Comparison of Hardness Enhancement and Wear Mechanisms in Low Temperature Nitrided Austenitic and Martensitic Stainless
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Comparison of Hardness Enhancement and Wear Mechanisms in Low Temperature Nitrided Austenitic and Martensitic Stainless Steel S. Mändl, D. Manova, D. Hirsch, H. Neumann, and B. Rauschenbach, Leibniz-Institut für Oberflächenmodifizierung, Permoserstr. 15, 04303 Leipzig, Germany ABSTRACT Energetic nitrogen implantation into austenitic stainless steel or nickel alloys leads to the formation of a very hard and wear resistant surface layer with an expanded lattice, while similar results are reported for selected martensitic steels. A comparison of the phase formation under identical process conditions at 380 °C using an austenitic (304) and a martensitic (420) steel grade (in an annealed ferrite/cementite condition) shows that the initial microstructure is retained in both of them. As the formation of dislocations and stacking faults cannot account for the dramatic hardness increase, the build-up of compressive stress is proffered as an explanation covering these two steel types. Furthermore the energetic nitrogen implantation apparently stabilizes the expanded lattice with suppressing the chemical transition to iron nitrides and, for steel 420, the metallurgical transition towards austenite at high nitrogen contents. INTRODUCTION Austenitic stainless steels can be hardened at temperatures around 350 – 400 °C using energetic nitrogen ion implantation while retaining the corrosion resistance [1,2,3]. The socalled “expanded austenite” is characterized by a concentration dependent diffusion coefficient, strongly increasing at higher nitrogen concentrations [4] and an anisotropic lattice expansion [5]. Selected results on nitrogen implantation into martensitic stainless steels show a comparable lattice expansion [6,7] while others do not show this effect [8,9]. Following a previous investigation focused on the nitrogen diffusion and the lattice expansion in austenitic and martensitic stainless steel, as observed by X-ray diffraction (XRD) [10], a more detailed insight into the respective surface morphology is presented here. Additionally, possible origins of the increased hardness and wear resistance are discussed. The method of choice for nitrogen ion implantation is plasma immersion ion implantation (PIII) allowing a fast and cost efficient implantation for complex 3D-objects [11,12]. EXPERIMENT This investigation focuses on austenitic stainless steel AISI 304 (X5CrNi18.10) and martensitic stainless steel 420 (X46Cr13), however additional experiments performed on 316Ti (X6CrNiMoTi17.12.2), 431 (X17CrNi16.2) and 430F (X14CrMoS17) did not yield significant differences. Samples prepared from 304 showed an austenitic structure with an average grain size between 10 and 15 µm whereas a ferritic microstructure with cementite inclusions was observed for 420 (see Fig. 1). Hence, an additional heat treatment step resulting in a change
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from the martensitic structure must have occurred prior to the receipt of the samples in the laboratory. The experiments were performed in a HV chamber at a base pressure below 5 × 10
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