Mechanical twinning in ferritic stainless steels

Scripta

METALLURGICA

V o l . 16, p p . Printed in

1225-1228, the U.S.A.

1982

Pergamon P r e s s Ltd. All rights reserved

MECHANICAL TWINNING IN FERRITIC STAINLESS STEELS

T. Magnin and F. Moret Dpt. Mat4riaux-Ing4nierie, Ecole Nationale Sup~rieure des Mines 158 cours Fauriel, 42023 SAINT-ETIENNE C~dex (France) ( R e c e i v e d J u n e 23, 1 9 8 2 ) (Revised September 14, 1 9 8 2 ) Introduction It has been recently shown that the stress corrosion cracking (S.C.C.) resistance of ferritic and austeno-ferritic stainless steels (Fe-Cr-Ni-Mo-Cu) is closely related to the nickel and copper contents of the ferrite phase (1,2,3,4,5). The addition of nickel and copper renders Fe-(21 to 30 %) Cr - (I to 5 %) Mo alloys more susceptible to S.C.C. in such media as H2S and boiling MgCI2, even if the general corrosion resistance is improved. These results, which have been recently described as being "at the heart of the problems of identifying mechanisms of S.C.C. in ferritic stainless steels" (i), lead us to study the mechanisms of plastic deformation of ferritic stainless steels containing (24 to 26 %)Cr - (0 to 6 % Ni) - (O to 1.5 %)Cu (I to 3.5 %)Mo. First experiments clearly demonstrated the occurrence of twinning (2). This mode of mation is investigated in further detail in the present paper. Particular attention the influence of temperature, strain rate, alloying elements (Ni and Cu) and ageing Mechanical results are discussed, mechanisms are proposed and their consequences on tance are emphasized.

plastic defor is paid to on twinning. S.C.C. resis-

Experimental The chemical compositions of the alloys used in this study and their respective heat treatments are given in table I : TABLE I Chemical compositions (weight %) of the alloys - (WQ = water quenched)

Alloy

Cr

Mo

Ni

Cu

A

25

3.5

6.6

1.5

B

25

3.5

6.6

C

25

3.5

D

26

1

E

26

1

1.5

3

C

N

0

0.003

O.O1

O.01

0.003

O.01

0.01

0.003

0.01

0.002

0.003

0.002

0.003

HEAT

TREATMENT

I IG R A I N

SIZE

WQ

1250°C

iOO ~m

WQ

1350°c

1 mm

WQ

1250°c

i00 ~m

0.01

WQ

1250°C

80 ~m

0.002

WQ

I050°c

60 ~m

0.002

WQ

1250°c

I00 ~m

Alloy A corresponds to the ferrite phase composition of an industrial austeno-ferritic stainless steel. Alloy D is a classical 26-I stainless steel. Other alloys are laboratory casts used to study the influence of the Ni and Cu contents. Some specimens were aged at a temperature of 4OO°C, within the theoretical (~i+~2) region of the Fe-Cr diagram (6). Tensile tests were conducted in an Instron machine using specimens of 6 diameter and a gauge length of 15 mm. The test temperatures range from room temperature to 6OO°C and strain rates from lO-6/s to iO-2/s.

1225 0@36-9748/82/111225-04503.00/0 Copyright (c) 1 9 8 2 P e r g a m o n Press

Ltd.

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Results The occurrence of mechanical twinning in alloy A is clearly shown in figure i which presents the tensile curve of this alloy at room temperature. Twinning, accompanied by its characteristic sound and load drops, appears well before the "yield stress", and is profuse up to necking. To estimate the quantity of twinning, we introduce the parameter : ~AE I m, m = ~ where Aelm is the plastic deformation corresponding to the i th drop in the tens sile curve and be the deformation at the start of necking. Thus, for alloy A at room temperature and at a strai~ rate ~ = i0-~/s (fig. i), 6 is about O.ii. m The influence of temperature on the amount of twinning is summarized by the figure 2. Two different regions can be observed : i) In the temperature range 20°C-150°C (region I) twinning is observed and decreases as usual with temperature ; 6 is only 0.02 at 150°C. 2) In the temperature m range 2OO°C-550°C (region If), twinning becomes more profuse (6 = 0.45 at 400°C) and completely m disappears at 550°C with ferrite transformation. Such results are new and rather surprising for b.c.c, alloys which generally exhibit twinning only at low temperatures. Moreover the general features of figure 2 suggest that twinning in region II can be related to microstructural changes in the ferrite phase. This idea is supported by the observed effect of ageing indicated in figures 3 and 4. When a specimen of alloy A is aged at 400°C for 20min and then deformed at IOO°C, twinning is much more pronounced than for a specimen directly deformed at IOO°C (fig. 3). Finally, a kinetic effect can be observed on specimens which are deformed at 4OO°C after different ageing times (fig. 4). In these latter tests, the flow stress increases with ageing time and a significant brittleness is observed after times longer than 20 hours. All the above results were obtained with a strain rate of IO-4/s. In figure 5, the influence of strain rate on the occurrence of twinning in alloy A at room temperature is described. A classical behaviour is observed : the higher the strain rate, the higher the frequency of twinning. It was also noticed that, with larger grain sizes, 6 increases and the stresses ~ at which twinning occurs are lowered, m To elucidate the occurrence of twinning in the ferritic stainless steel (alloy A), the influence of alloying elements was studied. It is well known that Fe-26Cr-iMo alloys (alloy D) and Fe-6Ni alloys do not exhibit twinning at room temperature (7,8). Thus, only the influence of Ni and Cu in the Fe-Cr matrix was examined. Tests on alloy C (which does not contain Ni) emphasize the role of nickel ; no twinning was observed, at any temperature. Moreover, tests on alloys B and E, with 6.6 % and 3 % Ni respectively, clearly show the influence of Ni on ~m (fig. 6) ; ~ is approximatively proportional to the Ni content. Finally, the influence of Cu can be describe~ as follows : no influence on twinning in region I, but an enhanced effect on twinning in region II. Discussion In b.c.c, crystals, deformation twinning is generally favored by conditions of low temperatures, high strain rates (as for shock loading) and by the presence of substitutional elements (9,10). Under these conditions, the mobility of screw dislocations is reduced (Ii), their dissociation is enhanced (12), and the nucleation of twins favoured (13). Thus, it is somewhat surprising to find extensive twinning in a ferritic alloy at temperatures up to 0.4 T . The study of the influm ence of temperature and alloying elements suggests that different mechanisms are at the origin of twinning. Twinning in region I, which usually decreases with temperature, can be considered as intrinsic, and caused by the presence of N~. Measurements of the effective stresses ~ by the strain rate change method (7) indicate that ~ increases with Ni content. But the physical process by which Ni promotes twinning in an iron-chromium matrix is still not understood. Nevertheless, our results clearly show the influence of Ni on the deformation mode of ferritic stainless steels. To our knowledge, this influence has never been reported in the literature. Twinning in region II is closely related to an ageing treatment near 4OO°C. It should be mentioned that such treatments are used industrially to obtain better hardness. Ageing of Fe-Cr binary alloys at 500°C, which has been extensively studied (14,15) is known to induce spinodal decomposition with chromium-rich zones in an iron-rich matrix. As a result of the decomposition, the dislocation movement by slip becomes more difficult in the chromium-rich zones (friction stresses are higher) and the mode of deformation changes from predominantly slip to predominantly twinning. This tendency has been observed in Fe-Cr alloys (16,17) and in a ferritic stainless steel (18), tested at room temperature after long ageing times at 475°C. Nevertheless our results i n d i c a t e that the decomposition kinetics are very much slower for Fe-Cr alloys than for alloy A (it must be pointed out that, in our case, the total time for a tensile test at 400°C is only 20 min,

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including the time required for temperature stabilisation). Moreover, twinning due to ageing is observed, in alloy A, not only at room temperature but even at 500°C, which has not been reported in references 16-18. Thus, the ability of twinning in region II can be attributed to a combination of sp~nodal decomposition and an intrinsic effect due to Ni. In fact, twinning is observed in alloy C (without Ni) for an ageing time of 48h at 4OO°C, only at room temperature. Finally, the secondary effect of Cu on twinning in region II can be related to the precipitation of the copper-rich £ phase, observed in such steels (19), which acts as strain concentrator and increases the amount of twinning. The consequences of the mechanical behaviour of alloy A in stress corrosion are now quite o b v i o u s Twinning, which occurs before the "yield stress", leads to an early depassivation and to localized corrosion. This plastic deformation mode can be considered to be the major factor controlling the premature failure of alloy A in corrosive media, as reported elsewhere (2). Acknowledgement The authors would like to express their gratitude to Drs A. DESESTRET and P. JOLLY Creusot-Loire) for several useful discussions.

(Soci@t4

References i. J.E. Truman, Int. Metals Reviews, 6, 301 (1981). 2. T. Magnin, J. Le Coze and A. Desestret - to be published in ASM conference on duplex stainless steels, St-Louis (October 1982). 3. A. Desestret and R. Oltra, 20, 799 (1980). 4. A.P. Bond and H.J. Dundas, S.C.C. and hydrogen embrittlement of iron base alloys, NACE, Houston, editor R.W. Stachle, 1136 (1977). 5. J. Hochmann, A. Desestret, P. Jolly and R. Mayoud, ibid., 956 (1977). 6. R.M. Fisher, G.J. Dulis and K.G. Carrol, Trans. AIME, 197, 690 (1953). 7. T. Magnin and J.H. Driver, Mat. Sci. Eng. 39, 175 (1979). 8. W.A. Spitzig and W.C. Leslie, Acta Met., 19, 1143 (1971). 9. S. Mahajan and D.F. Williams, Int. Met. Reviews, 18, 43 (1973). lO.J.O. Stiegler and C.J. Mc Hargue, "Deformation twinning", New-York, 203 (1964). II.G. Taylor and J.W. Christian, Phil. Mag., 15, 893 (1967). 12.M.S. Duesbery, V. Vitek and D.K. Bow, Proc. Roy. Soc., A332, 85 (1973). 13.S. Mahajan, Acta Met., 23, 671 (1975). 14.R. Lagneborg, Acta Met., 15, 1737 (1967). 15.T. de Nys and P.M. Gielen, Met. Trans., 2, 511 (1971).

a ~MPa 700

600 500

300

100

-j/I/

FIG.

1 -

,c~....

Tensile

curve

of

Z~(%)

I

I

10

20

alloy

A at

room

temperature

I "'-

30 and

o E =

10-q/s.

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IN STAINLESS STEELS

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16.M.J. Marcinkowski, R.M. Fisher and A. Szirmae, Trans. AIME, 30, 676 17.S. Jin, S. Mahajan and D. Brasen, Met. Trans. A, IIA, 69 (1980). 18.T.J. Nichol, Met. Trans. A., 8A, 229 (1977). 19.P. Jolly, Private communication.

16, No. II

(1964).

~MPa

~ra~

a

700

0,6. R6gion 0,5 ~i4 0,~,

II

(after ageing20 mn 400°C) (directlystrained)

t

R~gion II P-

/.00

O, 0,2_ 3_

200

0,1

t('C)

0

I

I

I

100

2OO

30O

FIG.

I

I

~0(]

50O

~¢(%)

10

2

FIG.

Influence of an ageing at 400°C on the plastic deformation o~ alloy A at IO0°C and ~ = i0-4/s

Influence of temperature on twinning intensity for alloy A at ~ = lo-4/s

o

8m I 1

0,5

MPo

700

"

6OO

10- Is

__10-6/s

5O0

Y

300

100

ageing time

10

20

3

102

(rnn)

103 FIG. 5

FIG. 4 Influence of ageing time at 4OO°C on for alloy A (~ = iO-4/s)

Influence of strain rate on the tensile properties of alloy A at room temperature

m

6m' 6,6 %Ni

0, 5 _

0,/._ 0,3. 0,2 _

o,1

o

ll,c1 I

I

I

I

I

100

200

300

400

500

FIG. 6 Influence of Ni content on twinning at different temperatures (~ = IO-4/s ; specimens in the as-quenched condition)