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E FFE CT O F RE T A I N E D A U ST E N I T E O N T H E

ST RE N G T H O F ST E E L

G . V . K oz yr e v a nd G . V . T opor o v

I M P A C T - F A T I G U E

UDC 620.178.3 85:66 9.14.018.25

In de te rmin ing the opt i mal s t ruc tu re of s tee l to obta in the highes t s t r engt h one must take into con-

s ide ra t i on the amount of r e ta ined aus teni te . Grea t e r a l loying of the s tee l and incre as in g the carbon content

of the case harde ned l aye r and the depth of the case ma y lead to large am ount s of retai ned au sten lte .

At pre sen t there is no agr eem ent on the relat i ve effect of retai ned aus teni te on the stat ic and fat igue

s t rengt h or impac t s t rength. Data given in [1] indica te lower ing of the bending s t rengt h and impac t s t rength

wi th decr eas i ng amou nts of r e ta ined aus teni te . The absence of re ta ined aus teni te in the ease hardened

la yer makes i t br i t t le and unstabl e with cyclic loading [2].

Howeve r , f rom an anal ysi s of dom esti c and foreign l i te rat ure i t was conclude d in [3] that retain ed

austeni te cannot be permi t ted in the case .

Some resul ts [4,5] indicate no effect o[ retai ned austen ite in amou nts not exceedi ng 15-20~ With

la rg er amounts of r e ta ined a us teni te the s t rength decr eases . On the cont ra ry, i t was found in [6] tha t

around I0~f0 retai ned aust enit e lower s the fat igue l i mit by 1 0 - 1 5 % , whi le la rger amounts have a negl igible

effect on the fat igue l imit .

The harmful e f fect of r e ta ined aus teni te on the fa t igue s t rength of ca rb ur iz ed sample s was a lso

reported in [7, 8].

The cont ra dic to ry resu l t s obta ined by di f fe rent authors can be expla ined by the compl ica ted and

imp rec i se methods of de te rmin ing the amoun t of r e ta ined aus teni te in the case , the amount of which var i es

wi th dis tance f rom the sur face .

Of prac t ica l in t e res t i s the e f fec t of r e ta ined aus teni te in the s t ru c tu re of tool s tee ls , the composi t ion

and s t ruc tu re of which a re s im i la r to the case hardened layer . These resul t s can be used to expla in the

prop er t ie s of tool s tee ls and carb ur iz ed s tee ls [9]. The amount of r e ta ined aus teni te can a lso vary wi thin

wide l imi ts in these s t ee ls .

Ther e is also no agre eme nt on the rea son for the differenc e in the effect o[ retain ed au sten ite on the

pr ope r t i e s o f c a r bur i z e d a nd tool s t e e l s . M ost i nve s t i ga to r s c ons ide r t hat a us t e n i t e, w i th a sma l l e r spe c -

i fi c vo lume than ma r t e ns i t e , l ow e r s the r e s idua l c ompr e s s ive s t r e s se s i n t he c a se. T he t r a ns f o r ma t io n

of re ta ined aus tenl t e in to mar t ens i te under load should incr ease the s t rength of par t s . Also, aus teni te has

a lower s t rengt h than mar tens i te . I t was repo r ted in [2] tha t the t r ansf orm at io n o[ re ta ined au s teni te in to

mar tens i te has a favorable e f fect , a l though the e f fec t of r e ta ined aus ten i te on com pres s iv e s t re sse s was

not ment ioned.

ITABLE 1

Quench- Retained l,~ d 2 an kg-m

i~ c g e m p a u s t e n i ~ e f c ~

~k[ il ] I g m m

070- -975 16 152 I 0 40

148 0 57

010--1015

1040- -1045 33 138 0 :61

1070- -1075 52 120 0 65

1100- -1110 72 86 0 75

The t ran sfo rmat ion of aus teni te in to mar t eas i te under load

was given in [10] as the re aso n for the reduct ion in stren gth.

In a l l cases the fa tigue tes t s were made wi th smooth cyl i ndr i -

ca l samples . The e f fect of r e ta ined aus teni te has not been inves t i -

gated und er condit ions of cyclic impact lo ading, which are widely

encounte red in se rvi ce . Impac t f a t igue tes t s were made only in

[2] , and only at one impa ct ener gy, and the refo re the res ult s cannot

T omsk E ng i ne e r ing - A r c h i t e c tu r a l I ns t it u t e . T r a ns l a t e d f r om M e tal l ove den i e i T e r mic h e ska ya

Obrabotka Metallov,-No. 12, pp. 45-48, December, 1973.

9 Con sul tan ts Bureau, a d ivis ion of Plen um Publ is hing Corporat ion, 227 g 'est 17th Street , New York, N. Y . 10011.

No part o f th is publ ic at ion may be reproduced, store d in a retrieval sy ste m, or t ransm it ted , in any form or by an) , mean s,

e l ec t ron i c , mecha n i ca l , pho t ocopy i ng , m i cro f i lm i ng , record i ng o r o t herw i se , w i t hou t wr i t ten permi ss i on o f t he pub l i sher . A

copy o f t h i s a r t i c l e i s ava i l ab l e f rom t he pub l i sher f o r $15 .00 .

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4 ~ , ~ 4 5 k g/.m m

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A ~ ~

F i g . 1 . E f f e c t o f r e t a i n e d a u s -

t e n i t e o n t h e im p a c t - f a t i g u e

s t r e n g t h o f s t e e l . T h e lo a d is

i n d i c a t e d o n t h e c u r v e s .

b e u s e d t o c h a r a c t e r i z e t h e e f f e c t of r e t a i n e d a u s t e n i t e i n a

w i d e r a n g e o f l o a d s .

W e i n v e s t i g a t e d t o o l s t e e l K h 1 2 M ( 1. 48 % C , 1 1 .5 % C r , 0 . 3 %

M n ~ 0 .2 79 o S i , 0 .2 % V , 0 .4 5 % M o ) , u s i n g c y l i n d r i c a l s a m p l e s 9 . 5

m m in d i a m e t e r w i t h a V - s h a p e d r i n g n ot c h 0 . 25 m m d e e p .

T h e h e a t t r e a t m e n t s w e r e s e l e c t e d s o a s t o o b ta i n s t r u c t u r e s

w i t h d i f f e r e n t a m o u n t s o f r e t a i n e d a u s t e n i t e , b u t w i t h a h i g h h a r d -

n e s s . T h e ra n g e o f q u e n c h i n g t e m p e r a t u r e s w a s e x p e r i m e n t a l l y

d e t e r m i n e d t o e n s u r e a h a r d n e s s o f H R C 5 7 -5 9 . T h e h i g h e s t

q u e n c h i n g t e m p e r a t u r e o f 1 07 5~ w a s r e s t r i c t e d b y t h e f a c t t h a t

n o s t r o n g g r a i n g r o w t h i s o b s e r v e d i n t h i s s t e e l u p t o t h i s t e m p e r -

a t u r e [1 0] . A c c o r d i n g t o [ l l ] , t e m p e r a t u r e s o f 9 7 5 - 1 0 7 5 ~

a c h a n g e i n t h e a m o u n t o f r e t a i n e d a u s t e n i t e f r o m a p p r o x i m a t e l y

6 t o 5 2% . O n e s e t of s a m p l e s w a s a l s o q u e n c h e d f r o m 1 1 0 0 -1 1 1 0 ~

t h e h a r d n e s s w a s ~ H R C 5 0 , t h e a m o u n t o f r e t a i n e d a u s t e n i t e N 7 2 % .

T h e p r o p e r t i e s o f f iv e s e t s o f s a m p l e s q u e n c h e d f r o m d i f f e r e n t t e m p e r a t u r e s a r e g i v e n in T a b l e 1 .

T h e b e n d i n g s t r e n g t h a n d th e i m p a c t s t r e n g t h w e r e d e t e r m i n e d o n s a m p l e s f o r fa t ig u e t e s t s w i th 8 0 m m

b e t w e e n s u p p o r t s .

T h e i m p a c t f a t i g u e t e s t s w e r e w i t h a m a c h i n e d e s c r i b e d i n [ 12 ]. T h e l o a d i n g f r e q u e n c y w a s 6 0 0 / m i n .

T h e s a m p l e s r e s t e d o n tw o s u p p o r t s a n d w e r e s u b j e c t e d to c o n c e n t r a t e d i m p a c t l o a d s . T h e b a s e o f t he

t e s t s w a s 5 - 10 5 l o a d i n g C y c l e s .

T h e e f f e c t o f r e t a i n e d a u s t e a i t e o n th e l i f e o f t h e s a m p l e s i n r e l a t i o n t o th e a c t i n g s t r e s s c a n b e s e e n

i n F i g . 1. A t h i g h s t r e s s e s l a r g e a m o u n t s o f r e t a i n e d a u s t e n i t e i n c r e a s e t h e i m p a c t - f a t i g u e r e s i s t a n c e o f

t h e s t e e l , a n d a t l ow s t r e s s e s l o w e r i t .

T h e r e s u l t s o f t h e f a ti g u e t e s t s a n d a l s o t h e v a l u e s o f t h e b e n d i n g s t r e n g t h a n d i m p a c t s t r e n g t h a g r e e

w i t h t h e d a t a i n [3 , ] 0 ] , b u t n o t w i t h t h e d a t a i n [ 1, 2 ]. T h e s t a t e m e n t i n [4 , 5 ] t h a t r e t a i n e d a u s t e n i t e i n

a m o u n t s u p t o 1 5 -2 0 % h a s n o e f f e c t o n f a t ig u e s t r e n g t h w a s v e r i f i e d i n o u r e x p e r i m e n t s o n l y f o r s t r e s s e s

o f 6 2 k g / m m 2 .

A s i s w e l l k n o w n [ 13 ], a t h i g h s t r e s s e s t h e r e s i s t a n c e t o r e p e a t e d i m p a c t d e p e n d s l a r g e l y o n t h e

d u c t i l e c h a r a c t e r i s t i c s o f t h e m a t e r i a l , b u t w i t h a l o n g li f e , i . e . , a t l ow s t r e s s e s t h e s t r e n g t h o f t h e m a t e r -

i a l i s t h e p r i n c i p a l f a c t o r .

O u r r e s u l t s a g r e e w i t h t h e d a t a i n [1 3] . A s c a n b e s e e n f r o m T a b l e 1 , w i t h i n c r e a s i n g a m o u h t s o f

r e t a i n e d a u s t e n i t e t he i m p a e t s t r e n g t h i n c r e a s e s a n d t he s t r e n g t h d e c r e a s e s . I n c r e a s i n g t h e a m o u n t o f r e -

t a i n e d a u s t e n i t e b y r a i s i n g t h e q u e n c h i n g t e m p e r a t u r e i s a c c o m p a n i e d b y a r e d u c t i o n i n t h e a m o u n t o f e a r -

b i d e s , w h i c h a l s o i m p r o v e s t h e d u c t i l e c h a r a c t e r i s t i c s a n d i n c r e a s e s t h e i m p a c t - f a t i g u e s t r e n g t h [1 4] .

T h u s , t h e p r o b l e m o f t h e p e r m i s s i b l e a m o u n t o f r e t a i n e d a u s t e n i t e ( i n c lu d i n g t h e p e r m i s s i b l e a m o u n t

i n t h e c a s e ) m u s t b e r e s o l v e d b y t a k i n g in t o a c c o u n t t h e s p e c i f i c o p e r a t i n g c o n d i t i o n s o f m a c h i n e p a r t s .

I t s h o u l d b e n o te d t h a t th e s t a t e m e n t in [ 2] , b a s e d o n r e s u l t s f r o m t e s t s a t o n e s t r e s s l e v e l , m a y l e a d

t o e r r o n e o u s c o n c l u s i o n s .

C O N C L U S I O N S

1 . T h e e f f e c t o f r e t a i n e d a u s t e n i t e o n t h e i m p a c t - f a t i g u e s t r e n g t h o f s t e e l d e p e n d s o n t h e a c t i n g

s t r e s s e s . A t a h i g h s t r e s s l e v e l , w h e n m a c h i n e p a r t s h a v e a s h o r t s e r v i c e l i f e , r e t a i n e d a u s t e n i t e i n c r e a s e s

t h e i m p a c t - f a t i g u e r e s i s t a n c e , b u t d e c r e a s e s i t a t l o w s t r e s s e s . T h e r e a r e a l s o i n t e r m e d i a t e s t r e s s e s a t

w h i c h t h e a m o u n t o f r e t a i n e d a u s t e n i t e h a s n o e f f e c t o n t h e i m p a c t - f a t i g u e r e s i s t a n c e o f a m a t e r i a l w i t h i n

a b r o a d r a n g e .

2 . T h e o p t i m a l a m o u n t o f r e t a i n e d a u s t e n i t e t o e n s u r e h i g h r e s i s t a n c e t o i m p a c t - f a t i g u e m u s t b e

s e l e c t e d i n v i e w o f t h e s p e c i f i c o p e r a t i n g c o n d i t i o n s o f m a c h i n e p a r t s g i v e n s e r v i c e l i f e a n d p o s s i b i l i t y o f

o v e r l o a d i n g ) .

1 0 6 5

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L I T E R A T U R E C I T E D

I . B . P r z h e n o s i l , C a r b o n i t r i d i n g [ in R u s s i a n ] , M a s h i n o s t r o e n i e , M o s c o w ( ] 9 6 9 ) .

2 . S . S . E r m a k o v a n d V . I . K o c h n e v , E f f e c t o f r e p e a t e d q u e n c h i n g t e m p e r a t u r e o n i m p a c t - f a t i g u e o f

e a r b u r i z e d s t e e l , M e t a l . i T e r m . O b r a b o t k a M e t a l . , 8 , 4 9 - 5 0 ( 1 9 5 9 ) .

3 . A . D . A s s o n o v , C h e m i c o t h e r m a l t r e a t m e n t o f t o o t h e d w h e e l s , i n : M o d e r n T e c h n i q u e s o f H e a t

T r e a t i n g M a c h i n e P a r t s [ in R u s s i a n ] , N o . I , M D N T P , M o s c o w ( 1 9 6 5 ) , p p . 7 5 - 9 2 .

4 . M . A . B a l t e r , E f f e c t o f c o l d t r e a t m e n t o n t h e p r o p e r t i e s o f h i g h - a l l o y s t e e l , M e t a l . i T e r m .

O b r a b o t k a M e t a l . , 5 , 3 3 - 4 0 ( 1 9 5 6 ) .

5 . N . B . S h u b i n a a n d V . I . M o r o z o v , I n c r e a s i n g t h e b e n d i n g f a t i g u e s t r e n g t h o f g e a r s b y c o m b i n e d

m e t h o d s o f h a r d e n i n g , i n : A u t o m a t i o n o f C h e m i c o t h e r m a l T r e a t m e n t s [ in R u s s i a n ] , M D N T P , M o s -

c o w ( 1 9 6 7 ) , p p . 9 6 - 1 0 4 .

6 . H . F r a n k e l , J . B e n n e t t , a n d W . P e n n i n g t o n , T A S M , 5 2 , 2 5 7 ( 1 9 6 0 ) .

7. B. G. Gurevich and S. F. Yur'ev , "The role of residual s tr es se s in incr easi ng the fatigue limit of

9 steel with che mic oth erm al tr eat ment ," in: Increa sing the Fatigue Strength of Machine Par ts by Sur-

face Treatment [in Russian], Mashgiz, Moscow (1952), pp. 43-63.

8. M. A. Balte r, Hardening of Machine Par ts fin Russian], Mashinost roenie , Moscow (1968).

9. I. S. Kozlovskii, Che mico the rmal Treat men t of Gears [in Russian], Mashinost roenie , Moscow (1970).

10. A. ~. Pav ara s and Yu. A. Geller, "Effect of retained austenite on the strength of tool stee ls," Metal.

i Term. Obrabotka Metal., 4, 45-48 (1965).

11. L. A. Poznyak, Die Steels for Cold Working [in Russian], Metallurgiy a, Moscow (1966).

12. L. A. Kolodeznyi et al . , "Impact-fatigue resistan ce of malleable cast iron at low temp erat ures ,"

Avtomobil ' naya Promysh lenn ost ' , 1, 38-39 (1969).

13. P. Fo rre st , Fatigue of Metals [Russian transl ation], Mashi nost roea ie, Moscow(1969).

14. G.V . Kozyrev , G. V. Toporov , and R. A. Kozyre va, "Effect of str uct ure of case on impact -fatigu e

res ist anc e of car bur ize d steel 12Kh3NA," in: Fatigue of Metals with Cyclic Impact Loads [in Rus-

sian], TISI, Tomsk (1971), pp. 58-64.

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