Plasma nitriding of 90CrMoV8 tool steel for the enhancement of hardness and corrosion resistance

• Published in: Surface & coatings technology Vol. 205; no. 19; pp. 4514 - 4520
• Main Authors: Nouveau, Corinne, Steyer, Philippe, Mohan Rao, K. Ram, Lagadrillere, Denis
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 25.06.2011; Elsevier
• Subjects: Alloy steel; Applied sciences; Corrosion; Corrosion environments; Corrosion resistance; Cross-disciplinary physics: materials science; rheology; Diffraction; Diffusion layers; Engineering Sciences; Exact sciences and technology; Hardness; Heat treatment; Ion nitriding; Iron nitride; Materials; Materials science; Metals. Metallurgy; Physics; Plasma treatment; Production techniques; Steels; Structural steels; Surface modification; Surface treatments; Thermochemical treatment and diffusion treatment; Wood; Alloy steel; Surface modification; Plasma treatment; Corrosion; Hardness; Iron nitride; Corrosion resistance; Thermochemical treatment; Plasma; Tool steels; Nitridation; Surface treatments; Electrochemical studies; Diffraction analysis; Nitrided layer; Cerium alloys; Nitriding treatment; Iron alloys; Hardening; Wood machining; Hardness and corrosion; X Ray Diffraction; Surface hardening; Compound layer; Crystallinity; Scanning Electron Microscopy; Structural and morphological properties; Crystallinities; Electrochemical properties; Electrochemistry; Surface Treatment; Hardness profiles; Microhardness measurement; SEM; Electrochemical test; Nitrogen plasma; Plasma applications; Morphological properties; White layer; Diffraction peaks; Chromium nitride; Machining; Plasma nitriding; Tool steel; Optimization; Nitrides; Mechanical Properties; Diffusion; Woodworking; Corrosion Resistance; Nitrided steels; Wood; Potentiodynamic polarization tests; Nitriding; Steel; Hardness values; Diffusion layers; Optimized parameter; Diffraction; Its efficiencies; Mechanical characterizations; Chromium; Electrolytes; Modified surfaces; Passive materials; Oak wood
• ISSN: 0257-8972; 1879-3347
• DOI: 10.1016/j.surfcoat.2011.03.087

The aim of the study is to apply a plasma nitriding process to the 90CrMoV8 steel commonly employed in wood machining, and to determine its efficiency to improve both mechanical and electrochemical properties of the surface. Treatments were performed at a constant N2:H2 gas mixture and by varying the temperature and process duration. The structural and morphological properties of nitrided layers were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) coupled with EDS microanalyses. Surface hardening and hardness profiles were evaluated by micro hardness measurements. To simulate the wood machining conditions, electrochemical tests were carried out with an oak wood electrolyte with the purpose of understanding the effects of the nitriding treatment on the corrosion resistance of the tool in operation. X-ray diffraction analyses revealed the presence of both γ′ (Fe4N) and ε (Fe2–3N) nitrides with a predominance of the ε phase. Moreover, α-Fe (110), γ′ and ε diffraction peaks were shifted to lower angles suggesting the development of compressive stresses in the post nitrided steel. As a result, it was shown that nitriding allowed a significant hardening of steel with hardness values higher than 1200HV. The diffusion layers were always composed of an outer compound layer and a hardened bulk layer which thickness was half of the total diffusion layer one. No white layer was observed. Similarly, no traces of chromium nitrides were detected. The temperature seemed to be a parameter more influent than the process duration on the morphological properties of the nitrided layer, while it had no real influence on their crystallinity. Finally, the optimal nitriding conditions to obtain a thick and hard diffusion layer are 500°C for 10h. On the other hand, to verify the effect of these parameters on the corrosion resistance, potentiodynamic polarization tests were carried out in an original “wood juice” electrolyte. After corrosion, surface was then observed at the SEM scale. Electrochemical study indicated that the untreated steel behaved as a passive material. Although the very noble character of steel was somewhat mitigated and the corrosion propensity increased for nitrided steels, the passive-like nature of the modified surface was preserved. For the same optimized parameters as those deduced from the mechanical characterization (500°C, 10h), surface presented, in addition to a huge surface hardening, a high corrosion resistance. ► Modification of 90CrMoV8 steel by plasma nitriding for wood machining. ► Enhancement of hardness and wear resistance without losing intrinsic protectiveness. ► Electrochemical tests in wood juice as the electrolyte. ► After plasma treatment high and durable corrosion resistance. ► Optimal nitriding parameters are 500 °C, 10 hours for best good behaviors.