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Experience with Duplex Stainless Steel Kraft Digesters

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NACE Corrosion 2004 Paper 04249, New Orleans, LA, March, 2004

1

EXPERIENCE WITH DUPLEX STAINLESS STEEL KRAFT DIGESTERS

Angela Wensley Pulp & Paper Corrosion Specialist Angela Wensley Engineering Inc.

15397 Columbia Avenue White Rock, BC, Canada V4B 1K19

ABSTRACT Duplex stainless steel has become the preferred material of construction for new kraft digesters, both for

reasons of excellent corrosion resistance and cost effectiveness compared with other materials of construction. Most duplex stainless steel digesters have been constructed using UNS S32205 (type 2205) stainless steel due to the greater availability of this material compared with other duplex stainless steel alloys. UNS S32304 (type 2304) duplex stainless steel has slightly superior corrosion resistance and is also a candidate material of construction for new kraft digesters. This paper describes experiences with duplex stainless steel digesters including replacement of a continuous digester top, replacement of an impregnation vessel, shop construction of batch digesters, and field replacement of a batch digester cylindrical shell.

Keywords: alkaline pulping liquors, corrosion testing, digesters, quality assurance, welding, white liquor.

INTRODUCTION

The superior corrosion resistance of duplex stainless steels in kraft digester liquors compared with other

materials of construction (carbon steel, austenitic stainless steels, nickel-base alloys) has been well-established for over two decades1-11. Despite this superiority, the North American pulp and paper industry has only begun using duplex stainless steels for new and replacement digesters within the past decade. There were many reasons for the slowness in the adoption of duplex stainless steel digesters including long delivery times for duplex stainless steel plates and the inexperience of fabrication shops in welding duplex stainless steels. The stainless steel suppliers have worked to shorten delivery times to acceptable values, and there have been great advances in the quality assurance procedures for duplex stainless steel welds12-17.

There has also been a formidable inertia slowing change from the old practice of constructing batch

digesters using carbon steel and then overlaying them using stainless steel once they had experienced corrosion thinning approaching their minimum allowable wall thickness. It was understood over 50 years ago that low-silicon content carbon steels had better corrosion resistance to kraft pulping liquors than did higher silicon-content carbon steels18. For many years type SA285-Grade C "MOD" carbon steel (UNS K02801) was the standard material of construction for batch digesters in North America. The "MOD" referred to the steel having a composition modified for digester service by having a silicon content below 0.02%. When SA516-Grade C carbon steel (UNS K02700) came into widespread use for the construction of pressure vessels (including

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digesters) in the 1970's, the use of the lower-strength low silicon steels for digester construction was abandoned. The medium silicon content (typically 0.25% Si) of SA516-Grade 70 carbon steel gives it poorer corrosion resistance and consequently a shorter service life before it was necessary to apply weld overlay. In turn, type 309 stainless steel weld overlays experienced rapid corrosion in batch digesters19, often necessitating re-overlaying. In recent years type 312 duplex stainless steel weld overlays have been applied in batch digesters and show improved corrosion resistance19. Digesters constructed using solid or clad duplex stainless steels require little or no maintenance compared with carbon steel or overlaid digesters. The required wall thickness to contain the design pressure is thinner for duplex stainless steel than for carbon steel. Further, smaller corrosion allowances (e.g., 1.4 inch or 6 mm) can be used for duplex stainless steel digesters than the thicker corrosion allowances (e.g., 1 inch or 25 mm) typical for carbon steel digesters. Since new duplex stainless steel digesters are thinner than new carbon steel digesters they can be cost-effective compared with new carbon steel digesters. The cost of a new duplex stainless steel batch digester over a projected service interval of 20 years is far less than that of a carbon steel digester over the same period when the costs of overlaying and re-overlaying are considered20.

This paper describes experience with digesters constructed using type 2205 duplex stainless steel (UNS

S32205). Although UNS S31803 also covers type 2205 duplex stainless steel, UNS S32205 is preferred for new duplex stainless steel digesters due to its having a higher minimum content of nitrogen (0.14% versus 0.08% for S31803). A nitrogen content of at least 0.14% retards the precipitation of detrimental chromium nitride intermetallic phase in the ferritic part of the duplex stainless steel microstructure. Other grades of duplex stainless steel such as type 2304 (UNS S32304) may also be suitable for digester construction.

Corrosion Testing

Laboratory corrosion testing in autoclaves containing batch and continuous digester liquors has repeatedly

demonstrated the superior corrosion resistance of duplex stainless steels compared with carbon steels and austenitic stainless steels6,8,11. Tables 1 and 2 summarize the maximum corrosion rates measured for different materials from several laboratory corrosion tests in batch and continuous digester liquors, respectively. The corrosion rates are those corresponding to the open-circuit corrosion potentials. Initially, batch digester liquors are mixtures of white and black liquors but during the course of a batch cook (e.g., 2 hours) the liquors become spent and ultimately resemble weak black liquor in composition. Continuous digester liquors vary in composition with elevation in the digester but are relatively constant in concentration for each elevation or zone. Some continuous digester environments (for example: extraction and wash zone liquors) can be much more aggressive to carbon steels than the most aggressive batch digester environment. Austenitic stainless steels and nickel-base alloys, on the other hand, have good resistance to corrosion in all continuous digester liquors but can experience high corrosion rates in many batch digester liquors.

The resistance to corrosion of stainless steels and nickel-base alloys in batch digester pulping liquors is

directly related to the chromium content8. Stainless steels containing at least 25% chromium such as type 2507 duplex stainless steel (UNS S32507) and type 312 duplex stainless steel weld overlay have the highest corrosion resistance. Although type 2304 duplex stainless steel was not tested in batch digester liquors, its corrosion rate is expected to be intermediate between types 2205 and 2507 on the basis of its intermediate chromium content. The corrosion rate of type 2205 duplex stainless steel in batch digester liquors is not negligible as is the case in continuous digester liquors. It is prudent to have a corrosion allowance of at least 1/8 inch (3 mm) or even ¼ inch (6 mm) for duplex stainless steel batch digesters. A smaller corrosion allowance of 1/16 inch (1.5 mm) or 1/8 inch (3 mm) is sufficient for continuous digesters and impregnation vessels.

Mixtures of white and black liquors can be more corrosive than pure white liquor at the same temperature,

suggesting that organic compounds play a role in the corrosion process. It is believed, however, that mixed liquors with a higher volume fraction of white liquor are more aggressive than those with a lower volume fraction of white liquor. Additional corrosion testing is required to establish the relationship between the volume ratio of white and black liquors and the corrosion rates of materials of construction for kraft batch digesters.

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Quality Assurance and Testing Welded duplex stainless steel pressure vessels need testing that is supplemental to the impact

requirements required by ASME Boiler and Pressure Vessel Code21. The metallographic, impact, and corrosion test requirements of ASTM standard test A92322 are particularly useful for detecting the presence of detrimental intermetallic phases in duplex stainless steel welds. Even though ASTM A923 was strictly developed for testing of annealed duplex stainless steel product such as plates, its application to welds is considered to be conservative. ASME UHA-51 requires impact testing of the plate, welds, and heat affected zones at a temperature no higher than the minimum design metal temperature, while ASTM A923 requites impact testing at -40ºC. Moskal and Davison also recommend that both ASME and ASTM impact testing be done at -40ºC to minimize the number of tests13,15,16. Welds that pass the less-stringent ASME UHA-51 impact requirement may fail nonetheless fail some of the requirements of ASTM A923. Fortunately, such situations are rare. If a production weld does not meet both the impact and corrosion test requirements of ASTM A923, there should be serious consideration about removing and replacing the weld. Table 3 gives some typical results from testing of run-out tabs and production plates during duplex digester projects.

A ferrite gauge is a particularly useful tool in quality assurance of duplex stainless steel welds and plate.

Although such gauges may not give completely accurate measurements of the ferrite content of welds and plates, they are able to detect problems quickly. These include detection of low ferrite content welds inadvertently made using austenitic stainless steel filler metals that were erroneously labeled, the detection of low-ferrite content plate that had been "annealed" at too low a temperature, and the detection of welds with high or low ferrite contents due to their having been welded outside the specifications of the welding procedure.

Inspection of duplex stainless steel plates at the rolling mill can avoid delivery of sub-standard product.

Duplex stainless steel plate can also have cosmetic problems due to imprinting of oxide scale or shot blast media while the plate was still hot after annealing. Rolled-in shot can have the same appearance as pitting corrosion. If the locations of rolled-in shot imprints are not documented at the time of construction they may be mistaken for in-service pitting corrosion during some future inspection of the vessel.

Additional quality assurance should also be done on cold formed heads and cones, since these can

experience cracking as a result of the forming operations. In addition to penetrant testing (PT) of the weld seams, complete PT of both inside and outside surfaces of cold formed heads and cones is recommended. ASTM A923 testing may also be useful for hot formed heads. If hot formed duplex stainless steel heads are not heated within the annealing range for a sufficient period of time they may contain significant amounts of intermetallic phases. The temperature of the furnace is not necessarily the temperature of the head. To know the temperature of the head it is better to attach a thermocouple directly to the head and cover it with some insulation23.

For duplex stainless steel batch digesters it is recommended that baseline ultrasonic testing (UT) be done

to measure the thickness of the wall (including the head, cylinder, cone, and bottom nozzle). Repeat measurements can be made using the same grid after the digester is in service to monitor any corrosion thinning that may be occurring. It is particularly important that UT measurements be made using a duplex stainless steel calibration block since use of a carbon steel calibration block will result in erroneous thickness measurements.

NEW DUPLEX STAINLESS STEEL DIGESTERS

The author has been involved with numerous duplex stainless steel digester construction projects, most

often with quality assurance both in the shop and during field erection. Discussed below are the highlights of these projects.

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Replacement Top for a Continuous Digester Severe stress corrosion cracking (SCC) in the top of a carbon steel continuous digester with twin top

separators necessitated replacing the top of the digester in 1997. Type 2205 duplex stainless steel was selected for the replacement top due to its high resistance to SCC in white liquor4 and in continuous digester liquors (unpublished corrosion testing). The carbon steel top had also experienced severe erosion corrosion due to high velocity liquor flows, and type 2205 duplex stainless steel is effectively immune to erosion corrosion in continuous digester service. The new top was 18 feet (5.5 m) in diameter and was fabricated in the shop and then transported to the site where the old carbon steel top was cut off and the new duplex stainless top was lifted into place during a scheduled shutdown. No attempt was made to take advantage of the superior strength of the duplex stainless steel and the wall thicknesses were the same as for the old carbon steel top, which was 1.5 inches (38 mm) at the weld joint attaching the new duplex top to the old carbon steel shell. Figure 1 is a view the new duplex stainless steel top just prior to installation.

Most of the duplex-to-duplex welding was done using the submerged arc welding (SAW) process. The

SAW procedure was qualified to the more stringent requirements of ASTM A923 and passed all tests. The new digester top was welded to the old SA516-Grade 70 carbon steel digester using a semi-automatic flux cored arc welding (FCAW) process with E2209 filler metal. There was difficulty in qualifying the FCAW procedure. It failed the corrosion test in ASTM A923, possibly due to the presence of tiny inclusions of flux in the weld cross section. Since the weld coupon had good metallographic appearance and also had impact properties acceptable to ASTM A923, it was decided to proceed with welding using the FCAW process. The duplex stainless steel joint had a 45º bevel from one side, while the carbon steel side of the joint was flat. The carbon steel side of the joint was first buttered using type E2209 filler metal before the new duplex top was lowered into position for completion of the weld joint. The duplex-to-carbon steel joint was located in the impregnation zone below the top separators, where the heat affected zone in the carbon steel would still be susceptible to SCC. A 1-foot (30 cm) wide band of Alloy 625 thermal spray was applied using the high velocity oxygen fuel process to protect this joint from SCC.

Inspection after 1 year revealed extensive "pitting" of the duplex stainless steel (Figure 2) that was

subsequently found to be an artifact of plate manufacture (shot blast imprinted by plate rolling after annealing). The "pitting" has been monitored in every subsequent internal inspection of the digester and has not changed in appearance. No corrosion or cracking whatsoever has been found in the duplex stainless steel top in the 6 years following its installation. The visual appearance remains as new.

Batch Digesters

Considerable quality assurance experience was gained during the shop construction of seven batch

digesters built for kraft mills in Canada and the US. These were all replacements for corroded carbon steel batch digesters. Three of the batch digesters had been repeatedly built up using carbon steel weld metal and four had been repeatedly weld overlaid using type 309 stainless steel. In all cases, type UNS S32205 duplex stainless steel was selected for the replacement vessels. The diameters ranged from 10.5 feet (3.2 m) and 13 feet (4.0 m). The plate thicknesses ranged from 7/8 inch (22 mm) to 1-¼ inch (32 mm). Three of the digesters (Figure 3) had internal screens and four of the digesters (Figure 4) had direct steaming through four nozzles in the bottom cone. SAW was the predominant welding procedure used for the duplex stainless steels due to its high production rate and very good track record for success, with few rejectable defects found in radiographic testing (RT). Some of the digesters were constructed using gas tungsten arc welding (GTAW) for the root passes. Some complete weld seams were also made using GTAW, FCAW, or gas metal arc welding (GMAW) processes.

The greatest problems with the batch digesters were encountered with the welded and cold formed heads.

The 2:1 ellipsoidal heads were made from two flat plates welded together using the SAW process (Figure 5). The weld seam was ground flat prior to the dishing and cold flanging operations that comprised the cold forming process. Figure 6 shows a completed head with PT of the weld seam. After delivery to the shop, some of the

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heads were found to have cracks in the base metal up to 8 feet long. The cracks were parallel to the weld seam and in some cases there were two parallel cracks equidistant from the weld seam on the inside surface. Figures 7 and 8 show cracking on the inside surface of the head that was ground out at a depth of 0.06 inch (1.5 mm). Figures 9 and 10 show cracking on the outside surface of a head that was ground out at a maximum depth of 0.14 inch (3.6 mm). This crack was removed by grinding and repaired using the GTAW process.

It was evident that the cracks originated at a linear scratch in the original plate. When the plate was cut

and welded together (Figure there were now effectively two scratches parallel to the weld seam. If the scratch was on the inside of the plate, the cold forming operation produced the shallow "braided cracking" observed in Figure 8. If the scratch was on the outside of the plate being formed it developed into the deep jagged cracking observed in Figure 10. The source of the scratches is unknown as both the head forming shop and the plate manufacturer declined to accept responsibility. This emphasizes the need for diligent third-party quality assurance including visual inspections of both sides of the plate before a head is cold formed. Complete PT of the inside and outside surfaces of cold formed heads is also strongly recommended.

Shallow linear cracking presumably resulting from original plate scratches has also been observed on the

outside surfaces of cold formed cones (Figure 11). This finding resulted in the performing of complete PT on the outside of one cone. This revealed a fine network of crack-like indications located away from a weld seam (Figure 12). These cracks were very shallow and easily removed. Their origin is unknown although they were most likely an artifact of the cold forming process.

An intermittent 14.5-foot (4.4 m) crack was also found on the outside of the SAW weld for a cold-formed

head (Figure 13). The crack was removed by arc gouging and repaired using the shielded metal arc welding (SMAW) process. Subsequent RT revealed cracking inside the weld necessitating re-gouging and re-repairing. Figure 14 shows the internal cracks revealed by PT after excavation. To simulate the double repair, one of the SAW run-out tabs from this head was treated in the same manner as if it had experienced cracking half-way through the weld on two different occasions, and two gouging excavations and SMAW repairs were made. The re-repaired weld passed all ASME and ASTM A923 tests. The cause of the cracking remains unknown. It is believed that the problem was due to the flux used during the SAW. The welding procedure specification had been qualified years before using different plate, welding wire, and flux than were used for the head. For future heads it was requested that the welding procedure be re-qualified using the same plate and welding consumables.

Two of the batch digesters were inspected after 1 and 2 years of service. Visually, there was some

staining of the duplex stainless steel. No evident corrosion attack was observed. Reference photographs of cosmetic defects in the plate ("pitting" caused by imprinting of shot blast media) showed no change in appearance. PT of the weld seams revealed no cracking. UT using the same grid as the baseline measurements revealed no discernable thinning, even in the 8-inch (20 cm) diameter bottom outlet nozzle.

Impregnation Vessel

A carbon steel impregnation vessel (IV) for a two-vessel kraft digester system was so extensively cracked

that it was decided to replace the entire IV using UNS S32205 duplex stainless steel. As if to underscore the urgency of the replacement, the IV experienced two through-wall leaks due to SCC in the year prior to its replacement. The new IV was 71.75 feet (21.9 m) tall and up to 12.5 feet (3.8 m) in diameter. It was built in two pieces in the shop (Figures 15 and 16). The SAW process was used for the pressure-retaining welds in the pressure shell of the new IV. The thinner shell of the duplex stainless steel IV (due to the higher strength of the duplex stainless steel compared with that of the SA516-Grade 70 vessel that it replaced) enabled the new IV to be installed using two lifts into the same location as the old IV. Figure 17 shows the bottom section being lowered into place.

The closing seam for the IV was made using the SMAW process and E2209 electrodes. The weld joint

was a double bevel. The weld was first welded from the outside followed by back-gouging, grinding, PT of the

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root, and completion of the weld from the inside. Significant lack of fusion (LOF) was encountered on PT of the root pass (Figure 18). The LOF was removed by grinding before proceeding with completion of the weld from the inside. PT of the completed weld cap revealed several minor defects that were easily removed by grinding. The ferrite content of the completed weld caps measured using a ferrite gauge was lower than expected (30%) for a duplex stainless steel weld. The low ferrite content was attributed to the weld being composed of numerous SMAW passes with the result that the cap had a composition more resembling that of undiluted E2209 filler metal instead of the type 2205 digester plate. No defects were found on the final RT. A production plate was used during the welding to simulate the actual welding process. As the welders in turn completed a pass they made a pass on the production plate. The production plate was impact tested at -40ºC and passed both ASME UHA-51 and ASTM A923 requirements.

Internal inspection by PT after 1 year revealed some fine pit-like indications on the caps of the weld

seams inside the IV (Figure 19). These were removed by light grinding. Plate clamp marks were also revealed as PT indications (Figure 20). These clamp marks were also removed by grinding. No cracks or corrosion of the duplex stainless steel were observed.

Partial Shell Replacement of a Batch Digester

A carbon steel batch digester had experienced severe corrosion thinning of much of the height of the

cylindrical section but the top head and top part of the cylinder were still in good condition, as was the bottom part of the cylinder and the bottom cone. It was decided to make a partial shell replacement using UNS S32205 duplex stainless steel. The cylinder was made in the shop and shipped to the mill site in two sections (Figure 21), necessary due to space limitations in the digester building. Short 6-inch (15 cm) stub rings of SA516-Grade 70 carbon steel were shop welded to the top and bottom of the duplex cylinder so that the field welds could be easily made by an inexperienced (with duplex) contractor. These carbon steel stub rings were in a location where the corrosion rate of the old carbon steel digester was very low. The duplex-to-carbon steel shop welds were made using the SAW process with E2209 wire. The carbon steel rings and the carbon steel-to-duplex circumferential welds were overlaid in the shop using the GMAW process with type ER309L stainless steel. Ferrite measurements revealed low ferrite content (as low as 5%) in part of one of these welds. This appeared to be a consequence of the dilution of the duplex stainless steel weld with the carbon steel. A cross section through the sample used to qualify the weld showed normal ferrite levels (Figure 22).

There were three field welds: two circumferential welds between the new section and the existing carbon

steel digester shell and a closing circumferential seam in the new duplex section. The carbon steel-to-carbon steel field welds were made using the SMAW process with E7018 carbon steel electrodes.

The field weld seam in the middle of the duplex replacement section was made using the GTAW process

with ER2209 filler metal. The duplex-to-duplex joint had a double bevel on the top section while the bottom section was flat. Welding was done first on the inside and completed from the outside. Figure 23 shows some the argon gas purge on the outside while the weld was being made on the inside of the digester. PT of the root of the completed inside weld showed significant LOF (Figure 24). A production plate was used to obtain a sample of the closure weld for testing (Figure 25). The final visual appearance of the weld was excellent and RT revealed no defects. Subsequent testing of the production plate revealed the weld met all impact requirements of both ASME UHA-51 and ASTM A923, but failed the ASTM A923 corrosion test. Figure 26 shows a cross section of the test specimen after corrosion testing. Since the results of the impact testing were excellent, suggesting that the presence of detrimental intermetallic phases was not a problem, the failure in the corrosion test was attributed to the presence of microscopic amounts of porosity in the weld cap, and the weld was accepted.

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DISCUSSION

Using duplex stainless steel for replacement of carbon steel digesters puts an end to the problems of SCC or rapid corrosion thinning that have plagued these vessels. There have been no reports of SCC, erosion corrosion, or appreciable corrosion thinning in any of the duplex stainless steel digester vessels described above. At this time, however, it is necessary to maintain quality assurance at a level of diligence that is greater than that typically performed during construction of carbon steel pressure vessels. The problem of cracking of cold formed heads is a persistent one and suggests that heads of this size are near the technical limits of cold forming. Hot formed heads and cones may be preferable, but diligent quality assurance is still necessary. Testing of the head (and welds) should be done in the hot formed condition. The reward is vessels that meet the most stringent quality requirements and as a result are essentially maintenance-free.

CONCLUSIONS

1. Duplex stainless steels have excellent resistance to corrosion in both batch and continuous digester liquors. 2. Laboratory corrosion testing suggests that a corrosion allowance of at least 1/8 inch (3 mm) should be

specified for duplex stainless steel batch digesters. 3. Third-party quality assurance is recommended during construction of duplex stainless steel batch digesters. 4. Cold formed heads are problematic for large duplex stainless steel pressure vessels. 5. SAW remains the most effective welding process for duplex pressure vessels, although welds can be made

successfully using other welding processes.

REFERENCES 1. J-P Audouard, A. Desestret, G. Vallier, et al, Study and Development of Special Austenitic-Ferritic Stainless

Steel Linings for Kraft Pulp Batch Digesters, Proc. 3rd Intl. Symp. Corr. in the Pulp and Paper Ind., p.30-39 (1980).

2. J-P Audouard, A New Special Austenitic-Ferritic Stainless Steel for Kraft Pulp Batch Digesters, Proc. 4th Intl. Symp. Corr. in the Pulp and Paper Ind., p.43-47 (1983).

3. J-P Audouard, Duplex Stainless Steels for Kraft Pulp Batch Digesters - Welding and Corrosion Performance, Proc. 5th Intl. Symp. on Corrosion in the Pulp and Paper Ind., p.193-199 (1986).

4. J-P Audouard, F. Dupoiron, D. Jobard, Stainless Steels for Kraft Digesters in New Pollution Free Mills, 14 pp. supplement to Proc. 6th Intl. Symp. Corr. in the Pulp and Paper Ind., (1989).

5. J-P Audouard, J. Charles, F. Dupoiron, High Chromium Duplex Stainless Steels Attractive Solution for Kraft Process Conditions, Stainless Steel Europe, Vol. 4(16), p.40-45 (1992).

6. A. Wensley, M. Moskal, W. Wilton, Materials Selection for Kraft Batch Digesters, Paper No. 378 presented at the NACE Corrosion 97 Conf., NACE, Houston, TX (1997).

7. J-P Audouard, Corrosion Performance of Duplex Stainless Steels for Kraft Digester Applications, Proc. 5th World Congress and Exposition on Duplex Stainless Steels (1997).

8. A. Wensley, Corrosion of Batch and Continuous Digesters, Proc. 9th Intl. Symp. Corr. in the Pulp and Paper Ind., p.27-37 (1998).

9. J. Olsson, B. Leffler, C. Jorgensen, Experiences of Duplex Stainless Steel in the Pulp and Paper Industry, Proc. 9th Intl. Symp. Corr. in the Pulp and Paper Ind., p.161-164 (1998).

10. E. Alfonsson, J. Olsson, Duplex Stainless Steel for the Pulp and Paper Industry, Paper No. 278 presented at the NACE Corrosion 99 Conf., NACE, Houston, TX (1999).

11. A. Wensley, Corrosion of Flash Tanks, Proc. 10th Intl. Symp. Corr. in the Pulp and Paper Ind., p.379-398 (2001).

12. P.H. Thorpe, Duplex Stainless Steel Pulp Digesters – Fabrication and User Experience in Australia and New Zealand, Proc. 8th Intl. Symp. Corrosion in the Pulp and Paper Ind., p.20-25 (1995).

13. M.D. Moskal, G. Cheetham, J. Paultre, W. Wilton, Quality Requirements for Duplex Stainless Steel Digester Fabrication, Proc. 9th Intl. Symp. Corr. in the Pulp and Paper Ind., p.67-73 (1998).

14. Welding of Duplex Stainless Steels, Technical Information Paper TIP 0402-23, TAPPI, Atlanta, GA (1998).

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15. R.M. Davison, M.D. Moskal, Qualification of Welding Procedures for Duplex Stainless Steels, Proc. TAPPI Eng. Conf., p.379-397 (1999).

16. Qualification of Welding Procedures for Duplex Stainless Steels, Technical Information Paper TIP 0402-29, TAPPI, Atlanta, GA (2001).

17. A. Wensley, Developments in the Quality Assurance of Duplex Stainless Steel Pressure Vessels for the Pulp and Paper Industry, Stainless Steel World America Conference Paper No. P0206, p.34-41 (2002).

18. C-G von Essen, Corrosion Problems in Sulphate Pulp Mills, TAPPI J. Vol. 33 No. 7 p.14A-32A (1950). 19. A. Wensley, Corrosion of Weld Overlay in Batch Digesters, Proc. TAPPI Eng. Conf., p.1193-1200 (1998). 20. Minutes of the TAPPI Digester Corrosion Information Meeting, Atlanta, GA, March 6, 2003. 21. Rules for Construction of Pressure Vessels, ASME Boiler and Pressure Vessel Code, Section VIII, Div. 1,

section UHA-51, American Society of Mechanical Engineers, New York, NY (latest edition). 22. American Society for Testing and Materials (ASTM) A923-94, Standard Test Methods for Detecting

Detrimental Intermetallic Phase in Wrought Duplex Austenitic/Ferritic Stainless Steels, Annual Book of ASTM Standards, Vol. 01.03, ASTM, West Conshohocken, PA.

23. R. Cordewener, M. Bors, Guideline for Manufacturing Duplex 2205 and Duplex 2507 Vessel Heads, Stainless Steel World America Conference Paper No. P0237, p.209-212 (2002).

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TABLE 1. MAXIMUM CORROSION RATES MEASURED AT THE OPEN-CIRCUIT CORROSION POTENTIAL IN

LABORATORY TESTING IN BATCH DIGESTER LIQUORS

Material

Liquor*

Temperature (ºC)

Corrosion Rate (mpy)

Corrosion Rate (mm/y)

Type 312 overlay 1.8 parts WL + 1 part BL 173 0.3 0.01 Type 2507 duplex 1.8 parts WL + 1 part BL 173 1.6 0.04 Type 2205 duplex 2 parts WL + 1 part BL 176 6.2 0.16 Alloy 82 overlay 3.5 parts WL + 1 part BL 180 9.4 0.24 Type 309 overlay 1.8 parts WL + 1 part BL 173 14.7 0.37 Alloy 625 overlay 1 part WL + 1 part BL 177 16.0 0.41 Type 316L austenitic 3.5 parts WL + 1 part BL 177 16.4 0.42 Type 304L austenitic 3 parts WL + 1 part BL 170 17.0 0.43 SA285-Grade C steel 3.3 parts WL + 1 part BL 170 66.0 1.68 SA516-Grade 70 steel 3 parts WL + 1 part BL 170 95.4 2.42 * Parts by volume of white liquor (WL) and black liquor (BL).

TABLE 2.

MAXIMUM CORROSION RATES MEASURED AT THE OPEN-CIRCUIT CORROSION POTENTIAL IN LABORATORY TESTING IN CONTINUOUS DIGESTER LIQUORS

Material

Liquor Temperature

(ºC) Corrosion Rate

(mpy) Corrosion Rate

(mm/y) Type 2304 duplex Softwood extraction liquor 175 0.01 0.0002 Type 312 overlay Softwood extraction liquor 160 0.04 0.001 Type 309 overlay Softwood extraction liquor 160 0.04 0.001 Alloy 625 overlay Softwood extraction liquor 160 0.08 0.002 Alloy 82 overlay Softwood extraction liquor 160 0.10 0.003 Type 2205 duplex Softwood extraction liquor 175 0.12 0.003 Type 304L austenitic Softwood extraction liquor 175 0.48 0.01 SA516-Grade 70 steel Softwood extraction liquor 175 402 10.2 SA285-Grade C steel Softwood extraction liquor 175 476 12.1

TABLE 3.

QUALITY ASSURANCE TEST RESULTS FOR DUPLEX STAINLESS STEEL DIGESTER WELDS

Welding

ASME UHA-51 Lateral Expansion(1)

ASTM A923 Impact Energy(2)

ASTM A923 Corrosion Test(3)

Average Ferrite(4)

Process mils mm ft-lb joules mdd % SAW 18 – 65 0.5 – 1.7 33 – 68 45 – 92 1.2 – 5.4 42

GMAW 47 – 90 1.2 –2.3 112 – 144 152 – 195 0.0 41 GTAW 39 – 87 1.0 – 2.2 92 – 144 125 – 195 1.5 – 31 48 FCAW 12 – 58 0.3 – 1.5 18 – 82 24 – 111 1.3 – 189 41 SMAW 24 – 30 0.6 – 0.8 37 – 53 50 – 72 3.7 30 (1) Acceptance criterion is 15 mils (0.38 mm) minimum. (2) Acceptance criterion is 25 ft-lb (34 J) minimum. (3) Acceptance criterion is 10 mdd (milligrams per square decimeter per day) maximum. (4) The ferrite content of duplex stainless steel welds is typically between 30% and 60%.

F

F

IGURE 1. Duplex stainless steel top for a continuous digester.

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IGURE 2. Imprinted shot blast media giving the appearance of pitting corrosion.

FIGURE 3. Duplex stainless steel batch digester.

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FIGURE 4. Duplex stainless steel batch digester.

FIGURE 5. Duplex stainless steel plates welded together using SAW, prior to head

forming.

F

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IGURE 6. Cold formed duplex stainless steel semi-elliptical head.

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FIGURE 7. PT revealing a crack on the inside surface of a cold formed head.

FIGURE 8. Closer view of the crack in Figure 7 showing a "braided" appearance.

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IGURE 9. Cracking on the outside of a cold formed duplex stainless steel head.

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IGURE 10. Closer view of the crack in Figure 9 (partly ground out).

FIGURE 11. PT revealing a crack on the outside of a cold formed cone.

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FIGURE 12. Crack-like indications on the outside of a cold formed cone.

FIGURE 13. Part of a long intermittent crack in the weld of a cold formed head.

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IGURE 14. PT revealing cracks in an excavated weld in a cold formed head.

FIGURE 15. Bottom half of a duplex stainless steel impregnation vessel.

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IGURE 16. Top half of a duplex stainless steel impregnation vessel.

FIGURE 17. Duplex stainless steel impregnation vessel being lowered into place.

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IGURE 18. PT revealing LOF in the root of a SMAW weld made using E2209.

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FIGURE 19. Fine pit-like indications found during PT of a duplex stainless steel

impregnation vessel after 1 year of service.

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IGURE 20. Clamp marks in the shell of a duplex stainless steel impregnation vessel.

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FIGURE 21. Two sections of duplex stainless steel being prepared for a partial shell

replacement in a batch digester.

FIGURE 22. Cross section of a duplex-to-carbon steel submerged arc weld showing

ferrite contents of the individual weld passes.

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FIGURE 23. Argon back purging for field welding of a duplex stainless steel batch

digester using the GTAW process.

FIGURE 24. LOF found during PT of the root of a GTAW weld made using ER2209.

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FIGURE 25. Production plate used during field welding of a duplex stainless steel

batch digester.

FIGURE 26. Cross section of the sample from the production plate in Figure 25.

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