E F F E C T O F C Y A N I D E C O A T I N G S ON T H E N O T C H

T O U G H N E S S O F H I G H - S P E E D S T E E L

G . K . S a v i n o v s k i i UDC 621.17:669.14.018.252.3 :621.785.533

By ana logy with the c a r b u r i z e d l a y e r on s t r u c t u r a l s t e e l s , the s u r f a c e h a r d e n e d l a y e r c r e a t e d by eyan id ing should i n c r e a s e the b r i t t l e n e s s of h i g h - s p e e d s t e e l s , a l though the ex t en t of the e f f ec t has not been dote t r a i n e d .

We i n v e s t i g a t e d the e f f ec t of cyan ide c o a t i n g s on the no tch t o u g h n e s s of h i g h - s p e e d s t e e l R18 (0.77~ C, 16.5~ ~ ' , 1.21c/c V). The t e s t s w e r e m a d e in the MK-15 i m p a c t t e s t e r .

S a m p l e s 10 x 10 x 55 m m (without a notch) w e r e p r e p a r e d f r o m p i e c e s s u b j e c t e d to d i f f e r e n t h e a t t r e a t m e n t s : 1) annea l ing at 840-860°C f o r 3 h, f u r n a c e coo l ing to 740 ° with ho ld ing a~ th i s t e m p e r a t u r e 3 h, and coo l ing to r o o m t e m p e r a t u r e in a i r (HB 229); 2) quenching f r o m 1260 ° in i n d u s t r i a l o i l and double t e m - p e r i n g at 560 ° f o r 1 h (HRC 63-65) .

Cyan id ing was conduc t ed at 560 ° in a bath of 75~c NaCN + 25~c Na~CO 3.

The dep th of the e y a n i d e d c a s e was 0.042 m m on annea l ed s a m p l e s and 0.042, 0.028, and 0.054 m m on quenched s a m p l e s . The h a r d n e s s d i s t r i b u t i o n th rough the c a s e depth i s shown in F i g . 1.

G r i n d i n g of the f a c e s p r o d u c e d e igh t t y p e s of s a m p l e s with the p o s i t i o n of the c y a n i d e d f ace d i f f e r i n g in r e l a t i o n to the d i r e c t i o n of the i m p a c t ( see Tab le 1).

The t o u g h n e s s of the two b a s i c t y p e s of s a m p l e s ( annea led and quenched), with d i f f e r e n t dep th s of the c y a n i d e d case , i s g iven in Tab le 1.

The p r e s e n c e of a c y a n i d e d e a s e s u b s t a n t i a l l y l o w e r s the t o u g h n e s s . I t i s known tha t a h a r d e n e d s u r - f ace l a y e r a c t s as a notch, r educ ing the work spen t on m a c r o p l a s t i c d e f o r m a t i o n ai, while the e n e r g y of i m p a c t i s spen t p r i m a r i l y on c r a c k p r o p a g a t i o n : a ~- ap (where a i s the t oughnes s , ap i s the w o r k of c r a c k p r o p a g a t i o n ) .

1975.

ilO0 y. ~ , yoga

900

600

700 [

800 g,~5 0,05 O, oa,5 mm

F i g . 1. H a r d n e s s d i s - t r i b u t i o n th rough dep th of the c y a n i d e d c a s e on quenched s t e e l R18.

an, kg-m/cm 2

1

1,o

52

0,o ¢_ ~ _ - _ _ £

e,2 1 o,2s o,5

---7 /

0,755 mm Notch radium

a i

ap

Fig. 2. Separation of notch toughness into its component

parts a i and ap.

a, kg-ln/cm 2

,0 , ! 2 _ l , , 8 l=b,-b, z,," "

4 f

0 0,1 42 43 40 0,5 0,s 0,7 ~,,mm Deformation

F ig . 3. D e f o r m a t i o n as a funct ion of t o u g h n e s s . 1) S a m p l e s of t y p e s 1 -6 ; 2) type 7; 3) type 8o

Translated from Metallovedenie i Termicheskaya Obr~a-Ilov, No. 9, pp. 6~ , Sop em or,

©19 76 Plenum Publishing Corporation, 22 7 West 17th Street, New York, N. Y. 10011. No part o f this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission o f the publisher. A copy o f this article is available from the publisher for $15.00.

799

TABLE 1

1

2

4

8

0

7

8

9iagram farrow indi- ca re.~ impact JirccEiol])

<D

i:]

Toughness, a (kg-ln/cm 2), at different case depths 5, mm o,04~ o,o42 1 o.o2s [ 0,054 (HB 299) tIRC 63-65

1,23 0,86 0,9I 0,76

1,I 0,82 0,6? - -

1,27 0,86 0,92 0,92

1,12 0,92 0,72 0,67

0,86 0,88 0,62 0,47

4,09 0,85 0,98 0,74

8,46 1,37 1,49 1,04

1,13 1,79 1,92 1,71

the highest deformation.

However, the toughness of smooth samples with a cyanided case (type 1) and the work of c rack propagation determined on samples with notches differing in root radius [1] differ (Fig. 2). In smooth cyanided samples the thickness of the case is inade- quate to prevent plastic deformation completely [2], and par t of the energy of impact is spent on plastic deformation before fo r - mation of a crack: a = ap + ai, where a i is the work of de forma- tion before c rack initiation.

The deformation of the samples was determined as the change in the c r o s s section f rom square o trapezoidal (Fig. 3). The measuremen t s were made by means of a mic romete r . The toughness of the different types of samples was compared with the plastic deformation.

For annealed samples (types 1-6) a l inear variat ion of toughness with deformation is observed (Fig. 3, line 1), i.e., the number of cyanided faces and their position with respect to the direct ion of impact affect the absolute values of the toughness. On these three types of samples the cyanide coating c rea tes the worst conditions for deformation - the absolute values of tough- ness are lowest.

In samples of type 6 the cyanide coating is in the tensile zone. However, the absence of a hardened surface on the lateral faces pe rmi t s absorption of large energies in deformation, which increases the absolute values of the toughness.

Samples of type 7 (with the hardened surface in the com- press ion zone) have the highest toughness, which cor responds to

The toughness and deformation of these samples are direct ly proport ional (Fig. 3, line 2). A l inear var ia t ion of deformation and toughness is also observed for samples of type 8 (Fig. 3, line 3), i.e., samples without a hardened surface.

Tests of samples with a hardened core showed that the toughness of samples with a cyanide coating decreases , but to a l e s s e r extent than for samples with an unhardened core ; the toughness dec reases least.

C O N C L U S I O N

1. A britt le hardened layer obtained by cyaniding lowers the toughness of high-speed steels due to reduction of deformation, i.e., the work of c rack initiation is lowered.

l e

2.

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

A. P. Gulyaev, Toughness and Cold Br i t t leness of Structural Steel [in Russian], Mashinostroenie,

Moscow {1969), p, 69. V. D. Zelenova, ,Res i s t ance to brittle f rac ture of steel with a surface-hardened layer, " Metal. i Term. Obrabotka Metal., No. 6, 49 (1970).

800