Influence of Cutting Speed on Residual Stresses by Machining of AISI 316L

Residual stresses, cutting speed, x-ray, orthogonal cutting RS have an important role in the performance of components and machined structures. The objective of this paper is to study the influence of cutting speed on RS in workpieces that are formed in orthogonal cutting. AISI 316L stainless steel since it has been used in many important industries such as chemical, petrochemical industries, power generation, electrical engineering, and food and beverage industry. Four cutting speeds are selected: (44, 56, 71 and 88) m/min. The alloy was machined by turning at constant depth of cut and various feed rate from (0.065 to 0.228) mm/rev. Residual stresses are examined by X-ray diffraction. The best results of RS obtained are (-3735.28, -1784.95, -330.142, -218.747, 890.758, -2999.632, -2990.401) MPa. Increasing the cutting speed from (44-56) m/min. reduces the compressive residual stress by (21.4 %), while from (71-88) m/min the RS is reduced by (19.3 %). Finally, the RS at cutting speeds are changed from compression to tension. How to cite this article: S.M. Lafta and M.A. Tawfiq, “Influence of cutting speed on residual stresses by machining of AISI 316L” Engineering and Technology Journal, Vol. 38, Part A, No. 03, pp. 394-401, 2020. DOI: https://doi.org/10.30684/etj.v38i3A.459


Introduction
Residual Stresses, or internal stresses, are the stresses which remain inside a flexible body without need for external loading applied to it. These stresses are one of the most important factors that characterize surface and subsurface properties and subsequent performance of any mechanical component. They affect fatigue life, cracking growth resistance, static strength, corrosion resistance and magnetic properties. Depending on the stress, distribution specified in any component, these properties could enhance the performance of the part or weaken it [1]. RS can be tensile or compressive and can have multiple stress layers depending on cutting conditions, work materials, the geometry of cutting tool and connection conditions in both of tool and workpiece interfaces. Compressive residual stresses improve component performance and overall life time because they enhance the tensile stress in service and prevent the crack core. The tensile residual stresses tend to increase service stresses leading to premature failure [2], as shown in Figure 1. The main effect of RS in machining is the cutting speed, when the cutting speed increases; it increases the mechanical load, thus causing the compressive residual stress on the workpiece. High speed also leads to higher temperature, thus increasing tensile residual stresses. This can lead to a critical cutting speed which surface residual stresses change from compressive to tensile [3]. At higher speeds, thermal effects predominate. This usually results in increased tensile stresses on the work surface and shallow penetration depth with increased cutting speed , because the high temperature at the cutting edge and the low time allowed for thermal diffusion leads to severe thermal gradients on the surface of the part [4]. Measuring residual stresses is not a trivial matter; so many researchers were closely interested in new or improved measurement techniques [5]. The measurement is based on diffraction methods, and x-ray diffraction may be the most common measure of stress. In all diffraction methods, the elastic strain of specific atomic lattice planes is measured. The X ray penetration depth in the steel is limited to about 5μm below the surface. This means that X-ray diffraction is suitable for residual stresses on the surface only [6]. Mohammadpour et al. [7] studied the influence of cutting speed on the surface, and the residual stress caused by the orthogonal cutting, AISI 1045 was selected, with the increased cutting speed, tensile residual stress was increased. Liu et al. [8] studied the effect of cutting speed on residual stress at orthogonal cutting. Titanium alloy TC4 was selected. The residual stress was compressive at low cutting and changed to the tensile stress when the cutting speed was increased. While the cutting operation was increased, the residual stress reduced to compressive again. Jacas-CabreraI et al. [9] studied the effect of cutting speed and feed rate on residual stress by using AISI 1045. The increasing in cutting speed caused little influence on the surface roughness little influence, with increasing cutting speeds from 100 to 451m /min, using feed rates in the range of 0,1to 0,2 mm / rev, the residual stress increased in the direction of traction, reflecting the direction to compression with increased of the cutting speed above 451m/min. For low feed rate (0,01 mm / rev) and any of the tested cutting speed, residual stresses were compressive. Bouacha et al. [10] used bearing steel (AISI 52100), machining process at higher cutting speeds, due to the impact of high influence of feed rate and cutting speed and little impact of depth of cut because it was remained constant, The results of the residual stresses showed that the influence of cutting speed and feed rate in both of the peripheral and axial stress behavior respectively. The measurement is based on diffraction methods, and x-ray diffraction may be the most common measure of stress. In all diffraction methods, the elastic strain of specific atomic lattice planes is measured. The X ray penetration depth in the steel is limited to about 5μm below the surface. This means that X-ray diffraction is suitable for residual stresses on the surface only [6]. Mohammadpour et al. [7] studied the influence of cutting speed on the surface, and the residual stress caused by the orthogonal cutting, AISI 1045 was selected, with the increased cutting speed, tensile residual stress was increased. Liu et al. [8] studied the effect of cutting speed on residual stress at orthogonal cutting. Titanium alloy TC4 was selected. The residual stress was compressive at low cutting and changed to the tensile stress when the cutting speed was increased. While the cutting operation was increased the residual stress reduced to compressive again. Jacas-CabreraI et al. [9] studied the effect of cutting speed and feed rate on residual stress by using AISI 1045. The increasing in cutting speed caused little influence on the surface roughness little influence, with increasing cutting speeds from 100 to 451m /min, using feed rates in the range of 0,1to 0,2 mm / rev, the residual stress increased in the direction of traction, reflecting the direction to compression with increased of the cutting speed above 451m/min. For low feed rate (0,01 mm / rev) and any of the tested cutting speed, residual stresses were compressive. Bouacha et al. [10] used bearing steel (AISI 52100), machining process at higher cutting speeds, due to the impact of high influence of feed rate and cutting speed and little impact of depth of cut because it was remained constant, The results of the residual stresses showed that the influence of cutting speed and feed rate in both of the peripheral and axial stress behavior respectively.

Experimental Work
A hollow shaft of stainless steel AISI 316L of 25 mm in diameter and 60 cm in length is selected, in order to achieve orthogonal cutting conditions. Table 1, 2 and 3, show the chemical composition, mechanical properties, physical and thermal properties of stainless steel 316L respectively.   The shaft is cut into 16 specimens each one having 25mm in length, the cutting process is done using Wire EDM technique, Figure 2 shows the specimen dimensions of the specimen after cut and before machining. The cutting conditions are selected as Vc= (44m/min, 56m/min, 71m/min, 88m/min) at constant depth of cut, const. feed rate = 0.228 mm/rev. Table 4 shows the machining conditions. Universal turning machine type sinus 330\3000 (SN-126130), is shown in Figure 3 was used to perform the experimental work. It has the following technical specifications being shown in Table 5. One type of cutting tool is used in this study. The insert used is coated carbide (PVD with TNMG) and it is selected as negative turning inserts according to specifications that are show in Table 6.     A tool holder type STACR\L 90 Deg. Screw clamp turning tool holder with dimensions as shown in Figure 5.
The residual stresses were examined by x-ray device (XRD-6000), Table 7 shows the specifications of the device. The device is shown in Figure 6.

Results and Discussion
Results obtained from X-ray diffraction: Table 8 show the results of residual stresses when feed rate was (0.228) mm/rev and Figure 7 show the values of residual stresses when feed rate was (0.228) mm/rev. Table 9 shows the results of residual stresses when feed rate was (0.16) mm/rev and Figure 8 shows the values of residual stresses when feed rate was (0.16) mm/rev. Table 10 shows the results of residual stresses when feed rate was (0.08) mm/rev and Figure 9 shows the values of residual stresses when feed rate was (0.08) mm/rev.       Table 11 shows the results of residual stresses when feed rate was (0.065) mm/rev and Figure 10 shows the values of residual stresses when feed rate was (0.065) mm/rev. The depth of cut was constant in each specimen and it was equal to 2 mm while cutting speeds were varied. The results in curves above show that the residual stresses started with compressive residual stresses because the cutting speed was low and then changed to tensile residual stresses with increasing the cutting speed gradually. At low cutting speed, the cutting force decreases and that the main reason to make the residual stresses in tension state, and that depending on Jomaa et al. [11] studied the effect of cutting parameters on the RS, and the results showed that Surface residual stress was tensile when the cutting speed was increased. Min wan et al. [12] studied the effect of different cutting parameters such as (cutting speed and feed rate), The results showed that when reduced in cutting speed and increased in feed rate, led to increase of compressive residual stresses on surfaces.

Conclusions
Tensile residual stresses generated by increasing of cutting speed, because it reduces the corresponding cutting forces and increasing in cutting speed leads to reduce the friction between the insert and the workpiece therefore the machining surfaces in high cutting speed be softer than surfaces machining in small cutting speed.