Phase constitution control of plasma nitrided layer and its effect on wear behavior under different loads
Introduction
Wear resistance can be considered as one of the most important properties for heavy load bearing components, and the wear behavior under heavy loads is extremely crucial, which involves abrasion, scuffing and contact fatigue, thus easily bringing about frequent and complex damage in surface layer. Therefore, surface modification is a necessary process to enhance the wear resistance for load bearing components [1,2].
Plasma nitriding, a typical thermochemical surface treatment, is widely used in steels for improving the hardness and wear resistance through the formation of a gradient structure with the outermost compound layer and subsurface diffusion zone [[3], [4], [5]]. Generally, the compound layer is consisted of ε-Fe2-3N and γ′-Fe4N dual phases, which can bring a positive contribution to wear resistance under light loads [6,7], unfortunately, the dual phase compound layer is friable and brittle, and will be crushed under conditions of impact and heavy loads, and thus, has to be removed or completely suppressed for heavy loads components. Although the compound-free nitrided layer has good toughness, its wear resistance is too poor to meet the requirements in some heavy load bearing cases due to its relatively lower hardness.
It was reported that γ′ single-phase compound layer has good combination of hardness and toughness, which can offer better toughness than that of double-phase compound layer, and higher surface hardness than that of compound-free nitrided layer [[8], [9], [10]].
In this study, AISI 4340 steel, a kind of typical nitriding steel is selected as the material, which is widely used as components requiring high wear resistance, such as plungers, high pressure valves and precision gears. Plasma nitriding is often adopted in order to meet the technical requirements of components.
Therefore, it is valuable to investigate the relationship between the phase constitutions of nitrided layer and wear resistance under different loads; the research goal is to obtain the optimal wear resistance under different loads by controlling the phase constitution of nitrided layer. For this perspective, plasma nitriding was performed for AISI 4340 steel by adjusting nitrogen ratio to get designed nitrided layers with different phase constitutions, the effect of phase constitutions of nitrided layer and wear resistance under different loads was investigated.
Section snippets
Materials and methods
The material used in this study was AISI 4340 steel with the following chemical compositions (wt%): C, 0.35–0.42; Cr, 1.35–1.65; Mo, 0.15–0.25; Mn, 0.30–0.60; Al, 0.70–1.10 and Fe balance. The specimens were machined to the dimensions of 10 mm × 10 mm × 5 mm, and heat treated at 930 °C, oil quenched, tempered at 600 °C to get tempered sorbite. The mechanically polished specimens were treated with emery papers of different granulometries (240, 500, 1000 and 2000 mesh) to achieve a fine finish,
Microstructure analysis
Fig. 1 shows the cross-sectional SEM image of samples after plasma nitriding with different nitrogen ratio at 510 °C for 4 h. It can be clearly seen that a compound-free nitrided layer is formed on the surface of sample at nitrogen ratio of 15% (Fig. 1(a)), a compound layer with a thickness of 1.5 μm is formed when the nitrogen ratio is increased to 20% as shown in Fig. 1(b). And the thickness of the compound layer increases with the increase of the nitrogen ratio, a thicker compound layer of
Mechanism discussions
The wear characteristics analysis results shown in Figs. 6(a–c) and 7 illustrate that the wear resistance increases with the hardness due to the dominant adhesive wear mechanism under light load of 200 g, and no crush and hard abrasive particles have been formed by compound layer spalling at this wear load [[18], [19], [20], [21], [22]], thus ε + γ′ dual-phase layer owns optimal wear resistance under light load of 200 g.
However, under heavy load of 400 g, ε + γ′ dual-phase compound layer is
Conclusions
In summary, in order to control the phase constitution of nitrided layer and investigate its effect on wear behavior under different loads, plasma nitriding was conducted at different nitrogen ratio under the same nitriding condition of 510 °C and 4 h by adjusting nitrogen ratio for AISI 4340 steel. The following conclusion could be drawn.
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Phase constitution of nitrided layer could be precisely controlled by adjusting nitrogen ratio during plasma nitriding, a compound-free layer; single γ′ phase
CRediT authorship contribution statement
Tiantian Peng: Writing - original draft, Writing - review & editing, Visualization, Validation, Investigation. Yao Chen: Validation, Investigation, Data curation, Writing - review & editing. Xiliang Liu: Supervision, Resources, Data curation, Writing - review & editing. Meihong Wu: Supervision, Resources, Data curation, Writing - review & editing. Yangyang Lu: Validation, Investigation, Data curation, Writing - review & editing. Jing Hu: Conceptualization, Resources, Writing - review & editing,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The research was supported by National Natural Science Foundation of China (51774052, 21978025), Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), Top-notch Academic Program Projects of Jiangsu Higher Education Institutions (TAPP) and Jiangsu Province Graduate Student Innovation Fund (SJCX18_0955& KYCX19_1754).
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