INVESTIGATION OF SOLIDIFICATION OF HIGH-STRENGTH STEEL CASTINGS

AMMRC CR 63-04/F

FINAL REPORT

f o r

October 1, 1966 - December 31, 1967

by

T. Z. Kat tamis and M. C. Flemings

J u l y 15 , 1968

Departmenr of Metallurgy and M a t e r i a l s Science Massachuse t t s I n s t i t u t e of Technology

Cambridge, Massachuse t t s 02139

Cont rac t N o . DA-19-020-AMC-5443(X) D/A P r o j e c t No, lC0244018328

AMCMS CODE 5025.11.294

Metals Research f o r Army M a t e r i e l

T h i s Document has been Approved f o r P u b l i c Release and Sale; i t s D i s t r i b u t i o n i s Unl imited

Army M a t e r i a l s and Mechanics Research Cente r Watertown, Massachuse t t s 02172

ABSTRACT

The morphology, s i z e and d i s t r i b u t i o n of s u l f i d e i n c l u s i o n s a r e s t u d i e d i n vacuum m e l t e d and s o l i d i f i e d A I S I 4330 low a l l o y s t e e l deox id ized w i t h v a r i o u s e lements such as A l , S i , Mn, e i t h e r i n d i v i d u a l l y o r i n v a r i o u s combina t ions , and B , Z r and C e . Types I , I1 and 111 i n c l u s i o n s a r e i n v e s t i g a t e d i n d e t a i l . Type I i n c l u s i o n s are i s o l a t e d , roughly s p h e r i c a l s u l f i d e s , Type I1 i n c l u s i o n s have a n i n t e r c o n n e c t e d r o d - l i k e morphology, and Type I11 i n c l u s i o n s are a n g u l a r and unconnected ( e x c e p t they are o f t e n connected t o Type I1 i n c l u s i o n s ) .

E f f e c t s of chemis t ry ( A l , S i , Mn) on i n c l u s i o n Type i s q u a n t i t a t i v e l y determined. The e f f e c t of c o o l i n g r a t e on i n c l u s i o n t y p e and s i z e i s examined f o r a few c a s e s . With d e c r e a s i n g c o o l i n g r a t e , t h e average s ize of Type I i n c l u s i o n s and t h e average spac ing of t h e e u r e c t i c Type I1 i n c l u s i o n s i n c r e a s e .

The o r i g i n of he v a r i o u s t y p e s of i n c l u s i o n s observed i s i n t e r p r e t e d w i t h the a i d of t h e Fe-MnS pseudo-binary phase diagram a n d - t h e Fe-Mn-S t e r n a r y phase diagram. M o d i f i c a t i o n s of t h e s e diagrams from a d d i t i o n of t h e v a r i o u s

.Y d e ~ x i d i z e r s are q u a l i t a t i v e l y p r e d i c t e d and t a k e n i n t o account . It i s concluded

-. that Type I i n c l u s i o n s form by e x s o l u t i o n of l i q u i d s u l f i d e poo ls from t h e m e l t , Type TI1 i n c l u s i o n s form by s o l i d i f i c a t i o n of s u l f i d e c r y s t a l s o u t of t h e melt and Type I1 i n c l u s i o n s form by e u t e c t i c s o l i d i f i c a t i o n of t h e s u l f u r - r i c h f i n a l

L'

liquid, I n t e r p r e t a t i o n of s t r u c t u r e s observed s u g g e s t s t h a t Type I and p o s s i b l y T y p e 111 i n c l u s i o n s are "pushed" ahead of growing d e n d r i t e s . Morphology of i n c l u s i o n s j,s adequa te ly p r e d i c t e d on t h e b a s i s of on ly t empera tu re and mode of s o l i d i f i c a t i o n ; such f a c t o r s a s s u r f a c e energy d i f f e r e n c e s between d i f f e r e n t i n c l u s i o n types need n o t b e p o s t u l a t e d t o e x p l a i n t h e i r d i f f e r e n t morphologies .

- I TABLE Of CONTENTS <- .

d

Chapter Page Number I Number

. . . . . . . . . . . . . . . ABSTRACT i

I11 EXPERIMENTAL PROCUDURE . . . . . . . . . . 8

. . . . . . . . . . . . . . . . I V RESULTS 11

S e r i e s I.AISI 4330 Specimens Containing Various Amounts of Su l fu r . . . . . . . . . . . . . . . 11

s e r i e s II.AIS1 4330 Specimens Containing 0 . 1 Percent S and va r ious Amounts of Oxygen . . . . 13

Se r iks III.ATS1 4330 Specimens Containing 0 . 1 Percent S and Deoxidized with Various Amounts of Aluminum . . 1 4

s e r i e s IV.AISI 4330 Specimens Containing 0 . 1 Percent S and Various Amounts of Aluminum and S i l i c o n . . . 1 4

S e r i , e s V.AISI 4330 Specimens Containing 0 . 1 Percen t S and ;Various Amounts of Aluminum and Manganese . . . 15

s e r i e s VI.AIS1 4330 Specimens Containing 0 . 1 Percent S and Various Amounts of S i l i c o n and Manganese . . 15

series VII.AISI 4330 Specimens Containing Various Amounts of S i l i c o n , Manganese and Aluminum . . . . 16

series VIII .AISI 4330 Specimens Containing 0 . 1 Percent S , a High Oxygen Concentrat ion and Various Amounts of S i l i c o n , Aluminum, and Manganese . . . 16

Chapter Number

iii . .

. . . . . . Page Number

Series IX,AISI Specimens Containing 0 .1 Percent S and Deoxidized with Ei the r Zirconium, Boron, Cerium. . , 17

Series X.AISI 4330 Specimens Containing 0.5 Percent . . . . . S and Various Amounts of Manganese and S i l i c o n 18

. . . . . . . . . . . . . . . . . . DISCUSSION 20

. . . . . . . . . . . . . . . CONCLUSIONS 30

REFERENCES . . . . . . . . . . . . . . 33

Figure Number

1,

2 0

3 .

4,

5 o

6.

7.

8.

9 .

LIST OF FIGURES IV

Page Number

Photomicrograph o f A I S I 4330 - 0.1% S specimen, s o l i d i f i e d at 40°C/mina (a) ~ i c h e d , 300X. Type I s u l f i d e inc lus ions . (b) Unetched, 200X. Mixed Ty$e 1-11 s u l f i d e inc lus ions . 40

~ h o t o m i c r o ~ r a ~ h s of two AISI 4330 specimens conta in ing 0.01% S and 1.0% S , r e s p e c t i v e l y , s o l i d i f i e d a t 4 0 ° ~ / m i n . , 1000X. (a) Type I s u l f i d e i n c l u s i o n s , (b) Type I1 s u l f i d e inc lus ions . 41

Photomicrographs of a n A I S I 4330-0.1% S specimen, s o l i d i f i e d a t 70°C/min., 200X. Unetched, showing i n c l u s i o n morphology~ on success ive s u r f a c e s obtained by po l i sh ing down the specimen. Note how inc lus ions t h a t appear rod- ' l ike i n one s e c t i o n appear as rods on ad jacen t s e c t i o n . Sec t ions approximately 200 microns a p a r t , 42

Photomic~ographs of AISI 4330 - 0.1% S specimen, 250X. (a) Unetched, (b) Same a r e a , e tched wi th Rosenhain's r eagen t , showing Type I1 i nc lus ions a l ignkd along d e n d r i t e arm boundaries .

Schematic r e p r e s e n t a t i o n of a c a s e of Type I1 e u t e c t i c i nc lus ions a t d e n d r i t e arm boundaries . 44

Average diameter of Type I s u l f i d e i n c l u s i o n s versus cool ing r a t e . A I S T -4330 - 0 .1% S specimens. 4 5

Photomicrographs of AISI 4330 - 0.1% S showing f i n e r Type I1 e u t e c t i c i nc lus ions a t h igher cool ing r a t e , 500X. Cooling r a t e s a r e : (a) 7O0C/min,, (b) 10°C/min, 4 6

Average spacing of Type I1 e u t e c t i c i n c l u s i o n s versus cool ing r a t e . A I S I 4330 - 0.1% S specimens. 4 7

Photomicrographs,of A I S I 4330 - 0.1% S - 0.08% 0 specimen s o l i d i f i e d a t 40°C/min., 1000X. (a) Center of t h e Ingo t , (b) reg ion a t h a l f d i s t a n c e between c e n t e r and edge of t h e i n g o t , (c) edge of t he ingo t . 48

10. Type of s u l f i d e i n c l u s i o n s versus A1 and S i add i t i ons . A I S I 4330 - 0.1% S specimens s o l i d i f i e d a t 40°C/min. 4 9

11, Photomicrographs of AISI 4330 - 0.1% S - 0.2T Si - 0.3% A 1 specimen s o l i d i f i e d a t 40QC/min. Mixed Type 11-111 s u l f i d e inc lus ions . (a) 500X, (b) 1000X. 50

3'

12. Photomicrographs ;of A I S I 4330 - O.l%S - 0.2 % S i - 0.4% A1 specimen s o l i d i v i e d at 4O0C/min., 5OOX. Type 111 s u l f i d e inc lus ions . 5 1 r.

13. Type of s u l f i d e i n c l u s i o n s versus A 1 and Mn add i t i ons . AISI 4330 - 0.1% S specimens s o l i d i f i e d a t 4O0C/min. 5 2

I Figure Number

Page Number

14. Type of s u l f i d e inc lus ions versus Mn and Si add i t i ons . AISI 4330 - 0.1% S specimens s o l i d i f i e d at 40°C/min. 53

15. Photomicrographs of AISI 4330 - 0.1% S - 0.3% Mn - 0.3% S i specimen s o l i d i f i e d a t 4O0Clmin., showing Type I r i n g - l i k e s u l f i d e i n c l u s i o n s ad j acen t t o Type I11 and Type I1 i nc lus ions . (a) SOOX, (b ) 500X, (c) I O O O X , (d) 1000X. 54

16. Photomicrograph of AISX 4330 - 0.1% S - 0.3% S i - 0.3% Mn specimen s o l i d i f i e d a t 40°C/min., 1000X. Type I1 sulfide i n c l u s i o n s showing i n t e r n a l e u t e c t i c s t r u c t u r e . 55

17. Type of s u l f i d e inc lus ions versus A l , Mn, and S i add i t i ons . A I S I 4330 - 0.1% S specimens s o l i d i f i e d a t 40°C/min. 5 6

18. Photomicrographs of AISI 4330 - 0.1% S - 0.4% Zr specimen s o l i d i f i e d a t 40QClmin., 500X. ( a ) , ( b ) , and (c) show s u l f i d e i n c l u s i o n s , (d) shows zirconium s i l i c a t e i nc lus ions (Z) ad jacen t t o s u l f i d e inc lus ions (S ) . 57

19. Photomicrographs of AISI 4330 - 0.1% S - 0.3% B specimen s o l i d i f i e d at 4 0 ° ~ / m i n . , (a) unetched, 200X, (b) etched, 200X, ( c ) and (d) unetched , 500X. 58

. .

20. Photomicrograph of AISI 4330 - 0.1% S - 0.1% C e specimen s o l i d i f i e d a t 4 0 ° ~ / m i n . , 1000X. Unetched. Type I mult iphase s u l f i d e inc lus ions . 59

21. Photomicrographs of A I S I 4330 - 0.5% S - 2% Mn specimen s o l i d i f i e d . a t 40" C/min. , showing mixed Type 1-11 s u l f i d e I n c l m i o n s . (a) 200X, (b) a d i f f e r e n t reg ion , 500X. 60

22 . Photomicrographs of A I S I 4330 - 0.5% S - 2% Mn - 0.3% S i specimen s o l i d i f i e d a t 40°C/min. showing mixed Type 11-111 inc lus ions . ( a ) 200X, (b) , ( c ) , and (d ) 500X. 6 1

23. Schematic r e p r e s e n t a t i o n of t h e Fe-Mn-FeS-MnS p a r t of t h e Fe-Mn-S phase diagram. 62

24. The four b ina ry phase diagrams of rhe Fe-Mn-FeS-MnS system and the b a s a l p r o j e c t i o n of t h e t h r e e e u t e c t i c valleys and of t h e contour l i m i t i n g t h e m i s c i b i l i t y gap. The p r o j e c t i o n of the t e rna ry e u t e c t i c p lane appears shadowed. 6 3

25. ( a ) Basal p r o j e c t i o n of t h e Fe-Mn-S system showing t h e m i s c i b i l i t y gap, t i e l i n e s across t h e gap, and t h e e u r e c t i c v a l l e y s . (b) Basal p r o j e c t i o n of the Fe(C saturated)-Mn-S system i n d i c a t i n g t h e s h i f t i n g of the m i s c i b i l i t y gap, of the tie l i n e s across t h e gap and of t h e e u t e c t i c v a l l e y s . 64

F f gure Numb er

26. (a) Schematic r e p r e s e n t a t i o n of the pseudo-binary Fe-MnS phase diagram obta ined by sec t ion ing t h e t e rna ry Fe-Mn-S phase diagram by the vertical plane whose trace on t h e basal plane i s the diagonal Fe-MnS, (b) Schematic s h i f t i n g of t h e equi l ib r ium l i n e s between the various phases by a d d i t i o n of deox id i ze r s ,

27. Schematic r e p r e s e n t a t i o n of the s o l i d i f i c a t i o n path of melt J. 28. Schematic representation of the s o l i d i f i c a t i o n paths of three

d i f f e r e n t m e l t s & ' ,

Page Numb e r

CHAPTER I - INTRODUCTION

Research has been conducted on s o l i d i f i c a t i o n of s t e e l c a s t i n g s a t

Massachuserts I n s t i t u t e of Technology s i n c e l a t e 1958. Research i n t h e yea r s 1958-

1963 was on foundry techniques f o r improving t h e mechanical p r o p e r t i e s of steel

castings. Techniques were developed, i nc lud ing " u n i d i r e c t i o n a l s o l i d i f i c a t i o n " ,

f o r ach iev ing major improvements i n p r o p e r t i e s , Seve ra l of t h e more g e n e r a l papers

desc r ib ing t h i s work a r e re fe renced (1-6)

Work i n the pe r iod 1963-1967 concent ra ted on s tudy of s t r u c t u r a l

v a r i a b l e s known t o i n f l u e n c e p r o p e r t i e s of steel c a s t i n g s . Detai led fundamental

s t u d i e s were conducted on formation of microsegrega t ion i n s teel c a s t i n g s .

Extens ive comparison of theory w i th experiment was made. Homogenization k i n e t i c s

were s t u d i e d , and compared wi th theory ; t h e s e s t u d i e s l e d t o t h e important works

of Quigley and he ern^ and o t h e r s showing t h e major improvements i n p r o p e r t i e s i n

low a l l o y steel ob ta inab le from h igh remperature homogenization t rea tments .

Microporosi ty was s t u d i e d and techniques developed f o r its eva lua t ion t h a t have

become u s e f u l i n d u s t r y t o o l s . This work has been summarized i n fou r r e p o r t s and

8- 18 seven t e c h n i c a l papers .

Research on t h i s program s i n c e late 1967 has been pr imar i l y on ano the r

s t r u c t u r a l . v a r i a b l e known t o be s t r o n g l y i n f l u e n t i a l on p r o p e r t i e s of s t e e l

c a s t i n g s ; i n c l u s i o n s . Work has been on i n c l u s i o n formation and growth, and on

in f luences of i n c l u s i o n morphology and chemistry on mechanical p r o p e r t i e s ; t h i s

is the f i r s t r e p o r t dea l ing wi th t h a t work. Work c u r r e n t l y i n progress on e f f e c t s

of i n c l u s i o n s on mechanical p ro 'per t ies will be summarized i n a la ter r e p o r t .

I n i t i a l work, and t h a t reported h e r e i n , has been p r imar i l y on developing a . . .

fundamental understanding of i n f luence of s o l i d i f i c a t i o n variables on morphology

of sulfide inclusions. Work.has been both experimental and t h e o r e t i c a l - t h e o r e t i c a l

work being pr imar i ly on detailed description of s eg rega t ion and s o l i d i f i c a t i o n i&'v

multicomponent ferrous a l l o y s (e.g., Fe-Mn-S).

CHAPTER I1 - LITERATURE SURVEY

A s i g n i f i c a n t amount of work has been publ ished on t h e morphology and

e f f e c t on mechanical p r o p e r t i e s of s u l f i d e inc lus ions i n i r o n , low and medium

carbon steel and low a l l o y steel (19-47) , ~ e n t r u p (19) s t u d i e d t h e formation of . .

i nc lus ions i n t he i ron - su l fu r (Fe-FeS), iron-sulfur-oxygen (Fe-FeO-FeS), i ron-

manganese-sulfur (Fe-FeS-MnS-Mn) and iron-manganese-oxygen~sulfur (FeO-MnO-FeS-

MnS) systems. Vogel e t a1 (20) s tud ied t h e iron-manganese-sulf u r sya tern i n more

d e t a i l , publ ished s e v e r a l s e c t i o n s of t h i s t e rna ry diagram, and proposed t h e

pseudobinary iron-manganese s u l f i d e diagram. Both Wentrup (19) and Vogel e t a 1 (20

found var ious s u l f i d e morphologies analogous t o t h e s u l f i d e types l a t e r def ined

by ~ i r n s ( ~ ~ ' and Sims and These au thors examined t h e e f f e c t s of va r ious

deoxid izers on the morphology of s u l f i d e inc lus ions i n low and medium carbon s t e e l s

i n the as-cas t condi t ion . Deoxidizers s t u d i e d were: manganese, aluminum,

t i t an ium, zirconium, and misch metal . They found t h a t , i n normal s t e e l s conta in ing

more than 0 . 5 . p e r c e n t manganese, s u l f i d e s are e s s e n t i a l l y MnS and can be c l a s s i f i e d

b r i e f l y as Types I, 11, and 111.

Type I inc lus ions a r e randomly d ispersed spheres having a wide range of

sizes; Type I1 i nc lus ions appear a s cha ins or s t r i n g e r s along g r a i n boundaries and

Type I11 i nc lus ions appear as randomly s c a t t e r e d , f ace t ed c r y s t a l s . Type I1

i nc lus ions are h igh ly undes i rab le lead ing t o a m a t e r i a l of poor d u c t i l i t y , Type I

inc lus ions l e a d t o a s u b s t a n t i a l l y more d u c t i l e material and Type I11 i nc lus ions

a r e no t as des irable a s Type I b u t a r e c e r t a i n l y p r e f e r a b l e t o Type 11, f o r a

given s u l f u r Manganese, though a weak deoxid izor , was found t o improve

(22 ) the a c t i o n o f s t r o n g e r d e o x i d i z e r s l i k e aluminumandsiliconwhenpresent .

Aluminum is a p o w e r f u l deoxid iaer b u t no t a s powerful a s zirconium.

Small add i t i ons of aluminum may b e s u f f i c i e n t f o r deoxfdat ion b u t lead t o Type I1

i nc lus ions . Larger add i t i ons of aluminum a r e necessary t o o b t a i n Type 111 i n c l u s i o n s ,

Titanium above 0.015 percent leads t o Type I1 i n c l u s i o n s , Type I11 i nc lus ions are

not obtained no ma t t e r how much t i tan ium i s added. Zirconium, on t h e o t h e r hand,

l eads t o Types I1 o r ILI according t o t h e amount added t o t h e melt. Misch me ta l

converts Types 11 and IT1 s u l f i d e s (formed by add i t i ons of aluminum and zirconium)

back t a Type I , However, i t has no e f f e c t on Type I1 i n c l u s i o n s o b t a i w d by

(23) deoxid iz ing the mel t w i th t i t an ium

Sims has attempted t o exp la in the formation of Type 111 s u l f i d e inc lus ions

by surface t ansfon considerat ions(22 ' . H e assumed an excess of aluminum o r zirconium

inc reased tha su l f ide-mat r ix i n t e r f a c i a l energy, r e s u l t i n g i n shr inkage and formation -

of l i q u i d pools a t reg ions where more chan two grain boundaries meet. ~ c h k m a n n I

s t u d i e d i n great d e t a i l the Fe-FeS-MnS-Mn phase diagram and i t s - m o d i f i t i a t l o n when

carbon i s added to t h e system'21). Van Vlack et a1 (24) exmined t h e dependence of

the mic ros t ruc tu re of sulfide i n c l u s i o n s upon steel composition and analyzed the

behavior of s u l f i d e s a t s t e e l r o l l i n g temperatures. The e f f e c t s of morphology and I

compasftion of s u l f i d e i n c l u s i o n s on hot -shor tness of s t e e l and on improvement of

m a c h i n ~ b i l i t y were a l s o s t u d i e d , and t h e e f f e c t of t h e s u l f u r l e v e l on the s u r f a c e

( 2 4 ) q u a l i t y of ho t - ro l l ed s t e e l s t r e s s e d

The nature, composition and phys i ca l p r o p e r t i e s (mainly microhardness) I

of s u l f i d e inc lus ions , both real and s y n t h e t i c , of the (Mn, Me)S type (where Me

is a transition meta l of t h e f i r s t long per iod l i k e chromium, t i t an ium, vanadium

o r i r o n ) were ex tens ive ly s t u d i e d by Kies s l ing e t a1 '25-28) and were c o r r e l a t e d

w i t h the domposition of the base-metal. The importance of t h e hardness and :, . .

p l a s t i c i t y of i n c l u s i o n s and of t h e dep le t ion of t h e ma t r ix around them wi th

r e spec t f o def o m a b i l i t y and machinabi l i ty of t h e mat r ix was s t r e s sed (25) . Some

( 2 6 ) types of duplex su l f ide-oxide o r s u l f i d e - s i l i c a t e i nc lus ions were examined ,

( 2 9 ) composition s t u d i e s of (Mn, Fe)S inc lus ions w e r e a l s o r epo r t ed by Matsubara . Boulger (30) examined t h e e f f e c t of s u l f u r and selenium on t h e machinabi l i ty and

t e n s i l e p r o p e r t i e s of a 5 percent chromium s t e e l . Gaydos (31) i nves t iga t ed

t h e e f f e c t s of s u l f f d e and s i l i c e o u s inc lus ions i n free-machining s t e e l s . He

examined t h e na tu re of duplex inc lus ions as a func t ion of t h e i r p o s i t i o n i n

t h e ingo t and c o r r e l a t e d i t t o t he oxygen level i n t h e s t e e l a t t h a t p a r t i c u l a r

p o s i t i o n , Detrez (32s33) redef ined the t h r e e types of s u l f i d e inc lus ions i n

-..J s t e e l s and found t h a t t h e c r i t i c a l aluminum l e v e l s f o r t he t r a n s i t i o n between

Types I and 11, and I1 and I11 t o occur depend, on t h e s u l f u r , oxygen, and t h e r e f o r e *

carbon l e v e l s , With a h igh s u l f u r l e v e l (above 0 , l percent ) and a very low carbon

level (below 0.05 percent ) i t was found almost impossible t o o b t a i n Type 111

s u l f i d e inc lus ions . Detrez (32s33) s t u d i e d a l s o t h e Charpy impact s t r e n g t h of

s t e e l a s a func t ion of t h e e x i s t i n g type of s u l f i d e inc lus ions .

Araki e t a1 ( 3 4 ) s t u d i e d the na tu re and p l a s t i c behavior of s u l f i d e

inc lus ions during forg ing of a r e s u l f u r i z e d low carbon s t e e l conta in ing chromium

and molybdenum. Melf ord (35) appl ied x-ray scanning microanalys is t o s tudy t h e

composition of duplex sulfide i nc lus ions i n h igh chromium s t e e l . Lichy e t a1 ( 3 6 )

* ..LJ s tud ied t h e c o n t r o l of s u l f i d e shape a f t e r r o l l i n g i n low carbon aluminum-killed

s t e e l . They found t h a t t h e heavy s t r i n g e r s u l f i d e s occurr ing i n non-zirconium s

bearing s t e e l after r o l l i n g were changed i n t o an i n t e r e s t i n g oval type when

zirconium 0.10-0.18 percent w a s p r e sen t , ~ g c k e r e t a1 (37' examined t h e n a t u r e

and composition of s u l f i d e inclusions i n r e s u l f u r i z e d c a s t s t e e l ' h a v i n g an

I.mpx.awd machinabil i ty . They a l s o inweistfgated the d i s t r f b u t i on of elements l i k e

manganese throughout t h e speci.men, mameby inside the matrix, t h e carb ide inclusions,

the slulfida f:n~clmsions and the oxides, Kondo et a1 (38) s tudf ed br ie ,£ ly t h e

ccmpl;~a'f~tji_on s f i nc lus ions in r e s u l f w i z e d high-speed steel and Araki et a1 (39)

~ ~ n d u c t a d similar studies on the e f f e c r of molybdenum, carbon and other elements

on s u l f i d e inch.mions i n haw carbon stee l , Salmon Cox e t a 1 (40' examined the

nature, t ype and d i s t r i b u t i o n of s u l f i d e s and duplex s u l f i d e - s i l i c a t e inclusions

across an i ngo t of 0,2 percent carbon s teel , They found t h a t Type I randomly

d i spe r sed sulfide inelusions csntainiwg a l so a s i l i c a t e phase are more f requent In

the wBmnar and branched r e g i m s of t h e i n g o t , whereas Type I1 e u t e e t i c inclusions

were m o w frequent in t he aquiaxed reg ion of t h e - i n g o t . They concluided that the

silicate phase has bean rejected from l i m i t e d s o l i d s o l u t i o n with the s u l f i d e

(411 ae opposed to Raxmazin who postulated that the s i l i c a t e phase is i n fact a

par t ic le acting as nucl@atl,ng agent of the s u l f i d e ,

The nntarre la t ionshfp between PeS and MnS i nc lus ions was examined by

Whickley . Nicodemil '43' examined the morphology and t h e effect of coa l ing rate

on non-metallic inclusions i n s t a i n l e s s steel castings, He observed t h a t the

s i z e of sulfides or duplex oxide-su l f ide inc lus ions decreases with increasing

zooling ra te , Analogous observa t ions were made by other investigators (34,441 I

who notdced the cossseapng of s u l f i d e p a r t i c l e s with d i s t ance from the i ngo t

edge, Y e 0 '45' s t u d i e d the effect of oxygen content and deoxideeion by silicon, I

boron, alminum and carbon in se su l fu r f aed s t e e l s on s u l f i d e morphology and

machinabilf ty, He found t h a t sulfide inclusions are hardened by oxygen dur ing I

r o l l i n g , s o tha t they remain more globular , , Dahl e t al (46' investigated in

detail the candi t ions of occurrence of t h e va r ious types of s u l f i d e i nc lus ions in

l o w carbon s teel and che a f fec t of various deox id i ze r s and attempted an

explanation of t h e var ious morphological observa t ions on t h e i n c l u s i o n s based on

the phase diagram of the system.

The effect on inc lus ions of t h e v a r i a t i o n i n i n i t i a l oxygen content has

been recognized and s t u d i e d by C r a f t s and ~ i l t ~ ' ~ ~ ) . These authors examined i n

d e t a i l t h e structure of i nc lus ions i n the iron-oxygen-sulfur system as w e l l as

i n the system modified by add i t i ons of manganese, aluminum, and s i l i c o n ,

CHAPTER 111 - EXPERIMENTAL PROCEDURE

Seve ra l s e r i e s of c y l i n d r i c a l i n g o t s (1.1" dfa . x 1.1" high) were prepared I

of AISI 4330 low a l l o y s t e e l whose nominal composition is given i n Table I. I n

these ingo t s t h e s u l f u r l e v e l w a s genera l ly brought up t o 0 .1 percent . Deoxidatian

was conducted using va r ious deoxid izers as i n d i c a t e d i n Table T I . Melting and

s o l i d i f i c a t i o n were conducted i n a Balzer USG 10 vacuum induc t ion u n i t wi th a

30 KW, 10 KC power source. Samples were hea ted i n d i r e c t l y using a g r a p h i t e

susceptor . Specimens of A I S I 4330 s t e e l weighing approximately 100 gms. were I

loaded i n fused s i l i c a c r u c i b l e s toge ther w i th t h e corresponding amount of s u l f u r

added as pure FeS, and ' i n some cases with the adequate amount of desx id i ze r .

The system was melted under 112 atmosphere of argon superheated t o approximately

1585'C (corresponding to about 100QC supe rhea t ) , he ld i so thermal ly f o r 10 minutes

i n order t o homogenize t h e mel t and subsequently cooled down f i r s t a t an average

r a t e of 15"C/min. by decreasing t h e power, and l a t e r on, a t t h e onse t of

nuc lea t ion of t h e ma t r ix , a t a r a t e of about 7O0C/rnin. by s h u t t i n g of f t h e power.

Undercooling be fo re nuc lea t ion of t h e s t e e l was in ten t ' iona l ly prevented by

nuc lea t ing t h e melt wi th an i r o n wi re .

Deoxidizers , such ' a s Mn, and S i , do no t r e a c t apprec iab ly wi th t h e

s i l i c a c ruc ib l e . Theywere added t o t he system d u r i n g . t h e melt-down period. I

Deoxidizers l i k e Al , z r , B which r e a c t with t h e s i l i c a c r u c i b l e were added a t

t h e end of the i so the rma l holding- per iod and were allowed t o d i s s o l v e and r e a c t

during the slow cooling per iod p r i o r t o nuc lea t ion of t h e mel t . One o r two weighed

p ieces of t h e deoxid izar were f a s t ened on a pure i r o n rod by means of pure i r o n wire.

The rod was lowered ingo t h e m e l t a t the appropr i a t e moment. Because t h e t i m e I

a v a i l a b l e f o r r e a c t i o n i s s h o r t , no apprec iab le r e a c t i o n i s be l ieved t o occur

\ * between s i l i c a c r u c i b l e and deoxid izer . Temperature was measured and recorded

using a ~ t / P t - 1 0 percent Rh thermocouple and a Moseley Autograf s t r i p c h a r t

recorder .

Specimens thus prepared were pol i shed and examined me ta l log raph ica l ly ,

gene ra l ly unetched but i n some cases etched wi th Rosenhain's reagent . This

e t c h a n t , used f o r observing t h e d e n d r i t i c s t r u c t u r e , c o n s i s t s of an aqueous

s o l u t i o n of copper, i r o n and s tannous ch lo r ides . Some specimens were s t u d i e d

using e l e c t r o n microanalys is . Inc lus ions were a l s o examined by x-ray scanning

a n a l y s i s .

Measurement of volume percent of i n c l u s i o n s w a s done by q u a n t i t a t i v e -1

metallography. A two-dimensional sys t ema t i c p o i n t count was used fol lowing

.L

procedure of Cahn and ~ i l l i a r d ' ~ ~ ) . A sys t ema t i c a r r a y of p o i n t s was used,

provided by corners of a two-dimensional 10 x 10 g r i d having 100 p o i n t s . The

g r i d w a s t r aced on t h e metal lograph on which t h e photomicrograph is p ro j ec t ed .

A coarse-mesh l a t t i c e c r i t e r i u m is t h e following:

P n = O f o r n > 2 -

where p i s t h e p r o b a b i l i t y t h a t a f e a t u r e w i l l occupy n l a t t i c e po in t s . n

a - I n c l u s i o n diameter measurements were c a r r i e d out by making a number of

random t r a v e r s e s ac ros s photomicrographs of t h e sample, counting t h e number of

i n c l u s i o n s , NL, crossed by rhe l i n e and measur ing t h e t o t a l l eng th of l i n e L

- i n t e r c e p t e d by t h e i n c l u s i o n s . The mean l i n e a l i n t e r c e p t , L, was obta ined from

-

i The average spacing, L , of Type I1 inc lus ions was measured by drawing * I j

a l i n e through t h e center of t h e inc lus ions and measuring the number of inc lus ions , -

NL, i n t e rcep ted pe r unit l ength of l i ne . The average spacing, L , is then given

by = ( 1 1 ~ ~ ) .

CHAPTER IV - RESULTS

The meta l lographic observa t ions made on var ious etched and unetched

specimens may be summarized a s fol lows:

S e r i e s I. A I S I 4330 Specimens Containing Various Amounts of Su l fu r

Specimens of A I S I 4330 low a l l o y s t e e l were prepared conta in ing 0.01,

0.02, 0.05, 0 .1, 0 .2 , 0.5 and 1.0 percent . As expected, i t was observed t h a t

t h e volume f r a c t i o n of s u l f i d e s i nc reases wi th inc reas ing amount of s u l f u r .

The type of i nc lus ions i s mixed; t h e r e i s a coexis tence of Type I and I1 i n c l u s i o n s ,

Inc lus ions of Type I a r e i n d i v i d u a l spheres o r sphero ids of a wide s i z e d i s t r i b u t i o n

s c a t t e r e d i n the i n t e r d e n d r i t i c spaces , Figure l a , Type I1 i nc lus ions appear as

r o s a r i e s of beads o r chains, or s t r i n g e r s , along g r a i n boundaries . I t i s sometimes

d i f f i c u l t t o d i s t i n g u i s h between Type I and Type I1 i nc lus ions by examining a s i n g l e

pol i shed s u r f a c e . Ambiguity i s e l imina ted by succes s ive ly polishing down the

specimen and examinfng t h e new s u r f a c e s , F igure lb is a t y p i c a l photomicrograph

of a specimen conta in ing 0 . 1 percent S , showing t h e coexis tence of t h e two types

of i nc lus ions .

With inc reas ing amount of s u l f u r , t h e r e i s an obvious s h i f t i n g of

i n c l u s i o n type from I t o 11, Figure 2 . Inc lus ion type is c l o s e t o I i n the

specimen containing 0 .01 percent S and c l o s e t o 11 i n t h a t conta in ing 1.0 percent

S . Because of t h e coexistence of i nc lus ions of Types I and 11 t h e s t r u c t u r e may

be cha rac t e r i zed by a number such as t h e " r e l a t i v e volume f r a c t i o n of Type I1 and

Type I i nc lus ions . " Thus, when t h e r e l a t i v e volume f r a c t i o n of Type I1 i n c l u s i o n s

i s h ighe r than 90 pe rcen t , the specimen is s a i d h e r e i n t o p re sen t Type I1 s u l f i d e s .

I f t h e r e l a t i v e volume f r a c t i o n of Type I1 i nc lus ions i s lower than 10 pe rcen t ,

t h e specimen is s a i d t o p re sen t Type I s u l f i d e s .

With inc reas ing s u l f u r l e v e l , t h e s i z e of i nc lus ions inc reases . A t

h igh magni f ica t ion , they appear t o c o n s i s t of two phases i d e n t i f i e d a s (Mn,Fe)S

and FeS*. Inc lus ions i n specimens conta in ing more than 0.05 percent S appear t o

have s i l i c a t e o r oxide cores which i n c r e a s e i n s i z e wi th s u l f u r l e v e l and

i n c l u s i o n size.

Because t h e s u l f u r l e v e l has such a s i g n i f i c a n t effect on t h e type

of i n c l u s i o n s , specimens i n which t h e e f f e c t of va r ious deoxid izers was s tud ied

were made wi th a cons tan t s u l f u r con ten t , namely 0 . 1 percent . The advantage of I

t h i s va lue i s t h a t f o r t h i s s u l f u r l e v e l t h e s u l f i d e morphology is q u i t e analogous

t o t h a t expected i n non-resulfur ized A I S I 4330 s t e e l specimens. Conclusions

based on morphological observa t ions made on specimens conta in ing 0 . 1 percent S 5 -

may, t he re fo re , be ex t r apo la t ed t o non-resulfur ized specimens. !

The bulk of t$e work r epor t ed i n subsequent s e c t i o n s - ( o n e f f e c t of

va r ious deoxid izers on i n c l u s i o n morphology) w a s conducted on 0 .1 percent S

mel t . De ta i l ed examination of t he morphology of Type I1 i nc lus ions i n specimens

conta in ing 0 . 1 percent S shows, f i r s t , an alignment of beads and s t r i n g e r s ,

F igure 3. Pol i sh ing dawn and examining t h e success ive su r f aces r evea l s t h a t i n 1

f a c t t h e beads and strihgers are in te rconnected a t ' some l e v e l ; F igure 3 shows

s e v e r a l of a large numb& of such s e c t i o n s s tud ied . The morphology of t h e , s u l f i d e s I (49) as a group is t h a t of a d e g e n e r a t e e u t e c t i c , very s i m i l a r t o t h a t of a

,- . "chinese scripturn" consis ring of i r r e g u l a r l y c y l i n d r i c a l rods. Thus, f o r t h i s y

p a r t i c u l a r case, t h e c l a s s i c a l "beads" a r e i n r e a l i t y i n t e r s e c t i o n s of " s t r i n g e r s " --

by . t he pol i sh ing plane. F igure 4 shows t h e same area. of t h i s specimen etched and

unetched. I t appears c l e a r l y that e u t e c t i c i n c l u s i o n s a r e spread along d e n d r i t e

* I n r e f l e c t e d n a t u r a l l i g h t (Mn,Fe)S has a dove gray co lo r , whereas FeS has a very l i g h t gray co lor .

arm boundaries and not along g r a i n boundaries a s gene ra l ly supposed. F igure 5

r ep re sen t s schemat ica l ly t h e morphology of the e u t e c t i c Type 11 i n c l u s i o n s around

a d e n d r i t e arm.

The examination of t h e samples conta in ing va r ious amounts of s u l f u r

showed t h a t wi th inc reas ing d i s t a n c e from t h e ingo t edge- o r from a cool ing

s u r f a c e t h e s i z e of i nc lus ions inc reases and t h a t there i s a progress ive s h i f t i n g

of i n c l u s i o n morphology towards Type I, I n order t o s tudy more schemat ica l ly

t h e s e observa t ions , a s e r i e s of fou r i ngo t s of AISI 4330 steel conta in ing 0 . 1

percent were s o l i . d i f i e d a t four different cool ing r a t e s , 70, 40, 20, and 1O0C/min

by adequately ad jus t ing the power during t h e cool ing per iod . F igure 6 shows t h e

decrease i n average diameter of Type I inc lus ions (measured by l i n e a l a n a l y s i s )

versus i nc reas ing cool ing rate. F igure 3 shows t h e decrease i n e u t e c t i c rod

diameter and i n average e u t e c t i c spacing wi th inc reas ing cool ing r a t e , The

v a r i a t i o n of spacing i s pl .ot ted i n Figure 8.

Series 11. A I S I 4330 Specimens Containing 0 . 1 Percent Su l fu r and- var ious Amounts d Oxygen

Two i ngo t s were s o l i d i f i e d conta in ing more oxygen than would correspond

t o equi l ibr ium wi th t h e s i l i c a c r u c i b l e a t the i so thermal holding temperature.

The oxygen w a s added t o the mel t as Fe 0 of known ana lys i s . I n one i n g o t , 0.04 2 3

percent oxygen was added; t h e i n c l u s i o n r e s u l t i n g type was mixed 1-11. A t t h e

cen te r of t h e ingo t t h e inc lus ions were over 90 percent Type I and a t t he edge

they were over 90 percent Type 11. I n t he o t h e r i n g o t , 0,08 percent oxygen was

added; i nc lus ions were Type I. Su l f ides con ta in dark oxide co res , Figure 9 .

S e r i e s 111. A I S I 4330 Specimens Containing 0.1 Percent S u l f u r and Deoxidized wi th " i'

Various Amounts of Aluminum

Five A I S I 4330 i n g o t s were made conta in ing 0.05, 0.3, 0 .4, 0 .5, and

0.8 percent A1 r e s p e c t i v e l y . Aluminum was added t o t h e mel t at: t h e end of t h e

i so thermal holding per iod . A t up t o 0.3 percent A1 a d d i t i o n s , t h e type of

i nc lus ions was mixed 1-11, Beyond t h i s l e v e l , i nc lus ions were e s s e n t i a l l y of Type

11. No t r a c e s of Type, 111 i nc lus ions were observed wi th A 1 add i t i ons as high a s

(23) 0.8 percent as would b e expected from work by Sims and Sims and Briggs ( 2 4 ) on

low and medium carbon steel .

S e r i e s I V . AISI 4330 Specimens Containing 0 . 1 Percent Sul fu r and Various Amounts of Aluminum and S i l i c o n

A s e r i e s of 16 ingo t s of A I S I 4330 were made, conta in ing 0 . 1 percent S +

and va r ious amounts o f A1 and S i , Table 11. A meta l lographic examination of t h e *"

Type of i n c l u s i o n s l e d t o the compilat ion of p o i n t s i n F igure 10. Three curves

a r e shown, d iv id ing t h e space of t h e quadrant i n t o four reg ions . I n t he f i r s t

reg ion , corresponding t o very low S i and A1 con ten t s , i nc lus ions a r e of mixed

Type 1-11, I n t h e second reg ion , corresponding t o low A1 and S i conten t , I

i nc lus ions a r e of Type 11. I n t h e t h i r d reg ion , corresponding t o h igher A 1 and

S i con ten t s , i n c l u s i m s a r e mixed Type 11-111, Figure 11. I n t h e fou r th reg ion I

corresponding t o s t i l l h i g h e r A1 and S i con ten t s , t h e r e l a t i v e volume f r a c t i o n

of Type I11 i n c l u s i o n s exceeds 90 percent and the. type of s u l f i d e s i s . t h e n def ined

as 111. Type I11 inc lus ions appear as i n t e r s e c t i o n s of idiomorphic cubic c r y s t a l s i-

with t h e p lane of po l i sh ing . They seem s c a t t e r e d throughout t h e d e n d r i t i c s t r u c t u r e

b u t are mainly concent ra ted i n the i n t e r d e n d r i t i c spaces. A c l o s e s tudy of t h e s e *,

i n c l u s i o n s shows that: : (1) t h e i r cubic p a t t e r n s a r e o f t e n d i s tu rbed by "dendr i t i c "

p r o j e c t i o n s , and (2 ) !hey o f t en . su r round reg ions of Type I1 i nc lus ions and p r o j e c t

arms which j o i n t h e s e i n t e r d e n d r i t i c e u t e c t i c networks, (3) They o f t e n appear

cracked, F igure 12,

S e r i e s V, AISI 4330 Specimens Containing 0 . 1 Percent Sulfur and Various Amounts of Aluminum and Manganese

A series of seventeen ingo t s of A I S I 4330 specimens were made conta in ing

0,l percent S and various amounts of A1 and Mn, T a b l e 11. Figure 13 summarizes

meta l lographic observarions r e l a t i n g t o t h e r e s u l t i n g i n c l u s i o n type. For very low

Mn and A1 a d d i t i o n s , i nc lus ions are of the mixed Type 1-11. For low Mn and A1

a d d i t i o n s , they are of Type 11, f o r h igher Mn and A 1 a d d i t i o n s , they are of mixed

Type 11-111 and f o r still h ighe r add i t i dns they are of Type 111.

S e r i e s V I . AISI 4330 Specimens Containing 0 . 1 Percent Sul fur and Various Amounts of S i l i c o n and Manganese

Six A I S I 4330 i n g o t s were prepared conta in ing 0 . 1 pe rcen t S and

deoxidized wi th va r ious amounts of S i and Mn. Figure 14 shows t h a t f o r very low

Mn and S i con ten t s , i n c l u s i o n s are of mixed ~ y p e 1-11, whereas f o r higher contents

they are of Type 11. For h igh Mn and S i con ten t s , a mixed Type 11-111 i s obtained.

For campositions as high as 1 percent Mn and 1 percent S i , i nc lus ions are s t i l l

of the mixed Type 11-111.

Photomicrographs of a pol i shed specimen of t h i s s e r i e s , conta in ing 0 . 1

percent S-0.3 percent S i a r e g iven i n F igure 15. An apparent ly new type of

inc lus ions can be seen. They are g lobu la r , r i n g - l i k e wi th a matrix core , s i m i l a r

to Type I i n c l u s i o n s , Type I1 and Type 111 i n c l u s i o n s l i e next t o them. Type II

i n c l u s i o n s of t h i s specimen, observed a t h igh magni f ica t ion , e x h i b i t an i n t e r n a l

e u t e c t i c s t r u c t u r e , F igure 16.

S e r i e s V I I . AISI 4330 Specimens Containing 0 , l Percent Su l fu r and Various Amounts of S i l i c o n , Manganese, and Aluminum

I

The information included i n F igures 10, 13, and 14 was compiled i n a

three-dimensional geometric r e p r e s e n t a t i o n of t h e composi t ional regions i n which

mixed Type 1-11, Type 11, mixed Type 11-111 and Type I11 of i n c l u s i o n may be

expected, Figure 17.

The curves shown a r e approximate s i n c e experimental po in t s have been

obtained t o , d a t e only along t h e t h r e e or thogonal ly o r i en t ed p lanes , Addi t iona l

work is c u r r e n t l y i n p rdg res s on alloy compositions w i t h i n the space enclosed

by t h e t h r e e axes. To date, t h r e e ingo t s have been made with a l l t h r e e elements

( S i , Mn, A l ) p r e sen t , able 11.

Series V I I I . AISI 4330 Specimens Containing 0 . 1 Percent S u l f u r , a High Oxygen I

concent ra t ion and Various Amounts of S i l i c o n , Aluminum, and Manganese

Three ingo t s oE AISI 4330 conta in ing 0 . 1 percent S were made. The oxygen

l e v e l was increased by a d d i t i o n of 0 -02 percent Oxygen as Fe 0 The amounts of 2 3 '

A l , S i , and Mn added were t h e same as those added i n t h e t h r e e ingo t s of t h e

previous s e r i e s V I I . Thus, t he compositions of t h e two s e r i e s of i n g o t s d i f f e r e d

only by their r e spec t iveoxygen l e v e l s . S e r i e s V I I contained approximately 0.008

percent oxygen and ~ e r i e i V I I I approximately 0 ,02 percent Oxygen. The e f f e c t on

i n c l u s i o n morphology of t h i s oxygen contear i n c r e a s e i s t h a t the type of i nc lus ions

i n t h e h igh oxygen ingot& i s mixed 11-111 i n s t e a d of 111. This shows c l e a r l y +

%-

t h a t an inc rease of oxygp(n content moves the s u r f a c e s between reg ions II and 11-111 i

on one hand, and between reg ions 11-111 and 111 on the o the r , towards h igher va lues I i I

of A l , S i , and Mn. A s i m i l a r e f f e c t can l o g i c a l l y be expected by decreasing t h e

carbon l e v e l af this s t e e ' l .

S e r i e s IX. A I S I 4330 Specimens Containing 0 . 1 Percent Su l fu r and Deoxidized wi th E i t h e r Zirconium, Boron, Cerium

Addit ions of 0 ,4 percent Z r t o an AISI 4330 s t e e l i ngo t conta in ing 0 . 1

percent S leads t o Type 111 s u l f i d e inc lus ions . Low add i t i ons of 0.05 percent Z r

do not seem t o a f f e c t t h e i n c l u s i o n Type and morphology. Addit ions o f 0 . 1 percent

Ce o r misch:metal t o t h e same allow convert Type I1 i nc lus ions i n t o Type I.

Addit ions of only 0.05 percent B do not affect t h e Type of i n c l u s i o n s , These

morphological changes w i l l b e descr ibed i n more detail below:

Specimen Containing 0 .4 Percent Zirconium

Photomicrographs of t h e pol i shed and unetched specimen a r e given i n

F igure 18 , Type 111 s u l f i d e s appear as idiomorphic hexagonal e rys tals , Figures 18a -JI:

and b , s c a t t e r e d mainly between d e n d r i t e arms and occas iona l ly w i t h i n the dendr i t e s

themselves, Type 11 s u l f i d e s appear as elongated s t r i n g e r s along i n t e r d e n d r i t i c

spaces. By successiveLy po l i sh ing down and examining a l a r g e number of s e c t i o n s

i t has been found t h a t i n r e a l i t y t h e s e s t r i n g e r s a r e narrow lamel lae , o f t e n

a t tached t o Type I1 i n c l u s i o n s , I n addition, t h e s i m i l a r i t y of o p t i c a l p r o p e r t i e s

(such as co lo r i n n a t u r a l and po la r i zed l i g h t and index of r e f r a c t i o n ) of

c r y s t a l s and lamel lae i n d i c a t e t h a t presumably t h e s e two Types of s u l f i d e s

have very nea r ly t h e same composition, Prel iminary e l e c t r o n microprobe examination

has shown t h a t t h e s e inclusims con ta in about 20 percent Zr bes ides Mn and S .

Some Type 111 i nc lus ions p re sen t a dark co re , Figure 18b, which i s q u i t e probably

a zirconium silicate. Idiornorphic zirconium s i l i c a t e c r y s t a l s appear s c a t t e r e d

i n c e r t a i n regions of the specimen, Figure 18d.

Specimen Containing 0 .3 Percent Boron

Photomicrographs of t h i s specimen are given i n F igure 19. The d e n d r i t i c

s t r u c t u r e appears r e a d i l y without e tch ing ,Figure 1 9 ( a ) , because of s i g n i f i c a n t

e u t e c t i c formation i n t h e i n t e r d e n d r i t i c spaces , This

(Fe, Mn, C r , N i , Mo) phase and an (Fe, Mn, C r , N i , MO)

J #

e u t e c t i c involves an

bo r ide -phase , Coring i n s i d e ,

t h e d e n d r i t e arms appears by e tch ing l i g h t l y t h e specimen with ~ a r b l e ' s r eagen t ,

Figure 19b. Type I11 inc lus ions cons i s t i ng of i d io rno rph ic~csys t a l s a r e s c a t t e r e d

i n t h e i n t e r d e n d r i t i c spaces, mainly along d e n d r i t e arm boundaries , The r e l a t i v e

p o s i t i o n of t hese s u l f i d e s with r e spec t t o the e u t e c t i e appears c l e a r l y i n Figures

19c and d.

Specimen Containing 0 . 1 Percent Cerium

~ h o t o r n i c r o ~ r a ~ h s of t h i s specimen a r e given i n Figure 20. S u l f i d e

i n c l u s i a n s belong t o Type I. They a r e globular and are s c a t t e r e d between d e n d r i t e . >

arms. More than one phase appears i n s i d e t h e i n c l u s i o n s , The very dark phase i s

presumably an oxide o r a s i l i c a t e conta in ing Ce, t h e dark-gray phase i s a cerium-

r i c h s u l f i d e (CeS) and t h e l i g h t dove gray phase i s t h e usual Mn-rich s u l f i d e .

I n a d d i t i o n , a s i n Figure 20 d , some of t he i ron - r i ch mat r ix m a t e r i a l occas iona l ly

appears w i t h i n t h e inc lus ion ,

S e r i e s X . AISL 4330 Specimens Containing 0 2- Percent . Sulfur- and- Various Amounts of Manganese and S i l i c o n

Photomicrographs of t h e specimen conta in ing 2 percent Mn are given i n

Figure 21. Some regiods of t he specimen e x h i b i t t h e s t r u c t u r e analogous t o t h a t

of Figure 21a. The i n4 lus ion Type i s mixed 11-111. Type I1 i nc lus ions a r e

s c a t t e r e d a t ' s i t e s whene two or t h r e e boundaries meet. Other reg ions of t he I

specimen e x h i b i t t h e ~ { r u c t u r e of F igure 21b, Here, t h e r e i s a continuous and

wide network of e u t e c t i c s u l f i d e inc lus ions along t h e d e n d r i t e arm boundaries .

Figure 22 shows photomicrographs of t h e specimen containing 2 percent

Mn-0.3 percent S i . Type 11 inc lus ions form wide e u t e c t i c networks between

dendrite arm boundaries. These:networks are surrounded by Type-111 inclusions

appearing as idiomorphic crys ta l s often with arms'projecting inside the eutectic

regions,

CHAPTER V - DISCUSSION

The o r i g i n and s o l i d i f i c a t i o n behavior of t h e va r ious types of s u l f i d e

inclusions in the AISI 4330 low a l l e y s teel w i l l be d i scussed in t h e present

Chapter, Low a l l o y s t e e l being a r a t h e r complex chemical s y ~ t e m , s i m p l i f i c a t i o n s

w i l l b e made i n o rde r t h a t a v a i l a b l e phase diagrams be used f o r understanding the

s o l i d i f i c a t i o n process ,

The va r ious types of s u l f i d e i n c l u s i o n s encountered i n t h e present s tudy

are b a s i c a l l y (Mn, P ~ ) S s o l i d s o l u t i ~ n s ; the average oxygen concen t r a t i on of t h e

melts, approximately 0.003 percent (30 ppm), was low enough t h a t oxygen i s

reasonably neg lec t ed i n discussnng t h i s mul t ic~mponent system, It i ~ ~ t h e r e f o r e ,

logical tha t the a n a l y t i c a l examination of the Fe-Mn-S system be t h e backbone of

t h e d i s cus s ion t o E o l l m ,

Figure 23 shows schemat ica l ly t h e Fe-Mn-MnS-FeS part of t h e Fe-Mn-S

phase diagram. The four s i d e s of the. diagram are the binaries: Fe-FeS, Fe-Mn,

Mn-MnS, and MnS-FeS '20'28'50) The Pa-FeS phase diagram is a s imple e u t e c t i c

diagram with p r a c t i c a l l y no i r o n s o l i d s o l u b i l i t y i n iron s u l f i d e . The s o l i d

s o l u b i l i t y of sulfur i n i r o n is a s low as 0,2 pe rcen t (51) a t room temperature .

The Mn-MnS phase diagram p re sen t s a e u t e c t i c , G , near ehe manganese s i d e , a

l i q u i d m i s c i b i l i t y gap, DK'E, and a monotect ic , E , near the MnS s i d e . The FeS-

MnS phase diagram is a e u t e c t i c type wi th no s o l i d s o l u b i l i t y of manganese i n i r o n I

s u l f i d e and a s i g n i f i c a n t s o l i d s o l u b i l i t y of i r o n i n manganese s u l f i d e . The I

Fe-Mn binary i s one of n e a r l y complete s o l i d s o h b i h i t y with narrow l i qu id - so l id

reg ion , The l fqu idus and s o l f d u s are s i m p l i f i e d t o a s i n g l e l i n e i n F igure 24 I I

f o r c l a r i t y ,

The l i q u i d m i s c i b i l i t y gap KMNCDK%OK has the shape of an oblong dome

of c r e s t - l i n e E' l i m i t e d by t h e contour EOKMNCD. The p r o j e c t i o n of t h i s contour

on t h e b a s a l p lane of t h e diagram i l l u s t r a t e s t h e . s l o p e s - of i t s , va r ious segments.

Thus, K i s a minimum p o i n t whereas C and P , i n t e r s e c t i o n s of t h e contour wi th t h e

v e r t i c a l plane.:whose t r a c e on t h e b a s a l p lane i s t h e d iagonal Fe-MnS, a r e maximum

- - po in t s . The segment is a monotect ic v a l l e y whereas OH, LH, and ?@ a r e

- e u t e c t i c valleys converging towards H , t h e t e rna ry e u t e c t i c . QG is also a e u t e c t i c

v a l l e y descending from Q , a maximum point: s i t u a t e d i n t h e same vertical plane as

P and C , t o G , t he b inary e u t e c t i c .

The b a s a l plane p r o j e c t i o n of t h e monotect ic and e u t e c t i c v a l l ~ y s

and of t h e l i m i t i n g contour of t h e l i q u i d m i s c i b i l i t y gap a r e a l s o shown schematical ly

i n Figure 24. F igure 25a shows some t i e l i n e s ac ros s t h e m i s c i b i l i t y gap, l ean ing

a g a i n s t t h e contour of t h e gap(21). F igure 25b shows t h e s h i f t of t h e m i s c i b i l i t y

gap, of t h e tie l i n e s ac ros s t h e gap and of t h e e u t e c t i c v a l l e y s when carbon i s added

t o t h e Fe-Mn-S The conf igura t ion of t he two b a s a l p ro j ec t ions of t he

systems Fe-Mn-S and Fe(C)-Mn-S being s i m i l a r , conclusions formulated by s tudying

t h e s o l i d i f i c a t i o n behavior of t h e f i r s t system should be q u a l i t a t i v e l y app l i cab le

. t o the second.

Figure 26 r ep re sen t s schemat ica l ly t h e pseudobinary phase diagram Fe-MnS

obtained by sec t ion ing t h e t e r n a r y Fe-Mn-S phase diagram by the v e r t i c a l p lane

whose trace on the basal plane is t h e d iagonal Fe-MnS. This pseudobinary i s used

below f o r a s i m p l i f i e d i n t e r p r e t a t i o n of t h e o r i g i n and s o l i d i f i c a t i o n behavior

of t h e va r ious types of s u l f i d e inc lus ions . A second and more r igorous way of

conducting t h i s i n v e s t i g a t i o n i s by us ing t h e b a s a l p r o j e c t i o n s of Figures 25a,

27, and 28 .

Experimental observa t ions made i n t h i s study, and which may be understood Ir

a t l e a s t q u a l i t a t i v e l y by t h e d i scuss ion t o fol low using t h e phase diagramsl of

Figures 23-26, inc lude t h e fol lowing:

(1) Type I i n c l u s i o n s appear as spheres of wide s i z e d i s t r i b u t i o n ,

s c a t t e r e d randomly i n i n t e r d e n d r i t i c spaces (no t throughout

t h e whole mat r ix a s widely be l ieved) . It is poss ib l e , though r a r e ,

t o see a f e w such i n c l u s i o n s w i t h i n the dendr i t e s .

( 2 ) Type I 1 , i n c l u s i o n s appear as a e u t e c t i c - l i k e network of

i r r e g u l a r c y l i n d r i c a l rods spread along i n t e r d e n d r i t i c spaces

(not j u s t along g r a i n boundaries a s widely be l i eved ) . The

c l a s s i c a l "beads" and " s t r i nge r s " are random i n t e r s e c t i o n s of

t h e e u r o c t i c network by t h e pol i sh ing plane.

(3) Type 111 i n c l u s i o n s appear as idiomorphic c r y s t a l s s c a t t e r e d

throughout t he whole d e n d r i t i c s t r u c t u r e , but f r equen t ly

s i t u a t e d i n i n t e r d e n d r i t i c spaces. They o f t e n p r o j e c t arms which

j o i n ~ ~ j e 11. i n c l u s i o n s .

(4) More than one type i n c l u s i o n is gene ra l ly observed i n a given

sample , but s m e , Type 11 is found i n a l l specimens.

I

( 5 ) Increas ing s u l f u r content favors formation of Type I1 i nc lua ions .

( 6 ) For an a l l o y containing 0 . 1 percent S c e r t a i n chemical v a r i a b l e s

favor the fol lowing t r a n s i t i o n s :

(a) ~ n c r e a s i n g amounts of A1 favor Type I + Type I1

(b) Inc reas ing amounts of A 1 and ~ a ' a n d / o r S i ' f a v o r Type II+

Type 111

(c ) Decrease in 0 l e v e l favors Type I+Type I1

(d) Decrease in 0 l e v e l and i n c r e a s e in A l , Mn and/or S i l e v e l s

favor Type II+Type 111

( e ) Addit ion of Z r o r B f avo r s Type II-tType 111

( f ) Ce a d d i t i o n s favor Type II+Type I

For q u a l i t a t i v e i n t e r p r e t a t i o n of t h e foregoing , cons ider f i r s t s e v e r a l

a l l o y s w i t h i n t h e pseudobinary Fe-MnS system. S ince , as noted i n Figure 25 and

elsewhere'21), t i e l i n e s during s o l i d i f i c a t i o n l i e approximately w i t h i n t h i s A

pseudobinary, w e need only cons ider t h e b ina ry when examining s o l i d i f i c a t i o n of

c. a l l o y s whose composition i n i t i a l l y l i e s along t h e l i n e j o in ing Fe and MnS; t h i s

pseudobinary i s sketched i n F igure 26,

The b ina ry of F igure 26 w i l l be used t o d i s cus s s o l i d i f i c a t i o n of three

d i f f e r e n t a l l o y s (Alloys 1, 2 , and 3 of Figure 2 6 ) , making t h e e s s e n t i a l pre-

suppos i t i ons t h a t (1) MnS-rich l i q u i d (L ) comprises Type I i n c l u s i o n s , ( 2 ) 2

the e u t e c t i c Type 11 i n c l u s i o n s form a t t h e e u t e c t i c Q - a s - L + Fe f MnS(so1id) and 1

(3) Type I11 i n c l u s i o n s form from t h e mel t L1 as L1+MnS(so l id) ; the equi l ib r ium

temperature range for t h i s r e a c t i o n l i e s above t h e e u t e c t i c Q and below t h e e u t e c t i c P .

The second pro-supposi t ion has been made e a r l i e r by ~ i r n s ' ~ ~ ) . Recent ly , Dahl e t a 1 ( 4 6 )

F made a l l t h r e e pre-supposi t ions .

Consider, first, s o l i d i f i c a t i o n of a l l o y 1, Figure 26, whose l i q u i d u s

is L Assuming n e g l i g i b l e undercooling before n u c l e a t i o n , i r on - r i ch d e n d r i t e s 1"

w i l l n u c l e a t e a t TI, and grow wh i l e the l i q u i d composition moves from L t o t h e 1 1

e u t e c t i c composition Q. At T t h e e u t e c t i c Fe-MnS forms ( e u t e c t i c Type I1 Q '

i n c l u s i o n s ) . This a l l o y , s o l i d i f i e d i n t h i s manne r ,w iP l . - cons i s t of only Type

IT i n c l u s i o n s i n an i r o n mat r ix .

Next, consider a l l o y 2 , a l s o s o l i d i f y i n g - w i t h n e g l i g i b l e undercooling

of a l l phases be fo re nuc lea t ion . Here, s o l i d Type 111 inc lus ions nuc lea t e a t

L' and grow whi le the l i q u i d moves t o t h e e u t e c t i c Q. F i n a l s t r u c t u r e c o n s i s t s 1

of Type 111 and Type I1 i n c l u s i o n s , w i t h the propor t ion of Type I1 i nc lus ions

decreasing as i n i t i a l l i q u i d composition moves from Q towards C.

Next, cons ider Alloy 3 , a l s o s o l i d i f y i n g with n e g l i g i b l e undercooling

of all phases be fo re nuc lea t ion , Liquid Type I s u l f i d e s grow above t h e e u t e c t i c f

temperature of l i q u i d L2; s o l i d Type I11 s u l f i d e s grow a t lower temperatures *

bu t above t h e e u t e c t i c of L and e u t e c t i c Type I1 s u l f i d e s grow a t the e u t e c t i c 1 '

of Liquid L ( a t Q ) . Hence, t h i s a l l o y , a s s o l i d i f i e d , conta ins Types I , 11, and 1

I11 s u l f i d e s .

F i n a l l y , i f nuc l ea t ion of s o l i d MnS i s supposed. to be hindered i n any

one of the t h r e e alloys discussed above, a d i f f e r e n t combination of inclusion

types w i l l r e s u l t . As example, consider Alloy 1. Suppose nuc lea t ion of s o l i d

MnS cannot occur a t Q , bu t l i q u i d L forms a t T' Thus, Type I inc lus ions form 2 Q '

and provided s o l i d MnS l a t e r n u c l e a t e s , Type I1 s u l f i d e s may a l s o form a f t e r

reca lescence a t o r near T S imi l a r ly Alloy 2 w i l l comprise Type I , I1 and I11 Q '

or Types I and 11 accor'ding t o t h e maximum reca lescence temperature reached (52)

The foregoing' s imple d i scuss ion exp la ins q u a l i t a t i v e l y the bu lk of

t h e observat ions made i n t h i s work, inc luding t h e gene ra l morphological f e a t u r e s

* of d i f f e r e n t types of i nc lus ions . One except ion i s

undercooling be fo re nuc lea t ion) i n c l u s i o n s of Types

t h e fact . t h a t (assuming no

I and 111 form be fo re t h e

s o l i d i r o n dendr i t e s , and would t h e r e f o r e b e expected t o be d i s t r i b u t e d more o r

l e s s a t random, no t concent ra ted p r e f e r e n t i a l l y a t d e n d r i t e arm boundaries ,

The observed concent ra t ion a t d e n d r i t e boundaries i s presumably due t o one o r

both of t h e fol lowing f a c t o r s :

(1) A f t e r i n c l u s i o n formation, and during growth and coarsening of

d e n d r i t e s , i nc lus ions a r e pushed ahead of t h e l i qu id - so l id i n t e r -

face i n t o i n t e r d e n d r i t i c spaces n This "pushing" has been demonstrated

(53) f o r S i O ' i n c l u s i o n s i n a n i r o n a l l o y a 2 -

(2 ) The Type I inc lus ions form, not according t o the equi l ibr ium

diagram, b u t after formation of some primary i r o n phase as d iscussed

above, Perhaps "hollow" i n c l u s i o n s a s shewn i n Figure 15 are found

i n t h i s way,

The pseudobinary phase diagram may a l s o be used t o q u a l i t a t i v e l y exp la in

the modif ica t ion of i n c l u s i o n t ype for a given s t e e l mel t , by a d d i t i o n of a deoxid izer ,

and i t s amount used, Assume , as example, t h a t deoxidat ion wi th s i l i c o n lowers t h e

equi l ibr ium l iqu idus l i n e s as i n d i c a t e d schemat ica l ly i n Figure 25b; t h e mel t of

composition 1 which i n absence of s i l i c o n gave Type I1 i n c l u s i o n s w i l l now lead

t o a mixed Type 11-111 i nc lus ions . There i s s u f f i c i e n t evidence that such a

s h i f t i n g of the equi l ibr ium l iqu idus might t ake p l ace (54 ) . More sys t ema t i c research

is necessary f o r q u a n t i t a t i v e informat ion about t h i s s h i f t i n g ,

For a l l o y s whose i n i t i a l composition i s no t exac t ly i n t h e b ina ry

Fe-MnS, t h e multicomponent diagram of Figures 23-25 must be used t o s tudy s o l i d i f i c a t i o n

( o r , for convenience, t h e schematic b a s a l p r o j e c t i o n s of Figures 27 and 28) .

Consider &st t h e m e l d o f hypoeutec t ic composition, F igure 27 .

Now assuming no undercooling be fo re nuc l ea t i on , t h e l i q u i d composition fol lows

t h e heavy pa th shown i n F igure 27 during s o l i d i f i c a t i o n . S o l i d i r o n dendr i t e s

nuc l ea t e a t t h e equi l ib r ium l i q u i d u s , and t h e l i q u i d moves along t h e s t r a i g h t - l i n e dl3 ( s i n c e s o l i d s o l u b i l i t y of s u l f u r is n e g l i g i b l e ) . A t 8 , e u t e c t i c

Type I1 i n c l u s i o n s nuc l ea t e and grow as l i q u i d composition moves toward t h e

t e r n a r y e u t e c t i c p o i n t H. F i n a l s t r u c t u r e comprises only Type I1 i n c l u s i o n s

i n t h e i r o n mat r ix .

The s a l i d i f i c a t i o n behavior of m e l t d of s l i g h t l y hype reu t ec t i c

composition l y i n g i n t h e f i e l d of primary s u l f i d e c r y s t a l l i z a t i o n , F igure 28a

may be summarized as fo l lows ( n e g l i g i b l e undercool ing): When t h e temperature

of t h e melt reaches t h e equ i l i b r ium l i q u i d u s , s o l i d manganese s u l f i d e c r y s t a l s

nuc l ea t e . With decreas ing temperature t h e l i q u i d as w e l l as t h e s o l i d i f y i n g

s u l f i d e become r i c h e r i n i r o n . When t h e l i q u i d composition reaches B on t h e

e u t e c t i c v a l l e y , i r o n nuc l ea t e s and t h e e u t e c t i c Type I1 s u l f i d e s grow,

e i t h e r by new nuc l ea t ion , o r by ex tens ion of the a l r eady e x i s t i n g primary

s u l f i d e s . The s o l i d i f i c a t i o n is aga in completed when t h e t e rna ry e u t e c t i c H

i s reached. The r e s u i t i n g i n c l u s i o n type i s mixed 11-111 (e .g. , Figures 11,

18, and 22) .

When the composition of t h e m e l t L f a l l s i n s i d e t h e l i q u i d

m i s c i b i l i t y gap, Figure 19b, the s o l i d i f i c a t i o n process is q u i t e d i f f e r e n t .

A s soon as t h e dome is keached, pools of a s u l f i d e r i c h l i q u i d L s e p a r a t e 2

ou t of t h e melt of composition L With decreas ing temperature L1 and L move 1" 2

i n oppos i te d i r e c t i o n s and L reaches p o i n t a when L reaches p o i n t a 1 1 2 2' a2al

forming a t i e l i n e , While t h e temperature decreases f u r t h e r , t h e l i q u i d L a2

moves along t h e monoteet ic l i n e PO whereas the mel t La moves along t h e l i n e 1 -

CN. L comprises t h e s u l f i d e - r i c h Type I i n c l u s i o n s forming as a l i q u i d a2

from t h e m e l t , Now, according t o s imple equi l ibr ium s o l i d i f i c a t i o n considera-

t i o n s La g ives monotec t ica l ly l i q u i d L + s o l i d (Mn,Fe)S and t h e l i q u i d is 2 al

e n t i r e l y consumed by the maslotectic r e a c t i o n , However, i t should be recognized . ,

t h a t a s wi th t h e case of t h e d i scuss ion on t h e s imple b ina ry , t h e Type I

s u l f i d e s comprise a very sma l l volume f r a c t i o n of t h e t o t a l me l t , and a f t e r

t h e i ron dendr i t e s begin to grow, t h e s e s u l f i d e s must be very quickly surrounded

-: and trapped, Hence, any changes t h a t occur t o l i q u i d L2 during cool ing from

a2 3 b2 w i l l b e 1ocal i ;zed w i t h i n these i s o l a t e d pools , w i l l no t change t h e +

L shape of t h e s u l f i d e s (Type I ) s i g n i f i c a n t l y , and w i l l r e s u l t i n no o v e r a l l

change i n i n c l u s i o n chemistry,

The o the r l i q u i d , L1, comprises a s i g n i f i c a n t l y l a r g e r volume

- f r a c t i o n than does L As t h i s l i q u i d cools a long blB, Type I11 s u l f i d e s 2 "

grow, and as i t proceeds along BH, Type I1 s u l f i d e s grow. A t t h e completion of

s o l i d i f i c a t i o n a t H, t h e r e s u l t i n g i n c l u s i o n type is mixed 1-11-111, Figure 15,

Type I being t h e inc lus ions which sepa ra t ed as l i q u i d pools and s o l i d i f i e d

between a2 and b 2 , III being t h e i nc lus ions which s o l i d i f i e d ou t of the mel t as

u s u l f i d e c r y s t a l s between b and B , and I1 being t h e e u t e c t i c i nc lus ions which 1

s o l i d i f i e d along t h e eurec t i c v a l l e y BH,

Figure 28c d e a l s wi th another case of a me1td:whose composition f a l l s

inside t h e m i s c i b i l i t y gap. The s o l i d i f i c a t i o n process is q u i t e similar t o t h a t

- examined i n t h e previous case, except t h a t b B is ze ro i n t h e p re sen t case. 1

Thus, when the l i q u i d L i s completely consumed a t b by t h e monotect ic r e a c t i o n , 2 2

any subsequent decrease i n temperature causes nuc lea t ion of t h e e u t e c t i c out of

t h e mel t Lb which then is a t p o i n t B. There are no primary s u l f i d e c r y s t a l s 1

i n t h i s case. The f i n a l i n c l u s i o n type i s mixed 1-11, Figure l b .

I n t h e case of the t e rna ry system, a s i n t h e pseudobinary system

discupsed e a r l i e r , the ' range of p o s s i b l e s t r u c t u r e s is increased i f t h e p o s s i b i l i t y

of hindered nuc lea t ion ' of Type I1 s u l f i d e s is considered. However, i n a l l c a ses ,

it: i s predic ted t h a t Type II e u t e c t i c i nc lus ions should be presenr t o . a t l e a s t

some small e x t e n t i n a l l specimens of t h e ana lyses considered. I t could -be

repea ted t h a t f o r a g iven s t e e l me l t , t h e type of i nc lus ions t o be expected

depends on t h e p o s i t i o n of the va r ious curves i n t h e b a s a l p ro j ec t ion . These +-

curves s h i f t acco rd ing t o t h e n a t u r e and t h e amount of deoxid izer used as i l l u s - .

t r a t e d i n Figure 26b f o r t h e case of carbon,

S o l i d i f i c a t i o n cons ide ra t ions based on t h e b a s a l pro3 e c t i o n method

provide a p o s s i b l e explana t ion t o va r ious observa t ions r e l a t i n g t o t h e e f f e c t

of cool ing r a t e on t h e type and s i z e of i nc lus ions . It was observed t h a t a

s lower s o l i d i f i c a t i o n decreased the r e l a t i v e volume fraction of Type 11 i n c l u s i o n s

i n specimens present inb a mixed Type 1-11"

In a l l t h e systems d iscussed h e r e i n , oxygen, as an element, was nor

taken i n t o account. The a n a l y s i s of cases i n which oxide o r s i l i c a t e cores appear

i n s u l f i d e inc lus ions should, however, cons ider oxygen as we l l . These cases

are i l l u s t r a t e d by Figures 9 and 2 , The presence of oxide o r s i l i c a t e core i n

Type I i n c l u s i o n s , Figure 9 , i s t o be expected because, i n t h i s specimen, t h e

oxygen level was i n t e n t i o n a l l y raised by addition of Fe 0 The presence of 2 3 "

oxide in the high sulfur specimens, Figure 2, could also b e expected because it

is well known (55' that wi th increasing sulfur levels the equilibrium s o l u b i l i t y of

oxygen i n molten steel increases.

CHAPTER V1 - CONCLUSIONS

1. S u l f i d e inc lus ions i n AISI 4330 vacuum melted and s o l i d i f i e d low a l l o y

s t e e l may be c l a s s i f i e d i n t o t h r e e types , I , 11, and 111. Type I inc lus ions

appear as i n d i v i d u a l sphero ids of a wide s i z e d i s t r i b u t b n s c a t t e r e d

randomly i n t he i n t e r d e n d r i t i c spaces (not through t h e whole ma t r ix a s

widely be l i eved ) . Type 11 i nc lus ions appear as a degenerate e u t e c t i c network

made of i r r e g u l a r c y l i n d r i c a l rods , spread out i n t he i n t e r d e n d r i t i c spaces ,

d e l i n e a t i n g the d e n d r i t e arm boundaries (no t j u s t a t g ra in boundaries as

widely b e l i e v e d ) , The c l a s s i c a l "beads" and " s t r i nge r s " observed are i n

r e a l i t y i n t e r s e c t i o n s of a c u t e c t i c network by t h e pol i sh ing plane. Type

I11 i n c l u s i o n s appear as idiomorphic c r y s t a l s scattered through t h e whole

d e n d r i t i c s t r u c t u r e , but with a h igher frequency i n t h e i n t e r d e n d r i t i c

spaces. More than one type of i n c l u s i o n usua l ly coex i s t i n a g iven sample.

The usua l occurrences are: Type 11, mixed Type 1-11, mixed Type 11-111,

and mixed Type 1-11-111..

2 . The s u l f i d e inc lus ions i n t he A I S I 4330 specimens (with no o the r add i t i on )

are of a mixed Type 1-11 and c o n s i s t of a (Mn, Fe, Cr)S s o l i d s o l u t i o n .

With inc reas ing S conten t t he volume f r a c t i o n of s u l f i d e inc lus ions inc reases

and t h e r e i s a morphological s h i f t i n g towards Type 11. With inc reas ing

amount of 0, t h e r e i s a r e v e r s e s h i f t i n g towards Type I inc lus fons .

3 . I n ingots deoxidized wi th A l , t h e r e i s a s h i f t i n g towards i n c l u s i o n Type T I

wi th i nc reas ing amount of A l . No Type 111 i n c l u s i o n s are observed by

a d d i t i o n of A1 alone1.

4 . A simultaneous deoxidarion of t h e mel t wi th A 1 and S f , o r A 1 and Mn, o r A l ,

S i and Mn may Lead t b formation of Type I11 i n c l u s i o n s . A three-dimensional ~

-31-

cornpositional diagram has been p l o t t e d , allowing t h e determinat ion of those

combinations of A l , S i and Mn contents which l ead t o a p a r t i c u l a r type of

i nc lus ions . An i n c r e a s e i n t h e 0 l e v e l displaces t h e var ious l i m i t i n g su r f aces

of the diagram i n such a way that higher amounts of A l , S i and Mn a r e requi red

f o r ob ta in ing Type 111 i nc lus ions . The oppos i te conclusion is expected t o

be v a l i d with an inc reas ing C l e v e l ,

5. Cerium used as a deoxid izer causes a s h i f t i n g of t h e - i n c l u s i o n Type towards

I. Zirconium add i t i ons l ead t o a mixed Type 11-111 s u l f i d e i n c l u s i o n

containing approximately 20 pe rcen t Zr. The Type 111 i nc lus ions appear as

idiomorphic crystals s c a t t e r e d i n the d e n d r i t i c s t r u c t u r e , bu t mainly

concentrated i n the i n r e r d e n d r i t i c spaces p r o j e c t i n g arms which j o i n the

Type I1 i nc lus ions . Boron add i t i ons lead t o a mixed Type of i nc lus ions

which is nonetheless very c l o s e t o Type 111, Idiomorphic c r y s t a l s appear

s c a t t e r e d almost exc lus ive ly i n t he i n t e r d e n d r i t i c spaces occupied by a

matr ix-boride e u t e c t i c .

6. With i nc reas ing amounts of S and Mn, t he Type 11 e u t e c t i c network becomes

wider. It i e surrounded by Type 111 i nc lus ions o f t e n showing d e n d r i t i c

tendencies with p ro j ec t ions jo in ing the e u t e c t i c network.

7 . With decreasing cool ing r a t e , t h e r e l a t i v e volume f r a c t i o n of Type I inc lus ions

and their average s i z e inc reases . The average spacing of t h e e u t e c t i c Type I1

i nc lus ions also i nc reases . For very low s o l i d i f i c a t i o n r a t e s , t he Type II

e u t e c t i c network degenerates i n t o i s o l a t e d rods o r spheroids.

8. I n a specimen cast with mixed Type 1-11-111 i n c l u s i o n s , Type I inclusions

appear t o b e r i ng - l ike . This p a r t i c u l a r morphology i s a t t r i b u t e d t o

undercooling.

9 . The o r i g i n of t h e va r ious Types of i nc lus ions has been explained by fol lowing

t h e s o l i d i f i c a t i o n process of t h e deoxidized a l l o y on t h e Fe-Mn-S ternary 1 I

phase diagram, o r , f o r g r e a t e r convenience, on the pseudo-binary Fe-MnS

phase diagram. The modi f ica t ion of t hese diagrams by a d d i t i o n of t h e var ious

deoxid izers is expected t o b e r e spons ib l e f o r t he morphological v a r i a t i o n

of the inc lus ions observed. It i s concluded t h a t Type I inc lus ions form by

exso lu t ion of s u l f i d e pools ou t of t h e s t e e l m e l t , Type I11 i nc lus ions form

by s o l i d i f i c a t i o n o'f sulfide c r y s t a l s ou t of the m e l t , and Type I1 inclusions

form by solidification of t h e s u l f u r - r i c h l i q u i d along a eutectic v a l l e y ,

Morphologies of the, d i f f e r e n t types of i nc lus ions are explained an t h e

b a s i s of phase diagram cons ide ra t ions alone, without need of p o s t u l a t i n g ,

f o r example, d i f f e r ences i n surface energy behavior of t h e d i f f e r e n t

i n c l u s i o n types. Hpwever, t o r a t i o n a l i z e t he f i n a l d i s t r i b u t i o n of i nc lus ions

observed, i t i s suggested that Type I, and t o a l e s s e r extent, Type 111

i nc lus ions are "pushed" by growing dendr i t e s .

TABLE I

Nominal Composition of A I S I 4330

Low Alloy Steel

0 . 3 1

1 .83

0,95

0.86

0.44

0.27

0.08

Balance

TABLE I1

Series I

S e r i e s I1

Series I11

S e r i e s I V

Specimens Prepared and Type of Inclusions Observed

Composition

AISI 4330-l-0.01s 0.02s 0 .O5 0.1s 0.1s 0.1s 0.1s 0.2s 0.5s 1.0s

Cooling Rate " ~ / m i n

Type of Inc lus ions

I-II towards I I-II towards I 1-11 towards I I-II in t e rmed ia t e I-II in t e rmed ia t e I-II towards I I-II almost I, coarser 1-11 intermediate I-II in t e rmed ia t e Almos t I I

AISI 4330+0.13+0.04 0 I-II. A t center of i n g o t , Type I , a t the edge, Type 11, sma l l e r s iee

I. Dark oxide cores

AISI 4330+0.1S+O.O5AI O.3Al P.4Al O.5Al D.8A1

AISI 4330+0.1SCQ.05Si-t0.1Al O.lSi+O. lA1 0 . lSi+O . 2A1 0.05Si+0.3Al

! .5Si ,O.lSi .2SifO. LA1 Q.ZSif0.3Al O.lSit0.35Al I). 5SitO. lA1 0.9Si

1 0.2SiN.4Al d .3Si+0.3Al 0.4Sif0.2Al

I 0.5S+tO. 3A1 I

1 0.7SPSO.lAl

I-II 1-11 A;Lmos t I1 I I 11. No t r a c e s of 111

1-11 I1 I1 I1 11, I-II 11-111 11-111 11-111 11-111 11-111 I11 I11 I11 I11 I11

TABLE I1 (continued)

Series V AISI 4330+0.1S+O . O5Mn+O. LA1 O.Mn+O. lA1 O.l.Mn+O. 2A1 O.Mn+0.3Al 0 .2Mn+O . U 1 0.1Mn+Om5A1 0,2Mn+002A1 0.2Mn4-0.4A1 O03Mn+0.3A1 O.05Mn 0.4Mn-kO. lA1 l,4Mn O.3Mn+O. SAl 004Mn+0. 3Al 0.5Mn+0.2Al 0.7Mn-t-O. 4A1 0,85Mn+O, 1Al

Series V I AISI 433O+O .lS+0.3Si+0.3Mn 0.3Sif0,7Mn 0 -45 S i+O .5Mn O.4OSi-kO .7Mn 0 .5S i+O .5Mn 1.0Sitl.OMn

Series VII

Series VIII

Series I X

Series X

AISI 4330+0,1S+0.1Sil-0.5Mn+0.4A1 0.2Si+Oa3Mn+Q.3A1 0.5Si+O.4Mn+O .1A1

- -. - -

AISI 4330+0.1SC0002 0+0,1SI+O.SMn+0.4Al OO2Si+0.3Mn+0.3A1 O.5Si+0.4Mn+Om lS1

AISI 4330+0,1S+O.OZr 0.3B O.1Ce

AISI 4330+0.5S+2Mn 0.5S+2Mn+O .3Si

-36-

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II W. Dahl, H. Hangstemberg, C. Duren, "Condi t ions f o r t h e Occurrence of the Various Types of S u l f i d e ~ n c l u s i o n s " , S t a h l und E i s e n , v. 86 , 1966, No. 1 3 , pp. 782-795.

W. C r a f t s , D. C. n i l t y , " S u l f i d e and Oxide Format ion i n steel", Proceed ings of E l e c t r i c Furnace S t e e l Conference, 1953, pp. 121-150.

48. J . E . H i l l i a r d , J. W. Cahn,,"An E v a l u a t i o n of Procedures i n Q u a n t i t a t i v e M e t a l l o g r a p h y f o r Volume F r a c t i o n Ana lys i s" , T r a n s a c t i o n s AIME, v. 221, 1961, pp. 3 4 4 - 3 5 2 .

49. G . A . Chadwick, E u t e c t i c Al loy S o l i d i f i c a t i o n , Pergamon P r e s s , Oxford, 1963. I 50. M. Hansen, C o n s t i t u t i o n of Binary A l l o y s , McGraw-Hill, New York, 1958.

51. E , T. Turkdogan, S, Igna towicz , J . Pearson, " S o l u b i l i t y of S u l f u r i n I ron and Iron-Manganese Alloys", J o u r . I r o n and S t e e l I n s t . , v. 180, 1955, p p . 349-354.

52. T. Z. Kat tamis , M. C. Flemings, " S o l i d i f i c a t i o n of High ly Undercooled Cas t ings" , Modern C a s t i n g , v. 52 , No. 1, 1967, pp . 191-198.

53. M. Myers, Department of Meta l lu rgy and M a t e r i a l s Sc ience , M . I . T . , Private Communication.

54. J . C. Yarwood, Department of Meta l lu rgy and Materials Sc ience , M.I.T., P r i v a t e Communication.

5 5 . D. C . Hilty, "Influence of S u l f u r on Oxygen S o l u b i l i t y i n S t e e l " , J o u r . Metals, v. 18, 1966, pp. 201-204.

COOLING RATE ,'C/MIN

Figure 6 : Average diameter of Type I s u l f i d e inclusions versus coo l i ng ra te . AISI 4330 - 0.1% S specimens.

Figure 7: Photomicrographs of AISI 4330-0.1% S showing finer Type I1 eutectic inc lus ions at higher cooling rate, 500X, cooling rates are: ( a ) 70°C/min., (b) lo°C/rnin.

COOLING RATE ,"C/MIN

Figure 8: Average spacing of Type I1 eutectic inclusions versus cooling rate. AISI 4330 - 0.1% S specimens.

0.7 -

o TYPE I - ll (MIXED) 0 TYPE D

0.5 A TYPE I I - I I I (MIXED)

0.4 TYPE17T

I I I I I I w

0 0.1 0.20.3 0.4 0.50.6 0.70,8 0.9 1.0 1 . 1 1.2 1.3 1.4 1.5

PERCENT SILICON ADDED

Figure 10: Type of s u l f i d e inc lus ions versus A1 and S i additions. AISI 4330 - 0.1% S specimens s o l i d i f i e d at 40°C/min.

o TYPE I-II (MIXED) T Y P E I I i

A TYPE II - 111 ( M IXED)

TYPE 171 -

-

-

-

m -

\ -

- I I

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4

PERCENT MANGANESE ADDED

Figure 13: Type of s u l f i d e i n c l u s i o n s versus A1 and Mn additions. AISI 4330 - 0.1% S specimens s o l i d i f i e d at 40°C/rnin.

o TYPE I -lf ( MI XED)

TYPE If:

A TYPE D-14. (MIXED)

\ I I I 1 \A - I I I

0 0.1 0.20.5 04 0.5 0.6 0.7 0.80.9 1.0 1.1 1.2 1.3 1.4 1.5

PERCENT SILICON ADDED

Figure 14: Type of sulfide inclusions versus Mn and Si additions. A I S I 4330 - 0.1% S specimens solidified at 4 0 ~ ~ 1 r n i n .

PERCENT ALUMINUM k,.

4" PERCENT SILICON

Figure 17: ~ ~ p e of sulfide inclusions versus A1 ,Mn ,and Si a d d i t i o n s . AISI 4330 - 0.1% S specimens s o l i d i f i e d a t 400C/min.

Figure 23: Schematic representation of the Fe - Mn - FeS - l4nS part of the Fe - Mn - S phase diagram.

Figure 24: The four binary phase diagrams o f the Fe - Mn - FeS - PlnS system and the basal projection of the three eutect ic val- l e y s and of the contour l imiting the m i s c i b i l i t y gap. The projection of the ternary eutect ic plane appears shadowed.

F e S L

Fe

( a

F i g u r e 25: (a) ~ a s b l p r o j e c t i o n of t h e Fe - Mn- S system showing t h e m i s c i b i l i t y gap,tie lines across the gap,and t h e eutectic valleys ' . (b) Basal p ro jec t ion of the Fe( C s a t u r a t e d ) -Mn- S systeh i n d i c a t i n g the s h i f t i n g of t h e m i s c i b i l i t y gap ,of t h e t i e l ines ac ros s the gap and of the e u t e c t i c va l l eys .

'-I+'-, - - - - - - - - - - --

L, + sol id M, S P~ P\

L,+ solid Mn S I I

--- - - ------- solid F e t solid M,S

1 pi

Figure 26: (a) Schematic representation of the pseudo-binary Fe - MnS phase diagram obtained by sectioning the ternary Fe - h- S phase diagram by the vertical plane whose trace on the basal plane is the diagonal Fe - MnS. (b) Schematic shifting of the equilibrium lines between the various phases by addi- tion of deoxidfzers.

Fes

Figure 27: Schematic representation of the solidification path of melt o C +

F i g u r e 28: Schematic representation of the s o l i d i f i c a t i o n paths of three different melts ..

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Cambridge, Massachusetts 02139 REPORT T I T Lk

INVESTIGATION OF SOLIDIFICATION OF HIGH-STmNGTH STEEL C A S T ~ G S

DESCRIPTIVE NOTES (Typo of npot t end lnclu*lvo da1.a)

Interim Repor t - October 1, 1966- December 31,1967 AUTHORCS) (Laat name, f l n t name, Inltl4ll

T. Z. Kattamis, and M. C. Flemings

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1. ABSTRACT

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Cast Steel Solidification Sulfide Inclusions Fe-Mn-S Phase Diagran Deoxidatkon

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ABSTRACT

The morphology, s i z e and d i s t r i b u t i o n of s u l f i d e i n c l u s i o n s a r e s t u d i e d i n vacuum mel ted and s o l i d i f i e d AISI: 4330 low a l l o y s t e e l deox id ized w i t h v a r i o u s e lements such a s Al , S f , Mn, e i t h e r i n d i v i d u a l l y o r i n v a r i o u s combinat ions , and B , Zr and Ce. Types I , I1 and I11 i n c l u s i o n s a r e inves - t i g a t e d i n d e t a i l . Type I i n c l u s i o n s are i s o l a t e d , roughly s p h e r i c a l s u l f i d e s , Type I1 i n c l u s i o n s have an i n t e r c o n n e c t e d rod- l ike morphology, and Type I11 i n c l u s i o n s a r e a n g u l a r and unconnected (except t h e y a r e o f t e n connected t o Type I1 i n c l u s i o n s ) .

E f f e c t s of chemis t ry ( A l , S i , Mn) on i n c l u s i o n t y p e i s q u a n t i t a t i v e l y determined. The e f f e c t of c o o l i n g rate on i n c l u s i o n type and s i z e i s examined f o r a few c a s e s . With d e c r e a s i n g c o o l i n g r a t e , t h e average s i z e of Type I i n c l u s i o n s and t h e average spac ing of t h e e u t e c t i c Type I1 i n c l u s i o n s i n c r e a s e .

The o r i g i n of t h e v a r i o u s t y p e s o f i n c l u s i o n s observed i s i n t e r p r e t e d w i t h t h e a i d of t h e Fe-MnS pseudo-binary phase diagram and t h e Fe-Mn-S t e r n a r y phase diagram. M o d i f i c a t i o n s of t h e s e diagrams from a d d i t i o n of t h e v a r i o u s d e o x i d i z e r s a r e q u a l i t a t i v e l y p r e d i c t e d and t a k e n i n t o account . It i s concluded t h a t Type I i n c l u s i o n s form by e x s o l u t i o n of l i q u i d s u l f i d e p o o l s from t h e m e l t , Type 111 i n c l u s i o n s form by s o l i d i f i c a t i o n of s u l f i d e c r y s t a l s o u t of t h e m e l t and Type I1 i n c l u s i o n s form by e u t e c t i c s o l i d i f i c a t i o n of t h e s u l f u r - r i c h f i n a l l i q u i d . I n t e r p r e t a t i o n of s t r u c t u r e s observed s u g g e s t s t h a t Type I and p o s s i b l y Type 111 i n c l u s i o n s a r e "pushed" ahead of growing d e n d r i t e s . Morphology of i n c l u s i o n s i s adequa te ly p r e d i c t e d on t h e b a s i s of on ly t empera tu re and mode of s o l i d i f i c a t i o n ; such f a c t o r s as s u r f a c e energy d i f f e r e n c e s between d i f f e r e n t i n c l u s i o n t y p e s need n o t b e p o s t u l a t e d t o e x p l a i n t h e i r d i f f e r e n t morphologies.