Presentation on theme: "G. SRINIVASAN SHAJU K. ALBERT A.K. BHADURI"— Presentation transcript:
1 NARROW GAP TIG WELDING AND DEVELOPMENT OF FILLER WIRES OF RAFMS FOR WELDING G. SRINIVASANSHAJU K. ALBERTA.K. BHADURIMaterials Technology DivisionIndira Gandhi Centre for Atomic Research Kalpakkam , India
2 BackgroundRAFM 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 350–550°CLower value being limited by irradiation-induced embitterment effectsUpper value by a strong reduction in mechanical strength.
3 ObjectivesTo develop and qualify procedures for joining of various components of TBM using RAFM steelNarrow Gap TIGLaserElectron BeamHybrid LaserDiffusion bondingTo develop consumables for NG-TIG and hybrid laserTo choose suitable joining process based onJoint DesignAccessibilityAssembly sequence
4 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 °CNo significant shift in Ductile Brittle Transistion TemperatureJoint should possess required strength, fracture toughness, creep and fatigue resistance to ensure adequate structural stability throughout the serviceFusion welds on the first wall facing plasma is not acceptable
5 Other Considerations in the Choice of the Welding Process Minimize the size of the fused metal zone and heat affected zone in the fabricated componentsMinimum distortion and low residual stress in the weldMinimize the use of edge preparation and filler wiresSuitable for the joint configuration and assembly sequence chosen
6 TIGArc 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 usedAutogenous welds do not require FMMost commonly used to weld thin sections of SS and light metals
7 NG-TIGNG-TIG is an advanced technique for higher productivity in the manufacture of thick-walled componentsDeveloped to achieve similar corrosion and fatigue properties for both weld and base metals without porosity or inclusionsElectrode 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.
8 NG-TIG Excellent mechanical properties- comparable to BM Ensures high weld quality and high efficiencyVolume of weld metal deposited and total heat input teo the weld are lower than in conventional TIGResults in a favorable residual stress profile in the HAZNG-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
9 Schematic of hot wire NG-TIG Oscillation of the torch tip to ensure side wall fusionAvoiding use of SMAW process, which has low weld metal toughness
10 TIG (Eurofer)TIG JOINT DESIGN FOR HORIZONTAL/VERTICAL STIFFEENING PLATES
11 TIG (Eurofer) FZ: Equiaxed grains of martensite laths No δ ferrite G/size µNo defects like cracks and inclusionsCross Section view of TIG weld
12 Improvisations in TIG Welding – Penetration Enhancing Flux Assisted (PEFA) TIG Welding (Patented) PEFA-TIG flux developed for single-pass autogenous weldingWeld bead penetration of upto 12 mm achievedSpecific AdvantagesUpto 50% in welding costs in bevel preparation in no. of weld passes in welding times in filler wire consumption Distortion in heat inputStraight edgesNo back gouging or grindingFull penetration in single passDiffused arc in Normal-TIGConstricted arc in PEFA-TIGWeld bead shape in Normal TIGWeld bead shape in PEFA-TIG12 mm
16 TIG – Comparison with other Processes HAZ width is high -3mmDistortion noticedFiller addition is required for more than 3mm thicknessPreheating and post heating generally employedResidual stress would be highVery low welding speedSuitable for site weldingAll position weldingWide experience is available
17 Concerns with respect to Joint Design and Assembly Sequence
18 Other Concerns with respect to Joining Use of preheating and post heatingPost Weld Heat Treatment ( /2h)It is essential, but extreme care shall be taken to ensure dimensional stability of the components and cooling channelsFor hybrid welding, solid state laser welding machine with fiber delivery may be requiredDissimilar welds involving RAFMS and 316L will have microstructure and properties different from the base metals
19 Development of RAFMS Filler Wires Requirements:Sound structural weldsFree from cracksHigh joint efficiencyLow pore levelsAmenable to automationin a spool form for both NG-TIG & Hybrid Laser welding
20 Characterisation Required Tests: Chemical Composition Soundness as per AWS SFA 5.28Mechanical PropertyAll Weld Tensile at RT and at 550°CDBTT <-45°C
21 Chemical Composition Elements Wt.% Cr 8.8-9.2 W 0.9-1.1 C 0.10-0.12 Ta MnNVO<0.01Nb<0.001Ni<0.005Mo<0.002CuCoPSi<0.05SAlAs+Sn+Sb+Zr<0.03
22 Mechanical Properties (RT) Tensile PropertyBMEurofer*IGCARWMUTS - MPa660925YS - MPa560835Elongation %182015Reduction in Area %80To be reported* Achieved
23 Mechanical Properties (550°C) Tensile PropertyBMEurofer*IGCARWMUTS - MPa410680420YS - MPa380555390Elongation %22To be reportedReduction in Area %9078* Achieved
25 SummaryIt 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 drawingChallenge would be actual fabricationJoint designAssembly sequencePWHTDistortionMock 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.