PSI - Issue 23
Barbara Nasiłowska et al. / Procedia Structural Integrity 23 (2019) 577– 582 Nasiłowska B./ Structural Integrity Procedia 00 ( 2019) 000 – 000
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Possible places for initiation of the material fatigue phenomenon development are local places of stress concentrations in the material (structural heterogeneity), geometric notches (at the place of cross section changes) or a change in the element curvature. In these places, the material yield strength is exceeded locally and permanent plastic strains (a basic condition for development of fatigue phenomenon of material and structure) occur. The magnitude of stress in the notch or value σ max of the maximum stress induced on the notch bottom depend on the notch geometry, characterized by a theoretical coefficient of stress concentrations α k (Cichański (1996), Kocańda (1985)). An influence of the notch on the maximum stress value is also described by Neuber coefficient for notch impact. The geometric and structural notch (material structural heterogneity) influences a local change in stress and strain distribution in the structural element. A method to estimate local stress and strain was provided by Neuber (1958). Damage to chemical and industrial instalations, due to long term service, results mainly from material defects, and welding notches and defects leading to initiation of fatigue fractures (Frazzini et al. (2007), Palmer et al. (2008)). Takakuwa and Soyama (2012) as well as Hatamleh (2009) pointed out the slowing of an increase in fatigue fracture in 1.4404 austenitic steel after shot peening. As a result of structural changes, hardness and residual stress, the experimental tests discussed in works (Gao (2008), Hassani-Gangaraj et al. (2014), Terres et al. (2012), Wei et al. (2002)) showed that the process of steel machining in the forms including shot peening, nitriding and hammering increases the fatigue life by approximately 30% , shifting the initiation of fracture deep into the material. 2. Methodology The research object was 1.4539 austenitic steel (with the following percentage: Ni 24-26, Cr 19-21, Mo 4-5, Cu 1.2-2.0, Mn ≤ 2.0, N ≤ 0.15, Si ≤ 0.7, P ≤ 0.030, S ≤ 0.010, C ≤ 0.02) welded with TIG and laser methods. Welding with a ϕ 2.5mm MTC MT-904L infusible tungsten electrode and with G/W 20 25 5 CuL (20% Cr, 25% Ni, 4.5% Mo, 1.5% Cu) weld was carried out in ZBACH, Capital Group of Zakłady Azotowe in Tarnow, according to production technology applied technically to perform, among others, chemical apparatus (Fig.1 a). The laser welding was carried out at the Metal Laser Processing Research Centre, Kielce University of Technology, with the use of CO 2 TRIUMF 1005 laser at the distance of the lens focal length equal to 260 mm with a spot on the sample surface ϕ 0.4 mm with power of P = 4.5 kW and nitrogen-shielding welding velocity v = 1.4 m/min. After welding, part of specimens was shot peened, in the Institute of Precision Mechanics in Warsaw, with a beam of spring steel (hardness of approximately 640 HV0.1) shot with a diameter of ɸ 0.8 mm under a pressure of 5 bar. Duration of exposition was 6 min, sample coverage was 100%. The shot peening intensity, determined with the use of Almen Strips strip, was 246 mm. such parameters were applied for numerous welded structures (Nakonieczny (2002)). The tests on low-cycle fatigue life of 1.4539 steel parent material for specimens, made with a laser beam, TIG method and after shot peening under conditions of axial tension on Instron 8802 hydraulic pulsator, were carried out at the level of 0.6, 0.7 and 0.8 of a tensile strength limit R m determined experimentally. The tests on fatigue life of welded joints made with TIG method and with a laser beam were conducted on flat specimens with a frontal weld before and after double-side shot peening. A coefficient of the cycle asymetry was R = 0.1 and the frequency was f = 4 Hz. Experimental tests showed that the parent material indicated the highest fatigue life while obtaining at the tested levels of nominal stress amplitudes Δσ an = 370, 430 and 500 MPa, the number of cycles to damage N f = 556 550, 281 537 and 125 766, respectively (Fig .1). A number of total cycles to failure of a specimen made of the parent mater was higher by approximately 800, 700 and 600%, at the level Δσ an = 375, 438 and 490 MPa, respectively, compared to the joints made with TIG method and by 400, 275 and 270 % compared to the joint made with a laser beam. The welded joints made with a laser beam were characterized with the fatigue life twice as high as the joints made with TIG method. Shot peening of the surface resulted in the grains deformation and an increase in microhardness and compression stress in the subsurface layers occurred (Nasiłowska et al. (2015)) .
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