PSI - Issue 23
Jozef Majerík et al. / Procedia Structural Integrity 23 (2019) 541–546
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Jozef Majerík et al. / Structural Integrity Procedia 00 (2019) 000 – 000
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and equipped with Triboscan software with F = 5000 µN test load force and t = 4 s holding duration. All quasi-static nanoindentation tests were realized at room temperature with Cube Corner indenter geometry application inside the CEDITEK (research center for quality testing and diagnostics of materials) Laboratory of Mechanical testing at Faculty of special technology in Trencin. Quasi-static nanoindentation measurements were performed in cross section measuring specimens. Quasi-static nanoindentation technique measurement requires pushing a diamond tipped indenter body into a specimen under specified load or displacement control. Displacement ( h ) is monitored as a function of the load ( P ) throughout the load-unload cycle where resulting ratio P - h is known as the nanoindentation curve. Plastic part of deformation is usually applied to designate Young’s Modulus, when the elastic-plastic part along with the indent surface is applied to rate nano-hardness. The surface terminated with both loading and unloading curves is then equivalent to dissipation energy, as mentioned Oliver & Pharr, Fisher-Cripps, Chen et al. and Iracheta et al. Within all realized nanoindenation measurements, load in common with displacement were noted, when the Cube Corner geometry of testing indenter was compressed onto the surface of measured sample with the standard loading and unloading profiles. The young modulus E r (GPa) was acquired together with the initial unloading stiffness S . The nanoindentation measurement was realized by sixteen punches at the selected microstructure site. Measured area is limited by the field of 75 × 75 μm 2 . Standard trapezoid with the maximum of F = 5000 μN and a pushing time of 4 seconds was applied as load curve. Quasi-static nanoindentation measurement of surface layers which were created after hard finish turning a grinding technology after machining represents the next aspect of the investigations. The subject of nanoindentation involved surface layers of three samples A, B, and C which were hard finish turned at three values of cutting speed v c1 = 100 m min – 1 , v c2 = 220 m min – 1 , and v c3 = 380 m min – 1 . The constant parameters were feed rate f = 0.1 mm. rev -1 and depth of cut a p = 0.5 mm. Samples D, E and F were grinded at v o1 = 450 m.min -1 , v o2 = 280 m.min -1 and v o3 = 112 m.min -1 . Depth of cut in grinding was constant parameter and the value was a p = 0.2 mm for all grinded samples. 3. Results and discussion Microstructure of fundamental material, which have been evaluated through the AFM microscopic analysis, is lightly heterogeneous, relatively coarse-grained and also is created by the highly tempered martensite and bainite with fine globular carbides. The middle size of original austenitic grain is approximately 50 µ m. Evaluated microstructure Therefore, the evaluated microstructure corresponds to the state after and high temperature tempering of the thick-walled Cr-Ni-Mo-V steels. Evaluated microstructure of surface layers, created by the plastic deformation after two different final machining operations can be seen in Fig. 1a-f. All the investigated subsurface layers of all machined surfaces showed influence of plastic deformation of all the investigated microstructures that was observable up to the depth of approx imately 5 μm and was microscopically noticed up to depth of just 20 μ m. The SPM (Scanning Probe Microscopy) mapping mode (gradient channel) was used of all the measured areas with the dimension of 75 × 75 µm 2 . All measuring positions (indents) are then characterized with light green points (from point No.0 to point No.15). All curves of obtained nano-hardness behavior then can be seen in Fig. 2a, b. All the curves were created by fitting experimental information through the nonlinear regression with the usage of exponential function. The first three courses (can be seen in Fig. 2a) after hard finish turning process were then characterized by initial high rate hardness decreasing in part to the depth of about 15 µm, and then followed by soft degressivity in nano-hardness values. The obtained nano-hardness values for cylindrically grinded surfaces (can be seen in Fig.2b) are characterized by a slight decrease on the surface samples compared to the base material, which is due to the high temperatures generated during the grinding process when applying high values of cutting speeds. Gradually, there is a slight increase in nano-hardness over the base material until it aligns with the untreated base material of the samples.This type of curve predicts a less rigid connection of the surface layer to the base material, which in turn reduces the operating life of the functional surfaces of the components thus produced. 4. Conclusion This investigation was analysed and compared the influence of selected finish machining process of 33NiCrMoV15 steel on AFM microstructural analysis and nano-hardness of obtained surface layers. Overall summarization of the all results is as follows:
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