Residual stress in grinding.pdf

Residual stress in grinding Bogdan W. Kruszyn ski*, Ryszard Wo jcik Technical University of o dz , Skorupki 6/8, 90-924 o dz , Poland Abstract Results of investigations on residual stress in surface grinding are presented in the paper. A coeffi cient B combining power density and wheel/workpiece contact time was developed. Experimental set-up and software to estimate the coeffi cient during grinding are described in the paper. Experiments were carried out for surface plunge grinding for several workmaterials in a wide range of grinding conditions. The infl uence of process parameters on the coeffi cient B as well as the relation between B and maximum residual stress were experimentally evaluated. The usefulness of the coeffi cient to predict residual stress in surface grinding was proved. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Residual stress; Grinding; Wheel/workpiece 1. Introduction Grindingisoneofthemostpopularmethodsofmachining hard materials. Because it is usually one of the fi nal opera- tionsofthetechnologicalprocess,propertiesofsurfacelayer created in grinding infl uence directly the functional proper- ties of the workpiece such as fatigue strength, abrasive and corrosion resistance, etc. Creating favourable surface integrity, especially in grind- ing with aluminium oxide grinding wheels is diffi cult due to two opposite tendencies. On one hand, high process para- meters are preferred in order to increase productivity. Unfortunately, such parameters usually lead to the increase of grinding power engaged in creation of the new surface of the workpiece. On the other hand, the increase of grinding power makes grinding temperatures grow, which may cause a serious damage to the surface layer created in grinding. Finding a compromise between high productivity and advantageous surface layer properties is extremely diffi cult due to the lack of relatively simple and universal routines, among others. Because of the importance of grinding opera- tion the investigations of this process are performed in many research centres. Some general approaches are observed in these investigations. The fi rst one, strictly analytical 4,5, is based on the mathematical description of physical processes involved in surfacelayercreation.Ingrindingthermaleffectsareusually described. On the basis of the calculations of temperature distribution in the workpiece, such changes in surface layer likemicrohardness,residualstresses,microstructure,etc.are estimated 5. Such an approach is very promising but at the present stage it is limited to theoretical investigations because of complex calculations and still limited knowledge about material behaviour in extreme grinding conditions. The experimental approach 1,7 aims at fi nding a corre- lation between grinding conditions and surface layer para- meters. This is a relatively simple method with some disadvantages. Experimental works are usually time- and capital-consuming which limits their application. Moreover, there is a limited possibility to extrapolate the experimental resultsondifferentgrindingmethodsandgrinding conditions. There is also a third approach to the problem of control of surface layer creation, which involves a search for such grinding coeffi cients, which are strongly correlated with surface layer properties 2,4. There are many such coeffi - cients existing. The most popular are: equivalent chip thickness (heq) and power density (P0). The former is proved to be useful in grinding ceramics, the latter is often applied when grinding with aluminium oxide grinding wheels is investigated 2. The main disadvantage of both coeffi cients is that to calculate them it is necessary to estimate the effective grinding depth or effective wheel/workpiece contact length. Both values are very diffi cult to estimate on-line grinding accurately. Thus, an easy-to-estimate grinding coeffi cient, which would be strongly correlated with surface integrity para- meters, is still lacking. The investigation on the correlation between the coeffi cient combining power density and the Journal of Materials Processing Technology 109 (2001) 254257 *Corresponding author. 0924-0136/01/$ see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S0924-0136(00)00807-4 wheel/workpiece contact time and residual stress in surface grinding is described below. 2. Grinding coeffi cient combining power density and contact time It was proved 3 that residual stresses in surface layer aftergrindingare closelycorrelated with maximum grinding temperature. The analysis of equations used for temperature calculation in grinding 6 indicates that it is not only the power density that infl uences the grinding temperature but there is also a second important factor wheel/workma- terial contact time. In surface grinding the contact time of the particular workpiece point with heat source (grinding wheel) can be easily calculated as tc? le vw (1) where leis an effective wheel/workpiece contact length and vwis the workspeed. The proposed grinding coeffi cient B is a product of power density P0and contact time tc: B ? P0tc? P bdle le vw ? P bdvw (2) where P is the total grinding power and bdthe grinding width. The fi rst advantage of this coeffi cient is that all quantities in this equation (grinding power, grinding width and work- speed) are easy to measure on-line in a grinding process. 3. Experimental set-up Experiments were carried out for the following grinding conditions. ? workmaterials:carbonsteel0.45%C,28HRC(markedS), alloysteel40H(0.38%C,0.9%Cr,0.28%Ni)48HRC(H), bearing steel H15 (equivalent to 100Cr6) 62HRC (L); ? grinding wheels: 38A60J8V (J), 99A80M7V (M); ? wheelspeed: 26 m/s (constant); ? grinding depth: from 0.005 to 0.06 mm; ? workspeed: from 0.08 to 0.5 m/s; ? grinding fluid: emulsion or none. Grinding parameters in these investigations were limited by the power of the main wheel drive, table speed regulation range and by the appearance of unacceptable changes in the surface layer, microcracks and burns. To estimate coeffi cient B it was necessary to measure grinding power, workspeed and grinding width. Grinding power was measured in two different ways: by the measure- ment of power consumed by wheel main drive (Pm) and simultaneous measurement of tangential grinding force Ft and wheelspeed vs. The grinding power can then be calcu- lated as Pc? Ftvs. The comparison of the results obtained from both methods is shown in Fig. 1. Avery good correla- tion can be seen from this fi gure, which proves that mea- surement of power consumption of wheel main drive is accurate enough to estimate coeffi cient B in the case when only grinding wheel is driven by this drive. The wheelspeed was measured by means of displacement transducer and grinding width was taken as a width of the sample being ground. 4. Experimental results On the basis of measured values of P, vwand bdin surface grinding, the coeffi cient B was calculated in each grinding test. Measurements carried out during grindingallowed,fi rst of all, to evaluate the infl uence of grinding conditions on the coeffi cient B, cf. Figs. 27. The linear dependence between effective grinding depth and B can be seen from Figs. 2, 4 and 6. Slopes of these lines depend mainly on grinding wheel, workspeed (Figs. 2 and 6) and on grinding fl uid (Fig. 4). The correctnessoflinear approximation was proved in a statistical way values of R2were higher than 0.9 in all cases. Fig. 1. Comparison of measured and calculated grinding power. Fig. 2. The infl uence of grinding depth and grinding wheel grade on coeffi cient B for carbon steel (S). B.W. Kruszyn ski, R. Wo jcik/Journal of Materials Processing Technology 109 (2001) 254257255 The infl uence of workspeed on coeffi cient B, Figs. 3, 5 and 7, is not as uniform as those obtained forgrinding depth. Much higher infl uence of vwon B is observed for a lower range of workspeeds. It indicates that there is a limited possibility to infl uence coeffi cient B by changes of the workspeed. Very similar dependencies were obtained for the third workmaterial investigated alloy steel (H). For all experiments, in which microcracks and/or burns were not present, residual stress distribution was measured by means of thewell-known materialremovalmethod. From residualstress vs.depth belowsurfacediagramsobtainedfor each grinding test, maximal residual stresses in the surface layer were determined. Usually, residual stresses reach their maximum (tensile values) close to the surface on depths of 1020 mm. Relations between coeffi cient B and maximum residual stress for investigated workmaterials are shown in Figs. 8 10. In these diagrams the results are summarised for each workmaterial regardless of other grinding conditions (grind- ingwheelproperties,grindingfl uid,grindingparameters).In each case the linear dependence was assumed which was proved in a statistical way (R2from 0.8529 to 0.9074). It results from these fi gures that the slopes of residual stress-coeffi cient B lines are characteristic for the given workmaterial and seem to be independent of other grinding conditions. The highest slope was obtained for bearing steel (L), Fig. 10, and the lowest one for alloy steel (H), Fig. 9. Fig. 3. The infl uence of workspeed and grinding wheel grade on coef- fi cient B for carbon steel (S). Fig. 4. The infl uence of grinding depth and grinding fl uid on coeffi cient B for carbon steel (S). Fig. 5. The infl uence of workspeed and grinding fl uid on coeffi cient B for carbon steel (S). Fig. 6. The infl uence of grinding depth and grinding wheel grade on coeffi cient B for bearing steel (L). Fig. 7. The infl uence of workspeed and grinding wheel grade on coeffi cient B for bearing steel (L). 256B.W. Kruszyn ski, R. Wo jcik/Journal of Materials Processing Technology 109 (2001) 254257 Some additional observations recorded during investiga- tions indicate that there is a possibility to use the coeffi cient B to predict and/or control such changes in surface layer like microcracks, burns or microstructure changes. Additional investigations are necessary to confi rm the usefulness of this coeffi cient in other grinding methods. 5. Conclusions 1. The grinding coeffi cient B combining power density and wheel/workpiece contact time was developed to predict residual stress in surface grinding. 2. A linear correlation between coeffi cient B and maxi- mum residual stress was found experimentally. It was confi rmed for several workmaterials. 3. The relation between coeffi cient B and maximum residual stress seems to be independent of grinding conditions. 4. Coeffi cient B increases linearly with the increase of grinding depth and decreases with the increase of workspeed. This decrease shows less intensity in the range of higher workspeeds. 5. The coeffi cient B is easy-to-estimate, even on-line, in industrial practice. 6. The coeffi cient B may be useful in predicting such surface layer properties in grinding like microcr