M

Sa

b

a

ARRAA

KMFCH

1

bfbastEp

coStaio(a[rhfI

0h

Fusion Engineering and Design 87 (2012) 1628– 1632

Contents lists available at SciVerse ScienceDirect

Fusion Engineering and Design

jo ur nal homep age : www.elsev ier .com/ locate / fusengdes

icrostructure and its influence on mechanical properties of CLAM steel

haojun Liua,∗, Qunying Huanga,b, Lei Pengb, Yanfen Lia, Chunjing Lia

Institute of Nuclear Energy Safety Technology, Chinese Academy of Sciences, Hefei, Anhui 230031, ChinaSchool of Nuclear Science and Technology, University of Science and Technology of China, Hefei, Anhui 230027, China

r t i c l e i n f o

rticle history:eceived 12 May 2011eceived in revised form 17 April 2012ccepted 22 June 2012vailable online 23 July 2012

a b s t r a c t

The microstructure of China Low Activation Martensitic steel (CLAM) and its influence on mechanicalproperties were investigated. The tensile test showed that the strength of CLAM (HEAT 0603A) washigher than that of HEAT 0408B at room temperature, and the reverse results were obtained at elevatedtemperatures. The results indicated that the microstructure was composed of dispersived carbide parti-

eywords:icrostructure

usionLAM steelot forging deformation

cles and lath martensite with high dislocation density. The main precipitation phases were Cr-rich M23C6

carbides precipitated mainly along the lath boundaries and prior-austenite grain boundaries and Ta-richMX particles precipitated mainly in the laths and lath boundaries. The finer lath was the main reason forthe higher strength of HEAT 0603A compared with HEAT 0408B at room temperature; contrasted withthe lower strength at high temperature. Heavier hot forging deformation degree was considered as themain possible reason for the decrease of martensite lath width in HEAT 0603A.

. Introduction

Reduced Activation Ferritic/Martensitic (RAFM) steels haveeen considered as the primary candidate structural materialsor the DEMO fusion reactor and the first fusion power plantecause of their better void swelling resistance, thermo-physicalnd thermo-mechanical properties compared with the austenitictainless steels [1]. During the past 30 years, a lot of R&D activi-ies on RAFMs, including JLF-1 and F82H in Japan, EUROFER 97 inurope, ORNL 9Cr2WVTa in USA and CLAM in China, have beenerformed in the world [2–4].

China Low Activation Martensitic steel (CLAM) with the nominalompositions of 9Cr–1.5W–0.2V–0.15Ta–0.45Mn is being devel-ped at the Institute of Plasma Physics, Chinese Academy ofciences (ASIPP) under wide collaboration with many other insti-utes and universities in domestic and overseas [5]. The R&Dctivities mainly cover composition design, melting and processing,mpurity control [5], performance test [6,7], fabrication techniquesf test blanket module (TBM) e.g. Hot Isostatic Pressing joiningHIP) [8] and Electron Beam Welding (EBW) [9], tritium perme-tion barrier coating, compatibility study with liquid metal LiPb10], irradiation experiments and activation analysis etc. [11]. Cur-ent test results show that CLAM steel has good properties and it

as been considered as the primary candidate structural material

or the FDS series liquid metal LiPb blanket designs [12–14] andTER TBM [15].

∗ Corresponding author. Tel.: +86 0551 5592424; fax: +86 0551 5593328.E-mail address: shaojun.liu@fds.org.cn (S. Liu).

920-3796/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.fusengdes.2012.06.008

© 2012 Elsevier B.V. All rights reserved.

Mechanical properties of structural materials strongly dependon their microstructure. In this paper, the microstructure of CLAMsteel and its influence on the tensile property of different heatswere investigated.

2. Experimental procedure

The materials used were the HEATs 0408B and 0603A. Theirchemical compositions are listed in Table 1. HEAT 0408B was aningot of 20 kg, while HEAT 0603A was a lager ingot of 300 kg. Bothheats were melted in a vacuum induction furnace, and then hot-forged at 1423 K and then rolled into 12 mm thick plates. The heattreatment was quenching at 1253 K for 30 min and then cooling byair and tempering at 1033 K for 90 min and then cooling by air.

The microstructure of CLAM steel was observed by opticalmicroscopy (OM), scanning electron microscopy (SEM), transmis-sion electron microscopy (TEM), selected area electron diffraction(SAED), X-ray diffraction (XRD) and energy dispersive X-ray spec-trum (EDS). The samples for metallographic observation andmicrohardness testing were etched in an alcohol solution of picricacid and muriatic acid. HEAT 0408B sample for precipitates obser-vation was electro-etched in an HCl–C2H5OH solution, while HEAT0603A sample was etched in an alcohol solution of picric acid andmuriatic acid.

The specimens taken from the 12 mm plate in parallel withrolling direction were used for tensile tests at temperatures ranging

from room temperature (RT) to 873 K. Miniature sheet type tensilespecimens with gauge of 5 mm × 1.2 mm × 0.25 mm were testedwith a strain rate of 6.67 × 10−4 s−1. The 0.2% proof strength wasmeasured as yield strength (YS).

S. Liu et al. / Fusion Engineering and Design 87 (2012) 1628– 1632 1629

Table 1Chemical compositions of HEATs 0408B and 0603A in wt%.

Cr W C Mn V Ta O P S Si N Fe

HEAT 0408B 8.91 1.44 0.12 0.49 0.20 0.15 <0.001 0.0035 0.0038 0.11 0.00988 Bal..15

Vt

3

3

psTh

csarrwwl

HEAT 0603A 8.94 1.45 0.13 0.44 0.19 0

The Vickers hardness was measured using a 402MVA Microickers hardness tester under a load of 200 g and with a dwelling

ime of 10 s at RT.

. Results and analysis

.1. Microstructure observation

Fig. 1 showed the tempered structure of the 12 mm HEAT 0603Alate. There was no remaining austenite detected by XRD in CLAMteel after heat treatment. Also, it can be inferred from the SEM andEM observations that the microstructure of CLAM steel after theeat treatment is fully lath-martensite phase.

TEM observations and SAED analysis of the two kinds of pre-ipitates with different morphologies, named M23C6 and MX, werehown in Fig. 2. They have also been detected on the quenchednd tempered condition like in most RAFM steels [16]. The resultsevealed that the larger precipitates (about 100–150 nm) were Cr-

ich M23C6 carbides, and the smaller precipitates (about 40–50 nm)ere Ta-rich MX particle, as indicated in Fig. 3. M23C6 carbidesere precipitated along the prior austenite grain boundaries and

ath boundaries, and MX particle were precipitated within laths.

Fig. 1. Tempered structure of CLAM (HEAT 0603A). (a) Back scatter elec

0.0017 0.0032 0.0042 0.10 0.00585 Bal.

Figs. 4 and 5 showed the metallograph and SEM images of theHEATs 0408B and 0603A. It can be seen that the two HEATs showedthe similar microstructure. Martensitic grain boundaries were dec-orated with carbides. The prior austenite grain size of HEAT 0408Bwas similar to that of HEAT 0603A. These precipitates were mainlyM23C6 type carbides with the similar mean size. The lath width ofHEATs 0408B and 0603A by TEM were shown in Fig. 6. The averagelath width was 300 and 200 nm, for HEAT 0408B and HEAT 0603A,respectively.

3.2. Tensile properties and hardness measurements

The tensile results from room temperature to 873 K were shownin Fig. 7. It can be seen that the YS and ultimate tensile strength(UTS) of HEAT 0603A were higher than that of HEAT 0408B at RT,and the reverse results were observed at high temperature. ForHEAT 0408B, the UTS was 670 MPa and the YS was 512 MPa at RT,while they were 373 MPa and 327 MPa at 873 K, respectively. The

differences in UTS and YS for HEATs 0408B and 0603A were 10 MPaand 64 MPa at RT, and 53 MPa and 45 MPa at 873 K, respectively. AtRT, the UTS of HEAT 0603A was slightly higher than that of HEAT0408B by 10 MPa; when the temperature was up to 373 K, there

tron image; (b) TEM image of lath-martensite; (C) XRD diagram.

1630 S. Liu et al. / Fusion Engineering and Design 87 (2012) 1628– 1632

F 03A).

T

wwHta0

0u

oH

ig. 2. TEM observations and SAED analysis for the precipitates in CLAM (HEAT 06EM image of precipitates.

as no obvious difference; and at 873 K, the UTS of HEAT 0408Bas higher than that of HEAT 0603B by 53 MPa. At RT, the YS ofEAT 0603A was higher than that of HEAT 0408B by 64 MPa; when

he temperature was up to 743 K, there was no obvious difference;nd at 873 K, the YS of HEAT 0408B was higher than that of HEAT603A by 45 MPa.

The total elongation of HEAT 0408B was higher than that of HEAT603A at all tested temperatures, and when the temperature was

p to 873 K, the total elongation showed no differences.

The average hardness value of HEAT 0603A was higher than thatf HEAT 0408B. The average hardness value of HEAT 0603A was 252V0.05, while it was 236 HV0.05 for HEAT 0408B.

Fig. 3. EDS analysis of the precipitates in CLAM (HEAT 0603

(a) Diffraction pattern of M23C6 carbide; (b) diffraction pattern of MX particle; (C)

4. Discussion

The results of tensile and hardness tests show that the tensilestrength of HEAT 0603A is higher than that of HAET 0408B at RT,while the tensile strength of HEAT 0603A are lower than those ofHEAT 040B at elevated temperatures. The main microstructuraldifference between the two heats is the lath width, which couldbe accounted for the differences in Vickers hardness and tensile

strength.

According to the Hall–Petch’s equation [17], a finer lath givesthe higher YS for martensitic steel at RT [18]. It has been mentioned[6] that difference in YS at RT between CLAM and JLF-1, had been

A). (a) Cr-rich M23C6 carbide; (b) Ta-rich MX particle.

S. Liu et al. / Fusion Engineering and Design 87 (2012) 1628– 1632 1631

Fig. 4. Metallographic observation of t

Fig. 5. SEM observations for the precipitates in HEATs. (a) 0408B and (b) 0603A.

Fig. 6. TEM observations of the HEATs. (a) 0408B and (b) 0603A.

he HEATs: (a) 0408B; (b) 0603A.

calculated as about 67 MPa. According to calculation method, thedifference in YS at RT between HEATs 0603A and 0408B, had beencalculated as about 66.8 MPa. The experimental difference in YSbetween the HEAT 0603A and 0408B is 64 MPa at RT, as shownin Fig. 7, which tallied with the calculated value. In Ref. [6], theexperimental difference in YS is smaller than the calculated resultdue to the difference in the content of W [19]. For HEATs 0603Aand 0408B, the content of W has no significant difference, so theexperimental result is in accordance with the calculated value byHall–Petch’s equation.

It has been reported [20,21] that there was a reduction in hightemperature strength with a decrease in grain size. The reason isconsidered to be that the strength of the grain boundary may belower than that of the grains at elevated temperature. Therefore,the finer lath width of HEAT 0603A was considered to be the main

possible reason for the higher hardness, tensile strength at roomtemperature contrasted with the lower strength at high temper-ature. Some earlier studies have been reported in Ref. [22] that

900800700600500400300200250

300

350

400

450

500

550

600

650

700

Str

eng

th,

MP

a

Temperature, K

UTS of HEAT 0408B

UTS of HEAT 0603A

YS of HEAT 0408B

YS of HEAT 0603A

300 400 500 600 700 800 9006

9

12

15

18

21

24

27

30

Tota

l E

lon

gati

on

, %

Temperature, K

HEAT 040 8B

HEAT 060 3A

Fig. 7. Tensile properties of HEATs 0408B and 0603A.

1 g and

wniHit

lciwtit

5

C

(

(

(

A

F

[

[

[

[[

[[

[[

[

[961–963.

[21] Y.S. Lee, D.W. Kim, D.Y. Lee, W.S. Ryu, Metals and Materials International A 7

632 S. Liu et al. / Fusion Engineerin

ith the increase of the deformation degree, the quantity of crystalucleus by nucleation of recrystallization increased, which resulted

n the grain refinement of alloy. The heat forging deformation ofEAT 0603A is larger than that of HEAT 0408B due to the lager

ngot of HEAT 0603A. One of the reasons for smaller lath width inhe HEAT 0603A is the increasing of hot forging deformation.

In the meantime, the total elongation of HEAT 0603A is muchower than that of HEAT 0408B, because the lager deforming extentould cause more non-moving dislocations or dislocation cellsn the HEAT 0603A specimens tempered with high temperature,

hich strengthen the grain boundary and lath boundary, and makehe dislocation become difficult to start. When the temperaturencreases to a certain value, the dislocation begins to move, andhus the elongation gradually increases rapidly.

. Conclusions

The microstructure and its influence on the tensile property ofLAM steel were studied. The following conclusions were obtained:

1) The microstructure of CLAM steel after the heat treatmentwas fully lath-martensite. Cr-rich M23C6 carbide with the sizeof 100–150 nm precipitated along the prior austenite grainboundaries and lath boundaries, and Ta-rich MX particle withthe size of 40–50 nm precipitated within laths.

2) The tensile strength and hardness of HEAT 0603A were higherthan those of HEAT 0408B at RT, while the tensile strength waslower than that of HEAT 0408B at elevated temperature. Theexperimental difference in YS between the HEATs 0603A and0408B was 64 MPa at RT, which tallied with the calculated value66.8 MPa by Hall–Petch’s equation.

3) The smaller lath width of HEAT 0603A was considered to be themain possible reason for the higher hardness, tensile strengthat room temperature, while the lower strength at high temper-ature. Higher heat forging deformation degree was consideredas the main reason for small martensite lath width in HEAT0603A.

cknowledgments

This work was supported by the China National Natural Scienceoundation with Grant Nos. 50901072, 91026002 and 51101148,

[

Design 87 (2012) 1628– 1632

the National Basic Research Program of China with Grant Nos.2009GB109000, 2011GB108001 and 2011GB113001. The authorswould like to thank Miss Fang Wang for her help with the pictureprocessing of the experimental data and Dr. Z.X. Xia for his help-ful discussion on this work, and also thank the great help from themembers of FDS Team in this research.

References

[1] N. Baluc, D.S. Gelles, S. Jitsukawa, A. Kimura, R.L. Klueh, G.R. Odette, et al., Journalof Nuclear Materials 367–370 (2007) 33–41.

[2] R.J. Kurtz, A. Alamo, E. Lucon, Q. Huang, S. Jitsukawa, A. Kimura, et al., Journalof Nuclear Materials 386–388 (2009) 411–417.

[3] T. Muroga, M. Gasparotto, S.J. Zinkle, Fusion Engineering and Design 61–62(2002) 13–25.

[4] R.H. Jones, H.L. Heinisch, K.A. McCarthy, Journal of Nuclear Materials 271–272(1999) 518–525.

[5] Q. Huang, C. Li, Y. Li, M. Chen, M. Zhang, L. Peng, et al., Journal of NuclearMaterials 367–370 (2007) 142–146.

[6] Y. Li, Q. Huang, Y. Wu, T. Nagasaka, T. Muroga, Journal of Nuclear Materials367–370 (2007) 117–121.

[7] Y.F. Li, T. Nagasaka, T. Muroga, Q.Y. Huang, Y.C. Wu, Journal of Nuclear Materials386–388 (2009) 495–498.

[8] C. Li, Q. Huang, P. Zhang, F.D.S Team, Fusion Engineering and Design 82 (2007)2627–2633.

[9] C. Li, Q. Huang, Q. Wu, S. Liu, Y. Lei, T. Muroga, et al., Fusion Engineering andDesign 84 (2009) 1184–1187.

10] Q. Huang, M. Zhang, Z. Zhu, S. Ga, Y. Wu, Y. Li, et al., Fusion Engineering andDesign 82 (2007) 2655–2659.

11] L. Peng, Q. Huang, C. Li, S. Liu, Journal of Nuclear Materials 386–388 (2009)312–314.

12] Y. Wu, S. Zheng, X. Zhu, W. Wang, H. Wang, S. Liu, et al., Fusion Engineering andDesign 81 (2006) 1305–1311.

13] Y. Wu, FDS Team, Fusion Engineering and Design 81 (2006) 2713–2718.14] Y. Wu, FDS Team, Journal of Nuclear Materials 367–370 (2007)

1410–1415.15] Y. Wu, FDS Team, Nuclear Fusion 47 (2007) S1533–S1539.16] P. Fernández, A.M. Lancha, J. Lapena, R. Lindau, M. Rieth, M. Schirra, Fusion

Engineering and Design 75–79 (2005) 1003–1008.17] N.J. Petch, Journal of the Iron and Steel Institute 173 (1953) 25–28.18] T. Nagasaka, H. Yoshida, K. Fukumoto, T. Yamamoto, H. Matsui, Material Trans-

actions, JIM 41 (2000) 170–177.19] S.G. Hong, W.B. Lee, C.G. Park, Journal of Nuclear Materials 288 (2001)

202–207.20] D.K. Matlock, W.D. Nix, Metallurgical and Materials Transactions B 5 (1974)

(2001) 107–114.22] F. Wang, F. Zhang, Y. Lai, W. Zhang, Y. Hou, P. Shi, et al., Materials Review 4

(2009) 13–15 (in Chinese).