Forming Welding & Heat treatment of ss equipment & components for Nuclear Recycle Plants

1. Forming, Welding & Heat Treatment of
SS equipment & Components for
Nuclear Recycle Plants
Authors
Santosh Takale,
Ramakant & D. S. Rana
Nuclear Recycle Group,
Bhabha Atomic Research Centre,
Trombay, Mumbai – 400 085.

2. Introduction
 Nuclear Recycle Group, involved in
• Design & detailed engineering
• Construction &
• Operation of Reprocessing & Waste Management Plants.
 Material of construction is SS 304L
for equipment, Piping & Cladding of Supports :
- Dissolvers - Process Columns
- Evaporators - Heat Exchangers
- Scrubbers - De-nitrators
- Conditioners - Annular Vessels
- Process Vessels - Storage Tanks

3. Operating Conditions
• Nitric acid medium : 0.1-12 M
• Temperature : Ambient to 1200C
• Levels of radioactivity : up to 250 Ci/lt.
• Operating pressure ≈ (-) 250 mm WC

4. Special Requirements
• Life time integrity
• Corrosion resistance
– Acidic Media ( upto 12 M HNO3 ).
– Presence of corrosion promoters
– Elevated temperature operations
• Operational requirement
– Redundancy & alternatives
– Purge instrumentation
– Remote sampling
– Decontamination
– Passive agitation & liquid transfer (No pumps)
The above resulting in 40-60 Nos. of nozzle openings

7. Formed Components
Components for equipment
fabrication are manufactured by
cold working process to have
smoother surface contours and
for minimum weld joints
• Dished ends
• Pipe bends
• Rolled shell sections

8. Formed Components
• Cooling / Heating coils
• Expansion bellows
• Conical section/
Toricone

9. Effect of cold work on SS
• Increase in susceptibility to sensitization.
• Impact on corrosion resistance of the material.
To ensure minimum cold work:
• KR of 10 % instead of 6% normally followed.
• Strict tolerance on die, punches & rollers.
• No, sharp pipe bends (pipe bends >D )
• Minimize the cold working by selection of technique.
Alternatives :
• Imposing limits on surface Hardness.
• Solution annealing heat treatment.

10. Considerations in Welding
• High thermal expansion in SS
• Low thermal conductivity of SS
• Sensitization
• High residual stresses
• Demand for low corrosion rates
(including HAZ & Weld Pool)
• Metallurgical cleanliness
• Controlled δ - ferrite requirement (4 to 8 FN)
• Stringent defect acceptance criteria
• Limitation on repairs.

11. Welding Techniques
• Use of GTAW welding (minimum defects, manual GTAW )
• Skip-welding, back-step welding
• Specially designed fixtures (reduced distortion, residual stresses)
• Use of low current & voltage to have controlled heat input
• IGC & Ferrite controlled welding filler wire
• Use of high purity argon gas (Shielding, Back purging)
• Use of HF units
• Qualified welders; qualification based on ASME Section IX criteria
& corrosion/delta ferrite limits
• Full penetration welds (back chipped & seal welded), inside weld
joints finished smooth

12. Aspects in Welding
• Solution heat treatment resorted to wherever possible.
• Delta ferrite content measurements.
• Correct fit-ups.
• Orientation of manpower by performing multiple
mock-ups.
• High purity Argon gas (>99.995%).
• Joint configurations to avoid crevices.
• Use of hollow bars nozzles.

13. Full Back-side Purging

14. Back-side Seal Weld

15. Hollow Bar Nozzles

16. Heat Treatment
• Solution heat treatment of ASS is intended to:
– getting alloying elements back to the solid solution in their free form
– nullify the effect of cold working and residual stress reduction
• To overcome the above detrimental effects, heat treatment process
is employed. However, the process should be such that it does not
itself induce ill effects. The following procedure is found to
provide acceptable results:
Loading Temperature : 150 oC – 200 oC
Rate of Heating : 150 oC/hr. above 300 oC
Soaking Temperature : 1050 oC + 10 oC
Soaking Time : minimum 30 minutes (1 hr. per inch of thk)
• Special Requirement : After soaking, the dished ends & formed
components is immediately quenched in adequate water within
90 seconds.

17. Heat treatment
– Calibrated Thermocouples of Nickel-Chromium / Nickel-
Aluminum of ‘K’ type.
– Minimum 6 thermocouples per charge.
– Components placed on appropriate SS fixture and kept
minimum 300 mm above the refractory bricks.
– Quench volume approx. 200 times job volume.
– Additional arrangement for water cooling and circulation.
– Placement arrangement to be as per approved sketch.
– Documents consist of Approved procedure, Heat treatment
specification, Calibration certificate of temperature recorder
and thermocouple, recorded Time-Temperature chart
– Chloride check on water (not to exceed permissible limit)
– Sulphur content for LSHS oil (not to exceed permissible limit)

18. Cleanliness & Contamination Checks
• Uniform surface contour
– Inside flush / merged with parent material
– Outside smooth
• Ensured pre & post cleaning
• Passivation of all surfaces.
• Care for cross contamination at any stages.
• Contamination checks:
– Iron Contamination Check
– Chloride Contamination Check

19. How important the cleanliness is?
(Initiation of corrosion due to use of CS contaminated grinding
wheel around the weld area)

20. How important the cleanliness is?
(Initiation of corrosion due to contamination)

21. Effect of wrong procedure
(Equipment failure even before use because of improper repair
procedure)

22. Stringent Quality Control
• Systematic quality assurance program during:
- Design
- Fabrication (Formimg/welding/Heat treatment)
- Material Storage & Handling
- Installation
• Fabrication in accordance to ASME Section III NC/ND.
• Stringent defect acceptance criteria in NDE.

23. Conclusion
 Considering the end use of the equipment in nuclear recycle
plant, special attention to forming, welding & heat treatment is of
utmost importance.
 Orientation of fabricator towards specific fabrication, Heat
treatment requirement and its implementation throughout the
work execution is the next issue to be handled.
 Equally critical is extensive mock-up trials for establishing
fabrication as well as inspection & test procedures, well before
start of work.
 Cleanliness, contamination control and surface finish remain the
central issue to be addressed throughout the fabrication work.

24. Acknowledgement
To the departmental metallurgists who have
guided us in developing the suitable procedure
for forming & heat treatment methodology.
&
To all of our vendors and departmental
Quality Assurance Surveyors who have
supported us in establishing these procedures.

25. Thank You
Santosh Takale, Scientific Officer, BARC.
Ph - 0-9967584554. santoshatbarc@gmail.com
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