Technical requirements and quality control for duplex stainless steel materials

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The use of duplex stainless steel is becoming increasingly widespread. To help users better understand, select, process, and apply this type of stainless steel, the International Molybdenum Association has published the latest edition of the \"Practical Guide to the Processing and Manufacturing of Duplex Stainless Steel.\" This guide outlines the various properties of duplex stainless steel and provides basic principles and practical information on processing and welding it, addressing one of the main challenges associated with its use. This article is taken from that publication and covers topics such as the properties and classification of duplex stainless steels, their development history, chemical composition, metallurgy, corrosion resistance, mechanical properties, physical properties, how to establish technical requirements, quality control, cutting, forming, welding, and applications. Due to the large amount of content, it will be published in several parts. Stay tuned! Technical requirements and quality control: Regarding the technical requirements and quality control for the processing and manufacturing of duplex stainless steel, a key practical issue is the preservation of properties after welding. The composition and manufacturing process of duplex stainless steel in its original condition must ensure that it retains good properties after welding (using appropriate welding techniques). 1 Standard test requirements 1.1 Chemical composition ASTM or EN standards are an appropriate starting point for selecting second-generation duplex stainless steels. Adding nitrogen to duplex stainless steel is beneficial, as it prevents an excessive amount of ferrite from forming in the heat-affected zone (HAZ) and also enhances the material’s metallurgical stability. The upper limit of the nitrogen content in duplex stainless steels is determined by the solubility of nitrogen in molten steel, which can be reflected by the maximum value within the nitrogen content range specified in the standards. However, the minimum nitrogen content listed does not necessarily represent the amount of nitrogen required to achieve optimal welding characteristics. For example, the original standard composition of 2205 duplex stainless steel was S31803. At the lower limit of the allowable nitrogen content range for S31803, which is 0.08% to 0.20%, the properties of 2205 after heat treatment and welding are not stable and consistent. Practical experience shows that for the welding of 2205 duplex stainless steel, a \"nitrogen content of at least 0.14%\" is necessary. Since this requirement often has to be specified, and to facilitate end-users who need welding, the S32205 version of 2205 was introduced into the duplex stainless steel standards. Super duplex stainless steels also have a higher range of nitrogen content, which reflects the recognition of the importance of nitrogen content. The technical requirements for duplex stainless steel in some end users are based on the “PREN” relationship. Although the PREN value is effective for comparing the corrosion resistance of different grades within a given steel type, altering the composition to achieve a specific PREN value does not necessarily result in an appropriate metallurgical balance. The PREN value helps in selecting one from a series of grades. Based solely on changes in composition, it might seem that chromium and molybdenum can be replaced by nitrogen in terms of pitting resistance; however, from a metallurgical perspective, chromium and molybdenum promote the formation of ferritic phases and intermetallic phases, while nitrogen promotes the formation of austenitic phases and prevents the formation of intermetallic phases. Therefore, it is best to select the composition of duplex stainless steel based on the standard grades specified in the relevant standards, with the nitrogen content for each grade possibly being set at the upper limit of the standard range. Regardless of how the material composition is specified, it should be the same as the material used in the welding procedure qualification, so that the qualification is meaningful for the desired manufacturing results. Interior of the cargo hold in a marine chemical tanker made of 2205 duplex stainless steel @Outokumpu: 1.2 solution annealing and quenching. In addition to the chemical composition, the actual annealing conditions of the rolled material are also important for welding. The purpose of annealing austenitic stainless steel is to recrystallize the metal and dissolve carbides. “Low-carbon stainless steel of grade L can be quenched in water or cooled more slowly in air, as it takes a long time for the formation of harmful carbides to occur. However, for duplex stainless steel, even with an ideal nitrogen content, staying within the critical temperature range for a few minutes has an adverse effect on its corrosion resistance and toughness. When the rolled material cools slowly in the rolling mill, it spends a certain amount of time at temperatures ranging from 700 to 980°C (1300 to 1800°F); it is no longer possible for it to be exposed to further heat in this temperature range, such as during welding. As a result, there is less time available for welders to create welds with a heat-affected zone (HAZ) that contains no intermetallic phases. Although standards such as ASTM allow certain duplex stainless steels to be \"quenched in water or cooled rapidly by other methods,\" the optimal metallurgical conditions for welding are achieved by quenching them as quickly as possible from the annealed temperature. However, this ignores the deformation and increase in residual stress caused by water quenching. For thin sheet production, air cooling is efficient in modern coil rolling lines ； However, for thick plates and products with thicker cross-sections, water quenching can achieve the optimal metallurgical conditions for welding. Cooling the sheet or component to 700–980°C (1300–1800°F) before quenching leads to the formation of intermetallic phases. Another method to ensure an optimal initial condition is to require inspection of the rolled material to ensure that it is free from harmful intermetallic phases. ASTM A 923 and ASTM A 1084 use metallographic examination, impact testing, or corrosion testing to prove that the intermetallic phases have not reached a harmful level. This test only considers whether harmful precipitate phases have appeared, without taking into account the quantity and degree of these harmful phases. By using this test, it can be demonstrated that the rolling process ensures that no harmful intermetallic phases are formed during rolling. This test is similar to the ASTM A 262 or EN ISO 3651-2 tests for determining whether austenitic stainless steels are sensitized due to the precipitation of chromium carbide. Standard ASTM A 923 includes only 2205, 2507, 255, and S32520, while standard ASTM A 1084 covers the cost-effective duplex stainless steels S32101 and S32304. Many processing manufacturers use these tests and similar tests, along with other acceptance criteria, as part of the weld qualification process for processing equipment components. ASTM A 923: Standard test method for detecting harmful intermetallic phases in austenitic/ferritic duplex stainless steels. ASTM A1084: Standard test method for detecting harmful phases in cost-effective austenitic/ferritic duplex stainless steels. ASTM A 262: Standard method for testing the intergranular corrosion susceptibility of austenitic stainless steels. EN ISO3651-2 – Determination of the intergranular corrosion resistance of stainless steels – Part 2: Ferritic, austenitic and ferritic-austenitic (duplex) stainless steels – Corrosion test in sulfuric acid medium 2 Special test requirements 2.1 Tensile and hardness tests Duplex stainless steels possess high strength compared to austenitic stainless steels; however, in some cases, end-user technical specifications specify maximum values for strength or hardness. The introduction of a maximum value for strength or hardness may be based on the experience with martensitic stainless steels, whose high strength and hardness stem from the untempered martensite. However, duplex stainless steel does not form martensite during cooling. Its high strength and hardness are attributed to its high nitrogen content, its dual-phase structure, and the work hardening that may occur during forming or straightening operations. Hardness testing is an effective method to prove that there was no excessive cold working during processing ； However, when a hardness test is used for this purpose, it is important that the measurement point be located between the surface and the center of the cross-section, rather than on the surface where local hardening or surface hardening has occurred. 2.2 Bending test The bending test can verify that the rolled material is free of rolling cracks, but it may be difficult for large pieces, small workpieces, or rolled materials with certain geometries. The bending test is not a conservative indicator of the quality of duplex stainless steel, as the bending point may not correspond to the defective area. Due to the directionality of bending, certain situations such as intermetallic phases on the center line cannot be detected. Bending tests are commonly conducted as part of the weld procedure qualification for austenitic stainless steels, because there is a risk of thermal cracking in the welds, especially in cases of severely restricted weld microstructures with high austenite content. Since duplex stainless steel does not have a tendency to develop thermal cracks, it is not very meaningful to use bending tests to evaluate weld quality. If the testing location coincides exactly with the affected area, the bending test may roughly detect an excess of ferrite, but it cannot detect the presence of small amounts of intermetallic phases that are harmful to corrosion resistance and toughness. 2507 stainless steel falling film evaporator © Gary Carinci TMR Stainless 2.3 Impact testing and metallographic examination of intermetallic phases There are two ways to use impact testing to define materials and evaluate processes: Tests are conducted under known conditions to identify materials that fail the inspection, such as those with an excessive amount of ferrite or intermetallic phases ； Prove that, for the intended operating environment, the performance of the manufactured product is sufficient to meet the requirements. For the first application of impact testing, ASTM A 923 specifies the acceptance criteria for duplex stainless steels and super duplex stainless steels, while ASTM A 1084 provides the acceptance criteria for economical duplex stainless steels. Method B of ASTM A 923 specifies that in the standard longitudinal Charpy impact test at –40°F/–40°C, if the result is less than 54 J (40 ft-lb), it indicates that the annealed rolled product is unsatisfactory. To ensure satisfactory heat treatment and quenching, as a means of production control, each batch of rolled products should be tested according to Method B of A 923 (or Method C, the corrosion test). ASTM A 923 allows metallographic examination (Method A) to be used as a screening test for the purpose of acceptance rather than rejection. Since Method A requires a high level of metallographic skills to be carried out, it is prudent for users to request the Charpy impact test using Method B in addition to the metallographic inspection. One advantage of ASTM A 923 Method A is that it allows for the identification of intermetallic phases in the centerline, as shown in Figure 7 of ASTM A 923. According to the screening method of Method A, intermetallic phases in the grain boundary will render the material unqualified, but this does not necessarily result in the material being rejected in the impact test of Method B for A 923. Since this type of central line intermetallic phase can cause delamination in medium and thick plates during forming, hot rolling, or welding, in addition to methods B or C, users should also request testing using method A; any material showing such central line intermetallic phase should be rejected. Although ASTM A 923 states that Method A is not intended for discarding materials, end-users are allowed to impose stricter requirements. Materials that exhibit intermetallic phases along the centerline near the middle thickness, as shown in Figure 7 of ASTM A 923, should be rejected. The second method for impact testing involves evaluating the base metal, weld zone, and heat-affected zone at temperatures lower than those expected in actual use, which may be a cautious and cost-effective approach. For the evaluation of welds, the test temperature and acceptance criteria must be determined for the specific type of welding and in conjunction with the service conditions. The toughness will not be as high as that of the duplex stainless steel rolled products subjected to solution annealing. Lower toughness of the weld metal does not necessarily indicate the presence of intermetallic phases; it is often caused by an increased oxygen content, especially in welding processes that use flux protection. ASME has issued new technical requirements for duplex stainless steels with a cross-sectional thickness greater than 9.5 millimeters (0.375 inches). These requirements specify that Charpy impact tests be conducted at or below the Metal Minimum Design Temperature (MDMT), with transverse expansion used as the acceptance criterion to prove that the toughness of the base material and the product welds is sufficient to meet the expected operating conditions. The difference between ASME testing and ASTM A 923 testing is that ASME testing requires the Charpy impact test to use three specimens, and it mandates that both the minimum value and the average value be reported. ASME requires testing of each batch of base material and filler material on the base material, weld, and HAZ (a total of 9 specimens). To achieve careful results while saving resources, the lower of the two test temperatures (–40°C/F per ASTM A 923 or the Minimum Design Metal Temperature MDMT per ASME standards) can be used, with the toughness being evaluated by measuring the impact energy and lateral expansion of three specimens. Installation of duplex stainless steel rebar on the bridge deck © Hardesty& Hanover, LLP 2.4 Determining phase equilibrium by metallographic or magnetic methods The austenite-ferrite phase equilibrium in duplex stainless steel rolled products varies little between different furnace batches or lots, as the chemical composition is kept within a very narrow range during production, and the annealing procedures are strictly specified. Generally, 2205 duplex steel contains 40% to 50% ferrite; therefore, determining the phase equilibrium of annealed rolled materials is of little significance. However, measuring the ferrite content is appropriate for evaluating welding processes, as it helps to prevent an excessive amount of ferrite from forming in the heat-affected zone. The accurate determination of phase equilibrium in duplex stainless steels generally requires metallographic examination and point-counting methods, such as the ASTM E 562 (manual) or E1245 (automatic) methods. Since duplex stainless steels are ferromagnetic and the spacing between austenite and ferrite is extremely fine, the reliability of magnetic testing methods is limited without reference standards consisting of specimens with the same geometry and metallographic measurement results showing phase equilibrium. AWS A4.2 and EN ISO 8249 describe the method steps for calibrating magnetic measuring instruments to measure ferrite at the welds of duplex stainless steel, with the results reported as the ferrite content FN. The acceptable range for phase equilibrium in the weld is much wider than that of the base metal. When the ferrite content is between 25% and 75%, duplex stainless steel can achieve ideal properties provided that ASTM A 923 tests show that the weld and heat-affected zone meet the requirements in terms of toughness and corrosion resistance. Magnetic measurements indicate that a ferrite content between 30 and 90 is considered acceptable. It is more costly to require phase equilibrium determination for materials that are already in the machining center or with distributors, compared to requiring the same for materials that are currently being produced in the factory. Obtaining samples and conducting separate tests also affects timely delivery. Since the intermetallic phases are non-magnetic, magnetic tests cannot be used to detect the s-phase and c-phase. However, if the duplex stainless steel shows a lower ferrite magnetic reading, it is possible that the ferrite has transformed into intermetallic phases. During heat treatment or cooling, if duplex stainless steel remains in the temperature range where intermetallic phases precipitate for too long, it may exhibit a low ferrite content. Super duplex stainless steel bolts are being installed to protect the ancient Swedish ship Vasa © Anneli Karlsson, Swedish Maritime Museum. 2.5 Corrosion testing – Conducting corrosion tests on solution-treated rolled materials according to ASTM A 923/A 1084 Method C is one of the most cost-effective ways to detect harmful conditions. The presence of intermetallic phases and chromium nitride, as well as excess ferrite, can all lead to a decrease in pitting resistance and thus can be detected. These phases result in a critical pitting temperature (CPT) that is 15°C or more lower than that of normally annealed materials. Determining the actual CPT cost of a sample is expensive, as it requires multiple tests in accordance with ASTM G 48, or a single test on the sample per ASTM G 150. At temperatures 10–15°C lower than the typical CPT of duplex stainless steels, the presence of harmful phases will be detected through a single corrosion test using the ASTM A923 method C. When corrosion testing is used to detect the presence of harmful phases, any pitting on the surface, sides, or edges should be considered as a reason for rejection. Although the edges may not be in contact with the medium in actual devices, the purpose of the test is to detect intermetallic phases, which are more likely to appear on the center line; therefore, the center line is evaluated by taking into account corrosion at the edges. Before the issuance of ASTM A 923, corrosion tests were typically based on the \"modified ASTM G48 test,\" but G48 is a laboratory testing method rather than a method for material acceptance. If a G48 test is required, but it is not specified which G48 procedure to use, nor are other test conditions indicated such as surface preparation, test temperature, test duration, whether edge corrosion is to be considered, and the definition of acceptance criteria, then such a requirement is incomplete. ASTM A 923 is an acceptance test designed to prove, in an economical and relatively fast manner, that rolled materials are free of harmful intermetallic phases. Method C of ASTM A 923 uses corrosion rate as the acceptance criterion. It may seem surprising to use this method for detecting pitting, but it is adopted for two reasons: 1. By assessing conformity based on weight loss, there is no need to deal with the complicated and potentially subjective issue of determining whether there are pits on the metal surface. The weight loss that meets the rejection criteria is large enough to be easily measured, yet it is small enough to allow for the easy detection of pitting types related to intermetallic phases within a 24-hour test. 2. Using the corrosion rate as a criterion, as long as the total surface area can be determined, samples of almost any size and shape can be tested. The corrosion test is a relatively cautious and conservative method, and it is not sensitive to the geometry and position of the specimen. In contrast, the Charpy test is susceptible to the direction and location of the notch, while the corrosion test is suitable as part of weld procedure qualification; it can also be used as an economical and effective quality control testing method for inspecting weld samples in product manufacturing. However, it is necessary to take into account the differences in corrosion resistance between annealed rolled products and welded joints; even a well-welded joint may have a critical pitting temperature (CPT) that is 5–15°C lower than that of the base metal, due to variations in welding techniques, shielding gases, and the grade of duplex stainless steel being welded. 2.6 Welding Production and Inspection: The problems that may occur with duplex stainless steel are not obvious to welders, nor can they be detected using non-destructive testing methods. Welders must understand that the overall quality of a weld depends on whether the welding procedure specifications are strictly followed, and it is measured by its toughness and corrosion resistance in service. Deviations from the specified process procedures may not be detected in the workshop, but every deviation represents a risk to the safe and economical use of the materials.