Effect of Regular Shot Peening and Semi-Random Shot Peening Conditions on Selected Properties of the Surface Layer of Gray Cast Iron

Both dispersed and concentrated shot peening can be an effective method for the finishing of machine components. This work investigates the effect of two different shot peening (SP) processes conducted with the same technologi - cal parameters on selected properties of the surface layer of gray cast iron EN-GJL 250. Specifically, regular shot peening (RSP) and semi-random shot peening (SRSP) were investigated in the study. The results demonstrated that the surface quality of EN-GJL 250 samples was higher after RSP than after SRSP. The analyzed surface roughness parameters were lower after RSP than after SRSP, with the exception of the Rvk parameter. As a result of RSP, the analyzed roughness parameters increased from 5% to 62% in relation to their values after pre-treatment. The lowest values of the surface roughness parameters were obtained after RSP conduced with the impact energy E = 100 mJ, the distance between the dimples x = 0.3 mm, and the diameter of the shot peening element d = 14.3 mm. Assess - ment of the 3D surface topography showed significant differences in the formation of machining traces depending on the employed surface treatment. In RSP, the traces were arranged in a uniform manner, with the assumed step, whereas in SRSP the shot peening traces had no set pattern of orientation. The application of RSP and SRSP caused an increase in surface microhardness. The maximum surface microhardness was 75 HV0.5 for RSP and 98 HV0.5 for SRSP. Residual stresses were higher after SRSP than after RSP. Compressive residual stresses were induced in both types of shot peening process.


INTRODUCTION
Shot peening (SP) and burnishing (B) are finishing methods for machine components, and they are primarily employed to improve operational properties of these components.The use of shot peening and burnishing makes it possible to increase fatigue strength [1,2].Their use also makes it possible to increase tribological wear resistance [3] and corrosion resistance [4].Burnishing and shot peening differ in the way the burnishing element affects the workpiece surface.In burnishing, the tool (e.g. a rotating roller or sliding spherical bowl) is pressed against the workpiece with an approximately constant force [5][6][7].The effects of burnishing and shot peening can be assessed by both experimental testing and numerical modeling [8÷10].Burnishing can also be combined with machining [11].In shot peening, the peening element hits the workpiece surface [12].Due to the impact of the shot peening element, traces are formed on the surface of this object.Shot peening kinematics is characterized by regular impacts of the peening element, which results in the formation of successive traces with a constant pitch.This type of shot peening is called concentrated shot peening.An example of concentrated shot peening can be impulse shot peening [13] or centrifugal shot peening [14].Shot peening can also be a result of impacts exerted on the machined surface by free peening elements (usually balls or shot).This type of shot peening process is performed on jet [15] or vibratory devices [16,17] and is known as dispersed shot peening.Brushing has features of both concentrated and dispersed shot peening [18,19].
In dispersed shot peening, the peening elements hit the surface randomly, causing the formation of dimples on the machined surface that are located at different distances from each other.An increase in the shot peening time leads to an increase in the coverage of the shot peened surface with the dimples.In both jet and vibratory shot peening processes, the shot peening elements move in different directions at different speeds.In order to enable dispersed shot peening with a known impact energy and a known topography of dimple distribution, a method of semi-random shot peening (SRSP) was developed [20,21].
Burnishing and shot peening are usually employed for machining elements made of materials with high ductility, such as steel [22], aluminum alloys [23], and titanium alloys [24].However, a vast number of elements exposed to variable operational loads and tribological wear are made of cast iron.Examples of such elements include shafts, cylinders, bodies, slideways, and gears.Therefore, research on the effectiveness of shot peening for cast iron objects was undertaken.A study [25] has shown that shot peening conducted with an intensity of 0.30 mmA causes an increase in the fatigue strength of ductile iron castings, for the as-cast surface and the machined surface a like.A beneficial effect of SP on the fatigue strength and the reduction of casting defects on the surface has been reported in [26].However, with considerable casting defects, the shot peening induced fatigue strength increase in cast iron elements is negligible [27].A work by Bagherifard et al. has shown that severe SP of nodular cast iron samples results in a greater increase in their fatigue strength than that observed after conventional shot peening [28].An increase in fatigue life as a result of SP has also been reported for austempered ductile iron [29].According to Silva et al. [30] the shot peening of ductile cast iron causes surface layer hardening and leads to increased surface roughness, which is undesired due to the resulting durability of the tested material.It has been shown that the removal of a material layer with a thickness of about 20 µm leads to reduced surface roughness, which results in an increase in wear resistance [30].The impact of shot peening on wear resistance has also been studied in [31,32].A beneficial influence of SP on the surface layer properties of laser-quenched cast iron has been demonstrated in [33].Maleki used artificial neural network modeling to investigate the severe shot peening of nodular cast iron with the ferrite-pearlite matrix [34].Feng et al. have shown that laser shock peening also has a beneficial effect on the surface layer properties and abrasion resistance of high-chromium cast iron [35].
A study [21] has shown that the use of semirandom shot peening (SRSP) makes it possible to compare the surface layer properties of aluminum alloy obtained after dispersed and concentrated shot peening processes conducted with the same technological parameters.A review of the literature has shown that favorable changes in the surface layer condition can also be obtained by shot peening low-ductility materials such as cast iron.The purpose of this study is to assess changes in properties of the surface layer of gray cast iron samples that were shot peened by the dispersed and concentrated methods, using the same values of impact energy, shot peening element diameter and impact density.

RESEARCH METHODOLOGY
Samples made of gray cast iron EN-GJL 250 (Table 1) were used in the study.EN-GJL 250 is characterized by good casting properties and good machinability.Its production cost is relatively low.Disadvantages of the EN-GJL 250 gray cast iron include low strength, low ductility and poor abrasion resistance.Advantages of this material include excellent noise and vibration damping properties.This material is used in the railway, machine (body casting) and automotive industries.Prior to shot peening, the EN-GJL 250 gray cast iron samples were ground.Peripheral grinding was performed on the SPC-20 grinder with the A-46-KVBE-33 grinding wheel with dimensions of 200×20×51 mm, using the following technological parameters: a grinding depth of a p = 0.3 mm, a feed speed of v f = 15 mm/min, and a rotational speed of the grinding wheel of n = 3000 rev/min.
Shot peening was performed on a specially designed test stand.In the experiment, two methods of applying dimples were used.The first one was semi-random shot peening (SRSP), which was implemented in such a way that the dimples were created at much greater distances than their diameter in the initial phase of the process and became compacted with the peening time.The other method of covering the surface with dimples involved applying dimples one next another with the assumed step x.This was a standard shot peening technique known as regular shot peening (RSP).Both methods of forming dimples have been described in detail and shown schematically in [21].
The experiment was conducted according to the research plan specified in Table 2.The variable parameters of the SRSP and RSP processes were: the impact energy (E), the diameter of the shot peening element (d) and the distance between the impact traces (x).Impact energy could be made variable thanks to the use of a cam mechanism and springs in the device.The exchangeable head allowed the use of a peening element of different diameters.The CNC table performed feed motion according to the assumed program at a fixed speed.The speed of the feed motion affected the value of shot peening density j (the number of impacts per unit area -Eq.1).
Hommel-Etamic's T800 RC120-400 was used to measure roughness and surface topography.The measurements were made in accordance with EN ISO 25178-2:2022 and EN ISO 21920-2:2022.A TKU300 measuring tip equipped with a measuring needle with a radius r k = 5 μm was used.The sampling length was lr = 0.8 mm, while measuring length ln = 4.8 mm.The surface roughness measurement speed was v t = 0.50 mm/s.
The analyzed surface roughness parameters are widely used in engineering practice, while the parameters related to the Abbott-Firestone curve allow for the assessment of functional characteristics of the surface.The scanned surface area was 1.5×1.5 mm.Surface microhardness was measured by the Vickers method using the Leco LM 700 at microhardness tester.The indenter weight was 500 g (HV 0.5).Residual stresses were measured by X-ray diffraction using a portable diffractometer, Theta-Theta EDGE.Measurements were made in one direction.A chrome lamp was used as the XRD beam source and a vanadium filter was utilized.A 0.5 mm collimator was used.The material model from the system library was used for measurements, it was "Class30_Grey-CastIron".The inter-measurement angle was 9°.The XRD beam exposure time was 60 s.
Figure 1 presents an overview of the research methodology and test stands that were used to perform semi-random shot peening and regular shot peening.-results in a higher strain of the EN-GJL 250 cast iron surface after grinding.The use of RSP leads to an increase in both Ra (from 7% to 12%) and Rt (from 5% to 14%) with respect to their values after grinding.The changes in the parameters Ra and Rt as a function of the impact energy after the SRSP process and their absence after RSP can be explained by the differences in dimple application in these processes, which, combined with the impact energy used in the SRSP experiment, can induce significant changes in surface quality.An increase in the diameter of the ball d used in the experiment leads to a decrease in the curvature of the peening element, which results in the

Measurement surface roughness and topography
Figure 2 shows the influence of energy on the parameters Ra and Rt.For the surfaces subjected to RSP, there are no clear differences in the values of Ra and Rt as a function of the applied energy.Regarding SRSP, the use of a higher energy value results in an increase in the tested surface roughness parameters.This increase can be explained by the fact that the higher impact energy leads to more intense deformation, which -in turn Figure 4 presents the effect of the distance between the dimples on the surface roughness parameters Ra and Rt.As expected, the use of a larger distance between the dimples results in a reduced degree of coverage and thus in more uneven deformation of the machined surface, and, consequently, in reduced surface quality.The best results (the lowest values of Ra and Rt) were obtained in the regular shot peening process conducted with x = 0.3 mm. Figure 5 presents the effect of the technological parameters of RSP and SRSP on the surface roughness parameter RSm.As expected, after both processes the micro-irregularities are spaced at a greater distance than after grinding.For all analyzed cases, the RSm parameter is higher after SRSP than after RSP, sometimes even more than twice.In SRSP, the surface of the workpiece material is deformed unevenly with multiple shots over a short period of time, which promotes the material flow and the formation of micro-irregularities with a greater spacing.In RSP, the surface is deformed in a controlled way (dimples are applied one next to another), which limits the "movement" of the material.The profiles shown in Table 3 confirm the above-observed changes.
A comparison of these results with the results of previous studies [13,21] devoted to the RSP and SRSP methods reveals that the Ra parameter of the EN-GJL 250 cast iron samples after RSP is lower than that obtained for the shot-peened EN-AW 7075 aluminum alloy [21], while for some conditions the Ra values are similar to those obtained after impulse shot peening of the Inconel 718 alloy [13].The effect of the tested technological parameters (E, d, x) on the surface roughness of EN-GJL 250 is similar to that described in [13] and [21].Referring the present results to those obtained in other studies on shot peening of cast iron, one can observe that the Ra and Rt parameters decrease for some technological parameters of RSP and SRSP, while in studies [30] and [36] the use of SP led to rise in surface roughness.
In terms of mating between two elements, the important surface roughness parameters allowing the evaluation of these characteristics are the Abbott-Firestone parameters, which are presented in Figures 6÷8 as a function of the applied technological parameters of shot peening.Regardless of the shot peening conditions used, the parameters Rpk (reduced peak height) and Rk (core depth) are lower after RSP than after SRSP, the exception being the parameter Rvk (reduced valley depth).This means that deeper dimples are formed on the shot-peened surface after RSP than after SRSP, and thus they may be potential lubrication pockets.The higher Rvk values after RSP result from the method in which dimples are applied: one next to another, with the assumed step.This way of covering the surface with dimples causes the material to be multiply deformed over a small area and within a short period of time, making it "spring back", which results in a greater share of plastic-elastic strains.
Compared to its value after grinding, the Rpk parameter, which indicates any increase in the wear resistance of the machined surface, is decreased after RSP conducted with the following parameters: E = 15 ÷ 185 mJ, d = const.= 10 mm, x = const.= 0.3 mm.On the other hand, compared to its value after grinding, the Rpk parameter shows a 15% decrease after the SRSP process conducted with E = 100 mJ, d = 14.3 mm and x = 0.3 mm.It can be observed that the surface load capacity, which is defined by the Rk parameter and shows whether a significant area of the surface is in contact with the surface of the mating element after a running-in period, improved after the RSP process conducted with the impact energy ranging E = 15 ÷ 185 mJ (d = 10 mm, x = 0.3 mm) and the ball with d = 14.3 mm (E = 100 mJ, x = 0.3 mm).The use of the ball with d = 3.95 mm (E = 100 mJ, x = 0.3 mm) in the RSP and SRSP processes causes larger depressions to be "knocked out" on the machined surface, which results in reduced surface quality.Nonethless, there is an improvement in the retention capacity of the lubricant.
The experimental results demonstrate that the functional parameters Rpk and Rk for the selected shot peening conditions increased from 15% to 62% in relation to their values after grinding.These changes are smaller than those reported in [2] and [14], which may result from the fact that the workpiece and pre-treatment types used in this study differed from those employed in [2,14].The surface roughness results are confirmed by the obtained surface topography (Table 4).On the surface subjected to RSP, the traces are arranged evenly, at regular intervals, with the assumed step.Regarding the surface after SRSP, the dimples do not have a set pattern.The method of surface treatment affects the deformation of surface micro-irregularities after grinding.The 3D parameters are lower after RSP than after SRSP, which is the same trend as that observed for the 2D surface roughness parameters.

Microhardness
The cyclic impact of the ball on the surface of the sample leads to an increase in the density of dislocations.The movement of the dislocations is stopped when they come across grain boundaries and precipitations, which results in an increase in surface microhardness.Both RSP and SRSP cause the surface microhardness to increase.The maximum surface microhardness increase is 75 HV0.5 for RSP and 98 HV0.5 for SRSP.These maximum microhardness values are greater than those obtained after RSP and SRSP for the elements made of aluminum alloy EN-AW 7075 [21].
The effect of the technological parameters of shot peening on the microhardness of gray cast iron EN-GJL 250 after the RSP and SRSP processes is plotted in Figures 9 ÷ 11.For the impact energy range E = 15 ÷ 100 mJ, there are no clear changes in surface microhardness, which suggests that the applied impact energy is too low to cause deformation and dislocation mobility.However, the use of a higher impact  energy of E = 185 mJ causes marked changes in the surface microhardness (Fig. 9).An increase in the ball diameter used in the experiment causes an increase in the indentation field resulting from the impact, as confirmed in [21].In effect, the concentration of energy transferred to the sample decreases, which results in reduced microhardness (Fig. 10).
An increase in the distance between the shot impact traces results in a reduced coverage of the machined surface.The uneven deformation of the surface results in reduced microhardness (Fig. 11).A four-fold increase in the distance between the dimples causes the microhardness to decrease by 46 HV0.5 for the RSP method and by 69 HV0.5 for the SRSP method.The microhardness results demonstrate that the greatest changes in their values compared to those observed after grinding occur for the extreme shot peening conditions (E = 185 mJ, d = 3.95 mm, x = 0.15 mm).

Measurement residual stress
Figure 12 shows the effect of the technological parameters of RSP and SRSP on the residual stress on the surface of EN-GJL 250.Compressive stresses occur regardless of the

CONCLUSIONS
This study investigated the properties of preground EN-GJL 250 gray cast iron samples subjected to regular shot peening (RSP) and semirandom shot peening (SRSP).Based on the obtained results, it can be concluded that: • the method of dimple creation affects the geometric structure, residual stress and microhardness of the surface; • RSP yields lower values of surface roughness parameters than SRSP, • the lowest values of surface roughness parameters were obtained after RSP conducted with the following technological parameters: E = 100 mJ, x = 0.3 mm, d = 14.3 mm, • the Abbott-Firestone curve parameters Rpk and Rk are lower after RSP than after SRSP; an opposite trend can be observed for the Rvk parameter, parameters Rpk and Rk are after RSP from 5% to 64% lower than after SRSP; • the microhardness and residual stresses on the surface of the EN-GJL 250 gray cast iron samples are higher after SRSP than after RSP (for microhardness the maximus difference is 7% and from residual stress the difference is from 2% to 16%).
The results have confirmed that the selection of the surface layer properties of gray cast iron EN-GJL 250 after regular shot peening and semirandom shot peening was well-chosen.However, for a wider application of these results, future works should investigate the RSP-and SRSPinduced residual stress of EN-GJL 250 together with the abrasive wear resistance of this material.

Fig. 3 .Fig. 4 .
Fig. 3.The surface roughness parameters Ra (a) and Rt (b) after RSP and SRSP in the function of the ball diameter d (E = 100 mJ, x = 0.3 mm)

Table 2 .
Technological parameters of SRSP and RSP