Structure and Properties of Gas-Nitrided, Precipitation-Hardened Martensitic Stainless Steel

Abstract

Nanoflex stainless steel is a promising material for medical applications. However, improvement of its mechanical properties without compromising its corrosion resistance is still a challenge. In order to investigate the effect of the nitriding process on the corrosion and wear resistance of Sandvik NanoflexTM steel, a number of processes were carried out in a gas atmosphere with differing ammonia contents in the temperature range of 425–475 °C for 4 h. The mechanical properties and wear resistance of the layers were tested using the nanoindentation and pin-on-disc methods, respectively. In order to assess corrosion resistance, potentiodynamic tests were carried out in Ringer’s artificial body fluid and in a 3% aqueous solution of sodium chloride. The results are discussed herein with respect to the microstructural characteristics of the layers studied using light and scanning electron microscopy, X-ray diffraction phase analysis and wavelength dispersive X-ray microanalysis. The structure of nitrided layers included three zones: the subsurface zone composed of nitrides and the zones composed of metastable phases, i.e., the S phase (γN) and expanded martensite (αN) with possible precipitates of nitrides. The third zone adjacent to the steel core was enriched with carbon. The nitrided samples showed significant improvement in the wear rate while maintaining good corrosion resistance in comparison to the non-treated steel. We concluded that nitriding should be carried out at a temperature below 450 °C and in an atmosphere containing no more than approximately 50% ammonia in order to avoid nitrides precipitation.

High-Efficient Gas Nitridation of AISI 316L Austenitic Stainless Steel by a Novel Critical Temperature Nitriding Process

“…A similar thin film was observed by Paweł Kochma ński et al when they treated martensite stainless steel by gas nitriding. The thin layer was identified as ε nitride [27]. The EDS map shows the uniform distribution of elements except nitrogen.…”

Enhancing Wear Resistance in Functionally Graded Metallic Components: Insights from Nanoindentation and Mechanical Analysis

“…Both the coating and CQT/NQT processes aim to increase the surface hardness. However, the effect of the increased hardness on the wear resistance of the CQT-treated and NQT-treated steels with functionally graded material (FGM) properties [17][18][19][20] has not been fully elucidated. Plastic deformation properties and wear resistance have been previously analysed using the H/E ratio, where E is Young's modulus.…”

Hydrogen Embrittlement of Nitrogenating Layer on Martensitic Alloys

“…Most stainless-steel alloys exhibit a three-layer structure during nitridation, which can be observed under optical magnification. The layers consist of the subsurface, typically composed of the ε phase, the second layer comprised of metastable phases (ϒ-N) and expanded martensite (α-N) with precipitates of MN-type nitrides (often CrN), and the deeper zone adjacent to the steel core enriched with carbon [5] [6]. Two mechanisms are considered for simultaneous penetration and the formation of a solid solution or interaction of nitrogen in the metal structure: physical diffusion and chemical gas-metal reaction.…”