Effect of drilling parameters on the hole surface integrity of low alloy steel for nuclear power during BTA deep hole drilling

Boring trepanning association (BTA) deep hole drilling is widely used in machining tube sheet of steam generator. In order to get a better service integrity, the surface quality after machining is required to be higher. In this paper, the effect mechanism of BTA deep hole drilling on the integrity and quality of the machined surface layer of low alloy steel SA508Gr.3Cl.2 for nuclear power is investigated. The results show that the gradient microstructure can be obtained by BTA drilling on the surface of the inner hole, including the recrystallized layer with grain refinement and the plastic deformation layer with high-density sub-crystal structure and grain distortion. With the increase of drilling speed and feed rate, the thickness of deformation layer increases. During the BTA deep hole drilling, the proportion of low-angle grain boundaries (LAGBs) increases with the increasing depth from the machined surface. The increase of drilling speed leads to the increase of recrystallization degree and the proportion of LAGBs in the machined surface. The effect of feed rate on the proportion of LAGBs is opposite. The machined surface is characterized by regular peak and valley, and there are typical surface defects mainly involving feed marks, surface tearing, and plowing grooves. With the increase of drilling speed, the surface roughness will decrease. The effect of feed rate on surface roughness is obviously lower than that of drilling speed. With the increase of drilling speed, and feed rate, the depth of hardened layer increases gradually, which is caused by dislocation strengthening and fine grain–strengthening effect during BTA drilling process. Higher drilling speed is recommended in forming a better machined surface with a strengthening layer of a certain thickness.


Introduction
Steam generator (SG) drives the turbine generator to generate electricity in a pressurized water reactor type of nuclear steam supply system and needs to withstand high temperature, high pressure, and strong radioactivity during the service [1][2][3]. As one of the key components in the steam generator, tube sheet is used to secure the heat transfer tubes and complete the heat exchange through tens of thousands of deep holes on the sheet [4,5]. The deep hole processing of tube sheet is difficult in nuclear power equipment manufacturing. The three classic deep hole drilling processes that are widely used in industry are single-lip deep hole drilling, ejector deep hole drilling, and BTA deep hole drilling [6][7][8][9]. Different from shallow hole drilling, there are many disadvantages in deep hole drilling process such as difficulty of chip removal, accumulation of cutting heat, poor rigidity of the process system, and the service life of cutting tools is short.
In order to improve the machining efficiency of deep holes with large length-diameter ratio (l b /D > 10), BTA deep hole drilling technology was first devised by Bessner in Germany in the 1940s [10]. The diameter of the hole can be machined by BTA method from 6 to 1500 mm [11]. In many important engineering fields, such as aeronautics [12], naval [13], automobile manufacturing [14], and nuclear and energy [15], BTA deep hole machining is of great value in 1 3 application. The research on BTA drilling mainly focuses on the following aspects, such as the role of guide pads [16,17], the vibrations occurring during machining [18], the chip formation process [19], and cutting force acting on the BTA drill [20]. Furthermore, a growing number of studies have concluded that the machined surface integrity (surface morphology, surface roughness [21], microstructure evolution [22,23], microhardness [24], and so on) of components significantly affects their integrity [25]. In the process of metal cutting, mechanical load and thermal load will superimpose on the workpiece surface, so that the microstructure of the workpiece surface layer is affected by plastic deformation and temperature change [26]. Thil et al. [27] studied the chip morphology on the macro and microscales when drilling 18MND5 steel with BTA, and results suggested that increasing the feed rate would promote chip fragmentation and improve the material removal rate, but too high feed rate would lead to serious flank wear. Strodick et al. [28] investigated that the surface integrity is affected by both the cutting edges and the guide pads during BTA deep hole drilling. Schmidt et al. [12,29] analyzed the roundness and roughness at different stages of the drilling process and measured the microstructure, hardness, residual stress, and other characteristics of the drilling surface and near sub surface area. Prior research has shown that the surface integrity of the hole is influenced by the BTA process. It is worth noting that machining parameters have significant influence on surface integrity in BTA drilling.
The main material of steam generator in nuclear power equipment is SA508Gr.3Cl.2. Generally, chip breaking and chip removal in BTA drilling are very sensitive to machining parameters and tool angle, and chip plugging is very easy to cause bit damage. Moreover, the optimal machining parameters vary when machining different materials. At the same time, because BTA drilling takes place in a closed and narrow space, it is extremely difficult to monitor the drilling process and measure the hole surface integrity. Therefore, it is essential to explore the characterization of the surface layer microstructure of BTA drilling holes and the relationship between surface integrity and machining parameters. This paper focuses on the effect of different machining parameters on the surface integrity of SA508Gr.3Cl.2 workpiece drilled by BTA, which is widely used in tube sheet machining. The grain size, grain misorientation, surface morphology, surface roughness, and microhardness distribution of the workpiece surface layer were characterized.
Firstly, electron back scattering diffraction (EBSD) imaging was used to obtain grain size and grain misorientation deviations perpendicular to the feed direction of the processed surface. Then, the detailed surface morphology and roughness of the machined holes were observed by laser microscope. The hardening effect of BTA deep hole machining was characterized by microhardness test of the surface layer. At last, the variation trend of surface integrity in BTA deep hole machining was obtained, which provides valuable theoretical guidance for the actual production of staggered tooth BTA deep hole drilling.

Workpiece material
The material used in the drilling experiment was SA508Gr.3Cl.2 high-strength steel with a low carbon content. The size of workpiece was 400 mm in length, 180 mm in width, and 80 mm in thickness. The chemical composition of the workpiece was measured by SPECTROLAB M12 direct-reading spectrograph, as shown in Table 1. Under the loading time of 15 s and loading force of 25 g, the microhardness of workpiece was 208 ± 5 HV. Figure 1 shows the initial microstructures of SA508Gr.3Cl.2 high-strength steel. The original material was etched using the nitrate alcohol formula with a volume fraction of 4% nitric acid and photographed under the scanning electron microscope (SEM), as shown in Fig. 1a. Figure 1b indicates the inverse pole figure (IPF) maps of EBSD. The grain of the original material is relatively distributed, and the average grain size is 0.98 μm. The initial micro-structural was dominated by featherlike upper bainite. The initial grain distribution is shown in Fig. 1b, and the average grain size is 0.98 μm, which is relatively uniform and small. Generally, the misorientation angle of high angle grain boundaries (HAGBs) is more than 15°, and the misorientation angle of low angle boundaries (LAGBs) is from 2° to 15° [30].

BTA deep hole drilling tests
The experiment was performed on a deep hole drilling machine (spindle motor power of 22 kW, with the maximum spindle speed of 1500 rpm, the maximum drilling depth of 1000 mm, the maximum cutting fluid pressure of 6.5 MPa, and flow rate of 120 L/min). The kinematic viscosity of drilling oil for deep hole drilling was 10 m 2 /s. Figure 2 shows the schematic of the deep hole drilling experiments. The BTA drill used in the experiment was the TiCrAlNcoated carbide series of ISCAR deep hole drilling products. The drilling diameter was 17.73 mm. According to the drilling diameter, three teeth chip breakers were brazed to the drilling head, which could meet the requirements of deep hole drilling of low alloy steel. To characterize the surface integrity of the machined holes under different deep hole drilling conditions, the drilling parameters in Table 2 were tested. New BTA drill has been used for each drilling test.

Sample preparation
After drilling, two specimens were extracted from the test workpiece by electrical discharge wire-cutting, which is on the circumference of each hole with a distance of 15 mm from the entrance shown in Fig. 3. One specimen was used to measure the machined surface quality, including surface topography and roughness. Another specimen was used to measure the microscopic properties of the machined surface layer, such as microhardness, metallographic structure, EBSD, and SEM test. After the sampling was completed, all specimens were ultrasonic cleaned with anhydrous ethanol and dried, and the specimens used for surface topography and roughness measurement could be directly observed on the processed surface. The specimens used for microhardness need to be set with the thermosetting resin and mechanically polished, while specimens used for metallographic structure observation need to be etched in addition to the above steps. The specimens for EBSD need to be vibrationally polished on the vibrating-polishing machine on the basis of the mechanical polishing specimens.

Measurements and characterizations
The microstructures of the specimens were tested on the SEM (JEOL JSM-7800F) equipped with the EBSD (Oxford Instruments NordlysMax3) detectors. The EBSD data of all specimens were collected at the scanning voltage of 20 kV and step length of 0.12 mm. The original specimens were characterized by EBSD with a sampling length of 60 μm and a sampling width of 50 μm, while the processed samples were characterized with a sampling length of 40 μm and a sampling width of 70 μm. The collected test data were analyzed using EBSD data processing software Channel 5. The surface topography was observed by Keyence VK-X200K laser microscope. The surface roughness parameters Ra and Rz values of each specimen were measured at different positions on the machined surface using the TIME3200 handheld roughness meter. The microhardness tester DHV-1000(Z)-CCD was used to obtain the microhardness of the machined surface layers during different drilling parameters. The parameters of surface roughness and microhardness were measured for 3 times and averaged.

Grain size
The depth and width of the scanning area of EBSD test are 60 μm and 40 μm, and the upper edge of the scanning area coincided with the machined surface of the specimens. Figure 4 shows the changes of grain microstructure and gradient distribution under different drilling speeds. According to the gradient change of material microstructure, the surface layer of drilling consists of two regions: recrystallized layer and plastic deformation layer. The recrystallized layer is characterized by uniform fine equiaxed crystal structure distributed closest to the machined surface. The appearance of the recrystallized layer is attributed to the serious plastic deformation of the material during drilling, with a large number of dislocations inside the grains. At the same time, the high temperature generated by drilling causes dynamic recrystallization of the deformed material, leading to grain refinement [31]. The plastic deformation layer is located below the recrystallized layer, and the deformation degree decreases with the increase of depth. The grain structure in this region shows clear streamlined distribution, which is related to the shear slip during machining [32]. In the plastic deformation layer, the grain extension direction is elongated, and the grain refinement degree is reduced [33]. The degree of deformation of the material decreased gradually from the machined surface to the interior. It was found from Fig. 4(a-b) that the recrystallized layer is ~ 12 μm in depth, and the plastic deformation layer is ~ 23 μm in depth, when the drilling speed is 61.27 m/min. As the velocity increases, the thickness of the two layers increases correspondingly. It was found from Fig. 4(c-d) that the recrystallized layer is ~ 15 μm in depth, and the plastic deformation layer is ~ 25 μm in depth, respectively. This is because as the drilling speed increased, the mechanical-thermal load is more obvious. The effect of drilling speed on the plastic deformation was obtained by quantitative analysis of grain proportion with diameter less than 0.5 μm in depth of 30 μm from machined surface. It was found from Fig. 5 that with the increase of the velocity, the proportion of grains less than 0.5 μm in the region 30 μm away from the machined surface becomes higher. It indicates that the grain refinement of the surface layer increases with the increase of the velocity. This may be because the increase in drilling speed leads to an increase in drilling temperature, allowing grain refinement to proceed more fully [34].
The changes of grain microstructure and gradient distribution under different feed rates are shown in Fig. 6. Similarly, the recrystallized layer and plastic deformation layer appear in the region 60 μm away from the machined surface. When the drilling speed is a fixed value of 66.84 m/min, the thickness of the recrystallization layer increases from 13 to 18 μm and the thickness of the plastic deformation layer increases from 25 to 26 μm with the increase of the feed rate from 0.12 to 0.18 mm/r. The fraction of grain less than 0.5 μm in the region 30 μm from the machined surface was counted, as shown in Fig. 7. The proportion of small size grains in the selected area increases with the increase of feed rate, which was increased from 67.2 to 75.7%. This is because the drilling force increases on the feed rate. The extrusion effect and friction force of the guide block on the machined surface increase, and the drilling temperature in this area increases, so the recrystallization effect is significant.

Grain misorientation
Under both thermal and mechanical loading, plastic deformation occurred on the surface layer of workpiece, among  which the distribution of grain misorientation is one of the typical characteristics. The misorientation angle distribution at the drilling speed of 66.84 m/min and the feed rate of 0.14 mm/r is shown in Fig. 8. Figure 8 shows the gradient-distribution of the misorientation angle under the drilling speed of 66.84 m/min, and the feed rate of 0.14 mm/r. The EBSD scan image could be evenly divided into four sections, and the depth of each section is 15 μm. By quantitatively calculating the relative proportion of the LAGBs in the four sections, the gradient distribution of misorientation angle was obtained. It is worth noting that with the increase of the depth from the machined surface, the proportion of LAGBs increases gradually. As can be seen from Figs. 1 and 8, the proportion of LAGBs in the original state of the workpiece is 66.4%, while the proportion of LAGBs in Sect. 1 is 36.1%. This is mainly due to the serious grain refinement of the recrystallized layer and the increase of the number of HAGBs. With the increase of the distance from the processed surface, the recrystallization phenomenon gradually weakens. However, the serious plastic deformation produces a large number of dislocations slip, and the number of sub-grain boundaries gradually increases, causing the proportion of LAGBs to gradually increase.
The gradient distribution trend of grain misorientation under different drilling parameters is depicted in Fig. 9. Under each set of drilling parameters, the proportion of LAGBs increases with the increase of depth from the machined surface. With the increase of drilling speed, the proportion of LAGBs closed to the machined surface area decreases. The main reason is that the thickness of the recrystallized layer increases with the increase of drilling temperature. With the increase of feed rate, the proportion of LAGBs closed to the machined surface area increases. This may be due to the increase of the feed rate; the tool on the machined surface is not sufficient. Fig. 8 The misorientation angle distribution at the drilling speed of 66.84 m/min and the feed rate of 0.14 mm/r

Surface topography
Feed marks, surface tearing, and furrow are typical characteristics of workpiece surface defects caused by drilling. Workpiece surface defect is the concrete embodiment of machining morphology and will directly affect the service life of the workpiece. The topographies of machined surfaces are directly affected by the variation of drilling parameters. The surface topographies of the workpiece processed with different drilling parameters are shown in Figs. 10 and 11. Obvious concave and convex peaks were observed on the machined surface, and there were extrusion marks trimmed by the guide block. When the drilling speed is low, the surface texture of the deep hole is obvious. With the increase of the drilling speed, the temperature of the contact area between the guide block and the hole wall increases. The extrusion effect of the guide block and the hole surface make the groove and feed ridge on the deep hole surface significantly reduce. From Fig. 11, it can be seen that at the feed rate of 0.14 mm/r, the peak valley height is higher than the other feed rates.

Surface roughness
The roughness of the machined surface is very important for the surface interaction between different parts, because it will affect the stiffness, fatigue strength, wear resistance, and tightness of the parts. Surface roughness Ra represents the average value of the absolute distance between each point on the actual profile to be measured and the midline of the profile within the sampling length. Surface roughness Rz represents the average height difference between the five largest contour peaks and the five largest contour valleys over the sampling length. Both can be used to assess surface roughness. Figure 12 shows the surface roughness under different parameters. Figure 12a shows the effect of drilling speed on the surface roughness. It is noted that with the drilling speed ranging from ~ 61 to ~ 78 m/min at the feed rate of 0.14 mm/r, the surface roughness Ra decreases from 1.3 to 0.5 μm and Rz decreases from 10.8 to 4.2 μm. This is because that the temperature of the drilling zone increases with the increase of the drilling speed, and the plasticity of the material increases. During the extrusion process, the material fluidity increases and the surface quality of the hole wall is improved. Figure 12b shows the effect of feed rate on the surface roughness. With the increase of the feed rate, the roughness of the machined surface increases gradually. It is worth noting that the surface roughness reaches the maximum under the drilling speed of 66.84 m/ min and the feed rate of 0.14 mm/r. This may be related to the characteristics of the machine tool. Through comparison, it is found that the effect of feed rate on surface roughness is obviously lower than that of drilling speed. Figure 13 shows the distribution of microhardness values in the direction of depth extension of the surface layer During the BTA deep hole drilling, the microhardness decreases along the depth direction. Within 10 μm from the machined surface, the microhardness changes insignificantly with depth. The microhardness values decrease greatly in the depth range of 10 ~ 30 μm from the machining surface. When the depth exceeds 30 μm, the hardness value changes slowly until it reaches the bulk material layer. In general, the increase of hardness of superficial layer could be related to the plastic deformation of grains therein allowing the accumulation of atomic dislocations [35]. Figure 14c shows the microhardness depth varies with drilling parameters. With the increase of drilling speed and feed rate, the depth of hardened layer increases gradually. This is related to the increase of the extrusion pressure of the outer teeth and the side of the guide block, which leads to the increase of the thickness of the plastic deformation layer [36].

Conclusion
In this paper, the effect of drilling parameters on the hole surface integrity during the BTA deep hole drilling has been investigated, and the main results can be addressed as follows: The cross-sections of the machined surfaces contain two deformation regions such as recrystallized layer and plastic deformation layer. Gradient microstructure of machined surface is influenced by the drilling speed and feed rate significantly. With the increase of drilling speed and feed rate, the deformation layer thickness will increase. 2. With the increase of the depth from the machined surface, the proportion of LAGBs increases gradually under each drilling parameter. This is related to the recrystallization of grains in the machined surface. 3. Regular concave valleys and convex peaks were observed on the machined surface. With the increase of drilling speed, the surface roughness decreases. The effect of feed rate on surface roughness is obviously lower than that of the drilling speed. 4. With the increase of drilling speed and feed rate, the depth of hardened layer increases gradually. It suggests that a thicker strengthening layer will be formed on the machined surface with the higher drilling speed.