The objective of the investigation was to identify surface roughness and surface topography parameters after the turning of Duplex Stainless Steel (DSS) with wedges of coated sintered carbide. The auto correlation and gradient distributions for variable cutting parameters were compared. An Infinite Focus Measurement Machine (IFM) was used for the surface texture analysis. The study was performed within a production facility during the machining of electric motor parts and deep-well pumps

Grzegorz Krolczyk

Stanislaw Legutko

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Grzegorz Krolczyk Stanislaw Legutko  ISSN 1333-1124 eISSN 1849-1391 INVESTIGATIONS INTO SURFACE INTEGRITY IN THE TURNING PROCESS OF DUPLEX STAINLESS STEEL UDC 669.14.018.8:621.941 Summary The objective of the investigation was to identify surface roughness and surface topography parameters after the turning of Duplex Stainless Steel (DSS) with wedges of coated sintered carbide. The auto correlation and gradient distributions for variable cutting parameters were compared. An Infinite Focus Measurement Machine (IFM) was used for the surface texture analysis. The study was performed within a production facility during the machining of electric motor parts and deep-well pumps. Key words:  Duplex Stainless Steel, optical microscopy, surface integrity, surface  roughness, surface texture, turning 1. Introduction Engineering surfaces generated by machining processes are usually intended for tribological applications such as bearings and shafts. Achieving the desired surface quality is of great importance for the functional behaviour of a part [1]. Machining process is the most common process in the production of machine parts [2]. Surface integrity measurements of any workpiece are among the most important ones in the length and angle metrology, both in theory and practice. A surface is not only a geometric entity but also a layer with its own structure and properties [3]. According to Zeleňák et al. [4], there is a growing demand for research on mechanical and tribological properties of surface layers of materials. The finishing operation is connected with the precision of produced parts, surface integrity, and required surface roughness [5]. According to Benardos and Vosniakos [6], surface roughness is a widely used index of product quality and in most cases a technical requirement for mechanical products. Roughness is a sum of surface imperfections measured on a small area [7]. The most commonly used surface roughness parameter in production is the arithmetic average deviation from the average line profile [8]. According to Mahovic Poljacek et al. [9], a precise characterization of roughness and surface topography is of prime importance in many engineering industries. Quality is defined as the scope of implementation of expected functions [10]. To ensure a better surface integrity, special attention must be paid to the selection of cutting parameters [11, 12], tool material and geometry [13], and tool coatings [14]. Surface integrity is important for the components adapting to high thermal and mechanical loads during their applications [15–17]. TRANSACTIONS OF FAMENA XXXVIII-2 (2014) 77G. Krolczyk, S. Legutko  Investigations Into Surface Integrity   in the Turning Process of Duplex Stainless Steel Development and application of new materials in mechanical engineering practice create a lot of questions concerning their technological applications. This paper focuses on research problems related to the surface integrity after the turning process performed by coated carbide tools. The main purpose of this study was to determine the quality of surface integrity by using a novel method of analysis. The workpiece material is duplex stainless steel because this stainless steel is widely used in many industrial applications due to its unique properties. The good combination of its mechanical properties and corrosion resistance makes the duplex stainless steel suitable for a wide range of applications. Due to its poor machining property, the surface and subsurface are easily damaged during the machining process. 2. Experimental techniques 2.1 Workpiece The machined material was 1.4462 (DIN EN 10088-1) steel with a ferritic-austenitic structure containing about 50% austenite. The ultimate tensile strength was UTS=700 MPa, and the Brinell hardness 293 HB. The elemental composition of the machined material and technical details of the cutting tools are given in Tables 1 and 2, respectively. Table 1  Chemical composition of 1.4462 duplex stainless steel [%] Element C max Si max Mn max P max S max Cr Ni Mo N Others wt. [%]  max 0.03 max 1.00 max 2.00 max 0.030 max 0.020 21.0 23.0 4.50 6.50 2.50 3.50 0.10 0.22 - Table 2  Cutting tool specification Tool Substrate Coatings Coating technique MM 2025 Code: T1 Hardness: 1350 HV3 Grade: M25, P35 Ti(C,N)-(2 µm) (Top layer) Al2O3-(1.5 µm) (Middle layer) TiN-(2 µm) (Bottom layer) CVD CTC 1135 Code: T2 Grade: M35, P35 TiN-(2 µm) (Top layer) Ti(C,N)-(2 µm) Ti(N,B)-(2 µm) TiN-(2 µm) Ti(C,N)-(2 µm) Ti(C,N)-(2 µm) (Bottom layer) CVD Cutting tool inserts of TNMG 160408 designation clamped in the tool shank of ISO-MTGNL 2020-16 type were employed. Based on industry recommendations and conclusions from the earlier own investigations [8, 11], a range of cutting parameters were selected:  vC = 100 m/min, f = 0.3 mm/rev, ap = 2 mm. The study was conducted within a production facility. The research program was carried out on a CNC 400 CNC Famot Famot - Pleszew plc lathe. 2.2 Surface integrity analysis Surface integrity analysis was performed using an Infinite Focus Measurement Machine (IFM). The IFM is an optical 3D measurement device, similar to a Scanning Electron Microscope (SEM), which allows the acquisition of datasets at a high depth of focus. The IFM method allows for the capture of images with a lateral resolution down to 400 nm and a vertical resolution down to 20 nm [18]. The IFM 3.2 software version was used for the measurements.  78 TRANSACTIONS OF FAMENA XXXVIII-2 (2014)Investigations Into Surface Integrity  G. Krolczyk, S. Legutko in the Turning Process of Duplex Stainless Steel 3. Results and discussion 3.1 Surface roughness By analysing the machined surface (Figs 1 and 2), one can state that it has an anisotropic and periodical structure. A structure of this type occurs on contactless surfaces, mostly unloaded ones and co-acting with various kinds of wave interaction. On loaded faces, this surface is often found in contacts between undeformable bodies and deformable ones. Such a structure is typical of the structure obtained after machining by turning.  Fig. 1  Surface texture of a sample turned with the T1 tool point in real colour  Fig. 2  Surface texture of a sample turned with the T2 tool point in real colour The shape of the load capacity curve mainly depends on the shape of irregularities in the direction perpendicular to the reference surface. A comparison of the load capacity curves depending on the cutting speed can be seen in Figure 3. Nielsen [19] found that the honing process can be controlled by Rk parameters. According to Sedlacek et al. [20], the Rvk and Rpk parameters could have an influence on friction. The representative measured values of roughness parameters and material ratio parameters (Rk parameters group) are listed in Tables 3 and 4 (the results are presented in the table as the arithmetic mean of three measurements). The contact angle was recorded at least thrice for each sample and at least two samples for each experimental condition were prepared. The biggest differences in the surface parameters due to the change of the cutting wedge were observed for the Rvk parameter. The Rvk parameter is a measure of the valley depths below the core roughness. By comparing the various parameters one may assess the uniformity of the surface peak and valley distributions relative to a particular direction. The Rvk parameter (Reduced valley height) is a parameter which is found from a measure of the valley depths below the core roughness. Rvk is a measure of the valley depths below the core roughness and is related to debris entrapment and lubricant retention. Table 3  Material ratio parameters Cutting tool Rk  µm Rpk  µm Rvk µm Rmr1 % Rmr2  % T1 11.329 3.3707 0.2316 12.19 99.2 T2 11.322 5.3377 2.2227 16.21 90.91 Table 4  Roughness parameters Cutting tool Ra µm Rq µm Rt  µm Rz µm Rmax µm Rp µm Rv µm Rc µm T1 3.4281 3.9656 14.772 12.305 14.175 8.2603 6.5116 14.270 T2 3.8218 4.7549 20.657 13.994 18.346 12.125 8.5316 19.237 Figure 4 shows the differences in roughness profiles of both analysed cases. The roughness profile of the duplex stainless steel surface after the turning with the T2 wedge has slightly deteriorated, which can be seen through tribological disturbances or through the wear of the cutting tool point. The higher values of the Rt, Rp, and Rv parameters on surface after the turning process using T2 wedge cause the occurrence of a sharp spike or burr on the surface. The sharp spike and burr cause damage to the seals and can cause cracks.  TRANSACTIONS OF FAMENA XXXVIII-2 (2014) 79G. Krolczyk, S. Legutko  Investigations Into Surface Integrity   in the Turning Process of Duplex Stainless Steel a) a) b) b) Fig. 3  Load capacity curves after turning a) T1 tool point b) T2 tool point Fig. 4  Surface roughness profiles after turning a) T1 tool point b) T2 tool point Figures 5 and 6 show histograms of the distribution of vertices and upgrades located on the analysed surfaces. It can be seen that the distribution of machined sample after the turning with the T1 wedge is characterized by a larger range of changes in the values of R group parameters.  Fig. 5  Parameter histogram of roughness profile of a sample turned with the T1 tool point  Fig. 6  Parameter histogram of roughness profile of a sample turned with the T2 tool point 3.2 Surface textures The surface texture analysis has been performed by means of an Infinite Focus Measurement Machine. The geometrical structures of the surface shown below have been observed after longitudinal turning. Fig. 7a shows the preferential direction of a periodically iterated surface structure whereas Fig. 7b shows the surface dominated by low frequencies. Sal parameter (Table 5) after the turning with the T2 wedge has a higher value. Str and Stdi parameters have higher values after the turning with the T1 wedge; therefore, Fig. 7a presents the surface without the dominated preferred direction.  Table 5  Calculated parameters for auto correlations Cutting tool Sal [μm] Str Std [°] Stdi T1 63.31 0.26833 -90 0.22242 T2 142.86 0.18289 0 0.09157  a) a) b) b) Fig. 7  Auto correlation of primary model after turning a) T1 tool point b) T2 tool point Fig. 8  Gradient distributions of primary model after turning a) T1 tool point b) T2 tool point Fig. 8 shows the gradients of the surface of ISO Gradient Distribution. Both samples exhibited surfaces with steep gradients. 80 TRANSACTIONS OF FAMENA XXXVIII-2 (2014)Investigations Into Surface Integrity  G. Krolczyk, S. Legutko in the Turning Process of Duplex Stainless Steel  Fig. 9  Spectrum of sample after the turning with the T1 tool point of primary model   Fig. 10  Spectrum of sample after the turning with the T2 tool point of primary model Figs 9 and 10 show two views of spectral distributions (Surface Texture Spectrum) of the selected region. The selected parameter was the Sa parameter (average height of selected area). Figs 9 and 10 show the selected parameter Sa in certain wavelength ranges. Due to the same machining parameters, the analysed surfaces have similar wavelength ranges but the sample turned with the T2 wedge is characterized by steep slopes. 4. Conclusions  Duplex stainless steel surfaces were machined by wedges with different cutting tool specification and with the same technological parameters. The machined surfaces were analysed using the novel method of analysis of surface integrity.  I. The examined surfaces are characterized by an anisotropic and periodical structure.  II. The roughness profile of the duplex stainless steel surface after the turning with the T2 wedge has slightly deteriorated, which can be seen through small tribological disturbances. III. Distribution of machined sample after the turning with a cutting tool coated with a ceramic intermediate layer is characterized by a larger range of changes in the values of R group parameters. IV. The surface after the turning with a cutting tool coated with a ceramic intermediate layer is characterized by preferential direction of a periodically iterated surface structure whereas the surface after the turning with the T2 tool point is characterized by a surface dominated by low frequencies. V. The turning process with analysed parameters of machined surface is characterized by steep slopes. 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[19] Nielsen, H.S.: New approaches to surface roughness evaluation of special surfaces, Precision Engineering, 10, 4, 209–213, 1988. [20] Sedlacek, M., Podgornik, B., Vizintin, J.: Influence of surface preparation on roughness parameters, friction and wear, Wear, 266, 482–487, 2009. Submitted: 20.01.2014  Accepted: 04.6.2014 Grzegorz Krolczyk Faculty of Production Engineering and Logistics. Opole University of Technology 76 Prószkowska Street, 45-758 Opole. Poland Stanislaw Legutko Faculty of Mechanical Engineering and Management. Poznan University of Technology 3 Piotrowo Street, 60-965 Poznan. Poland 82 TRANSACTIONS OF FAMENA XXXVIII-2 (2014)

Investigations Into Surface Integrity in the Turning Process of Duplex Stainless Steel

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