Cristiano José Scheuer

Low-temperature plasma nitriding of sintered PIM 316L austenitic stainless steel

This work reports experimental results on sintered PIM 316L stainless steel low-temperature plasma nitriding. The effect of treatment temperature and time on process kinetics, microstructure and surface characteristics of the nitrided samples were investigated. Nitriding was carried out at temperatures of 350, 380, 410 and 440 oC , and times of 4, 8 and 16 h, using a gas mixture composed by 60% N2 + 20% H2 + 20% Ar, at a gas flow rate of 5.00 × 10-6 Nm3s-1, and a pressure of 800 Pa. The treated samples were characterized by scanning electron microscopy, X-ray diffractometry and microhardness measurements. Results indicate that low-temperature plasma nitriding is a diffusion controlled process. The calculated activation energy for nitrided layer growth was 111.4 kJmol-1. Apparently precipitation-free layers were produced in this study. It was also observed that the higher the treatment temperature and time the higher is the obtained surface hardness. Hardness up to 1343 HV0.025 was verified for samples nitrided at 440 oC. Finally, the characterization of the treated surface indicates the formation of cracks, which were observed in regions adjacent to the original pores after the treatment.

Low-temperature Plasma Assisted Thermochemical Treatments of AISI 420 Steel: Comparative Study of Obtained Layers

Formation of metastable C–, N–, or even Formation of metastable C–, N–, or even C/N– expanded phases can be observed for typical non-equilibrium conditions attained at the plasma assisted thermochemical treatments when temperatures relatively low are used. In present work, kinetics data are considered in a comparative study comprising low-temperature plasma assisted carburizing, nitriding and nitrocarburizing of AISI 420 martensitic stainless steel samples treated at 350, 400, and 450 °C, aiming to put in evidence the main metallurgical differences of the obtained layers. Microstructural characterization and hardness measurement results for untreated and treated sample surfaces show significant difference for the carburized layer growth in relation to that verified for the nitrided and nitrocarburized layers. While the carburized layer is constituted of a thin outer layer and a deep diffusion layer, just the opposite was observed for the other two treatments, id est., formation of thicker outer layers and thinner diffusion layers. Finally, carbide-/nitride-precipitation-free layers were supposedly obtained for samples carburized, nitrided, and nitrocarburized at 350 °C temperature.

MICRO-ABRASIVE WEAR BEHAVIOUR OF LOW-TEMPERATURE PLASMA CARBURIZED AISI 420 MARTENSITIC STAINLESS STEEL

Experiments were carried out aiming to study the influence of micro-abrasive wear test variables and plasma treatment parameters, on the tribological characteristics of low-temperature plasma carburized AISI 420 steel. The first part of this study was conducted in order to evaluate the influence of abrasive particle size, normal load and counter body rotation speed on carburized sample wear behavior; and the second part was conducted in order to evaluate the treatment temperature and time effect on carburized samples wear behavior. Plasma carburizing treatments were performed at 8 and 12 h, for temperatures ranging from 350 to 500°C; and at constant temperatures of 400°C for times between 12 to 48h, and for 450°C for times of 4 to 16h. Micro-abrasive wear test were performed by mean of a calowear ballcratering equipment using a polished ball of AISI 52100 steel rotating at 80, 120 and 160 rpm, for sliding distances from 2.6 to 104.2 m. Alumina abrasive particles with sizes of 0.05, 0.3 and 1.0 m were employed. The normal loads applied on this study were 0.1, 0.3 and 0.5 N. Carburized sample was characterized by mean of OM, XRD and hardness measurements. Worn crater characteristics were evaluated by confocal laser microscopy. Results showed that the transient wear regime prevails along the outer layer length (composed by Fe3C and ’C), and the steady-state wear stabilizes in the diffusion layer region (composed by ’C). It was verified that the wear rate and wear coefficient decreases with treatment temperature in the range of 350–450°C and increase for 500°C. It was also verified that the wear rate and wear coefficient decreases with treatment time in the range of 12–36 h (for 400°C cycle) and 4–12 h (for 450°C cycle), and increase for larger treatment times due the chromium carbide precipitation. It was also found that the wear coefficient increases with the increase of abrasive particle size, and decrease with the increase in counter body rotation speed and normal load. Lastly, the analysis of the worn craters indicates the occurrence of grooving and micro-rolling abrasion wear mechanisms.