Effect of heat treatment on the mechanical state of 20% cold-worked type 316 austenitic stainless steel

Scripta

METALLURGICA

Vol. II, pp. 3 2 1 - 3 2 5 , 1977 P r i n t e d in the U n i t e d S t a t e s

Pergamon

Press,

Inc

EFFECT OF HEAT TREATMENT ON THE MECHANICAL STATE OF 20% COLD-WORKED TYPE 316 AUSTENITIC STAINLESS STEEL

H. Yamada Materials Science Division ARGONNE NATIONAL LABORATORY Argonne, Illinois 60439 (Received

Febrnary

II,

1977)

Introduction On the basis of the mechanical equation of state concept, Hart (i) proposed the following equation for stress, ~, and plastic strain rate, ~, which has been found to be applicable over a wide range of temperature, stress, and plastic strain rate: O = ~* exp[-(~*/~) %] + ~ ~l/M

(i)

o

where X and M are materials constants and o*, e*, and o are parameters that define the mechanical state of the material. The first term in the righ~ side of Eq. (i) represents plastic deformation controlled by diffusive processes, which dominate the deformation behavior at low strain rate. The second term represents plastic deformation controlled by dislocation glide, which dominates at high strain rate. The value of o* is the minimum stress necessary for the dislocation to overcome obstacles without thermal activation, and the parameter oo is a measure of resistance against dislocation-glide motion. Huang et al. (2) observed the deformation behavior described by Eq. (i) in aged Type 316 stainless steel over a wide range of strain rates at temperatures between 25 and 500°C. On the basis of fundamental deformation mechanisms, Hart (i) predicted that the change of o* due to mechanical work and thermal-aging treatment is given by d£n o*/dt = P(o*, o)$ -R(o*, T)

(2)

where P(o*, o) and R(o*, T) are work-hardening and recovery functions, respectively. The dependence of P on ~* and a was studied by Wire et al. (3) in Type 316 stainless steel, niobium, and Ii00 aluminum alloy. They found that the work-hardening function r depends solely on the current values of ~* and o. To date, no measurement of the recovery function R(~*, T) has been performed. In the present paper, the first such measurement to verify R(~*, T) in 20% coldworked Type 316 stainless steel is reported. Experiment The solution followed and 0.76

specimens tested in the present study were fabricated by alternate cold rolling and annealing of bar-stock material. Each solution anneal was conducted for I h at I025°C by a water quench. The gauge section of the specimen was 25.4 mm long, 5.1 mm wide, mm thick.

Load-relaxatlon experiments were performed at room temperature before and after the specimens were heat treated at high temperature. The specimens were strained in an Instron screwdriven mechanical testing machine ~ o d e l TM-S) using a crosshead speed of 2.03 mm/min to attain the desired stress level. To prevent any work-hardening during loading, the specimens were loaded just over the yield stress and the crosshead of the machine was stopped. The relaxation of the load was recorded digitally for up to 104 s after loading.

321

322

COLD-WORKED TYPE 316 STAINLESS STEEL

Vol,

11,

No, 4

To avoid misaligament, the specimens were heat treated in an electric furnace mounted on the Instron machine. The temperature was raised to the desired value at a rate of %10°C/mln and was maintained within 2°C for 25 h. The specimens were allowed to cool to room temperature in the furnace before the load-relaxatlon experiments were performed. One specimen (Number 4) was annealed at 600°C for i00 h before it was placed in the Instron machlne. Results and Discusslon Typical load-relaxation data in the form of log o versus log e at 25°C are shown in Fig. i. These data were obtained after the specimen was annealed at 600°C for 0, 25, 50, and 75 h. The values of o*, ~*, and o o were calculated numerically for all tests using a nonlinear least-squares fit of Eq. (i) to the data and are listed in Table i. Two materials constants,

TABLE I Mechanical State Parameters o*p ~*, and o

Specimen Number

Temperature of Heat Treatment (°C)

Duration of Heat Treatment (h)

o* ~Pa)

o

e* (s-1)

Go (MPa × s-7.6)

655.6 564.7 531.4 513.7

4.43 2.80 3.09 1.91

-20 10_20 10_20 i0 10 -20

108.9 iii.0

1

600

4

600

i00 125 150

473.0 482.0 471.0

8.48 x 10 -21 -21 8.45 x i0 1.03 x 10 -20

108.3 107.6 102.7

644.8 475.3 447.8 431.8 431.4

1.50 1.00 2.61 2.28 8.17

x x x x x

-19 10_20 10_20 i0 10 -20 10 -21

135.8 110.3

650

0 25 50 75 i00

655.0 384.4 363.0 341.2

3.94 2.10 5.57 2.99

x x x x

-20 10_21 10_21 i0 10 -21

134.5 90.3

700

0 25 50 75

2

3

× × × ×

139.3 121.4

0 25 50 75

101.4 i00.0 i00.0

88.9 86.2

F = 0.15 from high-temperature load-relaxation experiments (2) and M ffi 7.6 from anelastic deformation measurements (4), were selected for the numerical calculations. The value of o* is plotted as a function of the duration of heat treatment in Fig. 2. In the figure, the data at 600°C include those from both specimens 1 and 4. Since o* decreased monotonically to a saturation value with increasing duration of heat treatment, the rate of o* change with time~ t, is assumed to be proportional to the difference between the current and asymptotic values of 0*, namely,

(3)

do*/dt = -[0* - (O*)a]/~ 1 where (o*) a and T I are the asymptotic value of o* and a time constant, respectively. change of o* with time can be obtained by integrating Eq. (3):

The

Vol,

lit

No.

4

COLD-WORKED

TYPE

o* - (O*)a (0*)0 - (~*)a

316

STAINLESS

STEEL

323

(4)

= exp(-t/Zl)

where (0*) 0 is an initial value of 0*. The experimental data are fitted to Eq. (4) using a nonlinear least-squares method, and the results are shown by the solid curves in Fig. 2. Equation (4) appears to represent the data well. The n,,~erlcal values of (O*)a, (C*)o - (G*)a, and ~i are listed in Table II. From Eqs. (2) and (3), R(o*, T) is given:

R(G*, T)

i (O*)a - o* T1 o*

(5)

This recovery function, R, depends on temperature, T, through the temperature dependence of ~I and (G*) a. If the recovery of the material is governed by a single mechanism at temperatures between 600 and 700°C, Zl is expected to obey an Arrhenius-type temperature dependence.

TABLE II Preexponential Terms and Time Constant for o*

Temperature (°C)

(°*)a (MPa)

(0*)0 - (°*)a (MPa)

TI (h)

600

470

166

42.5

650

430

215

15.9

700

360

295

9.96

However, from the values for TI listed in Table II, a linear relationship between log T I and I/T is not observed. According to the time-temperature-transformation diagram for 20% coldworked Type 316 stainless steel proposed by Weiss and Stickler (5), the recovery of the material is expected to involve quite complicated processes. The parameter ~* is determined with less accuracy when compared with o* and o o. lated values of ~* listed in Table I exhibit little change with annealing treatment.

The calcu-

In addition to the change of o* with duration of heat treatment, the parameter Co has been observed to change monotonically to a saturation value. Values of o o are plotted as a function of heat-treatment time at 600, 650, and 700°C in Fig. 3. The experimental data at 600°C are for both specimens i and 4. Again, the rate of o o change with time, t is assumed to be proportional to the difference between the current and asymptotic values of Oo, namely, dOG/dr = -[o ° - (~o)a]/T 2 where (Oo) a and ~2 are the asymptotic value of 0 o and a time constant, respectively. of ~o with time can be obtained by integrating Eq. (6): o°

-

(Oo) o -

(6) The change

(Oo) a

(Oo) a = exp(-tl~2)

(7)

where (Oo) o is an initial value of ~o" The experimental data are fitted to Eq. (7) using a nonlinear least-squares method, and the results are shown by the solid curves in Fig. 3. The numerical values of (Go)a, (Oo) o - (Go)a, and T 2 are listed in Table III. The decrease of a o with annealing time is considered to be a consequence of the decrease of c*. The formation of M23C6 and ~-phase precipitates is expected to cause a decrease in the number of solute atoms dissolved uniformly in the matrix. Therefore, the glide motion of dislocations is facilitated because of less drag force exerted on them.

324

COLD,WORKED

TYPE 316

STAINLESS

STEEL

Vol,

11,

No,

4

TABLE III

P r e e x p o n ~ r t t i a i Terms and Time C o n s t a n t f o r ~

o

Temperature (°C)

(°o)a ~IPa x s-7.6)

6o)o - (Oo)a ~ P a × s-7.6)

T2 (h)

600

106

33

31.7

650

99

37

20.6

700

88

47

7.92

In conclusion, the change of mechanical state of 20% cold-worked Type 316 stainless steel with annealing treatment was studied using Hart's mechanical equation of state concept. The decrease in hardness, 0*, for an isothermal heat treatment of 20% cold-worked Type 316 stainless steel can be represented by an exponential decay function. In addition, resistance to dislocation glide, which is quantitatively measured by Oo, is observed to decrease with a decrease in 0*. The decrease of o o can also be represented by an exponential function. ~ e n t s The author is grateful to R. W. Weeks and R. B. Poeppel for the encouragement received throughout the course of this study. The author is also grateful to C. Y. Li of Cornell University for his helpful discussion. This work was supported by the U.S. Energy Research and Development Administration. References i.

E. W. Hart, Constitutive Relations for the Nonelastic Deformation of Metals, Report No. 75 CRD 168 D General Electric Corp. Corporate Research and Development Center, Schenectady, New York (1975).

2.

F. H. Huang, F. V. Ellis, and C. Y. Li, Plastic Equation of State in Type 316 Austenltic Stainless Steel, Report No. C00-2172-B, Cornell University Materials Science Center, Ithaca, New York (1975).

3.

G. L. Wire, F. V. Ellis, and C. Y. Li, Acta Met. 24, 677-685 (1976).

4.

N. Nir, F. H. Huang, E. W. Hart, and C. Y. Li, Relationship between Anelastic and Nonlinear Visco-plastlc Behavior of Type 316 Stainless Steel at Low Homologous Temperature, Report No. COO-2172-12, Cornell UnlversityMaterials Science Center, Ithaca, New York (1976).

5.

B. Weiss and R. Stickler, Met. Trans. 3, 851-866 (1972).

Vol.

II,

No.

l

4

COLD-WORKED

i

Z.ll4

i

I

l

j°*"

4

75

..~°

2,T7

STAINLESS

I

J

.•.°

2.B

316

oi

2:25 3 50

2.83

|

I

I : 0 (h)

TYPE

I

STEEL

I

I

325

I n :100%

I ( < 1 0 0 hi

O :(K)OeC(=,'100h)

8O=

o :@50"C 0 :TO0"C

7OO i 60(:

,2

...."" 4OO b

2.7~ 2.74

.... ....

2.7."

...

2.71

t

-I0

-9

I -8

I -7

LOG

;."

. 4

I -G

t --5

I -4

130

~ X

120

~ , , , ~

I

0 : 700 "C

o / ~

0

9o

bo

80

70 60 50

0

N

I

25

I

I

I

I

I

50 75 lOCI 125 150 DURATION OF HF.ATTREATMENT (h)

FIG. 2

I I I D :600 *C ( :600 *C (>lOOh) o :650 "C

=

D

0

-3

~ (STRAIN RATE IN S"1)

I

2O0 I00

FIG. i

I

B

•

•

Log a vs log ~ for 20% cold-worked Type 316 stainless steel after heat-treatment at 600°C for various lengths of time.

140

0

3OO

•

,." .~.

2.7£

%

.3

...-"

I I I I I I 25 50 75 I00 125 15o OURATION OF HEAT TREATMENT (h)

FIG. 3 Value of a o as a function of time at three heat-treatment temperatures.

Value of ~* as a function of time at three heat-treatment temperatures.