Temperature dependence of fracture toughness (J–R-curves) of a modified type 316L austenitic stainless steel

Int. J. Pres. Ves. & Piping 41 (1990) 59-74

Temperature Dependence of Fracture Toughness (J-R-Curves) of a Modified Type 316L Austenitic Stainless Steel K. G. S a m u e l , * O. G o s s m a n n & H. H u t h m a n n Interatom GmbH, 5060 Bergisch Gladbach 1, FRG (Received 15 January 1989; accepted 10 August 1989)

ABSTRACT The influence of temperature on the fracture toughness characteristics of a modified type 316L stainless steel has been investigated in the temperature range 25-550°C. Both multi-specimen and single specimen methods (using a direct current potential drop technique with an experimental calibration) were employed. The single specimen method of evaluating J-R-curves shows excellent agreement with the multi-specimen method. For the material used in this study the A S T M recommendation of the validity criteria and evaluation of the crack initiation fracture toughness were not found applicable. The initiation fracture toughness Jc which is estimated as thefracture toughness at 0.2mm crack growth beyond crack initiation is found to decrease with increase in testing temperature. This behaviour could be attributed to dynamic strain ageing effects which are known to occur in austenitic stainless steels in this temperature range. The crack growth resistance dJ/da is found to be independent of temperature up to 375°C and decreases by about 50% at 550°C.

NOTATION a0 aj a

Initial crack length Instantaneous crack length Final crack length

*Present address: Materials Development Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India. 59 Int. J. Pres. lies. & Piping 0308-0161/90/$03"50 © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain

60

K. G. Samuel, O. Gossmann, H. Huthmann

Aa

Change in crack length Crack length corresponding to 0"15-0.2mm exclusion line Constant in eqn (10) B Specimen thickness Net section thickness Bn Effective thickness Beff Remaining ligament length after prefatigue bo C Constant in eqn (10) Ci Specimen compliance at crack length a i a.* Constant in eqn (9) E Young's modulus J J-integral Elastic part of J L, Jol Plastic part of J Crack initiation fracture toughness L K, Ki Stress intensity factor m Constant in eqn (2) n Work hardening exponent Instantaneous load Pi Py Yield load for compact specimen q Constant in eqn (2) AU Change in potential drop W Specimen width O'y Yield strength Ten strength O'TS Average flow stress (=0.5(ay + aTS)) O'f V Poisson's ratio Aa¢ A

INTRODUCTION Austenitic stainless steels are important construction materials for liquidmetal fast reactors and chemical plants, owing to their excellent high temperature mechanical properties and corrosion resistance. To characterise the elastic-plastic fracture behaviour of stainless steels the widely accepted elastic-plastic parameter, the J-integral has to be used. To ensure that the J-integral characterises the stress fields ahead of both a stationary and a growing crack, the validity criteria have to be met. Such values are independent of specimen thickness and have general applicability to the analysis of fracture response of thick section structures. However, the fracture process in austenitic stainless steels involves stable tearing and yields a high fracture toughness well beyond the presently published validity

J-R-curves for a modified austenitic stainless steel

61

limits. Nevertheless an assessment of structural integrity at elevated temperature in the presence of crack like defects requires knowledge of the initiation fracture toughness and tearing modulus of the material used in the structure. This report presents the evaluation of J-R curves of a modified type 316L stainless steel as a function of temperature.

EXPERIMENTAL The material used in the study was a low carbon type 316 stainless steel with a high nitrogen content. The nitrogen is added to compensate for the loss in strength due to the low carbon level. The chemical composition and the relevant mechanical properties are given in Table 1. The fracture behaviour is characterised in terms of the variation of the fracture toughness J with the increase in crack length Aa at temperatures of 25, 170, 370 and 550°C. The material was tested in the 'as received' condition. The test temperatures were controlled within _+2°C. Side-grooved compact specimens with 2 5 m m thickness were used in the present study. The dimensions of the specimens used in the investigation are shown in Fig. 1. All specimens were fabricated in the T - L orientation. TABLE 1

Chemical Composition and Mechanical Properties of Modified Type 316L Stainless Steel (a)

(b)

Chemical Composition C Cr Ni Mo Mn S P

0'24 17-33 12.47 2'42 2'05 0"002 0.022

B

0-000 4

Si N Co Nb Ti

0'13 0"066 0"05 0.01 0-01

Mechanical properties

Yield strength (MPa) Tensile strength (MPa) Youngs modulus (GPa) Poisson's ratio

25°C 246 569 192 0.3

170°C 168 465 180 0'3

370°C 138 435 164 0"3

550°C 126 398 149 0'3

62

K. G. Samuel, O. Gossmann, H. Huthmann

~--

62.5

=i

50

~25~

I i

Q 325--~

Fig. l. Test specimen. Dimensions are in millimetres.

A fatigue pre-crack was introduced in all test specimens at room temperature from the end of the machined notch to give an identical starting crack length, a o. The initial ao/Wafter fatigue pre-cracking was 0"6. All test specimens had initial crack lengths in reasonably close agreement with each other. The maximum fatigue load started with 0"6Pyand was continuously decreased to 0.4Py, where Py for a compact specimen is given by

B( W - a) 2 PY= ( 2 W + a ) ' a y

(1)

where ay is the yield strength of the material. Crack length during precracking and testing was monitored and controlled by the DC potential drop method. All specimens were tested at a load line displacement rate of 0.016 mm/s using a load line displacement control in a 100 kN computer controlled servo-hydraulic testing system. Load line displacements were measured and controlled by averaging extensometer signals which senses the relative displacements of the top and bottom halves of the specimen along the load line. The load versus load-line displacements were recorded by an X-Yrecorder. Changes in the potential drop due to change in crack length were also recorded. All relevant data were stored by the computer. The extent of crack growth was marked by heat tinting at 650°C and post test fatigue cracking at room temperature to open the specimen. Crack lengths were measured optically by the nine point average method.

PROCEDURE FOR J-R-CURVE DETERMINATION Load-displacement diagrams showed intermittent load drops during the test. For the purpose of calculating the area under the curve these load drops

J-R-curves for a modified austenitic stainless steel

5

_i 3,6L M00

63

/jo

e,~

0

--

J

o~9

-J / i/

CRACKEXTENSION(OPTICAL) (mm)

Fig. 2.

Comparison of crack length measured optically and by potential drop technique.

were ignored and a smooth curve was assumed. The area under the load versus load-line displacements curves were determined by numerical integration in the appropriate energy units. During the course of the test the load, load-line displacement and the change in potential drop are continuously measured and stored by the computer. Therefore for a running crack at each arbitrary point, the relation between J and Aa can be obtained if a correct estimate of the crack length at that point can be made. The change in crack lengths during the tests was determined by establishing the relation between the final optically measured change in crack lengths and the corresponding changes in potential drop. This calibration between Aa and AU takes into account the crack tip blunting during the initial part of the test. The data points are found to obey a power law relation of the type Aa = m(A U)q

(2)

The optically measured change in crack length and the change in crack length derived from the potential drop method agree well as shown in Fig. 2. J is evaluated according to ASTM E813-87,1 J = Jel -{- Jpl

where Je~ is the elastic component of J and elastic component is given by

J°' -

(3)

Jpl is the plastic component. The

K?(1 -v 2) E

(4)

where K I = (PJ(BB. W)l/2)f(ao/W ) with

f(ao/W) = , ( 2 - - + - a ~ 2 (O'SS6 + 4"64(ao/W) --13.32(ao/W) 2 -

o/

J

+ 14"72(ao/W) 3 -- 5"6(ao/W) 4)

(5)

64

K. G. Samuel, O. Gossmann, H. Huthmann

and

qAP1(i)

Jpl -- B( W - ao)

(6)

where r/= 2 + 0.522(1 - a o / W ) and B = B° for side grooved specimens. The plastic area Ap,(i) is calculated by subtracting the elastic area from the total area measured at a constant displacement. The elastic area is calculated by evaluating the compliance function (W+a02

[

Ci = EBeff(W_ ai)2 (2-163 + 12.219(ai/W ) - 20"069(a~/W) 2 - 0"9925(a~/W) 3 + 20"609(a~/W) 4 - 9"9314(aJW) s]

(7)

where a~ is the instantaneous crack length. For the multi-specimen method only the final measured crack length a~ = a is considered. For the determination of a characteristic J-value for the initiation of crack growth, a blunting line describing the J-Aa behaviour before the onset of stable crack growth has to be established. The A S T M procedure gives the relation J = 2af" Aa

(8)

for the blunting line, where af = (ay + aTS)/2, the average flow stress. It is observed that this line has a slope which is much too low and cannot describe the blunting behaviour of this class of material. Cornec 2 has developed a procedure for the determination of the blunting line from the tensile data. The analytical blunting line is given as J = (E/0.4 d*) Aa

(9)

where the proportionality constant d* is a function of the strain hardening coefficient n and a/E, which are used to describe the stress-strain curve. The analytical blunting line is found to be more appropriate in this investigation and agrees well with the blunting line derived from the single specimen method. The A S T M standard advocates the use of a power law of the type

J = A(Aa) c

(10)

to fit the data points. The critical J-value characteristic of the initiation of crack growth is defined as Jc, which measures the fracture toughness at 0"2 mm crack growth beyond crack initiation. Initiation toughness values are termed as Je rather than JIc because they do not strictly meet the requirements of A S T M test procedure.

J-R-curves for a modified austenitic stainless steel

65

RESULTS A N D DISCUSSION Figures 3-6 show the J-R-curves for different test temperatures. The figures include the results from both the single and multi-specimen methods. The thick lines in the figures represent the curve fitted through the data points from the multi-specimen results and according to the curve fitting procedure laid down in the ASTM procedure. The values of the constants are given in Table 2. J-R-curves obtained by the single specimen method closely agree with the J-R-curve from the multi-specimen method. Typical SEM photographs of the fracture surface obtained after testing at room temperature and 550°C for different crack lengths are shown in Figs 7 and 8. These pictures were taken at the midpoint of the specimen thickness. It is observed that at the lowest crack length investigated, no real crack growth has occurred except blunting of the crack tip, as evident by the appearance of only a stretch zone. For larger crack lengths crack growth is 60

5O

TEMPERATURE 25"[ 316L MOD

~ 4o ~:

30

z

2o 10

0

I

I

I

I

2

3

I L

CRACK EXTENSION, Aa(mm}

Fig. 3.

J-R-curves for modified 316L stainless steel at 25°C.

60

5O

TEMPERATURE 170°C 316L M0D

E ~.0

3o u_l i-z

2£

10

0

~ I

ji'J

I 2

I 3

*j

i 4

CRACK EXTENSION,an(mm)

Fig. 4.

J-R-curves for modified 316L stainless steel at 170°C.

66

K. G. Samuel, O. Gossmann, H. Huthmann

60

TEMPERATURE 370"[ so

316L MOO

E

I--

_z

2

I

I

I

i

1

2

3

4

CRACK EXTENSION,

Fig. 5.

ao(mm]

J - R - c u r v e s for modified 316L stainless steel at 370°C.

6O

TEMPERATURE 5SO'[ sc

316L MOD

i=

0

[

I

I

I

I

2

3

4

CRACK

Fig. 6.

EXTENSION,

as{ram)

J-R-curves for modified 316L stainless steel at 550'~C.

TABLE 2 Values of the C o n s t a n t s of J R-Curves in the Relation J = A(Aa) c Temperature

A

C

Correlation coefficient

1.821 1.462 1-33 1'228

0'592 0'706 0.710 0.517

0-967 0"999 0.990 0-985

(c) 25 170 370 550

J-R-curves for a modified austenitic stainless steel

67

evident as seen by the dimpled fracture surface. Hence the use of a theoretical blunting line is justified in this case. The J-R-curves (Figs 3-6) reveal that crack growth behaviour of modified 316L stainless steel is influenced by temperature. The initiation fracture toughness Jc is plotted against the test temperature in Fig. 9. Variation of the slope of the power law regression line, dJ/da (which is proportional to the tearing modulus at various temperatures as a function of crack length) is given in Fig. 10. It is seen that the tearing modulus is more or less constant at different test temperatures except at 550°C where it decreases by 50%. According to the ASTM recommendation, Jc is equal to J~¢, the plane strain fracture toughness for crack initiation, if the following conditions are satisfied,

and

B, b o > 25 J~/af Jmax< boof/15 dJ/da at Aa¢ < af

It is clear that none of the validity conditions are satisfied. These conditions require, e.g. at 25°C a specimen thickness of 203 mm and allows a maximum J-value of 0.33 MJ/m 2. In practice no structural components of a nuclear reactor will have such high thickness, where fracture toughness evaluations have to be applied. The value of J¢ reported here should be considered valid only for a material with comparable specimen geometry. The transferability of these data to the design of structural components (e.g. pipes with 700 mm diameter) is under investigation. 3 Dependence of J~ with temperature (Fig. 9) shows a decrease with increase in test temperature. The ductile fracture resistance of a given material often varies inversely with flow strength. Generally it is observed that the ductility is improved when the flow strength is decreased. It has been observed that for the material in the present study, tensile, ductility, and impact toughness decrease with increase in temperature. 4 The decrease in fracture toughness with increase in temperature can be explained as being due to some mechanism which impairs ductility. Grain boundary weakening by precipitation of carbides at higher temperatures is ruled out because of the low carbon content of the alloy, short duration of test time and the test temperatures involved. In austenitic stainless steels another mechanism which reduces tensile ductility is dynamic strain ageing 5- 7 which occurs during the loading of the specimen. Dynamic strain ageing in austenitic stainless steels is caused by the interaction between dislocations and the interstitial solutes (carbon, nitrogen) or solute pairs consisting of one interstitial and one substitutional solute atom (chromium). The temperature domain of dynamic strain ageing depends on strain rate, grain size, solute content, etc. The characteristic features of dynamic strain ageing are peaks

Fig. 7.

Fracture surface o f a specimen tested at 25 C. (a) Aa = 0-721 mm.

(a)

fatigue

pre

blunting

fatigue

post

Fig. 7--contd.

(b) Aa = 1 ' 3 5 m m .

(b)

pre fatigue

blunting

crack growth

stable

post fatigue

2

7"

c~

~ig. 8.

Zracture s u r f a c e o f a s p e c i m e n tested at 550~C. (a) Aa = 0'723 ram.

(a)

itigue

re

1,.

,lunting

atigue

ost

~z

Fig. 8. contd

(b) A a = 2.859 ram.

(b)

fatigue

pre

blunting

crack growth

stable

fatigue

post

~n

2'

A

72

K. G. Samuel, O. Gossmann, H. Huthmann 3

316L MOD 2;', E A

0

_/',

I

I

I

I

I

~00

200

300

aO0

500

600

TEMPERATURE { ° [ )

Fig. 9. Temperature dependence of initiation fracture toughness Jc. 1000

\'k

\~..~.q..

800

,,0

~"

316

L MOD

"~

"~.~'~..

600

~oo .....

200-----

0 0

-

~70~ 370'~ 550~C

I 1

l 2

CRACK

I 3

EXTENSION,

I 4

aa(mm)

Fig. 10. Temperature and crack length dependence of dJ/da (proportional to tearing modulus).

in flow stress and work hardening, a ductility minimum and serrated stressstrain curves, s In a recent study s it has been shown that the serration of stress-strain curves of type 316 stainless steel can be suppressed by longterm ageing, which decreases the available interstitial carbon content by precipitating it out. Loss of fracture toughness of carbon steels in the dynamic strain ageing temperature range has been reported. 9'1° Miglin e t al. I° attempted to identify a tensile parameter that would reliably predict the loss in toughness due to dynamic strain ageing and concluded that the drop in crack initiation toughness with increasing temperature was associated with an increase in tensile strength with temperature, and was always accompanied by a loss in ductility. From the present results it is concluded that the loss of crack initiation toughness with increasing temperature in austenitic stainless steel could be due to dynamic strain ageing effects.

J-R-curves for a modified austenitic stainless steel

73

CONCLUSIONS 1.

2. 3.

4.

5.

Modified Type 316L steel in the 'as received' condition exhibits high fracture resistance for crack initiation which is beyond the limits of Jcontrolled crack growth. Fracture toughness for crack initiation is found to decrease with increase in temperature. The decrease in crack initiation fracture toughness with increase in temperature is believed to be due to dynamic strain ageing effects which decreases the tensile ductility for 316L austenitic stainless steels in this temperature range. The crack growth resistance dJ/da decreases with increasing crack length at all the test temperatures, and its variation with temperature shows no influence up to 375°C, whereas at 550°C a decrease of about 50% is observed. The single specimen method (DC potential drop with experimental calibration) of evaluating the J-R-curve shows excellent agreement with the multi-specimen method.

A C K N O W L E D G E M ENTS K.G.S. is grateful to Shri. C. V. Sundaram, Dr P. Rodriguez and Dr S. L. Mannan, all of the Indira Gandhi Centre for Atomic Research, Kalpakkam, India and the International Bfiro of the Nuclear Research Center Jfilich for their support during his stay at Interatom under the Indo-German bilateral agreement. REFERENCES 1. ASTM E813-87, Standard Test Method for Jtc, A Measure of Fracture Toughness. Annual Book of A S T M Standards. VoL 03.01. American Society for Testing and Materials, Philadelphia, PA, 1988, pp. 968-90. 2. Cornec, A., Heerens, J. & Schwalbe, K. H., Bestimmung der Rissspitzenaufweitung CTOD und Rissabstumpfung SZW aus dem J-Integral, Vortr/ige der 18. Sitzung des A rbeitskreises Bruchvorgdnge. Deutscher Verband fiJr Materialpriifung, Berlin, 1986, pp. 265-80. 3. Griiter, L., Debaene, J. P. & Faidy, C., Stable Crack Growth in Large Austenitic Pipes under Bending. ASME-PVP, 135 (1988) 65-70. 4. Design and Construction Rules for Mechanical Components of FBR Nuclear Islands RCC-MR, Section 1--Subsection 2: Technical Appendix A3. AFCEN, Paris, 1985. 5. Mannan, S. L., Samuel, K. G. & Roderiguez, P., Dynamic Strain Ageing in Type 316 Stainless Steel. Trans. Indian Inst. Metals., 36 (1983) 313-20.

74

K. G. Samuel, O. Gossmann, H. Huthmann

6. Mannan, S. L., Samuel, K. G. & Roderiguez, P., Influence of Temperature and Grain Size on the Tensile Ductility of AISI 316 Stainless Steel. Mater. Sci. Engng, 68 (1984-85) 143-9. 7. Ino, Y., Tensile Properties of Type 304 Stainless Steel in Temperature Range 77 to 1273K and Strain Rate 10 - 6 to 10-1S 1. Bulletin of JSME, 29 (1986) 355 61. 8. Samuel, K. G., Mannan, S. L. & Roderiguez, P., Serrated Yielding in AISI 316 Stainless Steel. Acta. Metall., 36 (1988) 2323 27. 9. Van der Sluys, W. A., Emmanuelson, R. H. & Futato, R. J., Elastic-Plastic Properties of Submerged Arc Weld Metal. A S T M S T P 8 5 6 , ed. E. T. Wessel & F. J. Loss. American Society for Testing and Materials, Philadelphia, PA, 1985, pp. 68-83. 10. Miglin, M. T., van der Sluys, W. A., Futato, R. J. & Domian, H., Effects of Strain Ageing in the Unloading Compliance J test, A S T M S T P 8 5 6 , ed. E. T. Wessel & F. J. Loss. American Society for Testing and Materials, Philadelphia, PA, 1985, pp. 15(~65.