The investigation of mechanical properties of ion-nitrided AISI 5140 low-alloy steel

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Abstract

The ion nitriding behavior of AISI 5140 low-alloy steel was investigated under different process parameters including time (1, 4, 8, and 12 h), temperature (400, 450, 500, and 550 °C), and gas mixture ratio (0.05, 0.33, and 3 N2/H2). The ion nitriding properties of AISI 5140 steel have been assessed by evaluating fatigue strength, hardness profile, compound layer thickness, and case depth by using a rotating bending fatigue machine, a microhardness tester, and scanning electron microscopy (SEM) with energy dispersive spectrometry (EDS). It was found that ion nitriding improves the fatigue strength, which depends on increasing the case depth, but the compound layer does not have a dominant effect on the fatigue strength. After the fatigue tests, sections of ion-nitrided specimens were observed to have failed by the fish eye phenomenon with the fatigue cracks originating from nonmetallic inclusions.

Introduction

Ion nitriding is a glow discharge surface modification technique, which is primarily used to increase the fatigue strength, wear and corrosion resistance, and surface hardness of ferrous alloy steels [1], [2]. Ion nitriding has recently received considerable industrial interest owing to its characteristic of faster nitrogen penetration, simplicity in application, economic and easier control of compound and diffusion layer formation compared with conventional techniques such as gas and liquid nitriding [3].

As a result of ion nitriding, different structures occur on the surface of steel, known as the compound layer and case depth from surface to core, respectively. Work has shown that the microstructure of the surface layers can be affected by process parameters such as temperature, time, and gas mixture ratio [4], [5], [6]. The change of microstructure of the surface layers changes the properties of fatigue resistance and surface hardness. The compound layer provides the materials with good physical properties against wear and corrosion. However, in the case of a very hard and brittle layer in a construction subjected to stresses, a thinner compound layer or no compound layer is necessary, since when the material is subjected to stresses, cracking begins in the layer [7]. The nitrided case depth, which encompasses comprehensive residual stresses and metal nitrides, provides enhanced fatigue resistance and surface hardness [8], [9].

In this study, AISI 5140 low-alloy steel was ion nitrided under different conditions. The process variables were investigated including time (1, 4, 8, and 12 h), temperature (400, 450, 500, and 550 °C), and gas mixture ratio (0.05, 0.33, and 3 N2/H2) under a constant pressure of 5 mbar. The effect on mechanical properties of the structures developed after ion nitriding was investigated by using a rotating bending fatigue machine, a microhardness tester, and scanning electron microscopy (SEM) with energy dispersive spectrometry (EDS).

Section snippets

Experimental details

A series of experiments has been carried out to investigate the ion nitriding response of AISI 5140 low-alloy steel with the following composition: 0.426 wt.% C, 0.77 wt.% Mn, 0.287 wt.% Si, 0.845 wt.% Cr, 0.134 wt.% Ni, 0.054 wt.% Mo, 0.0287 wt.% V, 0.064 wt.% S, 0.154 wt.% Cu, 0.0197 wt.% P. The specimens were normalized at 850 °C for 30 min, and then were cooled in air. After cleaning with alcohol, the specimens were placed into the plasma nitriding chamber and the chamber was evacuated to

Results and discussions

The compound layer thickness, case depth, surface hardness, and fatigue strength of ion-nitrided material depending on the process parameters are given Table 1.

The maximum case depth is obtained at a temperature of 550 °C, a time of 12 h, and a gas mixture ratio of 0.33 N2/H2. The case depth increases with increasing time and temperature as expected for diffusion-controlled growth. The decrease in case depth with increasing gas mixture ratio can be explained with the influence of changing

Conclusions

The following conclusions can be derived from the above results and discussion.

⋅ The fatigue strength of AISI 5140 steel was increased by up to 45% by an ion nitriding process. It was seen that the fatigue strength of the material depends on the thickness of the case layer. The fatigue strength increases by increasing the case depth.

⋅ The compound layer had no dominating effect on the fatigue behavior of ion-nitrided specimens.

⋅ The sections of ion-nitrided specimens after fatigue tests were

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