Hardness Enhancement Using Plasma Nitriding SKD11 Die Steel in an Atmospheric Pressure Plasma Jet

Abstract

Metals are typically altered for varying the color appearance and improving the physical properties of surface hardness, resistance of wear and abrasion, and anticorrosion. We employed an atmospheric pressure plasma jet (APPJ) to increase the surface hardness of SKD11 die steel. The H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> /N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> mixture gas was used as the nitriding gas for plasma nitriding process. Materials characteristics by Vickers hardness tester, optical microscope, and X-ray diffractometer (XRD) were employed. The optical emission spectroscopy (OES) was used to detect the optical emission of plasma species from atmospheric pressure plasma jet. The pristine SKD11 die steel represented a hardness of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$558\pm 5.9\text{HV}_{0.3}$</tex> , while plasma nitriding process apparently increased to reach the value around <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1095\pm 15.5$</tex> HV <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.3</inf> . The hardness enhancement was believed to be contributed from the self-growth of the nitride layer and diffusion layer with a total thickness of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$45\ \mu \mathrm{m}$</tex> above the matrix within 17 min analyzed by optical microscope. We also proposed that the accumulated heat induced by plasma bombardment under H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> /N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> mixture gas in an APPJ system was obtained, in which give a substrate temperature around 500°C detected by the thermal couple offering suitable nitriding temperature. Additionally, from OES observation, the NH radicals were the key species to control the hardening mechanism of SKD11 die steel, as well as the exposure time and substrate temperatures. Based on the thermochemical case hardening process using H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> /N2-APPJ system, the improved surface hardness can be used to promote high resistance to wear, scuffing, galling and seizure. Plasma nitriding process is implemented in typical applications in the manufacturing industries such as gears, valve parts, pressure-die-casting tools, forging dies, moulds, and engine parts. We also believed that plasma nitriding using H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> /N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> mixture gas in an APPJ system is a modern thermochemical treatment for offering short time operation on the nitridation of steel objects.