The development of a Vickers-type hardness tester for cryogenic temperatures down to 4.2 K

Cryogenics 36 ( 1996) 75-8 I 0 1996 Elsevier Science Limited Printed in Great Britain. All rights reserved 001 l-2275/96/$15.00 ELSEVIER

The development of a Vickers-type hardness tester for cryogenic temperatures down to 4.2 K A. Iwabuchi, T. Shimizu, Y. Yoshino, T. Abe, K. Katagiri, I. Nitta* K. Sadamori+

and

Department of Mechanical Engineering, lwate University, 4-5-3 Ueda, Morioka 020, Japan *Department of Mechanical Systems Engineering, Niigata University, 8050 lgarashi 2-nacho, Niigata 950-21, Japan +Product Development Department, Olympus Optical Co. Ltd, 2-3 Kuboyamacho, Hachioji, Tokyo 192, Japan Received 6 March

1995; revised 25 May 1995

Hardness is one of the most important parameters for evaluating the tribological properties of structural materials for magnets at cryogenic temperatures. We have developed a Vickers-type hardness tester for cyrogenic temperatures. This paper outlines the hardness tester and presents the results of hardness measurements. The temperature of specimens can be varied from 4.2 K to room temperature. A controlled d.c. servo motor and a ball-screw device drive the indentation system and control the velocity of the indenter, the applied load, and the loading rate. Another d.c. motor rotates the specimen which is placed at the bottom of a hollow cylindrical holder. The hardnesses of JNl, JN2, SUS304, SUS316L, Ti alloy, and copper were measured. Hardness increased with decreasing temperature to 77 K for all metals. The hardness of steels at 77 K is more than 1.8 times higher than at 293 K, and JN2 shows the greatest increase with a factor of 3.3. However, the hardness at 4.2 K does not always increase from 77 K. SUS304 shows the greatest increase (1.4 times). On the other hand, JN2, SUS316L, Cu, and CuNi soften between 77 and 4.2 K. Comparisons of hardness with yield stress and tensile strength are also made. Keywords: Vickers hardness;

hardness tester; cryogenic temperature

There is much interest in research and development of superconducting magnets for use in nuclear fusion reactors. From a mechanical engineering point of view, the structurai stability and reliability of magnets at cryogenic temperatures is important. Therefore, the mechanical properties of magnetic structural materials, such as yield stress, tensile strength, fatigue strength, and fracture toughness were studied’-“. Tribological properties, friction, and wear of the materials were also investigated to estimate the frictional heat relating to the nucleus of the quench of the superconducting magnet, and to analyse the stress distribution and deformation of the magne?‘. It is appreciated that the hardness of a material governs its tribological properties because it determines the real contact area relevant to the frictional resistance and wear damage of the contacting bodies. Conventional hardness testers are extensively used at room temperature and high temperature hardness testers are

also commercially available. There has not been a low temperature hardness tester available because low temperature hardness values are often not required from an engineering point of view. Only a few hardness data were reported above liquid nitrogen temperature (77 K)*-l*, however, magnetic structural materials have not been studied, and there is no data at liquid helium temperature (4.2 K). In order to consider the tribological properties of magnetic structural materials at cryogenic temperatures, the hardness needs to be known (noted above). In this paper, the development of the Vickers-type hardness tester is introduced, some hardness results are shown, and the relationships between hardness and other mechanical properties are discussed. Concept

of the

hardness

tester

There are many kinds of hardness tester, such as Brinell, Vickers, Rockwell, Shore, etc. In tribology, Vickers testers

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et al.

are extensively used because the real contact area between contacting bodies is evaluated as

rate, applied load, distance between neighbouring indentation scars, etc., conform to Japanese Industrial Standards (JIS B 7734 and Z 2244).

A, = PIHV

where A, is the real contact area, P is normal load, and HV is the Vickers hardness number of the softer body in contact. Therefore, the authors decided to develop a Vickerstype hardness tester, in which a pyramidal diamond indenter, with diagonal planes at 136” is used. HV is obtained as the normal load divided by the pyramidal contact area of an indentation scar. For conventional room temperature and high temperature hardness testers, measurement of the diagonals of the pyramidal scar is made with an optical microscope at the same temperature as for indenting. Since a measuring method has not yet been developed for measuring indentation scars at cryogenic temperatures in a cryostat, the measurement of the scars is made at room temperature after warming the specimens. A hardness tester consists of 4 systems, as shown in Table 1: an indentation system, including an actuator for an indenter and a loading system; a system for moving specimens; controlling and measuring devices for these systems, and a cooling system. Specimens are cooled down with liquid nitrogen and/or liquid helium in a cryostat. The temperature of the specimen is varied from 4.2 K to room temperature. It is easy to measure the hardness at 77 K and 4.2 K because the specimens are immersed in liquid gases. The loading method of a conventional Vickers hardness tester is a dead weight system with an oil damper to control the approaching velocity of an indenter. Since the specimen assembly is put into a deep cryostat, a long loading shaft must be inserted with some supports. Therefore, a ball screw device and a piezo-electric actuator with a load cell are chosen as the indentation mechanism, instead of the dead weight system. A conventional Vickers microhardness tester usually has an X-Y stage to move the specimen. In this case, the moving system is required to be structurally simple, and sufficient stiffness of the components is also required. As a result, we choose a cylindrical pipe as the holder support; rotating the cylinder provides rotation of the specimen holder and the specimens. The shape and approaching speed of the indenter, loading Table 1

Systems of the Vickers-type

Details of the hardness tester Figure 1 is a schematic diagram of the tester, and Figure 2 shows the details of the specimen and indenter assembly.

The height of the tester is about 2 m from the ground. The diamond pyramidal indenter used is the same as that in a conventional Vickers microhardness tester. The load cell is a ring of SUS304 steel, with strain gauges for cyrogenic temperatures. The indenter driving system consists of a d.c. servo motor and reduction gears, a ball screw device, a stage, a piezo-actuator and a loading shaft. The rotating motion of the d.c. motor is converted to vertical oscillations of the stage via the ball screw. The loading shaft is pressed onto the stage with a coil spring and the piezo-actuator. The load cell and the indenter are placed at the end of the shaft, which is supported by a polyimide sleeve inserted in a support pipe (Figure 2). The d-c. motor drives the indenter, which approaches the specimen, applying the load. The rotation of the d.c. motor is reduced with reduction gears to a factor of 100. The lead of the ball screw is 2 mm, and the feed per revolution of the motor is 20 pm. The allowable stroke of the stage is more than 10 mm. The piezo-actuator is expected to maintain the given load constant when the applied load fluctuates. The actuator consists of five piezo-elements in series, and the maximum displacement is 15 pm at 100 V for each element. However, this device was not used in this work. The specimen holder assembly consists of a hollow cylinder and a holder plate placed at the bottom of the cylinder. The rotation of the cylinder, driven by another d.c. servo motor through a transmission and gear system, rotates the specimens. The cylinder is supported by a couple of angular ball bearings. Measurements and control are achieved using a personal computer. Figure 3 shows the block diagram of the system. The two d.c. motors for the indentation and the movement of specimens are controlled independently. The rotating angle of the d.c. motor is detected by a rotary encorder, and,

hardness tester

System

Items

Remarks

Indentation

Indenting device Indenter Load cell

d.c. motor, ball screw, piezo-actuator Diamond micro-Vickers indenter Ring spring with strain gauges

Movement Holder support Specimen holder Driving device

Rotation Cylinder pipe Disk d.c. motor, reduction gears, transmission

Motor control

(See Figure 3)

Movement

of specimen

Measuring

and control

(1) Indentation

(2) Movement Load control Temperature Cooling

76

Cryogenics

Load cell (see Figure 3) Au-0.07% Fe versus chrome1 thermocouple Inner: Liq.He, Outer: Liq.N,

Cryostat Transfertube Lebel meter Sealing

1996 Volume

36, Number

50 cm length O-ring, Cu-gasket

2

Vickers-type

DC-Motor and reduction gear for indenting motion

hardness

,

et al.

tester: A. lwabuchi

DC-Motor and reduction gear for rotation

Piezo actuator for fine load control Stage for indenting motion Ball screw for indenting motion

?=T

Large gear for specimen rotation

Loading shaft Small gear for specimen rotation Rotating cylinder Pipe

Helium gas inlet valve

Load cell

Diamond indenter

Test specimen

I

Specimen holder

I

Figure 1

Schematic

diagram

of the developed

hardness tester

through a counter board, its output is stored on a personal computer. Output from the computer goes to the d.c. motor through a digital to analogue (D-A) board and a power amplifier. The applied load is measured by the load cell of a spring ring and its output comes into the computer via a strain amplifier and an A-D board. Calibration of the load cell was by the dead weight method, in which weight was applied at the top of the loading shaft at room temperature, 77 K in liquid nitrogen, and 4.2 K in liquid helium. Calibration curves between load, P [N],and strain, E, are obtained as

P=Ce

(2)

where C is a constant depending on temperature. regression gives the following equation:

Linear

C( T> = 6.395~1O-~T + 0.0282

(3)

where T is the temperature [K]. Two cryostats are used. Normally, liquid nitrogen is poured into the outer one and liquid helium into the inner one. As the inner cryostat is almost sealed from the atmos-

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hardness tester: A. lwabuchi

et al.

Support pipe

/

Polyimide sleeve

>;

.S 8

/

5

Gaugefor

load

\

J

Load Cell

3.0

4.0

Input voltage,

/

Figure4 Relationship between velocity of the indenter

\

Dummy specimen

Figure 2

Detail of the specimen

5.0

Diamond indenter

Test specimen

and indenter assembly

6.0

7.0

V

input voltage and approaching

Figure 4 shows the relationship between the input voltage to the d.c. motor and the approaching velocity of the indenter. Below 1.0 V, the motor is stopped by its friction, above 1.O V the relationship is linear, and at 1.5 V the velocity is 30 p ms-‘. According to JIS, the allowable approaching velocity range is lo-30 p ms-I, so that 1.5 V is applied to the motor in the approaching stage. When the normal load detected reaches 0.01 N or more, after the indenter contacts to a specimen, the control system is changed to the loading stage. Input voltage to the motor, VW, is determined as v, = kw(w, - w)

Figure 3 tem

Block diagram

of the measuring

and controlling

sys-

phere, liquid nitrogen is purged by helium gas before liquid helium is poured into the inner cryostat. A level meter for liquid helium is placed in it. The temperature of the specimens is regarded as equal to that of a dummy specimen (Figure 2). Temperature is measured using an Au-O.07 at %Fe-versus-chrome1 thermocouple, and a digital multimeter.

8.0

(4)

where w, is the target value, w is the moving average, and k, is proportional gain. This method provides a sufficient accuracy of load control for a hardness tester. When the applied load reaches the desired value, the maintaining stage is reached. The input voltage to the motor is reduced to 0 V and the holding time is 10 s. The applied load is controlled by the moving average of the last 10 data points without high frequency noise. Figure 5 is a typical example of the loading and maintaining stages of the indentation. The mean loading rate is 4.1 Ns-‘, in this case. The change in load is within 0.3% during the maintaining stage, which conforms to JIS specifications. The maximum load capability of this tester is about 30 N.

14 r

z

1

8

16

Measurement

4

Indentation The indentation process consists of three stages: approaching the indenter to the specimen, loading, and maintaining the load constant. The controlling system is changed for each stage. Sampling time is 5 ms for the first two stages and 100 ms for the last stage.

1 0

2

4

6

8

Time, s Figure 5

Example of loading against time

10

12

14

Vickers-type Table 2

Specimen

materials

and their compositions

Material

Composition

JNI JN2

4Mn-25Cr-15Ni-0.35N-Fe 22Mn-13Cr-3.1Ni-0.77Cu0.22N-Fe 18Cr-15Ni-3Mo-Fe 18Cr-8Ni-Fe 99.99cu 80Cu-20Ni 59Cu-2.6Pb-0.7Fe-0.7Sn-Zn 6.20AI-4.06V-O.O74Fe0.0081 Ni-Ti Glass cloth (80-87%)+epoxy

Type 316L (SUS316L) Type 304 (SUS304) Copper (Cu) Cupronickel (CuNi) Brass (CuZn) Ti-6AI-4V (Ti alloy) GFRP

Moving

(wt%)

d = do( 1+aAT)

(6)

where do is the length at the temperature indented, (Yis the thermal expansion factor, and AT is temperature rise. (Yis around lQ-’ K-l for metals, and AT is less than 300 K from liquid helium to room temperature, so that d = l.O03d, and the evaluated hardness number is reduced by about 0.6% of the real value. As a result, the effect on the hardness is negligible.

system of specimens

k,( e,( i)-e(

i))-Kd(

8( i)-e( i-l ))

(5)

where v, is the input voltage to the d.c. motor, e,(i) is the target value of the rotating angle, e(i) is the real value, ke is proportional gain, and Kd is velocity feedback gain. i denotes the number of the sampling (i.e. time). Sampling time in this case is at 1 ms intervals. At room temperature, the accuracy of the rotation of the holder is to within 0.1’. The specimen holder plate placed on the bottom of the cylinder is a disc with a diameter of 60 mm and a thickness of 8 mm, made of SUS304 steel. Eight cylindrical specimens, with a diameter of 10 mm and a thickness of 5 mm, are fixed at regular intervals on the circle of diameter 40 mm on the holder. According to JIS, the interval of the indentation required is more than 1.0 mm, equivalent to a rotating angle of 3” on the specimen in this work. Thus, it is possible to indent each specimen 7 times. Measurement

of indentation

scar

The indentation is carried out when the specimen temperature reaches a desired value. In this system, the diagonal of the indentation scar is measured at room temperature, using an optical microscope with a reading scale (x200 or x400). In this method, the effect of the thermal expansion of Table 3

et al.

the specimen or indentation scar on hardness should be taken into consideration. The measured diagonal length at room temperature, d, is written as

The rotation of the cylinder provides the movement of the specimen. Control of the d.c. motor driving the cylinder is carried out using the proportional gain and velocity feedback control, expressed as V@ =

hardness tester: A. lwabuchi

Hardness and standard deviation

Results and discussion Materials Table 2 shows the materials used in this study. SUS304 (type 304) and SUS316L (type 316L) are austenitic stainless steels. JNl and JN2 are newly developed cyrogenic steels’, which are also austenitic stainless steels. The copper is OFHC copper, and the two copper alloys, cupronickel and brass are the same as used before6. Titanium alloy (Ti6Al-4V) is also used at cryogenic temperature. GFRP is a magnet-insulating materia14.

Comparison of hardness with a conventional tester at 293 K Table 3 shows the results at 293 K in air, 77 K in liquid nitrogen and 4.2 K in liquid helium, where hardness numbers and standard deviations are shown. The measurement was carried out more than five times for one specimen at the same condition. The standard deviation is normalized by the mean hardness. Figure 6 shows the relationship between the hardness measured with the newly developed tester and a conventional Vickers microhardness tester, at room temperature (293 K) under a normal load of 9.8 N. Values are on the line with a gradient of unity. Furthermore, the standard deviations are small and comparable to the values obtained with a microhardness tester in Table 3. As a result, our device is satisfactory to be used as a Vickers hardness tester.

at 293, 77 and 4.2K

Material

Hardness” Standard at 293 K deviation (H,,MPa) (c/H,%)

Hardnessb Standard at 293 K deviation (H,,MPa) (u/H*%)

Hardnessb Standard Ratio at 77 K deviation (H,/H,)(H,,MPa) (U/H,%)

Ratio (H$H,)

Hardnessb Standard at 4.2 K deviation (H,,MPa) (a/Ha%)

JNI JN2 SUS316L SUS304 cu CuNi CuZn

2372 2283 2156 1833 1280 1294 1539

3.50 4.27 2.58 1.86 4.45 2.70 2.44

2450 2293 2205 17t*1 1313 1264 1578

1.26 4.00 4.02 1.78 5.30 7.58 2.81

0.968 0.996 0.978 1.027 0.978 1.023 0.975

6252 7487 6262 3361 1852 2538 2734

5.12 1.75 6.47 3.04 2.62 15.9 18.4

2.552 3.265 2.840 1.885 1.410 2.008 1.733

6468 5566 3793 4880 1715 2264 -

Ti alloy GFRP

3058 314

1.08 13.7

3116 304

1.68 14.1

0.981 1.032

4880 -

1.92

1.566

5978 _

_

“Measured “Measured

Ratio (HJH,)

Ratio (HJH,)

4.24 3.59 6.35 2.81 5.50 17.4 _

2.640 2.427 1.720 2.736 1.306 1.791

1.034 0.743 0.606 1.452 0.926 0.892

5.54

1.918

1.225 -

with Vickers microhardness tester with cryogenic hardness tester

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et al.

3500

3C03 -

n

JNI

a

cuzn

l

IN2

l

CuNi

c

Ti alloy

A 316L

2500

0

500

1000

1500

//

/

/

2ooo

/

2500

a

/

/ 4’

I

3ooo

3500

Hardness by developed tester, MPa Figure6 Comparison of the measured developed tester to Vickers microhardness

Hardness

b

hardness

by

the

at 293, 77 and 4.2 K

The hardness increases with a decrease in temperature from 293 to 77 K. The hardnesses at 77 K for steels are more than 1.8 times as high as those at 293 K, where JN2 increases by a factor of 3.3 and the hardness value reaches about 7500 MPa. While the hardnesses of copper and Ti alloy increase by about 0.5, those of copper alloys increase by 1.7-2 times. The normalized standard deviation also increases at 77 K, except for JN2 and copper; copper alloys show the greatest increase. In a previous paperI the authors obtained a relationship between hardness and temperature from 293 to 77 K, where the relationship was expressed using an Arrhenius equation. It is likely that the great increase in hardness at 77 K results from the high activation energy in the equation, which depends on the materials. The hardness at 4.2 K does not always increase compared to that at 77 K, whereas those of SUS304 and Ti alloy increase by 45% and 23%, respectively, and that of JNl is almost the same. The hardnesses of JN2, SUS316L and copper, and CuNi decrease. Indentation scars for JN2 at 293, 77 and 4.2 K are shown in Figure 7. The indentation is clear at each temperature. The reading error is one of the reasons for the great scatter of values at 77 K. For instance, a reading error of 1 pm corresponds to a change of about 200 MPa for a HV of 6000 MPa at 9.8 N. The higher the hardness, the greater the error. The change in microstructure might be another reason. For example, many cryogenic steels that are originally austenitic will transform locally to martensitic strnctures. The indentation scar is quite small, i.e. 55 pm for HV= 6000 MPa at 9.8 N, which is smaller than the average grain size. Therefore, results should be dependent on the microstructural uniformity in terms of grain size and shape. It was interesting that the indentation scar of GFRP at 77 K was not detected after a temperature rise to 293 K. This might be caused by the recovery of the epoxy during heating. Comparison of hardness tensile strength

to yield stress and

Figures 8 and 9 show the relationships between hardness and yield stress or ultimate tensile strength. The measured

80

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1996 Volume

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C

Figure7 SEM photographs of the indentation 293 K, (b) 77 K and (c) 4.2 K

for

JN1 at (a)

hardnesses at 293, 77 and 4.2 K are compared with yield stress (0.2% proof stress) and tensile strength data published in other referencesL”3-‘6. In Figure 8, the plotted points are on the line HV= 3a, for Ti alloy, while other points scatter along the line HV = 8u,. The results for hardness and tensile strength are also scattered between the lines HV= 3a,,, and 5u,,, in Figure 9. Yield stress and tensile strength tend to increase with decreasing temperature, but as noted above, hardness does

Vickers-type

hardness

tester: A. lwabuchi

et al.

Conclusion 8000

-

A Vickers-type hardness tester was developed to measure hardness from room temperature to 4.2 K in liquid helium, and the hardnesses of structural materials for superconducting magnets were investigated. The following conclusions are drawn:

I

;

.

’

+

JN2 Q,

*

SUS316L’”

-.-

SUS304”

--c

Cu”

-A-

Ti alloy ”

The developed apparatus performs well as a Vickers hardness tester. Hardness increases with decreasing temperature from 293 to 77 K. The measurement scatter also increases. For steels, the hardness at 77 K is about 2.5 times higher than at 293 K. Hardness does not always increase at 4.2 K, compared with 77 K. SUS304 and Ti alloy become 1.4 and 1.2 times harder at 4.2 K than at 77 K, respectively. However, the hardnesses of JN2 and copper at 4.2 K are lower than at 77 K. Relationships between the hardnesses measured in this work and published yield stress or tensile strength data are obtained. For yield stress, the data were plotted around HV= 8a, except for Ti alloy, with HV= 3a,. For ultimate strength, the data were plotted between HV= 3u,,, and 5a,,,.

I

’

-0

0’““““““““““~“~‘~” 0

1000

2000

3cQO

Yield stress, MPa Figure 8

Relationships

between

hardness and yield stress

t

,

8000

Acknowledgements

I I

-A-

SUS316L”

-m-

sus304

-.-

cu On

+-

Tialloy

The authors wish to thank Mr S. Iwasaki and Mr K. Fujima at Akashi Corp. for supplying the indenter. The authors also express their gratitude to Mr H. Mifune, Mr K. Sasaki, Mr M. Uchizaki and Mr 0. Tada at Iwate University for their assistance in preparing the specimens and .conducting the experiments. This work was carried out with the support of a grant-in-aid from the Ministry of Education, Japan (Number 05555047).

oaa

I

r

:,,,,,....,.,....,,,,,. 1000

2000

WJ .., 3m

Tensile strength, MPa Figure 9 Relationships sile strength

between

hardness

and ultimate

References tenI

2

not always increase with decreasing temperature below 77 K. As a result, the relationship for metals is not always a monotonic increase. It is said that hardness is about 3 times as high as yield stress for fully work-hardened metals’, and also is about 3 times as high as the tensile strength for metal. Oku et al.” noted that the relationships between hardness and ultimate tensile strength or compressive yield stress were HV= (2-5)~~,,, or HV= (3-4)~~ above 77 K. The yield stresses refered to above are not values for fully work-hardened metals, but mostly represent stresses for the annealed condition, which results in lower strengths and higher ratios of HVla,. On the other hand, tensile strength is obtained after full work-hardening, and the scattering is smaller than that for yield stress. Of course mechanical properties such as hardness, yield stress, and tensile strength, depend on microstructure, grain size, strain rate, heat treatment, processing, etc. These variables will affect the relationships shown in Figures 8 and 9. Therefore, a precise relationship should be obtained for the same ingot of a metal. After that, the mechanical properties at cryogenic temperatures could be estimated from cryogenic hardness measurements.

3 4 5

Suemura, K., Sakamoto, T., Ogawa, T., Okazaki, T., et al. Adv Cryog Eng (1988) 34 123-129 Takeuchi. M.. Shoii. T.. Takahashi. H. and Anavama. T. Tram ’ Jpn Sot kech’Eng ‘;1(1985) 51 2258-2263 Ogata, T., Ishikawa, K., Nagai, K. and Yuri, T. J. J Cryog Sot Jpn (1991) 26 190-196 Iwabuchi, A., Iida, S., Arai, H., Komuro, K., et al. J Cryog Sot Jpn (1992) 27 325-33 1 Iwabuchi, A., Arai, H., Iida, S. and Takahashi, H., Fusion Eng and Design

6 7 8 9 10 11 12 13 14

(1984)12

15 16

(1993)

20 333-338

Iwabuchi, A., Iida, S., Yoshino, Y., Shim@ T., et al. Cryogenics (1993) 33 1110-1115 Iwabuchi, A., Arai, H., Yoshino, Y., Shimizu, T., et al. Cryogenics (1995) 35 35-40 Bowden, F. P. and Tabor, D. The Friction and Lubrication of Solids, Pt. II Oxford Press, Oxford, UK ( 1964) 320-349 Oku, T. and Sato, S. J Sot Mnt Sci Jpn ( 1966) 15 ( 155) 547-554 Oku, T., Sato, S. and Fujimura, T. Nuclear Structural Eng (1965) 2 282-292 Oku, T. and Usui, T. J Nucl Mater ( 197 1) 40 93- 103 Shimizu, T., Iwabuchi, A., Yoshino, Y., Katagiri, K. and Nitta, I. Tram Jpn Sot Mech Eng A (1995) 61, 1431-1437 Suzuki, K., Fukakura, .I. and Kashiwaya, H. Adv Cryo Eng Mat (1992)38 149-158 Tobler, R. L. and Reed, R. P. J Testing and Evaluation. JTEVA 364-370

Handbook on Materials for Superconducting 04, Battele, Columbus, Ohio (1977) Nagai, K., Yuri, T., Ogata, T., Umezawa, Inst Jpn International

Cryogenics

(1991)

Machinery,

MCIC-HB-

0. et al. Iron and Sfeel

31 882-889

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