S. Howard, W. Daugherty, and C. Sessions
Westinghouse Savannah River Company
Aiken, South Carolina, 29802

This paper details three phases of testing to confirm use of a Gas Tungsten Arc (GTA) system for closure welding the 3013 outer container used for stabilization/storage of plutonium metals and oxides. The outer container/lid closure joint was originally designed for laser welding, but for this application, the gas tungsten arc (GTA) welding process has been adapted. The testing progressed in three phases: (1) system checkout to evaluate system components for operational readiness, (2) troubleshooting to evaluate high weld failure rates and develop corrective techniques, and (3) pre-installation acceptance testing.

A total of 190 can/lid welds were made and evaluated. During Phase I, weld failures were common due to pressure buildup and venting through the weld pool. During Phase II, characterization of the electrode contact to the weld pool and weld pool blowouts helped in the development of a corrective technique. During Phase III, a reduction in internal pressure, by controlling the final helium backfill of the can before welding, provided satisfactory weld results.

The work described was performed during 2002 pre-installation testing at the Savannah River Technology Center in Aiken, S.C. before installation of an Outer Can Welder (OCW) system at the Savannah River Site (SRS) plutonium processing facility. The first OCW system was originally developed at the SRS to support similar plutonium stabilization/storage efforts at the Hanford Site (operated by Fluor Hanford Corporation).

The Department of Energy (DOE) 3013 [1] outer container is the second of two containers used to seal plutonium metal or compounds. The 3013 outer container (Figure 1) is a 316L stainless steel cylindrical container that is closure welded to provide leaktight containment of the enclosed inner container, which is also closure welded. The 3013 outer container and lid are made of dual certified type 316/316L stainless steel, which combines the higher strength of 316 stainless steel with the low carbon content of 316L stainless steel. The design pressure is 4.8 MPa (699 psi) [1]. The joint design for the 3013 outer container does not have a designed gas vent path for the relatively small internal free volume that exists when the loaded outer container is closure welded. Closure welding for the 3013 containers is currently being accomplished at Rocky Flats and Lawrence Livermore National Laboratory (using different laser welding systems) and at Hanford (using an orbital GTA welding system [2-5]). A second GTA welding system similar to Hanford’s system was fabricated for use at the Savannah River Site. Testing of this system included application of a partial backfill technique instead of the full backfill technique that has been successful at Hanford.

The 3013 Standard specifies requirements and references national codes and standards for leak rates (ANSI N14.5 [6]) and design (American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (B&PVC), Sections VIII and IX [7,8]). Since neither the inner nor the outer container is actually used as a pressure vessel, not all of the B&PVC requirements apply. For instance, in production, neither post fabrication proof testing nor full radiographic inspection applies to all welds. Post fabrication proof testing is excluded since internal pressurization for testing is not possible for either of the sealed containers. DOE 3013 containers have been subjected to rigorous fitness for use tests such as drop, crush, impact and corrosion sensitization. These tests simulate potential accident and life storage scenarios.

The outer canister is 254 mm (10.0") tall (before welding the lid) and 125 mm (4.921") in diameter. The wall thickness at the weld is 3.0 mm (0.118"). The wall material is SA 312 Grade TP316/316L stainless steel seamless pipe. The base and lid material is SA182 Grade F316/316L bar. The GTA welding of a base to the canister body is inspected to the requirements of the ASME B&PVC, Section VIII [7], which specifies radiography. The thickness of the lid is 10 mm (0.393"). Chemistry specification for the can and lid material limits sulfur to 0.005-0.025 % for the can wall and 0.010-0.025 % for the lid based upon welding considerations [2,9].

The DOE 3013 closure weld is on a corner joint (Figure 1). The lid has 4 mm (0.157") of material extending above the joint with the 3 mm (0.118") canister wall. The top internal edge of the outer canister has a chamfer to facilitate an interference fit insertion of the lid into the container during encapsulation of an inner container [2], which contains the plutonium or plutonium oxide materials.

The closure-welded joint must meet two dimensional characteristics: penetration and aspect ratio. Additional requirements above code dictate that the weld must pass the B&PVC Section VIII [8] radiographic examination criteria and welded dimensional requirements on a sampling basis of every 25^th canister during radioactive production. A dummy canister (not containing plutonium, but containing surrogate material of equal weight and providing equivalent free gas volume) must be produced for the required radiographic and metallographic analysis of the weld bead quality and aspect ratio (see Figures 1 & 2).

An Arc Machines, Incorporated (AMI) Model 227, 225 Amp weld controller was used with a modified 9-7500 orbital GTA weld head. The OCW weld head was modified to accommodate a "snap-in" rather than a setscrew secured electrode. This modification minimizes errors in setting the arc gap and simplifies electrode change-out. The arc gap is fixed by the precision-machined electrode, whose length establishes a nominal operating arc gap of 1.651 mm (0.065").

The outer can welds were made with the current pulsed in synchronization with the rotational motion around the canister. The electrode moves during the low current pulse and is stationary during the high current pulse. To control the weld bead geometry (Figure 2), a chill block is required. The chill block was secured against the lid using a clamp that attaches to the pintle on the lid. The clamping tool design incorporates Belleville washers to maintain a constant contact force of 2,000 N (450 pounds) between the chill block and lid.

Most weld parameters for the welding system were held as constant as practicable by engineering and quality controls. Material type and composition were maintained within specifications as identified. Joint dimensions and fitup were controlled by precise dimensional control of the mating surfaces. Shielding gas composition was maintained consistent through procurement of custom mixed Argon/Hydrogen gas (Table 1). Tip positioning, chill block contact and joint positioning were controlled by built-in fixturing in the weld system. Other weld parameters such as rotational speed and current were controlled by weld controller programming. Arc gap was set by the total length of the snap-in tip assembly and cover gas flow rate was set by the operator. The partial helium backfill before lid insertion was controlled by the operator also.

Seven small tack welds were made sequentially and were placed symmetrically around the can; with the eighth position being the weld start position. The tack welds penetrate the can / lid to a depth of approximately 0.76 mm (0.03"). The tacks are fully consumed during the closure weld.

Real time monitoring and capturing of weld data was accomplished by the use of a passive data acquisition system. Weld parameters that were monitored and graphically displayed include current, voltage and travel speed, with calculated values of rotational position, energy input and resistance displayed and captured on magnetic storage media for records. The main welding parameters are given in Table 1. Acceptable average primary current was from 173 to 187 A. The AMI 227 weld controller will vary voltage as needed to deliver the programmed current. Experience has shown that the average primary arc voltage varies within 8.5 to 10.0 v for acceptable welds. Acceptable average travel speed was from 0.50 to 0.56 rpm for background travel rate during the first 135 degrees of travel, and from 0.57 to 0.63 rpm during the remainder of the weld. A 5 second preheat, consisting of pulsing to 90 A for 4 seconds, and pulsing to 180 A for 1 second was monitored also.

The weld procedure qualification met the methodology and rigor of ASME requirements, as specified in Reference 8.

The acceptance program for the system acceptance testing consisted of three phases; (1) a 100 can system component checkout phase, (2) a 50 can trouble shooting effort and (3) a final 40 can acceptance test Phase. The following six closure weld evaluations were performed:

1. visual evaluation,
2. review of the data acquisition system output,
3. helium leak test per ANSI N14.5 to <2E-7 atm. cc.He/s sensitivity [6],
4. digital radiography to ASME Section VIII, Division 1, UW-51 [7],
5. (conventional film) radiography to ASME Section VIII, Division 1, UW-51 [7] and
6. metallography to assure a weld bead penetration and aspect ratio per ASME Section VIII, Division 1, UW 13.2 (d) [7].

Visual evaluation was to verify freedom from each of the following:

1. any type of opening on the weld surface,
2. uneven or course ripples on the weld surface,
3. significant changes in bead width,
4. a consumed/rolled upper edge of the lid,
5. incomplete overlap by the down-slope weld bead,
6. cracks, lack-of-fusion or pinholes, and
7. undercut or joint thickness reduction > 0.012 inch

Phase I

The initial 100-can system component checkout used test cans (not meeting all production requirements, but acceptable for closure weld development) for most of the tests. Approximately 33% of the cans welded in Phase 1 were rejectable for a least one of the following reasons (in order of frequency of events): (1) electrode "touchout", (2) blowout of the molten weld zone within the first 60 degrees of weld, (3) porosity at the final weld closure position, and (4) loss of geometry control through partial consumption of the container lid edge. The occurrence of weld blowout and electrode contact with the weld pool (termed "touchouts") was more frequent than observed during initial welder qualification of the Hanford welder [2] or during current Hanford production welding. The system component checkout phase experienced unacceptable weld performance that identified the need for Phase II.

Phase II

Phase II of testing was a trouble-shooting campaign to identify and correct the reasons for recurring weld failures. Twenty-four percent (12 of the 50) of the tests resulted in failure to pass one or more weld acceptance criteria. The cause of failure was predominately electrode "touchout" near the start of the weld. This condition leaves the tungsten electrode contaminated with residue from the weld pool, causing unpredictable arc performance for the remainder of the weld.

Several influences were suggested as contributing to the high incidence rate of weld failures for the OCW system. Limited tests to identify the effect of variation in humidity, barometric pressure, and surface moisture were performed. Joint fit variation resulting from surface finish, flatness and machined diametrical dimensions were also evaluated. Attempts to correlate any of these influences to weld performance were not altogether successful, but they were not ruled out as influential.

To help aid the characterization of the weld closure dynamics, several tests were performed with a pressure transducer attached to a sampling tube welded to the bottom of the can. The internal outer can pressure was computer monitored during evacuation, backfill, lid press, welding and subsequent cooling. Typical pressure plots taken during the evacuation/backfill/lid press stage and in the tack weld/preheat/main weld and cooling phases are shown in Figures 3 and 4, respectively. In Figure 4, note that backfilling to one atmosphere of helium after the third evacuation cycle resulted in a pressure of about 1.2 Atm. (~2.5 psig) immediately after lid pressing. In Figure 4, note also that significant pressure loss occurs during each tack weld and during preheat. Since the weld interface region is isolated from the inside free volume of the outer container by a can-to-lid interference fit, pressure cannot be monitored at the weld interface (Figure 1). Local distortion from the weld heat allows venting from the internal free volume that is neither constant nor completely predictable. However, the pressure monitored inside the free volume is the best indicator of venting dynamics available to date. Tests indicate that the cans did not vent at the same rate, as might be expected during closure welding. A single test performed at Hanford for comparison purposes indicated a slower initial vent rate than that typically observed at SRS, but the reason is unknown. Figures 5 and 6 illustrate the internal pressure recorded over the backfill and weld cycle after adjustments were made to backfill to less than one atmosphere internal pressure. The backfill pressure was selected such that pressing the lid into place would bring the internal pressure to just over 1.0 Atm. Note that the amplitude of the change in pressure during lid press with less than one atmosphere (Figure 6) is less than that when one atmosphere was used (Figure 4). The increase in pressure due to lid press is proportional to the starting (backfilled) pressure.

Test results suggest that control of pre-weld internal free volume pressure influence the number of incidents of weld touchouts and blowouts. A partial backfill technique was developed which allowed the internal free volume pressure to be only slightly above ambient atmospheric pressure after the lid is pressed into place. After the weld is complete, and the can/lid cooled to room temperature, the pressure in the can is typically ~2 psi below atmospheric pressure, regardless of internal free volume pressure before welding. Use of this technique greatly reduced the failure rate of the closure welds, eliminating the tendency for touchouts and blowouts that adversely affected weld quality during Phases I and II.

Phase III

Phase III involved the pre-installation acceptance testing and performance demonstration of the OCW system. Test goals were to produce 25 acceptable production simulated closure welds and to demonstrate system capability to perform acceptable welds near parameter limits. Welds were made and screened as planned for production runs. Screening for acceptable welds included the use of visual evaluation, review of the data acquisition system print out, helium leak testing and digital radiography examination. Conventional (film) radiography and metallography were used for final acceptance of the test welds. The customer, as a part of the acceptance test program, dictated eight "stretch parameter" tests. The demonstration welds were to be made at high and low extremes of selected qualified welding parameters (cover gas flow rate, primary average current, arc gap and background average rotation). At these extremes, all other welding parameters were set at nominal values. A total of 40 welds (Table 2) were made for the demonstration, with seven screened out by digital radiography (DR). These seven welds included 3 that were identified as rejectable, and 4 that were conservatively set aside because they were close to acceptable limits. Even one weld failure to pass either film radiography or metallography would have been cause for having to rerun the 25 can qualification. Eleven tests were performed for demonstration of welds made at the eight parameter extremes. Of the eleven stretch parameter tests performed (Table 2), three were screened out, and one subsequently failed the (A+B)> 2t dimensional criterion and was not redone. That is to say, that digital radioagraphy screening did not provide data on the weld bead shape which would have allowed for us to screen out this one sample prior to performing the official metallographic examination. Overall, independent of screened out test cans, 36 of 40 test cans met all of the acceptance criteria for 90% acceptance or a 10% rejection rate. If one calculates rejection rates in light of screened out cans, one of 33 failed for 97% acceptance rate or a 3 % rejection rate.

Table 2 gives the results of the 40 tests. The acceptance test agreement with the SRS customer dictated that screening by visual evaluation, review of the data acquisition output, leak testing and digital radiography should be performed prior to submission of a welded joint for acceptance by film radiography and metallography.

Phase III also included the welding of several test welds with a special test configuration for weld procedure and weld operator qualification. The configuration included a standard test can and a specially fabricated lid with an extension. The test configuration does not allow for the use of the copper chill block. Evaluations for these qualifications included face and root bends, tensile tests, macro specimen examination and radiographic examination. Both procedure and operator performance qualifications were successfully performed.

The improved acceptable weld results are attributed to the helium partial backfill technique implemented with this system. Based upon the consistency of the data acquisition system voltage traces, the improved appearance of the weld surfaces and the absence of blowouts and touchouts, it is concluded that the adjustment made to the outer can free volume internal pressure is a step in the right direction towards improving the success rate for this closure weld.

The most common welding defects were electrode touchouts and weld blowouts. Adjusting the backfill pressure to provide a slightly positive pressure (vs. a relatively large positive pressure) in the can immediately prior to welding was successful in reducing the rejection rate. Some venting still occurred during welding.

The experience gained over the past three years with development and qualification of two GTA welding systems for Hanford and Savannah River Site has emphasized that consistently making acceptable closure welds on small volume containers without a designed vent path can be very difficult. Additional studies of the weld penetration and porosity are proposed to further decrease the weld rejection rate for closure welding the DOE 3013 containers.

We gratefully acknowledge the mechanical, electrical and manufacturing expertise brought to bear on the concepts and system designs for the OCW systems. The system designs came largely from the Equipment Engineering Department of the Savannah River Technology Center (SRTC) at SRS. James Tarpley, Brian Fiscus and Harold Peacock were instrumental in handling issues on a day-to-day basis. Without the assistance of many unmentioned support personnel within SRTC this work could not have been completed as scheduled. This work was performed under contract number DE-AC09-96SR18500 between Westinghouse Savannah River Company of Washington Group International and DOE.

1. DOE STANDARD 3013-2000 "Stabilization, Packaging, and Storage of Plutonium-Bearing Materials", December 2000.
   
2. G. R Cannell, W. Daugherty, L. Gaston, S. Howard. P. Korinko, D. Maxwell, G. McKinney, C. Sessions, & S. West, "GTAW Research and Development for Plutonium Containment", published in proceedings from 6^th International Conference on Trends in Welding Research, Callaway Gardens, Georgia, April 5-8, 2002, ASM International.
   
3. G. R. Cannell, W. L. Daugherty and M. W. Stokes, Welds Safeguard Plutonium-Bearing Containers", Welding Journal, Vol. 81, No. 7, July 2002.
   
4. W.L. Daugherty and G.R. Cannell, Analysis of Porosity Associated with Hanford 3013 Outer Container Welds.WSRC-MS-2002-00738, August 2002, submitted for publication in Practical Failure Analysis,
   
5. W.L. Daugherty and G.R Cannell, Mechanistic Modeling of Porosity in Hanford 3013 Outer Container Welds, WSRC-MS-2002-00693 Rev. 1, submitted for publication in Science and Technology of Welding and Joining
   
6. ANSI N14.5, Standard for Radioactive Materials Leakage Tests on Packages for Shipment, 1987.
   
7. ASME Boiler and Pressure Vessel Code Section VIII, Division 1 – Power Boilers, 2001
   
8. ASME Boiler and Pressure Vessel Code Section IX - Welder Qualification, 2001
   
9. P.S. Korinko and S.H. Malene, "Considerations for the Weldability of Types 304L and 316L Stainless Steels, Practical Failure Analysis, Vol. 1, Issue 4, Aug. 2001, pp. 61-68, ASM International.

Table 1- Welding Parameters

	1st. 135Deg.	Balance of Weld
Average Voltage	8.5-10.0 (avg.)	8.5-10.0 (avg.)
Average Primary Current	173-187 A	173-187 A
Primary Travel Speed	0.50-0.56 rpm	0.57-0.63 rpm
Preheat	4 sec @90A + 1 s @ 180A	N/A
Cover gas	Ar-2.6 to 2.9% H2	Ar-2.6 to 2.9% H2

Table 2. Results of Evaluation of 40 Cans Welded as Part of Acceptance Test Program.
(Except as Noted all Cans Passed all Acceptance Criteria.)

Can Number	Screened Out by DR	Pore Size (inch) Digital Radiography	Pore Size (inch) Film Radiography	Note
8 Can Stretch Run
1
2
3
4				(a)
5		0.019
6		0.015,0.019
7	X	0.034	0.035
7-1	X	0.019,0.034	0.025,0.035
7-2	X	0.038	0.038
7-3
8		0.019,0.023,0.023	0.020,0.030
25 Can Nominal Run
9		0.023	0.028
10
11		.
12
13	X	0.048	0.050
13-1		0.019
14		0.016
15
16
17		0.016
18
19	X	0.061	0.062
19-1	X	0.019,0.019,0.023	0.015,0.020,0.020
19-2
20
21
22
23
24
25
26
27
28
29	X	0.028	0.018,0.030
29-1
30		0.024
31		0.023	0.022
32		0.019
33		0.023	0.025

(a) Weld failed the aspect ratio test (A+B) >2t

Figure 1. 3013 Outer Container and Lid. An Interference Fit and
Square-Groove Weld Joint is Created when Lid is Inserted into the Outer Can.

Figure 2. Schematic of Weld Geometry Determined on
Metallographic Cross-Sections of Outer Can Welds. Acceptance Criteria
Require that the Penetration (A) be as Deep as the Can Wall Thickness (t) or
Greater, and that the Sum of Dimensions A and B be 2t or Greater.

Figure 3. Internal Outer Can Pressure Versus Time Plot for Three Cycle Evacuation and
Backfill with 1 Atmosphere of Helium (Part 1 of Overall Trace for 1 Atm. Backfill).

Figure 4. Internal Outer Can Pressure Versus Time Plot for Three Cycle Evacuation and
Backfill with 1 Atmosphere of Helium (Part 2 of Overall Trace for 1 Atm. Backfill).

Figure 5. Internal Outer Can Pressure Versus Time Plot for Three Cycle Evacuation and
Backfill with Less that 1 Atm. of Helium (Part 1 of Overall Trace for Partial Backfill).

Figure 6. Internal Outer Can Pressure Versus Time Plot for Three Cycle Evacuation and
Backfill with Less that 1 Atm. of Helium (Part 2 of Overall Trace for Partial Backfill).