PSI - Issue 51

Milan Uhríčik et al. / Procedia Structural Integrity 51 (2023) 166 – 172 M. Uhrí č ik et al. / Structural Integrity Procedia 00 (2022) 000–000

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1. Introduction For more than 100 years, researchers have been working on fatigue fracture, and it is still one of the most important topics in the field of materials. Nowadays, fatigue damage, in the biaxial or multiaxial state, is still studied primarily in the case of metallic materials. According to Major (2020), the fracture surface can be considered as a record of the degradation process and a lot of information about the material can be obtained from it. The good availability of SEM has opened the way to understanding fracture structures in different viewing directions and also enabled their quantification. Many different chemical-thermal treatments are used to improve the mechanical properties of material surfaces, such as hardness and wear resistance. Fernandes et al. (2008) and Fontes et al. (2014) mention that nitriding is a very often used material surface treatment to achieve improvements, mainly due to its ease of use, relatively low cost and good ability to improve surface properties. There are three different types of nitriding processes, namely liquid nitriding, gas nitriding and plasma nitriding. The last mentioned can be considered the best, because according to Fernandes et al. (2008) and Fontes et al. (2014) it has positive properties, such as precise control of surface layers, low energy and gas consumption, does not create environmental pollution and allows heat treatment at low temperatures (below 500 °C). Plasma nitriding is a chemical-thermal treatment process that induces surface hardening by interstitial diffusion of atomic nitrogen into metal surfaces. And according to Cocke et al. (1989) and Gontijo et al. (2004), this process causes the formation of a case layer, which can contain an oxide layer, a compound layer and also a diffusion zone. Fontes et al. (2014), Collins et al. (1996), Muñoz Riofano et al. (2006) and Borgioli et al. (2005) emphasize that temperature, pressure, processing time and the composition of the base material, all this affects the composition and thickness of nitride layers. Therefore, it is very important to follow the parameters of the nitriding operation in order to achieve the best performance of nitride components that are used in various industrial processes. According to Menthe et al. (1999), austenitic stainless steels are often used in industry as structural materials, especially in the chemical and food industries. The corrosion resistance of austenitic steels is excellent, but the hardness and wear resistance are relatively low. There have been many experiments on how to increase the wear resistance without deteriorating the corrosion resistance. Ichii et al. (1986), Dearnley et al. (1989), Saker et al. (1991) and Menthe et al. (1995) found that plasma nitriding improved wear resistance and corrosion properties, but only if the maximum process temperature was below 475 °C. This creates a surface layer with excellent properties. This study is focused on the analysis of the fatigue life of austenitic steel samples, which will be in the initial state and after chemical-thermal treatment, and the analysis of the fracture surfaces of samples after fatigue tests. 2. Material and testing setup The experimental material used in this work was an austenitic stainless steel corresponding to AISI 304 class. The chemical composition of samples is C-0.05 Cr-18.06 Ni-7.98 Mn-1.51 Si-0.34 Mo-0.23 S-0.04 Fe-balance in mass%. A Neophot 32 optical light microscope was used to evaluate the microstructure of the experimental material. According to Kucharikova et al. (2017) and Belan et al. (2020), samples for light microscope metallographic observation were prepared by standard metallographic procedures and etched with reagent Kalling´s 2. Three-point cyclic bending fatigue tests (Fig. 1a) were performed on two set of samples of AISI 304 stainless steel. The first set of samples was in the initial state and the second set of samples was after chemical-thermal treatment. An unconventional method was chosen as a chemical-thermal treatment, which is represented by plasma nitriding. According to Larisch et al. (1999), the plasma nitriding process was chosen, which took approximately 26 hours at a temperature of 450 °C. For experiments, test samples without any notch were used, which were cut from a rectangular bar with dimensions of 55x10x10 mm. The shape and geometry of samples is shown in Fig. 1b. Fatigue tests were performed by Vibrophores Amsler 150 HFP Zwick/Roell and testing samples were preloaded with 10 kN and then cyclic loaded with different amplitudes. The loading frequency was approximately 85 Hz.