E F F E C T OF D I A M O N D G R I N D I N G ON THE P H Y S I C O - M E C H A N I C A L P R O P E R T I E S OF THE H A R D ALLOY T 1 5 K 6 AND STEEL 4 0 K h V. I. Ivanets, M. G. Loshak, Yu. I. Babel, and G. V. Karpenko Fiziko-Khtmicheskaya Mekhanika Materialov, Vol. 2, No. 6, pp. 656-660, 1966 Grinding, which is one of the most widely used finishing machining operations, has a marked effect not only on the surface micro-geometry but also on the physico-mechanica! properties of machined parts. Grinding often produces work-hardening of the surface layers and, what is even more important, leads to the appearance of residual stresses which may be non-uniformly distributed in the surface layer. It is known [1, 2] that grinding may have-both beneficial and harmful effects on the performance of machined parts. The most damaging effects of grinding are residual compressive stresses, non-uniform work-hardening and structural changes, and the appearance of cracks and other surface defects. According to theoretical considerations, diamond grinding should not produce any of these harmful effects. If this grinding technique is not yet widely used in industry, it is because of the lack of data on its effect on the physicomechanical properties of metals and on the Service life of machined parts. There are two ways of improving the quality of machined parts by the use of this machining technique: 1) Diamond grinding of the part itself to reduce the degree of distortion of the crystal lattice of the metal surface layer and to prevent the formation of harmful residual stresses and surface defects; 2) diamond dressing of the cutting tools, which also leads to an improvement in the properties and homogenization of the structure of the surface layers of parts machined with these tools. The aim of the present investigation was to study the effect of diamond grinding on some physico-mechanieal properties of the hard alloy T15K6 and steel 40Kh, and to find how changes in these properties are reflected in the fatigue and corrosion-fatigue strength of steel 40Kh.

Fig. 1. Profllograms of steel 40Kh specimen surface after a) diamond and b) corundum grinding.

In one series of tests, specimens of the hard alloy T15K6 (type A, GOST 2209-55) were ground with a diamond wheel (AchK 150 x 20 x 3 x 82 AS25B1, K 100%), on a 3A64M universal machine with a UZP-1, universal dressing attachment* under the following conditions; Grinding wheel speed = 22.'/6 m/sec, longitudinal feed = 1.6 m/rain, and transverse feed = 0.02 mm/pass. For comparison, specimens from the same batch were dressed under the same conditions with an abrasive wheel (ChK 150 x 50 x 82 KZ25SMIK); to ensure constant cutting forces during grinding with diamond and abrasive wheels, the transverse feed in the latter case was 0.01 ram/pass. The elastic mode of grinding was used, with a cooling emulsion (95-97% water/3-5% industrial oil 12) consumed at a rate of 2-3 i/rain. The geometrical parameters of specimens after grinding were as follows: a = 8", a I = 15 ~ 71=57, ~,= 0~ ~= 0~ ~ =45", ~ 1 = 4 5 ~ r = 0.5 ram, bevel f = 2 mm.

Changes in the physico-mechanical properties of steel 40Kh (0.37%C, 0.67% Mn, 0.73%Cr, 0.88% Si) after diamond grinding were studied on 20 mm diam. fatigue test pieces, which had been oil'quenched from 840* C and tempered (2 hr at 180" C) to a hardness Fig. 2. Profilograms of hard alloy T15K6 HRC = 48-50. One half of the heat-treated test pieces specimen surface after a)diamond grinding was ground with a diamond wheel (APP 250 x 15 x 5 x 75 and b) grinding with a grade KZ wheel. ASP10B1, K 100%) at a wheel speed of 36.5 m/sec, a longitudinal feed of 0.4 m/min, and a transverse feed of 0. 005 m/pass. The remaining test pieces were ground with b

~

~

84184

* The attachment was developed at the Experimental Laboratory of the L'vov Diamond Tools Plant. 464

an EB25SM2 abrasive wheel under identical conditions.* The surface roughness of alloy T15K6 and steel 40Kh specimens was determined with a type 201 profilographprofilometer (feeler speed = 1 mm/min; vertical magnification = x20 000; speed of the graph paper in the recording instrument = 200 ram/rain). The results obtained for steel (see Fig. 1) show that diamond grinding produces Class 9a surface finish, and abrasive (corundum) grinding Class 9b. The slightly higher degree of roughness of diamond-ground specimens may be attributed to a lower number and better cutting properties of the diamond particles taking part in the removal of the metal. The surface finish of hard alloy specimens after diamond grinding is slightly better (Class 10b) than after grinding with KZ abrasive wheels (Class 9b); this is evidently because the corundum particles become more rapidly blunted by alloy T15K6 which is so much harder than steel. After grinding with grade KZ wheels, more than 80% of alloy T15K6 specimens had microcracks (Fig. 3) and many other surface defects, not observed on diamond ground specimens; the cracks are usually normal to the direction of longitudinal feed. In addition, the cutting edge is dressed to Class 9 surface finish after diamond grinding and to Class 7 after grinding with an abrasive wheel.

Fig. 3. Effect of grinding with a grade KZ abrasive wheel on the surface condition of alloy T15K6 specimens with a) uniform and b) nonuniform distribution of carbide particles. Metallographic examination of alloy T15K6 specimens showed that crack nucleation takes place mostly in the bonding metal (cobalt), whose strength (o b = 24.2 k g / m m 2) is much lower than that of tungsten and titanium carbides (35-40 and 40-50 k g / m m ~, respectively), With a uniform distribution of carbide particles in the alloy, when the degree of dispersion is 1 . 5 - 3 ~ and the average pressure exerted by the cutting tool (in the elastic mode of grinding) is more than 2 x 10 -2 k g / m m 2, isolated local cracks join to form a through crack which extends to the edge of the ground surface; this was demonstrated by experiment [1]. Non-uniform distribution of carbides in the bonding metal leads to changes in the direction of cracks, which may grow in a direction normal to the longitudinal feed or follow a path of reduced concentration of carbide particles (see Fig. 3). The formation of cracks in hard alloys during abrasive grinding may be explained in the following way. The temperature in the contact zone between the specimen and the grinding wheel reaches 1000~ ~ C which, owing to the low heat conductivity of the alloy (0.08 c a l / c m sec degree against 0. 928 c a l / c m sec degree in the case of copper), leads to the formation of a steep temperature gradient [3-7]. The temperature gradients and different thermal expansion coefficients of tungsten and titanium carbides and cobalt lead to the appearance of high local stresses and cracks, as a result of which the service life of cobalt-bonded carbide tools is reduced. The temperature in the cutting zone during diamond grinding reaches only 300~ ~ C [3, 4]. This is because of t h e better cutting properties of diamond particles, their higher heat conductivity (0.35 c a l / c m sec degree), and the smaller quantity of diamond particles Simultaneously engaged in the cutting action (for 10C~o concentration, a diamond wheel contains 25% cutting particles against a 60~ content of cutting material in a grade KZ abrasive wheel'). Stresses generated under these conditions are lower than the breaking stresses of the hard alloy constituents, and do not cause cracking. As a result, the life of tools tipped with diamond-dressed carbide T15K6 tips is increased by 5 0 100~ (The tool life was determined by measuring the machining time required to wear the back edge by 0.8 m m . ) Measurements, carried out by a method described in [8], revealed no residual stresses in steel 40Kh specimens ground with diamond or corundum wheels, because the "soft" grinding technique employed produced no substantial changes in the structure of steel surface layers; this was confirmed by micro-hardness measurements carried out after grinding. Fatigue tests were carried out on type IMA-80 machines [9] by a method described in [2]. The results showed that the fatigue strength of steel 40Kh in air is practically unaffected by the type of grinding wheel used to machine the test * Diamond wheels were used under the optimum grinding conditions, i . e . , under conditions of minimum specific consumption of diamond particles. 465

pieces; the fatigue limit of diamond-ground test pieces is only ffr/ohigher than that of test pieces ground with a corundum wheel. On the other hand, the corrosion-fatigue limit (determined in a 3~o NaCI solution on a basis of 50 x l0 s cycles) after diamond grinding is approximately 2~o higher than after grinding with a corundum wheel (see Fig. 4). These resuits may be attributed to the fact that the corrosion-fatigue strength of steel is more sensitive to the physico-mechanical properties of its surface layers than to the surface microgeometry [1, 2, 10]. High temperatures generated during corundum grinding [S, 11-13] produce undesirable changes in the fine crystal structure and in the electrochemical properties of the steel surface layers. Studies of differential polarization, analysis of anodic polarization curves, and meamrements of the difference in electrode potentials (e. g., ~ o f diamond and corundum-ground specimens were found to be - 5 7 8 and - 6 0 0 mV, respectively) showed that steel surfaces after diamond grinding are more corrosion-resistant than after grinding with a corundum wheel.

b"

,'

,2

'

7O

66

26

.f t Itt, ,

22

I

'-

i!i

~8

24

j

i

Summary

"1

'xi,,,i,lli] ,

20

l'

18 16 14

'll ] , t ,

12 0.2

ifill

0.4 a6" ~O

IIM\o,7= I

2

z, e

ll/I

It should be noted that grinding with corundum wheels was effected not under the optimum conditions, but under conditions identical with those chosen for diamond grinding. In practice, however, corundum grinding often produces tempering of the surface layers of quench-hardened steel, which leads to a sut~tantial reduction in its fatigue and corrosionfatigue strength [10, 11, 14].

:7

1. T o improve the performance of tools tipped with grade Ti5K6 Co-bonded carbides, diamond wheels should be used to grind the carbide tips. Grinding with grade KZ abrasive wheels produces a rougher surface and leads tO a deterioration in the properties of the alloy surface layers due to the formation of high-temperature gradients, residual stresses, cracks and other surface defects. 2. The corrosion-fatigue strength of heattreated steel 40Kh is higher after diamond grinding than after grinding with a grade EB abrasive wheel, even when the grinding conditions are identical. With diamond and abrasive grinding effected under (efficiency-wise) optimum conditions, a larger increase in fatigue and corrosion-fatigue strength of quench-hardened and tempered steel can be produced by changing over from abrasive to diamond grinding.

10 N.lOf:cycles

Fig. 4. Fatigue curves of steel 40Kh in air (Arabic numerals) and in 3~ NaC1 solution (Roman numerals). 1, 13 Diamond grinding; 2, II) corundum grinding. REFERENCES 1. G. V. Karpenko, Effect of Mechanical Treatment on Strength and Durability of Steel [in Russian], MoscowKiev, Mashgiz, 1959. 2. G. V. Karpenko, Strength of Steel in Corrosive Media [in Russian], Moscow-Kiev, Mashgiz, 1963. 3. M. F. Semko, V. A. Kacher, A. F. Rab, a n d M . D. Uzun'yan, Diamond Tools and Their Application in Mechanical Engineering [in Russian], Kharkov, Izd."Prapor," 1965. 4. M. V. Pilitsyn and A. K. Kiselev, Diamond Grinding and Dressing of Carbide-Tipped Cutting Tools at the Voskov Plant [in Russian], Leningrad, House of Technical Propaganda, 1965. 5. V. I. Tret'yakov, Cemented Carbides [in Russian], GNTIL Mining and Nonferrous Metallurgy, Moscow, 1962, 6. V. N. Bakul, Diamond Dre.~sing of Carbide-Tipped Tools [in Russian], Kiev, ITI, 1964. 7. A. M. Okon, D. D. Shchetinin, and L. F. Proskurin, "Mashinostroenie," no. 6, 1965. 8. I. V. Karpenko, B. F. Ryabov, M. F. Lutsiv, and Yu. I. Babel, FKhMM [Soviet Materials Science],no. 1, 1966. 466

9. Yu. I. Babei, collection: Machines and Instruments for Testing of Metals [in Russian], Izd. AN UkrSSR, 1961. 10. G. V. Karpenko, Yu. I. Babei, I. V. Karpenko, and E. M. Gutman, Strengthening of Steel by Mechanical T r e a t m e n t [in Russian], Izd. "Naukova Dumka," Kiev, 1966. 11. P. A. S h u r m a n and G. I. Val'chuk, FKhMM [Soviet Materials Science], no. 6, 721, 1965. 12. V. N. Bakul, I. P. Zakharenko, and g. I. Val, Tekhnologiya i organizapiya proizvodstva, no. 9., 1966. 13. B. I. Kostetskii, Grinding of Quench-Hardened Steel [in Russian], Kiev-L'vov, Gostekhizdat, 1947. 14. V. G. Delevi and P. A. S h u r m a n , MiTOM, no. 4, 1965. 17 May 1966

Institute of Physics and Mechanics, AS UkrSSR, L'vov Kiev Institute of Ultra-Hard Materials

467