Effect of Prior Cold Deformation and Nitriding Conditions on Microstructure and Mechanical Properties of Plasma Nitrided IF Steel

Abstract

The effect of prior cold deformation and nitriding parameters on the kinetics and mechanism of plasma nitriding and the resultant hardness, wear and corrosion properties of automotive-grade interstitial-free (IF) steel has been investigated. Following controlled prior cold deformation (uniaxial rolling), plasma nitriding was carried out in pulsed direct current glow discharge mode with applied voltage, current, temperature and time varied in the range 540 to 710 V, 3 to 6 A, 350 to 480 °C and 1 to 4 hours, respectively. The phases formed after nitriding were found to be mostly γ-Fe4N with a small volume fraction of Fe3N embedded in a ferrite matrix. Prior cold deformation increases the kinetics of the nitride formation. The volume fraction of nitride phases increased with an increase in nitriding temperature and time. Detailed characterization suggested that 80 pct cold deformation followed by plasma nitriding led to significant improvement in hardness and wear resistance of IF steel, particularly when nitrided at 480 °C for 4 hours. Moreover, plasma nitriding also enhanced the corrosion resistance of IF steel, enhancement being directly related to the nitride volume fraction at the surface. Hence, it was concluded that prior cold deformation was effective in enhancing the kinetics of plasma nitriding and in turn surface hardness and resistance to wear and corrosion of IF steel, which otherwise possess a fairly poor bulk strength and does not respond to usual bulk/surface-hardening treatments.