THE ENRICHMENT OF PHOSPHORUS NEAR Mo2 C PRECIPITATES IN STEEL

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THE ENRICHMENT OF PHOSPHORUS NEAR Mo2 C PRECIPITATES IN STEEL

R. Möller, S. Brenner, H. Grabke

To cite this version:

R. Möller, S. Brenner, H. Grabke. THE ENRICHMENT OF PHOSPHORUS NEAR Mo2 C PRECIPITATES IN STEEL. Journal de Physique Colloques, 1986, 47 (C2), pp.C2-277-C2-280.

�10.1051/jphyscol:1986241�. �jpa-00225675�

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THE ENRICHMENT OF PHOSPHORUS NEAR Mo,C PRECIPITATES IN STEEL

R. MOLLER, S.S. BRENNER' and H.J. GRABKE

Max-Planck Institut fur Eisenforschung, Diisseldorf, F.R.G.

'University of Pittsburgh, Pittsburgh, PA 15261, U.S.A.

A b s t r a c t

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Atom p r o b e a n a l y s e s o f Mo2C p r e c i p i t a t e s i n Fe-MO-C-P a l l o y s h a v e shown t h a t t h e c o n c e n t r a t i o n of p h o s p h o r u s i s e n h a n c e d i n t h e v i c i n i t y o f molybdenum c a r b i d e p a r t i c l e s a n d i t i s p r o p o s e d t h a t t h i s t r a p p i n g d e c r e a s e s t h e p h o s p h o r u s c o n c e n t r a t i o n a t g r a i n b o u n d a r i e s l e a d i n g t o a d e c r e a s e d t e m p e r e m b r i t t l e n e s s .

I

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INTRODUCTIOP?

The b e n e f i c i a l e f f e c t o f molybdenum o n t h e t e m p e r e m b r i t t l e n e s s o f s t e e l s i s we1 l known ( 1 ) b u t h a s n o t y e t b e e n s a t i s f a c t o r i l y e x p l a i n e d . S i n c e m o l y b d e n u m h a s l i t t l e e f f e c t o n t h e a m o u n t o f p h o s p h o r u s s e g r e g a t e d a t q r a i n b o u n d a r i e s i n c a r b o n - f r e e i r o n a l l o y s ( 2 ) i t i s q u i t e l i k e l y t h a t m o l y b d e n u m m u s t b e i n t h e f o r m o f c a r b i d e s t o b e a b l e t o s c a v e n g e t h e p h o s p h o r u s .

To t e s t t h i s h y p o t h e s i s w e h a v e u s e d t h e F I M / a t o m p r o b e t o m e a s u r e t h e c o n c e n t r a t i o n of p h o s p h o r u s i n molybdenum c a r b i d e and i n t h e r e g i o n s o f t h e m a t r i x s u r r o u n d i n q t h e p r e c i p i t a t e s .

I1

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EXPERIMENTAL

An a l l o y c o n t a i n i n g F e , M O , C , a n d P was p r e p a r e d b y vacuum c a s t i n g . I t s c o m p o s i t i o n i s g i v e n i n t a b l e 1. A f t e r f o r y i n c , t h e m a t e r i a l was c o l d drawn t o 0.2mm d i a m e t e r . The w i r e s w e r e t h e n s e a l e d i n q u a r t z c a p s u l e s a n d h o m o g e n i z e d a t 9 0 0 ~ ~ f o r 1 6 h o u r s . T h e y w e r e t h e n g i v e n a d d i t i o n a l h e a t t r e a t m e n t s a t l o w e r t e n p e r a t u r e s . Specimens f o r f i e l d - i o n m i c r o s c o p y w e r e p r e p a r e d i n t h e u s u a l manner

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111

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RESULTS

C a r b i d e p r e c i p i t a t e s s e v e r a l nm i n d i a m e t e r a s w e l l a s l a r e r o n e s ( > 3 0 nm) w e r e o b s e r v e d a f t e r a n n e a l i n g f o r 1 4 1 h a t 6 0 0 C.

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S e v e r a l c a r b i d e p a r t i c l e s i n c l o s e p r o x i m i t y t o e a c h o t h e r a r e shown i n f i g u r e 1. T h e a t o m p r o b e a n a l y s i s o f o n e o f t h e m ( l a b e l e d "l"), s h o w n i n f i g u r e 2 a , g a v e a c o m p o s i t i o n o f Mo2C. T h e r e was n o i n d i c a t i o n of d i s s o l v e d p h o s p h o r u s i n t h e c a r b i d e . The c a r b i d e t h u s d i f f e r s f r o m t h e n i t r i d e which d i s s o l v e s a s much a s 5 a t % p h o s p h o r u s ( 4 ) . T h e r e g i o n b e t w e e n t h e c a r b i d e s ( l a b e l e d " 2 " ) , h o w e v e r , e x h i b i t e d c o n s i d e r a b l e p h o s p h o r u s e n r i c h m e n t a s i n d i c a t e d b y t h e s p e c t r u m i n f i g u r e 2b. T h e c o m p o s i t i o n o f t h e i n t e r f a c e r e g i o n s o f t h e s e p a r t i c l e s c o u l d n o t b e m e a s u r e d b u t t h e a n a l y s e s o f o t h e r

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986241

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JOURNAL DE PHYSIQUE

precipitates showed no strong adsorption of phosphorus directly at the carbide-matrix interface and confirmed that there is a n appreciable enhancement of phosphorus in the matrix surrounding the particles.

The distribution o f the phosphorus in the carbide and matrix can be displayed most clearly i n a plot of the sequence of arrival of the ions a t the detector. Figure 3 shows such a plot of the data obtained from a 3nm diameter carbide particle formed during 380h annealing at 6 0 0 ~ ~ . The plot shows twenty atoms per line which was estimated to be approximately equal to the number of atoms collected from each evaporated layer of 0 . 4 ~ ~ thickness. No phosphorus atoms are observed in the carbide nor is there an appreciable enrichment of P at the interface. Instead, there is an enhanced phosphorus concentration of about 2 at.$ in a region approximately 15 nm wide surrounding the carbide.

IS'

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DISCUSSION AND SUMMARY

The atom probe results show that phosphorus is enriched i n the vicinity of Mo2C but not directly a t the interface between the precipitate and t h e matrix. A G i b b s e a n t y p e of e q u i l i b r i u m segregation due to a lowering of surface energies can therefore be ruled out. Since n o phosphorus was found within the carbide the heteroqeneous nucleation of the carbides at MO-P clusters is also not like1 y. The enrichment is probably caused by dislocations formed durinq the nucleation and growth of the precipitates or by the presence of the elastic strain fields surrounding the precipitates, as discussed by Johnson and Voorhees(5). Long-ranae interactions of solutes with carbide precipitates similar t o t h o s e observed in this study were reported previously by Piller, Miller and Brenner for the Fe

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^Tic

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Sb system (6). The effect of the phosphorus trapping on temper-embritt l ement wi l l be discussed e l sewhere.

REFERENCES

1. J i n Yu and C.J.McMahon, Jr., Met. Trans., 11A (1980) 277.

2. D.Y.Lee, E-V-Barrera, J.P.Stark, and H.L.Marcus, Met. Trans., 15A (1984) 1415.

3. c . z ~ r e n n e r and M.K.Miller, J. Metals, 35 (1983) 54.

4. S.S.Brenner and S.D.Walck, Proc. 27th International Field

Emission Svmposium, Tokyo, Japan, eds. Y-Yashiroo and N-Igata, (Publisher Tokyo University, 1980).

5. W.C.Johnson and P.W.Voorhees, Met. Trans. 16A (1985) 337.

6. J.Pil ler, M.K.Mil ler and S.S.Brenner, Proc. 29th

International Field Emission Symposium, eds. H.O.Andren and H.Norden, (Publisher Almquist & Wiksel l Int., Stockholm, Sweden 1982) 473.

TABLE I. Composition of Alloy (at.%)

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F i g u r e 2 a . Mass s p e c t r u m o f F i g u r e 2b. Mass s p e c t r u m o f c a r b i d e ( "1 " i n r e g i o n between f i g u r e 1 . ) showing c a r b i d e s ( " 2 " i n

a b s e n c e of P. f i g u r e 1) showing

P e n r i c h m e n t .

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JOURNAL DE PHYSIQUE

MCCM M M W C MM4

mmccc C C *M MCM M EmmCCM.CFIMCCMMMM

C W C W M ~ M M

C mFICMCCCCMMCMCMMMM

nm: cccm wmccncn M C K C M MCMMCCMCCMMM

~ m c . c c Km.

...

C..M...

...

... ... ...

^P. .S... ^P....

... ...

P . . .

..!...

P P . . .

...

... ...

P . . . .

...

P,...

... ...

..P. ...A

...

.

^P....

...

^P.

...

^p.

...

^P..

...

P.

... ...

^P...

...

^P...

...

... ...

...

carbide

MO,C

F i g u r e 3. F i e l d - e v a p o r a t i o n s e q u e n c e t h r o u g h c a r b i d e a n d r e g i o n s u r r o u n d i n g i t . " M " = molybdenum, " C " = c a r b o n ,

1, 11 = i r o n , " p " = p h o s p h o r u s .