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.
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 close to 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 (modified for digester service by having a silicon content below 0.02%) was the standard material of construction for batch digesters in North America. When SA516-Grade C carbon steel came into widespread use for the construction of pressure vessels (inclu ding digesters) in the 1970's, the use of the lower-strength low silicon steels for digester construction was abandoned. SA516-Grade 70 has a medium silicon content (typically 0.25% Si) steel and has poorer corrosion resistance resulting in a shorter service life before requiring overlay. In addition, type 309 stainless steel weld overlays experience rapid corrosion19 in batch digesters, necessitating re-overlaying. Duplex stainless steel digesters, on the other hand, require little or no maintenance. Due to the superior strength of duplex stainless steel the required wall thickness is much thinner than that for carbon steel. Therefore, construction of new duplex stainless steel digesters can be cost-effective compared with new carbon steel digesters20. 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 considered.
Type 2205 duplex stainless steel has two UNS numbers: S31803 and S32205. The S32205 grade is recommended 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.