PSI - Issue 33

Julio A. Ruiz Vilchez et al. / Procedia Structural Integrity 33 (2021) 658–664 Author name / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction Maraging steel 300 presents high values for mechanical properties, which are difficult to combine: mechanical strength, machinability, and good weldability capacity; the last property because the low carbon content. New trends in recent investigations have been oriented to improve the machining process, Bai et al. (2021), Sen et al. (2021); to incorporate new technologies in the manufacturing process, Turk et al. (2019), Mooney et al. (2020). Concerning mechanical properties, and particularly fatigue endurance on this steel, recent investigations have been conducted on conventional fatigue: tests in maraging specimens obtained by Selective Laser Melting (SLM), revealing orientation dependence on built SLM, when the testing conditions were: room temperature of 400  C, load ratio R=-1 and frequency of 50 Hz, Damon et al. (2019). Fatigue endurance improvement by heat post-treatment on the additively manufactured 18Ni300 maraging steel, Elangeswaran et al. (2020), with the testing conditions: hourglass shape specimen, room temperature, zero mean load ratio R = -1,and frequency of 30 Hz. Ultrasonic fatigue tests in this steel have been obtained with the use of intermittent ultrasonic fatigue tests in maraging steels, Ishii et al. (2002); concluding that fatigue life is related to the size of the nonmetallic inclusion associated with crack initiation; or the fatigue endurance improvement under ultrasonic fatigue tests with R= 0.1 of 18Ni maraging steel, by modification of the original TiN-inclusions, with less damaging aluminate inclusions, Karr et al. (2017). Under ultrasonic fatigue tests, high frequency induces an important decrease in the cyclic strength compared to its original static strength of this steel, which is associated with cyclic softening and growing of precipitates during cyclic deformation. A reduced number of studies have been developed on the ultrasonic fatigue endurance of maraging 300; the main purpose of the present work is to contribute to assess the ultrasonic fatigue behavior of this high strength steel. 2. Materials and methods Bars of maraging 300 of dimensions: 304.8 mm in length (12 inch), and 9.525 mm of diameter (3/8 inch), were bought to the American company Free Steel Company, Santa Fe Springs, CA, 90670, USA. The chemical composition in weight and main mechanical properties are shown in Table 1 and Table 2, respectively. Table 1. Chemical composition in weight of maraging 300. Fe Ni Co Mo Ti Al 63 Max. 18 Max. 12 Max. 4.8 Max. 1.4 Max. 0.1 Max. Table 2. Main mechanical properties of maraging 300. Density  y  u Poisson ratio Elasticity Module  Kg/m 3  8100  MPa  2020  MPa  2055 (-) 0.3  GPa  210 In order to carry out ultrasonic fatigue tests in this steel, the profile of testing specimen was obtained by modal numerical simulation to fit the resonance condition: the natural frequency of oscillation of specimen in longitudinal direction, must be close to the frequency of excitation of ultrasonic testing machine (20 KHz),  300 Hz. Testing specimen was chosen as hourglass shape, conserving the original diameter and modifying the length and the narrow section to meet the resonance condition. In Figure 1 are illustrate the Finite Element