Influence of Nb Addition on Sliding Wear Behavior of 25 Cr 7 Ni Cast Austenitic-Ferritic Steel

Austenite and ferrite phases share equal fractions in the steel and thus coins the name austenitic-ferritic steels, widely known as duplex steels, among researchers. The current study focuses on revealing the sliding wear behavior of the steels, which is of great industrial importance in oil & gas and chemical processing industries, where it is used as valves in pipelines. Austenitic-ferritic steel and its niobium (Nb) added castings were produced through the induction melting route. Thermodynamic analyses of the systems were performed using Thermo-calc software. Isopleths generated through thermodynamic simulation predicted niobium carbide (NbC) formation in the system, and the same was asserted through x-ray diffraction peaks. Scanning electron microscope (SEM) with energy-dispersive spectroscopy (EDS) study also confirmed the presence. The dry sliding wear performance of these castings was investigated. Experiments have been performed where cast austenitic-ferritic steel pins, slid against non-shrinking, oil-hardened steel disk, with sliding speeds 1, 1.5, and 2 m/s, under loads of 20, 30, and 40 N, without lubrication at room temperature. Variations in wear rate, friction coefficient, and volume loss were studied with respect to sliding distance, load, and sliding velocity. SEM was used to understand the wear phenomena and mechanisms involved, and white light interferometry analysis over the worn-out surfaces gives insight to the understanding. NbC formed in the system imparts stability to the steel against sliding wear. The results show that Nb addition can be beneficial in applications like valves, elbows, and other fittings in the pipelines, where degradation of a material due to wear is high and thus improves the productivity of the industry.