Machining studies of normal cryogenic treated P-40 tungsten carbide cutting tool inserts.
Tungsten carbides are a popular material choice for manufacturing tools, dies, and wear parts because of their resistance to wear, shock, corrosion and abrasion (Robert Powell, 2000) . A cryogenic treatment shows promise as a tool material treatment for increasing tool life (Harold A. Stewart, 2004) . Bill Bryson (1999) attributes the wear resistance, and hence the increase in tool life, of carbide tools to the improvement in the holding strength of the binder after cryogenic treatment . The cryogenic treatment also acts to relieve the stresses introduced during the sintering process under which carbide tools are produced.
Seah et al. (2003), reported that cryogenic treatment improved the wear resistance and overall tool lives of tungsten carbide tool inserts in turning . Yong et al. (2007), compared the performance of cryogenically treated and untreated tungsten carbide tool inserts during the high speed milling of medium carbon steel. The cryogenically treated tools exhibit better tool wear resistance than the untreated ones . In the present work, the turning studies of normal cryogenically treated P-40 tungsten carbide cutting tool inserts on machining C45 steel is reported.
In the present work, multilayer CVD coated ISO P-40, cemented carbide rhombic flat inserts of ISO specification CNMG 120408 with chip breaker and composition WC=75.13 wt %, Co=21.22 wt%, TiC & TaC=3.65 wt % and a net coating thickness of 25 [micro]m (1st layer-TiN-1.5[micro]m, 2nd layer-TiCN-12.5[micro]m, 3rd layer-[Al.sub.2][O.sub.3]-6[micro]m, 4th layer-TiN-5[micro]m), were used. The cutting tool inserts (Kennametal, India) of 4 mm thickness and rhomboidal in shape were subjected to normal cryogenic treatment as follows: a gradual lowering of temperature from room temperature to -110[degrees]C at the rate of about 2[degrees]C per minute holding the temperature at -110[degrees]C for 24 hours, then subsequently raising the temperature back to room temperature at the rate of 2[degrees]C per min. The electrical resistivity of the cutting tool inserts used in this study were measured with a standard four probe set-up. Hot hardness tests have been conducted on the substrate of all the inserts. The hot hardness of the inserts were measured using a Rockwell hardness testing machine with a load of 60 kgf.
The machining experiments were carried out using the P-40 tungsten carbide cutting tool inserts by turning operations using a high precision CNC lathe. The machining experiments were performed at a depth of cut 1 mm, feed rate of 0.22 mm/rev and cutting speeds of 200, 250, 300 and 350 m/min, under dry conditions. The workpieces selected in the present study is C45 steel, have dimensions of 60 mm diameter and 400 mm length, so that L/D ratio should not exceed 10 as per ISO 3685 standards. The tool holder used for the cutting tool insert is ISO PCLNR 2525M12 (Kennametal, India). The cutting conditions and tool geometry were held constant for all the experiments. The assessment of flank wear can be accomplished by a direct measurement as per the ANSI/ASME B94.55M-1985 standard, ASME, New York, 1985 . The flank wear of the inserts were measured using a tool makers microscope. The cutting forces were measured using a strain gauge type dynamometer. The surface roughness of the machined workpiece were measured using a perthometer.
Results and Discussions Flank Wear
Wear on the flank of a cutting tool is caused by friction of the newly machined surface and the contact area on the tool flank (Geoffrey Boothroyd, 1987) . Among the different forms of tool wear, flank wear is the significant measure as it affects the dimensional tolerance of the work piece. The dimensional accuracy of the work piece is controlled by flank wear of turning tools (Senthilkumar et al., 2003) .
The maximum width of flank wear ([VB.sub.B max]) at different cutting speeds of P-40 insert after 15 minutes of machining, is presented in figure 1.
[FIGURE 1 OMITTED]
As the cutting speed increases, wear increases due to increase of the sliding distance of cutting tool with increase in cutting speed for a given time. The increase of the cutting speed increases cutting temperatures, which leads to increase in wear and plastic deformation of the cutting edge. During metal cutting, high temperatures are generated in the region of tool cutting edge and these temperatures have a controlling influence on the rate of wear of the cutting tool. The rise in temperature adversely affects the wear resistance and hardness of the cutting tool. Increased heat causes dimensional changes in the part being machined, making control of dimensional accuracy difficult. The mean temperature in turning on a lathe is proportional to the cutting speed and feed as follows:
Mean Temperature [varies] [V.sup.a] [f.sup.b],
Where a and b are constants depends on tool and work piece materials, V is the cutting speed and f is the feed of the tool (Serope Kalpakjian, 1995) .
In general, the thermal conductivity of the cryogenic treated inserts are higher than that of the untreated inserts [10, 11]. The electrical resistivity and electrical conductivity of the cutting tool inserts are presented in table 1.
There is an increase in electrical conductivity from untreated to normal cryogenic treated inserts. The free electrons are primarily responsible for the electrical and thermal conductivity of metals and alloys, therefore, the wiedemann-franz-loranz Law can be applied to relate the thermal conductivity to the electrical resistivity. Wiedemann-franz-loranz Law states that for all metals at not too low temperature, the ratio of the thermal conductivity to the electrical conductivity is directly proportional to the temperature with the value of the constant proportionality independent of the particular metal.
The hot hardness of the cutting tool inserts are presented in table 2.
The hardness of the P-40 inserts decreases with increase in temperature. The hardness of the cryogenic treated inserts is slightly lower in comparison with untreated inserts at room temperature. However, there is a increase in hot hardness value of cryogenic treated inserts in comparison with untreated inserts with increasing temperatures.
The increase in thermal conductivity due to cryogenic treatment increases heat dissipation capacity of cutting tool and helps in decreasing the tool tip temperature, resulting in more hot hardness during machining, leading to lesser tool wear for cryogenic treated tools compared to untreated tools.
The cutting forces generally decreases as the cutting speed increases (Sornakumar and Senthilkumar, 2008) . Figure 2 shows the variation of main cutting force with cutting speed for untreated and normal cryogenic treated inserts at 15 minutes of machining. The lower cutting forces result in lesser distortion of work piece, which improves the surface finish while machining with the ceramic cutting tools. The main cutting forces for the cryogenic treated inserts is less when compared to untreated inserts. This is due to lower wear of cryogenic treated inserts on comparison to untreated inserts as observed during the turning process.
[FIGURE 2 OMITTED]
The surface roughness generally decreases with increasing cutting speed (Sornakumar and Senthilkumar, 2008) . The surface quality largely depends upon the form stability of the cutting nose. An ideal tool in turning is one which replicates its nose well on the work surface (Senthilkumar et al, 2003) . Figure 3 shows the surface roughness Ra at different cutting speeds of the P-40 inserts at 15 minutes of machining. The lower cutting force leads to a minimal vibration in machining thus lower surface roughness obtained on the machined C45 workpiece. The surface finish of the workpiece is better on machining with cryogenic treated inserts in comparison with untreated inserts at all cutting speeds. This is due to lower wear, less temperature at the tool tip and less distortion of cutting edge due to lower cutting force of cryogenic treated inserts on comparison to untreated inserts as observed during the turning process.
[FIGURE 3 OMITTED]
The flank wear of cryogenic treated inserts is lower when compared to untreated inserts in all cases. The main cutting forces for the cryogenic treated inserts is less when compared to untreated inserts. The surface finish of the workpiece is better, when the workpiece was machined, with cryogenic treated inserts in comparison with untreated inserts at all cutting speeds. In summary the turning studies revealed that cryogenic treatment of P-40 cutting tool inserts, may be used to the advantage of better machining performance.
 Robert Powell., 2000, "Stressing out over tungsten-carbide failure", American Machinist., 144 (9), pp.84-90.
 Harold A. Stewart., 2004, "Cryogenic treatment of tungsten carbide reduces tool wear when machining medium density fiberboard", Forest Products Journal., 54(2), pp.53-56.
 Bill Bryson., 1999, Cryogenics, Hanser Gardner Publications, Cincinnati. pp.81-108
 Seah K.H.W., Rahman M., and Yong K.H., 2003, "Performance evaluation of cryogenically treated tungsten carbide cutting tool inserts", Proc. Inst. Mech. Engrs., Part B: J. Engg. Manuf., 217, pp.29-43.
 Yong A.Y.L., Seah K.H.W., and Rahman M., 2007, "Performance of cryogenically treated tungsten carbide tools in milling operations", The Int. J. Adv. Manuf. Tech., 32, pp.638-643.
 American National Standard, "Tool Life Testing With Single- Point Turning Tools" ANSI/ASME B94.55M-1985, ASME, New York, 1985.
 Geoffrey Boothroyd., 1987, Fundamentals of metal machining and machine tools, McGrawHill book company, Singapore, Tenth printing, pp.108-124.
 Senthilkumar A., Rajadurai A., and Sornakumar T., 2003, "Machinability of hardened steel using alumina based ceramic cutting tools", Int. J. Refract. Met. & Hard Mater., 21, pp.109-117.
 Serope Kalpakjian., 1995, Manufacturing engineering and technology, Addison Wesley Publishing company, Reading, Massachusetts, Third edition, pp.594-634.
 Sreerama Reddy, T.V., Sornakumar, T., Venkatarama Reddy, M. and Venkatram, R., "Machinability of C45 steel with deep cryogenic treated tungsten carbide cutting tool inserts", Int. Journal of Refractory Metals & Hard Materials, In Press.
 Sreerama Reddy, T.V., Sornakumar, T., Venkatarama Reddy, M. and Venkatram, R., "Machining performance of low temperature treated P-30 tungsten carbide cutting tool inserts", Cryogenics, In Press.
 Sornakumar T., and Senthilkumar A., 2008, "Machinability of Bronze-Alumina Composite with Tungsten Carbide Cutting Tool Insert", J Mater. Process. Tech., 202, pp.402-405.
T.V. SreeramaReddy (1), T. Sornakumar (2), M. VenkataramaReddy (3), R. Venkatram (4)
(1) Assistant Professor, Department of Mechanical Engineering, Bangalore Institute of Technology, Bangalore-560004, India. E-mail: firstname.lastname@example.org
(2) Professor, Department of Mechanical Engineering, Thiagarajar College of Engineering, Madurai-625015, India. E-mail: email@example.com
(3) Professor, Department of Mechanical Engineering, Bangalore Institute of Technology, Bangalore-560004, India. E-mail : firstname.lastname@example.org
(4) Professor, Department of Mechanical Engineering, East Point College of Engineering and Technology, Bangalore-560078, India. E-mail: email@example.com
Table 1: The electrical resistivity and electrical conductivity of the P-40 inserts Condition of cutting Electrical Electrical tool insert, P-40 resistivity Conductivity ohm-m [(ohm-m).sup.-1] Untreated 2.517*[10.sup.-3] 397.2 Normal cryogenic treated 2.159*[10.sup.-3] 463.15 Table 2: The hot hardness of the P-40 inserts Rockwell Hardness, HRA Temperature [degrees]C P 40 Untreated P40 Normal cryogenic treated Room 84 84 temperature 100 83 83 200 81.5 81.8 300 78 79.2 400 76 76.8 500 71 73 600 59 63
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|Author:||SreeramaReddy, T.V.; Sornakumar, T.; VenkataramaReddy, M.; Venkatram, R.|
|Publication:||International Journal of Applied Engineering Research|
|Date:||Nov 1, 2008|
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