Development of modified SS 316 LN for SC magnet CICC jacket

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Development of modified SS 316 LN for SC magnet
CICC jacket

• B.B.Nayak
• Advanced Materials Technology Deptt.
• IMMT(formerly RRL) (CSIR)
• Bhubaneswar-751013

IPR collaborator S. Pradhan
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Outline

• Motivation
• Salient R D issues
• Candidate Materials
• Metallurgical characteristics of above.
• State-of-the-art scenarios issues
• Road map of investigations

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Motivation

• Current High Field High Current carrying Tokamak
  magnets employ Nb3Sn based Cable-in-Conduit-Conduc
  tors (CICC) at 13T or above (eg. ITER). CICC
  comprise of twisted strands and an outer jacket.
• In Indian scenarios, IPR is currently attempting
  to fabricate Fusion grade Nb3Sn strands and
  cables. So far, the jacket material development
  characterization in Nb3Sn CICC in Indian scenario
  has not been attempted.

Jacket
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RD Issues involved

• 1.Jacket is pulled over and roll formed on the
  twisted flexible green strands. Strands Jackets
  are together heat treated (625-7000C, 150-250
  hours) to form Nb3Sn phases of required super
  conductor (Jc, Tc, field and strain state)
  quality. (typical Jc1000 A mm-2 at 12 T, 4.5 K,
  0.7 strain)
• 2.Jacket forms the primary support for the SC
  strand/wire, it may be re-inforced with plates
  and bands. In the first case, primary loading is
  stress limited, in the second case, it is strain
  limited.
• 3. Thus, jacket requires a high yield stress and
  resistance to cyclic loading(a high fatigue life)

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Example of incompatibility of Strand jacket
650200h
Strand
Jacket cracked
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• 4.In Nb3Sn type SC, Jc and Tc are strain
  sensitive. Jacket material thermal expansion in
  the range 700-4K should match with that of Nb3Sn.
• 5.Jacket material should possess good
  fabricability and weldable properties. The
  properties of welds (Yield strength fatigue
  life) need to approach that of the base
  metal.Weld should not show Life Line Attack type
  corrosion.
• Candidate Materials
• SS 316 series
• Incoloy 908 (a specially developed Fe-Ni alloy)
• Ti its alloys

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• JAERI Report
• Yield strength at 4K 600 MPa
• Fracture toughness at 4K 80 MPa m1/2
• Fatigue crack growth rate
• Life 9 x 10 4
  cycles
• (safety factor 5)
  without
• unstable
  failure

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Difficulties encountered in the Metallurgy of the
Candidate Materials

• SS316 LN Although relatively immune to high
  temp. oxygen assisted cracking, during HT, it
  undergoes structural changes due to precipitation
  of inter-metallic compounds either at GB or
  within grains.
• Incoloy 908 Stringent protection against O2 (ppm
  level) is required to prevent inter-granular
  cracking during the initial stages of HT from
  oxidation along GB.

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• Ti its Alloys Stringent protection required
  against O2 , H2 N2 during HT as they get
  absorbed into metallic crystal structure to
  degrade ductility and produce brittle fracture.
• Choice Materials
• State of Art World Scenario
• MIT Plasma Science Fusion Centre (March 2007)
  National High Magnetic Field Laboratory (for
  Hybrid Magnets US ITER Central Solenoid
  Magnets) Use Modified SS316LN (NHMFL316).

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• JAEA ITER Super Conducting Magnet Technology
  Group (Jan,2007) (for ITER TF Conductors) 316LN
  (JK2LB) grade.
• Korea Superconducting Tokamak Advanced Research
  (KSTAR)(for magnets and ITER magnets) Modified
  SS316LN Incoloy908
• Jacket Failure Mechanism indicated
• SS316 LN
• HT in the range 600-800 0C precipitates Cr23C6 at
  the GB later inside grains, which reduce
  ductility, toughness , corrosion resistance,
  lowers the stability w.r to Martensite
  transformation .

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• Ultimately, the GB precipitation becomes
  extensive enough to establish a continuous low
  energy fracture path around the grains. The
  resulting transition in failure mechanism from
  trans-granular dimples to inter-granular facets
  at cryogenic temp., constitutes severe
  embrittlement.
• Fatigue crack growth rate (FCGR) at 4K takes
  place faster leading to failure.
• Composition of steel, unless strictly controlled,
  leads to drastic loss of cryogenic toughness
  after aging. Inter-granular fracture with time
  begins to dominate. Residual cold work
  accelerates precipitation by introducing
  dislocations which facilitates diffusion
• Section thickness which affect cooling rate , is
  also a potential variable.

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• Incoloy908 (Fe-Ni-4 wtCr)
• As a result of HT in O2 , embrittlement along GB
  is used to result.
• Stress accelerated GB oxidation (SAGBO) occurs in
  an environment of O2 H2O in excess of 5 ppm.

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• Materials Specs
• Base Metal
• AISI SS316LN () C 0.03, N 0.1-0.3, P 0.045, Ni
  10-14, Cr 16-18, Mn 2.0, Si 1.0, Mo 2-3 .
• Type Austenite Standard, Common name Chromium
  Nickel steel
• Vendors Osaka Stainless Co (Jpn.), Allied
  Electrode Pvt.Ltd, JindalStainless (India), Red
  Star Wire Mesh (China)
• Properties (at 250C) Density 7.7-8.03 T/m3,
  Poissons ratio 0.27-0.3, Elastic Modulus 190-212
  GPa,Tensile Strength 515 Mpa, Yield Strength 205
  MPa

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• Elongation 60, Reduction in area 70
• MIT(NHMFL 316) (Strip)() C 0.16, N 0.2, S 0.01,
  Nb 0.09-0.15, B 0.02
• Mechanical Properties of Base Metal (100 hr HT at
  7000C), Test done at RT
• Tensile Strength Off spec 1500 Mpa,
• In-spec1500MPa,
• Yield Strength Off-spec 1100MPa,
• In-spec950MPa,
• No CW, Elongation 30
• All CW, KIC 150 MPa-m 1/2 .

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• Test done at 77 K 4K
• KIC 150 MPa-m ½
• Fatigue limit Off spec achieves 3,00,000
  million cycles, In-spec results not available
• n o activation embrittlement probably not
  limiting
• 2.JAEA(JK2LB) (3-5 cm dia, 3mm length specimen
  tested)
• () C 0.05, N 0.24, S 0.001, B 10-40 ppm
• HT done at 650 0C, 11 Cold Drawing,240 hr HT
• YS 900 MPa at 4.2K, KIC(J) 130 MPa at 4K

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• Objectives in the NFP Project
• To produce the base jacket material SS316LN by
  melt-cast technique by suitably keeping control
  over elements like C, S,N,O,B and by micro
  alloying of Nb,Mo.
• To produce the base jacket material
  Incoloy908(Fe-Ni-4Cr) by keeping control over
  oxygen partial pressure and by making small
  addition of Zr and Cr at melting stage.
• To characterize the as-prepared ingots in respect
  of chemical composition, micro structure and
  mechanical properties

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• 4. To carry out lab scale CW of ingots followed
  by HT and evaluate the micro structural and
  mechanical properties(tensile strength
  fatigue).
• 5. Optimization of process parameters and
  production of optimized alloy ingots.
• 6. Proto type jacket fabrication in small size to
  be taken up in collaboration with IPR after
  receiving adequate/sufficient feed back from base
  material alloys.

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SS 301 Phase Diagram
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Action Plan Process Flow Sheet
SS 316 LN /Incoloy 908 master alloy
Melting in Vacuum induction furnace and Micro
alloying (Fe-Nb, Fe-Mo)
Casting, materials characterization Process
parameter Optimization
                           
Proto type small jacket fabrication and testing
in collaboration with IPR(after receiving
adequate materials feed back)
Cold Working (swaging/Roll forming Heat
Treatment
Evaluation of micro structural Mechanical
properties at RT and LT
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Micro Alloying of SS 316 LNElements Nb,
MoSpecification C P0.15 Ni12
NbLN chemical compositionImpact of Mo not yet
studied.Expected level of extra addition 0.7-1
wt
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Fracture Toughness of 316 LN steel after
aging(7000C, 200h)Material 4K Fract.
4K YS Failure Compos.
Toughness
Mechanism KIc(J)(MPavm)
MPa BAg AAg
BAg AAgPlate, 0.019C 239 67
1072 1135
IntergranularConduit 0.018C 232 80
900 930
,,Plate 0.05 Nb 210 160
1240 1260 DimplesPlate
NbS 264 167
1132 1125 ,,Conduit NbS
155 90 1354
1237 ,,Conduit, 0.016C -
214 - 982
Eq.gr.needle(Valinox, Sq.), LT

shaped second.

phase
particlesConduit, 0.012C - -
1100
-(Sandvik,Circ.)

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Tensile properties of as- welded aged specimen
(JAERI)
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Aged tensile specimen
Weld specimen
Fractography of ruptured surface
Weld J IC speci-men
Aged J IC specimen
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Vacuum Induction Furnace (to be installed at
IMMT shortly)

• Specs
• Capacity 3-5 kg SS charge
• Power rating 15 kW, medium frequency
• Vacuum 5 x 10 -6 mbar,
• Working10-2 to 10-3 mbar
• Working temp. Up to 1800 0C
• Induction coil size 175 mm ID, 200 mm height
• In-situ casting facility Co-axial feed through
• 
  with manual tilting

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Thank You