Low Temperature Effects on the Fracture Behavior of

Cold-worked STS 304 Stainless Steel for Membrane of LNG Storage Tank

Dosik Kim1,a

, Yong-Sun Choo1, Kwon-Pyo Hong

1, Jung-Kyu KimM

2 and Chul-Su Kim

3

1Irradiated Materials Examination Facility, Korea Atomic Energy Research Institute,

150 Duckjin-dong, Yuseong-gu, Daejeon 305-353, Korea

2School of Mechanical Engineering, Hanyang University, 17 Haengdang-dong, Sungdong-gu,

Seoul 133-791, Korea

3Department of Rolling Stocks Mechanical Engineering, Korea National Railroad College,

Wolam-dong, Uiwang-si, Gyeonggi-do 437-763, Korea

aCorresponding author: kimds@kaeri.re.kr

Keywords: cold-worked STS 304 stainless steel, low temperature, tensile properties, fracture toughness, critical stretch zone width

Abstract. The temperature dependence of the tensile properties and the fracture toughness of the

cold-worked STS 304 stainless steel have been examined in the temperature range of 293 K to 111 K.

The tensile strength significantly increases with a decrease in temperature, but the 0.2% yield strength

is relatively insensitive to temperature. The total elongation at 193 K abruptly decreases by 50% of

that at 293 K, and it decreases slightly at 193 K to 111 K. The strain hardening exponents at low

temperatures are about four times as high as that at 293K. Initiation fracture toughness (Jc) and tearing

modulus (Tmat) tend to decrease with a decrease in temperature. The Jc values exhibit an inverse

dependency on the effective yield strength (σflow) at all the test temperatures. Fractographic

examination revealed that the critical stretch zone width (SZWc) at room temperature was about three

times as large as that at 111 K. This indicates that the variation in fracture toughness according to

temperature corresponds to the decrease in SZWc with decreasing temperature.

Introduction

For the components of cryogenic structures such as LNG (liquefied natural gas) storage tanks, an

essential requirement in the design is to select the material having high strength, high stiffness and

high toughness at cryogenic temperatures [1]. To meet these requirements, the cold-worked STS 304

stainless steel has been developed by POSCO (Pohang Iron & Steel Co., Ltd.) in Korea as a material

for the membrane of LNG storage tanks. Using this material, KOGAS (Korea Gas Corporation) has

developed the new ring knot membrane for LNG tanks. They are also planning to construct large

capacity LNG storage tanks in the years ahead. Up to this time, however, efforts to obtain basic

knowledge and data on the cold-worked STS 304 stainless steel for research and development on

cryogenic appliances have not been sufficiently carried out.

In this study, the tensile and fracture toughness tests of the cold-worked STS 304 stainless steel for

the membrane of LNG storage tanks were conducted in the temperature range of 293 K (20 oC ) to 111

K (-162 oC). The effects of low temperature on strength, ductility and fracture toughness were

estimated experimentally. The correlation between the fracture toughness and the critical stretch zone

width was also examined by fractographic observation.

Experimental Procedure

The material used in this study was a 2mm thick plate of cold-worked STS 304 stainless steel produced

by POSCO in Korea. Its chemical composition is shown in Table 1.

Solid State Phenomena Vols. 124-126 (2007) pp 1345-1348Online available since 2007/Jun/15 at www.scientific.net© (2007) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/SSP.124-126.1345

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Table 1 Chemical compositions (wt%)

C Mn P S Si Cr Ni

0.05 1.2 0.021 0.008 0.41 18.02 8.6

Tensile and J-integral tests were conducted on a 100kN computer controlled servo-hydraulic

testing system with a cryostat. The specimen was kept inside the low temperature chamber where a

temperature was controlled within +/-2 °C by spraying LN2 gas. All the tests began after the

temperature had stabilized at 111 K (-162 oC, the service temperature of a LNG tank), 153 K (-120

oC),

193 K (-80 oC) and 293 K (20

oC, room temperature) and the extensometer output was constant. A

thermocouple was positioned at the center portion on the tensile specimen with a gage length of 50

mm and a width of 12.5 mm and near the crack growth path on the compact tension (CT) specimen

with a 2 mm thickness and a 40 mm width. The testing incorporates a personal computer to enable

digital data acquisition and plot a load-displacement curve and J-∆a curve during the tests. Three

identical specimens were tested at the same test temperature and the results are averaged.

Antibuckling plates were fitted to either side of the CT specimen, which prevent out-of-plane

displacement during J-integral test. In order to investigate the fracture mode at room and low

temperatures, fractographic observation of the fracture surfaces of the tensile and CT specimen was

carried out using a scanning electron microscope (SEM).

Results and Discussion

Tensile Properties. Fig. 1 shows the effects of temperature on the stress-strain behavior of the STS

304 stainless steel plate. As the temperature decreases, the stress-strain curves change from a smooth

parabolic to sigmoidal behavior. The observed variation in the stress-strain curve shape was

consistent with that observed by previous researchers for Fe-18Cr-8Ni alloys [2] and AISI Type 304

stainless steel [3].

0.00 0.10 0.20 0.30 0.40 0.50

0

200

400

600

800

1000

1200

1400

1600

1800

111K(-162 oC)

153K(-120 oC)

193K(-80 oC)

293K(20 oC)

Stress (MPa)

Strain (mm/mm)

50 100 150 200 250 300 350

0

500

1000

1500

2000

2500

3000

Strength (MPa)

Temperature (K)

0

20

40

60

80

Elongation

Yield strength

Tensile strength

Elongation (%)

Fig. 1 Engineering stress versus Fig. 2 The temperature dependence of yield

strain curves strength, tensile strength and elongation

The temperature dependence of tensile strength, 0.2% yield strength and total elongation are

shown in Fig. 2. The tensile strength at 111 K is about twice as high as that at 293 K, and the 0.2%

yield strength does not change noticeably as the temperature decreases. The total elongation, which is

commonly used as a measure of ductility, significantly decreases at 193 K by nearly 50% of that at 293

K, and then it decreases slightly at 193 K to 111 K. From the SEM fractographs of the fracture surface

of tensile specimen tested at 293 K and 111 K, it is found that fracture surfaces for both the specimens

1346 Advances in Nanomaterials and Processing

fractured at 293 K and 111 K are dominated by dimples produced by microvoid coalescence, and the

dimple size is slightly smaller for the specimen fractured at 111K. The strain hardening exponent (n)

can be expressed as a function of true stress (σT) and true plastic strain (εpT). In this study, the strain

hardening exponent (n) is 0.33 at room temperature (293 K), while n is relatively high ranging from

1.24 to 1.36 over the low temperature range of 193 K to 111K where significant strain-induced

martensitic transformation occurs. Based on the previous researchers’ study [2,3], it could be found

that the decrease in elongation and the increase in tensile strength as the temperature decreases in STS

304 stainless steel reflect the increased contribution of strain-induced martensite.

Fracture Toughness. In load and load-line displacement curves of the CT specimen, the pop-in,

which indicates the occurrence of unstable plastic deformation or unstable crack extension or both,

does not occur at each unloading point during testing. Thus, in this study, the microvoid coalescence,

which is the ductile micromechanism of fracture, is believed to be the dominant cracking mechanism

at all the test temperatures. Since the thickness of the specimen tested in this study does not meet only

the size requirement but also the requirements of the qualification of data recommended by ASTM [4],

the J-integral values for the onset of stable crack extension are termed as the initiation fracture

toughness (Jc), rather than the plane strain elastic-plastic fracture toughness (JIC). Initiation fracture

toughness (Jc) and tearing modulus (Tmat) [5] are shown as a function of temperature in Figs. 3 and 4,

respectively. It is shown in Fig. 3 that Jc considerably decreases as the temperature decreases to 193 K

and it does not change noticeably between 193 K and 111 K. On the other hand, Fig. 4 indicates that

the Tmat linearly decreases with a decrease in temperature.

50 100 150 200 250 300 350

0

200

400

600

800

1000

1200

Jc = 608.68 - 4.32 Temp. + 0.019Temp.

2

293K (σu = 720.5 MPa)

193K (σu = 1162.5 MPa)

153K (σu = 1245.0 MPa)

111K (σu = 1495.0 MPa)

Jc (kJ/m2)

Temperature (K)

80 90100 200 300 400

30

40

60

80

100

200

300

Tmat = 0.429(Temp.)

1.043

293K

193K

153K

111K

Tearing modulus, Tmat

Temperature (K)

Fig. 3 The effect of temperature Fig. 4 The effect of temperature

on initiation fracture toughness (Jc) on tearing modulus (Tmat )

The variations of Jc are plotted as a function of effective yield strength (σflow) in Fig. 5. In this

figure, the fracture toughness decreases with an increase in effective yield strength, which is

analogous to the trend for AISI 304 stainless steel [1].

Several researchers have studied quantitatively the relationship between the stretch zone width

(SZW) as a measure of the crack tip plastic blunting and the J-integral [6]. When a fatigue precracked

specimen in metal is loaded, the crack tip blunts, and the stable crack growth initiates at the critical

stretch zone width (SZWc). Fig. 6 shows typical SEM fractographs of the fracture surface at the

midpoint of the specimen thickness after J-integral testing at room (293 K) and low temperature (111

K). In this figure, the SZWc at room temperature is about three times as large as that at 111 K. The

J-test fracture surface at 111 K exhibits relatively small dimples compared with that at room

temperature. Through the fractographic examination, therefore, it is known that the degradation of Jc

and Tmat with decreasing temperature results from the decrease in the SZWc and the size of dimple

Solid State Phenomena Vols. 124-126 1347

with decreasing temperature.

400 600 800 1000

100

200

400

600

800

1000

2000

1200

Jc = 1.820E7(σ

flow)-1.580

293K (σu = 720.5 MPa)

193K (σu = 1162.5 MPa)

153K (σu = 1245.0 MPa)

111K (σu = 1495.0 MPa)

Jc (kJ/m2)

Effective yield strength, σflow (MPa)

Fig. 5 Correlation between initiation (a) 111 K, mSZW avec µ10)( . ≈ (b) 293 K, mSZW avec µ33)( . ≈

fracture toughness (Jc) and effective Fig. 6 Fracture surfaces of CT specimens tested

yield strength (σflow) at 111 K and 293 K

Conclusions

The cold-worked STS 304 stainless steel used for the material of the membrane of LNG storage

tanks was tested to investigate the temperature dependence of the tensile properties and the fracture

toughness in the temperature range of 293 K to 111 K (the service temperature of LNG tanks). The

tensile strength significantly increased with a decrease in temperature, and the 0.2% yield strength

was relatively insensitive to temperature. The total elongation at 193 K abruptly decreased by nearly

50% of that at 293 K, and it decreased slightly at 193 K to 111 K. The strain hardening exponents in

the low temperature range were about four times as high as that at room temperature. The variations in

tensile properties with temperature could be caused by the fact that the strain dependence of the

martensite transformation rate increased with a decrease in temperature. Initiation fracture toughness

(Jc) considerably decreased as the temperature decreased to 193 K, and it did not change noticeably

between 193 K and 111 K. In addition, the tearing modulus (Tmat) linearly decreased with a decrease in

temperature. Initiation fracture toughness (Jc) was inversely related to the effective yield strength

(σflow). Fractographic examination revealed that the critical stretch zone width (SZWc) at room

temperature was about three times as large as that at 111 K. This indicates that the variation in fracture

toughness according to temperature corresponds to the decrease in SZWc with decreasing temperature.

References

[1] R.L. Tobler, D.T. Read and R.P. Reed: ASTM STP 743 (1981), p. 250

[2] R.P. Reed and R.L. Tobler: Adv. in Cryogenic Engineering and Materials Vol. 28 (1982), p. 49

[3] G.L. Huang, D.K. Matlock and G. Krauss: Metallurgical Transactions A Vol. 20(A) (1989), p. 1239

[4] Standard Test Method for Measurement of Fracture Toughness, Annual Book of ASTM Standards

E 1820-01 (2001)

[5] P.C. Paris, H. Tada, A. Zahoor and H. Ernst: ASTM STP 668 (1979), p. 5

[6] P.R. Sreenivasan, S.K. Ray, S. Vaidyanathan and P. Rodriguez: Fatigue & Fracture of Engineering

Materials and Structures Vol. 19(7) (1996), p. 855

1348 Advances in Nanomaterials and Processing

Advances in Nanomaterials and Processing 10.4028/www.scientific.net/SSP.124-126 Low Temperature Effects on the Fracture Behavior of Cold-Worked STS 304 Stainless Steel for

Membrane of LNG Storage Tank 10.4028/www.scientific.net/SSP.124-126.1345

DOI References

[3] G.L. Huang, D.K. Matlock and G. Krauss: Metallurgical Transactions A Vol. 20(A) (1989), p. 1239

doi:10.1007/BF02647406