Studies on the Effect of Salt Bath Temperature and Nitriding Time on Corrosion Properties of Low Silicon Spring Steel for Automotive Suspension System

Leaf spring steels are indispensable in automobile suspension systems due to their high strength and fatigue resistance. This study focuses on low silicon spring steel grade SUP9, produced via the blast furnace-basic oxygen furnace route, continuously cast, and hot-rolled at the Visakhapatnam Steel Plant. Containing 0.6 wt.% carbon and 0.3 wt.% silicon, this high-carbon steel is used in load-bearing vehicles like trucks and vans, typically in a quenched and tempered state. Corrosion from contaminants such as dust and stagnant water affects the service life of suspension systems. Liquid nitriding was chosen as a surface hardening treatment to improve corrosion resistance and mechanical properties. The process involved a salt bath with potassium chloride, sodium carbonate, sodium cyanate, sodium cyanide, and urea, which was performed at two different temperatures and three different treatment times. The resultant samples were analyzed for case depth using optical microscopy, and X-ray powder diffraction (XRD) studies identified the phases formed. Corrosion behavior was assessed in a 3.5 wt.% NaCl solution before and after nitriding using potentiodynamic polarization (PDP) curves. Results indicated that nitriding case depth increased with treatment time and temperature, and liquid nitriding significantly enhanced corrosion resistance, making it a viable alternative to existing tempering treatments.