Duplex stainless steel digesters: A better choice?, Solutions!, Online Exclusives, April 2003

Duplex stainless steel digesters: A better choice?

By Max D. Moskal, M&M Engineering

After six years of operation in North America, duplex stainless steel digesters have proven to be more cost effective than digesters constructed from carbon steel, or those protected from corrosion using stainless steel weld overlay. This paper describes engineering considerations in design of DSS digesters, along with factors to consider in base material selection, welding characteristics and quality management.

For more than two decades, duplex stainless steel (DSS) alloys have been used for paper industry digesters in Europe, Australia and New Zealand [1-4]. The earliest digesters were constructed from DSS clad carbon steel, but later digesters were fabricated from solid DSS plate. Since 1997, several pulp and paper producers in North America have installed solid DSS digesters[5-6]. Their experiences have been excellent, and today, all users in North America should consider to construct digesters from DSS as an alternative to carbon steel.

Although the unit material cost for DSS is higher than for carbon steel, that higher cost is partially offset by the lower fabrication costs. The combined higher strength and superior corrosion resistance of the DSS allows for thinner plate to be used for the same digester pressure rating. Thinner plate means less welding time, along with lower shipping and handling costs[7].

The most significant advantage of DSS for digesters is its high corrosion resistance with resulting low in-service maintenance. Users report that after 3 to 5 years of operation, DSS digesters show little or no deterioration, and the cost and time for inspection is minimal. Life-cycle cost analyses indicate that, over a period of twenty years, mills may incur up to twice the cost for carbon steel digesters compared to DSS construction [8]. Further, the total cost for maintenance and repair for carbon steel digesters, including stainless steel weld overlay, may exceed the DSS cost after only 8 to 10 years.

It is significant to note that the practices used for plate production and weld fabrication of austenitic stainless steel (Grades 304L, 316L) are inadequate for DSS alloys. This is because the physical and mechanical characteristics of austenitic and DSS alloys are so different. Thermal expansion and heat conductivity properties of DSS, which are important to consider during welding, are intermediate between austenitic and carbon steels. Further, DSS alloys are more sensitive to detrimental reactions (precipitation of intermetallic compounds) at elevated temperatures encountered when producing the product form at the steel mill, or welding during fabrication. These reactions affect both the toughness and corrosion resistance of DSS alloys. DSS alloys are generally not prone to hot cracking during welding; but susceptibility to intermetallic precipitation, as noted above, limits the time permitted in the “red” temperature zone during welding. These factors all need to be taken into account when specifying the grade of DSS used, design criteria, fabrication techniques and quality tests for the digester.

Today, most fabricators in North America have experience with welding austenitic stainless steel grades, and increasing numbers are gaining welding experience with DSS. To avoid fabrication and in-service problems with DSS digesters, the purchaser should use experienced fabricators for construction work. Yet, because DSS is readily welded, fabricators without prior experience with these materials can obtain excellent results, provided that they follow certain guidelines. TAPPI TIP 0402-29, Qualification of Welding Procedures for Duplex Stainless Steels [9], prepared by the Corrosion & Materials Engineering Committee, provides general guidelines for welding DSS.

With this introduction in mind, it is possible to give some general guidelines for selection and welding of DSS, and then to apply this background to the fabrication of batch or continuous digesters.

Selection of mill product material
In North America, DSS alloys may be specified using the common name, the Unified Numbering System (UNS), or a manufacturer’s trade name. The UNS numbering system is specified when dealing with ASME Code designations, and is the preferred material designation. Nevertheless, the common name and manufacturer’s trade names also prevail. Both the common name for digester DSS alloys and the UNS system are used in this paper.

Table 1 shows the chemical compositions of the alloy grades referenced in this paper. A wide variety of DSS alloys are available, but UNS 31803, commonly referred to as “2205,” has become the current “standard” for digesters. Type 2205, containing 22% chromium, is used primarily because of its relatively low cost and good availability. A variation of this alloy is UNS 32205 (also referred to as “2205”), which has minor, within-grade element adjustments of chromium, molybdenum and nitrogen to enhance weldability. For digester construction, it is preferred that DSS products be dual-certified to both UNS 31803 and UNS 32205. Another DSS grade, UNS 32304 (common name “2304”), may in the future provide better cost efficiency in digester construction, but is currently not as readily available as the 2205 grades.

TABLE 1: Typical chemical composition of stainless steels referred to in text. Weight percent.

Attention to quality in production of DSS at the steel mill must be more exacting than for austenitic grades of stainless steel. For example, it is essential that the above-mentioned alloy balance be achieved. A second factor is heat treatment at the steel mill. Time in the red temperature range of 705 to 980°C (1300 to 1800°F) influences the material quality. Only a few minutes’ cumulative-time exposure to these temperatures results in detrimental intermetallic precipitation within the metal. Air cooling of the plate or forging, even when done rapidly, through the 705 to 980°C range will use up some of the subsequent time available for the welder to complete the weld before the detrimental reactions occur. Therefore, it is essential to purchase only DSS products, plate, pipe and forgings, that have been fully annealed and water quenched.

Welding characteristics of duplex stainless steel
All major welding processes have been successfully used in fabrication of DSS digesters. These processes include Submerged Arc Welding (SAW), Shielded Metal Arc Welding (SMAW), Flux-cored Arc Welding (FCAW), Gas Tungsten Arc Welding (GTAW) and Gas Metal Arc Welding (GMAW).

The need to clean the weld joint prior to welding applies to all stainless steels. The DSS joints are more sensitive to contamination, and especially to moisture, than the austenitic stainless steels. The chemistries of the base metal and the filler metal have been developed assuming there are no sources of contamination. Dirt, grease, oil, paint and other sources will interfere with welding operations and adversely affect the corrosion resistance and mechanical properties of the weldment. No amount of procedure qualification is effective if the material is not thoroughly clean before welding.

DSS requires good joint preparation. The weld joint design must facilitate full penetration and avoid autogenous regions in the weld solidification. Special attention must be given to uniformity of the weld preparation and the fit-up. For an austenitic stainless steel, a skilled welder can overcome some deficiencies in joint preparation by manipulation of the torch. For duplex stainless steel, these techniques can cause a longer than expected exposure in the harmful temperature range, leading to results outside those of the qualified procedure. Examples of joint designs used with DSS are shown in the above referenced TAPPI TIP 0402-29.

Compared to austenitic stainless steels, DSS can tolerate relatively high weld heat inputs. Typical heat inputs are 0.5-4.0 kJ/mm, but will vary according to the welding process. To avoid problems in the the heat-affected zone (HAZ), the weld procedure should allow rapid (but not extreme) cooling of this region. The temperature of the work piece is important because the plate itself provides the most effective cooling of the HAZ. The weld interpass temperature is typically limited to 150°C (300°F). That limitation should be imposed when qualifying a weld procedure, and production welding should be monitored to assure that the interpass temperature is no higher than that used in the qualification. As a general rule, preheating of DSS is not recommended because it slows the cooling of the HAZ. Preheating should not be part of a procedure without a specific justification. For example, preheating to 100°C (212°F) may be desirable for GTAW welding of heavy sections; otherwise, initial welds may cool too rapidly, resulting in excessive ferrite in the weld.

Post-weld stress relief is not necessary or useful for DSS. Thermal stress relief, or long periods of exposure in the range of 340 to 980°C (644 to 1800°F), initiates precipitation reactions, resulting in loss of ambient temperature ductility, toughness and corrosion resistance. This is why continuous service of DSS is generally limited to temperatures below about 340°C (644°F).

Grade 2205 DSS mill products are balanced to have about 40 to 50% ferrite, with the remainder being austenite. This ferrite balance provides the characteristic benefits of good strength, toughness, and resistance to corrosion and stress corrosion cracking. The ferrite in the as-deposited weld metal is typically specified to be in the range of 25 to 60%. There are no reports of problems associated with the ferrite contents at the lower end of this range, such as hot cracking typically seen in SMAW or SAW welds.

DSS can be readily welded to carbon steel or austenitic stainless steel, and this is a consideration during digester design and fabrication of external components and attachments. Table 2 shows the recommended filler metals for welding DSS to DSS, to austenitic stainless steel and to carbon steel.

TABLE 2. Welding consumables used for dissimilar welding of DSS.

Repair welds in DSS digester vessels should be limited to avoid excess time in the red temperature zone. A typical specification will limit one repair weld for a given region on the inside (process side), and two repair welds on the outside. Separate qualification of repair welds using the ASTM A 923, Method C [10], corrosion test should be considered.

Digester design considerations
In North America, fabricators typically build digesters according to ASME Code, Section VIII, Division 1, requirements. The ASME Code requires impact testing for DSS parent metal, weld and HAZ when the section thickness is above 3/8 inch.

Both laboratory corrosion testing and experience show that corrosion rates of up to 0.25 mm/year (0.009 inch/year) might be expected in 2205 DSS batch digesters in kraft service[6]. Corrosion-erosion can be experienced in the lower cone and outlet nozzle regions, especially if significant abrasives (sand) are present in the chips. DSS batch digesters typically are designed with 6 to 9 mm (0.25 to 0.38 in.) corrosion allowance. Corrosion of 2205 DSS has been found to be negligible in kraft continuous digesters. Nevertheless, a modest corrosion allowance (2 to 3 mm) should be specified for continuous digester service.

Digester external attachments are typically made from DSS in an effort to match the coefficient of thermal expansion. Temporary attachments that are welded directly to the digester pressure envelope should also be made from DSS.

Despite the improved resistance to stress corrosion cracking of DSS, cold-formed components and weld joints may be susceptible to cracking under insulation. To minimize the chance for cracking under insulation, DSS digesters may be painted entirely on the exterior. An alternative is to paint only cold-formed components such as the top head.

Mills should also consider post-weld cleaning requirements when specifying DSS for digesters. The extent of cleaning is dependent on the service environment. The process side of vessels should always be cleaned free of deposited or imbedded iron, rust, scale, weld slag, fluxes, dirt, oil and grease. Removal of heat tint produced from welding is costly, and for kraft digester service, the heat tint need not be removed. However, service in acid sulfite liquors is more stringent, especially when chloride is present in the liquor [11]. Removal of heat tint with pickling paste is recommended in acid sulfite environments to enhance pitting resistance.

Quality control
Good results in DSS digester construction do not come by chance, but by adhering to a good quality assurance program. Poor practices that are forgiven with carbon steel or austenitic stainless steel fabrication can result in costly degradation of the superior physical and mechanical properties of DSS. While ASME Code requirements cover a certain level of quality, additional measures are warranted for DSS construction. Examples are the qualification of mill product materials and welding procedures to ASTM A 923, Method C, and the follow-up testing of shop- and field-production weld coupons. Routine monitoring of welding heat input and interpass temperatures should also be performed. Ferrite balance of welds can be made using a portable meter such as the Fischer Feritscope®. Production joint fitup and intermediate steps in the welding process should be verified and documented by quality control signoff.

The mill product material quality should be similarly monitored. Mills should give attention to material surface finish and post-fabrication cleaning. Additional information on specifications and testing of DSS materials are shown in the TAPPI TIP 0402-29.

Summary
Since 1997, many new digesters in North America have been fabricated from Type 2205 DSS. Experience shows that the alloy has excellent resistance to corrosion, erosion and stress corrosion cracking in the digester environment. Although the initial DSS digester cost is marginally greater than for carbon steel, a cost advantage is realized in just a few years due to the low cost for maintenance and inspection of DSS.

DSS alloys are readily welded, but fabricators must recognize the differences between DSS materials and the more familiar austenitic stainless steels. Welding heat input and interpass temperatures are critical to avoid excessive time at elevated temperatures. Excessive time in the “red” temperature zone can result in a loss of toughness and corrosion-resistance properties. The fabricator’s quality control program should include routine tests for heat input and interpass temperature, as well as production coupon evaluation for corrosion using the ASTM A923, Method C test. A modest corrosion rate will occur in service of DSS digesters, especially in kraft batch digesters, and the design should incorporate sufficient corrosion allowance.

REFERENCES

1. Thorpe, P.H., "Duplex Stainless Steel Pulp Digester Fabrication and User Experience in Australia and New Zealand," Proc., 8th International Symposium on Corrosion in the Pulp & Paper Industry, Stockholm, pp 20-25, May (1995).
2. Dupoiron, F., Audouard, J. P., Schweitzer, G. and Charles, J., "Performances of Special Stainless Steels and Alloys in Pulp and Paper Industry: Industrial References and Field Test Results," Corrosion/94 NACE Paper No. 420 (1994).
3. Audouard, J. P., "Corrosion Performance of Duplex Stainless Steels for Kraft Pulp Digesters Application," Paper D97-010, Stainless Steels97 - 5th World Conference, Maastricht, the Netherlands, October (1997).
4. Olsson, J., Leffler, B., and Jorgensen, C., "Experiences of Duplex Stainless Steel in the Pulp and Paper Industry,” Proc., 9th International Symposium on Corrosion in the Pulp & Paper Industry, Ottawa, pp 161-164, May (1998).
5. Moskal, M., Cheetham, G., Paultre, J. Wilton, W., “Quality Requirements for Duplex Stainless Steel Digester Fabrication,” Proc., 9th International Symposium on Corrosion in the Pulp & Paper Industry, Ottawa, pp 67-73, May (1998).
6. Wensley, A., Moskal, M., and Wilton, W., "Materials Selection for Kraft Batch Digesters," Corrosion/97, NACE Paper No. 378 (1997).
7. Audouard, J-P., Grocki, J., “Duplex Stainless Steels for Tanks,” Corrosion/2002, NACE Paper No. 02135 (2002).
8. “Stainless Steels and Specialty Alloys for Modern Pulp and Paper Mills”, Reference Book Series No. 11025, The Nickel Development Institute, Toronto, Ontario, pp. 6-7, 2002.
9. “Qualification of Welding Procedures for Duplex Stainless Steels,” TAPPI Technical Information Paper TIP 0402-29 (2002).
10. American Society for Testing Materials, 1916 Race Street, Philadelphia, PA.
11. Ahlers, P-E, "The Corrosion of Stainless Steels in Passive and Active States Under the Conditions of Sulfite Cooking," Pulp & Paper Industry Corrosion Problems, Vol. 3, p. 66, NACE, 1980.

About the author: Max D. Moskal is principal metallurgical with M&M Engineering, which is headquartered in Austin, Texas, USA. Formerly with Smurfit-Stone Container, Moskal has 41 years experience working on all critical equipment problems in paper mills. He has been a member of TAPPI since 1972 and has served in all chairs of the Corrosion and Materials Engineering Committee. He can be reached at max_moskal@mmengineering.com.