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LW01-004@40Cr调质钢磨削强化温度与强化效果试验研究,机械毕业设计 论文

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1 Journal Article | Print Published: 04/01/2003 | Online Published: 03/20/2003 Pages: 245 - 259 DOI: 10.1081/AMP-120018908 Materials and Manufacturing Processes , Volume 18 , Issue 2 Experimental Studies on the Grind-Hardening Effect in Cylindrical Grinding V. S. K. Venkatachalapathy *Corresponding B. Rajmohan *Corresponding V. S. K. Venkatachalapathy *Send correspondence to: rajmohan51 Department of Mechanical Engineering V.R.S. College of Engineering & Technology Arasur Villupuram (Dt.) Tamil Nadu India Search ALL works by V. S. K. Venkatachalapathy B. Rajmohan *Send correspondence to: rajmohan51 Department of Production Technology MIT, Anna University Chromepet Chennai Tamil Nadu India Search ALL works by B. Rajmohan Abstract In recent years high-strength and high-temperature alloys are used for structural and other applications. These newer high-performance materials are inherently “more difficult to machine” and also necessitate the need for higher dimensional and geometrical accuracy. Grinding is one of the most familiar and common abrasive machining processes used for the finishing operation. Compared to other machining processes such as turning, milling, etc., the specific energy developed during grinding is very high. At a critical level of specific grinding energy, the temperature rise (1) experienced by the workpiece may be such that thermal damage is induced. Heat damage induced by the grinding process is well documented and may be categorized by temper colors that are at least unsightly and probably indicative of more serious damage, including thermal cracks, tempered zone, etc., (2) which can lead to catastrophic failure of critical machine parts that shortens the life of products subject to cyclic loading. In this work, a new heat treatment process called “grind hardening” and a mathematical model are introduced, and this work deals with how the in-process energy in grinding can be effectively utilized to improve the surface hardness and surface texture, and also to prevent damages. An experimental study has also been carried out in grinding AISI 6150 and AISI 52100 steels with an alumina wheel, and the results are discussed. Keywords Cylindrical grinding, Grinding heat, Partition ratio, Surface texture, Surface hardening nts2 1 Introduction Grinding is a versatile and also final machining process in the production of components requiring close dimensional tolerances, geometrical accuracies, and a smooth surface finish. There are no processes that can compete with grinding for precision machining operations. It is a major manufacturing process that accounts for about 25% of the total expenditure on machining operations in industrialized countries. Almost all products have either been machined by grinding at some stage of their production or have been processed by machines, which owe their precision to abrasive operations. Grinding removes metal from the workpiece in the form of small chips by the mechanical action of abrasive particles bonded together in a grinding wheel. However, it is a slow way to remove the stock, thus other methods are used to bring the work quite close to its required dimensions and then the work is ground to achieve the desired finish. With the advent of abrasive machining, grinding is also accepted as a dependable process for higher material removal rates. Parts can now be produced more economically by this process to the size and finish that is not possible by any other method. Grinding often permits heavy stock removal and good finish on the same machine, even without changing wheels. (3) In the past, many scientists investigated the dissipation of heat in grinding and the resulting influence on the surface finish of the workpiece. Depending on the grinding conditions, the heat flux mainly takes part via the workpiece and leads to a large thermal loading in the surface. This thermal load is superimposed by a mechanical load, causing a high temperature in the surface. This thermomechanical load may cause some undesired alterations in the surface layer like cracks, tempered zones, or white etching areas (WEA). If the material in the surface layer is heated above the upper critical temperature (910C) during grinding, diffusion and phase transformation take place. Shaw and Vyas gave an impressive theoretical description of metallurgical damages in ground surfaces. (2) Under abusive grinding conditions, the formation of a heat affected zone (HAZ), which damages the ground surface, was observed. (4) A thermally damaged component may, therefore, incur a significant cost to the manufacturer in failing a quality standard. The aim of most investigations was the prediction of undesired alterations in order to avoid thermal damages when grinding hardened steels. In any case, the quantities of generated heat in grinding are considered as a restricting factor. By summarizing todays experience in heat treatment and grinding, three important limitations can be identified: i There are many heat-treatment processes for surface hardening, like induction hardening, case hardening process, etc., but they are very difficult to integrate into the production line, ii These surface-hardening processes cannot be done perfectly as in the case of irregular- and contour-shaped objects and iii Subsequent to heat treatment, structural parts are subjected to grinding, in the course of which impairment of hardened materials can arise because of the thermomechanical influences of the grinding processes. The above said problems caused the authors to investigate how this process-generated heat energy can be effectively utilized for quality improvement in cylindrical grinding. nts3 2. Selection of the workpiece material 3 Influence Of The Process Parameters Cylindrical grinding has many parameters that can be varied, but only three are very important: i) depth of cut; ii) feed speed; and iii) number of passes. The heat generated is proportional to the depth of the cut at the contact zone, because higher depths of cut result in longer heat treatment duration. Increasing depths of cut lead to higher quantities of energy entering into the workpiece. This will lead to a large amount of thermal damage on the surface of the workpiece. In addition to causing surface damage, grinding heat can also affect the precision of the workpiece. Increasing the feed speed is generally connected with increasing process forces. The two main effects of feed speed are i At very low feed speed, the traveling energy is high, due to lower cutting power, the extent of hardened layer is reduced. ii At very high feed speed, the cutting power increases, but due to the decreased contact time and lower traveling energy, the extent of the hardened layer is once again reduced. Thus, a moderate or medium feed speed is always preferable to produce maximum hardness in the surface. Increasing the number of passes increases the hardness at the surface only to a certain depth of cut, after that, it decreases. Because of an increase in contact time and traveling energy, excessive heat transfer takes place. Due to this, the specific energy in grinding reaches very close to the melting energy of the workpiece material in a short period. So the hardness decreases after a certain number of passes. This excessive heat transfer affects the surface finish of the component due to thermal expansion and distortion of the workpiece. 4 Temperature Modeling nts4 Usually the validity of a thermal approach is substantiated by means of evaluating the various thermal properties using various correlation theories and experimental results. In this case, the grind-hardening effect is being quantified by means of heat entering into the workpiece. The effective work surface temperature is principally influenced by the process parameters, and the thermophysical properties of the work and abrasive materials were described by Shaw. (7) Most of the observations suggest that the mechanical and metallurgical characteristics of abrasively machined surfaces can be produced by controlling the effective work surface temperature. Rowe et al. (8) determined the thermal properties by steady-state experiments for alumina and silicon carbide wheels. The contact zone models consider the partitioning of energy over the whole grinding contact zone. The wheel-bulk-property model assumes the workpiece and the grinding wheel are subjected to a sliding heat source. Shaw used an area ratio factor to correlate grain properties with such a model. Rowe et al. (9) proposed a grain model in which the partition ratios (partition ratio is defined as the proportion of heat entering into workpiece to the total heat developed) were considered without considering the energy convected away by the chips and coolant. Rowe et al. (10) have investigated the significance of the grinding-wheel thermal properties on the surface texture of the workpiece. The approach was to measure temperature and analyze the proportion of grinding 5. Terminology Involved Partition ratio (R): The partition ratio is the proportion of the heat entering the workpiece to the total heat. where QwAmount of heat entering the workpiece (J) QtTotal heat produced (J) For a square law distribution, the partition ratio is given by where ( c)wThermal contact coefficient for workpiece (J m-2 s-0.5 K-1) bGrinding width (m) VwWork speed (m/sec), leGrain contact length (m) mBackground temperature (C), nts5 QtTotal heat produced (J) Justification for using the square law distribution is based on the assumption that the heat distribution is uniform along the contact area and that the flow is radially inward to the work. energy entering the workpiece. 6 Theoretical Model Theoretical models are required to predict the partition ratio and workpiece temperature. First, using a grain contact zone model proposed by Rowe et al. (9) and by considering the partitioning of energy over the whole grinding contact, the partition ratio for various materials was found. where ( c)sWheel bulk thermal coefficient (J m-2 s-0.5 K-1) VsSpeed of the grinding wheel (m/s), VwSpeed of the workpiece (m/s) The bulk thermal coefficient for alumina wheel The bulk thermal coefficient for each workpiece material is found by using their physical properties The partition ratios for various workpiece materials are as follows. nts6 For AISI 52100 On solving, R =0.90948 0.91. For AISI 6150 Similarly, on solving, R =0.9246 0.925. If the allowance for the energy convected away by the chips (ecc) and coolant (ecf) is considered, then the predicted partition ratio is reduced, i.e., where ecSpecific chip energy (J/mm3). According to Howes et al., (11) a typical value of ecc is 6 J/mm3 and ecf tends to be very small where fluid boiling occurs (ecf=0). Thus, considering the allowance, the original partition ratio can be given by The effect of the chip energy becomes increasingly significant at lower specific energies. According to the grain contact model, the specific energy of the chip is assumed to be Thus, the partition ratios for the workpiece material were found as nts7 Therefore, R=R 0.8: Using the grain contact model developed by Rowe et al., (9) the solution for partitioning of heat between the wheel and the workpiece is where kgeThermal conductivity of the grinding wheel (Wm-1 K-1) kge=35 W m-1 K-1 (for alumina wheel) roRadial depth of cut ( m) For AISI 52100 On solving, ro=2.7110-6=2.71 m. Optimal grain contact length (le), It is known that where deEquivalent diameter (m) dsDiameter of wheel (m), dwDiameter of workpiece (m) On solving, de=0.0318 m. Therefore, le= (2.7110-6)(0.0318) =0.293510-3 m=0.29 mm. Grain contact time (t), t=le/Vs, on solving, t=9.66 sec. nts8 7 Calibration Of Total Heat Entering Into The Workpiece From the square law distribution, Therefore, For AISI 52100 Total heat entering into workpiece On solving, Qw=300 J. It is known that where kThermal conductivity (W m-1 K-1) 43.3 W m-1 K-1 A=Area (m2)=6.59710-3 m2 T=300/(6.59710-3)(43.3) On solving, T=1050C For AISI 6150 nts9 on solving, ro=2.018810-6=2.0 m. Optimal grain contact length (le) Grain contact time(t) For AISI 6150 The total heat entering into the workpiece On solving Qw=338 J. It is known that Q=kA T/dx(dx=1), where k=53.6 W m-1 K-1 and A=6.59910-3 m2. On solving, T=955C, T1=987C This temperature developed at the contact area is the real cause for the phase transformation, i.e., austenite into fine martensite. 8 Experimentation Investigations were carried out experimentally with varying depths of cut, feed, and number of passes. For the attainment of grind hardening, a standard alumina wheel was employed, and rough grinding conditions were selected. This means that a high-specific-metal removal rate was necessary to induce martensitic phase transformation. In this experiment, the number of passes were varied according to the rough and finish grinding. The grinding conditions are given in Table 1 . Table 1. Grinding conditions Process : Cylindrical grinding Grinding wheel : Alumina A46L5V nts10 Materials : AISI 6150 & AISI 52100 Cutting speed : 30 m/s Coolant : Emulsion The various roughness parameters like centerline average Ra, peak to valley height Rt, and average peak to valley height Rz were measured. The surface cracks were analyzed with the use of electromagnetic crack detector. The results are discussed. 9 Results And Discussions It is evident from the microstructures (shown in Figs. 1 2) that the bulk of chip produced during machining has etched darkly, but white etching bands are also present. This means that the carbide atoms are almost fully segregated all along the ferritic matrix. (i.e., a fine martensitic structure is generated). Figure 1. Microstructure of ground specimen AISI 6150. Figure 2. Microstructure of ground specimen AISI 52100. This etching response suggests that a temperature of about 810C has been reached in the bulk of chip and about 950C or more in the white etching bands, which may be expected as reported by Doyle and Dean. (5) Thus, the temperature at the workpiece surface could have an important influence on metal removal at a large wheel depth of cut. However, heating and cooling of the surface would occur rapidly, and this may have the influence of producing phase changes in the surface. The theoretical model developed in this present work also gives the same temperature that developed at the contact area. The following graphs (shown in Figs. 3 4) show the coarse hardness of the ground material at various depths beneath the surface. nts11 Figure 3. Hardening results depending on number of passes (AISI 6150). Figure 4. Hardening results depending on number of passes (AISI 52100). The higher hardness obtained during this grind hardening effect when comparing ground specimen with turned specimen is shown in Figs. 5 6 (number of passes 10 in Fig. 3 and number of passes 14 in Fig. 4 ). The experiments were carried out with the application of a coolant, however the coolant does not have much impact in the grind-hardening effect. Figure 5. Comparison of hardness of turned and ground specimen (AISI 6150). Figure 6. Comparison of hardness of turned and ground specimen (AISI 52100). With the higher depth of cut and increased number of passes (shown in Figs. 7 8), the area and the time for heat transfer is increased due to the increase (to certain depth of cut only) in traveling energy. A further increase in the total depth of the cut (increase in number of passes) decreases the hardness. This is because nts12 the increase in the total depth of the cut after a certain period decreases the specific cutting energy. The grind-hardened components were also inspected using an electromagnetic crack detector, and no flaws were found. Figure 7. Influence of number of passes (AISI 6150). Figure 8. Influence of number of passes (AISI 52100). 10 Control Of Surface Texture Surface texture is defined as the inherent or enhanced condition of a surface produced in a machining or other surface-generating operations. The nature of the surface layer may have a strong influence on the mechanical properties of the material. This association is more pronounced in some materials and under certain machining operations. Nam et al., (12) have stated that the surface finish is a major concern in manufacturing. Yet very little attention has been given to elucidate the relationship between the characteristics and the functional requirements of surfaces. A recurring problem in the specification of surface characteristics, and the design and manufacture of tribological surfaces has been the lack of a clear understanding of the friction and wear phenomena. Consequently, despite the engineering importance of low-friction and low-wear surfaces and the resulting economic benefits, so far it has been impossible to design and manufacture sliding surfaces optimally. On structural applications, the nature of declivity troughs of the surface is the most important, while on bearing applications, the nature and number of crests on the surface is more significant. Therefore, the surface roughness of the ground specimen was also measured and the results were plotted (Figs. 9 10). nts13 Figure 9. Surface roughness values of a ground specimen (AISI 6150). Figure 10. Surface roughness values of a ground specimen (AISI 52100). The results are at acceptable levels according to Japanese International Standard. (The acceptable range of finish Ra in grinding is 0.1 to 0.16 m). 11 Conclusion Experimental investigations have shown that the generated heat in cylindrical grinding could be effectively utilized as a new heat treatment process. With the existing knowledge in grind hardening and based on the test results, the following findings are presented: The grind hardened parts are cha