Industrial Applications of Rare Earth Elements - ACS Publications

2 The Role of the Rare Earth Elements in the Production of Nodular Iron

Downloaded by UNIV ILLINOIS URBANA on August 22, 2013 | http://pubs.acs.org Publication Date: September 3, 1981 | doi: 10.1021/bk-1981-0164.ch002

H. F. LINEBARGER and T. K. McCLUHAN Union Carbide Corporation, Metals Division, P.O. Box 579, Niagara Falls, N Y 14302

Before discussing, i n any detail, the role of rare earths i n the production of nodular i r o n , it is important to arrive at some basic understanding regarding the metallurgy of t h i s material. It is also appropriate to discuss the rare earth materials being used commercially. Both gray and nodular irons are examples of iron-carbon-silicon a l l o y s . Gray iron i s one of the oldest ferrous alloys cast. Evidence indicates that iron was founded i n China over s i x centuries before Christ. Effective use of iron casting technology was a driving force behind the i n d u s t r i a l revolution. On the other hand, nodular iron is one of the newest alloys to achieve utility as an engineering material i n the metals casting industry and, in reality, is an outgrowth from gray i r o n . Gray irons nominally contain from two-to-four percent (by weight) carbon, and from one-to-four percent s i l i c o n . The composition of nodular iron is more r e s t r i c t e d . Typical nodular irons contain from three-to-four percent carbon, and two-to-four percent s i l i c o n . Carbon and silicon are important elements i n the iron because of their effects upon microstructure and the resulting physical properties of the castings. To the continual consternation of foundrymen, the properties of a l l the cast irons are also s i g n i f i c a n t l y affected by the presence of minute concentrations of many elements other than carbon and s i l i c o n . For example, the presence of sulfur, i n concentrations from 0.01% to 0.1%, substantially affects the graphite morphology i n the s o l i d i f i e d cast iron (1). The presence of rare earth elements or the presence of magnesium, i n amounts from about 0.02% to 0.1% (by weight) i n irons which have a low (~0.01%) s u l fur concentration, can change the entire growth pattern of the graphite as w i l l be described l a t e r . Certain of these minor elements are intentionally introduced, as when treating base irons with magnesium or rare earth elements to produce nodular i r o n . Other trace elements, such as sulfur, are at times introduced 0097-6156/81/0164-0019$06.00/0 © 1981 American Chemical Society In Industrial Applications of Rare Earth Elements; Gschneidner, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


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Figure 1. Scanning electron micrographs of flake graphite etched in bromine and methanol: a, Type D graphite—175X; b, Type D graphite—500X; c, Type B graphite—175X; d, Type B graphite—500 X ; e, Type A graphite—500 X . Graphite types are representative of a range of cooling rates: Type D graphite is typical of the most rapid cooling, then Type B, and then Type A.

In Industrial Applications of Rare Earth Elements; Gschneidner, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


2.

LINEBARGER AND McCLUHAN

Nodular

Iron

Production

21

i n t e n t i o n a l l y o r a t other times a r e c a r r i e d along i n the raw mat e r i a l s used i n producing the i r o n . For example, s u l f u r i s a cons t i t u e n t of the coke used i n cupola m e l t i n g operations and there i s some s u l f u r pickup from t h i s source. Elements, such as chromium, phosphorus and t i t a n i u m are picked up during the scrap r e m e l t i n g process. M i c r o s t r u c t u r e s i n cast i r o n s a r e a l s o d r a m a t i c a l l y i n f l u enced by c o o l i n g r a t e s . I f c o o l i n g i s r a p i d , no g r a p h i t e p r e c i p i t a t e s . Rather, the a l l o y s o l i d i f i e s i n the metastable Fe-Fe3C s t a t e . In that s t a t e , the carbon i s combined w i t h i r o n as i r o n c a r b i d e s . The f r a c t u r e d s u r f a c e o f c a r b i d i c cast i r o n i s w h i t e . Such i r o n s a r e hard and a r e not r e a d i l y machined. C a r b i d i c i r o n c a s t i n g s are used f o r some s p e c i a l a p p l i c a t i o n s , when abrasion r e s i s t a n c e i s important. Graphite p r e c i p i t a t e s only i f the l i q u i d i s allowed t o s o l i d i f y and c o o l as the thermodynamically s t a b l e i r o n - g r a p h i t e system. In a s o - c a l l e d gray i r o n , the f r a c t u r e d surface i s gray because the carbon has p r e c i p i t a t e d as g r a p h i t e and the g r a p h i t e i s v i s i b l e on the f r a c t u r e . Depending upon the heat t r a n s f e r , the graphi t e morphology i s a l t e r e d . C h a r a c t e r i s t i c forms of g r a p h i t e i n gray i r o n are t h e r e f o r e a f u n c t i o n o f t h e c o o l i n g r a t e of t h e metal (see Figure 1 ) . From a p r a c t i c a l standpoint, there i s l i t t l e a foundryman can do t o c o n t r o l t h e c o o l i n g r a t e of i r o n i n a given c a s t i n g . He can, however, add n u c l e a t i n g agents t o the i r o n which minimize the c o n s t i t u t i o n a l undercooling a s s o c i a t e d w i t h the formation o f t h e i r o n - c a r b i d e phase. This i s accomplished by a p p r o p r i a t e s e l e c t i o n of n o d u l i z i n g and i n o c u l a t i n g a l l o y s which promote v e r y h i g h degrees o f n u c l e a t i o n . The degree o f n u c l e a t i o n i s measured by nodu l e counts i n nodular i r o n o r c h i l l depth i n gray i r o n . In gray i r o n s , the g r a p h i t e grows upon the prism faces of the hexagonal c r y s t a l (see F i g u r e 2 ) . This growth i s not uniform b u t , r a t h e r , i s a f f e c t e d by mechanisms which r e s u l t i n the i r r e g u l a r s t r u c t u r e s observed i n F i g u r e 1. The g r a p h i t e forms a n e a r l y continuous network w i t h i n the numerous e u t e c t i c o r s o l i d i f i c a t i o n c e l l s . An o v e r s i m p l i f i c a t i o n may be made: that gray i r o n i s a s t e e l m a t r i x i n t e r r u p t e d by l a r g e g r a p h i t i c i n c l u s i o n s . When a sample of gray i r o n i s p o l i s h e d and examined u s i n g a microscope, the two-dimensional cut through the g r a p h i t e causes i t t o appear as i s o l a t e d f l a k e s (see F i g u r e 3 ) . The morphology of the f l a k e g r a p h i t e , p a r t i c u l a r l y the sharp edges, a r e thought t o serve as i n t e r n a l notches i n the m a t e r i a l (2). Cracks can r e a d i l y nucleate a t the ends and edges of t h e g r a p h i t e , and propagate through the g r a p h i t e o r along the i r o n g r a p h i t e i n t e r f a c e . Gray i r o n s f r a c t u r e r e a d i l y under s t r e s s . They a r e c h a r a c t e r i z e d by r a t h e r low t e n s i l e strengths (150-300 MNt/m ) n e g l i g i b l e d u c t i l i t y , and low impact s t r e n g t h (_3). These m a t e r i a l s a r e r a t h e r s o f t and r e a d i l y machined. On the p o s i t i v e s i d e , m a c h i n a b i l i t y i s enhanced by the l u b r i c a t i n g e f f e c t o f t h e g r a p h i t e . Y i e l d s (mass of c a s t i n g d i v i d e d by t o t a l mass of metal 2



Figure 3.

Appearance of graphite (200X) in a polished specimen of gray iron: a, Type D; b, Type B; c, Type A



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i n the mold) when producing gray i r o n are r e l a t i v e l y h i g h because of e u t e c t i c g r a p h i t e s o l i d i f i c a t i o n . The m e l t i n g p o i n t i s r e l a t i v e l y low compared t o other f e r r o u s a l l o y s ; which means that l e s s energy input i s r e q u i r e d during production than f o r some other f e r r o u s m a t e r i a l s . In s p i t e o f the l i m i t a t i o n s of i t s p h y s i c a l p r o p e r t i e s , gray i r o n i s a u s e f u l engineering m a t e r i a l . About 60% of the t o t a l c a s t i n g tonnage shipped i n the United States i s gray i r o n (4). Mechanical p r o p e r t i e s of gray i r o n s can be i n t e n t i o n a l l y a l t e r e d by the use of a l l o y i n g elements, such as copper, t i n , manganese, n i c k e l o r chromium. A l l o y i n g a f f e c t s the mechanical p r o p e r t i e s o f the m a t r i x more so than the morphology of the graphite. However, the p h y s i c a l p r o p e r t i e s of the i r o n are p r i m a r i l y a f f e c t e d by g r a p h i t e morphology, whether nodule o r f l a k e ; and, s e c o n d a r i l y , the m a t r i x s t r u c t u r e . This r e s u l t s i n p r a c t i c a l l i m i t a t i o n s upon the u t i l i t y o f a l l o y i n g i n the m o d i f i c a t i o n of physi c a l properties. I t was known f o r some time that i f the g r a p h i t e morphology could be a l t e r e d , c e r t a i n p h y s i c a l p r o p e r t i e s of the cast i r o n s could be improved. The m a l l e a b l e i r o n i n d u s t r y ( s p e c i f i c a l l y , t h e b l a c k - h e a r t m a l l e a b l e i n d u s t r y ) had long made use of annealing procedures a p p l i e d t o c a s t i r o n s t h a t had i n i t i a l l y s o l i d i f i e d as the metastable Fe-Fe3C system (white i r o n ) . Heat t r e a t i n g procedures a p p l i e d t o these white i r o n c a s t i n g s caused the i r o n c a r b i d e t o break down. The carbon d i f f u s e d through the m a t r i x and was accreted a t i s o l a t e d s i t e s . This g r a p h i t e took the form of spheroids r a t h e r than f l a k e s . Irons produced i n t h i s manner had^ higher t e n s i l e strengths than gray i r o n ( t y p i c a l l y 365-700 MNt/m ) and showed a p p r e c i a b l e e l o n g a t i o n (2% t o 18%). The change i n g r a p h i t e morphology, from f l a k e t o s p h e r o i d a l , was the cause i n the improvements i n these mechanical p r o p e r t i e s . There were e s s e n t i a l l y two problems a s s o c i a t e d w i t h m a l l e a b l e i r o n s . One was that the heat treatment procedures were c o s t l y . Secondly, i t was imperative that the i r o n s o l i d i f y i n the metas t a b l e s t a t e . That requirement made production of h e a v i e r , slowcooled s e c t i o n s i m p o s s i b l e s i n c e they would l i k e l y c o n t a i n f r e e g r a p h i t e . However, the higher t e n s i l e strengths and d u c t i l i t y of m a l l e a b l e i r o n s were d e s i r e d i n some cases. Therefore, the problem c o n f r o n t i n g the foundry i n d u s t r y was to somehow produce s p h e r o i d a l g r a p h i t e without heat treatments o r c o n s t r a i n t s upon s e c t i o n t h i c k n e s s e s . I t was from these c h a l lenges that nodular i r o n was developed and i t s use has grown. The r a r e earths have assumed a c r u c i a l r o l e i n the process. One o f the workers attempting t o produce as-cast i r o n c o n t a i n i n g nodules was Henton Morrogh of the B r i t i s h Cast Iron Research A s s o c i a t i o n . Morrogh experimented w i t h a v a r i e t y o f a d d i t i o n s o f a l l o y i n g e l e ments based upon three assumptions 05). He assumed that the nodu l i z i n g element need be an i r o n c a r b i d e s t a b i l i z i n g element, that the element be capable o f d e s u l f u r i z i n g the i r o n , and t h a t t h e element r e a d i l y d i s s o l v e i n i r o n . H i s e f f o r t s r e s u l t e d i n the



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Figure 4.

Spheroids of graphite (250X); etched in bromine and methanol solution

Figure 5.

Appearance of polished nodular iron microsample (200X); 2 % nital. Matrix structure is pearlite.

etched in



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Iron

Production

25

d i s c o v e r y t h a t a d d i t i o n s o f cerium would r e s u l t i n s p h e r o i d a l g r a p h i t e s t r u c t u r e s i n n i c k e l - c a r b o n and i r o n - n i c k e l - c a r b o n a l l o y s . He continued these experiments and u l t i m a t e l y d i s c o v e r e d that s p h e r o i d a l g r a p h i t e could be r o u t i n e l y produced, i n the l a b o r a t o r y , i n i r o n s having 0.02% cerium introduced as mischmetal. The mischmetal was introduced i n t o h y p e r e u t e c t i c i r o n - c a r b o n s i l i c o n a l l o y s c o n t a i n i n g l e s s than 0.06% s u l f u r i n the base, o r untreated i r o n . Morrogh f u r t h e r r e s t r i c t e d the phosphorus l e v e l to l e s s than 0.1% ( 6 ) . Morrogh announced h i s d i s c o v e r y i n the United States a t t h e American Foundrymen's S o c i e t y convention i n P h i l a d e l p h i a i n 1948 (7). I n t e r e s t i n g l y enough, d u r i n g the d i s c u s s i o n f o l l o w i n g h i s p r e s e n t a t i o n , T.H. Wickerdon o f the I n t e r n a t i o n a l N i c k e l Company announced t h a t h i s company had been s u c c e s s f u l i n producing nodul a r i r o n i n a production environment. I n t h e i r process, they had used a d d i t i o n s o f magnesium. This magnesium was contained i n a nickel-magnesium a l l o y . Both hypo- and h y p e r - e u t e c t i c base i r o n s had been s u c c e s s f u l l y t r e a t e d u s i n g the nickel-magnesium a l l o y . The nodular i r o n i n d u s t r y was born. However, there was much work yet t o be done. The acceptance of nodular i r o n as an engineering m a t e r i a l was n e i t h e r overwhelming nor immediate ( 8 ) . The m a t e r i a l remained l a r g e l y a l a b o r a t o r y c u r i o s i t y f o r some time. Studies were undertaken i n order t o e x p l a i n the mechanisms of the g r a p h i t e growth. Even today, the phenomenon o f producing s p h e r o i d a l g r a p h i t e has not been unambiguously e x p l a i n e d . I t has been speculated that t h i s e f f e c t i s the r e s u l t o f i n t e r r e l a t e d r o l e s played by what can be c a l l e d the n o d u l i z i n g elements ( c a l c i u m , magnesium, the r a r e e a r t h s ) . By the formation of o x i d e s , s u l f i d e s o r o x y s u l f i d e s , t h e s u l f u r and oxygen can be removed from the melt. As a r e s u l t , the g r a p h i t e growth p a t t e r n i s a f f e c t e d . I t i s now known that spher o i d a l g r a p h i t e i s c h a r a c t e r i z e d by growth upon the b a s a l planes of the g r a p h i t e c r y s t a l r a t h e r than upon the prism faces as i n gray i r o n . Numerous f i b e r s of g r a p h i t e grow r a d i a l l y from a given n u c l e a t i o n s i t e . As a r e s u l t , a spheroid o f g r a p h i t e i s formed (see F i g u r e s 4 and 5). Other researchers have detected i n c l u s i o n s a t the geometric center of a t l e a s t some of the spheroids of g r a p h i t e i n the nodul a r i r o n s t e s t e d . I n one reported study, u s i n g t r a n s m i s s i o n e l e c t r o n microscopy, these i n c l u s i o n s were analyzed as being the oxi d e s of the r a r e e a r t h elements cerium and lanthanum, as w e l l as magnesium ( 9 ) . Use o f the e l e c t r o n microprobe allowed other workers t o i d e n t i f y s u l f i d e s of cerium, lanthanum and magnesium (10) as w e l l as calcium-magnesium s u l f i d e s (11) a t the centers of nodules (see Figures 6 and 7). I t appears probable t h a t such i n c l u s i o n s provide heterogeneous s u b s t r a t e s on which a t l e a s t some of the g r a p h i t e can r e a d i l y n u c l e a t e . The r o l e o f the n o d u l i z i n g elements can, t h e r e f o r e , be specul a t e d as t w o - f o l d . They cleanse the melt of elements such as oxygen o r s u l f u r which p r o h i b i t s p h e r o i d a l g r a p h i t e growth. The



RARE EARTH ELEMENTS

Cast Metals Research Journal

Figure 6. Photographs of CRT output from characteristic x-ray scans for the various elements listed using electron microprobe Particle scanned was a graphite nodule. Scans verify presence of a heterogeneous inclusion in the nodule. Iron was treated with magnesium and a rare earth silicide. The element distribution pictures were taken at 1800X-



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compounds thus formed provide heterogeneous s u b s t r a t e s f o r graphi t e nucleation. The spheroids of g r a p h i t e no longer serve as i n t e r n a l notches i n the m a t r i x . Consequently, the m a t e r i a l i s not as e a s i l y f r a c tured as gray i r o n . The t e n s i l e s t r e n g t h s , y i e l d s t r e n g t h s , and percentages of e l o n g a t i o n i n c a s t i n g s c o n t a i n i n g s p h e r o i d a l graphi t e are d r a m a t i c a l l y increased over those p r o p e r t i e s i n gray i r o n . For example, nodular i r o n s have t e n s i l e strengths from 300-400 MNt/m and percentages of e l o n g a t i o n from 6% t o 18% i n the as-cast c o n d i t i o n . These q u a n t i t i e s are on the order of those observed i n m a l l e a b l e i r o n s . These p h y s i c a l p r o p e r t i e s can be f u r t h e r modif i e d by a l l o y i n g o r heat t r e a t i n g procedures, j u s t as i n gray i r o n . The m a c h i n a b i l i t y o f nodular i r o n c a s t i n g s i n g e n e r a l l y not as good as f o r gray i r o n c a s t i n g s . Y i e l d s are somewhat lower because o f a l t e r e d s o l i d i f i c a t i o n (around 50% as compared t o 60% t o 70% f o r gray i r o n ) . The good p h y s i c a l p r o p e r t i e s of nodular i r o n coupled w i t h i t s good founding q u a l i t i e s , have e s t a b l i s h e d nodular i r o n ' s place as an engineering m a t e r i a l . As a f i n a l i n t r o d u c t o r y p o i n t , i t should be noted that there i s some confusion w i t h i n the foundry i n d u s t r y , and i t s l i t e r a t u r e , regarding the s p e c i f i c r a r e earths being employed. E a r l y work i n t h i s f i e l d was conducted u s i n g mischmetal. However, i n many i n stances, o n l y the cerium l e v e l was reported i n these t e s t s . The presence of the other r a r e earths was ignored. Even today, t h e elements most o f t e n mentioned are the f i r s t four l a n t h a n i d e s : lanthanum, cerium, praseodymium and neodymium. That i s not t o say that the e f f e c t s o f the other elements i n the s e r i e s would not be s i m i l a r t o those o f the f i r s t four o r that they could not be u t i l i z e d . Rather, t h e i r r o l e s have not been s t u d i e d i n d i v i d u a l l y . I t should be kept i n mind that commercially used r a r e e a r t h sources c o n t a i n s m a l l q u a n t i t i e s o f these other r a r e e a r t h s . From t h i s e a r l y a v a i l a b i l i t y and use of mischmetal grew a demand f o r b a s t n a s i t e ore. I n these a l l o y s , about one-half o f t h e present r a r e earths are cerium. I n the m i d - t o - l a t e s i x t i e s , a more economical source o f cerium was introduced which was, i n essence, a concentrate from which the lanthanum had been removed. This m a t e r i a l allowed f o r the production of a l l o y s whose r a r e e a r t h c o n c e n t r a t i o n was about 90% cerium. These r a r e e a r t h bearing m a t e r i a l s have the approximate analyses shown i n Table I and are now used commercially, w i t h the h i g h cerium source predominating i n the United S t a t e s . The v a r i o u s r a r e earths are used i n the foundry i n d u s t r y as r a r e e a r t h s i l i c i d e s , i n which the r a r e e a r t h content i s about 30%. Other a l l o y s are used i n which the l e v e l of r a r e earths i s about 10% (10% cerium, 2% other r a r e earths) w i t h s i l i c o n and i r o n comprising the bulk o f the remaining elements. I n the magnesiumf e r r o s i l i c o n a l l o y s , the r a r e earths are present i n amounts from about 0.1% t o 1.0%. These a l l o y s are used d i f f e r e n t l y by t h e v a r i o u s consumers. However, the e f f e c t s of the r a r e e a r t h e l e ments, introduced by whatever means, are the same.


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TABLE I Analyses o f Rare E a r t h Sources

Approximate Analyses> %


Mischmetal or Low Cerium Rare E a r t h s High Cerium Rare E a r t h s Cerium

50

90

Lanthanum

33

5

Neodymium

12

2

Praseodymium

4

1

Other Rare E a r t h s

1

2

Transactions of the American Foundrymen's Society

Figure 7. Photographs of CRT output of electron microprobe depicting heterogeneous particle at the center of two graphite nodules (11). Particles were identified as calcium-magnesium sulfides: a, 1375X; b, 5000X; c, 1250X; d, 5000X-


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The r a r e earths p l a y three r o l e s i n the production o f nodular i r o n . These r o l e s are as a n o d u l i z i n g element (or as the growth m o d i f i e r ) as a means o f enhancing the nodule count (or n u c l e a t i o n ) and, f i n a l l y as c o n t r o l l e r s o f d e l e t e r i o u s elements. The use o f the r a r e earths f o r each of these purposes w i l l be described i n d e t a i l i n the f o l l o w i n g s e c t i o n s .


The Rare Earths as N o d u l i z e r s i n the Production o f Nodular Iron The modern foundry process f o r producing nodular i r o n can be o v e r s i m p l i f i e d by d e s c r i b i n g i t as the treatment o f a base i r o n (3% t o 4% carbon, 1% t o 2% s i l i c o n ) having low (0.005% t o 0.05%) s u l f u r l e v e l s and c o n t a i n i n g l i t t l e (

Industrial Applications of Rare Earth Elements - ACS Publications

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