JOURNAL OF MATERIALS SCIENCE LETTERS 14 (1995) 577-579

Diffusion coefficients of Sb and Si in 21Cr-1Mo-NiNb stabilized ferritic steel

M. AHMAD, K. A. SHOAIB, M. A. SHAIKH Radiation Damage Group, Nuc/ear Physics Division, Pakistan/nstitute of Nuc/ear Science and Techno/ogy, P.O. Ni/ore, /s/amabad, Pakistan

K. EHRLICH /nstitut fur Materia/- und Festkorperforchung 1, Kemforchungszentrum Karlsruhe, Germany

Temper embrittlement has been recognized as a problem in alloy steels for well over 50 years, but an understanding of its causes has begun only recently. Generally, it is assumed that segregation of trace elements or impurities, such as P, Sb, Sn and As, to the grain boundaries induces temper embrittlement when the steels are held within the embrittling temperature fange 370-600 °C [1-3].

It has been well established that the amount of Sb at grain boundaries depends on the presence of Ni [4]. Both Mn and Si strongly promote phosphorus segregation, whereas Mo plays a beneficial role up to a certain extent [5, 6]. In addition, impurity segregation can also be influeneed by rather complex interactions with other alloying elements, by involvement with precipitates [7] and possibly with lattice defects [8].

Many of the transformation processes occurring in the solid state, especially those involving precipi- tates, growth of second phases and segregation of trace elements or impurities to the grain boundaries [9], are diffusion controlled. The availability of appropriate diffusion data can help in the interpreta- tion of embrittlement kinetics and may, in some instances, highlight certain details of the embrittle- ment mechanisms. However, most diffusion data are obtained at higher temperatures and this necessit- ates extrapolation towards the lower temperature where embrittlement occurs. Furthermore, in high alloyed materials, volume diffusion coefficients have been shown to depend upon the actual composition of the alloys [10-12]. It is, therefore, necessary to obtain diffusion data for each individual alloy of interest. The diffusion of Sb in steel is also of technical interest in view of Sb coating of steel to improve corrosion resistance [13].

In this work the diffusivity of Sb and Si was studied in a 2¼ Cr- lMo ferritic steel (DIN 1.6770) which is Nb stabilized and contains small additions of Ni. The diffusion measurements were performed in the temperature regions 475 to 600 °C or 800 °C for Sb and Si, respectively.

Chemical composition of the steel investigated is given in Table I. Samples of size 10 x 7 x 3 mm were austenitized at 1150 °C for 1, 5 and 24 h to investigate the influence of austenitization time on grain size. The grain size was determined to be 50 #in for the samples heat treated for 24 h. The

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T A B L E I Chemical composition of 2¼ Cr- lMo-NiNb steel (DIN 1.6770)

Element wt % Element wt %

Cr 2.42 Co 0.011 Mo 1.00 Ta 0.010 Nb 0.78 N 0.009 Mn 0.65 S 0.008 C 0.063 B <10 ppm P 0.012 Fe balance

specimens were polished on three sides to obtain flat surfaces for the diffusion measurements.

Two methods were used to produce diffusion couples. In the first series, steel specimens, sur- rounded by Sb or Si powder of 100 mesh were enclosed in quartz capsules. The capsules were filled with helium gas (99.999% purity) and sealed. The annealing time was 120 h and the temperature was 475 or 600 °C for specimens embedded in Sb and 475 or 800 °C for those embedded in Si. The capsules were quenched in cold watet after diffusion anneal- ing. After heat treatment nearly 1 mm of one of the polished surfaces was removed by grinding and was re-polished to reveal the diffusion interface.

In the second series of experiments, specimens were prepared by vacuum evaporation of thin layers of Sb or Si on a well polished side of the specimen. The above mentioned process was adopted for polishing and for the prevention of oxidation of the specimens. These diffusion couples were annealed in the fange 550-600 "C for the Sb-coated and 475 °C for the Si-coated specimens.

The diffusion couples produced by both methods were also etched to show the grains and concentra- tion was measured, away from grain boundaries, across the interface of Si or Sb with the largest grain.

Measurement of the concentration gradient c(x) was carried out on a scanning electron microscope Philips SEM 505 with an EDAX energy dispersive microanalyser attached. All the data were taken at an accelerating voltage of 30 kV. The concentration profiles were then taken through the largest grain in steps of 1 or 1.5 #m along the x-axis. The data was taken to the point where the concentration of Si or Sb reduced to zero.

Diffusion coefficient for infinite couples can be determined from the well known equation [14]

577

derived from Fick's second law of diffusion

c(x) - c(O) c~~, c~ ~[~ er~(~)] where x is the distance normal to the interface and the interface of the diffusion couples lies at x = 0, c(0) is the concentration of the solute at x < 0, c(1) is the concentration of the solvent at x » 0. For the evaluation of D those ranges were taken, where the concentration c(x) of the solute varies linearly with the distance x. Concentration profiles, c(x) versus x, were plotted for this purpose.

For the diffusion couples prepared by the coating of thin film the expression used for evaluation is

c ( x ) - 2~-D~C°nst exp --4--~-t

D can thus be determined from the slope of the straight line obtained by plotting, for a given time, log c(x) versus x 2.

The diffusion coefficients D of Sb and Si so determined in the steel are given in Table II. Where a direct comparison is possible, the D values determined with infinite couples are slightly higher than those for thin film coated specimens. Also we can state that the diffusivity of Sb is higher than that of Si for the temperature investigated.

The values of the activation energies for the diffusion of Sb and Si in 1.6770 steel were calculated using the relation

log (Dl/D2) = - Q/R(1/T1 - l/T2)

where D1 is the diffusion coefficient at temperature T~, D 2 is the diffusion coefficient at temperature T2 and Q is the activation energy. The frequency factor D 0 was calculated for both Si and Sb by plotting log D versus 1/T and extrapolating the curve to 1/T = 0. The calculated values of D0 and Q are given in Table III.

In Fig. 1 the experimentally determined diffusion data are plotted for Sb and Si as a function of 1lT. In addition the already published data [15, 16] on the diffusivity of both elements in ol-Fe are extrapolated down to the temperature range of interest, i.e. to 475 °C. The published diffusion coefficients of Sb in a dilute Fe-0.69 wt % Sb alloy [17] are also included in Fig. 1. Our results show that diffusivity of Si in 1.6770 steel is different from the diffusivity of Si in oL-Fe. The diffusivity of Sb in 1.6770 steel is very much higher than that in o~-Fe at lower temperature (temper embrittlement region). The deviation in

T A B L E I I I Calculated frequency factor D0 and activation energy Q for Sb and Si in the DIN 1.6770 sample

Tempera ture D o Q Solute range (°C) (m2/s) (eV)

Si 475-800 3.4 x 10 -~2 1.0 Sb 475-600 3 x 10 .9 1.3 Sb 550-600 6.6 x 10 .4 2.3

10-14

10-15

10-16

10 -17

10-18

10-19

10 -20 9

~---_.._...--.. "-....

- - Q ~ - - > ~ ~ .... "~ ~ ) x " ò ' " & . . . . -

~ ' ~ Q . ~ . . . \ -"--'-Zzz:0

[ I I I " - . 2 - . . . ,

10 11 12 13 14 104/T(K -1 )

Figure 1 Diffusion coefficients D versus I/T: - - 0 - - Sb in 1.6770 steel (embedded in powder); - O - Sb in 1.6770 steel (coating); . . . . . C)-- Si in 1.6770 steel (embedded in powder); -- - -- S b - F e alloy in o;-Fe [16] (extrapolation to lower temperature) ; - - - - - - S i -Fe alloy in oL-Fe [16] (extrapolation to lower tempera- ture); - - 0.69 at % S b - F e alloy [17].

diffusivity can be explained on the basis of a difference in concentration of Sb at lower tempera- ture. According to Nishida et al. [15] the value of diffusivity changes rauch more at lower temperature than that at higher temperature, with change in concentration gradient. In the present study the difference in concentration is expected because of the application of a different technique.

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257. 2. L . G . E M M E R , C. D. C L A U S E R and J. R. LOW Jr, ibid.

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Metall. 30 (1982) 1309. 5. J. C. M U Z A and C. J. M c M A H O N Jr, J. Eng. Mater.

Technol. 102 (1980) 369.

T A B L E II Measured diffusion coefficients for Si and Sb in the steel

Type of Tempera ture Anneal ing D Solute couple (°C) time (h) (m2/s)

Si infinite 475 + 2 120 1.2 x 10 -18 Si thin film 475 + 2 240 1.1 x 10 -I8 Si infinite 800 + 2 120 1.3 x 10 «6 Sb infinite 475 + 2 120 2.2 x 10 « s Sb thin film 550 -+ 2 120 6.25 x 10 -is Sb infinite 600 + 2 120 4.0 x 10 «7 Sb thin film 600 + 2 120 3.5 x 10 «7

578

6. T. WADA and W. C. HAGEL, Metall. Trans. 7A (1976) 1419.

7. C .L . SMITH and J. R. LOW Jr, Metall. Trans. 5 (1974) 279-287.

8. B.R. BANERJEE, J. Iron Steellnst. 203 (1965) 166-174. 9. N. SARAFIANOS, Mater. Sci. Eng. 80 (1986) 87-91.

10. J. C. SMITHELLS, "Metal reference book", 5th edn (Butterworth, London, 1976) p. 64.

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13. B. JONES and J. D. D. MORGAN, ibid. 1 (1934) 115. 14. J. CRANK, "The mechanics of diffusion" (Oxford Univer-

sity Press (Clarendon), London (1975). 15. K. NISHIDA, H. MUROHASHI and T. YAMAMTO,

Trans. Jap. Inst. Metals 20 (1979) 269-275. 16. H. OIKAWA, Technol. rep., Tohoku Univ. 47 (1982)

215-224. 17. P. GAS, S. POIZE and J. BERNARDINI, Acta Metall. 34

(1986) 395-403.

Received 18 Augus t

and accepted 28 October 1994

579