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LITERATURE CITED

i. L. E. Rudin, Effect of micros tructu re and hardness on the characteristics of the

nitrided case, in: Transaction s of the Leningrad Institute of Precision Mechanics

and Optics [in Russian ], No. 16 (1955), p. 45.

2. C. Hodgs on and H. Waring, Steel, 116, No. 20, 118 (1945).

3. C. Ho dgson and H. Baron, J. Iron Steel Inst., 18..2 No. 3, 256 (1956); 29, No. 14, 623.

4. A . A . Shmykov, Heat Treatm ent Hand book [in Russian] , Mashgiz, Mosc ow (1961), p. 124.

FORMATION OF DIFFUSION LAYERS DURING GAS NITRIDING

J. Zysk, J. Tatsikovski,

and I. Sulkovski

UDC 621.785.532.062.5:620.18

Deni tridi ng in an atmo sphe re of H2--N2 is used to reduc e the brittl e surface zone of

iron carbonitrides (g phase) [1-4]. The best results are obtained with nitriding in an

atmosphe re of ammonia with additions of gases that lower the nitride potential [i] . Such

studies have been made at the Institute of Precision Mechanics, Warsaw.

It is known [5, 6] that wit h incr easing nitr iding times the porous surface layer

grows rapid ly (Fig. i).

X-ray analysis (powder patterns) w ith successi ve remova l of layers (~0.005 mm each)

revea led e phase in porous zones and particle s of e + y' phas e beneath the e phase (y'

phase is prec ipit ated in the proces s of cooling). A conti nuous layer of y' phas e after

nitriding was observed only on Armco iron and low-carbon steel with 0.1-0.2% C, for which

the coefficient of nitrogen di ffusion is fairly high. The distribu tion of phases in the

surface layer on steels 45 and 40Kh after nitridin g at 5800C in relation to nitriding time

is shown in Fig. 2. With ni tridi ng for more than 4 h the thickness of the hard (HV 800-1200)

zone of E + y' phase does not change, only the thic kness of the porous E phase increasing.

Near the ~-(Fe, N) bounda ries the columns of s + y' phase are perpend icul ar to the surface.

With increasing concentrations of nitrogen in the atmospher e this arrangement is lost.

The zone of ~ + y' phase has a far higher wear re sist ance than the porous zone of e

pha se (Fig. 3, curv es 1 and 2).

Analysis of the distribution of nitrogen and carbon diffusing from the steel into the

zone of e phase indicates (Fig. 4) that the concentra tion of these elements in the zone of

E + y' phase, with excellent operating characteristics, does not exceed 8-8.5%. It follows

that to avoid the formation of a porous zone the total concen tration of carbon and nitrogen

should not exceed 8.5%, but at this concentration the surface layer should be dense, with

Fig. I. Micro struct ure of Armco iron after nitriding at

580 ° in an atmosphere of partially dissociated ammonia

(S~l~ micrograph), a) Nit rid ed 4 h (1500x); b) I0 h

(600×).

Institute of Precisi on Mechanics, Warsaw. Translated from Metall oveden ie i Termicheskaya

Obrabotka Metallov, No. 6, pp. 12-15, June, 1980.

392 0026-0673/80/0506- 0392507.50 © 1981 Plenum Publishing Corporation

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~I1T1

o

g5 ~ - -

40 I

4 a I f 16 20 24 4 8 12 iG 20 h

Nitriding t ime

F i g . 2

Mm

21 / 4

16 /;

7

i /

4 i °'

io

f2

7

p

(

30 50

rain

Test ing t ime

F i g . 3

F i g . 2 . D i s t r i b u t i o n o f p h a s e s i n t h e n i t r i d e d c a s e o n s t e e l s 4 5 ( a)

a n d 4 0 K h ( b) . N i t r i d i n g a t 5 8 0 ° i n p a r t i a l l y d i s s o c i a t e d a m m o n i a.

F i g. 3. W e a r o f st e e l 4 0 K h in f r i c t i o n t e s t s i n r e l a t i o n t o n i t r i d i n g

t i m e at 5 8 0 ° i n p a r t i a l l y d i s s o c i a t e d a m m o n i a . T e s t s i n t h e 4 7 - K 5 4

m a c h i n e u n d e r a l o a d o f 4 0 k g f / m m 2, 5 . 5 m / m i n , c o u n t e r b o d y s t e e l 4 5

( H RC 3 0 ) , l u b r i c a t i o n w i t h o i l , n = 5 7 6 r p m . N i t r i d i n g t i m e a n d t h i c k -

n e s s o f z o n e s : i ) 2 h , a = 0 . 0 0 1 m m , a + Y ' = 0 . 0 1 1 m m ; 2 ) 4 h , e =

0 . 0 0 5 m m , e + y ' = 0 . 0 1 7 m m ; 3 ) 1 2 h , ~ = 0 . 0 3 0 m m , e + y ' = 0 . 0 3 0 m m ;

4 ) 2 4 h , e = 0 . 0 8 0 m m , e + y ' - 0 . 0 2 5 m m .

m + o , .~ c

I

qN+C

7

Z ~ A C

o ~ ~ 2¢

. . . . . . o - - -

o

U O ~ 2 3 ~ 5 GO 7 8 im~

Distance from surface

F i g . 4 . D i s t r i b u t i o n o f n i t r o g e n a n d c a r b o n

d i f f u s i n g f r o m t h e s t e e l ) i n t h e l a y e r o b -

t a i n e d a f t e r n i t r i d i n g o f s t e e l 4 5 a t 5 8 0 ° i n

p a r t i a l l y d i s s o c i a t e d a m m o n i a i n r e l a t i o n t o

n i t r i d i n g t i m e ( g i v e n o n t h e cu r v e s ) .

n o m i c r o p o r e s , w i t h a s t r u c t u r e o f c p h a s e a n d p a r t i c l e s o f y ' p h a s e t h a t p r e c i p i t a t e d u r i n g

s l o w c o o l i n g .

W i t h r a p i d c o o l i n g t h e y ' p h a s e m a y n o t b e p r e c i p i t a t e d a n d t h e s t r u c t u r e o f t h e l a y e r

w i t h ~ 8 . 5 ( N + C ) w i l l b e s i n g l e - p h a s e ( E p h a s e ) . T h e o p e r a t i n g c h a r a c t e r i s t i c s o f t h e

s t e e l a r e n o t i m p a i r e d i n t h i s c a se .

U n d e r c o m m e r c i a l c o n d i t i o n s t h e q u a l i t y o f t h e n i t r i d e d c a s e i s c o n t r o l l e d b y m e a n s o f

e x a m i n a t i o n i n a m i c r o s c o p e o r m e a s u r e m e n t s o f s u r f a c e h a r d n e s s u n d e r l o a d s o f 0 . 5 - 1 k g f .

3 9 3

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N ~m - 7 8 ' I

. ~ / ~ o = ~ ~ -.~ L ~ I ~ ~ ~

/ ] ' l

0 ~ ~

. . . . . . _ \

~ . o o , o , l - ~ . = o ,7 ' , ' l " %

~ .~ o 10 20 Jo 7 9 71 U r5 17 is 21 2,1 25 Z T m

N i t r i di n g t i m e I n c r e m e n t o f d i a m

Fig. 5 Fig. 6

Fig. 5. Thick ness of carb onit ride zone (i) and zone of internal

nitrid ing (2) with two-s tage p roces sing (stage I at 505 ° for 12h,

Np = 5-7; stag e II at 530 ° for 30 h, Np = 0. 2-0.4).

Fig. 6. Var iat ion of the increme nt of diamete rs of journa ls

(solid lines) and cranksh afts (dashed lines) nitride d by the old

(i) and ne w (2) meth ods.

With >8.5 (N + C) in the case and exce ssive poros ity the hardness measur ed under such

loads will be too low.

Tests were made on steels 25Kh3M (0.2-0.3 C, 2.9-3. 5 Cr, 0.4-0.7 Mo) and 2TsM (0.3-

0.4 C, 1.9-2. 4 Cr, 0.25-0.3 5 Mo), whi ch are used for cranks hafts in diese l engines.

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

layer with a zone of e carbon itrides . Nitridi ng was cond ucted in an atmospher e of HNa--H2--N=

at 505-530°C. Nitri ding of Cr--Mo steels wit h a consta nt low nitroge n potential (Np ~ i),

even for a long time, does not ensure the neces sary hardne ss or case depth. Satura tion

in a med ium wit h Np > 1 leads to form ation of a surfac e zon e of E carbon itride s and a zone

of internal nitridi ng -- equally uns atis fact ory results. Thus, wit h low valu es of the nitro-

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

to forma tion of pores in the surfa ce layer.

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

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

W e i n v e s t i g a t e d t h e s t r u c t u r e a n d g r o w t h r a t e o f t h e c a s e o n s t e e l s 2 5 K h 3 M a n d 2 T s M

nitride d at 505 ° for 12 h wit h a high nitro gen poten tial (Np = 5-7). With in creasing

n i t r i d i n g t i m e t h e z o n e s of ~ c a r b o n i t r i d e s a n d i n t e r n a l n i t r i d i n g i n c r e a se d . T h e h i g h e s t

d e n s i t y o f t h e ~ c a r b o n i t r i d e z o n e w a s o b t a i n e d a t t h e t i m e t h e b a t c h r e a c h e d n i t r i d i n g

temperature.

In the first st age of nitridin g (5 h) the thicknes s of the zone of E carbon itride s

reach ed ~i0 Bm, with sur face hardn ess HV: = 940-980 for steel 25Kh3M and 840-880 for steel

2TsM, and the thicknes s of the nitride d case was ~5 0 of the thickness of the case obtain ed

in the second stage after severa l tens of hours.

In the second stage of nitrid ing at 530 ° Np was taken as 0.2-0.4. At this potent ial

o f t h e a t m o s p h e r e o n l y t h e z on e o f i n t e r n a l n i t r i d i n g i n c r e a se s , w h i l e t h e zo n e o f ~ c a r bo -

nitri des remains almost unchanged (Fig. 5). The carbonitr iding time in the second stage of

t h e p r o c e s s d e p e n d s o n t h e t h i c k n e s s o f t h e n i t r i d e d c a s e r e q u i r ed .

W i t h d e c r e a s i n g v a l u e s o f t h e n i t r o g e n p o t e n t i a l i n t h e s e co n d s t a g e o f t h e p r o c e s s

to 0.4 the thicknes s of the ~ earbo-

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

s t a g e o f t h e p r o c e s s a t w h i c h t h e e c a r b o n i t r i d e z o n e r e m a i n s u n c h a n g e d i s n a r ro w .

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

c a s e d e p th , s a t i s f a c t o r y d i s t r i b u t i o n o f h a r d n e s s t h r o u g h t h e s e c t i o n of t h e c as e , a n d a

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

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

thickne ss of the case in the second stage. The param eters of the proce ss depend on the

requ irem ents of the trea tment and the type of st eel as w ell as on the size of the load. It

was found that steel 2TsM requi res a longer sec ond stage tha n steel 25Kh3M.

394

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O n th e b a s i s of t h e p r e l i m i n a r y t e st s , w e s e l e c t e d t h e f o l l o w i n g n i t r i d i n g c o n d i t i o n s

for cran ksha fts: sta ge I, 505 ° , 5 h, Np = 3 for bo th s teels; sta ge II, 530 ° , 38 h,

Np = 0.3 for st eel 25Kh 3M and 530 ° , 45 h, Np = 0.4 for steel 2TsM.

T h e s u r f a c e h a r d n e s s o f c o n t r o l s a m p l e s o f s t e e l 2 5 K h 3 M w a s H V: = 8 5 0- 9 0 0 , w h i l e H V I =

7 0 0 - 7 5 0 f o r st e e l 2 T s M; t h e h a r d n e s s o f c r a n k s h a f t s a n d j o u r n a l s m e t t h e s p e c i f i c a t i o n s .

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

n i t r i d i n g r e d u c e s t he v a r i a t i o n o f t h e i n c r e m e n t o f t h e d i a m e t e r o f c r a n k s h a f t s a n d j o u r n a l s t o

to almost one-ha lf for steels 25 Kh 3M an d 2TsM -- from %17-2 3 to 9-14 ~m (Fig. 6).

F a t i g u e t e s t s of c o n t r o l s a m p l e s s h o w e d t h a t w i t h N > 1 0 7 i n t o r s i o n a l b e n d i n g t e s t s

t h e f a t i g u e l i m i t i s 6 5 k g f / m m = f o r s a m p l e s n i t r i d e d b y b o t h t h e o l d a n d n e w m e t h o d s . H o w -

e v er , t h e c o n t a c t f a t i g u e s t r e n g t h i n t e s ts b y t h e f o u r - r o l l e r m e t h o d u n d e r a l o ad o f ~ 3 0

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

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

w a s ~ 6 0 m i n a f t e r t w o - s t a g e n i t r i d i n g , a s c o m p a r e d w i t h 2 m i n a f t e r n i t r i d i n g b y t h e o l d

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

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

zone. The shafts are not ground after this treatment, but only polished. In bench tests

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

this method.

C O N C L U S I O N S

i. I n t h e p r o c e s s o f t w o - s t a g e n i t r i d i n g a l a y e r w i t h a d e n s e z o n e o f E c a r b o n i t r i d e s

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

i s t i cs . G r i n d i n g c a n b e r e p l a c e d w i t h l i g h t p o l i s h i n g .

2. The properti es are best for the nitri ded ca se with a thin (several ~m) dense layer

o f e c a r b o n i t r i d e s . T h e p r e s e n c e o f a d e n s e l a y e r a c c e l e r a t e s i n t e r n a l n i t r i d i n g d e s p i t e

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

3. To obta in a nitri ded layer of smalle r thic kness (parts of mach ines not subjec ted to

o v e r h a u l ) o n e c a n u s e b r i ef g a s n i t r i d i n g w i t h a g i v e n n i t r o g e n p o t e n t i a l f o r e a c h st e e l .

1

2.

3.

4.

5.

6.

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

Y u . M . L a k h t i n a n d Ya . D. K o g a n , N i t r i d i n g o f S t e e l [ i n R u s s i a n ] , M a s h i n o s t r o e n i e ,

Mo sc ow (1976), p. 225.

A . A . Y u r g e n s o n , N i t r i d i n g i n P o w e r M a c h i n e r y C o n s t r u c t i o n [ in R u s s i a n ] , M a s h g i z ,

Mos cow (1962), p. 132.

J . Z y s k, N o w o c z e s n e P r o c e s y O b r o b k i C i e p l n o - C h e m i c z n e j , W a r s z a w a , I MP , Z e s z y t No . 3/ 7

(1973).

B . C l a y t o n a n d K. S a c h s , R e d u c t i o n o f t h e w h i t e l a y e r o n th e s u r f a c e o f n i t r i d e d c o m -

p o n e n t s , H e a t T r e a t m e n t 7 6, 1 6 t h In t. H e a t T r e a t m e n t C o n f . , S t r a t f o r d ( 1 9 76 ) .

J. Zysk, Met alo zna wst wo i Obrob ka Cieplna, No. 4, 2 (1973).

J. Zysk, Har ter ei- Tec hni sch e Mitte ilun gen, No. 3, 137 (1976).

395