Study of the Effect of Gas Nitriding Time on Microstructure and Wear Resistance of 42CrMo4 Steel

Prior studies have noted that gas nitriding has a considerable effect for wear resistance. The aim of this paper is to study the influence of gas nitriding time (12, 24, 36 and 48 h) in the wear behaviour of 42CrMo4 steel. It has been assessed by micro hardness, pin-on-disc tribosystem, and SEM through the nitrided layer for each nitriding time. The study relates to the performance of the compound layer and the diffusion layer with respect to adhesive wear. The results were analyzed in terms of the weight lost during wear, for nitrided steel with and without the compound layer, and for untreated steel. It has been observed that wear rate varies as a function of the tests conditions due to the presence of different wear mechanisms. Thus, for short tests conditions wear rate depends on two mechanisms: plastic deformation and adhesive wear, whereas for large tests conditions the mechanisms controlling wear rate are abrasive and oxidative wear. Furthermore, this study contains an analysis of the wear mechanisms of a nitrided part, founded on scanning electron microscopy (SEM) observations of the wear traces at various stages of the evolution of wear. The SEM examination of worn surfaces revealed signatures for the adhesion, abrasion, delamination and tribochemical (oxidative) modes of wear. This is an important issue for future research.


Introduction
Nitriding process is a chemical-heat treatment consists in introducing nitrogen into metallic materials to improve their surface hardness, fatigue life, wear and corrosion resistance [1] [2] [3]. In engineering the depth of nitriding is generally about 0.3 -0.4 mm; this thin harder surface layer may cause surface layer spall from matrix because of high-applied load or great stress gradient in surface layer [3] [4] [5] [6].
During nitriding, two different structures are formed from surface to core, known as the compound layer and diffusion region. Wear characteristics of the compound layer depend on many factors such as composition ε/γ'), thickness and mode of mechanical loading [7] [8] [9] [10]. The principal reasons why the surface of steel benefits from nitriding, are the fact that wear and friction properties are improved by structure and microstructure modification, especially via a hardness increase due to the formation of an interstitial solid solution or compound layer [11] [12] [13] [14]. The parameters governing the wear resistance are multiply depending on relative speed, contact pressure, lubricant, and properties of the material itself (hardness surface and residual stress) [15] [16] [17] [18] [19].
The purpose of this work is to study the tribological properties of gas-nitrided 42CrMo4 steel, and to determine the influence of the compound and diffusion layers on wear behavior. 42CrMo4 steel is gas-nitrided under different time (12,24, 36 and 48 h). The effects on tribological properties of the structure formed after gas nitriding are investigated using a pin-on-disk wear test, micro hardness tester, and scanning electron microscopy (SEM).

Material and Techniques
The material used in this study was a 42CrMo4 low-alloy steel with the following composition (wt%): 0.41% C; 0.77% Mn; 0.28% Si; 1.02% Cr; 0.16% Ni; 0.16% Mo; 0.25% Cu and balance Fe. The material was quenched at 850˚C, cooled in oil and tempered at 580˚C. The metallurgical structure obtained is a tempered martensite.
Cylindrical samples (15 mm × 16 mm) were cut out and rectified under identical conditions to obtain a uniform surface quality for all the studied parts. The average roughness (R a ) was 0.5 μm. One series of specimens were submitted to surface treatment by gas nitriding (temperature (θn) = 525˚C; Ammonia dissociation degree (τ) = 35%) under different time (12,24, 36 and 48 h).
Nitrided layer were observed by using a scanning electron microscope (SEM) and X-ray diffraction (XRD). Investigations of compound layer were carried out using a Siemens Analytical D5000 diffractometer with Cu anode. The HV 0.05 micro hardness profile of the nitrided layers was obtained in the transverse section by means of Dhimadzu MMV-2 instrument. Three indentations were placed into the compound layer at equal depth in order to define the micro hardness profiles. Residual stress measurements were obtained using the X-ray diffraction method. The diffraction conditions are reported in Table 1.

The Tribosystem
The experimental set-up used to investigate wear significantly affects the me-chanisms of wear recorded later on. The device used in this work is based on the initial by linear cylinder/plan contact represented in Figure 1; it is frequently seen in industrial applications such as cams, push rods and rollers. This configuration induces adhesive wear. The cylinder of diameter 200 mm is made from X200CrMoV12 steel. The parameters considered are: • The normal loading (50 N < F N < 225 N) and relative speed (3.1 m/s < V < 8.3 m/s) which are two energy parameters that influence the response of the material.
• The nature of the surface layer. Three distinct surface qualities were studied: untreated 42CrMo4 steel, treated steel with its compound layer, and treated steel without a compound layer. This layer was eliminated by grinding.
The weight loss was measured by weighing with a command precision of 10 −4 g. All measurements were carried out under dry conditions. During each test the friction coefficient was recorded. Finally, the worn surface of nitrided samples was characterized by scanning electron microscope "SEM" observations to investigate the wear mechanisms.

Micro Structural Characterization
Vickers micro hardness was evaluated at the cross-section of the specimens for  Number of θ angles both treated and non-treated specimens. The results are plotted in Figure 2 against the distance from the surface; the depth hardness profile as a function of nitriding time. A maximum value of hardness of about 1100 HV was observed near the surface within the white layer. In the diffusion layer, the hardness decreases with the distance from the surface. For depths above 0.6 mm, the hardness remains at 355 HV. The micro hardness measurements were repeated for several specimens and no significant differences were found. Comparing the hardness of the treated and the untreated specimens for depths greater than 0.6 mm, similar values of about 355 HV were observed.
The metallurgical investigations carried out on the surface layer ( Figure 3) and scanning electron microscopy "SEM" (Figure 4), show the existence of a compound layer, made up of two phases, γ' (Fe 3-4 N) and ε (Fe 2− N), and a hardened diffusion layer. Table 2 shows the change of the compound layer thickness, case depth, sur-    The case depth increases with increasing time, as expected for diffusion-controlled growth. The surface hardness of 42CrMo4 steel was increased up by the gas nitriding process. The maximum surface hardness was observed at a treatment time of 24 h.

Wear Resistance Improvement
Results of adhesive wear tests on the 42CrMo4 steel, with and without gas nitriding treatments are show in Figure 5. Weight losses are less for the gas- nitrided test specimens than for the quenched and tempered specimens which demonstrates the effectiveness of the gas-nitriding treatment on wear resistance.
Wear resistance of the nitrided specimen was due to increased resistance against plastic deformation by increasing surface hardness. The improved wear resistance is considered to be related to the combined effect of the solid solution of

Untreated T2
Cr and the high chemical stable phases γ' and ε formed on the steel surface during gas nitriding.
When it is compared to the nitrided steels in Figure 6, it is observed that the 24 h nitrided samples resulted in improved adhesive wear resistance than this improvement is due to the higher surface hardness values. Nitrogen atoms take interstitial places among the parent α structure and form surface layers by expending the lattice. The possibility of CrN formation increases with increasing treatment time and therefore these layers dissolve. Chromium rich oxide film formed on the surface is a continuous layer and if its thickness, intensity and adhesion are well enough, it causes an increase at the wear resistance of the specimens. Figure 7 shows results of adhesive wear test with and without com-

Wear Mechanisms of the Nitrided Parts
An analysis of the wear mechanisms of the nitrided parts was carried out using    Figure 9 shows a wear crack with a compound layer. In the initial period of sliding, the brittle compound layer with high stress fractured and then transformed into abrasive particles. Higher magnification reveals that this layer initially cracked and broke into pieces. The SEM analysis of the wear remains enabled us to note that the wear particles with compound layer are detached from scales because of the porosity and the brittleness of this layer.
These observations revealed that the compound layer is delaminated as dice during the first cycles of wear. This observation is confirmed for all the loading levels applied and for all speeds used, as illustrated in Figure 10. In this figure, one notes that the worn thickness is greater than the thickness of the compound layer even for the low loading levels. SEM images of the wear debris collected during the first cycles are presented in Figure 11. The general aspect observed shows that this debris was removed in the form of plates per delamination. It is noticed that for severe conditions the number of folds multiplies with the presence of micro cracks on the level of the compound layer. Indeed the matter re-    moved during the wear test is stuck on surface used creating the third body necessary to improve the wear resistance of the nitrided parts ( Figure 12).
The SEM observations of the transition zone worn surfaces/no worn surfaces showed that the matter is removed in the shape of the plates by delamination.
One can deduce that the compound layer has a weak adhesion with the sub-

Discussion
The microstructure of the layer obtained by gas nitriding treatment is composed of a diffusion zone and a compound layers. Uniform and thick layers and un-   When a normal load was applied without relative movement between the bodies, the compound layer did not separate from the substrate but hammering traces were observed, the breadth of which depended on the load level [

Conclusions
In the present work, the wear behaviour of 42CrMo4 steel has been studied according to the nitriding time used during gas nitriding. The most important conclusions obtained are: 1) Nitriding results in a considerable increase of hardness and the formation of compressive residual stress. ε (Fe 2+3 N) and γ' (Fe 4 N) nitrides are indispensable to produce the highest hardness. The nitrided case supports sufficiently the hardcoating.
2) Compared to the base line, quenched and tempered samples, adhesive wear tests demonstrated that the application of gas nitriding was effective in enhancing the hardness and adhesive wear resistance of the samples composed of the low alloy 42CrMo4 steel. The formation of a thin compound layer resulted in optimum wear properties. It is found that the compound layer is eliminated very quickly. It is thus vulnerable to delamination. The resulting particles constitute a third hard body which contributes to the wear by diffusion layer.
3) It is possible to obtain the desired compound of hardness, structure and thickness of the compound layer leading to optimal wear resistance of the 42CrMo4 steel reported here, by controlling the gas nitriding time.

4) Adhesive wear resistance increases with hardness increasing. When oxide
was produced on the worn surface, the governing wear mechanism was tribooxidation, which causes less wear. On the contrary, when oxide was absent the governing wear mechanisms were abrasion and adhesion.
By referring to the experimental results of this study, the wear rate can be predicted by numerical models based on the neural network to better describe wear resistance behaviour of nitrided specimens as a function of the conditions test.