The internal friction of the pearlitic, bainitic and martensitic transformations in FeNiC alloys

Acta metall, mater. Vol. 38, No. 11, pp. 2337-2342, 1990

0956-7151/90 $3.00 + 0.00 Copyright © 1990 Pergamon Press plc

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THE INTERNAL FRICTION OF THE PEARLITIC, BAINITIC A N D MARTENSITIC TRANSFORMATIONS IN Fe-Ni-C ALLOYS W E I Z H O N G C H E N ' , T. Y. H S U (XU Z U Y A O ) ' , S H U C H U A N C H E N t and J I H U A Z H A N G 2 'Department of Materials Science, Shanghai Jiaotong University, and 2Shanghai Institute of Metallurgy, Chinese Academy of Sciences, P.R. China

(Received 15 December 1989) Abstract--The low frequency internal friction of the pearlitic, bainitic and martensitic transformations in three Fe-Ni-C alloys has been studied. The results show that the internal friction of the martensitic transformation is in correspondence with the model based on the hysteresis loss mechanism associated with stress-induced motion of the interface dislocations, while that of the pearlitic and bainitic transformations obeys Postnikov's model. At a constant frequency, the internal friction peaks of the pearlitic and bainitic transformations increase as the cooling rate increases, but the temperature of the internal friction peaks decreases, and the Q ~ is directly proportional to the ~/fm Tin. When the frequency increases, the Qm2~ of the pearlitic and bainitic transformations lowers apparently, and the temperature of the internal friction peaks increases. The internal friction of the isothermal bainitic transformation in a Fe-Ni-C alloy has also been studied, which proves that there appears a damping peak in the incubation period, and which may be attributed to the nucleation of bainite. Consequently it infers that the mechanism of the bainite reaction seems similar to that of pearlite. R6sum6---On 6tudie le frottement int~rieur ~ basse fr6quence des transformations perlitique, bainitique et martensitique dans trois alliages Fe-Ni-C. Les r6sultats montrent que le frottement interieur de la transformation martensitique correspond h u n mod61e bas6 sur le m6canisme de pertes d'hyst6r6sis qui est associ6 au mouvement, induit par la contrainte, des dislocations d'interface, tandis que le frottement int6rieur 1i6 aux transformations perlitique et bainitique ob~it au mod61e de Postnikov. A fr6quence constante, les pics de frottement int6rieur des transformations perlitique et bainitique augmentent lorsque la vitesse de refroidissment croit, mais la temperature des pics de frottement int6rieur d6croit et Qm~xest proportionnel fi ~/fm Tin. Lorsque la fr6quence augmente, Qm~xdes transformations perlitique et bainitique semble diminuer et la temp6rature du pic augmente. On &udie aussi le frottement interne de la transformation bainitique isotherme dans un alliage Fe-Ni-C: on prouve qu'il apparait, pendant la p6riode d'incubation, un pic d'amortissement qui pourrait &re attribu6 fi la germination de bainite. Ainsi, on en d6duit que la m6canisme de la r6action bainitique est semblable fi celui de la r~action perlitique. Zusammenfassung--Die niederfrequente inhere Reibung wurde w/ihrend der perlitischen, bainitischen und martensitischen Umwandlung in drei Fe-Ni-C-Legierungen untersucht. Die Ergebnisse zeigen, dab die innere Reibung bei der martensitischen Umwandlung mit dem Modell, welches auf dem mit der spannungsinduzierten Wanderung der Grenzflfichenversetzungen zusammenh/ingenden Hystereseverlust aufbaut, gut iibereinstimmt; die innere Reibung bei der perlitischen und bainitischen Umwandlung gehorcht dagegen dem Modell von Postnikov. Bein einer konstanten Frequenz nehmen die Maxima der inneren Reibung der perlitischen und bainitischen Umwandlung mit der Abkiihlrate zu, die Temperatur der Maxima allerdings nimmt ab; Qg2x ist direkt proportional zu 1"/frnTin. Mit zunehmender Frequenz sinkt Q,~x -' der pearlitischen und bainitischen Umwaudlung; die Maxima verschieben sich zu niedrigen Temperaturen. Die innere Reibung bei einer isothermenbairtitischenUmwandlungwurde auch untersucht. Hier tritt ein D/impfungsmaximumw/ihrend der Inkubationsperiode auf. Dieses kann der Keimbildung des Bainits zugeschriebenwerden. Folglich heiBt das, dab der Mechanismus der Bainitreaktion dem des Perlits ~ihnlichzu sein scheint.

1. INTRODUCTION Based on the thermodynamic calculations [1] and the study of the relationship between the austenite strength and the martensitic and bainitic transformations [2], a conclusion has been drawn that the bainitic transformation should be classified as diffusional at least in the temperature range from B~ to the nose temperature.

The internal friction of the martensitic transformation has been studied frequently [3], while that of the bainitic transformation has been investigated recently [4, 5], and that of the pearlitic transformation was still unavailable in the previous literature. This article attempts to compare the internal frictions among the pearlitic, bainitic and martensitic transformations, and may shed a light on the mechanism of the bainite reaction.

2337

2338

WEIZHONG CHEN

et al.:

INTERNAL FRICTION OF TRANSFORMATIONS IN Fe-Ni-C 2. EXPERIMENTAL MATERIALS AND 'METHODS The chemistry of the F e - N i - C alloys tested" are 0.72 C-4.95 Ni (No. 1), 0.36 C-9.73 Ni (No. 2) and 0.15 C-16.05 Ni (No. 3) in wt.%. The internal friction tests have been conducted in an automatic inverted torsional pendulum. The CCT curves and the incubation periods were measured using a LK-02 dilatometer with vacuum at 10-1pa. All the specimens with 1 mm in diameter were austenitized at 710°C for 10 min during the tests, while they had been annealed in vacuum at 880°C for 35 min before the tests. The heating rate for the tests was 5°C/s. The internal frictions of the phase transformations in the three F e - N i - C alloys have been measured under various cooling rates. The isothermal internal friction of the alloy No. 2 was investigated only at 480 and 500°C, because of the limited cooling capacity of the instrument. The Ms temperatures of the materials tested and the CCT curves of the alloys Nos 1 and 2 were determined by dilatometry. The cooling rates selected were correspondent with those used in the internal friction tests. The errors of temperatures measured were within + 2°C. In addition, metallographic examinations have also been accomplished.

¢,1

3. EXPERIMENTAL RESULTS

= o -r"

.2

l-

..= E

7 E •

~ - ~ •~ C ~

i ~:~l

The main experimental results of F e - N i ~ alloy Nos 1 and 2 under continuous cooling are listed in Table 1, and those of alloy No. 2 under the isothermal transformation in Table 2. In the tables, Q max -1 is the value of the internal friction peak, Q b I refers to the background value of the internal friction, ~ to the cooling rate, fro, Tm and tm are respectively the frequency, temperature and time correspondent with the internal friction peak. Q -1 and F × F a s functions of temperature in alloy No. 1 under the cooling rate of 15°C/rain and in alloy No. 2 under 17°C/rain are shown in Figs 1 and 2 respectively. Figure 3 shows Q - I as a function of time during the process of isothermal holding at 500°C for 25 min, and then cooling to 412°C with a rate of 22°C/min in alloy No. 2. Figure 4 shows the internal friction as a function of temperature in alloy No. 3 under cooling from austenite to martensite with a rate of 2.9°C/min. From the metallographic observation and the CCT curves, it is proved that the product transformed from austenite of alloy No. 1 between 620 and 500°C under continuous cooling is pearlite, and the Widmannst/itten structure does not appear under the selected cooling rates for internal friction tests• The product transformed from austenite of alloy No. 2 Table 2. The main experimentalresults of No. 2 alloy under isothermal transformation Isothermal Incubation temperature fro(l/s) tin(S) Qmalx Q~-] periods 480"C 2.14 0 0.079 0.037 9s 1.89 0 0.145 0.061 500°C 1.84 768 0.159 0.054 1500s

W E I Z H O N G C H E N et al.:

I N T E R N A L F R I C T I O N O F T R A N S F O R M A T I O N S IN F e - N i - C

0.620

6.5 -o ------°--" --'~3----__.

/ ~ / \

x o FxF o A Q-' 5.6

x

. . . .

x

]-O

X~x~

4.7

x --,.x~

0.310

f

,T K

3.8

u_

2.9

0,001 300

380

I

I

I

460

540

620

2.0 700

Temperature (*C)

Fig. 1. Q - i and F x F as function of temperature in alloy No. 1 under the cooling rate of 15°C/min. (a) f = 1.81 - 1.87 l/s; (b) f = 2.01 - 2.22 1/s.

0.150

7.0 x [] F X F

0~}~o..."

~3~'[3-~

O A 0-1

0.092

5.2 ~"

~'~.o

0.063

"X-Xx~x~ ~ . ~

0.034

~

0.005 290

/

~

/

I 370

4.3

3.4

"×~..~x

I 450

I 530

I 610

2.5 690

Temperature (°C)

Fig. 2. Q - I and F x F as functions of temperature in alloy No. 2 under the cooling rate of 17°C/min. ( a ) f = 1.89 - 1.97 l/s; ( b ) f = 2.23 - 2 . 4 0 1/s.

0.160

(0) il1 :::

0.136

0.112

5.0 (b) 4.6

4.2 I

L

0 x~x~

0.088

K

3.8

0.064

3.4

0.040 0

I

I

I

I

398

796

1194

1592

3.0 1990

Time (s)

Fig. 3. Q -~ as a function of time in alloy No. 2 during the process of isothermal holding at 500°C (a) and then cooling to 412°C with a rate of 22°C/min (b).

2339

2340

WEIZHONG CHEN et al.:

INTERNAL FRICTION OF TRANSFORMATIONS IN F e - N i ~ 5.5

0.012 0 0 -1 ~ , x ~ j T , ,"X'x.-'~

0.010

0.008

x

x

~ "x--x~x'--'x---,x

;/

T

3.3

FxF

3.1

%

---._=

~.~

-~

0.006

2.9 ~.

0.004

2.7

0.002

I 114

45

I 183

2.5 252

321

390

Temperature (°C)

Fig. 4. Q - ~ and F x F as functions of temperature in alloy No. 3 under cooling from austenite to martensite with a rate of 2.9°C/min.

800

800

600

600

G o

iE

400

400

E l--

I--

200

200

O

0.5

[

1

J

I

l

10

I

f

I

10 2

l

I

I

10 3

t

I

I

t

I

10 4

I

10 5

t 0.5 1

t

t

I t 10

i I f 10 2

r

I 10 3

I I i 10 4

= I 10 5

Time (s)

T i m e Is1 Fig. 5. T h e CCT d i a g r a m o f a l l o y N o .

0

1.

between 490-300°C is bainite, and no other diffusional transformation occurs above the bainite region. Usually in alloy No. 3, austenite can only transform into martensite, and the Ms temperature is 195°C. The C C T curves of alloy No. 1 and 2 are shown in Figs 5 and 6 respectively. 4. DISCUSSION The experimental results show that the internal friction of all the transformations under continuous cooling reaches its maximum after the transformation. At a constant frequency, the damping peaks of the pearlitic and bainitic transformations increase as the cooling rate increases, but the temperature of the internal friction peaks decreases, and the Qmax -l is directly proportional to the 7"/fmTm, as shown in Table 3. Besides experiments by changing frequency revealed that the increment of frequency would lead

Fig. 6. The CCT diagram of alloy No. 2.

to the apparent decrease of the damping peaks of the pearlitic and bainitic transformations but the increase of the temperature of the damping peaks, as shown in Figs 1 and 2. It can be concluded that the internal friction of the pearlitic and bainitic transformation does not coincide with the mode of hysteresis, and is unlike that of the martensitic transformation which obeys the mode [6]. Based on the fluctuation of nucleation in the first order transformations, Postnikov [7] proposed

Q-I = G~a2 M (kT¢o)-1

(1)

Table 3. Qma= -] as function of 'p/fmTm in the pearlitic and bainitic transformations No. f(l/s) Q max-'~ "p]fmTm (s/min) ! 1 1.81-1.87 Q,~ax = 0.1292 + 52.787 "p/fm Tm 2.01-2.22 Q~2, = 0.0609 + 32.324 "P/f, Tm 2 1.89-1.97 Qmx-~= 0.0266 + 6.387 "p/fm Tm 2.23-2.40 Q~,2~= 0.0413 + 4.089 'p/f~ T,~

WEIZHONG CHEN et al.: INTERNAL FRICTION OF TRANSFORMATIONS IN Fe-Ni-C in which G "refers to shear modulus, fl is nucleus volume of the new phase, a is nonelastic strain in the transformation, co the circular frequency of vibration, the rate of transformation. The internal friction of the pearlitic and bainitic transformations under continuous cooling can be best explained from this mode. Generally, the rates of the pearlitic and bainitic transformations are directly proportional to the cooling rate. Therefore we can deduce Q-lctGfla2]'(kTco) -1 from equation (1). G, fl and a are nearly constant for a same material in a small temperature range. Thus Qmax - is linear with ]'~fro Tm for a certain frequency, which coincide with the experimental result. From equation (1) it can be also deduced that the internal friction decreases as the frequency increases. When the oscillating frequency increases, the energy inputted from outside becomes larger, which leads to a greater nucleating rate. Therefore M and a increase, and the temperature at which A;/ and a reach their maximum values also increase. Then it can be concluded that the damping peaks of the pearlitic and bainitic transformations lower apparently as the frequency increases, but the temperature of the damping peak becomes higher. It is known from the CCT curves that the beginning temperatures of the pearlitic and bainitic transformations and the temperatures at which M and a reach their maximum values decrease as the cooling rate increases. It reveals that the higher the cooling rates, the lower the damping peaks, when the frequency does not vary greatly. As to the martensitic transformation in alloy No. 3 (Fig. 4), its damping peak should be much higher than those of the pearlitic and bainitic if it were correspondent with the Postnikov's mode, because of much larger ~ / a n d a as well as much lower T in the martensitic transformation. However the experimental results revealed that its damping peak is nearly of one order lower than those of the pearlitic and bainitic transformations in other two alloys. It may result in that the internal friction of the martensitic transformation does not coincide with the Postnikov's mode. Yang et al. [8] believed that the internal friction of the martensitic transformation was due to the mobile interface under the vibrated stress. They put forward a model based on the hysteresis loss mechanism associated with stressinduced motion of the interface dislocations. From this model Q - l = 2n -

3/2

).bpt Si ni/rflc,

(2)

in which, #t is the torsional modulus, Si is the area of all the interfaces in the sample, n i is the total length of the twin dislocations on the unit interfaces, r is the direction factor, b is Burgers vector, 2 is the wave length of the stress wave, c' is ( c H - c~2)/2. The damping peak of the martensitic transformation in alloy No. 3, shown in Fig. 4 can be explained with the hysteresis loss mechanism associated with the interface dislocations. Suppose that the strain in

2341

grains of the sample is homogeneous, using Voigt's averaging

(3)

~, = (3c4, + 2 c 3 / 5

c11, c~2, c ~ are elastic constants which does not vary.

Substituting equation (3) into equation (2), we know that the damping peaks will appear at Si.... and c~in i.e. -1 _ 27z-3/Z~'bSi .... ni ~" 3c44 2 t Q max-/~r [ 5--~. + ~ , . (4) If we suppose the stress in every grains of the sample is homogeneous, using Reuss's averaging I~t = 5 c ~ c ' / ( 3 c ' + 2c44).

(5)

Substituting equation (5) into equation (2), we have 2~ -3/2,~b S i....

Q~a~x -

fir

ni ~"

5c44

"~

[3Cmin + 2C44J"

(6)

Nevertheless the above mode can not be used to explain the internal frictions of the pearlitic and bainitic transformations because they do not obey the mode of hysteresis as stated before. Consequently, it seems that the internal friction of the martensitic transformation in F e - N i - C alloys is in agreement with the model based on the hysteresis loss mechanism associated with stress-induced motion of the interface dislocations, and the interface effect is an important factor. However the mechanism of the internal friction of the pearlitic and bainitic transformations in F e - N i - C alloys obeys Postnikov's mode, in which the volume effect plays an important role. Thus it infers that the mechanism of the bainitic transformation should be similar with that of the pearlitic transformation, and should be classified as diffusional. Chang et al. [4, 5] have revealed that the internal friction peak appeared in the incubation period of the isothermal bainitic transformations, and they thought it was due to the nucleation process. The measurement of the internal friction of the isothermal bainitic transformation in F e - N i - C alloy No. 2 confirms the above concept. Figure 3 and Table 2 show the experimental results of the isothermal internal friction. The magnitude of the internal friction also decreases as the frequency increases. The nucleation rate of the isothermal bainitic transformation at 480°C is higher than that at 500°C, because the background value of the internal friction at 480°C is larger than that at 500°C. Besides it can be seen from Fig. 3 that after the isothermal damping peak at 500°C of alloy No. 2 there appears another internal friction peak in the process of cooling to 412°C, which infers that the internal friction of the bainitic transformation on continuous cooling does not depend wholly on the nucleation rate, as the nuclei generated at the isothermal holding would lessen the later chances of nucleation.

2342

WEIZHONG CHEN et al.: INTERNAL FRICTION OF TRANSFORMATIONS IN Fe-Ni-C 5. CONCLUSIONS

From the experimental results of the F e - N i - C alloys, we can reach the following conclusions: I. The internal friction of the martensitic transformation seems to be in agreement with the hysteresis loss mechanism associated with stress-induced motion of the interface dislocations, and the mechanism of the internal friction of the pearlitic and bainitic transformations coincides with Postnikov's mode. 2. The internal friction peak may appear in the incubation period, which can be attributed to the nucleation of bainite. 3. When the frequency increases, the Q max -~ of the pearlitic and bainitic transformations lowers apparently, and the temperature of the damping peaks increases. 4. The mechanism of the bainitic transformation seems to be similar to that of the pearlitic transform-

ation, and bainite reaction should be classified as diffusional.

REFERENCES

1. T. Y. Hsu (Xu Zuyao) and Yiwen Mou, Acta metall. 32, 1469 (1984). 2. T. Y. Hsu (Xu Zuyao) and Weizhong Chen, Scripta metall. 21, 1287 (1987). 3. Yening Wang, Yifeng Zou and Zhifang Zhang, Acta physica sinica 29, 1535 (1980). 4. Jihua Zhang, Shuchuan Chen and T. Y. Hsu (Xu Zuyao), Aeta metall. 37, 241 (1989). 5. Jihua Zhang, Shuchuan Chen and T. Y. Hsu (Xu Zuyao), Metall. Trans. 20A, 1169 (1989). 6. E. Miiller Scheil, Arehs Eisenhiitt. 27, 801 (1956). 7. V. N. Belko, B. M. Darinskii, V. C. Postnikov and I. M. Sharshakov, Phys. Metals Metallogr. 27, 141 (1969) (in Russian). 8. Zhaojin Yang, Yifeng Zou, Zhifang Zhang and Yening Wang, Aeta metall, siniea 18, 22 (1982).