From Metallovedenie i Term. Obrabotka Metal lov, Dec. 1959, #12, 39-u,3 HB No. 5008

T H E S T R U C T U R E AND H A R D N E S S O F S U R F A C E - H A R D E N E D

S P H E R O I D A L G R A P H I T E CAST IRON

Cand . t ech . s c i . M. M. K l i m o c h k i n C e n t r a l R e s . Ins t . of T e c h n o l o g y and M a c h . D e s i g n

It was found from preliminary experiments that surface fusion of speci- mens during induction heating occurs at 1150 ~ C for pearlitic and at IZ00 ~ C for

ferritic cast irons. Upon quenching from temperatures below 900 ~ C the struc-

ture of the metal remained unchanged. Accordingly, specimens of pearlitic cast

irons were quenched in water from 900 ~ , 950 ~ , i000 ~ 1050 ~ and ii00 ~ C, and

specimens of ferritic cast irons from 900 ~ , 950 ~ , 1000 ~ , 1050 = , 1100 ~ and 1150 ~

C. The heating rates were I0 ~ 25 ~ 50 ~ and I00 ~ C/sec which are typical in

surface hardening practice. The heating rates were determined in the range of

phase transformations, i.e. above the Curie point (-~ 770 ~ C). The temperature

was measured with an optical pyrometer.

The specimens for studying the structure and hardness of the hardened

surface were 30 mm in diam. and Z0 mm thick. The hardness change across the

thickness of the hardened layer was determined on 70 mm diam. specimens by

successive grinding. In this case the specimens vcere quenched from 950 ~ I000 ~

1050 ~ and II00 ~ C. The depth of the hardened layer was 3 ram. The amount of

retained austenite was determined on a ballistic magnetometer. The microhard-

ness of the structural components was measured on a PMT-3 instrument. After

quenching, the specimens were tempered at 140 ~ - 160 ~ C for Z hrs. Pearlitic

(90~- 95~o pearlite), pearlito-ferritic (60~- 90~ pearlite), ferrito-pearlitic (Z0~-

40~ pearlite) and ferritic cast irons(5~o- Z0~ pearlite) of a composition listed in

Table 1 were investigated. The structure of quenched pearlitic cast iron consis-

ted of a subcrystalline or fine, acicular martensite. Quenching from 900 ~ C gave

a structure of bainite and martensite. Quenching of a pearlito-ferritic iron re-

sults in a mixture of martenslte with small patches of residualferrite. The con-

tent of this ferrite decreased with increasing hardening temperature and decreas-

ing heating rate.

T a b l e I .

c

Structure c~ E

I Pearl i t i cii0, 6 Pearl i t o - ] F e r r i t i c 0.5 Fer r i to - 1 Pearl i t ic tO. I~ F e r r i t i c ]00l

I

Composition, %

Si Mn P S o [

i l '"J o.oo

', o.

3.2/%7 0.5]0,2 I 0,007 3:3j ~ .s i o.6ro.17, ' o,oo4

H o w e v e r , e v e n a f t e r h a r d e n - ing f r o m 1000 ~ 1100 ~ C s u c h p a t c h e s w e r e p r e s e r v e d due to the h igh s i l i c o n con t en t (2 - Z. 2%) w h i c h o b s t r u c t e d the d i f f u s i o n of c a r b o n in the m e t a l l i c m a t - r i x .

F i g . 1 p i c t u r e s the p e r m i s s i - b le h a r d e n i n g t e m p e r a t u r e c o m p i l e d f r o m an a n a l y s i s of the s t r u c t u r e s of p e a r l i t i c c a s t i r o n s . The a r e a of sub - c r y s t a l l i n e m a r t e n s i t e g r a d u a l l y c h a n g e s in to f ine a c i c u l a r and t r o o s t o m a r t e n s i t e .

In the transition areas the structure has a mixed character. An investigation of

the structure of surface-hardened pearlitic irons showed that the hardened layer

consisted of martensite, ferrite and graphite.

43

,000

850 I e

Heating Rate, C/sec

F i g . 1. D i a g r a m of o p t i m u m h e a t i n g t e m p s . in s u r f a c e h a r d e n i n g of p e a r l i t i c i r o n c a s t i n g s : a - s u r f a c e fu s ion ; b - f ine a e i c u l a r m a r t ' e ; c - s u b c r y s t a l l i n e " d - t r o o s t o - m a r t e n s i t e ; e - no h a r d e n i n g e f f ec t .

In f e r r i t o - p e a r l i t i c c a s t i r o n the m a r t e n s i t e f o r m e d b a s i c - a l l y as a r e s u l t of d i s s o l u t i o n of g r a p h i t e i n c l u s i o n s . S m a l l p e a r l i t e r e g i o n s in th is i r o n a l s o t e n d e d to i n c r e a s e the a m o u n t of m a r t e n s i t e in the s t r u c t u r e . With i n c r e a s i n g t e m p e r a t u r e and d e c r e a s i n g h e a t i n g r a t e , t h e s e p r o c e s s e s a c c e l e r a t e d , F i g . 2. H o w e v e r the s t r u c t u r e c o n t a i n e d f e r r i t e

F i g . 2. M i c r o s t r u e t u r e e v e n a f t e r q u e n c h e s f r o m 1100 ~ of f e r r i t o - p e a r l i t i c c a s t C. When f e r r i t i c c a s t i r o n is i r o n s u r f a c e - h a r d e n e d q u e n c h e d , t he c a r b o n con ten t f r o m 1100 ~ C. H e a t i n g of the a u s t e n i t e i n c r e a s e s r a t e i00 ~ C / s e c . p r a c t i c a l l y on ly on a c c o u n t of

the d i s s o l v i n g g r a p h i t e . T h i s i s why l e s s m a r t e n s i t e was

f o r m e d in the h a r d e n e d l a y e r in t h i s c a s e than d u r i n g h a r d e n i n g of f e r r i t o - p e a r l i t i c c a s t i r o n .

T h e a m o u n t of f e r r i t e in the h a r d e n e d l a y e r of f e r - r i t i c c a s t i r o n s i n c r e a s e d wi th i n c r e a s i n g h e a t i n g r a t e b e c a u s e l e s s t i m e was a v a i l a b l e f o r g r a p h i t e d i s s o l u t i o n .

The p r o c e s s of d i s s o l u t i o n of g r a p h i t e i n c l u s i o n s and c a r b o n d i f f u s i o n in the e n t i r e a u s t e n i t e v o l u m e r e q u i r e s no t on ly h igh t e m p e r a t u r e s but a l s o m u c h t i m e s i n c e the g r a p h i t e i n c l u s i o n s a r e f a r a p a r t . The h e a t i n g r a t e s n o r m a l l y u s e d in s u r f a c e h a r d e n i n g of s t e e l p r o d u c t s (25 ~ C / s e c and above) a r e h e r e too f a s t to o b t a i n a r n a r t e n s i t i c h a r d e n e d l a y e r . To p r o d u c e in f e r r i t i c i r o n s a h a r d e n e d l a y - e r wi th a m a r t e n s i t i c s t r u c t u r e it is n e c e s s a r y , t h e r e f o r e , to e m p l o y low hea t i ng r a t e s ( i 0 ~ C / s e c o r l e s s ) and h igh h a r d e n i n g t e m p e r a t u r e s ( l l 0 0 - l l 5 0 ~ C). It was found a f t e r q u e n c h i n g tha t s o m e g r a p h i t e i n c l u s i o n s w e r e not s u r r o u n d e d by m a r t e n s l t e w h e r e a s o t h e r s , w e r e . H e n c e a s m a l l p a r t ( 1 0 ~ - 15%) of the g r a p h i t e i n c l u s i o n s t a k e s no p a r t in s a t u r a t i n g the a u s t e n i t e w i t h c a r b o n . T h e t h i c k n e s s of the m a r t e n s i t i c e n v e l o p e s is the s a m e a r o u n d l a r g e and s m a l l g r a p h i t e p a r t i c l e s a l i k e . In add i t i on , t h e y h a v e a p r a c t i c a l l y i d e n t i c a l w id th a long the p e r i m e t e r , tha t i s , the s o - c a l l e d G e v e l i n g e f f e c t was not o b s e r v e d . ( G e v e l i n g had c l a i m e d tha t a h e t e r o g e n e o u s s t e e l s t r u c t u r e is h e a t e d u n e v e n l y by e l e c t r i c c u r r e n t s ~

T h e a m o u n t of r e t a i n e d a u s t e n i t e in the h a r d e n e d l a y e r of p e a r l i t i c i r o n s p e c i m e n s i n c r e a s e d wi th i n c r e a s i n g t e m p e r a t u r e and d e c r e a s i n g h e a t i n g r a t e prior to hardening, Fig. 3. When the retained austenite content reached 25~-30~ the hardness of the quenched layer fell by 4 - 5 R C. Retained austenite was ob- tained also in quenching pearlito-ferritic cast irons but in smaller amounts (8~ - 12~).

Upon h a r d e n i n g f r o m a f u r n a c e hea t , the a m o u n t of 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 d by a f a c t o r 1 .5 - Z c o m p a r e d wi th i n d u c t i o n h e a t i n g to s i m i l a r t e m p e r a - t u r e s . In add i t ion , the r e t a i n e d a u s t e n i t e con t en t was p r a c t i c a l l y i n d e p e n d e n t f r o m the i n i t i a l s t r u c t u r e .

T h u s , the s t r u c t u r e of s u r f a c e - h a r d e n e d p a r t s f r o m p e a r l i t i c c a s t i r o n s c o n s i s t s e s s e n t i a l l y of m a r t e n s i t e , a u s t e n i t e and g r a p h i t e , T h e s t r u c t u r e of s i m - i l a r l y h a r d e n e d p r o d u c t s f r o m f e r r i t i c i r o n s c o n s i s t s of f e r r i t e , m a r t e n s i t e and g r a p h i t e . T h e s e s t r u c t u r a l d i f f e r e n c e s a r i s e d u r i n g r a p i d hea t i ng s i n c e the cond i - t i ons f o r c o m p l e t i o n of c a r b o n d i f f u s i o n t h r o u g h the e n t i r e a u s t e n i t e v o l u m e a r e m u c h m o r e a d v a n t a g e o u s in p e a ~ l i t i c t han in f e r r i t i c c a s t i r o n s .

44

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

In t h i s w o r k , t h e h e t e r o g e n e i t y of m a r t e n s i t e of a p e a r l i t i c c a s t i r o n w a s f o l l o w e d u p b y 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 . B e t w e e n Z5 a n d 30 r e a d i n g s w e r e t a k e n o n d i f f e r e n t a r e a s of t h e m e t a l l o g r a p h i c s p e c i m e n w h i c h g a v e s u f f i c i e n t d a t a

T h e s e m i c r o h a r d n e s s d a t a a r e c o n t a i n e d i n T a b l e 2.

I f ~ Table 2 / 4

, /

f o r t h e p u r p o s e .

#50 90g 95a lOOO I050 husteni t iz ing Temp., C

F i g . 3. E f f e c t of m e t h o d a n d r a t e of h e a t i n g o n 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 q u e n c h e d p e a r l i t i c c a s t i r o n : 1 - f u r n a c e h e a t i n g , 30 r a i n . h o l d ; Z - i n d u c t i o n h e a t i n g a t 10 ~ C / s a c ; 3 - i n d u c t i o n h e a t - i n g a t 100 ~ C / s e c .

:10

25

50

t00

Austenitizing Hicro- Temperature, C

hardness

900' 95010001105011~

656 774 780[724 666 HaximumHinimum 1394 50415041433 373 Difference 12721270 275 291 293

.ax m.o 666 i 74417341691i600 Minimum 336i464 433 405 344 Difference 1330L280 30112861322

Maximum 6661691 724 724 724 Ninimum 2901363 405 4201405 D fference 376i328 319 304 319

.ox,moo 6661666 66616911 24 Minimum 268'244 465 394[394 Difference 398 322 261 297 330

A j u x t a p o s i t i o n of t h e m i c r o h a r d - n e s s f i g u r e s w i t h t h e a p p r o p r i a t e m i c r o - s t r u c t u r e s s h o w e d t h a t d u r i n g s u r f a c e h a r d e n i n g , t h e h i g h e s t h a r d n e s s i s d e v - e l o p e d i n t h e s u b c r y s t a l l i n e m a r t e n s i t e . T r o o s t o - m a r t e n s i t e a n d f i n e , a c i c u l a r m a r t e n s i t e w e r e s o m e w h a t s o f t e r . T h e l o w e r v a l u e s i n t h e l a t t e r c a s e a r e a s - c r i b e d t o t h e r e t a i n e d a u s t e n i t e . I n f e r r i t i c c a s t i r o n s , t h e m a r t e n s l t i c p a t c h e s a n d r i n g s a r o u n d g r a p h i t e i n c l u s i o n s h a d a h i g h m i c r o h a r d n e s s ( 5 0 0 - 8 0 0 D P N ) . T h e h e t e r o g e n e i t y of m a r t e n s i t e d u r i n g s u r f a c e h a r d e n i n g w a s h i g h e r t h a n a f t e r h a r d e n i n g f r o m f u r n a c e h e a t . F o r t h e s u b c r y s t a l l i n e m a r t e n s i t e t h i s d i f f e r e n c e w a s 30% a n d f o r t h e a c i c u l a r v a r i e t y , 50%. T h e s e d i f f e r e n c e s a r e r e l a t e d t o t h e h i g h h e a t i n g r a t e s f o r h a r d e n i n g .

T a b l e 3 .

Structure

Pearl i ti c

Pearl i to- Ferri t ic

Ferri to- Pearl i t i c

Ferr i t ic

•gg

lO 25 5o

lO0

lO 25 5o

10o

lO 25 50

lOO

IO 25 5o

lOO

Austenitizing Temperature,r

900 950 1000

48--55 I 55--50 53--50 50--56 53--60 52--50 43--58 50--58 50--58 32--58 48--58 ~ 54--58

42--48 j 46L54 52--57 35--45 42--52 48--55 30--44 40--52 47--57 25--40 35--50 45--56

17--24 27--39 16--23 23--37 10--22 15--32 5--21 8--24

7--13 18--28 1--10 10--20 1--12 6--15 1--10 3--13

45

1050

49--56 48--57 50--59 50--58

50--55 52--58 52--58 50~57

30--43 40--52 28--42 34--45 23--38 32--42 12--30 2 5 4 0

1100

46--54 45--55 48--58 48--58

48"56 48--58 49--58 50--57

l

48--57 43--52 35--48 36--48

17--80 I 26--39 | 26--39 i 12--24 20--35 30--43 i 5--17 14--29 I 24--37

1150

48--59 45--54 40--52 40--52

44--49 40--47 38--.48 37--44

S u r f a c e - h a r d e n i n g of p a r t s m a d e of f e r r i t i c c a s t i r o n s did not r e s u l t in i n c r e a s e d m i c r o h a r d n e s s of the f e r r i t e r e t a i n e d in the h a r d e n e d l a y e r . The f e r - r i t e had n e a r l y i d e n t i c a l h a r d n e s s e s b e f o r e and a f t e r q u e n c h i n g (195-Z14 DPN) . T h e h a r d n e s s e s of s u r f a c e - h a r d e n e d s p e c i m e n s a r e l i s t e d in T a b l e 3.

S u r f a c e h a r d e n i n g of p e a r l i t i c c a s t i r o n s r e s u l t e d in a h igh h a r d n e s s c l o s e to the h a r d n e s s of s u r f a c e - h a r d e n e d s t e e l . F o r e a c h h e a t i n g r a t e t h e r e was an a p p r o p r i a t e h a r d e n i n g t e m p e r a t u r e w h i c h g a v e the h i g h e s t u n i f o r m h a r d n e s s . In p e a r l i t i c c a s t i r o n s th i s was o b s e r v e d w h e n the h a r d e n e d l a y e r c o n s i s t e d of sub - c r y s t a l l i n e m a r t e n s i t e and in p e a r l i t o - f e r r i t i c i r o n s w h e n f e r r i t e was a b s e n t f r o m the h a r d e n e d l a y e r . T h e o p t i m u m t e m p e r a t u r e of h e a t i n g f o r h a r d e n i n g i n c r e a s e d wi th h e a t i n g r a t e . A m a x i m u m h a r d n e s s of s u r f a c e - h a r d e n e d p a r t s f r o m p e a r l i t i c c a s t i r o n w a s o b t a i n e d a f t e r h a r d e n i n g f r o m t e m p e r a t u r e s e x c e e d i n g t h e u s u a l t e m p e r a t u r e a f t e r f u r n a c e p r e - h e a t i n g by 75 ~ - 1Z5 ~ C f o r p e a r l i t i c , and by 150 ~ - Z00 ~ C f o r p e a r l i t o - f e r r i t i c i r o n . P e a r l i t i c c a s t i r o n s wi th m o r e than 0. 1 8 ~ - 0 . 2 % P d e v e l o p e d s t r u c t u r a l m i c r o c r a c k s w h e n q u e n c h e d f r o m 1050 ~ - 1100 ~ C.

T h e s e c r a c k s cou ld be d e t e c t e d by a m a g n e t i c f l a w d e t e c t o r . The h a r d n e s s of the h a r d e n e d s u r f a c e of f e r r i t i c i r o n s i n c r e a s e d wi th

t e m p e r a t u r e & w i t h d e c r e a s i n g h e a t i n g r a t e . M a x i m u m h a r d n e s s e s w e r e found a f t e r q u e n c h i n g f r o m 1 1 0 0 - 1 1 5 0 ~ The a s - q u e n c h e d h a r d n e s s h e r e w a s l o w e r in f e r r i t o - p e a r l i t i c i r o n b e c a u s e in f e r r i t i c i r o n the a u s t e n i t e r e c e i v e s c a r b o n v i r t u a l l y on ly f r o m d i s s o l v i n g g r a p h i t e i n c l u s i o n s . The h a r d n e s s of s u r f a c e - h a r d e n e d f e r r i t i c c a s t i r o n s d e p e n d e d on the q u a n t i t a t i v e r e l a t i o n s h i p of m a r t e n - s i t e and f e r r i t e in the q u e n c h e d l a y e r . At h igh h a r d e n i n g t e m p e r a t u r e s the a m o u n t of m a r t e n s i t e e x c e e d e d tha t of f e r r i t e a n d the h a r d n e s s w a s h i g h e r than a f t e r q u e n c h i n g f r o m l o w e r t e m p e r a t u r e s .

N e v e r t h e l e s s , the m a x i m u m h a r d n e s s of f e r r i t i c c a s t i r o n s q u e n c h e d f r o m h igh t e m p e r a t u r e s was l o w e r than of t h o s e q u e n c h e d f r o m l o w e r t e m p e r a t u r e s . T h e s c a t t e r of h a r d n e s s v a l u e s i n c r e a s e d wi th hea t i ng r a t e . The h a r d n e s s of the q u e n c h e d s u r f a c e a f t e r h a r d e n i n g f r o m a f u r n a c e hea t was p r a c t i c a l l y i n d e p e n d - e n t f r o m the i n i t i a l s t r u c t u r e of the c a s t i r on . S p e c i m e n s wi th v a r i o u s i n i t i a l s t r u c t u r e s had, a f t e r q u e n c h i n g , n e a r l y i d e n t i c a l h a r ' d n e s s e s .

r t~

2m 30 [ ~o = 20 i

10 0 I 2. 3 o ~ 3

o ~"[ !t,o~c t I - ~

30 kx~ ~ I

t 7 3 ~ ! 2 ~m Y" Depth of Hardened Layer, m Depth of Hardened Layer,

a) b) c) d)

F i g . 4. H a r d n e s s c h a n g e a c r o s s s u r f a c e - h a r d e n e d l a y e r f o r v a r i o u s a u s t e n i t i z i n g t e m p e r a t u r e s (dep th of l a y e r - 3 ram}: ( a ) - p e a r l i t i c ; ( b ) - p e a r l l t o - f e r r i t i c ; ( c ) - f e r r i t o - p e a r l i t i c ; ( d ) - f e r r l t i c c a s t i r o n ,

The h a r d n e s s c h a n g e s a c r o s s the q u e n c h e d l a y e r of n o d u l a r i r o n a r e in F i g . 4. In p r o d u c t s m a d e of p e a r l i t i c i r o n s , the h a r d e n e d l a y e r has a h igh h a r d n e s s z o n e ( t r u l y h a r d e n e d l a y e r ) and a s o f t e r t r a n s i t i o n l a y e r . In p r o d u c t s f r o m f e r r i t i c i r o n s , the h a r d n e s s of the q u e n c h e d l a y e r g r a d u a l l y d e c r e a s e s f r o m the s u r f a c e to the i n i t i a l s t r u c t u r e of the m a t e r i a l .

46