PSI - Issue 5

Marek Smaga et al. / Procedia Structural Integrity 5 (2017) 989–996 Marek Smaga et al. / Structural Integrity Procedia 00 (2017) 000 – 000

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stainless steels this technology allows the net shape machining with simultaneous formation of martensitic layers at the specimen surface (Fig. 1b). The present paper is focused on the characterization of cryogenic turning-induced surface morphologies produced under different turning conditions in metastable austenitic stainless steel AISI 347. Experimental data on the influence of various surface morphologies on the cyclic stress-strain behavior and fatigue life are reported.

2. Experimental setup and investigated material

2.1. Cryogenic turning

For cryogenic turning, a CO 2 snow cooling system including two nozzles with an exit diameter of 10 mm each was applied. Due to its chemical and physical properties, the CO 2 reached the workpiece as a solid-gas mixture with a temperature of -78 °C. One nozzle was applied for a pre-cooling the workpiece in front of the cutting zone and one nozzle was oriented behind the tool to cool the workpiece behind the cutting zone. To assure the required deformations, the passive force, being the crucial factor influencing the mechanical load in the workpiece surface layer, had to be enlarged. Therefore, a chamfered cutting edge (CNMA120416T02020), a negative tool orthogonal rake angle of -6 ° and tool cutting edge inclination of -6 ° were applied. The cemented carbide tool was equipped with a multilayer coating (TiN/TiCN/Al 2 O 3 ). A comparably low cutting speed of v c = 30 m/min was applied to keep the process energy and, therewith, the generated heat and resulting temperature at low levels. As the cooling penetration of the workpiece decreases with increasing distance from the workpiece surface, a relatively low cutting depth of a p = 0.2 mm was chosen, as higher depths of cut would remove a larger previously cooled workpiece volume. More details about the cryogenic turning process were published elsewhere, see e.g. Aurich et al. (2014), Mayer et al. (2014). To vary the surface morphology generated in cryogenic turning process, two different feed f = 0.15 mm/rev and f = 0.35 mm/rev were selected to manufacture fatigue specimens with Martensitic Surface Layer after turning using the above mentioned CO 2 snow-cooling system: MSL t, f=0.15, and MSL t, f=0.35 . Futhermore, specimens of the type MSL t, f=0.15 were additionally polished mechanically and electrolytically (MSL p ) to eliminate turning induced surface roughness. During polishing the layer with a thickness of 25 – 35 µm was removed. The diameter of each specimen was thus carefully measured using an optical system with a resolution of 1 µm before fatigue testing. Specimens with an Austenitic Surface Layer after turning (ASL t ) with the same parameters as MSL t, f=0.15 specimens but without the C0 2 snow-cooling were manufactured as a reference. In addition, specimens of the ASL t type were polished mechanically and electrolytically (ASL p ). In summary, specimens with five different surface morphologies were obtained, namely (i) MSL t, f=0.15 , (ii) MSL t, f=0.35 , (iii) MSL p , (iv) ASL t, f=0.15 and (v) ASL p . Tensile tests were performed in a Zwick electro-mechanical testing system with a maximum load capability of 250 kN. Cylindrical specimens for tensile and cyclic tests were machined from the central part of bars. Specimens with a diameter of 6 mm were used for monotonic tensile tests in agreement with the geometry requirements of the German Institute for Standardization (DIN) standards DIN 50125. Stress-controlled fatigue tests were carried out using a servohydraulic testing system MTS 810 with the load ratio of R  = – 1 and frequency of 5 Hz at ambient temperature (AT) and 300 °C, respectively, in air. For characterization of cyclic deformation behavior by means of stress-strain hysteresis, two extensometers were used. In fatigue tests at ambient temperature an extensometer with a gauge length of 8 mm and in fatigue tests at elevated temperature an extensometer with ceramics clips and a gauge length of 12 mm. For fatigue tests, specimens with a gauge diameter of 7.6 mm and different surface morphologies were manufactured applying the turning parameters specified above with and without CO 2 snow cooling (see Fig. 1b). Optical micrographs before mechanical loading were taken with Zeiss Axio Imager Vario Z2 and Leica DM 6000M microscopes. A scanning electron microscope FEI Quanta 600 FEG equipped with EBSD (electron backscattering diffraction) technique was used for detailed characterization of microstructure and orientation of grains. The topography of the investigated specimens was measured using a confocal microscope (CM), Nanofocus, µSurf Explore. Ferromagnetic  ´-martensite fraction was determined by a Feritscope™ magnetic sensor. Even though there 2.2. Mechanical testing and analytical methods