THE SILICON EFFECT IN THE TEMPERING OF MARTENSITE IN STEELS

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THE SILICON EFFECT IN THE TEMPERING OF MARTENSITE IN STEELS

Li Chang, G. Smith

To cite this version:

Li Chang, G. Smith. THE SILICON EFFECT IN THE TEMPERING OF MARTEN- SITE IN STEELS. Journal de Physique Colloques, 1984, 45 (C9), pp.C9-397-C9-401.

�10.1051/jphyscol:1984966�. �jpa-00224454�

JOURNAL DE PHYSIQUE

Colloque C9, supplement au n°12, Tome 45, decembre 198* page C9-397

THE SILICON EFFECT IN THE TEMPERING OF MARTENSITE IN STEELS

Li Chang and G.D.W. Smith

Department of Metallurgy and Soience of Materials, University of Oxford, Parks Road, Oxford 0X1 3PH, U.K.

Résumé - L'exceptionnelle résistance à la trempe des aciers à haute teneur en silicium est connue depuis longtemps, mais les raisons de cet effet demeurent incomprises. Nous décrivons ici une étude du troisième stade de trempe de ces aciers, au cours duquel la cémentite se forme. Dans un acier allié à 0,75 1 en poids de C - 1,4 % en poids de Si, trempé une heure à 380°C, on trouve que les atomes de silicium sont rejetés de la cémentite en cours de croissance vers la matrice de ferrite, pour former une atmosphère enri- chie en silicium autour des particules. La concentration de silicium dans la zone de ségrégation atteint 12 % dans une région de 1-2 nm de large adjacente à la cémentite. Les résultats sont en accord avec un mécanisme dans lequel la région riche en silicium constitue une barrière cinétique à la croissance ultérieure de la phase cémentite, et où la diffusion du silicium en dehors de l'interface contrôle les processus de croissance et d'épaississement.

Abstract - High-silicon steels have long been known to possess exceptional resistance to tempering, but the reasons for this effect were unknown.

We report here a study of the third stage of tempering of these steels, in which cementite is formed. In a 0.75 wt% C - 1.4 wt% Si alloy steel, tempered for 1 hour at 380°C, it is found that silicon atoms are rejected from the growing cementite into the ferrite matrix, and form a silicon- enriched atmosphere around the particles. The concentration of silicon in the segregated zone reaches levels of 12% in a region 1-2 nm wide adjacent to the cementite. The results are consistent with a mechanism in which the silicon-rich region is the kinetic barrier to the further growth of the cementite phase, and the diffusion of silicon away from the interface controls the growth and coarsening processes.

1 - INTRODUCTION

High silicon steels are known to possess exceptional resistance to tempering - hence they are of considerable interest in industrial applications.

Although the retarding effect of silicon has long been recognised, the mechanism by which silicon affects the tempering process has been the subject of dispute.

One school of thought has been that silicon may diffuse into the epsilon carbide phase during the first stage of tempering and stabilize it against the transition to cementite [l,2]. An alternative view was that the rejection of silicon ahead of growing cementite particles was the dominant factor in the retarding process [3]. In 1981, Barnard and Smith reported the use of the atom probe to

investigate the first stage of tempering in these steels, and demonstrated that there was no significant effect of silicon on the formation and stability of epsilon carbide [4],

This result implies that the most likely explanation of the silicon effect on tempering resistance is the retardation of cementite growth in the third stage.

However Barnard and Smith did not carry out any direct experiments to demonstrate this. We now report a study of the third stage of tempering, in which cementite is formed, using the atom probe.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984966

C9-398 JOURNAL DE PHYSIQUE

2

-

The commercial silicon steel, EN45 (0.6 wt% C

-

Instead, a pure 0.75 wt% C

-

-

FIM specimens were prepared from heat treated materials by a standard two-stage polishing method. The first stage consisted of 25% perchloric acid in acetic acid, floating on carbon tetrachloride, with a polishing voltage of 25 volts at room temperature. The second stage was carried out in 2% perchloric acid in 2- Butoxyethanol, with a polishing voltage of 30 volts at a temperature below O°C.

Atom probe analyses were carried out using a pulse fraction of 20% of the D.C. tip voltage. This was to minimise errors due to the anomalous evaporation behaviour of silicon in iron [5]. Specimens were cooled with liquid nitrogen, and analyses were carried out in a vacuum of torr. Both a time-of-flight atom probe

(TOFAP) and an imaging atom probe (IAP) were used in this work.

3

-

The IAP was used to show the spatial distribution of carbon, silicon and iron.

The dark areas in FIM image, Fig.1, are the cementite particles and the bright area is the ferrite phase. The gated images of carbon and silicon are strikingly different, showing clearly that silicon depleted areas correspond to the cementite regions.

Fig.1

-

F.

F E R R I T E

I

C E M E N T I T E

Fig.2

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Typical TOFAP spectra from the f e r r i t e and the cementite phases are shown i n Fig.2. The measured concentrations of s i l i c o n and carbon i n each phase are l i s t e d i n Table 1 .

JOURNAL DE PHYSIQUE

CEMENTITE FERRITE

a

3 8 0 ~ ~ 1 1 hour

C a r b o n

n u m b e r o f i o n s ( ~ 1 0 0 ) 1 5 -

1 0 a t %

5 0

4 0 o 0 c l l hour

I+ INTERFACE

I

I .

-

. . . . . . .

- . . ' . . _{. .}

. .

. . . .

-.:

. .

I c INTERFACES +I

a t %

FERRITE a

(b)

I

I

I I

0 5 10 ^I 15 ^I 2 0 ^I

number of ions ( ~ 9 0 0 )

Fig.3

-

(a) at 380°C/1 hour (b) at 40O0C/lhour

Table 1 Tempering

T OC Si at % C at X Total ions Ferrite 380 3.05 f 0.33 0.33

+

400 3.31 f 0.14 0.22 f 0.04 17118 Cementite 380 0.75 f 0.33 19.3 f 1.5 694

400 1.20 f 0.45 21.3 f 1.7 588

Composition profiles across interfaces between the ferrite and cementite phases are shown in Fig.3. A localized silicon-rich region 1-2nm wide can be seen in the ferrite phase adjacent to the cementite. The silicon concentration in such segregated zones was usually as high as 12 f 3% after tempering for 1 hour at 380°C and 9 f 3% after tempering at 400% for the same period. (The comparison is made on the basis of the same sample size of 100 ions). The silicon enhancement factor near the interface is about 4

-

4

-

From the above results, it is apparent that silicon atoms are rejected from the cementite phase into the ferrite, and form a silicon-enriched region near the interface, at least during the initial part of the third stage of tempering.

As the solubility of silicon in the cementite at metastable equilibrium is low and the diffusion of silicon as a substitutional atom in the ferrite is very slow at low temperatures in comparison with carbon diffusion, the silicon-enriched ferrite region around the cementite must represent a kinetic barrier to the further growth of the carbide. Also the silicon-enriched atmosphere around the cementite may slow down the transfer of silicon atoms from the cementite to the ferrite, so that the first formed cementite still contains appreciable quantities of silicon. It requires long-term tempering at high temperatures to complete this transfer.

Then, the presence of silicon in the cementite will decrease the stability of the precipitate. Principally, however, the diffusion of silicon away from the interfacial region will control the growth and coarsening of the cementite phase.

Acknowledgements

Financial support for this work was provided by the Science and Engineering Research Council (SERC). Li Chang wishes to thank the Ministry of Education, Taiwan, ROC for the provision of a graduate studentship.

References

[I]

227,

[2] Gordine J. and Codd I., J.I.S.I.,

207,

[3] Owen W.S., Trans. A.S.M.,

46,

[4] Barnard S.J. and Smith G.D.W., 28th I.F.E.S., 1981, Portland, Oregon, U.S.A.

[5] Miller M.K. and Smith G.D.W., J.Vac.Sci.Techno1.

19,