Stainless Steel Nitriding With A High Nitrogen Ion Beam Flux And A High Power Plasma Torch

Stainless Steel Nitriding With A High Nitrogen Ion Beam Flux And A High Power Plasma Torch

J. P. Riviere^a, C. Templier^a, G. Abrasonis^{a, b}, L. Pranevicius^b, L. L. Pranevicius^b, D. Milcius^c, J. Nomgaudyte^b

^aLaboratoire de Métallurgie Physique, Université de Poitiers, Bd M. et P. Curie, B.P. 30179, 86962 Futuroscope Chasseneuil Cedex, France ^bVytautas Magnus University, 8 Vileikos St, LT-3035 Kaunas, Lithuania ^cLithuanian Energy Institute, 3 Breslaujos St, LT- 3035 Kaunas, Lithuania

Abstract. Low energy-high flux (1,2 keV, 10¹⁶cm^–2s^–1) nitrogen ion implantation and high flux plasma torch nitriding (10²¹cm^–2s^–1) of AISI 304 L stainless steel have been carried out in the temperature range 400° C–600° C. Thick nitrided layers of high nitrogen content (25–30 at %) have been formed and the nitrogen depth profiles determined by nuclear resonance analysis (NRA) and/or glow discharge optical spectroscopy (GDOS). The phases formed were identified by X-ray diffraction at glancing incidence. Below 430° C, the formation of a solid solution or expanded austenite g_N phase has been observed; for higher temperature, the precipitation of chromium nitride occurs together with the formation of a a bcc Fe (Ni) phase. An important increase of hardness was obtained at both temperatures as well as a significant and durable wear resistance.

INTRODUCTION

Austenitic stainless steels exhibit an excellent corrosion resistance but their surface hardness is low and their wear resistance is not sufficient for many applications. There has been an increasing interest for developing hardening processes for austenitic stainless steels. Nitriding is a well-known surface treatment, but in the case of austenitic stainless steels conventional methods are not working because of the formation of a surface oxide layer acting as a diffusion barrier. New processing techniques using more or less energetic nitrogen ions with high fluxes have proved to be very powerful for producing at moderate temperatures thick nitrided layers [1, 2, 3] with an increased hardness (factor 6–7) and a considerable reduction of the wear rate (2 orders of magnitude). The formation of these thick nitrided layers and the deep atomic nitrogen transport are not well understood; however there is strong experimental evidence indicating that the rate of nitrogen supply and the treatment temperature are two key parameters [4]. For practical applications it is important to develop simple and economical techniques delivering high nitrogen flux (ions, molecules or atoms).

We have investigated the structural and mechanical properties of AISI 304L stainless steel after nitriding using two different techniques: low energy — high flux nitrogen ion implantation and plasma torch treatment at atmospheric pressure

Experimental conditions

The samples were prepared from commercial AISI304L bars containing 19.7 at % Cr and their surface was mechanically polished to a final roughness of 0. 01 mm. Nitrogen implantation was performed with a Kaufman type ion source and the nitrogen ions (N₂⁺ and N⁺) were accelerated with a voltage of 1.2 kV. The initial vacuum in the chamber is 10^–4 Pa and 10^–2 Pa during implantation. The majority of the experiments were performed with an ion flux of 10¹⁶ cm^–2s^–1 and a processing time of one hour.

The plasma torch was generated in the nitrogen working gas at atmospheric pressure [5] using a power density in the range of 10⁵–10⁷ W×m^–2 and the flux of atomic particles was about 10²¹cm^–2×s^–1. The mean temperature of the gas at the exit of the gun was about 3000–3500 K while the mean velocity was equal to 350–400 m×s^–1. No external heating was necessary since the plasma torch heated the samples, which are loaded on a sample holder cooled with water from the back. The temperature of samples was changed by controlling the distance between plasma gun and substrate and the temperature was measured with a thermocouple fixed into a hole drilled in the sample approximately 1 mm from the surface. For preventing surface oxidation during the treatment, an additional flow of nitrogen was directed onto the substrate surface.

Nitrogen depth profiles were determined by nuclear reaction analysis (NRA) and glow discharge optical spectroscopy (GDOS) and the microstructural state of the modified layers was analysed by XRD.

results

Plasma torch nitriding

The scanning electron microscopy (SEM) observations show that the roughness increases with both increasing treatment time and increasing temperature [6]. For a temperature lower than about 430° C, the formation of a nitrogen solid solution of f.c.c. structure called expanded austenite γ_N with a larger lattice parameter than the initial f.c.c. γ austenite is identified on the X-ray diffraction patterns. Figure 1 shows a glancing-angle XRD pattern obtained at incident angle a=5° for a sample nitrided 10min at 350° C (Figure 1a). The pattern is typical to the high N f.c.c. expanded austenite phase, with a set of broad peaks (labeled g_N) shifted towards lower diffraction angles by comparison with the initial austenite structure (labeled g). At temperatures higher than 430° C, glancing-angle XRD shows a gradual transformation of the high N f.c.c. phase into a b.c.c. a-ferritic phase and the precipitation of the f.c.c CrN phase. After 10 min at 550° C a full set of a peaks develops (Figure 1b). No g_N peaks are evident. High-energy flux in the plasma torch promotes early decomposition of the supersaturated f.c.c. phase.

An estimate of the thickness of the nitrided layer was made by a cross-sectional metallography. The scanning electron micrographs (SEM) in Figure 2 show that the modified layers are homogeneous and uniform in thickness. Extremely thick nitrided layers have been obtained as we can see on the SEM cross sections of samples nitrided at different temperatures.

Implantation-diffusion nitriding

All the nitrogen implanted samples exhibit non diffusional type of profiles with a quasi linear and slow decrease followed by a sharp leading edge [6, 7]. The phases formed in the nitride layer by implantation are similar to those observed after plasma torch nitriding: expanded austenite g_N. for temperature lowers than 430°C, and fcc CrN and a b.c.c. a-ferritic phase with a lattice parameter close to Fe which corresponds to a solid solution of Ni in Fe. However the nitrided layers are much thinner than those formed by plasma torch; a value of 3.5 mm is obtained for one hour at 400° C and 6 mm for one hour at 500° C. The hardness increases from 2.8 GPa to 15 GPa for the implantation at 400° C and to 18 GPa for the implantation at 500° C and a considerable reduction of the wear rate by a factor up to 80 is obtained [9]. The SEM micrographs of Figure 3 show that for the unimplanted steel, the wear tracks are wide and typical of an abrasive wear process of a ductile material. On the contrary for the implanted samples, the wear tracks are very narrow and smooth with a very weak contrast.

Figure 1. Glancing angle X-ray diffraction patterns (α=5°) of AISI 304 L samples after plasma torch nitriding: a — 10 minutes at 350° C, b — 10 minutes at 550° C