Investigation of Machinability of Cooled Microalloy Stell in Oil After The Hot Forging with Coated a

2.1. Hot forging and microstructure

The chemical composition of the Ø60 × 6000 mm sections of 38MnVS6 microalloyed steel (Çelik Makine San. ve Tic. A.Ş) used in the experiments is given in Table 1. Before forging, the materials had been turned to the size of Ø46.7 × 240 mm. Table 1. Chemical composition of 38MnVS6 steel used in this study C 0.418

The samples were heated to 1200 °C in the induction annealing system. After the samples were annealed, the diameter was reduced to 35 mm by hot forging in a 1600-ton eccentric press closed mold. The final temperature of the samples after forging was measured by an infrared laser thermometer as 1150 ± 20 °C. After forging, the samples were quenched in oil. At the end of the forging process, metallographic examinations and machinability tests were carried out. In order to remove the oxides and decarburization areas formed on the surfaces of the samples, the surfaces were turned and a 1-mm chip removed to ensure that the tests were accurately performed. The samples for the microstructure studies were prepared via conventional metallographic methods by etching with 2% Nital solution. Surfaces of the treated samples were cleaned with alcohol and examined under an optical microscope. The Vickers hardness method was used to measure the hardness values of the samples. The hardness measurements were carried out by applying a load of HV1 (1000 g). The average of 10 hardness measurements taken for each sample was determined as the hardness value. The experimental steps for forging and machinability testing are schematically shown in Figure 1.

2.2. Experimental measurement system and procedures

Turning experiments were carried out on a Johnford TC-35 industrial CNC lathe under dry cutting conditions without the use of cooling fluid. The cutting parameters used for the turning experiments are given in Table 2. The turning experiments used polycrystalline cubic boron nitride (PCBN) cutting tools. These included the quality grade inserts KB5610, coated with TiAlN via the Kennametal PVD method, and KB1610 (uncoated), both having CNGA120404S01025FWMT geometry (Kennametal). In Figure 2, descriptions of the cutting tools and tool holder used in the turning experiments are given in detail. These cutting tools are used in the machining of hardened steels and cast iron. The PCNNR 2525M12 Kenlever external turning tool holder (Kennametal) was chosen to firmly attach the cutting inserts. Turning tests were carried out on the 38MnVS6 steel samples, which had acquired martensite morphology as a result of the oil quenching after its subjection to hot forging. Cutting force measurements were performed during the turning experiments using a Kistler 9257A dynamometer and the cutting force factors were recorded. During turning experiments, the maximum cutting force is usually Fz (Fc) at the basic cutting force, followed by the feed force Fx (Ff) and the radial force Fy (Fp).

Figure 2. Cutting tools and tool holder used in the turning experiments Measurements of surface roughness values were made during the processing of the 30 × 35 mm samples. In the turning experiments, the Mahr Perthometer M1 instrument was used to measure the roughness of the machined surfaces. These measurements were made parallel to the sample axis without removing the machined samples from the CNC lathe to prevent damage being caused to the machined surfaces during their removal. Surface roughness values were determined by taking the average of four measurements made on each sample.

3.1. Microstructure and hardness

The formation of the martensite can be seen in the microstructure images taken of the 38MnVS6 steel samples obtained by oil quenching after hot forging (Fig. 3). This structure resulted from the fact that the cooling rate of the oil quenched 38MnVS6 steel was higher than the critical cooling rate. The alloying elements are effective on the critical cooling rate in steels. Alloying elements reduce the critical cooling rate by shifting the continuous cooling diagrams over a long time and thus facilitate the formation of the martensite structure [16-17]. The martensite transformation takes place via cooling above the critical cooling rate. Otherwise, bainite, residual austenite and ferrite are formed instead of the martensite phase, resulting in the loss of strength [18]. The hardness value of the hot forged oil quenched 38MnVS6 sample was measured as 587 HV1. The higher hardness value resulted from the fact that the microstructure of the sample oil quenched after hot forging exhibited martensite morphology. The most important reason for the high hardness value of the martensite formation was that the cage structure was excessively distorted. The distortion of the cage structure also caused a significant increase in the hardness and strength of the steel cooled at high cooling rates, making dislocation movement difficult or disrupting it [16].

Figure 3. Microstructure view of 38MnVS6 steel samples quenched in oil after forging

3.2. Cutting forces

The effects of cutting forces generated during the cutting of the hot forged oil quenched 38MnVS6 steel using coated and uncoated CBN cutting tools at different cutting rates (120, 150, 180, 210 and 240 m / min) are given in Figure 4. The cutting forces measured for the coated CBN cutting tool averaged 8.4% higher than those measured for the uncoated CBN cutting tool. Because coated cutting tools have a low coefficient of friction, lower cutting temperatures occur during machining. For this reason, it was expected that the cutting force levels for the coated cutting tools would be lower. However, in this study, the cutting force values measured on the coated CBN cutting tool were higher than the values of those measured on the uncoated CBN cutting tool. The high build-up of cutting forces in the machining with the coated CBN cutting tool can be explained by the larger cutting edge radius due to the coating [19]. The highest cutting forces were obtained in turning experiments at a cutting speed of 120 m / min. The cutting forces were measured at 163.5 N and 150.75 N in the coated and uncoated CBN cutting tools, respectively (Fig. 4). The high cutting forces at low cutting speeds can be attributed to the fact that less heat was generated around the slip area, and therefore the hardness of the workpiece did not decrease, resulting in higher cutting forces [20]. In the tests performed with increasing cutting speeds (150, 180, 210 and 240 m / min), the cutting forces measured with the coated and uncoated CBN cutting tools decreased regularly. With a cutting speed increase from 120 to 240 m / min, there were cutting force reductions of 9.6% (149.2 N) and 8% (139.49 N) with the coated and uncoated CBN cutting tools, respectively. This reduction in cutting forces with the increase in cutting speed caused a decrease in the length of the cutting tool-chip contact area and due to the rising temperature with increasing cutting speed, reduced sliding resistance of the material in the slip area of the tool-chip surface resulted in a reduction in cutting strength [21]. As the length of the cutting tool-chip contact area decreased, the temperature increased with the increasing cutting speed.

Figure 4. Effect of cutting speed on cutting force of 38MnVS6 specimens with coated and uncoated CBN cutting tools

3.3. Surface roughness

The effects of cutting speed (120, 150, 180, 210 and 240 m / min) on the average surface roughness values (Ra) using the coated and uncoated CBN cutting tools in the turning of the 38 MnVS6 steel samples obtained after forging are given in Figure 5. In the turning experiments, the surface roughness values measured with the coated CBN cutting tool were about 103% higher than those measured using the uncoated CBN cutting tool. The high surface roughness values in the experiments with the coated CBN cutting tool can be explained by the increase of the cutting edge radius due to the coating on the cutting tool. For the turning experiments at a cutting speed of 120 m / min, surface roughness values were measured as 0.907 μm for the coated CBN cutting tool and 0.341 μm for the uncoated CBN cutting tool. The surface roughness values were about 250% higher in the experiments with the coated CBN cutting tool than in those with the uncoated CBN cutting tool. However, this difference decreased in the experiments in which the cutting speed was increased. The minimum surface roughness values were obtained in the experiments where the cutting speed was increased to 180 m / min. At the cutting speed of 180 m / min, surface roughness values of 0.367 μm were obtained for the coated CBN cutting tools and 0.164 μm for the uncoated CBN cutting tools. Consequently, the surface roughness values had been reduced by about 60% with the coated CBN cutting tools and by about 50% with the uncoated CBN cutting tools. The reduction in surface roughness with increasing cutting speed was due to the thermal softening property of the test samples at high cutting speeds. Increased cutting speed resulted in easy chip removal and reduced surface roughness values [22-23]. The reduced tool-chip contact area due to the temperature at the high cutting speed of 180 m / min led to reduction of friction in the tool-chip contact area. This in turn led to the reduced surface roughness values of the machined surface [24].

Figure 5. Effect of cutting speed on surface roughness in machining of 38MnVS6 samples with coated and uncoated CBN cutting tools However, when the cutting speed was increased from 180 to 240 m / min, the surface roughness values showed a significant tendency to rise. The reduction of surface roughness values with increasing cutting speed has been observed in a number of studies [25]. Conversely, with a further increase of cutting speed (from 180 to 240 m / min), the surface roughness values for the coated and uncoated CBN cutting tools increased by approximately 110% and 290%, respectively. The maximum surface roughness for the coated CBN cutting tool was seen at 120 m / min cutting speed, whereas the maximum surface roughness for the uncoated CBN cutting tool was found at 240 m / min. The increase of the surface roughness values at the cutting speeds of 210 and 240 m / min can be explained by the wear mechanisms on the cutting tools during the experiments. In the study, samples of 38MnVS6 microalloyed steel oil quenched after heat treatment and having a martensite structure were processed with carbide cutting tool at different cutting speeds. The martensite structure, being resistant to abrasion, was found to be more durable, leading to significant wear in the carbide cutting tools used in the turning experiments [12] (Fig. 6). In these cases of processing high hardness materials, CBN cutting tools are recommended. The wear mechanisms in the coated and uncoated CBN cutting tools are visible in the scanning electron microscopy (SEM) images in Figure 7a, b, c and d. In this case, it can be said that the results from the processing of high hardness materials showed the resistance against wear to be better with the coated and uncoated CBN cutting tools than with the carbide cutting tools. However, at high cutting speeds, minor wear, chip adhesion and coating deposits occurred on the cutting edges of the CBN cutters, although this was not so great as the wear on the carbide cutters.

Figure 6. Cutting tool wear with water-quenched material processed at different cutting speeds; (a) 150m / min and (b)180 m / min. [12]

Figure 7. SEM images of wear mechanisms of coated and uncoated CBN cutting tools