PSI - Issue 13

M. Seleznev et al. / Procedia Structural Integrity 13 (2018) 2071–2076 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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improve core and surface hardness of specimens. As chemical composition slightly differs for every FT batch, core hardness and nitriding hardness depth (NHD) of fatigue specimens also differs as shown in Table 1. Detailed description of the fatigue specimen ’ s preparation along with the sectional profiles of hardness (Vickers indentation) and internal stresses (X-ray diffraction) are given elsewhere [15]. 2.2. Experimental techniques Fatigue tests were performed using an ultrasonic fatigue testing (USFT) machine (BOKU, Vienna, Austria) enabling testing frequencies of about 20 kHz. The tests were carried out under symmetrical push – pull loading conditions (R = -1) at room temperature in pulse-pause mode under forced air cooling. Fractographic analysis of each specimen was performed using field-emission scanning electron microscope (SEM, Tescan, Czech Republic) with energy dispersive X-ray (EDX) chemical analysis system (EDAX, Ametek). 3D scanning of crack initiation sites was performed using confocal laser scanning microscope (CLSM, LEXT OLS 4000, Olympus). The sectional data was obtained with the help of an optical semi-automatic measurement system (Particle analyzer, Olympus).

3. Results and discussion 3.1. VHCF testing results

S-N curves after USFT testing of 42CrMo4 FT-batches show typical hardened steel behavior with FGA presence at ≥ 10 6 cycles and fatigue limit at 10 7 - 10 9 cycles (Fig. 1a). Despite the reported reduction of NMI population due to FT [7], fatigue strengths do not show any noticeable improvement (Fig. 1). Experimental fatigue limit σ f , estimated from runouts (tests without failure with cycle number of ≥ 10 9 ) is either on the same level (FT3, 4) with the reference batch without filter (FT2), or lower (FT5-8).

Fig. 1. Ultrasonic fatigue testing results for specimens after finger tests (differentiated by color) of sulfur-lean (circles) and sulfur-rich (squares) 42CrMo4 steel according to Table 1 (a) and comparison of experimental fatigue limits (triangles) with ones calculated from areas of failure response NMIs, using eq. (1) according to Murakami (dot lines) and its modification eq. (2) according to Masumoto (straight lines) (b)