NARROW GAP TIG WELDING AND DEVELOPMENT OF FILLER WIRES OF RAFMS FOR WELDING

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NARROW GAP TIG WELDING AND DEVELOPMENT OF FILLER
WIRES OF RAFMS FOR WELDING

• G. SRINIVASAN
• SHAJU K. ALBERT
• A.K. BHADURI
• Materials Technology Division
• Indira Gandhi Centre for Atomic Research
  Kalpakkam-603102, India

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Background

• RAFM steels are the candidate structural
  materials for TBM to be installed in the ITER
  Fusion Reactor.
• RAFM steels have a chemical composition similar
  to modified 9Cr-1Mo steel.
• Mo and Nb that produce long living radioactive
  isotopes in the reactor environment are replaced
  with W and Ta.
• Temperature window for use of these steels are
  presently about 350550C
• Lower value being limited by irradiation-induced
  embitterment effects
• Upper value by a strong reduction in mechanical
  strength.

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Objectives

• To develop and qualify procedures for joining of
  various components of TBM using RAFM steel
• Narrow Gap TIG
• Laser
• Electron Beam
• Hybrid Laser
• Diffusion bonding
• To develop consumables for NG-TIG and hybrid
  laser
• To choose suitable joining process based on
• Joint Design
• Accessibility
• Assembly sequence

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Requirements of Weld Joints in TBM Fabrication

• Microstructure of the joint shall be resistant to
  the effects of neutron irradiation in the
  temperature range of 350-550C
• No significant shift in Ductile Brittle
  Transistion Temperature
• Joint should possess required strength, fracture
  toughness, creep and fatigue resistance to ensure
  adequate structural stability throughout the
  service
• Fusion welds on the first wall facing plasma is
  not acceptable

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Other Considerations in the Choice of the Welding
Process

• Minimize the size of the fused metal zone and
  heat affected zone in the fabricated components
• Minimum distortion and low residual stress in the
  weld
• Minimize the use of edge preparation and filler
  wires
• Suitable for the joint configuration and assembly
  sequence chosen

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TIG

• Arc welding process, uses a nonconsumable
  tungsten electrode to produce the weld.
• Weld area is protected from atmospheric
  contamination by an inert gas and a filler metal
  is normally used
• Autogenous welds do not require FM
• Most commonly used to weld thin sections of SS
  and light metals

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NG-TIG

• NG-TIG is an advanced technique for higher
  productivity in the manufacture of thick-walled
  components
• Developed to achieve similar corrosion and
  fatigue properties for both weld and base metals
  without porosity or inclusions
• Electrode will be oscillated in the narrow groove
  by twisting the torch tip in which the tungsten
  electrode tilted and the pulse energizes and
  preheats the filler wire prior to its contact the
  weld puddle.

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NG-TIG

• Excellent mechanical properties- comparable to
  BM
• Ensures high weld quality and high efficiency
• Volume of weld metal deposited and total heat
  input teo the weld are lower than in conventional
  TIG
• Results in a favorable residual stress profile in
  the HAZ
• NG-TIG technique is considered for the
  fabrication of the various components of TBM
  especially in site welding where other joining
  techniques cannot used.
• For PFBR steam generator fabrication this welding
  process is chosen by the fabricators instead of a
  combination of TIG and shielded metal arc welding
  (SMAW) proposed.
• Ease of Automation

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Schematic of hot wire NG-TIG

• Oscillation of the torch tip to ensure side wall
  fusion
• Avoiding use of SMAWprocess, which has low weld
  metal toughness

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TIG (Eurofer)
TIG JOINT DESIGN FOR HORIZONTAL/VERTICAL
STIFFEENING PLATES
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TIG (Eurofer)

• FZ Equiaxed grains of martensite laths
• No d ferrite
• G/size 40-100µ
• No defects like cracks and inclusions

Cross Section view of TIG weld
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Improvisations in TIG Welding Penetration
Enhancing Flux Assisted (PEFA) TIG Welding
(Patented)

• PEFA-TIG flux developed for single-pass
  autogenous welding
• Weld bead penetration of upto 12 mm achieved
• Specific Advantages
• Upto 50 ? in welding costs
• ? in bevel preparation
• ? in no. of weld passes
• ? in welding times
• ? in filler wire consumption
• ? Distortion
• ? in heat input
• Straight edges
• No back gouging or grinding
• Full penetration in single pass

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Multi-pass TIG Vs. PEFA-TIG
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PEFA-TIG Welded 9 mm thick 316LN SS Square-Butt
Joint
Top face
Bottom face
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PEFA-TIG Welding Implemented at CWD/IGCAR
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TIG Comparison with other Processes

• HAZ width is high -3mm
• Distortion noticed
• Filler addition is required for more than 3mm
  thickness
• Preheating and post heating generally employed
• Residual stress would be high
• Very low welding speed

• Suitable for site welding
• All position welding
• Wide experience is available

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Concerns with respect to Joint Design and
Assembly Sequence
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Other Concerns with respect to Joining

• Use of preheating and post heating
• Post Weld Heat Treatment (730-760/2h)
• It is essential, but extreme care shall be taken
  to ensure dimensional stability of the components
  and cooling channels
• For hybrid welding, solid state laser welding
  machine with fiber delivery may be required
• Dissimilar welds involving RAFMS and 316L will
  have microstructure and properties different from
  the base metals

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Development of RAFMS Filler Wires

• Requirements
• Sound structural welds
• Free from cracks
• High joint efficiency
• Low pore levels
• Amenable to automation
• in a spool form for both NG-TIG Hybrid Laser
  welding

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Characterisation

• Required Tests
• Chemical Composition
• Soundness as per AWS SFA 5.28
• Mechanical Property
• All Weld Tensile at RT and at 550C
• DBTT

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Chemical Composition
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Mechanical Properties (RT)
Achieved
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Mechanical Properties (550C)
Achieved
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DBTT

• WM
• BM
• EUROFER
• Below RT

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Summary

• It is possible to develop welding procedure to
  produce defect free welds of RAFMS using any of
  the processes considered for TBM fabrication.
• Indigenous development of filler wire is feasible
  and M/s MIDHANI has the technology for melting
  and wire drawing
• Challenge would be actual fabrication
• Joint design
• Assembly sequence
• PWHT
• Distortion
• Mock up trails shall be carried out for actual
  joint configuration and fabrication procedure
  established before component fabrication is taken
  up (including heat treatment)
• Weld joints needs to be characterised in detail
  for dissimilar joints involving austenitic
  stainless steels and RAFMS and produced by EB or
  Laser Welding without filler addition.

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