7/27/2019 1-s2.0-S0011227504001031-main

1/4

Small punch testing for determining the cryogenic fractureproperties of 304 and 316 austenitic stainless steels

in a high magnetic field

Yasuhide Shindo *, Yoko Yamaguchi, Katsumi Horiguchi

Department of Materials Processing, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan

Received 22 December 2003; received in revised form 12 April 2004; accepted 16 April 2004

Abstract

This paper examines the effect of magnetic field on the fracture properties of austenitic stainless steels at liquid helium tem-

perature (4 K). Small punch tests were performed on cold-rolled 304 and 316 austenitic stainless steels. Previously proposed

correlation for small punch and elasticplastic fracture toughness test methods was applied to predict a small punch test-based

fracture toughness from equivalent fracture strain.

2004 Elsevier Ltd. All rights reserved.

Keywords: Metals (A); Structural materials (A); Liquid helium (B); Mechanical properties (C); Superconducting magnets (F)

1. Introduction

Austenitic stainless steels are the primary structural

materials for the superconducting applications, in which

they are subjected to high magnetic fields at liquid he-

lium temperature (4 K). The presence of a strong mag-

netic field enhances the strain-induced martensitic

transformation in some of these materials at low

temperatures. If the structural materials selected for

superconducting applications undergo martensitic

transformation under service conditions, there may be

unanticipated effects, such as changes in the fracture and

deformation properties that can potentially degrade the

performance of the device [1].

Experimental efforts have been made to examine theeffect of magnetic field on the tensile and fracture

properties of metastable austenitic stainless steels at 4 K.

Fultz and Morris [2] studied the plastic deformation of

AISI 304L and AISI 304LN stainless steels in magnetic

fields as large as 18 T (tesla) at temperatures of 4, 77 and

290 K. They found that the effects of high magnetic

fields on the deformation behavior were probably too

small to be of engineering importance in the design of

large superconducting magnets. Fukushima et al. [3]using compact (CT) specimens precracked at 77 K,

suggested that there may be a significant decrease in the

fracture toughness of 304 stainless steel at 4 K in a 9 T

magnetic field. Murase et al. [4] also observed that an 8

T magnetic field decreased the 4-K fracture toughness of

304 CT specimens precracked at 77 K. However, Chan

et al. [5] using CT specimens precracked at room tem-

perature, found that an increase in the fracture tough-

ness of 304 CT specimens tested at 4 K in an 8 T

magnetic field was observed relative to the fracture

toughness of CT specimens tested in 0 T. They con-

cluded that this improvement is expected as a result of

magnetostatic effects and transformation strain differ-ences due to the excess martensite formed within the

magnetic field, and the increase in strain hardening

rates. The direction of fracture toughness change is

influenced both by the stability of the alloys and by the

specimen preparation conditions, such as precracking

temperature. The stability of martensitic transforma-

tions and thus the mechanical properties of 304 are

sensitive to factors such as its C, N, and Ni content [6]

and grain size. The C and Ni content in the alloy used in

Ref. [5] was a slightly lower than that in the alloys used

by Fukushima et al. [3] and by Murase et al. [4]. Chan

* Corresponding author. Tel./fax: +81-22-217-7341.

E-mail address: shindo@material.tohoku.ac.jp (Y. Shindo).

0011-2275/$ - see front matter 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.cryogenics.2004.04.008

Cryogenics 44 (2004) 789792

www.elsevier.com/locate/cryogenics

http://mail%20to:%20shindo@material.tohoku.ac.jp/http://mail%20to:%20shindo@material.tohoku.ac.jp/

7/27/2019 1-s2.0-S0011227504001031-main

2/4

et al. [1] examined the fracture behavior of CT speci-

mens made from austenitic stainless steels of differing

stability in a 4 K, 8 T magnetic field environment. The

least stable alloy showed a large reduction in the 4-K

fracture toughness with an 8 T magnetic field. The

amount of fracture toughness reduction with an 8 T

magnetic field decreased as the stability of the specimens

increased. They found that this difference in fracture

behavior is attributed to the enhancement of martensitic

transformation about the crack tip during the fracture

process in a magnetic field.

Because of the limited space available in high-field

mechanical property testing facilities, a test specimen

much smaller than the standard CT specimen would be

extremely desirable. Shindo et al. [7] examined the use of

small punch (SP) testing to estimate fracture toughness of

austenitic stainless steels and weld metals at 4 K, and

assessed correlation between equivalent fracture strain

and elasticplastic fracture toughness JIC. Following JIS

Z 2284 [8] standard test method, all JIC data were ob-tained using 25-mm-thick CT specimens. Shindo et al. [9]

using circumferentially notched bar specimens, also

investigated the cryogenic fracture toughness of austen-

itic stainless steels and weld metals. Recently, Yamaguchi

et al. [10] investigated the effect of magnetic field on the

cryogenic fracture toughness of alloy 908, a ferromag-

netic austenite, using SP and notch tensile specimens. The

4-K fracture properties of alloy 908 were not changed

significantly by magnetic field. The theoretical model [11]

predicted a negligible magnetic field effect on the stress

intensity factor for a crack in low-permeability materials.

The purpose of this study is to examine the effect

of magnetic field on the cryogenic fracture properties

of austenitic stainless steels, and establish the suitabil-

ity of SP testing technique for fracture characterization

of cryogenic structural materials at 4 K in magnetic

fields. SP tests were performed with thin plate specimens

at 4 K in magnetic fields of 0 and 6 T. Two austenitic

stainless steels of differing stability, SUS304 and SUS316

were selected for this test series. A method outlined by

Shindo et al. [7] was applied to predict a SP test-based

fracture toughness from equivalent fracture strain.

2. Experimental procedures

2.1. Materials and specimen

The compositions of the commercial SUS304 and

SUS316 plates used in this work are listed in Table 1.

SUS304 is metastable with respect to austenitic-

to-martensitic transformation and undergo a phase

transition from an fcc structure to a more stable bcc

martensite on deformation at low temperatures; SUS316

is stable with respect to austenitic-to-martensitic trans-

formation. Both were obtained as 10 mm thickness plate

in the cold-rolled condition. The initial volume fraction

of a0 martensite was determined using the relationship

between saturation magnetization and amount of a0

martensite obtained by X-ray diffraction [12]. The sat-

uration magnetization was measured using a vibrating

sample magnetometer. The percent martensite is 0.8%

for 304 and 0.4% for 316. Small, thin plate specimens

of 10 10 0.5 mm were sliced by electro discharge

machining and tested in the as-received condition. The

SP specimens were oriented with the thickness direction

parallel to the rolling direction.

2.2. Testing method

The high-field SP testing was done using a load frame

designed and constructed to fit into the bore of a 8 T

superconducting magnet with a 77 mm diameter work-

ing bore. The punch and the specimen holder, designed

for SP tests, are shown in Fig. 1. The SP specimen

holder consisted of an upper and lower die and four

clamping screws. All test fixtures were fabricated using

SUS310 that is completely stable with respect to mar-

tensitic transformation.

SP tests were performed at 4 K in magnetic fields of 0

and 6 T. The specimens were oriented such that the axis

of the solenoid field was parallel to the specimen loaddirection. The stroke rate was 0.2 mm/min. Displace-

Table 1

Chemical compositions of SUS304 and SUS316 stainless steels (wt%)

C Si Mn P S Ni Cr Mo

SUS304 0.06 0.47 0.89 0.028 0.001 8.54 18.28

SUS316 0.05 0.43 1.04 0.023 0.001 13.69 16.46 2.51

Fig. 1. Schematic of small punch test device.

790 Y. Shindo et al. / Cryogenics 44 (2004) 789792

7/27/2019 1-s2.0-S0011227504001031-main

3/4

ment was measured by measurement of the motion of

the punch relative to the lower die using a 20-mm-

diameter ring-shaped clip-on gage. The clip-on gage

response with and without a 6 T magnetic field was

measured at 4 K. The effect of magnetic field on the clip-

on gage response was negligible.

3. Results and discussion

All loaddisplacement curves for both SUS304 and

316 SP specimens are represented in Figs. 2 and 3. In the

4-K SP tests of austenitic stainless steels and weld met-

als, an approximate definition of equivalent fracture

strain eqf was adopted [7]:

eqf 0:0756dmax

t0

1:83; 1

where dmax is the displacement at peak load and t0 is theinitial thickness. The SP test-based fracture toughness

JCSP can be estimated from the equivalent fracturestrain as

JCSP 957:7eqf 29:5; 2

where JCSP has units of kJ/m2. Standard deviation is

24 kJ/m2. The JCSP values of SUS304 decrease 30%from approximately 303 to 213 kJ/m2 on going from 0 to

6 T, while those values of SUS316 decrease 5% from 0 T

condition (274 kJ/m2) to 261 kJ/m2 at 6 T. Decreases in

fracture toughness are detected with an applied mag-

netic field, depending on the alloy stability. The lessstable SUS304 has the large percentage reduction while

SUS316 has not changed significantly. The transfor-

mation related factor important to the fracture behavior

is the formation of lower toughness a0 martensite. A

material that transforms easily produces a brittle zone,

reducing its measured fracture toughness.

Our results showing a decrease in fracture toughness

with application of a magnetic field are consistent withsome experimental data obtained from CT specimens

[3,4]. At present, fracture toughness is usually deter-

mined using CT specimens, which are rather expensive

to manufacture and require relatively sophisticated

laboratory equipment to test. The advantages of the SP

test are reduced specimen machining costs compared to

CT specimens and simplified test methodology. The SP

test approach is shown to be a viable option for evalu-

ating the material toughness at 4 K in magnetic fields.

4. Conclusions

The 4-K fracture toughness of austenitic stainless

steels, SUS304 and SUS316, in magnetic fields are

characterized by small punch test. The results are sum-

marized as follows:

1. SUS304 shows a decrease in the measured fracture

toughness at 4 K with the application of magnetic

field.

2. The magnetic field has a measurable influence on the

4-K fracture toughness of SUS304.

3. For SUS316, the magnetic field effect is not large

enough to affect mechanical design.4. The magnitude of change in 4-K fracture toughness

with the application of magnetic field is a function

of the stability of the alloy.

References

[1] Chan JW, Chu D, Sunwoo AJ, Morris Jr JW. Metastable

austenites in cryogenic high magnetic field environments. Adv

Cry Eng 1992;38:559.

[2] Fultz B, Morris Jr JW. Effects of high magnetic fields on the flow

stress of 18-8 stainless steels. Acta Metall 1986;34(3):37984.

0.5 1.0 1.5

0.5

1.0

1.5

0

0 T

6 T

Displacement (mm)

Load(kN)

SUS304

SP test

4 K

Fig. 2. Loaddisplacement curves for SUS304 SP specimens.

0.5 1.0 1.5

0.5

1.0

1.5

0

0 T

6 T

Displacement (mm)

Load

(kN)

SUS316

SP test

4 K

Fig. 3. Loaddisplacement curves for SUS316 SP specimens.

Y. Shindo et al. / Cryogenics 44 (2004) 789792 791

7/27/2019 1-s2.0-S0011227504001031-main

4/4

[3] Fukushima E, Kobatake S, Tanaka M, Ogiwara H. Frac-

ture toughness tests on 304 stainless steel in high magnetic

fields at cryogenic temperatures. Adv Cry Eng 1988;34:367

70.

[4] Murase S, Kobatake S, Tanaka M, Tashiro I, Horigami O,

Ogiwara H, Shibata K, Nagai K, Ishikawa K. Effects of a high

magnetic field on fracture toughness at 4.2 K for austenitic

stainless steels. Fus Eng Des 1993;20:4514.

[5] Chan JW, Glazer J, Mei Z, Kramer PA, Morris Jr JW. Fracturetoughness of 304 stainless steel in an 8 Tesla field. Acta Metall

Mater 1990;38(3):47987.

[6] Pickering FB. Physical metallurgy of stainless steel developments.

Int Mat Rev 1976;21:22768.

[7] Shindo Y, Horiguchi K, Sugo T, Mano Y. Finite element analysis

and small punch testing for determining the cryogenic fracture

toughness of austenitic stainless steel welds. ASTM J Test Eval

2000;28(6):4317.

[8] JIS Z 2284, Method of elasticplastic fracture toughnessJIC testing

for metallic materials in liquid helium. Japanese Standards

Association; 1998.

[9] Shindo Y, Mano Y, Horiguchi K, Sugo T. Cryogenic fracture

toughness determination of a structural alloy weldment by notch

tensile measurement and finite element analysis. ASME J Eng

Mater Technol 2001;123(1):4550.

[10] Yamaguchi Y, Horiguchi K, Shindo Y, Sekiya D, Kumagai S.

Fracture and deformation properties of NiFe superalloy in cryo-genic high magnetic field environments. Cryogenics 2003;43(8):46975.

[11] Shindo Y. The linear magnetoelastic problem for a soft ferro-

magnetic elastic solid with a finite crack. ASME J Appl Mech

1977;44(1):4750.

[12] Kurita Y, Emura S, Fujita K, Nagai K, Ishikawa K, Shibata K.

Effects of magnetic fields on martensitic transformation and

serration of austenitic FeNi and FeCrNi steels at 4 K. Fus Eng

Des 1993;20:44550.

792 Y. Shindo et al. / Cryogenics 44 (2004) 789792