RESIDUAL STRESSES ON HIGH-SPEED MILLING OF HARDENED STEEL USING CBN CUTTING TOOL Hadi Sutanto Department of Mechanical Engineering Atma Jaya Catholic University Jl. Jend. Sudirman 51, Jakarta 12930 [email protected] Abstrak Proses pemesinan akan menghasilkan benda kerja dengan bentuk geometri dan permukaan tertentu. Permukaan benda kerja tidak hanya ditentukan oleh kekasaran permukaan, juga oleh kekerasan mikro, tegangan sisa dan struktur mikro pada daerah dekat permukaan. Perubahan hasil proses pemesinan tersebut akan menentukan umur lelah benda kerja dalam pemakaiannya. Tulisan ini akan membahas perubahan tegangan sisa pada permukaan benda kerja akibat proses freis kecepatan tinggi terhadap material baja diperkeras (60 HRC). Eksperimen menggunakan mesin freis CNC dengan pahat CBN (cubic boron nitride) dan kondisi pemotongan tertentu. Hasil eksperimen menunjukkan bahwa tegangan sisa hasil proses freis pada baja diperkeras berupa tegangan tekan dan maksimum pada permukaan benda kerja. Kata kunci: freis kecepatan tinggi, baja diperkeras, tegangan sisa, pahat CBN.

1. Introduction With the results of recent advances in machine tools and cutting tool materials, high-speed machining of hardened steel became a cost-effective manufacturing process to produce parts with high precision and surface quality. Furthermore, machining of alloy steels in hardened state and at high cutting speeds offers several advantages such as: reduction of finishing operations, elimination of distortion if the part is finish-machined after heat treatment, achievement of high metal removal rates, lower machining costs and improved surface integrity [1], [2].

Figure 1. Cutting speed for high-speed machining with different materials.

The definition of high-speed machining is based on the criteria to determine the boundary between conventional and high-speed machining. The workpiece material grade can be accepted as the criteria to define high-speed machining. Figure 1 show the high-speed region for machining different materials related with cutting speeds [3]. For instance, a cutting speed of 100 m/min is considered high-speed machining for cutting nickel alloys where as this speed is considered conventional in cutting aluminum alloy. The state of the part surface as a result of machining processes determines the functional behavior of that machined part when it has high load applications. A modern design of machine elements with high demands on endurance and reliability has to take into account the influence behavior of the surface properties. Surface integrity is the term used to define the geometrical and the physical development of the surface [4]. Surface integrity as the relationship between functional behavior and machining process is shown in figure 2. As shown in the figure, the properties of the machined surface have to be defined related with the cause of part failure. Some of the failures related with surface properties are corrosion, fatigue life, and wear. FUNCTIONS OF SURFACES

MACHINING

CAUSE OF FAILURE

PHYSICAL

PROCESS

PROPERTIES

Figure 2. Relationship between machining process and functional surfaces. Residual stresses in the surface and sub surface are a function of it machining processes. Surface layers produced by machining are usually plastically deformed and work hardened [5]. The layers may contain residual stresses and micro-hardness changes. Residual stresses can enhance or impair the functional behavior of the machined part. The tensile residual stress on the surface can reduce the fatigue life of a component, while the compressive stress on the surface has the opposite effect. The tensile residual stress on the surface can also lead to stress corrosion cracking if the machined surface is placed in the corrosive environment. The objective of the present work is to investigate the effect of cutting speeds and feed rates during hard milling process on the characteristic of residual stresses. The experiment was done by the method of stress relieving on the machined parts. The method is based on that the removal layers of material will relieve a portion of the residual stresses and disturb the existing condition of equilibrium. Then the remaining stresses will redistribute themselves and attain a new equilibrium by producing a change of the deflection of the workpiece. The changes of the workpiece can be measured and used to compute the residual stress distribution.

2. Experimental work Machining tests were conducted on CNC milling machine, type FV 25 CNC. The cutting tool insert used was circle CBN (cubic boron nitride) – RCHT 12 04 MO CB50 from Sandvik Coromant. The tool holder used for single toothed cutter (flymill) was Coromill 290, R290 – 080Q27 – 12M (Dc = 80 mm). Figure 3 shown the geometrical dimension of the tool insert used in the experiment. The machining condition was dry cutting with cutting speed vC = 115 – 140 – 160 m/min, feed rate ft = 0.07 – 0.10 -0.12 mm, and depth of cut aP = 0.2 mm. The work material was CSN 14 109.4 (~ AISI 52100) hardened steel (60 HRC) with the chemical composition as in table 1 [6]. The workpiece materials supplied in block sizes 110 mm x 7 mm x 6 mm. The experiment used the electrolytic etching process for residual stress determinations [7]. The test-piece was placed in the location of the electrolytic etching apparatus and the etching rate was measured to determine the metal removal rates. The curves were made to obtain the relation between the layer removal thickness and time for all work materials.

Figure 3. Geometrical shape of tool insert (iC = 12 mm, di = 4 mm, S = 4.76 mm, α = 70, γ = 00 ). Table 1. Composition of CSN 14 109.4 hardened steel (60 HRC). Element C Mn Si Cr Ni Cu (Ni+Cr) S Fe

% weight 0.90-1.10 0.30-0.50 0.15-0.35 1.30-1.60 0.30 0.25 0.50 0.30 balance

The deflection of the free end of parts was measured as a function of time when the electrolytic etching process was started. The deflection of work parts as a function of the thickness of removal material used to compute the residual stress distribution in the

machined surface. The actual residual stress is the sum of the residual stress in a n-th layer with thickness dt (σ1,n ) and the residual stress in the removal of previous layers (σ2,n ). The second theorem of Castigliano was used to calculate the residual stress as a function of depth beneath the machined surface. Figure 4 shows the machined workpiece with residual stress. One end of the part is fixed in a rigid support and the other is free. The deflection δ happened when a layer of the material was removed from the machined surface.

Figure 4. The machined workpiece with residual stress [6]. Hence, the stress in the n-th layer due to the removal layer is

σ 1,n =

E[2t 0 − (2n − 1)dt ] 2 (δ n − δ n −1 ) 2dt.L2

(1)

where E = modulus of elasticity of work material, t0 = original thickness of workpiece, and L = length of workpiece. If the layer of the work material was removed from the machined surface, the stress distribution changed in the remaining layers due to the removal previous layers. Therefore, the stress in the n-th layer as the sum of the removal successive layers is n

σ 2,n = ∑ n =1

E (t 0 − n.dt )(δ n − δ n −1 ) L2

Then, the total residual stress in the n-th layer is σn = σ1,n + σ2,n

(2)

(3)

3. Results and discussions Residual stresses as a result of machining processes can be produced due to homogenous plastic deformation induced by mechanical and thermal event in the process. The plastic deformation related with the process of chip formation and the interaction between the tool cutting edge and freshly machined work surface.

The mechanical deformation in metal cutting that caused residual stresses consists of two parts. The first part of deformation is due to the cutting action of the tool cutting

edge and the second part is the result of the rubbing and burnishing effect on the tool flank. In the metal machining processes, the workpiece material ahead of the cutting point experiences compressive plastic deformation. In contrary, the material behind the cutting point has a tensile plastic deformation. The residual stress will be compressive if the tensile deformation is higher than the compressive deformation. On the other side, the result is tensile if the compressive deformation is higher than the tensile deformation. Some effects of rubbing or burnishing in the compressive residual stresses is similar with surface rolling or shot-peening process. The heating of the surface will produce compressive plastic deformation by thermal stresses, and then the tensile residual stresses upon cooling [8]. Figure 5 – 7 show the experimental results of residual stresses for different cutting speeds and feed rates. As seen in figure 5 – 7, the residual stresses are high at the surface of the work material and decrease with an increase in depth below the surface. The maximum of residual stress is – 375 MPa (compressive stress) when cutting at cutting speed 160 m/min and feed rate 0.07 mm. The residual stress are generally negative (compressive stress), but results of milling with cutting speeds 140 / 160 m/min and feed rates 0.12 / 0.10 mm show positive results (tensile stress). The tensile residual stresses shown are relatively small, approximately + 100 MPa.

fz=0.07 mm

fz=0.10 mm

fz=0.12 mm

Residual stress (MPa)

50 0 -50 0

0.5

1

1.5

2

-100 -150 -200 -250 -300 -350 Depth (mm)

Figure 5. Residual stress distributions beneath the machined surface produced at the cutting speed of 115 m/min for different feed rates.

fz=0.07 mm

fz=0.10 mm

fz=0.12 mm

Residual stress (MPa)

200 100 0 -100

0

0.5

1

1.5

2

-200 -300 -400 Depth (mm)

Figure 6. Residual stress distributions beneath the machined surface produced at the cutting speed of 140 m/min for different feed rates. fz=0.07 mm

fz=0.10 mm

fz=0.12 mm

Residual stress (MPa)

200 100 0 -100

0

0.2

0.4

0.6

0.8

1

1.2

1.4

-200 -300 -400 Depth (mm)

Figure 7. Residual stress distributions beneath the machined surface produced at the cutting speed of 160 m/min for different feed rates. The residual stress will be tensile or compressive (positive or negative) depends on the extent of the depth of the permanent plastic deformation zone during machining. This zone depends on the stress generated by the mechanical and thermal load in the machining process. If the stress does not reach the yield point of the work material, a compressive residual stresses will exist on the workpiece surface [9]. However, a detailed explanation for the effects of cutting parameters on the characteristics of residual stress distribution in the surface region has not been advanced.

4. Conclusions An experimental work of high-speed milling of hardened steel was conducted to determine residual stresses on the machined surface. The experiment used CBN cutting

tools with different cutting parameters. The results of the experiment due to the mechanical and thermal effects of the chip formation processes can be concluded as 1. Residual stresses measured in the experiment of high-speed hard milling are higher at the machined surface ( ~ - 375 MPa, maximum) and lower below the surface. 2. In general, the residual stresses are negative (compressive stress). 3. For higher feed rates and cutting speeds, the surface layer is affected by low tensile stress ( ~ 100 MPa). 4. There was no tendency for the effects of cutting parameters on the characteristics of residual stress distribution. Acknowledgement

We would like to thank to Prof. J. Madl, Dr. V. Cmelik, and Dr. M. Pavel of the Department of Manufacturing Technology of the Czech Technical University in Prague for their contributions to the experimental works.

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[2] Konig, W., Berktold, A., and Koch, K.F., Turning versus Grinding – a Comparison of Surface Integrity Aspects at Attainable Accuracies, Annals of the CIRP 42 91), 1993, pp. 39-43. [3] Schulz, H., High-speed Machining, Annals of the CIRP, 41 (2), 1992, pp 637 – 643. [4] Field, M., and Kahles, J., Review of Surface Integrity of Machine Components, Annals of the CIRP, 20 (2), 1971, pp. 153 – 163. [5] Tonshoff, H.K., Arendt, C., and Ben Armor, R., Cutting of Hardened Steel, Annals of the CIRP, 49 (2), 2000, pp. 547 – 566. [6] Kriz, R., and Tricka, J., Tabulky materialu pro strojirenstvi I (Table of engineering materials), Montanex a.s., Czech Republic, 1999. [7] Madl, J., Experimentalni metody v teorii obrabeni, (Experimental methods in the machining theory), Faculty of mechanical Engineering, Czech Technical University in Prague, 1988. [8] Colwell, L.V., Sinnot, M.J., and Tobin, J.C., The Determination of Residual Stresses in Hardened, Ground Steel, Trans. of ASME 77, 1955, pp. 1099 – 1105. [9] El-Wardany, T.I., Kishawy, H.A., and Elbestawi, M.A., Surface Integrity of Die Material in High Speed Hard Machining, part 2: Micro-hardness and Residual Stresses, Trans. of ASME, 122, 2000, pp. 632 – 641.