Mechanical tests on the ITER PF 316L jacket after compaction

Cryogenics 51 (2011) 234–236

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Mechanical tests on the ITER PF 316L jacket after compaction H.J. Liu a,*, Y. Wu a, Q.Y. Han a, Zh.X. Wu b, L.F. Li b a b

Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, PR China Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China

a r t i c l e

i n f o

Article history: Available online 7 July 2010 Keywords: CICC ITER Mechanical properties 316L

a b s t r a c t This paper focuses on mechanical tests on the ITER PF jacket 316L stainless steel material. As requirement of ITER, the conductor will be compacted after cable insertion. Young’s modulus, yield strength (0.2% offset), ultimate tensile strength and elongation at failure shall be reported. For researching an effect of compaction on mechanical properties, the three jacket sections which were cut from the same jacket were compacted at 0%, 4.77% and 5.58% in cross section. The mechanical properties of materials were measured at 4.2 K, 77 K and 300 K. The effects of compaction on materials properties were discussed. TEM images showed that there were stress-induced martensite phase after compaction. It is concluded that the results are accordant with requirement of ITER. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction ITER as a full superconducting coil tokamak is under construction [1,2]. It is a large-scale scientific experiment intended to prove the viability of fusion as an energy source. The six poloidal field (PF) coils are designed to adopt the cable in conduit conductor (CICC) with NbTi strands [3]. The Institute of Plasma Physics of Chinese Academy of Sciences is in charge of the manufacture of PF2-5 conductor. The 316L stainless steel materials will be used as the PF jacket. Since the conductor has to undergo compaction after inserting the superconducting cable into the jacket, the effect of compaction on the mechanical properties of jacket shall be investigated. It is also necessary to evaluate the mechanical test at low temperature because the CICC is operated at about 4.5 K. The tensile properties, Young’s modulus, yield strength, etc. were measured at 4.2 K, 77 K and room temperature in this paper.

2. Sample preparation The specimens for tensile test were obtained from the center of each side and in the longitudinal direction of jacket section. Round bar specimens with a diameter of 6.25 mm and length of 80 mm were used for the tensile test in accordance with relevant ASTM standards [4–6]. The half-size, 5-mm-thick specimens were used for K1C test as requirement of JIS standard [7]. The specimens were taken out of the mid-section of the parallel part in the longitudinal direction of jacket section. The direction of crack propagation is the longitudinal direction. It needs pre-generate fatigue crack, in the * Corresponding author. E-mail address: [email protected] (H.J. Liu). 0011-2275/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.cryogenics.2010.06.017

specimens, and then, machine side grooves in accordance with relevant JIS standard. The sampling location was shown in Fig. 1. 3. Experimental procedure The process for base material production underwent an electroslag remelting (ESR) step. The heat number is 0812703012701-03. The jacket was solution heat treated at 1060 °C for 20 min. The chemical composition of jacket is listed in Table 1. The Ni equivalent amount was 13.75. The jackets were compacted into different dimension in a single step using tens of sets of rollers. The original dimension of jacket was h54.2 mm  U37.8 mm  6 m and finally dimension as requirement was h51.9  U35.3. The jacket dimensions before and after compaction are listed in Table 2. The absorbed energy was measured at low and room temperature. When tested at low temperature, the sample was put in a thermal-insulator sleeve and cooled by liquid helium or liquid nitrogen. The tensile tests were carried out in a testing machine (MTS-SANS CMT5000). The sample was installed and immersed in liquid helium or nitrogen. 4. Results and discussions There was not any trace of d-ferrite visible on micrographs at a magnification of 500 as shown in Fig. 2a. Fig. 3 shows the testing results of Charpy impact test. The three differently deformed specimens were tested at 4.2 K, 77 K and 300 K. The absorbed energy increased as testing temperature decreasing. It shows that compaction can enhance the impact property of 316L materials. There was not a linear relationship between compaction variation and impact strength at low

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Jacket Fig. 1. Sampling map (left: charpy specimen, middle: tensile specimen, right: fracture toughness specimen).

Table 1 Chemical composition of stainless steel jacket.

no compaction 4.77% compaction deformation 5.58% compaction deformation

4500 ITER requirement

0.018 0.32 1.60 0.031 0.0006 16.62 12.41 2.08 0.069

<0.030 <0.75 <2.00 <0.03 <0.01 16.00–18.50 11.0–14.0 2.00–2.50 <0.10

4000 2

Product analysis

C Si Mn P S Cr Ni Mo Co

Absorbed energy kJ/m

Element wt.%

3500 3000 2500 2000 1500 1000

Table 2 The jacket dimensions before and after compaction.

500

Deformation

Cross section

Area variation (%)

1 2 3

h54.2  U37.8 h52.2  U35.6 h51.9  U35.3

0 4.77 5.58

temperatures. The 4.77% variation sample is larger than those of 0% and 5.58% variation sample at low temperature. Tensile test results were shown in Table 3 at 4 K, 77 K and room temperature. The elongation at failure is decreasing as the increase of deformation. The 0.2% yield strength, ultimate tensile strength and Young’s modulus are increasing as deformation variation increasing. Fracture toughness of two 5.58% area variation samples at 4.2 K was also tested. The average value of K1C is 200 MPa m1/2 at 4.2 K. All the test results satisfied the requirement of ITER. Fig. 4 shows the typical fracture surfaces of the tensile specimen at 300 K and 4.2 K. The dimples, which represent ductile failure, were observed in four specimens with 5.58% deformation variation. The SEM images show that there exist some cracks which indicate the inter-grain failure when tested at room temperature (Fig. 4a). The dimples were distributed homogenously when tested at 4 K and there is almost no existence of crack (Fig. 4b). Fig. 5 shows the TEM images of three differently deformed jackets. The samples were cut from the center of jacket surface. Fig. 5a shows the microstructure of jacket before compaction. It was

0

100 µm

4.2 K

Fig. 3. Absorbed energy measured at 4.2 K, 77 K and 300 K.

Table 3 The data for tensile test (E.L. is elongation at failure, Y.S. is yield strength, U.T.S. is ultimate tensile strength, and Y.M. is Young’s modulus). Temperature

Sample

E.L./%

Y.S./MPa

U.T.S./MPa

Y.M./GPa

300 K

1 2 3

55 46 40.5

442.5 580 602.5

620 657.5 667.5

193.5 194.5 195.5

77 K

1 2 3

46 45.5 48.5

582.5 645 645

1345 1360 1355

203 204 205.5

4.2 K

1 2 3

45 43 43

710 740 788.7

1600 1620 1635

205 205 208

typical austenite phase without martensite phase. Fig. 5b and c shows the images after 4.77% and 5.5% compaction respectively. There are some lath shaped deformation twins. It showed that twins were body center cubic structure as indicated in selected area electron diffraction (SAED) patterns (see Fig. 5b). It is stressinduced martensite phase which cause an increase in absorbed

(c)

(b)

(a)

77 K

300 K

100 µm

100 µm

Fig. 2. The micrographs of different deformed sample, (a) the 0% variation sample, (b) the 4.77% variation sample, (c) the 5.58% variation sample.

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Fig. 4. The SEM of tensile specimens of 5.58% deformation when tested at 300 K (a) and 4.2 K (b).

Fig. 5. The TEM images of different compaction deformation, (a) is the original jacket, (b) and (c) are compacted at 4.77% and 5.58%, respectively.

5. Conclusion The ITER PF conductor jacket was prepared and compacted at different deformation ratios. The mechanical properties were tested at 4.2 K, 77 K and room temperature. The effect of compaction on the mechanical properties was discussed. The test results showed that it reached requirement of ITER. The TEM images show the lath-shaped martensite was formed after compaction. There were lots of stacking faults and dislocations, which were formed in the process of compaction. Acknowledgements The authors acknowledge the support by the Chinese ITER Specific Foundation (2008GB101000) and the Knowledge Innovation Program of the Chinese Academy of Sciences (085FCQ0127). Fig. 6. The images of dislocation after compaction, (a) shows glide dislocations and (b) shows disordered dislocation.

energy. There are also lots of glide dislocations and stacking faults as shown in Fig. 6a and disordered dislocation as shown in Fig. 6b. The higher strengths or lower elongation after compaction is caused by introduced dislocation. Based on the above analysis, we believe that the compaction processing can contribute 316L steel both the strength and impact toughness. As we know, 316L is not perfectly steel. It will undergo a stress-induced transformation from austenite to martensite at cryogenic temperature, and the resulting transformation will enhance strength and impact toughness.

References [1] Shimomura Y. ITER towards the construction. Fusion Eng Des 2005;74(1/ 4):9–16. [2] Holtkamp N. The status of the ITER design. Fusion Eng Des 2009;84(2/ 6):98–105. [3] Libeyre P, Decool P, Guerin O, Perrella M, Bourquard A. Industrial engineering studies for the manufacture of the ITER PF coils. Fusion Eng Des 2007;82(5/ 14):1561–6. [4] Standard test method for tension testing of structural alloys in liquid helium. ASTM E 1450; 2003. [5] Standard test methods and definitions for mechanical testing of steel products. ASTM A 370; 2005. [6] Standard test method for tension testing of metallic materials. ASTM E8M; 2004. [7] Method of elastic–plastic fracture toughness J1C testing for metallic materials in liquid helium. JIS Z 2284; 1998.