KNOWLEDGE FOR A BRIGHTER FUTURE Stainless steels and specialty alloys for pulp, paper and biomass conversion A PRACTICAL GUIDE FOR MILL ENGINEERS Nickel Institute Technical Series No 11 025 2 Stainless steels and specialty alloys for pulp, paper and biomass conversion A PRACTICAL GUIDE FOR MILL ENGINEERS 2nd Edition Published 2017 The material presented in this publication has been prepared for the general information of the reader and should not be used or relied on for specific applications without first securing competent advice. The Nickel Institute, its members, staff and consultants do not represent or warrant its suitability for any general or specific use and assume no liability or responsibility of any kind in connection with the information herein. Prepared by a Task Force of the Metals Subcommittee of the Corrosion and Materials Engineering Committee of the Technical Association of the Pulp and Paper Industry and the Nickel Institute. Senior Editor: Andrew Garner, P.Eng. 2. Characteristics of stainless steels and other corrosion resistant alloys 3 Contents 1. Introduction ......................................5 2. Characteristics of stainless steels and other corrosion resistant alloys ................. 7 3. Digesters........................................19 4. Oxygen delignification and brown stock washing .........................33 5. Chemical recovery................................37 6. Tall oil manufacture ..............................49 7. Sulphite process .................................55 8. Neutral sulphite semichemical pulping..............61 9. High yield mechanical pulping .....................65 10. Fiber recycling ...................................69 11. Bleach plant and pulp/paper stock preparation.................................71 12. Paper machine ...................................79 13. Suction rolls.....................................87 14. Threaded fasteners ...............................93 15. Welding........................................103 16. Abrasion .......................................113 17. Corrosion ......................................117 18. Risk based inspection (RBI).......................123 19. Biomass conversion and biorefineries ..............127 20. Appendix tips...................................139 21. Abbreviations...................................141 4 Stainless steels and specialty alloys for pulp, paper and biomass conversion mills 5 1. Introduction by Andrew Garner This Nickel Institute Reference Book is a major update of the is driving this transformation. The use of trees grown in first edition which was published in 2000 and edited by Art sustainable forests is considered carbon-neutral because any Tuthill. The first edition described the use of stainless steels net carbon release is recaptured after replanting. and other corrosion resistant alloys in pulp and paper mill equipment. Since then there have been two major trends Stainless steel equipment is used to make many of the new from a corrosion and materials perspective: almost every new green products: choosing the right grade saves capital and pulp mill has been constructed in South America and Asia; maintenance costs. A main objective of this revision is to and the family of commercially available duplex stainless describe where newer alloys should be considered. Notable steels has expanded significantly, now including several lower examples are the excellent resistance of lean duplex grades cost lean grades with lower Cr, Ni and Mo content. in alkaline pulping and recovery environments, and good resistance of super-duplex grades in acidic oxidizing chloride Another notable industry trend is the use of advanced containing environments. automation and improved sensor technology to operate mills with much better control of the process and with fewer In summary, this reference book spans changes in materials of personnel. At the end of the 20th Century the focus was on construction driven by three recent drivers: ecological clean- controlling effluents to minimize pollution and water usage. up, cost efficiency, and new bioeconomy products. Now the focus has shifted to controlling product quality and improving the cost effectiveness of manufacturing and maintenance. Safety and reliability are the principle drivers for maintenance of fixed and rotating equipment, and there is now a much more systematic attempt to identify the critical damage mechanism, particularly the corrosion mechanisms which are covered extensively in this reference book. The 21st century also marks the initial development of the bio-economy and new types of products that can be made from trees. Processes are being introduced to make new materials such as cellulose nanocrystals, cellulose fibrils and new products from lignin. New thermo-chemical technologies such as pyrolysis, torrefaction and gasification are coming to the fore as the environment-friendly capability of the forest sector comes of age. Consumer concern about global warming 1. Introduction 6 Stainless steels and specialty alloys for pulp, paper and biomass conversion mills 7 2. Characteristics of stainless steels and other corrosion resistant alloys by Arthur H. Tuthill 2.1 DESIGNATIONS, PROPERTIES AND the finishing treatment. Stainless steels are subdivided SPECIFICATIONS into four different groups, based on their microstructure: austenitic, ferritic, martensitic, and duplex. Each group’s This section acquaints mill engineers with the principal distinctive characteristics are discussed below to provide characteristics of the different families of stainless steels a better understanding of these terms and general and other corrosion resistant alloys used in the pulp and properties as they are encountered in the literature and paper industry. Alloy tables in the back of this section give discussions on corrosion. the common name, the Unified Numbering System (UNS), and the European Number (EN), which is similar to the German 2.2 AUSTENITIC STAINLESS STEELS (DIN) designation, for the alloys used. These tables help in identifying and specifying the alloys regardless of where the Most stainless steels traditionally used in pulp and paper equipment was made. The compositions and properties of equipment are austenitic grades. Austenitic alloys, which wrought alloys are shown in Tables 2-1 and 2-2, and of cast have a face-centred cubic (FCC) crystal lattice, are non- alloys in Table 2-4, which also shows the wrought equivalent magnetic, tough, ductile and easily welded because the of the less familiar cast alloys. The common designation of weld and heat-affected zone microstructures are not greatly the wrought alloys and the cast grade of the cast alloys is affected by welding. Austenitic alloys are hardened by cold used throughout the text, followed by the UNS designation work, not by heat treatment. Their corrosion resistance is in parentheses ( ) on first mention in each section of the due primarily to an extremely thin chromium oxide surface reference book. film. Molybdenum (Mo) and Nitrogen (N) additions enhance The composition of special-purpose alloys used in rolls, the resistance of these alloys to localized corrosion in fasteners, welding filler metals, and special applications are chloride-containing solutions. given in the individual sections of this bulletin. Stainless steels are normally produced in the “annealed” The pitting resistance equivalent number (PREN) for wrought condition. The term anneal, also solution anneal, used for alloys is shown in Table 2-2 and for the cast alloys in stainless steels means heat to 1040˚C or higher and water Table 2-4. Generally speaking, a higher PREN means the quench to optimize the microstructure. [Annealing a carbon alloy is more resistant to pitting corrosion in oxidizing acid steel involves heating above 750˚C and slow cooling in the chloride environments. furnace to produce its softest condition.] For any given stainless steel, the mechanical properties, The basic 18-8 austenitic grade, Type 304 (UNS S30400), has weldability, corrosion resistance, and wear and abrasion 18% Cr, 8% Ni and up to 0.08% C. Type 316 (UNS S31600) resistance depend to a large extent upon its microstructure, is Type 304 with more nickel, and >2% Mo to improve which in turn depends upon the alloy composition, the resistance to localized corrosion, especially by chlorides. More steel making or casting practice, the thermal history, and than 0.03% C in Type 304 and 316 makes them vulnerable 2. Characteristics of stainless steels and other corrosion resistant alloys 8 on welding to “sensitization”, which reduces the corrosion susceptible to caustic stress corrosion cracking in hot caustic resistance of the grain boundaries, making intergranular environments above about 120˚C, depending on the tensile corrosion possible. Heat of welding causes chromium to stress level. combine with carbon and precipitate chromium carbide at grain boundaries, depleting chromium from the general Austenitic grades with higher Mo, e.g., Type 317L (UNS S31703) microstructure in the weld’s heat affected zone (HAZ). This with >3% Mo, and Type 317LMN (UNS S31726) with >4% makes the affected zone susceptible to intergranular corrosion, Mo, resist localized corrosion better than 316L. The 6% Mo, or intergranular attack (IGA). super-austenitic alloys are even higher on the ‘ladder of resistance to localized corrosion’, as indicated by their higher Adding Nb or Ti to the base composition to combine with PREN. Superaustenitics, especially grades with 1–2% Cu, carbon prevents