Abstract
The aim of this work was to study the microstructure and friction coefficient of hybrid surface layers, produced by a controlled gas nitriding and laser modification. Nitriding is well-known technique of thermo-chemical treatment, applied in order to produce the surface layers of improved hardness and wear resistance. The phase composition and growth kinetics of the diffusion layer can be controlled using a gas nitriding with changeable nitriding potential. 42CrMo4 steel was treated by composite technology of gas nitriding and laser hardening. The nitriding processes were carried out at temperature of 580 °C for 8h. Next, the nitrided layer was laser-modified using laser TRUMPF TruDiode 3006 with maximal power of 3 kW using the two laser beam powers (P): 0.53 kW and 0.62 kW. Then, the microstructure and properties of the laser-modified nitrided layers were investigated using optical microscopy, Vickers hardness tester and friction wear testing machine. The nitrided layers were subjected to wear tests using a ball-on-disc method at room temperature. The results showed that the microstructure of the produced hybrid layers consisted of the re-melted and heat-affected zones in which martensite mainly occurred. Additional laser treatment effectively increased the hardness, especially in heat-affected zone, as well as the depth of the hardened layer. The layer after modification laser hest tretment were good friction coefficien. The curve of the friction coefficient after the nitrided layer was characterized by large fluctuations. Compared with nitriding technology , the hybrid treatment technology can effectively increase the hardness and wear resistance of the 42CrMo4 steel surface. The effect of LHT on the tribological properties was ambiguous. Although the relatively low value of the average friction coefficient (0.46) was calculated for nitrided layer, the course of friction coefficient was characterized by large fluctuations and the extended grinding-in time. Simultaneously, the course of friction coefficient was very smooth after nitriding and LHT. However, the average friction coefficient were higher, obtaining 0.60 and 0.57 for the hybrid layers, produced using P=530 W and P=620 W, respectively. Keywords: Nitriding; Laser Heat Treatment; Microstructure; Wear Resistance; Microhardness
Highlights
Laser surface modification is increasingly used in industrial applications
Nitriding process is well-known as a method of surface treatment which improves wear resistance properties [4]
Controled gas nitriding enabled the control and regulation of the growth of the nitrided layer using the changeable value of the nitriding potential
Summary
Laser surface modification is increasingly used in industrial applications. Laser hardening or laser alloying with different elements greatly increased the hardness and wear resistance of machine parts, especially those containing carbides, nitrides and borides [1,4]. Laser materials processing can be carried out in three different ways: without re-melting, with re-melting or with vaporization. Such a treatment is usually performed to improve the wear characteristics of materials [1]. Nitriding process is well-known as a method of surface treatment which improves wear resistance properties [4]. The nitriding potential is very important to control the growth kinetics of the diffusion zone, and the thickness and phase composition of the iron nitride zone (compound zone) [10]. As a consequence of gas nitriding, the two zones usually appear in the surface layer: a compound zone on the top surface and a diffusion zone beneath it [7]. The compound zone consists of intermixed iron nitrides, ɛ (Fe3N) and γ’ (Fe4N) phases, while the diffusion zone consists of a nitrogen rich solid solution zone with precipitates of nitrides [8,11,12]
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