Development of modified SS 316 LN for SC magnet

CICC jacket

B.B.NayakAdvanced Materials Technology Deptt.

IMMT(formerly RRL) (CSIR)

Bhubaneswar-751013

IPR collaborator: S. Pradhan

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

Outline

Motivation

• Current High Field High Current carrying Tokamak magnets employ Nb3Sn based Cable-in-Conduit-Conductors (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

Similar Thermo-Physical

characteristics

Reduced strainState

(< 0.1%)

Load bearingStructure

(~ 1 GPa, 4.5K)

Cryo-Compatibility(4.5 K,14 bar)

CompatibilityTo Heat

TreatmentSchedule

ConduitJacket

R&D 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 Jc~1000 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)

Example of incompatibility of Strand & jacket

Strand Jacket cracked

650+200h

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

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

•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.

Difficulties encountered in the Metallurgy of the

Candidate Materials

• 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).

•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 .

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.

Incoloy908 (Fe-Ni-4 wt%Cr)

•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.

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, Poisson’s ratio 0.27-0.3, Elastic Modulus 190-212 GPa,Tensile Strength 515 Mpa, Yield Strength 205 MPa

Elongation 60%, Reduction in area 70%

1. 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-spec>1500MPa,

Yield Strength: Off-spec 1100MPa,

In-spec>950MPa,

No CW, Elongation >30%

All CW, KIC: 150 MPa-m 1/2 .

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

Objectives in the NFP Project:

1. 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.

2. To produce the base jacket material Incoloy908(Fe-Ni-4%Cr) by keeping control over oxygen partial pressure and by making small addition of Zr and Cr at melting stage.

3. To characterize the as-prepared ingots in respect of chemical composition, micro structure and mechanical properties

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.

SS 301 Phase Diagram

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

Cold Working (swaging/Roll forming & Heat Treatment

Evaluation of micro structural & Mechanical properties at RT and LT

Proto type small jacket fabrication and testing in collaboration with IPR(after receiving adequate materials feed back)

              

Action Plan: Process Flow Sheet

Micro Alloying of SS 316 LNElements: Nb, Mo

Specification: C< = 0.01 wt% P< = 0.01 N<= 0.15 Ni>=12 Nb<= 0.05 in addition to usual 316 LN chemical composition

Impact of Mo not yet studied.Expected level of extra addition: 0.7-1 wt%

Fracture Toughness of 316 LN steel after aging(7000C, 200h)

Material 4K Fract. 4K YS Failure Compos. Toughness Mechanism KIc(J)(MPa√m) 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 Nb+S 264 167 1132 1125 ,,Conduit Nb+S 155 90 1354 1237 ,,Conduit, 0.016C - 214 - 982 Eq.gr.needle(Valinox, Sq.), LT shaped second. phase particlesConduit, 0.012C - - 1100 -(Sandvik,Circ.)

Tensile properties of as- welded & aged specimen (JAERI)

Fractography of ruptured surface

Aged tensile specimen

Aged J IC

specimen

Weld specimen

Weld J IC speci-men

Vacuum Induction Furnace (to be installed at IMMT shortly)

Specs:1. Capacity: 3-5 kg SS charge2. Power rating: 15 kW, medium frequency3. Vacuum: 5 x 10 -6 mbar, Working:10-2 to 10-3 mbar4. Working temp.: Up to 1800 0C5. Induction coil size: 175 mm ID, 200 mm height6. In-situ casting facility: Co-axial feed through with manual tilting