Welded Stainless Steel Tube&Pipe

Properties, Specifications and Experience of Welded Stainless Steel Pipes in the grades

UNS S31254, N08904, S32304, S31803 and S32750

Göran Gemmel M Sc. Metallurgy

Manager Market Development Avesta Sandvik Tube AB 64421 Torshälla, Sweden

Abstract Properties, specifications and case stories of welded stainless steel pipes in the grades UNS S31254, N08904, S32304, S31803 and S32750 are presented. The pipes are used in all types of corrosive environments in industries such as, chemical, metallurgical, energy, oil &gas, pulp & paper, ships, desalination, and food. More than 13,000 tons of welded pipe and 5500 km of welded heat exchanger tubes in UNS S31254, N08904, S32304 and S31803 are now in service, without any reports of failure due to corrosion attack of the longitudinal weld seam. Improved design properties and much higher corrosion resistance than the conventional Stainless Steels’ 304L, 316L and 317L, are major benefits of these new super austenitic, duplex and super duplex grades. Introduction Welded stainless steel tubes and pipes are used in a number of applications where high demands are set on reliability, strength and corrosion resistance. Chloride environments give however a risk for pitting, crevice and stress corrosion for standard stainless steels. Avesta and Sandvik have developed more chloride resistant grades such as the austenitic UNS S31254 and N08904 and the ferritic austenitic UNS S32304, S31803 and S32750. Keys to successful performance are computer designed chemical compositions with tight limits on each element in order to optimise not only the properties but also manufacturing and welding. Table I: Designations and AST’s nominal chemical composition of the presented grades. Grade UNS Cmax Cr Ni Mo Cu N PREN* Duplex and super duplex grades SAF 2304 S32304 0,025 23 4 0,3 0,3 0,10 25 2205 S31803 0,025 22 5,5 3,0 - 0,17 35 SAF 2507 S32750 0,025 25 7 4,0 - 0,27 43 Super austenitic grades 904L N08904 0,020 20 24,5 4,2 1,5 0,05 35 254 SMO S31254 0,020 20 18 6,1 0,7 0,20 43 654 SMO S32654 0,010 24 22 7,3 0,4 0,50 56 * %Cr+3,3x%Mo+16x%N

High Performance Grades Austenitic stainless steels such as 304(L) and 316(L) have been the dominant structural steels in most of the process industry. The success of these steels is due to their many excellent properties. However, their shortcomings are low resistance to localised corrosion, such as pitting, crevice corrosion and stress corrosion cracking in chloride rich acid environments. During the 1980s, several new stainless steels with both improved resistances to corrosion and higher strength were introduced. The austenitic grades UNS S31254, S32654 and N08904 and the duplex grades S32304, S31803 and S32750 with higher contents of chromium, molybdenum and nitrogen are now used in all types of process industries where problems have occurred with type TP 304(L), 316(L) and other materials. Corrosion Resistance of the High Performance Stainless Steels All of the stainless steels shown in table I have very good resistance to chloride induced corrosion, demonstrated not only by laboratory tests but also through years of experience. Table II. As all these grades are very resistant to stress corrosion cracking, the choice of grade for a certain application is most dependent on its resistance to pitting and crevice corrosion. This is directly related to the Cr, Mo and N- contents or PREN-values. Table I and III. Table II: Pitting and Stress Corrosion resistance of some stainless steels. SCC, TTF hours CPT °C NaCl at 200 °C Grade UNS 6% FeCl3 0.5 x Rp0,2 AST TP Base metal Weld metal* Base metal SAF 2304 S32304 20 10 350 2205 S31803 35 25 350 SAF 2507 S32750 70 45 >500 17-11-2L TP 316L 5 2 100 904L N08904 40 25 >500 254 SMO S31254 70 45 >500 654 SMO S32654 Bp. 90 >500 * Heat-treated. Table III. Guide lines. Selection of Stainless Steels in Cl-containing neutral waters Temp < 70 °C Grade Cl-conc. AST TP or UNS PREN* Max ppm 18-9L, 18-10Ti 304, 304L, 321 18 200 17-11-2L, 17-11-2Ti 316, 316L, 316 Ti, S32304 25 500 2205, 904L S31803, N08904 34 2000 SAF 2507, 254 SMO S32750, S31254 43 20000 654 SMO S32654 56 >20000 *%Cr+3,3x%Mo+16x%N Welding. The high performance grades are alloyed with higher contents of molybdenum and nitrogen in order to improve the corrosion resistance and the strength, and must be welded with overalloyed filler metals and/or with nitrogen added to the shielding gas, in order to become as high pitting corrosion resistance as possible in the weld. Pipes made of nitrogen alloyed chloride resistant grades in the presented applications are welded with the addition of nitrogen in the shielding gas. Most of the pipes are also welded with filler metal and heat treated when specified.

Corrosion resistance of welds The main reason for using stainless steel is for its corrosion resistance. For welded stainless steel pipe, there are three types of corrosion, which are likely to occur in the weld or weld area. Intergranular corrosion in the HAZ, selective corrosion and pitting corrosion The standards seldom prescribe any corrosion test but that for intergranular corrosion (IGC). Intergranular corrosion Testing the resistance to IGC was worthwhile 20 years ago when carbon contents were above 0.05%. Today most stainless steels have carbon contents below 0.05% and the L-grades below 0.02%, making IGC-testing more or less worthless. 99% will pass this test independent of the manufacturing history. Therefore, heat treatment of welded austenitic stainless steel pipe to reduce the risk for IGC is seldom necessary. Selective corrosion due to ferrite in the weld When austenitic stainless steels solidify after welding, small portions of ferrite form in the weld, normally in the range of 1-6%. In some acids, this may cause corrosion unless the tubes are solution annealed. For example, improperly heat-treated seam welds in heat exchanger tubing may be attacked when cleaned with HCl. One way to reduce this risk is to use bead worked solution annealed tubes. Pitting corrosion due to Cr- and Mo-segregations. Autogenous welds of Mo-alloyed austenitic stainless steels suffer from segregation, reducing the resistance to pitting corrosion in chloride environments. Higher Mo-contents give more severe segregation. A solution anneal evens out the segregation and a proper heat treatment will give the weld a pitting resistance equivalent to the base metal. Welding with the addition of a filler metal with higher Cr and/or Mo-content will also have the same effect. Strength and calculation of wall thickness An additional advantage of the new corrosion resistant stainless steels is their high strength, which makes it possible to reduce wall thickness when compared to standard austenitics. Table IV. If the duplex SS UNS S31803 is used instead of TP 316L, the wall thickness can in some cases be reduced by more than 50% when the German DIN-rules for calculation are used instead of ANSI. Table IV: Calculated Pipe Wall Thickness (mm) Calculated according to DIN 2413, and ASME B31.1. For a design pressure of 100 bar at 200 °C OD, mm 168 508 762 NPS 6 20 30 Grade DIN ASME DIN ASME DIN ASME TP 316L 7,3 9,6 22,1 29,0 33,1 43,5 S31254 6,4 6,8 19,2 20,5 28,9 30,8 S31803 4,0 6,5 12,0 19,7 18,0 29,5 Table V: Minimum mechanical strength of the presented special grades Grade UNS Yield Tensile Elong. MPa MPa % SAF 2304 S32304 410 600 25 2205 S31803 450 720 25 SAF 2507 S32750 550 800 25 17-11-2L TP 316L 170 485 35 904L N08904 220 520 35 254 SMO S31254 300 650 35 654 SMO S32654 430 750 35

Joint efficiency factor or weld factor. ASME/ANSI rules allow a joint efficiency factor of 0.85 for hydro tested welded pipe and 1.0 for 100%-radiographed pipes. This means that when calculating the wall thickness for a specified internal pressure, 100% of the strength can be utilised for welded pipe, the same as for seamless pipe, when the pipe weld is X-rayed 100%. German (TÜV) and Swedish (ASS) authorities give after audition, time limited approvals to pipe manufacturers to deliver pipe with a weld factor (joint efficiency factor) of 1.0 when the pipe has been tested and approved according to DIN 17457 or SS 219711 respectively. TÜV allows both ET and RT. as NDT, while ASS only approve ET for weld factor 1.0. Weld factor =1.0 approved by TÜV and ASS can only be used for pipes manufactured and tested to German or Swedish standards. ASTM Standards for Welded Austenitic Stainless Steel Pipe All of the mentioned grades are included in the most common ASTM, ASME and ANSI standards. The 2 most common ASTM specifications for welded austenitic stainless steel pipe are A 312 and A 358, and for duplex SS they are A 790 and the new A 928 for duplex pipe welded with filler. Table VI: ASTM-standards for welded pipe of high performance grades. Grade UNS ASTM ASTM ASME SAF 2304 S32304 A 790 A 928 SA-790 2205 S31803 A 790 A 928 SA-790 SAF 2507 S32750 A 790 A 928 SA-790 904L N08904 B 673 - SB-673 254 SMO S31254 A 312 A 358 SA-312 654 SMO S32654 A 312 A 358 - The important differences between ASTM A 312/A790 and A 358/A928 are: ASTM A 312/ A 790 ASTM A 358/ A 928 Filler metal No Yes Weld classes One Five Non destructive test, NDT Hydrotest or EC HT or EC / Radiogr.exam. Heat treatment Yes Yes/no Thickness tolerance -12,5% -0,3 mm/ -12,5% These differences may be necessary by the law for the US-market, but they complicate life for both users and producers in countries where the ASTM specifications are used, but not necessary by the law. How to select a suitable pipe specification. Some important parameters to consider when specifying pipe systems are: • Dimension and tolerances • Welded or seamless • Grade • Execution. - Can filler metal be accepted? - Is heat treatment necessary? - Type of heat treatment? - Type of surface treatment? - Type of bevelling? • Non destructive testing. Type and amount - Eddy Current. - Radiographic.

- Hydro pressure

• Mechanical and corrosion testing • Approvals from authorities or customers These parameters are covered to different degrees in the various national standards, however there are some significant differences, which should be pointed out. A specifier who wants one specification for a package of pipe sizes like 21.34-914.9 mm (NPS 1/2-36) in sch 10S and 40S will have problem if ASTM standards are used. A 312, an often-used ASTM specification includes both seamless and welded pipe with no addition of filler metal. When A 312 is specified problems with undercut, incomplete penetration and lack of fusion may occur in pipes with wall thickness’ above about 6 mm (1/4 ”) if normal welding techniques, like GTAW and PAW are used. When pipes are welded from plate it is very difficult to get enough power to press the plate edges together in order to properly fill the weld gap. The quality of such welds is questionable because of the higher risk of weld defects. It is much better to add filler metal when welding heavy walled pipe because it will minimise the risk with lack of fusion and an incomplete fill up of the weld. Filler metal also provide the opportunity to improve corrosion resistance through the use of higher alloy filler metals. Above all, there are only advantages in using filler metal on thick walled pipe. Alternatives to A 312 for heavy wall pipe are ASTM A 358 or A 409; but as these have quite different requirements on heat treatment, NDT and tolerances their use will create new problems for the manufacturer. The pipe producer must propose deviations for some sizes because of restrictions regarding the addition of filler metal and the amount and type of testing coupled to this. cont. How to select.. Few end users and consultants have the knowledge to approve such deviations. European standards like the German DIN 17455 and DIN 17457 do not have this problem because in these standards there is no difference in how the pipes are welded, only in the amount and type of testing required. My advice is that the pipe specification should have options to use either seamless or welded, and also options to use EITHER A 312/A790 or A 358/A928. This will increase the availability without influencing the quality. Heat treatment of welded SS pipe. The main reason for heat treating austenitic SS welded pipes with low C- and high Cr- and Mo-contents is to reduce the segregations of Cr and Mo which have occurred during the solidification of the weld metal. For the ferritic-austenitic grades also the balance of Cr, Mo and N in the ferrite and austenite must be equalised in order to optimise the corrosion resistance. If overalloyed filler metal is added, heat treatment of pipes with wall thicknesses above about 3 mm is not always necessary. By optimising the welding procedure it is possible to secure that the necessary corrosion resistance and strength are achieved without heat treatment. Other reasons for heat treatment which often are mentioned are to - Reduce stresses and hardness caused by the forming of the strip/plate to a pipe. - Dissolve carbides and other precipitates in the HAZ. These reasons are not always relevant and the small improvements must be judged compared to the possible disadvantages, which may occur, such as distortion, slow cooling rates and/or catastrophic oxidation. In ASTM A 312 and A 790 heat treatment of the pipe is mandatory. In A 358 and A 928 heat treatment can be excluded by agreement. However, there are circumstances when a heat treatment is not necessary because the properties are only marginally improved, or even impaired. Heat treatment of welded pipe should only be required when there are enough evidence that an improvement is achieved and necessary for the application. Depending on how and where the pipes will be used it is up to the end user to decide which type of heat treatment is necessary. For heat-treated pipes there are no standard, which prescribes the holding time. Pipes delivered to the same specification can have varying corrosion resistance, and mechanical properties can differ considerably, depending on whether the pipes have been induction heated in line (5-10 sec) or solution annealed (about 3-5 min). Pipes alloyed with Molybdenum are often selected

because of there higher pitting corrosion resistance, however, this resistance may be reduced by the shorter annealing times associated with in line induction heat treatments. IGC-test results do not indicate inadequate corrosion resistance when Mo-segregation is the problem. Case Histories Chemical Industry. Pipes for transport of phosphoric acid. Grade: Alloys 2205 and 904L Product: Welded pipes, with filler. Size: 2205: ANSI NPS 10 and 12 x 5,38 and 5,97 mm. 904L: ANSI NPS 6 and 8 x 5 mm. References: 2205:IBN-Al Baytar, Algeria. 904L: Enichem, Italy Experience: Both these grades have a very high resistance to different types of technical phosphoric acids. 2205 also has the advantage of allowing design with a thinner wall, with lower cost as a consequence. Energy Production Seawater pipeline at a nuclear power station. Grade: Alloy 254 SMO Product: Welded pipe, with filler, not annealed. Size: 206-406 x 3 mm References: Sydkraft, Oskarshamn, Sweden. Medium: Chlorinated brackish water, max temp 25° C. Experience: Pipes for the transport of chlorinated brackish cooling waters are severely attacked by pitting corrosion if SS of type 316 or 316L are used. 254 SMO has shown excellent corrosion resistance in this type of environment in Sweden and Finland. After 5 years in service with yearly inspections, no corrosion has been observed. Oil & Gas Seawater- ballast- and fire fighting systems on offshore platforms On offshore platforms the standard materials in these pipe systems were galvanised steel or Cu-Ni alloys. The corrosivity of chlorinated seawater creates corrosion problems for these materials resulting in high maintenance costs and loss of production. 254 SMO solves these problems, and thanks to its higher strength also smaller pipe sizes can be used reducing total cost and total weight. Another advantage is the ability to use higher water velocities. Grade: Alloy 254 SMO Product: Welded pipes, with filler. NPS 4: annealed, NPS>4: not annealed. Size: ANSI 21,34-1067 mm (NPS 1/2-42) x 1,65-25,4 mm References: Statoil, Shell, Mobil, Elf, Hydro, Marathon, Conoco and Aramco. Experience: Since 1982, welded pipe in 254 SMO has been in service on over 20 platforms on the North Sea. 1993 254 SMO was installed by Aramco in the Gulf. Until today, not one single corrosion attack has been reported on pipes and longitudinal welds in service under normal conditions. Crevice corrosion have however occurred on a few flanges and screwed fittings, and also pitting on welds welded onsite with improper welding consumables. (Less than 0.01% of all installed flanges were attacked, mainly through conditions above the design temperature)

Pipes for transport of oil and gas. Grade: Alloy 2205 and SAF 2507 Product: Welded pipes, with filler, annealed. Size: ANSI NPS 4-24” x 3,05-35 mm References: Shell-NAM Netherlands, Shell UK, Elf Congo, Statoil, and Hydro. Experience: Duplex stainless steels replace carbon steels in process systems and flowlines where both unprocessed and processed Oil&Gas are transported. Welded pipes of alloy 2205 and SAF 2507 were selected due to their high strength in combination with the overall very good corrosion resistance to oil and/or gas contaminated by seawater, H2S and CO2. Pulp & Paper Industry Transport of pulp. Grade: Alloy 2205 Product: Welded pipes, with filler and not annealed. Size: 101,6-355,6 x 1,6-3,5mm Reference: ICAL Kimberly Clark Australia. Tubeside: Pulp Experience: The standard material for this application is 316. Problems with fatigue, pitting and stress corrosion on TP316 made 2205 a better alternative. The high strength made it possible to reduce the wall thickness with decreased costs as a consequence. Its overall very good corrosion resistance in pulp and chloride solutions made 2205 the perfect material. Desalination Pipes for transport of seawater at reverse osmosis plants. Grade: Alloy 254 SMO Product: Welded pipes, with filler. NPS 4: annealed, NPS>4: not annealed. Size: ANSI NPS 1/2-16” x Sch 10S, 40S. References: Canary Islands Spain, Gaviota USA. Experience: Earlier austenitic 316L, 317L or Cu-Ni pipes were used for seawater distribution, but after a short period of time, these materials were attacked by pitting and/or crevice corrosion. Welded pipes of 254 SMO have been in service since 1984 in about 20 reverse osmosis plants mainly around The Gulf, and so far no sign of corrosion attack has been observed. Food Industry Transport pipes at processing of potatoes for french fries. One of the largest producers of raw potatoes for french fries in Denmark had severe problems with wear when the uncleaned potatoes were transported through austenitic SS pipes to the pealing process. Tests with 2205 showed good results, thanks to the higher strength and hardness, which provide excellent wear resistance. Grade: Alloy 2205 Product: Welded pipe, with filler, not annealed. Size: 212 and 312 x 6 mm Tubeside: Raw uncleaned potatoes Reference: Flendsted A/S Denmark Experience: Standard austenitic grades suffered from bad wear resistance, while the duplex 2205 has shown considerably less wear.

Table VII: Amounts of welded tubes and pipes in special grades delivered from AST.

Grade Ton pipes Km HEX tubes 2205 6000 2500 904L 1500 1800 254 SMO 5500 1200 Summary • Properties and pipe specifications for AST 2205, SAF 2304, SAF 2507, 904L and 254 SMO, are presented. • The major ASTM standards for welded SS and duplex pipe are discussed and compared. • The reasons for heat treatment are discussed. • Case histories of special grades within the oil & gas, chemical, pulp & paper, energy and food industries, are presented. • Significant cost reductions and reduced Life Cycle Costs can be achieved by - using new stainless steel grades with higher strength and improved corrosion resistance. - using welded pipe instead of seamless. - using European rules and values for calculating pipe thickness. Literature review. Göran Gemmel: Alternative Design and Specifications for Welded Stainless Steel Pipes, and Alternative Grades to TP 304L, 316L and 317L. First International Symposium on Process Industry Piping Dec 1993 Orlando, Fa Göran Gemmel: Applications of welded stainless steel tubes and pipe in AST 904L, 254 SMO and 2205.Application of Stainless Steel ’92. 9-11 June 1992, Stockholm. Vol.2 p 661-672. Avesta Sandvik Tube: Chloride resistant welded stainless Tubes and Pipes. Brochure, AST 90 10 65 E. Ove Jonsson et al The role of nitrogen in longitudinal welding of tubing in duplex stainless steels. Duplex Stainless Steel 1991, Beaune, France Mats Liljas: The welding metallurgy of duplex stainless steel. Paper KV. Duplex 94, Stainless steels. Glasgow, Scotland 13-16 Nov. 1994 Jan Olsson: UNS S32654, a New Super austenitic Stainless Steel for harsh environments. Acom 1-1995 Avesta Sheffield Info