Choice of Specifications and Design Codes for Duplex Stainless Steels

Transcription

1 acom AVESTA SHEFFIELD CORROSION MANAGEMENT AND APPLICATION ENGINEERING Choice of Specifications and Design Codes for Duplex Stainless Steels By Mats Liljas, Avesta Sheffield AB, and Göran Gemmel, Avesta Sandvik Tube AB, Sweden Duplex stainless steels have gained a large commercial interest in the latest decades due to their good combination of strength and corrosion resistance. Being a relatively new type of material to most users, this family of steels has been subjected to a large variety of tests and specifications. The paper reviews and discusses some international standards and special requirements regarding manufacturing process, composition, microstructure, corrosion tests and mechanical requirements. Available international standards are generally sufficient to specify for most applications. The use of quantitative metallography in acceptance criteria is a source of confusion and alternative methods are possible. It is recommended that acceptance of a certain product or procedure should be based on testing of engineering properties of practical concern. One advantage of using duplex steels in loaded constructions is the possibility to reduce the wall thickness compared to austenitic stainless steels. It is well known that European design rules allow thinner sections and thus better cost benefits than ASME codes. Different regulations and their consequences for duplex steels are discussed. stainless structural steel, representing a potential area for further growth. In general the service of duplex installations has been very successful resulting in wider use. A rapid growth has involved many new operators manufacturing and fabricating duplex steels including those with limited experience. The main problems encountered with duplex stainless steels have been related to heat treatment and welding. Unsuitable heat treatment has resulted in precipitation of intermetallic phase and deterioration of toughness and corrosion resistance (2,3). Although most welding methods can be used to weld duplex steels, they require special procedures for the retention of properties after welding. Deviations from established welding procedures have caused problems (4). As a result users have applied several restrictions and tests that in some way could be considered as over-specifying. This paper reviews some of the specifications and design codes and discusses their applicability. INTRODUCTION Duplex stainless steel is the current commercial designation of this steel family originally introduced as ferriticaustenitic stainless steels. In some standards they are also denominated austenitic-ferritic steels. The first duplex stainless steels were produced commercially almost seventy years ago but did not appear in national standards until around The main advantages identified with the duplex microstructure were the high strength and improved resistance to intergranular corrosion compared to austenitic grades. The development of this family of stainless steels into the second generation of duplex steel has been described elsewhere (1). It was not until the 1980 s, coinciding with the gradual introduction of the second generation of duplex stainless steels, that they found a wider use and were manufactured in larger tonnage by several producers. Important applications were pipeline systems for oil and gas industry, tanks for marine chemical carriers and pressure vessels for pulp industry. These areas are still dominating but many new application areas have been added. Standards, specifications and special requirements have developed alongside with the growth of this family. However, lack of data in some areas and, to some extent, conservatism restricts their full use. The high mechanical strength of duplex steels makes them very attractive as a GENERAL SPECIFICATIONS FOR DSS The two principal international standards describing duplex stainless steels are ASTM/ASME and EN. Some of the standards are presented and briefly discussed below. To limit the length of this paper the discussion is mainly confined to three duplex grades often listed as 2304, 2205 and 2507 respectively. Other common designations are 23Cr, 22Cr and 25Cr. These grades cover most application areas although similar grades are available. The international steel numbers of these steels are listed in ASTM and EN standards in Table 1 where the composition ranges are shown.

2 Chemical composition There is no difficulty in meeting standard specifications of chemical compositions. Individual steel producers have narrow target compositions within the specification to meet different criteria. Duplex steels are sensitive to variations in composition, particularly of those elements controlling the phase balance. Therefore, the relatively broad chemical limits permit large variation in properties. It is thus more important to specify what property profile is expected from the product rather than a certain composition range. To specify the minimum pitting resistance equivalent (PRE) value based on compositional condition to attain certain corrosion resistance may result in harmful deviation in other properties. The PRE concept can be used only in ranking alloys processed to optimise pitting corrosion resistance and should not be included in specifications. By applying narrowed composition ranges such as in the new standard UNS S32205 such problems in over-specification are avoided. The specified minimum mechanical properties at room temperature for plate and pipe in both standards are shown in Table 2. The values are quite similar in both standards the only substantial difference being the tensile strength of the superduplex grade UNS S MANUFACTURING PROCESSES General The process routes for the production of duplex steels are, to a great extent, similar to those for austenitic stainless steels. Individual process steps influence the product properties in various ways. However, experienced producers have established procedures for optimum properties at justified production costs. Heat treatment One of the most crucial process steps influencing the product properties is the final heat treatment. Duplex stainless steels are more vulnerable to deviations in heat treatment than austenitic stainless steels and several problems have been reported from different projects (3). The low strength at solution annealing temperatures involves a potential for undesired deformation of the product. A great concern is the risk of precipitation of intermetallic phases with detrimental effects on toughness and corrosion resistance. In most cases excursions from the specified temperatures are the cause of the failure. Another reason could bee too short holding times insufficient to dissolve intermetallic phases. This may occur in a highly loaded furnace where the heating time is long and the material is exposed long time to a temperature region where intermetallic phase precipitates. Recommended temperature ranges are listed in most standards (Table 3). It should be noted that individual manufacturing procedures require a smaller range for optimum and consistent result. At least for thin walled material the annealing times are short and if the heat treatment is carried out in a continuous furnace, the upper part of the temperature range specified is usually preferred or even Table 1. Specification limits of selected elements in modern grades of duplex stainless steels. Weight compositions, maximum unless otherwise stated. UNS/EN C Si Mn Cr Ni Mo N Cu S S S S Table 2. Mechanical properties at room temperature according to ASTM 1 and EN 2 standards of plate and pipe Grade YS, MPa TS, MPa Elongation, % Charpy V, J Standard ASTM EN ASTM EN ASTM EN EN S S S (800) ASTM: A240 plate, sheet. A790 welded and seamless pipe. 2 EN: plate, sheet and strip. EN /7 welded and seamless pipe. 2

3 exceeded. This is accepted and expressed in a footnote in the EN standard. The lower temperature regions could be on the borderline for dissolution of intermetallic phase and should be avoided, particularly for the higher alloyed variants. The cooling rate from the solution annealing temperature is also of importance and more critical for the higher alloyed grades. Extremely rapid quenching, particularly from the upper solution temperature region, can result in precipitation of nitrides and loss in properties. If the cooling rate is too low there will be a risk of precipitation of intermetallic phases and a risk of impaired properties. A slow cooling may result in a precipitate free microstructure but less exposure time available in the critical temperature range ( C) for further processes such as welding. This is of most concern for the highly alloyed superduplex grades. For above described reasons there are specifications requiring water quenching of duplex products. For type 2205 the propensity for this behaviour is hardly a great problem. It has been shown that also after a moderate cooling this steel can be welded with very high arc energies without any sign of loss in toughness and pitting corrosion resistance (5). One effect of the quenching is that the Table 3. Heat treatment of duplex plate and pipe material will deform heavily and the products have to be straightened. Heat treatment of welded pipes The main reason for heat treating high alloy austenitic welded pipes with low C- and high Cr- and Mo-contents is to restore the resistance to pitting corrosion of the weld by reducing the segregation of Cr and Mo which have occurred during solidification. For the duplex grades the partitioning of Cr, Mo and N between the ferrite and austenite must be balanced in order to optimise the corrosion resistance in the weld. If over-alloyed filler metal is added, heat treatment of pipes with wall thickness above about 3 mm is not always necessary. By optimising the welding procedure it is possible to ensure that the necessary corrosion resistance and strength are achieved without heat treatment. Other reasons for solution heat treatment often mentioned are: Reduced stresses and hardness caused by the forming of the strip/plate to a pipe. Dissolution of 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 collapse of thin walled pipe, slow cooling rates or surface defects due to Steel grade ASTM A790 EN Cooling 1 S C C Air or water S C C Air or water S C C Air or water 1 Cooling rate shall be rapid. Table 4. Temperature limits for duplex steels in pressure vessels. EN UNS VdTÜV WB EN ASME S C C F S C C F S C C F catastrophic oxidation. ASTM A790 requires heat treatment of the pipe. In A928 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 is 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 standards, which prescribe the holding time. Pipes delivered to the same specification can have varying corrosion resistance, and the 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). Bright annealing of duplex welded tubes, in line or separately, is not recommended as this often means too slow cooling rates, with impaired corrosion resistance as a consequence. WELDING General Fusion welding of duplex stainless steels has been the subject of substantial research during the last decades. One reason is that it is a fascinating metallurgical topic with many interesting reactions occurring. Another reason, naturally, is that this area is crucial to control for fabrication of sound constructions. The area is now well understood and welding procedures are established. The essential aim is to control that parent steel properties are retained in the weld. Available matching filler metals are designed to produce a weld metal with adequate properties and should be used for most practical joints. Nitrogen addition to the shielding gas is currently used in automatic welding to control the nitrogen level in the weld metal and hence the weld properties. Well known are the effects of low heat input giving a risk of high ferrite in the high temperature heat-affected zone (HAZ). This is a much lesser 3

4 problem in second generation duplex grades and particularly in superduplex grades. Too high heat inputs may cause formation of intermetallic phase in the low temperature (~ C) HAZ. As mentioned above, this is seldom a problem in lean or medium alloyed duplex grades but of some concern for the superduplex steels. In principle, all methods used for welding austenitic stainless steels can be used for duplex steels, provided the specific limits for duplex steels of each process are known. For example, gas metal arc welding (GMAW) has not been allowed for certain projects with the argument that this method involved a risk of lack of fusion. Naturally, it is required to perform a weld procedure qualification (WPQ) for any process to verify that desired properties are attained. Use of hydrogen addition to the shielding gas is frequently prohibited due to the risk of hydrogen embrittlement. With control of steel and weld chemistry and of the procedures this risk is minimal and miles of welded and annealed pipes have been manufactured using hydrogen in the shielding gas with no problems. Examples of specifications for welded duplex pipe Almost each oil company has its own pipe specification for 22Cr duplex stainless steels (Table 5). The requirements are usually a mixture of own experience and consultant s ideas. All requirements are set in order to secure the material properties, which are: Strength Toughness Corrosion resistance Chemical composition, welding and heat treatment are process parameters that determine the properties. To control all these parameters, some specifications are over-specified, which sometimes lead to conflicting requirements. One example is when an over-alloyed filler wire with 9% Ni is required for the longitudinal weld, and nitrogen is added to the gas for pitting resistance, in which case the ferrite content after heat treatment may fall below 30% in the weld. Most specifications allow minimum 30% ferrite in the weld, due to an expected risk for SCC, in spite of that the risk for SCC being negligible in the intended process environment. The NORSOK specification allows min. 25% ferrite, which is a reasonable and safe level to achieve the expected resistance to SCC. Specification of microstructure will be discussed further below. Our opinion is that mechanical and pitting corrosion tests are enough to secure a product with the necessary and expected properties. SPECIAL REQUIREMENTS AND METHODS FOR ASSESSMENT In many projects using duplex stainless steels several special requirements are included in the specifications. This may be needed, as there is an ambition to control those suppliers and fabricators that are handling a less mature steel family properly but the result is also that the costs increase and the availability will be restricted. An excellent review of testing methods applicable to duplex stainless steels was made in 1997 (6). The following criteria on special requirements were put forward: Conservative capability to address the property of interest. Definition of a test sample, location, test frequency and acceptance criterion. Applicability to all product forms and constructions of interest. Cost. Some further aspects of special requirements will be discussed below. Microstructure Naturally, the microstructure is of great importance for duplex steels as most properties are closely related to the phase balance between austenite and ferrite. However, microstructure is not a property but a state of the metal and much confusion emanates from unclear Table 5. Specifications for 22Cr welded pipe. Pitting YS, Hard- Chemical Ferrite % test min. UTS ness Toughness CTOD Hydropr. Specifier comp. PREN base weld G48A MPa MPa max. base weld FL base test A N> C J at 40 C B N> /22 C Rc 60 J at 95% x YS 50 C C N> C Rt 0.5 yes >450 D C Rc 45 J at yes yes yes 95% x YS 35 C E N> No Rc 45 J at yes yes 46 C 4

5 or inadequate requirements on microstructure. There are relevant technological tests to verify that a product in a duplex stainless steel is suitable for its purpose without the microstructure documentation. Because the phase balance in annealed mill products has very low heat-to-heat variation, specification or determination of a certain range is of limited value. PHASE BALANCE For duplex steels the ferrite content is largely depending on the chemical composition and the thermal history. For the parent steel it is controlled within a fairly narrow range. In many cases the measuring methods commonly used give a greater variation than the process variation. The influence of variation (+/ 15%) of the phase balance on properties such as tensile strength and corrosion resistance is small. The second generation of duplex stainless steels is in general aimed to contain % ferrite in the parent steel resulting in optimum properties. No ferrite content is prescribed in the international standards. There are, however, other specifications requesting a ferrite range. The most common specified ranges are 40 60% for the parent metal and 30 60% for the weld area. However, some of the most experienced end users of duplex and superduplex steels are using the Norwegian standard NORSOK that specifies 35 55% for the base metal and 25 60% for the weld area. The general opinion is that a too high ferrite content, i.e. >70%, decreases the toughness and pitting resistance, and a too low ferrite content, i.e. < 25%, decreases the SCC-resistance. What really matters is that the corrosion resistance and mechanical properties fulfil the engineering requirements. The ferrite content is not a property, but a way to check that the welding and heat treatment have been properly done. Therefore the limits should be within a reasonable range and be used for control only, and in case of deviation lead to an extra check of the material properties. MEASURING FERRITE As stated above, measuring the ferrite content and intermetallic phases can be done for monitoring consistency in processing, but the microstructure should not be a cause for rejection, as long as the engineering properties are within the specified limits. Our recommendation is that assessment of microstructure should be of informative character and not a cause of rejection. Magnetic methods are attractive, as they are non-destructive and inexpensive. However, they only give a qualitative measure even if proper calibration standards are used. For welds where ferrite numbers (FN) are specified magnetic methods are regularly recommended using for example a measurement system defined in ISO 8249 and AWS A4.2 (7). The ferrite number is then rather a physical property and cannot directly be translated into a ferrite content. A more direct method for measuring the volume fraction of ferrite is metallographic examination. The manual procedure according to ASTM E562 is currently referred to in specifications but it is time consuming and to some extent subjective. As the method involves a relative error of ±10%, at the best, the requirements should involve an average value and tolerated extremes. As an example, if a minimum ferrite content of 40% is required and the metallographic assessment gives 35 ± 5% the product should be accepted. Automatic methods adopting image analysis systems are used regularly. However, there is no standard directly written for duplex microstructures. ASTM E1245 is primarily designed for measuring inclusion content and is not fully relevant for ferrite measurement. The etching technique used is very critical to the result and details should be included in such a standard. PRECIPITATES As shown in several papers many different phases can occur in duplex steels and extensive research has been devoted to defining their conditions and effects (8, 9). Duplex steels are more susceptible to precipitation of intermetallic phases than austenitic stainless steels due to the high Cr- and Mo-contents and high diffusion rates in the ferrite phase. Problems have occurred with reduced toughness and corrosion resistance due to excessive amounts of intermetallic phase (10). The fact that such properties are directly effected also makes it possible to control the level of impairment by technological test methods. The formation conditions of intermetallics may vary considerably depending on process thermal cycles resulting in different particle size, amount and compositional gradients. A certain percentage of second phases has an ambiguous correlation to a property (11, 12). Thus, it is hardly possible to judge from the microstructure what amount of intermetallic phase is acceptable for a certain purpose. It is also unrealistic to require a microstructure completely free from precipitates as this is not quantifiable and therefore a requirement difficult to interpret. A paper, recommended for publication by International Institute for Welding (IIW), Commission IX, regarding metallography of weldments, supports this view (13). A concept analogous to ASTM A262 for austenitic stainless steels is used in ASTM A 923; metallographic examination is used as a screening test and an affected microstructure is further evaluated by other methods. In this standard, mill products cannot be rejected by the metallographic examination per se but presence of detrimental intermetallic phases is tested with impact testing or corrosion testing. The standard thus offers different methods to verify that an affected microstructure has no detrimental effect on the material and, depending on the conditions, either method can be used and be reported. This standard should be used only to assure the absence of harmful intermetallic precipitates and not to assess the suitability for any service. For welds the situation is more complex and A923 is less applicable as a test for presence of intermetallic phase, as other microstructure conditions such as non-metallic inclusions, nitrides or segregation effects also could impair the properties. 5

6 Charpy impact testing Toughness criteria for steel are traditionally set to assure that the risk of brittle fracture is avoided. Austenitic stainless steels do not show a brittle transition and in general there are no requirements on toughness testing on parent material. In absence of complete data for duplex stainless steels specifications have been based on those for ferritic structural steels. This is a conservative approach considering the gradual transition behaviour for duplex steel with substantial amounts of the ductile austenite. EN specifies minimum Charpy V impact toughness Number of CVN 60 J at room temperature as shown in Table 2 (transverse direction, CVN 100 J in longitudinal direction). These requirements are reasonable. In the coming EN pressure vessel standard 60 J will be required in the joints. However, this could restrict the possible welding methods. Recent investigations at TWI conclude that a relevant requirement on duplex steels is CVN 40 J at minimum operating temperature (14). Substantial documentation of thick plate confirms that duplex steel and welds having CVN 35 J still show ductile behaviour (15). It is suitable to use a different criterion when the purpose is to verify that the product has undergone correct heat treatment. In ASTM A923, for example, the 2205 parent metal shall pass an impact test (CVN 54 J at 40 C) for acceptance. In national standards there are no or only room temperature toughness requirements on duplex stainless steels (Table 2). For certain application areas, particularly oil and gas industry, low service temperatures have necessitated more stringent specifications. Frequent requirements for parent metal and welds are minimum 45 J average at 46 C for both duplex and superduplex steels. CTOD Crack tip opening displacement (CTOD)-testing is a fracture toughness test developed for carbon steel and its relevance and usefulness for specification of duplex steels is under dispute. Fracture toughness data are necessary for design purposes and should be collected for different steel types and products. Correlation between CTOD and impact energy have been developed for duplex steels (14) and therefore it would be sufficient to specify impact toughness testing as a routine check of the material. Hardness measurement Some pressure vessel standards include a maximum hardness value to verify that the material has been properly heat-treated and has not been subjected to excessive cold deformation. EN has no such requirement and it could be argued if such a requirement will assure any improvement of the steel. There are also hardness requirements for service in environments containing hydrogen sulphide involving a risk of sulphide stress corrosion cracking. This requirement is based on the condition the actual steel was documented according to NACE MR0175 (16). As described below under corrosion testing it should be noted that each steel grade has its individual environmental limits listed in NACE MR0175. NACE maximum requirements are 36 HRC for UNS S31803 and 32 HRC for UNS NORSOK requires for both parent and weld metal 28 HRC for UNS S31803 and 32 HRC for UNS (17). Corrosion tests Corrosion testing is frequently used to rank alloys concerning their resistance in different aqueous solutions. Data are available in corrosion tables or curves and diagrams (18, 19). If an end user wants to have information concerning the performance of a steel in a certain service environment a tailor made test can be performed. Such testing is seldom included in a specification. Corrosion testing is also used to verify that the steel has been properly manufactured to achieve optimum properties. Classical corrosion tests for austenitic stainless steels are the methods to assess the susceptibility to intergranular corrosion. The duplex microstructure possesses a much higher resistance to this form of corrosion and the standard tests such as ASTM A262E will not give any response on duplex steels. PITTING CORROSION TESTS ASTM G48A Duplex steels are more frequently specified to meet requirements regarding pitting corrosion. The most common specified corrosion test is ferric chloride testing, e.g. ASTM G48A, and modified versions. This test is considered to be a check of the metallurgical condition of the steel rather than of its performance in any service environment. This is for example the case of ASTM A923. The ferric chloride solution is very aggressive and resembles very few actual industrial service environments. Specified test temperatures vary considerably and are C for 22Cr duplex and C for 25Cr duplex. Although a 72hour test period is stipulated in ASTM G48A, a 24hour exposure is usually specified. For 22Cr duplex, the experience from service as well as from testing according to various specifications is extensive and shows that a test temperature of C is suitable to ensure that materials properties after welding and heat treatment operations are within the limits expected for 22Cr duplex. For 25Cr duplex, a combination of laboratory test results and service experience, although not as extensive as for 22Cr duplex, strongly indicate that 50 C is a suitable test temperature for solution annealed products. Evaluation criteria vary between test procedures. The most objective and reproducible criterion for approval is a maximum tolerable weight loss, as in e.g. ASTM A923, although it is not a fully relevant quantification of pitting corrosion. STRESS CORROSION CRACKING (SCC) It is well established and experienced in practice that duplex stainless steels have a superior resistance to chlorideinduced SCC compared to conventional austenitic grades. The SCC resistance depends on many environmental and metallurgical factors. There is, however, no simple accelerated laboratory test available today which generates a ranking valid for all chloride 6

7 environments (20). Tests that have been specified in certain projects for duplex steels are thus questionable and are not recommended. Another cracking mechanism of concern for duplex steels is sulphide induced stress corrosion cracking (SSCC). This is of relevance in the oil and gas industry handling hydrocarbons containing hydrogen sulphide (sour environment). The susceptibility to SSCC depends on environmental factors such as hydrogen sulphide partial pressure and steel condition. Several duplex steels are approved for use in sour service and are listed in NACE MR0175. The approvals can be regarded as procedure approvals and are based on SSCC testing in certain environments and in different cold-worked conditions. This is an extensive and costly documentation. Each steel grade is listed with allowed environmental conditions and maximum product hardness accepted. Project specifications may include SSCC testing in cases when the environment is beyond that in NACE MR0175 but this is a very timeconsuming and expensive procedure. DESIGN RULES Temperature limitations Duplex steels are being used more and more in critical components such as pressurised vessels and pipe systems. Naturally, restrictions in environment have to be set for various applications. The nature of the micro-duplex structure limits the use of duplex steels both to low and elevated temperatures. Use of the duplex steels outside the permitted range could involve hazards. The German pressure vessel codes (VdTÜV WB) specify different temperature ranges for parent steels of various duplex grades as shown in Table 4. For welds TÜV specifies allowed maximum temperature 30 C lower than for the parent material due to a higher susceptibility to embrittlement in the weld metal. In ASME and the coming EN standards for pressure vessels the temperature limits are not explicitly listed. However, the designer can conclude from the temperature range that strength requirements are listed what temperature is allowed. The maximum temperature appears to be 250 C for all three grades. This is at large in accordance with the existing TÜV rules for welded constructions. Wall thickness It is well known that the pressure vessel designs using EN or ASME rules differ substantially as the European system is based on yield strength values while ASME rules are based on tensile strength (21). The different concepts are both results of old traditions difficult to change. According to EN the exchange of 316 with 2205 gives a reduction in wall thickness of up to about 35% while ASME rules only permit about 20% thinner wall. A certain vessel in 22Cr duplex designed according to EN will have 40% less wall thickness than designed according to ASME (21). This difference remains with the change in July 1999 of ASME rules using a safety factor against the tensile strength of 3.5 instead of 4. The different rule systems may be one reason why duplex steels have been used more extensively in Europe. For offshore pipelines some oil companies are using own design criteria more based on European codes to take full advantage of the material. Table 6. The important differences between ASTM A790 and A928. ASTM A 790 ASTM 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 Dynamic loads There are limited data in the literature for stainless steels in general to be used in structural codes, particularly for welded joints. The very limited data for duplex stainless steels indicate comparable performance to those of structural steels (22). Recent European work on several joint configurations show that fatigue data of duplex stainless steel fall within the scatterbands enclosing the extensive databases for structural steels. The design S-N curves for welded structural steels are thus applicable also for duplex steels (23). Welded pipe issues ASTM STANDARDS FOR WELDED DUPLEX STAINLESS STEEL PIPE All of the mentioned grades are included in the most common ASTM, ASME and ANSI standards. The two most common ASTM specifications for welded duplex stainless steel pipe are A790 and the new A928 for duplex pipe welded with filler. The important differences between ASTM A790 and A928 are described in Table 6. 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 NPS 1/2 36 ( mm) in Sch 10S and 40S will have problem if ASTM standards are used. A790, an often-used ASTM specification includes both seamless and welded pipe with no addition of filler metal. When A790 is specified for thick walled welded pipe, problems with undercut, incomplete penetration and lack of fusion may exist on thickness above about 6 mm (1/4 inch) if normal welding techniques, like GTAW and PAW are used. When pipes are welded from plate it may be 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 7

8 lack of fusion and an incomplete fill up of the weld. Filler metal also provides the opportunity to improve corrosion resistance through the use of higher alloy filler metals. Principally, there are only advantages in using filler metal on thick walled pipe. An alternative to A790 for duplex heavy wall pipe is ASTM A928. However, as these two specifications have different requirements on use of filler metal, heat treatment and NDT, a change 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. Contractors and package builders seldom have the authority to approve such deviations. A possibility is that the pipe specification should have options to use either seamless or welded, and also options to use either A790 or A928. This will increase the availability without reducing the quality. JOINT EFFICIENCY FACTOR OR WELD FACTOR ASME/ANSI rules allow a joint efficiency factor of 0.8 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%. HYDRO TEST (HT) HT is used both as a tightness test and a mechanical test of the pipe body. There are mainly two common requirements. Either a test pressure according to the ASME or API rules is required, or a pressure able to strain the pipe up to 95% of the Y.S. A pressure test up to 95% of the Y.S. can result in distortion of the pipe that leads to out of tolerance. Therefore the test can produce more problems than it solves. CONCLUSIONS Duplex stainless steels have been commercially available several decades but gained large interest first in the 1980 s and 1990 s. This makes this family of steel less mature than the austenitic family and many different specifications have emerged. The conclusions of this paper can be summarised as follows. The international standards should be used as far as possible. They include requirements on chemistry and mechanical properties. Recommended procedures for heat treatment in standards and specifications are not sufficient to guarantee a good result. Each manufacturer has to develop specific procedures for optimum properties of each product. Specification on manufacturing and fabrication outside the standards should be based on relevant properties of interest for the application. Requirements on microstructure are seldom necessary and if qualification of mill products is requested technological testing of properties is preferred. A pipe specification should have the option to use either seamless or welded pipes for best availability and quality. Duplex stainless steels have a very attractive combination of mechanical properties that makes them suitable as a stainless structural steel. Design data indicate that they so far are used too conservatively. 8

9 REFERENCES 1. Olsson J, Liljas M, 60 years of duplex stainless steel applications, Corrosion 94, NACE, paper No Tystad M, Application of duplex stainless steels in the offshore industry, 5th World Conference on Duplex Stainless Steels, Maastricht, 1997, Book 1, p Egan F, Service experience of superduplex stainless steel in seawater, Stainless Steel World, December, 1997, p Johansson KA, Duplex stainless steels in offshore applications experiences from projects and operations, 4th International Conference, Duplex Stainless Steels 94, Vol.3, ISBN , Glasgow, November, 1994, paper No. KVII 5. Vishnu R, to be published 6. Redmond JD, Davison RM, Critical review of testing methods applied to duplex stainless steels, 5th World Conference on Duplex Stainless Steels, Maastricht, 1997, Book 1, p Kotecki DJ, Ferrite determination in stainless steel welds advances since 1974, Welding Journal, Vol. 76, No.1, ISSN: , 1997, p.24-s 8. Nilsson J-O, Super duplex stainless steels, Materials Science and Technology, Vol.8, August 1992, p Nilsson J-O, The physical metallurgy of duplex stainless steels, 5th World Conference on Duplex Stainless Steels, Maastricht, 1997, Book 1, p Bowden PL, Ward JL, Experiences in welding 25Cr superduplex stainless steel for topsides hydrocarbon piping, Conf. Proc. 25th Annual OTC, May, 1993, Houston, Texas, OTC 7316, p Karlsson L, Duplex stainless steel weld metals effects of secondary phases, 5th World Conference on Duplex Stainless Steels, Maastricht, 1997, Book 1, p Ginn BJ, Gooch TG, Effect of intermetallic content on pitting resistance of ferritic austenitic stainless steels, Proc. Stainless Steel 99, Science and Market, Chia Laguna, Sardinia, ISBN , June 1999, Vol. 3, p van Nassau L, Melker H, Position statement on the specification of metallographic properties of weldments in duplex and superduplex stainless steel, Welding in the World, Vol. 43, No. 2, 1999, p Wiesner, CS, Toughness requirements for duplex and super duplex stainless steels, 5th World Conference on Duplex Stainless Steels, Maastricht, 1997, Book 2, p Deleu E, Dhooge A, Fracture toughness of welded thick walled duplex stainless steels, 5th World Conference on Duplex Stainless Steels, Maastricht, 1997, Book 1, p NACE Standard MR , Standard Material Requirements, Sulfide Stress Cracking Resistant Metallic Materials for Oilfield Equipment, NACE International, ISBN , NORSOK Standard, Norwegian Technology Standards Institution 18. Avesta Sheffield Corrosion Handbook, 8th Edition, Avesta Sheffield AB, ISBN , MTI Publication No.47, Corrosion testing of iron- and nickel-based alloys, Part II, Test Data, Materials Technology Institute of the Chemical Process Industries, Inc., Jargelius-Pettersson RFA, Linder J, Hertzman S, Ranking the resistance of duplex stainless steels to chlorideinduced stress corrosion cracking, 5th World Conference on Duplex Stainless Steels, Maastricht, 1997, Book 2, p Jonson J, Stainless steel design stresses in EN and ASME pressure vessel codes, Proc. Stainless Steel 99, Science and Market, Chia Laguna, Sardinia, ISBN , June 1999, Vol. 3, p Razmjoo G R, Design guidance on fatigue of welded stainless steel joints, Proc. OMAE, Vol. III, Materials Engineering, ASME, 1995, p Maddox S J, Fatigue design of welded stainless steels, EUR ECSC Information Day; Stainless Steels-New Product and Market Developments, 6 October 1998, Seville, Edited by D. Naylor, E Nägile, ISBN , 1999, p.63 This paper was earlier presented at the Duplex America 2000 Conference in Houston in March. Published with the kind permission of Stainless Steel World. 9

10 EDITORIAL Dear Reader, There have been some remarks from alert readers concerning data presented in acom No. 2, Corrosion testing in flash tanks, earlier this year and I agree that some comments are worthwhile. First of all, Table 2, which gives the chemical compositions of grades included in the test. The composition given for 316L is not a 316L composition but rather 2304, i.e. the grade presented on the next line. The composition given for 2304 reflects the content of The consequence is that there is no composition for 316L in the table so we have to assume a typical one, low carbon and around 17Cr-11Ni-2.2Mo. A second remark is about the isocorrosion diagram for 304 and 316 in sodium hydroxide, Figure 8. The 1 mpy curve is rather generous and should preferably not be used for the selection of stainless steels in this type of environment. The isocorrosion diagrams shown in Avesta Sheffield Corrosion Handbook are not only more conservative; they are more realistic as well. I also want to take this opportunity to thank the readers for being observant. That strengthens my opinion that most of you find acom worthwhile reading. Yours faithfully, Jan Olsson Technical Editor 10

11 Although Avesta Sheffield has made every effort to ensure the accuracy of this publication, neither it nor any contributor can accept any legal responsibility whatsoever for errors or omissions or information found to be misleading or any opinions or advice given. 11

12 acom is distributed free of charge to persons actively involved in process industry development and other areas where stainless steels are important. acom appears four times a year, and we welcome applications from all interested parties for additions to our mailing list. Name: Please type or write legibly. Position: Company activity: Company: Mailing address: Postcode/City: Country: Please, add my name to your mailing list I have changed my address as shown above. My previous label is enclosed. acom All rights reserved. Comments and correspondence can be directed to Jan Olsson, Technical Editor, Avesta Sheffield AB, R&D, SE Avesta, Sweden. Tel. +46 (0) Fax +46 (0) Avesta Sheffield AB Research and Development SE Avesta, Sweden Tel. +46 (0) Fax +46 (0) ISSN Teknisk information/centrumtryck Avesta 2000