SURFACE RESIDUAL STRESSES IN DRY TURNING OF 0.45% C STEEL

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1 Copyright JCPDS-International Centre for Diffraction Data 9 ISSN SURFACE RESIDUAL STRESSES IN DRY TURNING OF.45% C STEEL Tadeusz Leppert 1, Ru Lin Peng 2 1 Department of Production Engineering, University of Technology and Life Sciences, Bydgoszcz, Poland, 2 Department of Management and Engineering, Linköping University, SE Linköping, Sweden ABSTRACT Residual stresses in a surface layer determine many exploitation characteristics of machined surface. Depending on their kind and the type of applied loading, the influence of residual stresses can be negative or positive. Tensile residual stresses usually exert highly detrimental impact on several functional aspects such as strength, fatigue life, corrosion, wear resistance, etc., whereas compressive residual stresses are considered to have a beneficial effect on these features. The residual stresses found in mechanical parts are mainly generated in the final steps of machining process and are highly dependent on the machining conditions used. The increasing trend in industrial practice to eliminate cutting fluids from machining processes substantially changes machining conditions and influences the relationship between factors causing residual stresses. The main purpose of the presented investigations was to identify the relationship between residual stresses in a surface layer and the method of cooling employed. Using the X-ray diffraction method, residual stresses generated in both the cutting (circumferential) and feed (axial) directions were analysed. On the basis of empirical results the influence of turning with and without the application of emulsion on residual stresses was identified for a broad range of cutting parameters. The results showed that the method of cooling and cutting parameters exert a substantial influence on residual stresses. In the used range of cutting conditions, tensile residual stresses were detected. The elimination of cutting fluid from a turning process of C45 steel increased the magnitude of residual stresses in the surface layer. Residual stresses in the cutting direction turned out to be higher than in the feed direction in most cases of cutting conditions. INTRODUCTION Residual stresses in a surface layer determine many exploitation characteristics of machined parts. Depending on their kind and the type of applied loading, the influence of residual stresses can be negative or positive. Tensile residual stresses usually exert highly detrimental impact on several functional aspects such as strength, fatigue life, corrosion, wear resistance, etc., whereas compressive residual stresses are considered to have a beneficial effect on these features. Furthermore, the residual stresses in surface layer after machining may also cause dimensional instability of components (distortion) [1,2,4]. Therefore, the knowledge of residual stresses in the surface layer is important when critical structural components are machined, especially if the objective is to reach high quality and reliability levels. The direct impacts of residual stresses on the functional behaviour of a part are relatively well known [5,8,1]. Nevertheless, a number of questions regarding the generation of residual stresses in machining, their type, values and distribution persists [4,1,11].

2 This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This document is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific community. All copyrights for the document are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website ICDD Website -

3 Copyright JCPDS-International Centre for Diffraction Data 9 ISSN The residual stresses found in mechanical parts are mainly generated in the final steps of machining processes and are highly dependent on the machining conditions used. The increasing trend in industrial practice to eliminate cutting fluids from machining processes substantially changes machining conditions and affects the relationship between factors causing residual stresses. Therefore, considering the significance of residual stresses for reliability and longevity of a part, a lot of attention is paid to the residual stresses issue in terms of machining zone cooling and lubrication medium application together with the process parameters selection. Unfortunately the mechanism of residual stresses generation is still not completely recognized, particularly the influence of reduction or elimination of cutting liquids. For that reason the understanding and proper control of residual stresses in machining are a prerequisite in order to enhance components performance and minimize risk of failure or dimensional instability. The main reasons of residual stresses initiation in machining operations are mechanical and temperature impacts which change the subsurface physical and mechanical properties. Their influence depends to a large extent on the machining conditions, mainly selected cutting parameters and cooling and lubrication of cutting zone, which determine physical phenomena on the tool - workpiece interface [1,12]. Therefore, considering the significance of residual stresses for mechanical parts durability the main purpose of the presented investigations was to determine residual stresses in the surface layer generated in turning C45 steel in dry and wet conditions, taking into account a large range of cutting parameters. EXPERIMENTAL PROCEDURE Cylindrical turning tests were carried out on a lathe TUD 5 with 6.7 kw main motor. Medium carbon C45 steel bars of 6 mm in diameter with separated segments of 15 mm length for each cutting test were prepared (Fig. 1). Before the machining, the test specimens were annealed and then pre-machined with 1 mm cut to remove any possible surface irregularity and to ensure a similar surface condition for all the. The specimens for the residual stresses measurement were cut off from the machined bar. The chemical composition and mechanical properties of C45 steel presents table 1. Fig. 1. Specimen and the residual stresses measuring directions Sintered carbide inserts of SPUN 1238 covered by PVD method with (TiAlSi)N coating were employed, fixed in a tool holder of CSSPR 2 x 2mm. The following cutting point geometry was used: the orthogonal rake angle γ =5, clearance angle α =1, cutting edge angle χ r =45, cutting inclination angle λ s = and corner radius r ε =.8mm. The quoted inserts were recommended for machining of anticorrosive, austenitic or heat resistance steels as well as soft steels machined with medium cutting speeds. In order to minimize the influence of cutting tool

4 Copyright JCPDS-International Centre for Diffraction Data 9 ISSN wear on investigated quantities, each set of turning experiments was conducted using a new insert. Table 1. Chemical composition and mechanical properties of C45 steel Designation C45 Chemical composition, % C Si Mn P S,42-,5,17-,37,5-,8,4,4 Cu Cr Mo Ni,3,3,1,3 Mechanical properties Re, MPa Rm, MPa A5, % HB The experiments were planned according to the design of experiments technique [6]. For dry cutting the following cutting parameters were used: cutting speed vc= 25.5, 129.3, and 255 m/min; cutting feed rate f=.8,.27, and.47 mm/rev; and depth of cut ap = 1 and 2 mm. The same feed rates were used for turning with emulsion. However, the cutting speed was restricted to and 255 m/min while an additional cutting depth ap=,5 mm was also included. The following designations for cutting cooling and lubrication conditions were used: E refers to wet turning with a flood supply of 6% emulsion with a basis of emulsifying oil ARTEsol Super EP and a flow volume of 4 l/min, D denotes dry turning (turning without the application of any emulsion). The emulsifying oil ARTEsol Super EP containing up to 46% concentration of mineral oil and 15% additives to increase lubricity is recommended for multi machining operations of steel, cast iron and nonferrous metals. With water it constitutes a transparent emulsion. For the residual stresses measurement, specimens were fixed in the specimen holder on a Seifert 4-axis X-ray diffractometer (Fig. 2). The Cr-Kα radiation was used, together with a 2 mm pinhole collimator to limit the measurement area on the specimen surface. The residual stress components in the cutting (hoop) and feed (axial) directions in the surface layer were analysed by the sin2ψ method [7]. For each stress component, the Fe-211 peak from eleven sample directions, spreading between Ψ = -55o and +55o, was measured. By fitting a Pseudo-voigt function to the experimental data, the diffraction peak centre was determined and used for calculating the residual stress. The X-ray elastic constant is 5.8 x 1-6/MPa [3]. Fig. 2. Residual stresses measurement set up

5 Copyright JCPDS-International Centre for Diffraction Data 9 ISSN RESULTS AND DISCUSSION The analysis of the results showed that tensile residual stresses prevail in the surface for all the applied cutting conditions in both the cutting (hoop) and primary feed motion (axial) directions. However their values varied in a wide range, from 2 to 55 MPa. The circumferential stresses were generally greater than the axial stresses. The average standard error associated with the measured data was 68 and 2 MPa for the former and the latter, respectively. Cutting parameters as well as cooling and lubrication method exerted a strong influence on residual stresses in both directions as analysed below. In dry turning a substantial influence of cutting speed and feed rate on residual stresses in the hoop and axial directions was identified. Increasing cutting speed or feed rate implies an increase in hoop residual stress which was found to vary from 19 to MPa (Fig. 3a), depending on the cutting speed and feed rate. The impact of increased cutting speed was stronger at lower cutting speed (from 25.5 to m/min compared with from to 255 m/min), while the influence of increased feed rate was more significant for higher cutting speed. (a) (b) Hoop residual stress (MPa) D,8 D,27 Cutting speed (m/min) D,47(mm/rev ) Axial residual stress(mpa) D,8 D,27 D,47 (mm/rev ) 1 Cutting speed (m/min) Fig. 3. Influence of cutting speed and feed rate on hoop (a) and axial (b) components of residual stresses in dry turning In the axial direction, the residual stresses varied between 89.9 to MPa (Fig. 3b) for the studied range of cutting parameters. The increase of cutting speed in the range of 25.5 to m/min produced larger values of axial residual stress but with further increase to 255 m/min its influence was diminished and a significant reduction of axial residual stress was observed for the feed rate of.27 mm/rev. The increase of feed rate enlarged the axial residual stress in all the span of cutting speeds. Comparing the hoop and axial components of residual stresses in dry turning it can be noticed that the axial stresses were larger than the hoop stresses for the low cutting speed, 25.5 m/min (Fig. 4). The difference decreased with increasing cutting speed and eventually the reverse relationship between the hoop and axial stresses was observed. The values of hoop stresses considerably exceeded the axial stresses at cutting speed 255 m/min. The results indicate changes in the condition for chip generation and flow on the rake plane as well as in the relationship between the cutting force and its components. With an increase of feed rate a growth of cutting force follows in both the analysed directions [9]. Further explanation of this phenomenon needs more comprehensive investigations. For practical reasons, the selection of cutting speed regime

6 Copyright JCPDS-International Centre for Diffraction Data 9 ISSN for dry turning ought to be restricted to lower cutting speed in order to reduce the magnitude of residual stresses. The same remark is also valid for the choice of feed rate, where the increase of feed rate from.8 to.47 mm/rev made a considerable increase of residual stresses (Fig. 4b) D25,5hoop D129,3hoop D255hoop D25,5axial D129,3axial D255axial,8,27,47 Feed (mm/rev) D25,5hoop,8,27,47(mm/rev) D25,5axial D129,3hoop D129axial D255hoop D255axial Cutting speed/stress direction Fig. 4. Comparison of hoop and axial components of residual stresses in dry turning Similar results are observed in the case of turning with a conventional emulation flood supply (Fig. 5). In general, an increase in feed rate also contributes to the growth of hoop and axial residual stresses. In the cutting speed range to 255 m/min the largest impact on the hoop component appeared at the lowest feed rate,.8 mm/rev. At the feed rate of.27 mm/rev, increasing the cutting speed resulted in a reduction in both the hoop and axial stresses. The results suggest that a small feed rate should be used to reduce residual stresses in both directions. (a) (b) (c) 6 5 Hoop residual stress (MPa) Cutting speed (m/min) E,8 E,27 E,47(mm/rev) Axial residual stress (MPa) 4 1 E,8 E,27 E,47(mm/rev) Cutting speed (m/min) ,5 1 1,5 2 2,5 Depth of cut (mm) E129,3/,8 hoop E129,3/,8 axial Fig. 5. Influence of cutting speed and feed rate on (a) hoop and (b) axial components of residual stresses in turning with emulsion. (c) shows the surface residual stresses as a function of depth of cut Fig. 5c shows that the influence of depth of cut on residual stresses is substantial. The hoop stress increased considerably from to MPa and axial stress from 12.3 to MPa when the depth of cut was increased from.5 to 2 mm. In the case of axial residual stresses, an increase of depth of cut from.5 to 1 mm, at a feed rate.8 mm/rev, did not have a significant effect. For finishing operations the reduction of residual stresses in a surface layer requires selection of smaller values of depth of cut (less than 1 mm).

7 Copyright JCPDS-International Centre for Diffraction Data 9 ISSN The application of emulsion as a cooling and lubrication medium changed the relationship between hoop and axial residual stresses. As a consequence, the circumferential residual stresses can be considered as the most critical to a component E129,3hoop E255hoop E129,3axial E255axial ,8,27,47(mm/rev),8,27,47 Feed (mm/rev) E129,3hoop E129,3axial E255hoop E255axial Cutting speed/stress direction Fig. 6. Comparison of hoop and axial components of residual stresses in turning with emulsion The influence of emulsion can be derived from Fig. 7, in which the residual stresses are compared for turning without and with emulsion. For the same cutting speed and feed rate, the elimination of emulsion tends to increase the axial stress (Fig. 7b). On the other hand, dry turning may lead to an increased or decreased hoop stress, depending on the feed rate and cutting speed. At the feed rate.8 mm/rev and cutting speed interval from m/min to 255 m/min the hoop residual stress values in dry machining were bigger than those when emulsion was applied (Fig. 7a). With the increase of feed rate to.47 mm/rev at the cutting speed m/min, the change of relationship between dry and wet hoop residual stresses took place and the wet hoop stresses were higher than dry. (a) (b) Hoop residual stress (MPa) E129,3hoop E255hoop D129,3hoop D255hoop,8,27,47 Feed (mm/rev) Axial residual stress(mpa) E129,3axial E255axial D129,3axial D255axial,8,27,47 Feed (mm/rev) Fig. 7. Comparison of hoop (a) and axial (b) components of residual stresses in turning dry and with emulsion

8 Copyright JCPDS-International Centre for Diffraction Data 9 ISSN Experiments aiming at the definition of the influence of depth of cut on residual stresses in dry and wet machining were performed at the cutting speed m/min and feed rate.8 mm/rev (Fig. 8). Its impact on residual stresses in both directions depended to a large extent on adapted values of depth of cut. At 1 mm depth of cut the removal of emulsion from cutting led to larger hoop and axial residual stresses. However, at 2 mm depth of cut the relationship was reversed and lower hoop and axial residual stresses were found in dry cutting. It is also worth to notice that when the depth of cut was increased from 1 to 2 mm the magnitude of residual stresses decreased for dry cutting but increased for wet cutting. 25 E129,3/,8hoop E129,3/,8 axial D129,3/,8hoop D129,3/,8axial Depth of cut (mm) Fig. 8. Influence of depth of cut on hoop and axial components of residual stresses in turning dry and with emulsion CONCLUSIONS An experimental study of residual stresses induced by cylindrical turning of C45 steel in dry and wet cutting conditions were performed in the present investigations. Considering the obtained results the following conclusions can be drawn: The cutting zone cooling and lubrication method exert a substantial influence on the residual stresses in the machined surface layer. The magnitude of hoop and axial stress components depends to a large extent on the applied cutting parameters. In both dry and wet cutting conditions tensile residual stresses appear in both the circumferential and axial directions regardless of cutting parameters used in the experiments. Residual stresses in circumferential direction exceed the axial residual stresses when emulsion is supplied to the cutting zone as well as in dry turning, when lower cutting speed is used. The increase of cutting speed and feed rate produce an increase of residual stresses values for most cutting parameters. The influence of cutting speed on hoop and axial residual stresses is more intense at lower cutting speeds (25.5 to 129.3) and is diminished with its increase leading to the decrease of their values. The values of residual stresses depend substantially on the applied feed rate. The depth of cut in the applied range of cutting speed and feed exert a substantial influence on residual stresses in circumferential and axial direction.

9 Copyright JCPDS-International Centre for Diffraction Data 9 ISSN The results of presented investigation can make up the guidelines for cutting parameters selection taking into account the residual stresses constituted in the cutting process. REFERENCES 1. Capello E., Residual stresses in turning Part I: Influence of process parameters, Journal of Materials Processing Technology, 16 (5), p Capello E., Residual stresses in turning Part II. Influence of the machined material, Journal of Materials Processing Technology, 172 (6), p Cullity, B.D., Stock, S.R., Elements of X-Ray Diffraction, Third Edition, 1, Addison- Wesley 4. Leskovar P. (1), Ferlan D., Kovac M. Residual stresses as essential criteria for the evaluation of production processes CIRP 1987/36, 1, p Mamalis A.G., Kundrak J., Gyani K., On the dry machining of steel surfaces using superhard tools, International Journal of Advanced Manufacturing Technology, 19 2 (3), p Montgomery D.C. Design and Analysis of Experiments. John Wiley & Sons Inc, Noyan I.C., Cohen J.B. (eds.) Residual Stress Measurement by Diffraction and Interpretation. Springer-Verlag, Outeiro J.C., Umbrello D., M Saoubi R., Experimental and numerical modelling of the residual stresses induced in orthogonal cutting of AISI 316L steel, International Journal of Machine Tools & Manufacture, 46 (6), p Saglam H., Yaldiz S., Unsacar F., The effect of tool geometry and cutting speed on main cutting force and tool tip temperature, Materials and Design, 28 (7), p Saoubi R.M., Outeiro J.C., Changeux B., Lebrun J.L., Morao D.A., Residual stress analysis in orthogonal machining of standard and resulfurized AISI 316L steels, Journal of Materials Processing Technology, 96 (1999), p Sasahara H., The effect on fatigue life of residual stress and surface hardness resulting from different cutting conditions of.45%c steel, International Journal of Machine Tools & Manufacture, 45 (5), p Yang D.Y., Watkins T.R., Kozaczek K.J., Hubbard C.R., Cavin O.B., Surface residual stresses in machined austenitic stainless steels, Wear, 194 (1996), p