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Paper on Forming, Welding & Heat treatment of SS equip & component for NRP


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Paper on Forming, Welding & Heat treatment of SS equip & component for NRP

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Paper on Forming, Welding & Heat treatment of SS equip & component for NRP

  1. 1. Forming, Welding & Heat Treatment of SS Equipment & Components for Nuclear Recycle Plants Santosh H Takale, Ramakant and D. S. Rana Nuclear Recycle Group, Bhabha Atomic Research Centre, Trombay, Mumbai-400 085.Key Words: Stainless steel, NAG, corrosion, IGC, forming, Knuckle, SF, expansionbellows, solution annealing, GTAW, QA/QC.AbstractBack-end Fuel Cycle Plants play a vital role in Nuclear Power Programme as these enablerecovery & recycle of useful materials from the discharged spent fuel from nuclear reactors.The recovery process entails handling of high concentration of nitric acid solutions coupledwith high levels of radioactivity. Austenitic stainless steel (SS) generally withstands theseaggressive process conditions. Apart from material selection, all manufacturing stepsinclusive of forming, heat treatment, joining, cleaning & contamination checks play veryimportant role in lifetime integrity & functioning of process equipment. Years of experiencein the use of such equipment has shown that unattended microscopic defects as well as lockedup stresses at the stage of manufacturing lead to much detrimental effect and at times failuresin the operational stage. To ensure lifetime integrity of the process systems comprisingequipment, piping and assemblies a well defined & established manufacturing set updedicated to such special production is required.The paper shares the experiences on various aspects pertaining to fabrication of austeniticstainless steel equipment with emphasis on forming and heat treatment.1. Introduction:Nuclear Recycle Group (NRG) of Bhabha Atomic Research Centre (BARC) functionallyconsist of design, engineering, construction and operation of reprocessing & wastemanagement plants. Separation of fertile materials from spent fuel is done by chemicalprocessing, which results in generation of high level liquid radioactive wastes. The separationprocess is carried out within RCC shielded cells having wall & roof thickness ranging from900 mm to 1500 mm. The main process medium is nitric acid with various levels ofconcentration, containing a host of isotopes some of which are highly corrosive in nature.Presence of radioactivity makes equipment and systems inaccessible for in-servicemaintenance. Hence, lifetime integrity of fabricated equipment is a prerequisite.The special equipment viz. dissolver assembly, extraction columns, thermo-syphonevaporators, conditioners, denitrators, annular vessels, condensers, scrubbers, etc. employedin reprocessing of spent fuel and waste management demand utmost attention & due care atmanufacturing stage mainly during forming (cold working), heat treatment & welding. Eachof the above equipment and vessels is designed for different process conditions ranging fromnormal temperature to boiling condition operations & 0.1 M to 12 M of HNO3 concentrations.Hence it is essential to control cold working on formed components & various parameters inwelding technique, apart from maintaining the surfaces contamination free. The detail ofvarious aspects with respect to forming, joining and heat treatment of SS equipment &components is elaborated in succeeding paragraphs. 1
  2. 2. 2. Forming:The Austenitic stainless steel being very ductile can be easily made to flow in desired sizeand shapes. Most of components for equipment fabrication are manufactured by cold workingprocess to have smoother surface contours and for minimum weld joints. Process vessels &equipment are constructed from Shells, Dished ends, Expansion bellows, Nozzle pipe bends,Cooling/Heating/Sparger coils & Pull outs, which are all formed components. The pull-outprovides smother geometry for nozzles and eases the complete draining of the equipment. Itmakes joint accessible for radiography and better result interpretation. The degree of coldwork depends to great extent on the method of forming, which in turn have significant impacton corrosion resistance of the material.2.1 Effect of cold work:The cold working of SS material results in increase in susceptibility to sensitization betweentemperature range of 450 to 800 0C [6]. Further, the susceptibility to sensitization ofaustenitic stainless steels increases with increasing cold work [4]. Cold work levels up to 15-20% increase retained energy in the material and as a result the energy required to nucleatecarbide at grain boundary decreases, [1]. The cold working also affects the susceptibility toLow temperature sensitization (LTS). Also machining and cold working results in martensiteformation due to plastic deformation within grains.It is evident from the studies that the corrosion resistance of SS is impaired because of coldworking hence care has been taken to keep degree of cold work to minimum level. Thedished ends geometries are maintained to knuckle radius (KR) of 10 % of ID whereas theconventional torispherical dished ends have KR of 6 % [9]. Higher KR gives addedadvantage of reduced thinning at knuckle region. Various processes adopted for forming ofdishes are die punch technique (for small diameter up to 350 mm), spinning (up to 2000 mm),point pressing for higher diameters. The care has been taken to maintain strict tolerance ondie, punches & rollers used as well surface finish to obtained better product quality in termsof dimensions, surface finish & uniformity in thickness. Similarly, sharp bends in pipecomponents are avoided. The radius given to pipe bends is always more than 5D (D isnominal diameter of pipe). The techniques of material forming are carefully selected tominimize the cold working. e. g. Coils (cooling/heating/sparging) are always manufacturedon coiling machine consisting of three rollers, which produces coils with better dimensioncontrol, meets tolerances and minimizes induced stress. The acceptable values of ovality afterbending of the pipe components are kept less than the codal values. The hardness valuesobtained after cold working are restricted to 200 BHN. Solution annealing heat treatment isgiven to the components exceeding the hardness value of 200 BHN [9]. The details are asmentioned in heat treatment section ahead.3. Welding:The equipment used in nuclear recycle plants are of fully welded construction to provide highreliability and also due to inaccessibility for any repair and maintenance once put into use.Austenitic stainless steels have excellent weldability and can adapt to various availablewelding process viz. GTAW, SMAW, SAW, FCW, EBM, LBM etc. The high thermalexpansion and low thermal conductivity of stainless steel demands controlled heat inputalong with special jigs and fixtures during welding process. GTAW ensures minimum defectsin welding due to its inherent features such as absence of flux/slag, inert gas shielding, nonconsumable electrode, etc. The manual GTAW adds to quality by providing easy arcmaneuvering, welder controlled feed rate etc. Manual GTAW over automatic welding 2
  3. 3. processes was also preferred due to presence of complex internal geometry of the equipment.The quality of the welding is affected by the base material & filler wire used. The corrosionrate of material is restricted to 12 mpy when a sample having all the structures i.e. weld pool,heat affected zone (HAZ) & parent metal is tested in accordance with ASTM A 262 Pr. ‘C’[7]. This has been achieved by modifying the composition of Austenitic Stainless Steel andmaintaining the metallurgical cleanliness i.e. Inclusion-rating (A+B+C+D) < 4.5 [8].The purity of argon gas for welding is also found to have great impact on the weld jointquality. Therefore, only high purity argon gas (>99.995%) for shielding as well as for backpurging was used during all the welding operations [9]. Care was taken to envelope allsurface expose to weld heat during and after welding to avoid any heat tint or oxidation. HFunits were used for arc initiation and to minimize the inclusion due to non consumableelectrode. Only qualified welders were permitted to do welding including temporary andattachment welds. Weld procedures were qualified not only to meet ASME Section IXcriteria but also to meet the requirements of corrosion, controlled content of delta-ferrite etc[9]. To the extent possible all the welds were full penetration joints, back chipped & re-welded. Inside weld joints were finished smooth with the parent metal and outside contoursof the welds were made smooth. This way it was ensured that no crevices are left duringfabrication, which otherwise is highly detrimental during the use of the equipment because ofcorrosive environment inside the equipment. Reinforcement on welds was not allowed toexceed 10% of the material thickness. Consideration has been given to fact that stressespresent due to cooling from high temperatures during welding in constrained geometriescause deformation in materials. Skip-welding, back-step welding, specially designed fixturesetc. were used to avoid distortion and to control residual stress. The precise control overwelding parameters (Voltage & Current) was also maintained. Generally the current andvoltage used on lower side to minimize the heat affected zone and to reduce the extent ofpossible sensitization of the material.Reduction in residual stresses introduced during forming, welding etc. was one of the criticalrequirements of fabrication, as this may result in stress related corrosion during use. Solutionheat treatment was resorted to wherever it was found necessary. Wherever the formedcomponents were of joined type, the representative sample coupons with same heats of thematerial were tested for corrosion tests stipulated as above.Ferrite number or delta ferrite content was measured at random places on welds by contacttype ferrite gauge of certified calibration. The value of ferrite content in the weld both for thewelding procedure qualification as well as production welds should not be less than 4 FN andshall not exceed 8 FN [9].4. Heat Treatment:The heat treatment of Austenitic stainless steel has two fold intentions - getting alloyingelements back to the solid solution in their free form & to nullify the effect of cold workingand stresses induced.The process of formation of chromium depletion zones is called sensitization. In thetemperature range 500-800 0C, the chromium and carbon of the austenitic stainless steel reactfavorably with each other to form chromium carbides (Cr23C6) at the grain boundaries. Thus anarrow chromium depleted zone is produced adjacent to grain boundaries [2, 3]. The abovementioned temperature range is often encountered during welding in which the materialremains for considerably longer durations. Sensitization increases the susceptibility tointergranular corrosion and this will occur when the steel is heated to temperatures in the 3
  4. 4. above range. The cold working also increases susceptibility of ASS to above phenomenon aswell as for low temperature sensitization (LTS). Sensitization occurs in stainless steelscontaining carbon levels as low as 0.02 wt%. The probability of IGSCC occurrence increasesrapidly with carbon concentrations above this level up to 0.055% and there after maintains anapproximate steady rate [5]. The welding of stainless steel welded components produces anarrow band of chromium carbide in an area close to the weld, which has been heated into thesensitization range and the cracks occur primarily in this area.To overcome some of the above detrimental effects, heat treatment process is employed.However, the process should be such that it does not itself induce ill effects. The followingprocedure is found to provide acceptable results.4.1 Typical furnace Specifications: Furnace type : Gas-fired Furnace (Electrical type are preferred) Furnace size : Depending on Job size. [Typ : 5000mm (L) X 5000mm (W) X 2000mm (H)] Furnace insulation : 250mm ceramic fiber on walls and roof of furnace. Mode of fire : High velocity oil fired burners (HSD) Max. operating temp. : 1150 oC Min. thermocouples : 6 nos. per charge (Calibrated by approved agency).4.2 Heat treatment Specifications: 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 section thickness) Special Requirement : After soaking, the dished ends & formed components is immediately quenched in water within 60 - 90 seconds. Automated quenching mechanism is preferred.4.3 Furnace calibration & Setup: All thermocouples are of Nickel- Chromium/ Nickel-Aluminum of ‘K’ type. The calibrated M.I. thermocouple are held against dummy plate of same thickness with the help of T/c nuts placed on dished ends & formed component. Minimum 6 thermocouples per charge installed on the dishes. All dished ends & formed components are placed on appropriate SS fixture and kept minimum 300 mm above the refractory bricks. The fixture is supported in such a way that after completing the soaking period the job can be immediately quenched in water tank [Approx volume 200 times job volume] within 90 seconds (max.). After quenching temperature of job need to be below 200o C. The water tank capacity should be adequate around 45 Cu. meter (min.) with additional arrangement for cooling and circulation. Placement arrangement for dished ends to be as per approved sketch. Potentiometric self-compensating, type ‘K’ duly calibrated recorder, traceable to national standards. Certificate confirming the same to be handed over to QA personnel. Documents consist of Approved procedure, Heat treatment specification, Calibration certificate of temperature recorder and thermocouple, recorded Time-Temperature chart duly signed by concern authority, HSD Test Certificate & chloride level test certificate for water. The sulphur content in oil not to exceed 0.5% wt [9]. 4
  5. 5. 5. Cleanliness:The parts & assemblies after solution annealing need thorough cleaning for removal of hardscales formed during heat treatment process. Earlier experience from the failure analysis ofsome of the equipment reveled that the contamination at any stage in fabrication stage affectsreliability & its performance under use. Hence proper cleaning of all the surfaces is veryimportant. The raw materials were thoroughly cleaned before use in order to remove traces ofpaint, oil, grease, dust or any other contamination. Cleaning of prepared weld edges andcompleted weldments was also done on immediate basis. Wire brushes used were of stainlesssteel and reserved specifically for SS jobs to avoid contamination of the weld surfaces bycarbon steel material. All scales, dents, burrs, weld spatter, oxide, oil and other foreignmaterials were also completely removed from inside and outside of the vessel. The Chloride& Iron contamination checks were carried out on a continuous basis.Scales and Oxide films occurring during the welding & solution heat treatment are subjectedto pickling subsequent to mechanical cleaning in order to attain a metallic bright surfacewhich is finally passivated to attain evenly corrosion resistant oxide layer on all areas. Thematerials are also be subject to pickling treatment before commencement of heat treatmentoperations. All external and internal irregularities like tacks, dents and other effects if any andalso scale formation due to solution annealing are removed mechanically. Any blemishes onthe parent material were rectified and inspected by DP test. Oil, dirt, welding spatters andany other foreign matters adhering to the surface are cleaned by DM water and whenrequired also by grinding, brushing etc. The grinding wheels employed were silicon basedand were reserved for the SS jobs only.Chemical composition of pickling solution is 20 % HNO3 (70 % Concentration) with upto 2% HF (if required) and remaining DM Water. Pickling time depends on the condition ofpickling solution and the extent of thickness of scale. However, minimum 30 minutes arerequired at normal temperatures. During pickling, the residual slags and all black spots arecleaned by scrubbing the surfaces and the welds thoroughly with a nylon brush. Finally, alltraces of acid are removed by DM water rinsing.Passivation solution consists of 15-20 % HNO3 (70 % concentration) and remainder DMwater. Passivation solution is applied for 20 to 30 minutes at a temperature of 40-500 C. Nobrushing is done during the passivation. After passivation, all traces of acid are removedthoroughly by DM water (chloride less than 25 ppm) [9]. Finally, the surfaces are completelydried.6. Contamination Check:Diameter of 5 sq. inch circular area of the surface to be checked is scoured with 10 % HNO3solution laden swab. The free liquid is reabsorbed by swab and dropped into glass test tubecontaining 5 ml. of DM. water. Few drops of AgNO3 are then added to the solution.Evaluation of the surface contamination is done by comparing the turbidity of the spot checktest solution with control sample solution. If turbidity of the spot is more than the controlsolution then the surface is deemed contaminated and in this case the test area should becleaned again and re-checked to ensure effective removal of chloride contamination [9].The check for iron-contamination consists of dropping 10 ml. of CuSO4 solution on a cleansurgical swab and scouring a 5” Dia. circular area of the surface to be tested. The surface isthen observed for change in colour of CuSO4 solution. If the colour of the swab changes togolden brown, the surface may be deemed contaminated with Iron and in this case the 5
  6. 6. affected area should be cleaned and re-checked to ensure effective removal of Ironcontamination followed by passivation [9].7. Quality Assurance:The final result is determined through quality surveillance and control followed duringforming, heat treatment & welding operations. Procedurally the detailed quality assuranceplan was established & approved before the start of activity. Requisite inspection & testswere pre-qualified on mock-up trial pieces so as to achieve satisfactory results on productionjob. Apart from other non-destructive examinations visual inspection was given significantimportance. Acceptance criteria specified for other NDE’s were more stringent than the codalrequirement [9]. These modifications are based on hands on experience for more than 300equipment fabrication.8. Conclusion:It is clear that considering the use of equipment in radioactive & highly corrosiveenvironment, special attention is required in forming, welding & heat treatment steps.Orientation of fabricator towards specific requirements is very much desired. Equallyimportant is extensive mock-up trials to evolve right methodology to be adopted in forming,heat treatment & welding operations.9. References:1. A. H. Advani, L. E. Murr, D. G. Atteridge and R. Chelakara. Metallurgical Transactions 22A, December 1991, p. 2917.2. C. L. Briant, R. A. Mulford and E. L. Hall (1982), ”Sensitization of Austenitic Stainless Steels, I. Controlled Purity Alloys”., Corrosion, vol.38 No. 9 September 1982, p. 4683. F. B. Pickering, `Physical metallurgy of stainless steel developments’, Int. Met. Rev., 21, pp 227-268, 1976.4. H.D. Solomon, Corrosion Vol. 41 No. 9 September 1985, p. 512.5. R. M. Horn and G. M. Gordon, F. P. Ford and R. L. Cowan “Experience and assessment of stress corrosion cracking in L-grade stainless steel BWR internals ”, Nuclear Engineering and Design, Vol. 174 (1997), p. 3136. Mars G. Fontana, Book on Corrosion Engineering, Ed. 3 page 74.7. Standard Practices for detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels, ASTM A 262-02a8. Standard Test Methods for Determining the Inclusion Content of Steel, ASTM E 45-97 (re-approved 2002).9. Technical specifications for equipment fabrication, [TSE-05/P3A/183766/TECH SPECS, (R1)]Regards.Santosh Takale,Scientific Officer, BARCPh - 0-9967584554.santoshatbarc@gmail.comPrint only if essential.......SAVE TREES" Go GREEN, Save Earth " 6