ALLOY CARBIDE PRECIPITATION IN A HIGH COBALT-NICKEL SECONDARY HARDENING STEEL

HAL Id: jpa-00225932

https://hal.archives-ouvertes.fr/jpa-00225932

Submitted on 1 Jan 1986

HAL

is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire

destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

ALLOY CARBIDE PRECIPITATION IN A HIGH COBALT-NICKEL SECONDARY HARDENING

STEEL

J. Liddle, G. Smith, G. Olson

To cite this version:

J. Liddle, G. Smith, G. Olson. ALLOY CARBIDE PRECIPITATION IN A HIGH COBALT-NICKEL

SECONDARY HARDENING STEEL. Journal de Physique Colloques, 1986, 47 (C7), pp.C7-223-C7-

231. �10.1051/jphyscol:1986739�. �jpa-00225932�

ALLOY CARBIDE PRECIPITATION IN A HIGH COBALT-NICKEL SECONDARY HARDENING STEEL

J . A . L I D D L E , G.D.W. SMITH and G . B . OLSON

Department of Metallurgy and Science of Materials, University of Oxford, Parks Road, GB-Oxford OX1 3 P H , Great-Britain

Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, M A 02139, U.S.A.

A b s t r a c t

-

-

-

-

-

1%Mo

-

1

-

High Co-Ni secondary h a r d e n i n g s t e e l s e x h i b i t o u t s t a n d i n g c o m b i n a t i o n s of s t r e n g t h and toughness. The HP94X s t e e l s were t h e f i r s t of t h i s t y p e . These were f o l l o w e d by HY180 [ I ] and t h e n by AF1410 [2]. The l a t t e r e x h i b i t s t h e b e s t c o m b i n a t i o n of s t r e n g t h and t o u g h n e s s of any commercially a v a i l a b l e s t e e l . It i s t h e r e f o r e i m p o r t a n t t o d i s c o v e r t h e r e a s o n s f o r i t s o u t s t a n d i n g p r o p e r t i e s .

A p r e l i m i n a r y FIM/AP i n v e s t i g a t i o n by Chang e t a l . [ 3 ] l o o k e d a t m a t e r i a l i n t h e s t a n d a r d h e a t t r e a t e d and overaged c o n d i t i o n s , and r e v e a l e d t h e p r e s e n c e of s m a l l r o d - l i k e c a r b i d e p a r t i c l e s . A TEM e x a m i n a t i o n of h e a v i l y overaged m a t e r i a l , by Krzanowski ( u n p u b l i s h e d ) showed t h e p r e s e n c e of much c o a r s e r c a r b i d e s . The p r e s e n t work i s i n t e n d e d t o p r o v i d e a more s y s t e m a t i c s t u d y of t h e growth of t h e s e a l l o y c a r b i d e s a t t h e s t a n d a r d h e a t t r e a t m e n t t e m p e r a t u r e .

2

-

The m a t e r i a l examined was s u p p l i e d by C a r p e n t e r S t e e l Co., Reading, P e n n s y l v a n i a , U.S.A. Four p i e c e s 12.5mm s q u a r e and lOOmm l o n g were c u t from a n i n g o t of c o m p o s i t i o n .163C, 1.03M0, 2.10Cr, 10.21Ni, and L4.24Co (wt%) b a l . Fe.

The i n i t i a l h e a t t r e a t m e n t u s e d on t h e s e samples was 1 . 5 h r s a t 830°C; o i l quench; 1 h r a t -70°C; a i r warm. With a n a g e i n g t r e a t m e n t of 5 h r s a t 510°C t h i s m a t e r i a l g a v e K I C v a l u e s o f 200MPa m1/2 a n d U.T.S. v a l u e s o f 1.65GPa. M a t e r i a l f o r e x a m i n a t i o n i n t h e TEM and FIM was tempered i n t h e form of s m a l l s h e e t s (0.5mm x 5mm x 35mm). For t i m e s up t o 1 5 mins samples were t r e a t e d i n a s a l t b a t h , w h i l e

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986739

C7-224 JOURNAL DE PHYSIQUE

f o r l o n g e r t i m e s samples were f u r n a c e - t r e a t e d a f t e r s e a l i n g i n t o s i l i c a t u b e s u n d e r .25 atmosphere of A r , i n o r d e r t o p r e v e n t d e c a r b u r i z a t i o n and o x i d a t i o n . A l l tempering was c a r r i e d out a t 510°C, and samples were quenched i n t o i c e d water.

O p t i c a l microscope, SEM, TEM, FIM and atom p r o b e e x a m i n a t i o n of t h e samples was c a r r i e d o u t . Both t h e o l d e r Oxford FIM/AP w i t h a 125cm s t r a i g h t f l i g h t t u b e , a n d t h e newer FIM 100 w i t h a 226cm curved f l i g h t t u b e w i t h a P o s c h e n r i e d e r t y p e energy compensator were used i n t h i s work. These i n s t r u m e n t s have been d e s c r i b e d i n d e t a i l e l s e w h e r e [ 4 ] [5]. FIM specimens were p r e p a r e d by a s t a n d a r d two s t a g e e l e c t r o p o l i s h i n g t e c h n i q u e ; t h e f i r s t s t a g e u s i n g a 25% p e r c h l o r i c a c i d 75% a c e t i c a c i d e l e c t r o l y t e , and t h e second s t a g e 2% p e r c h l o r i c a c i d i n 2-butoxyethanol.

3

-

Hardness Measurements

Hardness v a l u e s a g a i n s t a g e i n g time a t 510°C were o b t a i n e d f o r t o t a l a g e i n g t i m e s of up t o 1 6 h r s (See Fig.1). As c a n be s e e n t h e h a r d n e s s of t h e a s r e c e i v e d m a t e r i a l i s h i g h , a t 530HV. On tempering t h e r e i s a r a p i d i n i t i a l i n c r e a s e 4%

h a r d n e s s , followed by a s l o w e r r i s e t o a peak h a r d n e s s of 574HV a f t e r 1 5 mins. The h a r d n e s s f a l l s s l o w l y u n t i l 5 h r s , and t h e n d e c l i n e s s h a r p l y between 5 and 1 6 h r s

-

s u g g e s t i v e of e i t h e r a r e c o v e r y p r o c e s s coming i n t o p l a y , o r t h e o n s e t of r a p i d c o a r s e n i n g , o r both.

-

FIM p i c t u r e s of AF1410 were t a k e n i n t h e a s r e c e i v e d c o n d i t i o n , and a f t e r a g e i n g a t 510°C f o r 1, 5, 15, and 30mins and 1, 3, 5 , 8 and 1 6 h r s ( s e e Fig.2). The photographs show t y p i c a l s t r u c t u r e s s e e n a t t h e v a r i o u s a g e i n g t i m e s . I n t h e a s r e c e i v e d c o n d i t i o n t h e most n o t i c e a b l e f e a t u r e s a r e t h e s m a l l randomly d i s t r i b u t e d b r i g h t l y imaging s p o t s , b e l i e v e d t o b e Mo atoms. A f t e r tempering f o r 1 and 5mins s m a l l c l u s t e r s of b r i g h t atoms a p p e a r

-

Comparison w i t h t h e g r a p h of h a r d n e s s vs a g e i n g time shows t h a t t h e i n i t i a l c l u s t e r f o r m a t i o n c o r r e s p o n d s t o t h e rise t o peak h a r d n e s s , w h i l e t h e development of t h e r o d - l i k e morphology up t o tempering times of 5 h r s i s a s s o c i a t e d w i t h a r e l a t i v e l y s m a l l d e c r e a s e i n h a r d n e s s , and t h e s p h e r o i d i s a t i o n and c o a r s e n i n g c o r r e s p o n d s t o t h e r a p i d f a l l i n h a r d n e s s between 5 and 16hrs.

It h a s been assumed t h a t t h e a l l o y c a r b i d e s i n s u c h s t e e l s a r e n u c l e a t e d on d i s l o c a t i o n s . It i s sometimes p o s s i b l e t o s e e i f t h i s i s t h e c a s e u s i n g t h e FIM, b e c a u s e when d i s l o c a t i o n s i n t e r s e c t a prominent p o l e t h e c o n c e n t r i c r i n g s round t h e p o l e w i l l be c o n v e r t e d i n t o s p i r a l s i f t h e d i s l o c a t i o n s have a Burgers v e c t o r component normal t o t h e s u r f a c e . However, i t i s n o t p a r t i c u l a r l y e a s y t o o b s e r v e t h i s because b r i g h t l y imaging atoms t e n d t o draw imaging g a s from t h e s u r r o u n d i n g a r e a a n d s o r e n d e r t h e r e l e v a n t r e g i o n of m a t r i x b a r e l y v i s i b l e . Also d i s l o c a t i o n s d o n o t always c o n v e n i e n t l y i n t e r s e c t prominent p o l e s . . D e s p i t e t h e s e d i f f i c u l t i e s however, p a r t i c l e s were s e e n t o l i e on d i s l o c a t i o n s i n some i n s t a n c e s , but t h i s i s by no means always t h e c a s e .

p a r t i c u l a r micrograph, d i v i d i n g by t h e f i e l d of view, and d i v i d i n g a g a i n by t h e a v e r a g e p a r t i c l e s i z e ; t h i s l a s t because, a s t h e p a r t i c l e s g e t l a r g e r e a c h one h a s a g r e a t e r chance of i n t e r s e c t i n g a g i v e n sampling plane. (The p a r t i c l e s i z e was t a k e n t o be t h e a v e r a g e of t h e l a r g e s t and s m a l l e s t dimensions of a p a r t i c u l a r s e t of p a r t i c l e s ) . It s h o u l d be p o i n t e d o u t t h a t t h e r e a r e s e v e r a l s o u r c e s of e r r o r i n v o l v e d i n such measurements. At s h o r t t i m e s , i.e. 1 and Smins, i t i s very d i f f i c u l t t o d e c i d e what i s and i s n o t a p a r t i c l e . T h i s i s n o t h e l p e d by t h e f a c t t h a t a s t h e f i e l d of view i n c r e a s e s w i t h i n c r e a s i n g v o l t a g e , t h e p a r t i c l e s a p p e a r s m a l l e r ; and what may have passed a s a c l u s t e r of atoms a t a lower v o l t a g e now a p p e a r s t h e same s i z e as a s i n g l e atom d i d p r e v i o u s l y . However, a s t h e a g e i n g t i m e s g e t l o n g e r , and t h e p a r t i c l e s become more c l e a r l y d e f i n e d , t h i s c e a s e s t o be a problem. U n f o r t u n a t e l y , though, t h e number of p a r t i c l e s s t a r t s t o f a l l r a p i d l y , f o r a g e i n g t i m e s of l h r o r more, and s o t h e s t a t i s t i c a l e r r o r becomes more s i g n i f i c a n t .

To g i v e some i d e a of t h e weight t h a t s h o u l d b e g i v e n t o e a c h p o i n t on t h e graph, t h e numbers of p a r t i c l e s examined f o r each h e a t t r e a t m e n t were, on a v e r a g e , 1 7 0 f o r t i m e s of 1-30mins and 25 f o r t i m e s of 1-8hrs. A l l t h e p o i n t s a r e p l o t t e d w i t h e r r o r b a r s of one p o p u l a t i o n d e v i a t i o n on e i t h e r s i d e . The l i n e of b e s t f i t c a l c u l a t e d from t h i s g r a p h f o r a g e i n g t i m e s between 15mins and 1 6 h r s h a s t h e e q u a t i o n :

where t i s i n h o u r s .

The c o r r e l a t i o n c o e f f i c i e n t c a l c u l a t e d was 0.965, i n d i c a t i n g a good f i t t o t h i s l i n e .

A g r a p h of a p p a r e n t p a r t i c l e s i z e vs a g e i n g time was o b t a i n e d (Fig.4) by measuring t h e p a r t i c l e image s i z e s from t h e micrographs. (The l a r g e s t d i m e n s i o ~ i of e a c h p a r t i c l e h a s been t a k e n a s i t s s i z e ) . The a d d i t i o n a l m a g n i f i c a t i o n f a c t o r makes i n t e r p r e t a t i o n of t h e r e s u l t s d i f f i c u l t , s i n c e a t s m a l l p a r t i c l e s i z e s t h e e x t r a e f f e c t w i l l be l a r g e , b u t a s t h e p a r t i c l e s approach t h e s i z e of t h e t i p t h e i r a p p a r e n t s i z e w i l l t e n d towards t h e i r a c t u a l s i z e . The change i n morphology i s a l s o a n i m p o r t a n t f a c t o r . However, i t i s s t i l l p o s s i b l e t o make some g e n e r a l comments on t h e graph.

At t h e s h o r t e s t a g e i n g t i m e s t h e p a r t i c l e s i z e a p p e a r s t o be roughly c o n s t a n t , b u t , a s a g e i n g c o n t i n u e s t h e r e i s a smooth i n c r e a s e i n p a r t i c l e s i z e , w i t h a s m a l l d i s c o n t i n u i t y a t 5 h r s ( p a r t i c l e s i z e 120 A). I f a l i n e of b e s t f i t i s c a l c u l a t e d f o r t h i s graph, f o r times of 15mins onwards, i t s e q u a t i o n is:

where r i s i n A, and t i s i n h r s .

The c o r r e l a t i o n c o e f f i c i e n t c a l c u l a t e d f o r t h i s l i n e i s 0.987, which a g a i n i n d i c a t e s a good f i t t o t h e d a t a .

C7-226 JOURNAL DE PHYSIQUE

Atom Probe Analysis

Atom probe analysis was carried out on all heat treatments examined by FIM.

The compositions of the particles probed for each heat treatment are given below:

Heat treatment Fe:Cr:Mo M:C lhr 75 ions .94: 1:.53 7.6:1

69 ions .17: 1:.83 4.0:1 116 ions 0.0: 1:2.0 7.5:1 3hrs 73 ions 2.9: 1:1.2 15.7:1

67 ions 1.0: 1:1.3 11.5:1 5hrs 192 ions 0.0: 1:.47 4.4:1 8hrs 308 ions* 0.0: 1:0.5 3.0:1 504 ions+ 0.0: 1:.92 2.3:1 16hrs 150 ions+ 0.0: 1:.75 2.2:1

(• = With image gas, + = Without image gas)

Since at the shorter ageing times the particles are relatively small, the probe hole covers part of the matrix, and so matrix material is analysed along with the particle. In order to obtain a more accurate composition, in these cases, it was assumed that of the elements Fe, Nl and Co, only Fe would actually be part of the carbide particle. To correct for the Fe analysed from the surrounding matrix the numbers of Co and Ni ions appearing in each particle analysis were added and the appropriate number of Fe ions (%Fe/(%Co + %Ni) =3) were subtracted from the analysis.

As can be seen from these results, the quantity of carbon being analysed from each particle is well below what would be expected from M2C carbides that are formed in Mo/Cr containing steels. It was suggested that the presence of image gas might somehow affect the amount of carbon detected. This same effect has been observed in the analysis of bainite [6]. To see if this was indeed the case, it was decided to probe two carbides in the 8hr aged sample; one with, the other without image gas present. As is evident from the table there is a significant difference between the two cases.

Despite the uncertainty of these results some interesting trends can be seen. At shorter ageing times it appears that there is a relatively high Fe content to the particles, but this falls to zero as the particles begin to coarsen rapidly. It is also clear that, even allowing for the possible effects of probing with image gas present, the compositions of the particles are not close to an M2C stoichiometry. This is particularly pronounced for samples aged for short times.

The composition tends towards M^C as ageing proceeds. In addition, the Cr/Mo ratio varies as a function of ageing time; at short times the ratio is close to, or even less than 1, but this is reversed for longer times, as was shown by Chang et al.

[3.].

Random area analyses were also made of material heat treated for times of less than lhr. In the samples aged for 1 and 5mins, the carbon was not evenly distributed in the material, because the amount picked up was a factor of two too low. This suggests that, at these ageing times the carbon is "locked up" in specific areas; perhaps in cementite particles. At ageing times of 15 and 30mins the carbon analyses are much closer to what would be expected from the bulk composition. This suggests that the carbon is now more evenly distributed in the material - which is consistent with the appearance of the first fine alloy carbides

(i.e. the analysis integrates over many fine clusters).

Boundaries decorated with brightly imaging atoms were found in samples aged for 1, 5 and 8hrs. It proved possible to analyse the boundaries in the 1 and 8hr specimens. Segregation of Cr, Mo and C to the boundaries was observed. In both instances the problem of the probe hole covering the matrix as well as the area of interest was encountered, and quantitative estimation of segregation was difficult.

The p r e c i p i t a t i o n sequence o c c u r r i n g i n AF1410 i s complex, i n v o l v i n g t h e t r a n s f o r m a t i o n of a l l o y c a r b i d e s from c l u s t e r s t o r o d s t o s p h e r o i d s and t h e n c e t o l a r g e r r o d s , a s shown s c h e m a t i c a l l y i n Fig.5. It i s i n t e r e s t i n g t o compare t h i s s t u d y w i t h t h e work of Davies [ 9 ] a n d Davies and Ralph [ l o ] , who examined a 3.5Mo 0.2C s t e e l . A comparison of t h e t i m e s and t e m p e r a t u r e s s t u d i e d i s g i v e n i n t h e t a b l e below:

AF1410

Temp OC 5 1 0 593*

Time ( s )

1 0 s o l u t e atoms

102

s m a l l c l u s t e r s

s m a l l r o d s

s p h e r o i d s s p h e r o i d s

rods

s p h e r o i d s s p h e r o i d s

r o d s

r o d s

1

o6

(*TEM by J.Krzanowski a t AMRRC, Watertown, U.S.A.).

It i s i n t e r e s t i n g t o n o t e t h e c o n s i d e r a b l y s l o w e r k i n e t i c s of p r e c i p i t a t e growth a p p a r e n t i n t h e 3.5Mo 0.2C a l l o y ; a peak h a r d n e s s of 574HV i s r e a c h e d i n 15mins by AF1410 a t 510°C w h i l e t h e model a l l o y t a k e s l O h r s t o r e a c h peak h a r d n e s s a t 575°C. (A s i m i l a r 4.OMo 0.2C a l l o y examined by I r a n i e t a l . [ l l ] gave a peak h a r d n e s s of 400HV a f t e r 5 h r s a t 600°C). U n f o r t u n a t e l y , i t i s not p o s s i b l e t o compare t h e r e s u l t s o b t a i n e d f o r AF1410 a t s h o r t a g e i n g t i m e s w i t h ones from t h e 3.5Mo 0.2C a l l o y , s i n c e , perhaps s u r p r i s i n g l y , no work was done on t h i s a l l o y a t s h o r t tempering times. The r e a s o n f o r t h e l a r g e d i f f e r e n c e i n k i n e t i c s i s t h o u g h t t o be due t o t h e much . f i n e r d i s p e r s i o n of a l l o y c a r b i d e s produced i n AF1410, which r e s u l t s from t h e h i g h e r d i s l o c a t i o n c o n t e n t , and t h e g r e a t e r d i f f u s i v i t y of Cr compared t o Mo. The d i s l o c a t i o n s w i l l p r o v i d e s h o r t c i r c u i t p a t h s f o r d i f f u s i o n of s o l u t e atoms t o p a r t i c l e s ( e s p e c i a l l y d i s l o c a t i o n n u c l e a t e d o n e s ) , and t h e h i g h e r d i f f u s i v i t y of C r w i l l i n c r e a s e t h e f l u x of s o l u t e atoms f o r p r e c i p i t a t e growth s t i l l f u r t h e r .

Once t h e l e v e l s of C r , Mo and C have r e a c h e d s u f f i c i e n t l y h i g h v a l u e s i n t h e c l u s t e r s , c o h e r e n t , r o d - l i k e p r e c i p i t a t e s s t a r t t o form. A s tempering c o n t i n u e s , t h e r o d - l i k e p a r t i c l e s i n AF1410 grow u n t i l t h e y r e a c h a c r i t i c a l l e n g t h of a b o u t 120 8 . At t h i s p o i n t l e n g t h e n i n g a p p e a r s t o c e a s e and t h i c k e n i n g t a k e s p l a c e . The l a t t i c e c o n s t a n t of t h e f e r r i t e m a t r i x a l o n g [100] i s c a l c u l a t e d a s 2.872 A and t h e l a t t i c e c o n s t a n t of M2C a l o n g [ 1 1 2 0 ] i s 2 . 9 6 8 a s m e a s u r e d by Krzanowski from c a r b i d e s e x t r a c t e d from overaged AF1410). The l a t t i c e mismatch i s t h e r e f o r e p r e d i c t e d t o r e a c h one r e p e a t d i s t a n c e i . e . 2.96 A i n a l e n g t h of 33.6 u n i t c e l l s , o r a b o u t 100 A . Given t h a t t h e Fe c o n t e n t i n c a r b i d e s produced a f t e r s h o r t a g e i n g t i m e s w i l l probably r e d u c e t h e mismatch between c a r b i d e and m a t r i x f u r t h e r , and C r i s known t o r e d u c e t h e l a t t i c e p a r a m e t e r of M2C c a r b i d e s [12], t h e n t h e observed

C7-228 JOURNAL DE PHYSIQUE

c r i t i c a l l e n g t h of 120 A i s i n good agreement w i t h t h a t c a l c u l a t e d . For t h e p r e c i p i t a t e s t o grow any f u r t h e r they m s t g e n e r a t e i n t e r f a c e d i s l o c a t i o n s t o accommodate t h e i n c r e a s i n g mismatch. However, t h e Burgers v e c t o r s of t h e s e i n t e r f a c e d i s l o c a t i o n s

-

-

The g r a p h of a p p a r e n t p a r t i c l e s i z e vs a g e i n g time g i v e s r - a s p r o p o r t i o n a l t o However, a s was mentioned e a r l i e r , t h e d i f f e r e n c e between a c t u a l and a p p a r e n t p a r t i c l e s i z e s d e c r e a s e s a s t h e p a r t i c l e s i z e s i n c r e a s e

-

Acknowledgements

T h i s work was s u p p o r t e d by t h e United S t a t e s Army Research Development and S t a n d a r d i s a t i o n Group under c o n t r a c t No.DAJA45-86-M-0187.

R e f e r e n c e s

[ I ] Speich G.R., Dabkowski D.S. and P o r t e r L.F., Met. Trans.

4

[ 2 ] General Dynamics, U.S.Patent, No. 4,076,525. (1978).

[ 3 ] Chang L., Smith G.D.W. and Olson G.B., J. De Physique Supplement t o No.3

47

(19.86) 265.

[ 4 ] Cerezo A., Smith G.D.W. and Waugh A.R., J. De Physique Supplement t o No.12

2

(1984) 329.

[ 5 ] M i l l e r M.K., Beaven P.A. and Smith G.D.W., S u r f a c e and I n t e r f a c e A n a l y s i s ,

1

(1979) 149.

[ 6 ] Rose, D . J . , J . Appl. Phys.

7

[ 7 ] B i r d s e y e P.J., Smith D.A. and Smith G.D.W., J. Phys. D. (Appl. Phys.) l ( 1 9 7 4 ) 1642.

[ 8 ] W o r r a l l G.M., P r i v a t e Communication.

[ 9 ] Davies D.M., D.Phil T h e s i s , Cambridge (1970).

[ 101 Davies D.M. and Ralph B., J.I.S.I.,

210

[ 111 I r a n i J. J. and Honeycombe R.W.K., J.1.S .I.

203

[ 1 2 ] Morikawa M., Komatsu H. and Tanino M., J. E l e c t r o n Microscopy,

22

420

i

Fig.3 Particle number density vs ageing time.

0

- I ₁

3 hrs

Fig.4 Apparent particle size vs ageing Fig.5 Evolution of particle morphology

time

.

30URNAL DE PHYSIQUE C7-230

a)As received 10-9 Kv b)1min 100 Kv

c) 5mins 14-36 Kv d) 15 mins 80 Kv

e) 30 mins 12-1 Kv f) 1 hr 8-86 Kv

Fig.2 Sequence of FIM images showing development of particles as a function of ageing time at 510°C. All micrographs were recorded using neon image gas.

g) 3hrs 7-5 Kv h) 3hrs 7-5 Kv

i) 5hrs 5-6 Kv j)5hrs8-15Kv

Fig.2 Continued.