Plasma nitriding behavior of AISI 5140 steel

The plasma nitriding behavior of a low-alloy steel, AISI 5140, has been examined under various process conditions. The process variables studied include time (l-12h), temperature (450-550°C) and nitrogen partial pressure (10-60voL%N2) of nitriding atmosphere, which consisted of a mixture of N2+H2 with a total pressure of lOmbar. Microhardness measurements, optical microscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques have been utilized to investigate the nitriding response of the steel The work has focused on the surface hardening behavior of the steel, the composition and morphology of the nitrided layers, and studies of fatigue and wear properties of plasma nitrided specimens. A reversed bending fatigue testing machine was used in fatigue tests. Wear tests were carried out under dry sliding conditions using a pin-on-ring configuration. The maximum surface hardness was obtained in the sample plasma nitrided over a period of 4h at 500°C in a nitriding atmosphere containing 20voL% N2. For shorter treatment times and/or lower temperatures the amount of hardening was low due to the lower nitrogen uptake and precipitate density. Due to the coarser precipitates, extended nitriding periods and/or higher treatment temperatures led lower surface hardness as well XRD studies revealed that the outermost layer or compound layer of samples are made up of a mixture of ¦/-nitride and E-nitride phases. Formation of a predominantly ¦/phase compound layer structure is favored at high temperatures, for long treatment times and by the use of low nitrogen atmospheres. The case depth and compound layer thickness were found to increase with increasing process temperature and time. Nitrogen content of the plasma nitriding atmosphere while increasing the compound layer thickness, slightly decreased the case depth. The fatigue and wear tests revealed substantial improvements depending upon the structural characteristics of the component ie. plasma nitriding process conditions. It was found that fatigue strength of a plasma nitrided sample was determined by its hardness and case depth. Fatigue fractography study using SEM revealed subsurface crack initiation especially on samples tested at low stress levels. Compound layer thickness, which seemed to have no significant effect on fatigue test results, played an important role in wear tests. Wear rates were observed to increase with increasing compound layer thickness and decreasing surface hardness.