E F F E C T OF DIAMOND GRINDING ON THE STRENGTH OF STEEL IN CORROSIVE MEDIA Yu. I. Babei, V. I. Ivanets, and M. G. K h i t a r i s h v i l i F i z i k o - K h i m i c h e s k a y a Mekhanika lViaterialov, Vol. 6, No. 2, pp, 22-25, 1970 UDC 621.795.2; 621.79.02; 669.018.24

The life of p a r t s working in various media depends on the physicomechanical state of m e t a l surface l a y e r s [1-3]. Grinding is often the finishing machining operation and, as such, d e t e r m i n e s the exploitation c h a r a c t e r i s t i c s of m a c h i n e parts. The wide range of a b r a s i v e s produced for grinding wheels makes it possible to select the optimum grinding conditions in any given case. However, the s c a r c i t y of r e l i a b l e data on the effect of methods of grinding on physicomechanical p r o p e r t i e s of metals m a k e s it i m p o s s i b l e fully to utilize such p r o m i s i n g u l t r a h a r d m a t e r i a l s as Elbor or synthetic diamonds and to adopt grinding as a fast and heavy-duty machining operation. This a r t i c l e d e s c r i b e s the r e s u l t s of an investigation of the effect of diamond grinding on the fatigue and c o r r o s i o n - f a t i g u e strength of steel and on its r e s i s t a n c e to s t r e s s - c o r r o s i o n cracking. The steels investigated included m e d i u m - c a r b o n 40Cr steel, h i g h - c a r b o n b a l l - b e a r i n g C r l 5 S i steel, and an austenitie 1 C r l S N i l 0 T i steel. The s p e c i m e n s were subjected to the following heat t r e a t m e n t : 40Cr s t e e l - - o i l - q u e n c h i n g f r o m 840-850 ~ C and 2 hr t e m p e r i n g at 180 ~ C (HRC = 49-50); C r l 5 b a l l - b e a r i n g s t e e l - - o i l - q u e n c h i n g from 840-850 ~ C and 2 hr t e m p e r i n g at 150 ~ C (HRC = 60-62); 1 C r l S N i l 0 T i s t e e l - - w a t e r - q u e n c h i n g f r o m 1050-1080 ~ C and 2 hr t e m p e r i n g at 600 ~ C (HB -= 137). After heat t r e a t m e n t the specimens were ground on a 3B12 grinding machine with a diamond wheel (ASP24, BI, 100%, A P P , 300 • 15 • 27) and a borazon wheel (BO25, B1, 100%, BP, 300 • 15 • 127). The grinding conditiohs were as follows: grinding wheel speed = 39.5 m/sec; work piece speed = 2.3 m/sec; longitudinal feed = 0.15 wheel width/ revolution, t r a n s v e r s e speed = 0.006 r a m / d o u b l e p a s s . A 3% e m u l s i o n w a s used as a cooling medium. F o r c o m p a r i s o n a batch of specimens of each steel was ground under the same conditions with an e l e c t r o c o r u n d u m wheel (I~B25, SM1, K). The surface finish of specimens after grinding was d e t e r m i n e d on a 201 p r o f i l o g r a p h - p r o f i l o m e t e r . The surface of diamond ground s p e c i m e n s had r e l a t i v e l y shallow s c r a t c h e s and low c r e s t s , while after e l e e t r o c o r a n d u m grinding the p r o f i l o g r a m s showed much deeper n o n u n i f o r m l y distributed s c r a t c h e s formed as a r e s u l t of nonuniform blunting of e l e c t r o e o r u n d u m grains. M i c r o h a r d n e s s m e a s u r e m e n t s on s p e c i m e n s ground with an e l e c t r o c o r u n d u m wheel showed that in this case secondary t e m p e r i n g of their surface l a y e r s to a depth of 110-150 # took place (their m i c r o h a r d n e s s was reduced by 17-22%). T h e r e was nO noticeable reduction in m i c r o h a r d n e s s of s p e c i m e n s ground with a borazon wheel, while a c e r t a i n i n c r e a s e in m i c r o h a r d n e s s of steel surface l a y e r s 5 - 7 ~ deep was observed after diamond grinding (Fig. 1). The reduction in m i c r o h a r d n e s s during grinding with e l e c t r o c o r u n d u m wheels is attributable to high t e m p e r a t u r e s and p r e s s u r e s produced during cutting with e l e c t r o c o r u n d u m g r a i n s , while in the case of grinding with diamond wheels, c h a r a c t e r i z e d by a m o r e favorable g e o m e t r y of the cutting p a r t i c l e s and their high wear r e s i s t a n c e , the p r e s s u r e in the contact zone is lower and the t e m p e r a t u r e does not exceed the allotropic t r a n s f o r m a t i o n points (Ac,) The d e t e r m i n a t i o n of the r e s i d u a l s t r e s s e s of the I - s t kind* showed that diamond grinding produces in the s u r f a c e l a y e r s c o m p r e s s i v e s t r e s s e s r e a c h i n g 90-120 kg/mmZ; borazon grinding produced c o m p r e s s i v e s t r e s s e s not exceeding 55 kg/mm 2, while tensile r e s i d u a l s t r e s s e s reaching 37-57 k g / ~ m 2 were observed after grinding with e i e e t r o e o r u n d u m wheels. The appearance of c o m p r e s s i v e r e s i d u a l s t r e s s e s after diamond grinding is attributable to plastic deformation and favorable phase t r a n s f o r m a t i o n in s p e c i m e n surface l a y e r s , which leads to a reduction in the quantity of r e s i d u a l austenite; the quantity of r e s i d u a l austenite after grinding with e l e c t r o e o r u n d u m wheels is i n c r e a s e d [4]. The r e s u l t s of the d e t e r m i n a t i o n of the quantity of r e s i d u a l austenite in the surface l a y e r s of steel specimens ground with different wheels a r e given in Table 1. Fatigue t e s t s on s p e c i m e n s (with a 9 - m m - d i a m e t e r gauge portion) were c a r r i e d out on NU machines. The test

*This was done with t e n s o m e t r i c gauges which m e a s u r e d s t r a i n s after dissolving s e m i c y l i n d r i c a l s p e c i m e n s u r f a c e s [3]. 154

base was 20.106 cycles in air and 50,10~eyclesina 3% NaCl solution (simulating sea water)~ Test results showed (Table 2) that the fatigue and corrosion-fatigue strength of all the steels studied is higher after diamond grinding than after grinding with borazon or electrocorundum wheels. Thus, the fatigue limit in air of BB-CrI5Si steel after diamond grinding is 6% higher than that of specimens ground with eleetrocorundum wheels; the corresponding increase in the corrosion-fatigue limit is 3.7-fold. The beneficial effect of diamond grinding increases with increasing purity of steel (in terms of nonmetallic inclusions). For instance, the fatigue limit of BB-CrI5Vi steel after diamond grinding is increased by 25%, while the corresponding increase in the fatigue and corrosion-fatigue limit of 40Cr steel is 8 and 25%, respectively.

6gO

JD

J

~

JE

40

80

12o

~,o

2~0

h, ,

Fig. 1. Distribution of mierohardness in steel surfaee layers after grinding with D) diamond, ]3) borazon and E) eleetrocorundum wheels: 1) ball-bearing Cr15Si steel; 2) 40Cr steel; 3) 1Cr18Ni10Ti steel. Diamond grinding increases the fatigue limit of iCrl8Nil0Ti steel by 6% and its corrosion-fatigue limit by 15% over the corresponding values recorded for specimens ground with eleetrocorundum wheels. Grinding with borazon wheels ensures intermediate levels of fatigue strength of the steels studied.

401

~I

I

/0 T, h r

Fig. 2. Stress-corrosion cracking curves of 40Cr steel specimens tested in i , 2) 20% H2SO4 and I, If) boiling 42% MgCI2 solutions: l, I) grinding with eleetrocorundum wheels; 2, II) diamond grinding. The above-noted improvements in the properties of the surface layers of diamond ground specimens should increase the resistance of these specimens to stress-corrosion cracking. In fact, uniaxial tension tests on diamondground 40Cr steel specimens (18-ram-diameter gauge portion) in 20% H2SO 4 and boiling 42% MgCI 2 solutions showed that their resistance to stress-corrosion cracking is noticeably higher than that of specimens ground with an electroeorundum wheel (Fig. 2). Although the increase in the stress-to-rupture recorded for diamond ground steel in 100-hr tests in a sulfuric acid solution was not very large (5 kg/mm2), the time-to-rupture at high applied stress levels was substantially increased. Since curves I, 2 and I, II in Fig. 2 do not intersect each other, it is possible to compare time-to-rupture values obtained for specimens ground with diamond and eleetrocorundum wheels and tested at the same stress levels. Data reproduced in Table 3 show that, other factors being equal, the time-to-rupture of diamond ground specimens is 80-125 times that of specimens ground with an electrocorundum wheel. The relatively small increase in the stress-torupture after diamond grinding is attributable to the fact that the character of the distribution of residual stresses in

155

Table 1

Steel grade

Quantity of residual austenite after grinding, %

40Cr BBCr 15 Si BBCrI 5Vi* lCrl 8NilOTi

ASP

BO

Not detected.

Less than

6

15

~ EB

6

ASP

BO

EB

--93

.+37.6 +57.7

I 4 '15

--88.4 --121.5

I 17

--90

1,0o

1~

1oo

Residual stresses after grinding, kg/mm 2

--55 ---

-

--

-

*Conditional designation of a bearing steel smelted from virgin materials and

twice remelted in vacuum.

Table 2

Steel grade

40Cr BBCr 15 BB BBCr 15 Vi lCr 18NilOTi '

I After grinding with Structure after ASP heat treatment t ~ ~'~Z~ ~

Martensite Mart ensit e Martensite Austenite

Fatigue limit of steel, kg/mm 2 After griridAfter grind~ ing with ing with BO ~~ EB

~

~

~176

72 76.5 102

5.5

9a 9a 9a

73 76

3,5 2.5

8h 8c

94

16

9a

~

i6

8a

~

7266.5 4.75 82

22,5

14

i

Table 3

*,kg/mm 2

90 55

40

Time-to-rupture (min) of specimens ground with electrodiamond corundum wheel wheel

~A

1

22

156

300

1200 18O0

8b 8c 9h 8a

specimen surface layers undergoes changes with increasing time under load in a corrosive medium: dissolution of metal surface layers takes place leading to a redistribution of, and gradual increase in, residual compressive stresses [5].

r, hr

Fig. 3. S t r e s s - c o r r o s i o n c r a c k i n g c u r v e s of 1 C r l 8 N i l 0 T i steel s p e c i m e n s tested in a boiling 42~ MgClx solution: 1) grinding with e l e e t r o c o r u n d u m wheels; 2) diamond grinding. An i n c r e a s e in the s t r e s s - c o r r o s i o n r e s i s t a n c e of d i a m o n d - g r o a n d s p e c i m e n s was observed also in the case of 1 C r l S N i l 0 T i steel ( 1 8 - r a m - d i a m e t e r gauge portion) tested in a boiling 42% MgC12 solution (Fig. 3). Thus, the s t r e s s t o - r u p t u r e of diamond ground s p e c i m e n s tested for 240 hr was 35% higher than that of specimens ground with an e t e c t r o c o r u n d u m wheel. The i n c r e a s e in the fatigue and c o r r o s i o n - f a t i g u e strength of steels and in their r e s i s t a n c e to s t r e s s - c o r r o s i o n c r a c k i n g after diamond grinding is attributable to a favorable state of metal surface l a y e r s : appearance of r e s i d u a l c o m p r e s s i v e s t r e s s e s , reduction in the quantity of r e s i d u a l austenite and absence of secondary t e m p e r i n g effects, b u r n s , m i c r o c r a c k s , and other defects. Grinding with e l e c t r o c o r u n d u m wheels produces s e c o n d a r y t e m p e r i n g , leads to the appearance of tensile r e s i d u a l s t r e s s e s , r e d u c e s m i c r o h a r d n e s s , i n c r e a s e s the quantity of r e s i d u a l austenite and leads to the a p p e a r a n c e of b u r n s and m i c r o c r a c k s . It should be s t r e s s e d that diamond grinding e n s u r e s a better surface finish than that produced by grinding with e l e c t r o c o r u n d u m wheels. Thus, diamond grinding can be used to increase the fatigue and corrosion-fatigue strength of machine parts and to increase their resistance to stress-corrosion cracking. The beneficial effect of diamond grinding is much more pronounced in the case of parts working in corrosive media than in air.

REFERENCES 1. G. V. Karpenko, Effect of Active Liquid Media on E n d u r a n c e of Steel [in R u s s i a n ] , Izd. AN UkrSSR, 1955. 2. G. V. Karpenko, Effect of Machining on Strength and E n d u r a n c e of Steel [in Russian], Mashgiz, 1959. 3. G. V. Karpenko, Yu. I. Babel, I. V. Karpenko, and l~. M Gutman, Strengthening Steel by Mechanical T r e a t m e n t [in Russian], Izd. Naukova dumka, 1966. 4. G. V. Karpenko, V. I. Ivanets, Yu. I. Babel, M. G. K h i t a r i s h v i l i , ]~. M. Gutman, and B. T. Dyadchenko, collection: I n c r e a s i n g the Wear R e s i s t a n c e and Life of Equipment in the Foodstuff I n d u s t r y [in Russian], part II, Moscow-Kiev, 1968. 5. M. F. Lutsiv, B. F. Ryabov, M. G. Khitarishvili, Yu. I. Babel, FKhMM [Soviet M a t e r i a l s Science], no. 4, 1968. 15 July 1969

Institute of P h y s i c s and Mechanics, AS UkrSSR, L'vov

157