Dynamic fracture toughness of X70 pipeline steel and its relationship with arrest toughness and CVN

Materials and Design 23 (2002) 693–699

Dynamic fracture toughness of X70 pipeline steel and its relationship with arrest toughness and CVN Yongning Liua,*, Yiaorong Fenga, Qiurong Mab, Xiaolong Songa a

State Key Laboratory of Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, PR China b Tubular Goods Research Center of CNPC, Xi’an, 710065, PR China Received 21 May 2002; received in revised form 5 August 2002; accepted 11 September 2002

Abstract The dynamic fracture toughness K1d and J1d , arrest toughness K1a and Charpy V-notched impact toughness (CVN) of a pipeline steel, X70, were studied at different temperatures. It was found that fracture toughness was strongly affected by temperature and loading rate. The fracture toughness decreases with decreasing temperature from 213 to193 K and increasing loading rate from ˙ to 105 MPa1y2 sy1. At constant temperatures, only increasing loading rate can induce the transition from ductile to brittle. Ks1.0 There exists a fracture transition caused by loading rate. Through thermal activation analysis, a quantitative relationship has been ˙ ˙ 0.1ynexpŽQfynkT.. It can describe the fracture process at different temperatures and loading rates. At a derived: J1dsJaqJ0ŽKyK ˙ MPa m1y2 sy1, the relationship can predict arrest toughness well. It provides the possibility of measuring loading rate of Ks15 arrest toughness with small size specimen. An empirical equation has been derived: CVNs4.84=106 T y2.8 K1d (K1a ), which correlates K1d and K1a with CVN in one equation. This means that we can calculate K1d and K1a when we get CVN. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Pipeline steels; Fracture toughness; Loading rate; Thermal activation

1. Introduction Fracture toughness is a very important materials parameter in safety design and running of pipelines. However, fracture toughness is strongly influenced by temperature and loading rate w1,2x. Generally, temperature effect was considered more than that of rate effect. Cracked Charpy specimen was often adopted to measure J1c in a series temperatures and K1c then could be obtained by conversion from K1cswEJ1c y(1yy2)x1y2

(1)

where J1c is fracture toughness in the elastic–plastic state for small size specimen. K1c is fracture toughness in the plan strain state of specimen, E and y are Young’s modulus and Poisson ratio of materials. In this way, some empirical equations of fracture toughness with temperature were obtained w2–4x K1csKminqAwexp(CT)x *Corresponding author. E-mail address: [email protected] (Y. Liu).

(2)

Where Kmin, A and C are materials constants. T is temperature. By this equation, fracture toughness can be obtained at different temperatures after the materials constants A, C and Kmin have been obtained. However, loading rate has a strong effect on fracture toughness and strength of steels w5,6x and it has not been considered in this equation and furthermore, this equation has not disclosed the mechanism of fracture transition related to loading rates and temperatures. A research on plain low carbon steel indicated that fracture toughness would decrease with increasing loading rate at constant temperatures. A ductile to brittle fracture transition was induced by only loading rate w5x. The analysis indicated that the fracture transition is tallied with the thermal activation process and a quantitative relationship had been obtained through thermal activation analysis w5,7x. B

1

˙ En B Q E K f F J1dsJaqJ0C ˙ F expC D K0 G D nkT G

(3)

where, J1d is dynamic fracture toughness, Ja is fracture ˙ is stress intentoughness in an athermal component, K

0261-3069/02/$ - see front matter 䊚 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 1 - 3 0 6 9 Ž 0 2 . 0 0 0 7 7 - 8

Y. Liu et al. / Materials and Design 23 (2002) 693–699

694

Table 1 The chemical compositions of steel X70 (wt.%) Elements

C

Si

Mn

P

S

Cr

Contents

0.051

0.2

1.56

0.014

0.0029

0.026

Elements Contents

Mo 0.21

Ni 0.14

Nb 0.045

V 0.032

Ti 0.016

Cu 0.18

Table 2 Mechanical properties Tensile strength sb Yield strength ss Elongation Ratio of ssysb (MPa) (MPa) (%) 638

525

39

0.82

especially arrest toughness? Is there any connection between CVN and K1c and K1a? These are interesting problems and this paper attempts to answer these questions. 2. Experimental procedures The steel used is a Chinese-made pipeline steel in grade X70. The chemical compositions are shown in Table 1. The steel is in hot rolling state and the microstructure is composed of ferrite and pearlite. The ordinary mechanical properties are shown in Table 2. The Charpy specimen is in a standard V-notched geometry with dimension 10=10=55 mm3. The measurement of arrest toughness is according to ASTM E1221-88 standard, the specimens are in CCA (Compact Crack Arrest) geometry as shown in Fig. 1. The dimension of the specimen is 140=140=14.5 mm. The depth of the side grooves is 2 mm in each side. The crack propagation direction is parallel to the axis of the pipeline, K1a is calculated by: KsEf(x)d(ByWyB0)1y2

(4)

and f(x)s2.24(1.72y0.9xqx2)(1yx)1y2 y(9.85 y0.17xq11x2) Fig. 1. Geometry, crack and specimen orientation of arrest toughness specimen.

˙ 0 and n is materials constant, Qf is fracture sity rate, K activation energy. This equation correlates loading rate with temperature in one equation. In the pipeline industry, Charpy impact energy is the materials parameter used often in permit hoop stress designing and crack arresting evaluation w8x. Comparing Charpy energy measurement, the fracture toughness is difficult to measure, especially for crack arrest toughness. In order to satisfy the demand for the plain strain condition, the specimen size is usually much larger. A huge specimen, 0.1=1=1 m, had been reported to satisfy the specimen size requirement in a measurement of K1a for a pressure vessel steel A533 w9x. It is impossible to carry out this experiment in an ordinary laboratory and thus brings the difficulty in measurement and usage of arrest toughness in engineering. Is there any simple way to measure the fracture toughness,

(5)

where K is stress intensity, Xsayw, a is crack length and W is width of specimen from loading center to the edge, d is crack opening, B is thickness of specimen, B0 is the net thickness of the side groove. Three points bend specimen were adopted in dynamic fracture toughness measurement. The crack direction is also along the axis of the pipeline. The geometry of the specimen is shown in Table 3. The side grooves were cut on both sides along the direction of the crack to increase stress concentration. When the depth of the side grooves reaches 25% of the depth of thickness B, the fracture toughness can be calculated by w1x Jmaxs2Amax y w(Wya)B0x

(6)

where Amax is the maximum work under load–displacement curves. We can get the fracture toughness when the specimen dimension satisfies criterion, Wya, W, B0)25(Jq ysy)

(7)

where JQ is the fracture toughness without dimension check and sy is the yield strength.

Table 3 The dimension of fracture toughness specimen (mm) Width W

Thickness B

Net thickness B0

Crack length a

Ligament length Wya

Length of specimen L

Depth of side groove b

28 mm

14 mm

10.5 mm

14 mm

14 mm

132 mm

1.75 mm

Y. Liu et al. / Materials and Design 23 (2002) 693–699

695

Fig. 3. Dynamic fracture toughness of X70 steel with loading rates at constant temperatures.

Fig. 2. Impact Charpy energy with temperatures.

The experiment was carried out on testing machine MTS800. Loading rates were changed from vs0.01, 0.1, 1, 10, 100 to 1000 mmys at several constant temperatures. The temperatures were controlled by alcohol with liquid nitrogen from 193 to 253 K in four different temperatures. The specimens were dipped in the low temperature alcohol for more than 10 min before starting tests. 3. Results Fig. 2 shows the Charpy impact energy with temperatures. The results indicate that the quality of the steel is high. The Charpy energy keeps very high, 288 J, even in 193 K. After temperature increases to 293 K, it reaches to 367 J. The temperature region in this test satisfies the demand of pile line design of this steel. Table 4 shows the measurements of arrest toughness. We only get K1a values at temperature below 233 K. Over this temperature, no unstable preparation takes place and only stable cracking goes with load increasing. Therefore, K1a values cannot be obtained over temperature 233 K with this size of specimens. This result shows again the difficulty in K1a measurement. The valuable data of K1a in Table 4 indicate a trend that the K1a increases with increase of temperature. The testing data of dynamic fracture toughness J1d

are shown in Fig. 3. It is clear that there exists a transition from ductile to brittle induced by increasing loading rate only. Most published data on fracture transition were related to the variation of temperature at constant loading rates w1,2,7x. Here, loading rate shows a similar effect as the variation of temperature in fracture transition. Therefore, we should also pay more attention to the rate effect. The temperature effect on fracture transition is shown clearly also in Fig. 3. Increasing temperature makes the transition to a higher loading rate. That means the transition is difficult to happen as temperature is increasing. When temperature is over 253 K, no transition takes place within the loading-rate regions in this test. The trend lines in the figure were drawn approximately through the average points of the tested data for each constant temperature. The dash line in the figure indicates the dimension criteria for fracture toughness. Below this line, J1d satisfies the dimension requirement, otherwise, J1d is not effective. Comparing to the results of Charpy impact energy, the fracture toughness varies by approximately 10 times as loading rate or temperature variations. The Charpy energy changes only by 30% for temperatures increasing from 193 to 293 K. This indicates that there will be a great difference if we design the pipeline stress with different materials parameters of K1d or CVN.

Table 4 The measurements of arrest toughness K1a Specimens

Temperature (K)

d mm

F(x)

Xsayw

B mm

BN mm

K1a (MPa m1y2)

6噛 5噛 4噛 1噛 3噛 2噛

233 233 213 213 193 193

1.91 1.77 1.81 1.73 1.85 2.16

0.166 0.178 0.133 0.165 0.132 0.082

0.580 0.538 0.700 0.585 0.705 0.850

14.12 14.16 14.26 14.2 14.3 14.12

10.62 10.66 10.76 10.7 10.8 10.62

259.0 256.9 195.8 232.5 198.5 125.4

696

Y. Liu et al. / Materials and Design 23 (2002) 693–699

Fig. 4. Load–time curves for steel X70 at temperature 193 K. (a) loading rate is 0.003 mmys, (b) loading rate is 1000 mmys.

4. Discussions 4.1. Ductile to brittle transition The above results indicate that there exists a fracture transition caused by increasing loading rate at constant temperatures. This phenomenon can be displayed more clearly by load–time curves and SEM photographs. Fig. 4 is a pair of load–time curves at a temperature of 193 K for two different loading rates. The loading rate for Fig. 4a is vs0.003 mmys and the loading rate for Fig. 4b is 1000 mmys. In slowly loading, the material exhibits very good plasticity. The SEM photograph, Fig. 5a, displays the corresponding fracture surface. The plastic fracture mechanism has been displayed. The

dimple is not so dipper as that in a relative higher temperature as shown in Fig. 6a, which is fracture surface at temperature 213 K and loading rate vs0.01 mmys. A very typical plastic fracture mechanism has been displayed. In high loading rate, Fig. 4b displays the brittle mechanism. No plasticity has been displayed and the maximum load at this rate is also rather lower than that for slowly loading, Fig. 4a. The corresponding fracture surface has been shown in Fig. 5b. The cleavage fracture mechanism has been shown in this figure and also in Fig. 6b. The loading rate for Fig. 6b is vs1000 mmys and temperature is 213 K. Only changing the loading rate can induce the fracture transition. This phenomenon exists also in other steels w5x and even in polymer materials such as Nylon w10x and PP w11x. However, the loading rates in these tests were not so high and generally lower than 1000 mmys. This reminds us of the need to pay more attention to the loading rate effect in structure design. 4.2. Thermal activation analysis Above experimental results indicate that loading rate and temperature have a similar effect on fracture transition. Even non-crystal materials such as polymer showed the similar fracture transition phenomenon w10,11x. Arrhenius Law is the best way to unify temperature and rate effects, and also it is suitable for all kinds of materials no matter what they are, crystal or

Fig. 5. The fracture surface of SEM photographs for steel X70 at a temperature of 193 K. (a) loading rate is 0.003 mmys, (b) loading rate is 1000 mmys.

Fig. 6. The fracture surface of SEM photographs for steel X70 at temperature 213 K. (a) loading rate is 0.01 mmys, (b) loading rate is 1000 mmys.

Y. Liu et al. / Materials and Design 23 (2002) 693–699

697

Table 5 ˙ 0 with J1d Relationship of K9 J1d

0.20

0.15

0.10

31.82

34.40

0.075

.

lnK90

31.0

35.27

fracture toughness. Eq. (10) is plotted into Fig. 8 with data in Table 5. A linear relationship has been shown ˙ 0 and n can be obtained, which are K ˙ 0s and K 10 1y2 y1 6.185=10 MPa m s , nsy3.428. Now taking Eq. (9) and Eq. (10) into Eq. (8), it gives B

.

Fig. 7. Loading rate K with 1yT at several J1d levels.

B

E

(8)

.

˙ 0 is constant, Qf where K is the stress intensity rate, K9 is the fracture activation energy, k is the Boltzmann constant and T is temperature. ˙ 1 coordinator in Eq. (8) can be plotted in a lnKy T Fig. 7 with temperature and loading rate values obtained in Fig. 3 at several constant J1d levels (J1ds0.75, 0.1, 0.15, 0.2 MPa m1y2), which are values which satisfy the dimension requirement. Nearly a group of parallel lines has been displayed in Fig. 7. When measuring the slopes, we can get the fracture activation energy from Eq. (8). It gives Qfs0.53 eV. From the intersections of ˙ 0 and they the lines with ordinate, we get the constant K9 are shown in Table 5 ˙ 0 increases as J1d decreasing. It needs to find The K9 ˙ 0 with J1d in the fracture process the relationship of K9 before we can use Eq. (8), The fracture toughness can be divided into two parts w2,5,7x: J1dsJtqJa

B

E

B

˙ E1yn B Qf E K C F J1dsJaqJ0 ˙ F expC D K0 G

D nkT G

B ˙ E1yn B Q E K f F K1dsKaqK0C ˙ F expC D K0 G D nkT G

(13)

Eqs. (12) and (13) are a quantitative relationship of fracture toughness with temperature and loading rate. Fig. 9 is a plot of Eq. (13) with temperature at several constant loading rates for steel X70. The data of arrest toughness of the steel have been plotted into the figure also. It is noticed that K1a is in coincident with K1d ˙ s15 MPa m1y2 sy1. This coinciwhen loading rate K dence means that K1a has the same temperature dependence as K1d and both share probably the same thermal activation temperature mechanism. If we find this rate we can get the K1a with K1d by Eq. (13). For steel X70, ˙ s15 MPa m1y2 sy1. However, this does this rate is K not mean that the steel arrests at this rate. Exact rate for arresting needs to be measured. This result only provides a possible way to measure the arrest toughness with small size specimen.

(9)

(10)

˙ 0 is a material constant and J0 is a unit of Where K

(12)

if we take Eq. (1) into Eq. (12), it gives

Where Jt is the thermal component and it relates to temperature and loading rate effects, Ja is an athermal component related to the elasticity property of fracture and irrelevant to temperature and loading rate. When temperature is low enough, the loading rate is high enough and the fracture process displays total cleavage mechanism, we considered the fracture process as an athermal process. At present state, we take Ja value at temperature 193 K and loading rate vs1000 mmys, which gives Jas0.03 MPa m. Now it is supposed: ˙ 0sK ˙ 0(Jt yJ0)n K9

(11)

and

non-crystal. In fracture situation we can write the Arrhenius Law in the following form w5,11x ˙ ˙ 0 expC yQf F KsK9 D kT G

En

˙ ˙ 0C J1dyJa F expC yQf F KsK D D kT G J0 G

˙ 0 with JtyJ0. Fig. 8. The relationship of lnK9

698

Y. Liu et al. / Materials and Design 23 (2002) 693–699

Fig. 9. Calculated fracture toughness and tested arrest toughness with temperature at 5 loading rates.

where A(T) is a temperature function, taking Eqs. (14) and (15) into Eq. (16), it gives

4.3. The relationship of CVN with K1d and K1a CVN, K1d and K1a are three independent material parameters in fracture mechanics. The CVN is notched specimen and it is easy to be machined and to be measured, therefore, is used often in engineering. K1d and K1a are dynamic initiation and arresting toughness, respectively, with cracked specimens. Relatively, they are difficult to measure, but they have a very tight theory base and are most precise in prediction of the fracturing process. The relationship between the three parameters is an interested problem and there were a lot of published works to disclose the relationship between these three parameters for different steels w12–14x. CVN is a material parameter used most frequently in the pipeline industry w8x. The arresting CVN values calculated by three independent institutes, BMI, AISI and BGC, are 119 J, 87 J and 68 J, respectively, for an X70 steel made UOE pipe with dimensions of diameter 1219.2 mm and thickness 18.3 mm. However, the explosion test result was 180 J w15x. There is a large difference, not only in the calculations between institutes, but also in the predictions and the measurement. This reminds us that it is necessary to design the pipes based on fracture mechanics and needs to find the links between CVN, K1d and K1a. Fig. 10 is a plot of log CVN and log T with data in Fig. 2. Fig. 11 is a plot of log K1a and log T with data ˙ s15. Good linear relationships have been in Fig. 9 at K displayed and we can get CVNs15.5T0.55

Fig. 10. Log CVN vs. Log T.

A(T)s4.84=106Ty2.80

(17)

then CVNs4.84=106Ty2.8K1d

(18)

Fig. 12 is a plot of CVN and K1d with both calculated and tested data. There is a good coincident between ˙s predictions and experiments. When loading rate is K 1y2 y1 15 MPa m s , we can calculate arrest toughness with Eq. (18), it gives .

K1as2.065=10y7T2.8CVN (Ks15)

Fig. 13 is a plot of the result. There is a good agreement between calculation and testing. 5. Conclusions 1. There exists a ductile to brittle fracture transition induced by loading rate for steel X70. Fracture toughness decreases with an increase of the loading

(14)

and K1ds3.2=10y6T3.35

(15)

It is supposed CVNsA(T)K1d

(16)

(19)

Fig. 11. Log K1d vs. Log T.

Y. Liu et al. / Materials and Design 23 (2002) 693–699

699

K1a with CVN values. This will provide convenient in use of fracture and arrest toughness in engineering. References

Fig. 12. The relatioship between CVN tested and calculated with K1d.

rate. Increasing temperature makes the transition take place at a higher rate. CVN, arrest toughness and dynamic fracture toughness show the similar trend that they increase as temperature increases. 2. A quantitative relationship of fracture toughness with loading rate and temperature has been obtained. At ˙ s15 MPa m1y2 sy1, arrest toughness loading rate K K1a shows the same temperature dependence with K1d. The K1a value can be obtained by this equation with small size specimens. 3. An empirical relationship of CVN, K1d and K1a for steel X70 has been obtained. It can predict K1d and

Fig. 13. Arrest toughness predicted by CVN.

w1x Server WL, Wullrert RA, Ritchie RO. On the use of sidegrooves in estimating J1c fracture toughness with Charpy-size specimens. Trans ASME J Eng Mater Tech 1980;102:192. w2x Holzmann M, Dlouhy I, Brumovsky B. Measurement of fracture toughness transition behaviour of Cr–Ni–Mo–V pressure vessel steel using pre-cracked Charpy specimen. Inter J Pressure Vessels Piping 1999;76:591 –598. w3x Dlouhy I, Kozak L, Valka L, Holzmann M. The susceptible of local parameters to steel microstructure evaluated using Charpy type specimen. J de Physique IV 1996;c6:205. w4x Wallin K, Planman T, Valo M, Rintamaa R. Applicability of miniature size bend specimens to determine the aster curve reference temperature. Eng Fracture Mech 2001;68:1265 –1296. w5x Liu YN, Zhou HJ. Ductile to brittle fracture transition of a plain carbon steel and its thermal activation model. Eng Fracture Mech 1994;48:773 –782. w6x Radone JG, Ruben DJ. The effect of temperature and strain rate on yield strength of high strength steels. High Temp Technol 1994;15(2):216 –228. w7x Li CJ. Effects of temperature and loading rate on fracture toughness of structural steels. Mater Des 2000;21:27 –30. w8x R Eiber, L Carlson, B Leis. Fracture control requirements for gas transmission pipelines. In: International Pipeline Conference, ASME, Calgary, Canada, 1998. w9x Schwartz CW, Bass BR. Crack speed relations in inferred from large sen specimen. Eng Fracture Mech 1989;34(5–6):1209 – 1223. w10x Liu YN, Song XL. Dynamic fracture study of nylon-6 at different temperature. J Mater Sci Lett 1995;15(14):1032 – 1033. w11x Liu YN, Song XL. Thermal activation analysis of ductile to brittle transition of polypropylene. Inter J Fracture 1996;77:R61. w12x Barsom JM. The development of AASHTO fracture toughness requirements for bridges steels. Eng Frac Mech 1975;7(3):605 –618. w13x Barsom JM, Rolfe ST. Correlation between K1c and Charpy Vnotch test results on the transition temperature range. Impact testing of Materials, STP 466. Philadelphia: SATM, 1979. p. 281 –302. w14x JA Begley, WA Logsdon, Correlation of fracture toughness and Charpy properties for rotor steels, WRL Scientific Paper 711E7-MSLRF-P1, Westinghouse Research Laboratory, Pittsbourgh, 1971. w15x Li HL. Hot topic in study and application of steel line pipe for nature gas transportation. 1st National Symposium of Tubular Goods mechanical and Environmental Behavior Research. Beijing: Petroleum Industry Press, 2001. p. 3 –19.