Fusion Engineering and Design 58–59 (2001) 713–717

Mechanical properties of hot isostatic pressed type316LN steel after irradiation to 2.5 dpa

A. Lind a,*, U. Bergenlid b

a Studs�ik Eco & Safety AB, SE-611 82, Nykoping, Swedenb Studs�ik Material AB, SE-611 82, Nykoping, Sweden

Abstract

Hot isostatic pressing (HIP) of powder is considered as a tentative manufacturing method for primary wallcomponents of ITER. The mechanical properties of unirradiated specimens and specimens irradiated to a dose of 0.7dpa at 290 °C from HIPed powder and from wrought, reference, type 316 LN ITER grade steel have been reportedearlier. Complementary tensile, low cycle fatigue and fracture toughness tests of the materials were performed afterneutron irradiation to a dose of 2.5 dpa at 290 °C. The results of these tests compared to those reported previouslyindicate that at the lower dose the HIPed steel shows more irradiation hardening and less elongation compared to thewrought material but after 2.5 dpa the properties are almost identical again. No significant difference in fatigueendurance (at a single strain range of 0.8%) was observed at a dose of 0.7 dpa. After 2.5 dpa the HIPed steel has ashorter average life, but the variation in the results was less compared to the wrought reference steel. The wroughtsteel behaved noticeably tougher than the HIPed after 0.7 and 2.5 dpa (JQ�3×JQHIP). The two steels fractured ina ductile mode. Valid J1c data could not be obtained owing to specimen size limitations. © 2001 Elsevier Science B.V.All rights reserved.

Keywords: Hot isotatic pressing (HIP); ITER; Irradiation; Elongation

Nomenclature

area reductionAuniform area reductionAu

�a crack extensioncompact tensionCTdisplacement per atomdpahot isostatic pressingHIPJ value at the intersection between the power law regression line and the 0.2 mm offsetJQ=J0.2BL

line

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* Corresponding author. Tel.: +46-155-221961; fax: +46-155-221616.E-mail address: anders.lind@studsvik.se (A. Lind).

0920-3796/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.

PII: S0920 -3796 (01 )00541 -5

A. Lind, U. Bergenlid / Fusion Engineering and Design 58–59 (2001) 713–717714

the critical value of J near the onset of stable crack extensionJ1c

LCF low cycle fatiguepart per millionppmultimate tensile strengthRm

Rp0.2 yield strength

1. Introduction

Powder metallurgy hot isostatic pressing (HIP-ing) is under consideration as one possiblemethod for the manufacture of primary wall mod-ules for ITER. Powder HIPing involves the com-pression of metal powder under high isostaticpressure and high temperature.

Several ITER-related demonstration blocks, upto a size of 1940×960×465 mm have been suc-cessfully produced by HIPing of steel powder. Inthe production process, welded cooling tube gal-leries have been included and show excellentbonding to the matrix material. In the unirradi-ated condition the mechanical properties of theHIPed steel were well within the ITER specifica-tion [1].

While likely effects of neutron irradiation havebeen discussed by [2], experimental data are lack-ing. Tensile, low cycle fatigue and fracture me-chanical tests were performed after irradiation todamage levels 0.7 and 2.5 dpa and reported by [3]and by [4]. These results are summarised in thepresent paper.

2. Specimens

Specimens were cut out of a steel block in grade316LN IG prepared by HIPing at 140 MPa,1160 °C for 2 h, from steel powder delivered byANVAL (charge 84043) with composition givenby [3].

The oxygen content of the powder, 195 ppm,was close to the preliminary upper limit of the316LN ITER powder grade specification (200ppm).

Miniature specimens were prepared from theHIPed block as well as from the European type316LN IG wrought reference steel as follows:

� Tensile specimens with gauge length 20 mmand diameter 3 mm

� Low cycle fatigue specimens of hour-glassshape with a minimum diameter of 3.2 mm

� Compact tension fracture mechanics specimenswith thickness B=6.25 mm and width W=12.5 mm. The reference material specimenswere taken out in L-T orientation. MaterialsHIPed from powder are isotropic.Specimens of each kind (2–5) were irradiated in

a pressurised water loop in the Studsvik R2 reac-tor. The specimen temperature during the irradia-tion was 290 °C.

3. Testing procedures and results

3.1. Tensile tests

Two specimens of each variant were tested at290 °C. Almost identical results were obtained foreach pair of specimens and also for all fourunirradiated specimens. The results are shown inTable 1 below and in Fig. 1, as stress vs.crosshead displacement traces. As can be seenfrom Fig. 1, the HIPed specimens irradiated to 0.7dpa show slightly larger increase of yield stressand reduction of elongation compared to thespecimens of the wrought reference material. Af-ter 2.5 dpa the curves are almost identical again.

3.2. Low cycle fatigue tests

The tests were performed under a constantdiametral strain amplitude of 0.17% for all speci-mens giving axial strain ranges of about 0.8% (0and 0.7 dpa specimens) and 0.85 (2.5 dpa speci-mens). A triangular shaped diametral strain witha frequency of 0.125 Hz was applied. The testingafter 2.5 dpa was performed at a slightly increased

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Table 1Tensile test results Tirr= t-test=290 °C

Unirrad. [3] 0.7 dpa [3]Materials 2.5 dpaData

219SS ref. wrought 426Rp0.2 (MPa) 695Rm (MPa) 486 595 702

28 18Au (%) 10A (%) 19Z (%) 92 94 79

213SS-HIP 493Rp0.2 (MPa) 710494 632Rm (MPa) 715

Au (%) 28 16 10A (%) 17

85 79Z (%) 61

strain range (0.85%) compared to the testing ofunirradiated specimens as well as those performedearlier in [3] on material with low neutron dose.

The fatigue endurance of specimens at differentneutron doses are reviewed in Table 2. Hysteresiscurves at NF /2 (NF, cycles to failure) are shownin Fig. 2. The average fatigue endurance (at thesingle strain range of 0.8%) for the steels arecomparable after a dose of 0.7 dpa. The decreasein average life for the reference steel after 2.5 dpais explained by the slight increase in the strainamplitude but the reduction of average fatigueendurance for the HIPed steel is probablysignificant.

3.3. Fracture toughness tests

Miniaturised CT specimens with W=12.5 mmof each steel variant were irradiated to 2.5 dpa atthe Studsvik test reactor. The pre-cracking wasperformed at room temperature after the irradia-tion while the testing was performed at 290 °C.The testing was controlled by the displacement atthe knife edges via a computer program for deter-mination of fracture toughness according toASTM E813. Load, crosshead displacement, anddisplacement of the knife edges was recorded. Theaxis of rotation was assumed to be located at(a+W)/2 from the load line and a modifiedelastic modulus EM according to ASTM 813 wasused. Crack lengths were determined by means ofelastic compliance. The compliance was correctedfor rotation according to ASTM standard. Valid

J1C data could not be obtained owing to thelimited size of the specimens in relation to theirtoughness. JQ (J0.2BL) values were evaluated. Theresults from the testing of the steels are sum-marised in the Table 3 below. The table includesthe results after 0.7 dpa presented by [3]. As canbe seen from Fig. 3, not only the JQ but also theslopes dJ/da are lower for the HIPed steel com-pared to the wrought reference.

4. Examination of fracture surfaces

The fracture surfaces of the tensile specimensdiffered in appearance between the reference andthe HIPed steel. The reference specimens frac-tured after extensive necking almost without anyshear lips. The HIPed specimens showed lessnecking, and had large shear lips. Dimples were

Fig. 1. Tensile behaviour at 290 °C of wrought and HIPed316LN steel before and after irradiation.

A. Lind, U. Bergenlid / Fusion Engineering and Design 58–59 (2001) 713–717716

Table 2Results of low cycle fatigue tests

Materials 316LN IG Cycles to failure NFdpa

Test 1 Test 2 Average

Wrought 0 10 994 16 147 13 60014 808HIPed 11 1330 13 000

9009 18 4030.7 13 700Wrought [3]0.7HIPed [3] 12 358 16 425 14 400

13 381Wrought 76672.5 10 5008941 75682.5 8300HIPed

Table 3Results of fracture toughness tests

dpa JQ kJ/m2Materials

Average316LN IG0.7 1230Wrought [3]

HIPed [3] 0.7 3809032.5Wrought

2.5 255HIPed

tensile properties of the HIPed steel, resulting in alarger increase of yield stress and larger reductionof uniform elongation compared to the wroughtsteel.

After irradiation to 2.5 dpa at 290 °C the ten-sile yield increased up to around 700 MPa for thetwo materials. No work hardening occurred andfurther reduction in ductility was observed. TheHIPed and the reference material behavedsimilarly.

Low cycle fatigue testing at irradiation temper-ature revealed average fatigue endurance to bealmost the same after 0 and 0.7 dpa. After 2.5dpa, the HIPed steel had a shorter average life.However, the variation in results from the HIPedmaterial was not as large as those from thewrought reference steel.

The fracture toughness test results indicated aconsiderable toughness difference. Both materialswere ductile after irradiation (0.7 and 2.5 dpa) but

visible at all the fracture surfaces. The HIPedmaterial showed generally smaller dimples thanthe reference material. No significant effect of theirradiation on the fracture appearance of the ma-terials was noted. Differences in fracture surfaceappearance of the CT specimens were similar tothose of the tensile specimens and turned up assmaller amount of necking and 45° shear lips onthe HIPed material.

5. Conclusions

The tensile properties of unirradiated HIPedtype 316LN steel were very similar to those of thewrought 316LN IG reference material at 290 °C.

Irradiation to a moderate displacement dose of0.7 dpa had a somewhat larger effect on the

Fig. 3. J-integral versus crack extension (�a) for powderHIPed and reference 316 LN IG steel irradiated to 2.5 dpa at290 °C. t-test=Tirr.

Fig. 2. Influence of irradiation on hysteresis curves fromwrought and HIPed (bold) steels at NF/2.

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the HIPed steel shows mean JQ values only about30% of those for the wrought steel.

The reduced ductility of the HIPed steel may berelated to the oxygen content of the metal pow-der. Finely dispersed oxygen containing precipi-tates are expected to form in the HIPed materialand were consequently found by AUGER exami-nation. The neutron irradiation causes not onlyformation of interstitial and vacancy clusters butcan also induce redistribution and further disper-sion of the precipitates.

Acknowledgements

This work was supported by the EuropeanCommunities under an association contract be-tween EURATOM and the Swedish Natural Sci-

ence Council. The authors would like to thankJakobsson for performing the mechanical testing.

References

[1] A. Lind, Shield fabrication development of ITER primarywall modules by powder HIP. ITER Task T 216-Subtask3E1. Studsvik Report, Studsvik Material AB, 1997/88.

[2] D.R. Harries, Hot isostatic pressed type 316L(N) austeniticsteel: assessment of the structure, properties and effects ofirradiation. Studsvik Report, Studsvik Material AB, 1996/28.

[3] A. Lind, U. Bergenlid. Mechanical properties of hot iso-static pressed type 316 LN steel after irradiation. J. Nucl.Mat. Proc. 9th Int. Conf. on Fusion Reactor Materials,CO USA, 1999, to appear.

[4] A. Lind, Testing of powder HIPed stainless steel andCuCrZr/stainless steel joints after irradiation. Studsvik Re-port, Studsvik Eco & Safety AB, 2000/18.