitrogen. This is attributed to the strengthening effect of N and the lower stacking fault energy (SFE) leading to an increased resistance to fatigue. A new development utilizes the joint addition of carbon and nitrogen. This results in even better mechanical properties, combining a tensile strength of >1000MPa with a uniform elongation of >70%, which in turn promise an outstanding cavitation resistance. In this study, different high interstitial CrMnCN steels were tested in an ultrasonic vibratory cavitation rig. Results were correlated with mechanical properties derived from different test methods like tensile and fatigue tests. The alteration of the surface zone was investigated by instrumented indentation testing and electron microscopy. The cavitation tests reveal a superior resistance of the high interstitial CrMnCN steels compared to e.g. AISI304 and Hadfield Steel. They exhibit a significantly longer incubation time and a lower maximum erosion rate. Cavitation resistance generally increases with increasing content of C+N. In order to explain the findings, cavitation resistance was correlated with different materials properties like tensile strength and specific tensile energy absorption. Through instrumented indentation, a relationship between cavitation resistance and hardness and elastic indentation energy was derived. Indentation of eroded samples showed a work hardened surface layer of 50μm due to intense twinning. SEM analysis revealed material removal by both debris flaking and formation of cavities for high interstitial steels. Contrarily, in Hadfield Steel material removal almost exclusively results from cavities. High interstitial CrMnCN-steels can be produced by conventional metallurgy and offer a very high strength and toughness as well as a good corrosion resistance. Thus, they appear to be very suitable for the use in parts that are at risk for cavitation.