Grain growth in austenitic stainless steels

Grain growth in austenitic stainless steels

235 METALLOGRAPHY 11,235-239 (1978) Grain Growth in Austenitic Stainless Steels R. M. GERMAN Metallurgy and Electroplating Division 8312, Sandia La...

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235

METALLOGRAPHY 11,235-239 (1978)

Grain Growth in Austenitic Stainless Steels

R. M. GERMAN Metallurgy and Electroplating Division 8312, Sandia Laboratories, Livermore

In the case of both metallic and ceramic systems, exposure to elevated temperatures generally results in grain growth with a concomitant loss in strength and fracture toughness. In many instances, grain growth is also undesirable for reasons other than diminished mechanical properties. For example, grain growth during sintering reduces the number of diffusion vacancy sinks, which results in a diminished densification rate [i]. Alternatively, grain growth during the processing of high coercivity magnets can greatly diminish their value as permanent magnets [2]. Such considerations indicate that a knowledge of grain growth kinetics is fundamental to material processing. Recent studies on the sintering of 304L stainless steel powder compacts showed an intimate coupling of the grain growth and densification processes [3]. The effect was particularly dominant in the intermediate stage of sintering in which the pore structure became rounded (i.e., temperatures in excess of 1330K and times in excess of one hour). Metallographic examination of the sintered 304L specimens suggested that the pore and grain structures were interacting in a predictable manner, giving a possible means of estimating the activation energy for grain growth from the shrinkage rate temperature dependence [4-6]. The determination, based on a model by Coble [4], gave the activation energy for grain growth as 285 ± 35 kJ/mol. In an attempt to assess the relative merit of this determination, a literature review was conducted to locate data on the grain growth of austenitic stainless steels. This review revealed that, although little data exists on the activation energy for grain growth [7], Stanley and Perrotta [8] did tabulate the mean grain size versus annealing time and temperature for four austenitic stainless steels. The present communication relates a kinetic analysis of Stanley and Perrotta's data whereby the time dependence and activation energy are estimated for each of the four alloys. O Elsevier North-Holland, Inc., 1 9 7 8

0026-0800/78/0011-0235501.75

236

R. M . G e r m a n

The empirical observations on grain growth and the theoretical treatments generally support a phenomenological equation of the form [4, 5, 9, 10] D n - Do n = Atexp [ - Q / R T ]

[I]

where Do is the initial grain size, D is the grain size after an isothermal exposure at a temperature T for a period t, Q is the activation energy, R is the gas constant, and n and A are constants for a given experimental situation. The grain growth data of Stanley and Perrotta for 304, 316, 32 i, and 347 stainless steels have been analyzed for a best fit to Eq. 1. A modified Newton-Raphson nonlinear least squares computer analysis gave the best Q value for an assumed integer n. The spectrum of best fits to the digital data are shown in Fig. I, where the process activation energy is shown as a function of the kinetic exponent n. Interestingly, the data follow linear relations over the range in n covered by this analysis. In alloy systems, n usually ranges between three and five, with a theoretical lower bound of two [11]. Values of n larger than two indicate a grain boundary drag process associated with

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Short Communication

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inclusions or chemical segregation at the boundaries [5, 10]. By calculating the difference between the predicted grain size and the observed grain size for a given t and T, the relative accuracies of each fit to the data can be assessed. It was found that Eq. 1 gave best agreement with the experimental grain size data when n was four. Based on a constant value of n = 4, Fig. 2 was constructed to show the wide experimental range over which Stanley and Perrotta's data agree with Eq. 1. From Fig. 1, the respective activation energies for n = 4 have been compiled in Table 1. The errors in Q are estimated by the variation in Q from n = 3 t o n = 5. It is significant that the activation energies for the 321 and 347 stainless steels are very large. The experimental data show essentially

R. M. German

238 TABLEI Activation Energies ~rGrain Growth Alloy

304 316 321 347

Q, kJ/tool

280 ± 310 ± 420 ± 650 ±

70 80 100 150

no grain growth at the lower annealing t e m p e r a t u r e s for these two alloys, in contrast to the behavior of 304 and 316. Since 321 and 347 are titanium and niobium stabilized, respectively, it is not surprising to o b s e r v e such hindered grain growth kinetics [10]. Possibly, the actual growth process is dependent on dissolution of grain boundary segregates or carbides before significant growth can occur. Thus, the high activation energies for these two alloys probably represent the case where the carbides cause grain b o u n d a r y drag at the lower temperatures. With increasing temperature, such pinning points will dissolve into the lattice while also exhibiting an increased mobility. The net result would then be an abnormally rapid i n c r e a s e in grain size with temperature. This acute t e m p e r a t u r e dependence manifests itself as a high apparent activation energy. Alternatively, 304 and 316 appear to b e h a v e in a predictable manner, giving activation energies near that for lattice diffusion in these alloys [I 2, 13]. The value for 304 stainless steel agrees favorably with the determination on 304L from sintering kinetics [3]. The computational assistance o f R. E. Huddleston is gratefully acknowledged. This study was supported by the U.S. Energy Research and Development Administration.

Received March, 1977

References I. 2. 3. 4. 5. 6.

B. A. Alexander and R. W. Balluffi. Acta Met. 5, 666 (1957). R. E. Cech..I. Appl. Phys. 41, 5247 (1970). R. M. German. Met. Trans. A 7A, p. 1879 (1976). R. L. Coble..l. Appl. Phys. 32, 793 (1961). F. A. Nichols, .1. Appl. Phys. 37, 4599 (1966). : J. H. Rosolowski and C. Greskovich. J. Amer. Ceram. Soc. 58, 177 (1975).

Short Communication 7. 8. 9. 10. I I. 12. 13.

A. N. Popandopulo, M¢,talloved¢,ni¢, i T(~rm. Obrabot. Metallov 7, 71 (1974). J. K. Stanley and A. J. Perrotta, ft4etallography 2, 349 (1969). J. E. Burke, ,1. Appl. Phys. 18, 1028 (1947). M. Hillert, A¢ta Met. 13, 227 (1965). J. E. Burke and D. Turnbull, Prog. Metal Phy.~. 3, 220 (1952). P. Guiraldenq and P. Poyet, M¢,m. Sci. Re~'. ~4etalho'g. 70, 715 (1973). R. A. Perkins. R. A. Padgett. Jr.. and N. K. Tanali, M~,t. Trans. 4, 2535 (1973).

Re~'eiv~,d Mar~'h, 1977

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