Effect of the Cryogenic cooling on. Surface Quality of Ground AISI Steel

Transcription

1 Effect of the Cryogenic cooling on Surface Quality of Ground AISI Steel P. Prudvi Reddy 1, A.Ghosh 2* 1 Dept. of Mech. Engg., Indian Institute of Technology Madras, Chennai, , pprudvi108@gmail.com 2* Dept. of Mech. Engg., Indian Institute of Technology Madras, Chennai, , amitava_g@iitm.ac.in Abstract Cryogenic cooling with LN 2 is recognized as a green solution for effective control of machining zone temperature, thereby resulting in substantially enhanced tool life. Although literatures mostly indicate sustainable character of LN 2 and report its positive effect as green cooling medium in machining, there are possibilities of potential adverse effects due to extreme low temperatures. Present work makes an attempt to explore the negative side of cryo technology in a case study, where hardened bearing steel (AISI 52100) was ground by an alumina wheel with chilled N 2 in both gas and liquid (LN 2 ) jet form. It was observed that the ground specimen suffered a significant dimensional deviation with the liquid jet with respect to dry and soluble oil environment. In a similar fashion, micro hardness of work piece notably was changed and so was the deterioration of surface finish. On the contrary, G-ratio was found to be remarkably improved, which is in line with information in the available literatures. Extent of those adverse effects could be controlled by using chilled N 2 gas instead of LN 2 jet however with a compromise on G-ratio. Keywords: Grinding, LN 2, dimensional inaccuracy, micro hardness, G-ratio. 1Introduction High specific energy consumed in grinding process, finally gets transformed to heat and a major portion of it is transferred to the work piece. The excessive heat at wheel- work interface not only affects the grinding wheel life but also impairs the ground surface quality by introducing thermal defects such as surface burning, dimensional distortions, metallurgical damages such as phase transformation and change in micro hardness etc (Jin and Stephenson, 2003). Introducing cutting fluids in the machining zone dissipates heat resulting in good surface quality, high wheel life and reduced thermal defects. Despite many significant advantages of cutting fluids in grinding, there is more concern about its ecological and economical impacts. Most of the conventional cutting fluids are not bio-degradable. These cutting fluids vaporizing from grinding zone and forming mist, ultimately causes several health problems to the operators. At the same time, chances of bacteria growth in stored cutting fluids bring down their effectiveness by reducing the lubricity. In addition, Howes (1990) reported that these cutting fluids are effective only up to their film boiling temperatures and if the temperatures in the cutting zone are more than the respective film boiling point, then these cutting fluids become ineffective. Several methodologies like Minimum Quantity Lubrication (MQL) and Cryogenic Machining (CM) have been reported in recent years as alternatives to flood cooling technique. In MQL, even though cost and energy requirements are reduced, it releases cloud of fluid droplets and vapors which cause health problems to operators. Development of cryogenic technology opened up a new horizon for the researchers. In cryogenic machining, cryogenic coolants such as Liquid Nitrogen (LN 2 ) and chilled carbon dioxide gas (CO 2 ) are introduced into the cutting zone in order to reduce the cutting zone temperature. Carbon dioxide is considered as pollutant and since it is heavier than air, it causes the oxygen deficiency problems in machining environments. On the other hand, Liquid nitrogen is not air pollutant and can be produced easily from the air (79% of the atmospheric air is nitrogen). In addition, with boiling temperature as low as C, LN 2 is a very good alternative to conventional cutting fluids. When the LN 2 enters into the cutting zone, it control the temperatures drastically and quickly evaporates into the atmosphere without leaving any residue on the work piece, wheel and keeps machine environment clean and safe. So, this process admirably recognized as sustainable machining process being environmental friendly. Application of cryogenic coolants plays very important role in machining of difficult to machine materials, such as titanium and nickel based alloys, in controlling excessive tool wear at high process temperatures (Shokrani et al. 2012). Under cryogenic 389-1

2 Effect of the Cryogenic cooling on Surface Quality of Ground AISI Steel conditions, presence of low temperature restores cutting tool strength and there by the wheel life. Hong (2006) proved that cryogenic coolants like LN 2 not only acts as coolants but also exhibits good lubrication characteristics. Cryogenic Machining is now becoming a wellrecognized process in machining industries and several beneficial effects of application of LN 2 in various machining processes have been reported. Paul et al. (2001) revealed that the substantial benefit of cryogenic machining was realized on tool life and surface quality over dry and soluble oil machining in turning of AISI 1060 steel. Shane Hong and Ding (2001) reported the improvement of tool life and so as found by Khan and Ahmad (2008) in turning of titanium. A recent work by Biček et al. (2012) also supports the same. Investigations by Wang (2002) concludes that application of LN 2 in turning of tantalum could control tool wear significantly compared to conventional machining. Shane Hong et al. (2001) reported that friction co-efficient on the tool-chip interface was substantially reduced in cryogenic machining. Studies of Paul et al. (1993) concluded that oxidation and surface burning on ground surface were completely absent and surface quality of ground steel was improved due to absence of cracks. Similar beneficial effect of LN 2 was observed even in milling, in terms of reduced surface roughness, tool flank wear by Ravi and Pradeep (2011). Biermann and Hartmann (2012) introduced chilled CO 2 gas in drilling, and it helped to control the burr formation. In grinding, cryogenic machining is supportably proven to be significantly beneficial due to its extraordinary ability in controlling the temperatures, reducing cutting forces, retaining wheel wear rate. However, reported investigations mostly demonstrated the beneficial role of LN 2, its adverse impact on dimensional deviation and possible metallurgical damage of surface and subsurface zone is not investigated in depth. Present work focuses on assessing overall surface quality of ground specimen under cryogenic medium, when applied in liquid and gas phase. It brings out the negative impact of LN 2 on dimensional deviation, spark at prolongation, surface roughness and unfavorable metallurgical changes on ground specimen when hardened AISI was ground by Al 2 O 3 wheel. It further looks for a possible scope of avoiding the above predicament by using chilled N 2 jet instead of LN 2 and demonstrates the extent of compromise in obtaining improved G-ratio. 2 Experimental Procedure Grinding experiments were carried on a reciprocating surface grinder (Make: Alex Machine Tools Pvt Ltd, India; Model: NH 500). A dedicated set up was built up for the delivery of LN 2 to the grinding zone. Liquid nitrogen, stored in and pressurized to 1.5 bar within a self-pressurized dewar (Make: Wessington cryogenics; UK Model; TPV-60), was delivered through suitably insulated SS- pipe network having a solenoid valve (Make: Jefferson, USA) followed by delivering nozzle at the exit end. Nozzle was fixed at a standoff distance of 50 mm and an angle of 15 o with respect to longitudinal feed direction. Figure 1 represents the schematic view of the cryogenic delivery system. LN 2 was supplied in the form of jet in both liquid and gas- alone phase. Grinding velocity and down feeds were varied to observe the effect of the change in grinding Table 1: Chemical compositions for raw sample (Test Method: OES- ASTM E ) Material C Si Mn Cr Nominal Actual environments. The Experimental conditions are briefed in Table 1. AISI steel plates (100 mm X 80 mm X 10 mm) with initial hardness of 190 HV5 were heat treated to final hardness of 798 HV5. Elemental analysis of the sample was carried using Optical Mission Spectroscope (OES) and the results were presented in table 2 along with the nominal range. Experiments were conducted using an alumina abrasive grinding wheel with a mesh size of 60. Before each test, the wheel was dressed using a single point diamond. Surface roughness experiments were conducted under each condition and the centre line average of surface roughness (R a ) was measured across the work piece length (transverse to the direction of table feed) using a perthometer (Make: Mahr, Germany,Model: MarSurf XR20). After each experiment, a micro dialindicator (Make: Mitutoyo) of 1µm L.C was used Table 2 Experimental conditions Reciprocating surface Machine tool grinder (Make: Alex Machine Tools Pvt Ltd, India, Model: NH 500) Work Piece Material AISI 52100, 62HR c Wheel specifications A60K5V8 Process parameters Grinding 20 and 30 m/s velocity Table speed 6 and 12 m/min Down feed 10 µm Grinding environment Dry, Wet, LN 2 gas and LN 2 jet LN 2 delivery Pressure: 1.5 bar LN 2 consumption rate in gas form: 15 LPH LN 2 consumption rate in jetform : 23LPH 389-2

3 to measure the amount of the material removed and dimensional deviation was calculated. grits, which make wheel life shorter. However, the moment cryogenic jet was introduced substantial rise GrindingWhee Work- piece Nozzle Figure 1 Schematic diagram of the cryogenic machining set up Closer view of the setup during cryogenic jet machining Grinding ratio was measured at selected parameters by allowing the work material to be ground for 1000 passes, to observe the performance of alumina grinding wheel under each environment. Wheel wear was measured using the same micro dial indicator. Figure 2 compares the grinding ratios, obtained under each condition. Micro hardness tests were conducted on the cryo machined (both gas and jet machined) samples.after necessary preparations, micro hardness was measured on the ground surface using a vickers micro hardness tester by applying a load of 500 grams with a dwell time of 10 sec. The results are presented in Fig. 6. Figure 2 Grinding ratio under different environments 3 Results and Discussions Figure 2 compares the grinding ratio of alumina wheel under different grinding environments. It can be clearly seen that grinding ratio under dry and soluble oil conditions are 19 and 24 respectively. Blunting of the grits at high temperature causes increase of force and subsequent rapid loss of the in the grinding ratio was realized, almost 2.5 and 2 times of that under dry and soluble oil conditions respectively. As compared to liquid jet, in cryo gas conditions temperature was not controlled to extreme but it provides much better conditions than dry and soluble oil machining hence long lifer of wheel was observed. Since heat dissipation ability of soluble oil is far inferior to cryo-environments, it could only reasonably enhance as found under dry, but certainly not to extent achieved under gas and liquid jet of nitrogen. Figures 3 and 3 show the variation of center line average roughness (R a ) and ten-point roughness (R z ) respectively with change in grinding velocity and table feed under different machining conditions. From Fig. 3, it is clearly observed that average surface roughness is increased as table speed increases. It is because, as table speed increases, the maximum uncut chip load per grit increases and there by surface roughness. Similar phenomenon can be observed at the grinding velocity of 30 m/s. But when the grinding velocity is increased from 20 m/s to 30 m/s at a given table feed and down feed, R a values were found to be decreased. As the grinding velocity increases, grit penetration becomes shallower, which results in low roughness values. It has been discussed earlier that LN 2 offer best protection for thewheel wear and subsequently resulted in achieving the highest G-ratio amongst all the conditions. However, when surface roughness results are analyzed, it is seen that significantly finer finish was produced by dry and soluble oil environment. Soluble oil conditions exhibited relatively less surface roughness values for any given conditions. Cryo gas conditions produced significantly higher surface roughness with respect to dry and soluble oil environments.surface was the roughest under cryo jet conditions. This trend has been found irrespective of set of grinding parameters

4 Effect of the Cryogenic cooling on Surface Quality of Ground AISI Steel Figure 3 R a R z surface roughness values of the ground samples under different environments (Note: 20_6_10 signifies 20 m/s grinding velocity, 6 m/min table speed and 10 µm down feed) As discussed earlier surface LN 2 (c) (d) Figure 4Surface roughness profiles of the ground samples at table speed 12m/min and down feed 10µm under different environments[ at 30m/s under cryo gas at 30m/s under cryo Jet (c) at 20m/s under cryo gas (d) at 20m/s under cryo Jet conditions] Figure 5 Dimensional deviation of the ground samples under different environments. is capable of offering the best production of sharpness retention of grits, it is reasonable to apprehend that the ground surface was produced with sharper grit under both gas and liquid jet conditions within the short spell of grinding. Quick dulling of grits within few passes of grinding under dry conditions led to better Figure 6 Micro hardness of the ground samples under cryo conditions. finish with respect to the cryo conditions.however under soluble oil conditions, the roughness was minimum since it is aided with flushing mechanism of chips. The relative performance of the environments are repeatedly pronounced on both CLA and 10 point roughness parameters. Figure 4 brings out sample 389-4

5 roughness profiles under four different environments at table speed of 12 m/min with down feed 10µm and cutting velocity varied of 20 m/s and 30 m/s to realize the trend as discussed above on the microscopic extent. Under cryo jet conditions, it was also intended to explore if process is effected the product quality adversely due to the extreme cooling of LN 2. When it was intended to explore the residue amount of material to be removed, it was found that both cryo gas and liquid jet left negative impression with almost 25% deviation of the dimension under cryogenic jet. Dry and soluble oil environments exhibited closer performance and found to be substantially better with respect to the cryo conditions. With soluble oil the dimensional deviation was found to be least. The same effect was also realized when number of spark out phases were monitored to be the largest with cryo jet condition. It may be noted that it is a practical difficulty for the operator to understand how many number of spark out phases to be prescribed for cryo conditions especially when it is a jet. Hence, even though cryogenic jet is found to be superior to offer the best protection to the wheel wear it increases the production time. Further, when micro hardness of ground surface was investigated, there is a significant difference in micro hardness of samples under cryo gas and jet conditions. There is an approximately increase of 10% micro hardness when thecryo gas was replaced by cryo jet. This is also clear indication of change of micro structural configuration of material. Further study is required to investigate the cause of this change. However, it is left out of the scope of present paper. 4 Conclusions Jet of nitrogen in gas and liquid form could be delivered successfully with the present setup. Major conclusions of the present work are Sustainable character of cryogenic application could be very realized in both gas and liquid form of nitrogen. But liquid jet was found to offer the best performance in terms of substantial enhancement in grinding wheel life due to its extraordinary cooling ability resulting in protection of grit surface. In short spell grinding better surface quality was obtained in soluble oil with respect to cryo jet environment, due to slow but progressive dulling of grit types and good flushing of chips. Adverse effect of the liquid nitrogen jet was pronounced on dimensional deviation and notable change in the micro hardness of work specimen. These could be reduced with the application of the gas instead of cryo jet, however with a slight compromise on wheel wear. References Biermann, D. and Hartmann, H. (2012), Reduction of burr formation in drilling using cryogenic process cooling, Procedia CIRP, Vol. 3, pp Biček, M., Dumont, F., Courbon, C., Pušavec, F., Rech, J. and Kopač, J. (2012), Cryogenic machining as an alternative turning process of normalized and hardened AISI bearing steel, Journal of Materials Processing Technology, Vol. 212, pp Hong, S. Y. and Ding, Y. (2001),Cooling approaches and cutting temperatures in cryogenic machining of Ti-6Al-4V, International Journal of Machine Tool and Manufacture, Vol. 41, pp Hong, S. Y., Ding, Y. and Jeong, W. (2001), Friction and cutting forces in cryogenic machining of Ti 6Al 4V, International Journal of Machine Tool and Manufacture, Vol. 41,pp Hong, S. Y.(2006), Lubrication mechanisms of LN 2 in ecological cryogenic machining, Machining Science and Technology, Vol. 10,pp Howes,T.(1990), Assessment of the cooling and lubricative properties of grinding fluids, Annals of the CIRP, Vol.39. Jin, T. and Stephenson, D. J. (2003), Investigation of the heat partitioning in high efficiency deep grinding, International Journal of Machine Tool and Manufacture, Vol. 43,pp Khan, A. A.and Ahmed, M. I.(2008), Improving tool life using cryogenic cooling, Journal of Materials Processing Technology, Vol. 196,pp Paul, S., Bandyopadhyay, P. P. and Chattopadhyay, A. B. (1993), Effects of cryo-cooling in grinding steels, Journal of Materials Processing Technology, Vol. 37, pp Paul, S., Dhar, N. R. and Chattopadhyay, A. B.(2001), Beneficial effects of cryogenic cooling over dry and wet machining on tool wear and surface finish in turning AISI 1060 steel, Journal of Materials Processing Technology, Vol. 116,pp Ravi., S. amd Pradeep Kumar., M (2011), Experimental investigations on cryogenic cooling by liquid nitrogen in the end milling of hardened steel, cryogenics, Vol. 51, pp Shokrani,A., Dhokia, V. and Newman, S. T. (2012), Environmentally conscious machining of difficult-tomachine materials with regard to cutting fluids, International Journal of Machine Tool and Manufacture, Vol. 57, pp Wang, Z. Y., Rajurkar. K.P., Fan, J. and Petrescu, G. (2002), Journal of Manufacturing Processes, Vol.4, pp