Machinability Study on Hard Milling of Ultra-high Strength Steel 30Cr3SiNiMoVA

Machinability Study on Hard Milling of Ultra-high Strength Steel30Cr3SiNiMoVAMing Chen 1, 2, a , Jinyang Xu 2, b , Zhiqiang Liu 2, c and Qinglong An 2, d1College of Electromechanical Engineering, Hunan University of Science and Technology,Xiangtan, 411201, P. R. China2School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai, 200240, P. R. Chinaa mchen@,b xujinyang@,c liuzhiqiang09@,d qlan@,Keywords: Ultra-high strength steel, Hard milling, MachinabilityAbstract. The superior mechanical and physical properties of ultra-high strength steel30Cr3SiNiMoV A have awarded the material such desirable and increasing demands in aerospaceand aviation industries. Due to its excellent properties, 30Cr3SiNiMoV A is usually classified as adifficult-to-cut material. The machining process of ultra-high strength steel is often characterized byhigh cutting forces, low surface finish and severe tool wear. In actual production, hard cutting whichis a profitable alternative to finish grinding is usually employed in order to improve machiningefficiency and reduce processing cost. Therefore, this research concerns the machinabilityevaluation in hard milling of 30Cr3SiNiMoV A by using TiAlN coated carbide tools. Themachinability of the steel was investigated with respect to cutting forces, surface roughness, chipmorphology and tool wear, respectively. Finally, a comparative test was also conducted betweenhard milling and grinding process. The results pointed out that hard milling and coated tools weresuitable, beneficial and effective for manufacturing ultra-high strength steel 30Cr3SiNiMoV A.IntroductionUltra-high strength steel 30Cr3SiNiMoV A has excellent comprehensive mechanical properties suchas high strength (MPa b 1715>σand MPa 1370s >σ), high hardness (HRC50-55), good impacttoughness and fracture toughness, etc. On the one hand, these unique properties have ensured itswide application of manufacturing wing beams, bolts and landing gear in aviation industry. On theother hand, these superior performances also have come with its relatively poor machinability. Themain drawbacks of machining of 30Cr3SiNiMoV A steel can be summarized as follows.(1) High cutting forces. The high tensile strength and shear strength of 30Cr3SiNiMoV A steelusually resulted in severe shear deformation. When under the same cutting condition, the unitcutting force obtained from the ultra-high strength steel is 2-3 times larger than that obtained from45 steel. Moreover, the high cutting forces also increase the risk of tool chipping failure during thecutting process [1].(2) High cutting temperature. 30Cr3SiNiMoV A steel has low thermal conductivity which isabout 1/5 of 45 steel. The low thermal conductivity often leads to a large amount of cutting heataccumulated on the tool-chip interface which then results in continuous increase of cuttingtemperature.(3) Severe tool wear and short tool life. 30Cr3SiNiMoV A steel not only has high strength, highhardness, but also has good toughness. In the machining process, the cutting tool usually sufferssevere crater wear under the condition of high temperature and pressure, which acerbated the toolwear and chipping failure [2].Therefore, manufacturing ultra-high strength steel 30Cr3SiNiMoV A has become a high-costprocess and a time-consuming task [3]. In order to tackle these problems, hard cutting and coatedtools are applied to improve cutting condition and reduce tool wear [4]. Hard cutting technique isconsidered an attractive alternative to traditional finish grinding operations because of the highflexibility, the capability to achieve comparable workpiece quality, the ability to achieve highermetal removal rate and to operate without the use of coolants [5, 6].The main objective of this research is to investigate the machinability of 30Cr3SiNiMoV A steelin hard milling condition by using TiAlN coated tools. The TiAlN coating was confirmed to have excellent performance when used in high-speed machining ascribed to their high oxidation resistance, high-temperature chemical stability and low thermal conduction [7, 8]. Machinability of the steel was studied in terms of cutting forces, surface roughness, chip morphology and tool wear, respectively.Experimental ProcedureWork Material. Machining tests were carried out on a XH714 vertical milling center, and cutting fluid was not used in the operation. Fig. 1 shows the experimental set-up in processing site. The workpiece used in the milling experiments was a 30Cr3SiNiMoV A block with the size of 100mm × 100mm × 50mm. The chemical composition and mechanical properties of the steel are given in Tables 1 and 2, respectively. It should be emphasized that the addition of Mn, Cr, Ni, Mo and V alloying elements play the enhanced role in improving the mechanical properties and reducing thermal conductivity of the material. Fig. 2 presents the microstructure of 30Cr3SiNiMoV A after quenching treatment. It is observed that various needle-like martensites were formed in the matrix after the heat treatment. These martensites are the enhanced phase in the matrix which can greatly improve the hardness and strength of the steel.Fig.1 Experimental setup in processing site Fig.2 Microstructure of 30Cr3SiNiMoV A Table 1 Chemical composition of 30Cr3SiNiMoV A (wt. %)CSi Mn Cr Ni Mo V S P 0.28/0.34 0.9/1.2 0.5/0.8 2.8/3.2 0.8/1.2 0.6/0.8 0.05/0.15 <0.02 <0.02Table 2 Mechanical properties of 30Cr3SiNiMoV AWork material []b MPa σ 0.2[]MPa σ 5[%]δ [%] Ak [J] V model30Cr3SiNiMoV A 1854 1640 12.8 53.8 80Test Condition. Orthogonal methodology was adopted in the experimental design. Cutting speed(v c ), feed per tooth (f z ), axial depth of cut (a p )and radial width of cut (a w ) were selected as factors at four levels (v c of 40, 60, 80 and 100m/min, f z of 0.04, 0.06, 0.08, 0.10mm/z, a p of 10, 20, 30, 40mm and a w of 0.15, 0.30, 0.45, 0.60mm). A standard L 16 (44) orthogonal array was selected to conduct the hard milling experiments. CVD coated carbide tool was applied in the following experiments. The cutter has a diameter of 12mm, two cutting teeth, rake angle of 10°and helix angle of 40°. The TiAlN coating has good chemical stability, high hardness and excellent wear resistance. The good tractility of the TiAlN coating also ensures its suitable application for intermission milling process. The cutting forces and surface roughness were measured by a Kistler (model 9272)dynamometer and a portable SJ201 profilometer, respectively. Nikon microscope was used to observe the tool edge morphology and wear pattern, and tool flank wear was measured by self-developed software. Eventually, both the S150 scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) were applied to fulfill the chemical composition and wear mechanism analysis.Results and DiscussionCutting Forces. Ascribed to the severe chip deformation and intensive sliding friction, much more energy would be consumed in hard milling process, which consequently led to high cutting forces. In side milling process, resultant force (F s) may have the dominant effect on machining process in contrast with feed force (F x), radial force (F y) and axial force (F z), respectively.Fig. 3 presents the main effect plots-mean values for resultant force F s. It is observed that F s decreased gradually in the interval 60-100m/min with the increase of v c,which indicated that high-speed machining was helpful for reducing cutting force in hard milling process. This phenomenon can be explained that when under high cutting speed the strain rate in the primary shear zone would be very high and workpiece was sheared off at a fast rate, thus much more heat was generated. The large amount of cutting heat induced the increase of the cutting temperature which softened the metal and thus required less friction forces.Fig.3 Main effect plots-mean values for resultant force (F s) In addition, a w was observed to have the dominant influence on F s followed by f z, a p and v c, respectively. v c=100m/min, f z=0.04mm/z, a p=10mm and a w=0.15mm were found to be the optimal factor level combination in theory. Therefore, the ideal combination of cutting parameters for lowest resultant cutting force should consist of maximum v c, minimum f z, a p and a w.Surface Roughness. Surface roughness of the machined surface is an important indicator of the surface quality, and also a key factor reflecting tool wear correspondingly. Fig.4 illustrates the effects of the cutting parameters (v c, f z, a p and a w) on surface roughness (Ra).Fig.4 Main effect plots-mean values for surface roughness Ra Basically, v c was observed to favor reduction of Ra especially for speeds up to 60m/min as shown in Fig.4. The phenomenon can be attributed to the drop of resultant cutting force that translates a relative stability of the machining system. Alternatively, an increase of f z, a p or a w will deteriorate surface finish with a w noted as a determinative factor. It is well known that the generated surface comprises helicoid furrows resulting from the tool shape and the form of tool-part movements. In side milling process, surface roughness usually has a close relation with f z and a w. When factors of f z and a w were high, cutting forces and tool wear were exacerbated which consequently undermined the machined surface and deteriorated the surface finish.In summary, the effects of cutting parameters on the measured Ra indicated that changing f z from 0.04 to 0.10mm/z, a p from 10 to 40mm and a w from 0.15 to 0.60mm resulted in direct roughness increase. Otherwise, an augmenting v c from 60 to 100m/min caused an obvious drop of surface roughness value.Chip Morphology. In hard milling of 30Cr3SiNiMoV A, a large amount of cutting heat would be generated on tool-chip interface and then formed a localized peak-temperature zone on tool rake face. Since the bulk heat generated in metal cutting is carried away by chips and dissipated into the air, the high cutting temperature favored the oxidation reaction on chip surface, making the chip shows different colors under different cutting conditions (f z=0.10mm/z, a p=40mm and a w=0.60mm).Fig.5 Chip type under different cutting speed: Fig.6 Typical serrated chip(a) v c =40m/min, (b) v c =60m/min, (c) v c =100m/min As presented in Fig.5, the obtained chips basically kept the same curling pattern belonging to aspiral chip type. Chip colors changed from the original silvery white, to slight yellow and eventually to purple color with the increase of cutting speed. The results can be associated with the formation of oxide layers (especially iron oxide) on chip surface. The potential gas-metal oxidation reaction between Fe and O can be represented as follows:22()()2()Fe s O g FeO s +→ (1)2234/3()()2/3()Fe s O g Fe O s +→ (2)2343/2()()1/2()Fe s O g Fe O s +→ (3)Since the colors of FeO, Fe 2O 3 and Fe 3O 4 are blue, red and black, respectively, the obtained chipcolors varies with the presence of specific iron oxide or their combinations, which depend on the degree of oxidation. With the increase of cutting speed, more cutting heat would be generated through the chip surface, making the chip suffer higher cutting temperature and severe oxidation reaction. According to S. Zhang [9], the higher temperature (especially exceeds 1478℃) may reduce Fe 2O 3 to FeO and Fe 3O 4. The more oxide composition FeO and Fe 3O 4 makes the chip color to become darker and darker. In addition, sawtooth chip was another typical characteristic in hard machining process due to the adiabatic shear band formation which was caused by the localized shear deformation resulting from the predominance of thermal softening over strain hardening. As presented in Fig.6, obvious serrated morphology was observed under the cutting condition of v c =100m/min, f z =0.10mm/z, a p =40mm and a w =0.6mm.As illustrated before, this phenomenon can be ascribed to the existence of shear slip bandsformation caused by the localized shear deformation. When in hard milling process, ultra-high strength steel was deformed at sufficiently high strain rates; the plastic strain was accommodated in narrow localized adiabatic shear bands. Therefore, this adiabatic shear band formation led to the formation of the sawtooth chip in hard milling.Tool Wear. In these experiments, tool wear was measured as the width of the wear land on theflank surface, VB , as a function of cutting length (l ) as shown in Fig.7. The wear experiments was carried out under the cutting condition of v c =100m/min, f z =0.10mm/z, a p =40mm and a w =0.6mm. Figs.8 and 9 present the SEM and EDS analysis of the worn tool surface, respectively.Crater wear A(a)(b)Coating peelingMicro chipping BFig.7 Tool flank wear curve Fig.8 SEM images for TiAlN coated tool: (a) rake face;(b) flank face Advanced Materials Research Vol. 565499Fig.9 EDS analysis for TiAlN coated tool: (a) position A; (b) position B It is observed that TiAlN coated tool had underwent the initial, normal and rapid wear stages in the entire milling process. When in the initial wear stage, tool wear rate was rapid which can be explained that surface roughness value of antithesis surfaces was high, and the actual contact area suffered powerful pressure and severe friction in their early running. Before l reached 30m, the cutter suffered a steady wear stage, and tool flank wear was controlled below 0.135mm which can be ascribed to the effective protection and excellent wear resistance of TiAlN coating material. When l reached over 30m, micro chipping was found to take place on tool nose, which made the cutter quickly enter the rapid wear stage.Based on Fig.8, it is evident that no BUE was found along the main cutting edge in hard milling of 30Cr3SiNiMoV A. Obvious crater wear was observed on tool rake face which was mainly caused by high cutting temperature localized on tool-chip interface. Via the EDS analysis, the main elements of position A were W and C together with very small amount of Fe and O elements, which indicated that coating peeling had took place on the crater area. The occurrence of coating peeling may result in minor effects of lubricants and ineffective heat-dispersion between tool-chip interfaces, which thereby deteriorated the cutting condition of the second shear zone. In addition, tool flank face had occurred abrasive wear where many grooves were found on its surface as presented in Fig.8 (b). This abrasive wear was mainly caused by numerous hard particles (particularly the martensite hard phase) in work material that were able to cut into the coating material. Due to severe contact and friction between the machined surface and tool flank face when machining ultra-high strength steel, adhesive wear is particularly active on tool flank face with the help of high stress and temperature. The large amount of Fe element on position B had already confirmed its occurrence (as shown in Fig.9(b)). Because the adhesive wear usually underwent an adhesive-peeling-adhesive dynamic process, it would take away parts of the tool material periodically. Ascribed to the negative effects of micro chipping and coating peeling on cutting edges, the cutter was soon deprived of the protection from coating material and failed quickly in its rapid wear stage.Comparison between hard milling and grinding process. It is well known that surface integrity induced in hard turning process is an important factor for assuring the quality and fatigue life of the machined components in order to substitute grinding process. The key advantage of hard cutting alternative to grinding lies in that hard cutting can obtain comparable excellent surface finish and higher compressive stress. A preliminary test had been carried out to determine proper cutting parameters for comparative experiments between hard milling and grinding. The detailed cutting parameters were v c=80m/min, f z=0.05mm/z, a p=30mm and a w=0.15mm for hard milling; v s=25m/min, v w=25m/min and a p=0.05mm for grinding. Table 3 presents the results between hard milling and grinding of 30Cr3SiNiMoV A in terms of tangential force per unit width (F c*), radial force per unit width (F r*) and surface roughness (Ra).Table 3 Experimental comparison between hard milling and grinding of 30Cr3SiNiMoV AMachining types F c*(N) F r*(N) Ra (µm)Hard milling 12.7 14.3 0.43Grinding 37.7 72.4 0.40500Advances in Abrasive Technology XVIt is notable that hard milling process obtained extremely lower F c* and F r*, probably 1/3 and 1/5 of that acquired from grinding operation, respectively. This phenomenon demonstrated that in hard milling process the removal of unit volume of material consumed less power and less cutting heat was generated compared with grinding operation, which would have less adverse influence on the machined surface. In addition, hard milling also obtained equivalent low surface roughness; only minor higher than that of grinding process. Fig.10 presents the residual stresses induced by grinding and hard milling on the machined surfaces.As shown in Fig.10, compressive residual stresses induced by hard milling on machined surface in both feed direction and perpendicular to feed direction were higher than that induced by grinding process. This phenomenon indicates that the workpiece machined by hard milling may obtain longer rolling contact fatigue (RCF) than grinding one.SummaryBased on the results acquired, the major conclusions can be summarized from this study.(1)Increase of v c may favor the reduction of both F s and Ra. In addition, a w has the dominant influence on F s and Ra followed by f z, a p and v c, respectively. Optimal factor level combination (v c=100m/min, f z=0.04mm/z, a p=10mm and a w=0.15mm) was obtained through the method of extremum difference analysis.(2) The occurrence of oxidation reaction on chip surfaces had made the chips show different colors, and the colors also varied with the cutting speed. Spiral type and serrated shape were the typical chip morphology in hard milling of 30Cr3SiNiMoVA.(3) The main wear mechanisms for TiAlN coated tool were crater wear, adhesive wear and abrasive wear. Micro chipping and coating peeling were confirmed to be the main tool failures in the hard milling process.(4) Hard milling was confirmed to be a suitable and effective method for manufacturing 30Cr3SiNiMoV A compared with grinding process.AcknowledgementThe work is supported by National Natural Science Foundation of China (No.50935001 & No. 51105253), National Key Basic Research Program under grant (No.2010CB731703 & No.2011CB706804), “Shu Guang” project supported by Shanghai Municipal Education Commission and Shanghai Education Development Foundation (2011-2012), and Important National Science & Technology Specific Projects (2011ZX04015-031, 2012ZX04003-051, 2012ZX04012-021 & 2012ZX04003-031).References[1] R. Q. Chen: Principles of Metal Cutting (China Machine Press, Beijing 2005).[2] R. Wang, J. Liu, X. B. Wang and H. J. Yang: Tool Engineering Vol. 39 (2005), p. 25[3] X. Q. Zhang, A. Senthil Kumar, M. Rahman, C. Nath and K. Liu: J. Mater. Process. Tech., Vol.211 (2011), p. 1701[4] H. K. Tönshoff, C. Arendt, and R. Ben Amor: Ann. CIRP., Vol. 49 (2000), p. 547[5] T.Tamizharasan, T.Selvaraj and A. N. Haq: Int. J. Adv. Manuf. Technol., Vol. 28 (2006), p. 671[6] C. Cappellini, A. Attanasio, G. Rotella and D. Umbrello: Int. J. Mater. Form Vol.3 (2010),p.455[7] G.S. Fox-Rabinovich, A.I. Kovalev and M.H. Aguirre: Surface & Coatings Technology Vol.204 (2009), p. 489[8] Y.S. Liao, H.M. Lin and Y.C. Chen: Int. J. Mach. Tools. Manuf., Vol. 47(2007), p. 1667[9] S. Zhang, Y. B. Guo: Int. J. Mach. Tools. Manuf., Vol. 49 (2009), p. 805Advances in Abrasive Technology XV10.4028//AMR.565Machinability Study on Hard Milling of Ultra-High Strength Steel 30Cr3SiNiMoVA 10.4028//AMR.565.496DOI References[3] X. Q. Zhang, A. Senthil Kumar, M. Rahman, C. Nath and K. Liu: J. Mater. Process. Tech., Vol. 211 (2011), p.1701.doi:10.1016/j.jmatprotec.2011.05.015[5] T. Tamizharasan, T. Selvaraj and A. N. Haq: Int. J. Adv. Manuf. Technol., Vol. 28 (2006), p.671.doi:10.1007/s00170-004-2411-1[6] C. Cappellini, A. Attanasio, G. Rotella and D. Umbrello: Int. J. Mater. Form Vol. 3 (2010), p.455.doi:10.1007/s12289-010-0805-1[7] G.S. Fox-Rabinovich, A.I. Kovalev and M.H. Aguirre: Surface & Coatings Technology Vol. 204 (2009), p.489.doi:10.1016/j.surfcoat.2009.08.021。