Qualification Testing of Lean Duplex Stainless Steels for the Process Industry

A CORROSION MANAGEMENT AND APPLICATIONS ENGINEERING MAGAZINE FROM OUTOKUMPU 3/214 Qualification Testing of Lean Duplex Stainless Steels for the Process Industry

3/214 2 Qualification Testing of Lean Duplex Stainless Steels for the Process Industry Rachel Pettersson, Carolina Canderyd, Jan Y. Jonsson, Avesta Research Centre, Outokumpu Stainless AB, Avesta, Sweden, Poul-Erik Arnvig, Outokumpu Stainless North America, Schaumberg, IL, USA. Abstract The materials specifier needs accepted methods for qualification or assurance testing to verify that supplied materials have acceptable properties. For duplex stainless steels the ASTM A923 standard has found extensive use as a qualification test for the duplex UNS S3225 and superduplex S3275 grades, where the primary cause for concern is the precipitation of intermetallic phases. However, lean duplex grades such as UNS S3211 or UNS S3234 present more of a challenge because these steels are much less sensitive to intermetallic phase precipitation than the higher alloyed duplex grades. The small microstructural changes associated with improper heat treatment are challenging to detect and impact toughness acceptance limits need definition. The ferric chloride immersion test used in ASTM A923 results in sub-ambient temperatures for lean duplex grades and is therefore in many instances impractical to use. The approach explored in the present work is the use of an inhibited 5% ferric chloride solution containing additions of 1% sodium nitrate. This leads to critical temperatures for the onset of pitting which are around or above ambient. Results are presented showing the influence of the level of nitrate additions on the corrosion performance and also correlated to impact toughness data. Application to various product forms is discussed, together with proposed acceptance criteria. Finally, a limited laboratory intercomparison of the test method is presented. These results represent steps towards development of a new variant of ASTM A923 for lean duplex stainless steels, which is aimed to remove any remaining uncertainty when specifying such grades. Key words: stainless, testing, corrosion, sensitization, lean duplex, qualification, acceptance Introduction Qualification or acceptance testing is often required by the materials purchaser in order to be able to verify that delivered material is in an acceptable condition and will fulfill the property requirements placed on the grade. The ASTM (1) A923 standard is an established test for the standard duplex grade UNS S3225 and the superduplex S3275. Both these grades are sensitive to the precipitation of intermetallic phases, typically in temperature range 8 1 C, Figure 1. The ASTM A923 test describes three methods. Method A involves electrolytic etching in sodium hydroxide solution to reveal microstructural changes. Intermetallic phases such as sigma phase are usually seen as discrete particles at grain and phase boundaries and can easily be detected using the required magnification of 4 5x. According to the standard this type of screening test can be used to pass material which has an acceptable, unaffected structure, but if the microstructure appears affected or possibly affected this must be followed up by testing according methods B or C. Method B involves Charpy-V impact testing at -4 C (-4 F) while Method C involves immersion testing in 6% ferric chloride solution, with the requirement that the weight loss at the specified temperatures should not exceed 1 mdd (mg/dm 2 /day) measured in a 24 hour test. In practice, these two readily quantifiable tests are often used in preference to the metallographic screening. Temperature ( C) 11 1 9 8 7 6 S3275 S3225 S3211 S3234 5 4 3.1 (36 s).1 (6 min) 1 1 1 Time (h) (1) ASTM International, 1 Barr Harbor Drive, PO Box C7, West Conshohocken, PA, 19428-2959, USA. Figure 1 TTT diagram shown here as the times to obtain 5% reduction in impact toughness. [2]

3/214 3 The lean duplex grades such as UNS S3211 and UNS S3234 show a very different type of behavior if they are subject to inappropriate heat treatments. Most rapid precipitation of secondary phases occurs at lower temperatures, typically 6 18 C, as seen in Figure 1. This is primarily associated with the precipitation of chromium nitrides and carbides. [2] Intermetallic phases appear first after much longer ageing times, typically in excess of 1 hours at the most sensitive temperature, and are thus very unlikely to be an issue. The methodology described in ASTM A923 is unfortunately difficult to apply to lean duplex grades for a number of reasons. Firstly, the sodium hydroxide etching in Method A is not suitable for revealing the fine phase boundary precipitates which are involved, even though this can be done by an experienced microscopist working at fairly high magnification. The impact toughness in Method B is not particularly sensitive for lean duplex grades at -4 C [3] while the ferric chloride testing in Method C results in sub-ambient temperatures which are impractical to work with in many laboratories. The approach addressed in this work is the use of an inhibited ferric chloride solution containing additions of sodium nitrate. The sodium nitrate acts as an inhibitor, and increases the critical temperatures for the onset of pitting to around or above ambient, thus providing the possibility for a simple and practical acceptance test. This concept has been addressed in a number of recent works on lean duplex grades, [3, 4, 5] and was also discussed many years ago within the ASTM G48 working groups, for lower alloyed austenitic grades.[6] The aim of the present work is to apply the inhibited ferric chloride test to a range of product forms in the two lean duplex grades UNS S3211 and UNS S3234 order to examine its validity and usefulness. Materials and Experimental Procedures Materials The investigated materials were UNS S3211 (EN 1.4162, Outokumpu LDX 211 (2) ) and UNS S3234 (EN 1.4362, Outokumpu 234 (2) ), with the typical compositions given in Table 1. Various product forms were investigated as specified in Table 2. These were investigated in the mill annealed condition, and also after a number of laboratory heat treatments designed to simulate the variation in annealing conditions which are likely to be encountered in different mills. In addition, sensitizing heat treatments were performed at 7 C, which is the temperature of most rapid precipitation of secondary phases according to Figure 1. The sensitizing times ranged from 1 minute to 3 hours and are the holding times at the respective temperature, as detailed in previous work. [3] Product Steel grade and dimension (mm) UNS S3211 UNS S3234 Plate 12, 3 2, 3 Sheet.5 1.5 1.5 6 Tube Ø27x2 Ø27x1.5 Rebar/bar Ø16 25 Table 2 Tested products from the two grades. UNS EN C N Cr Ni Mo Mn S3211 1.4162.3.22 21.5 1.5.3 5 S3234 1.4362.2.1 23 4.8.3 1.5 Table 1 Nominal compositions of the grades investigated. Inhibited Ferric Chloride Tests An inhibited test solution comprising 5 wt% FeCl 3 + 1 wt% NaNO 3 was used as described previously. [3, 5] Samples were cut to approximate dimensions of 5 x 25 mm. All surfaces were dry ground to a 12 grit finish as used in ASTM A923, followed by an acetone rinse. The exceptions were some tube, rebar and welded samples which were pickled, rather than being ground, in order to retain the original surface which is typical for the products in question. According to ASTM A923 testing of a specimen in the as-fabricated condition, rather than undergoing the prescribed grinding to 12 grit, is permitted if this is relevant to the application. All samples were weighed to the nearest.1 g and the dimensions measured. The majority of testing was performed at the Avesta Research Centre laboratories (denoted Lab A); while Lab D and Lab N participated in the inter-laboratory comparison. Testing was performed in glass beakers with a minimum of 6 ml of test solution (15 ml at Lab N). The test temperature was controlled to ± 1 C. Each test involved immersion for 24 hours, after which the exposed samples were cleaned, rinsed and dried. Samples were then re-weighed and the corrosion rate calculated in units of mg/dm 2 /day (mdd). Each condition was tested at various temperatures to determine the pitting temperature as a function of the ageing time. A corrosion rate greater than 1 mdd was used as a threshold for unacceptable performance, which is the same acceptance criterion used in the ASTM A923 Method C. The critical temperature was defined as the highest temperature at which the corrosion rate was below this limit. In order to assess the electrochemical behavior of lean duplex grades in nitrate-inhibited chloride solutions, polarization curves were also obtained at a scan rate of 2 mv/minute in 5.5 wt.% NaCl solutions which were acidified to ph 1.3 and modified by various additions of NaNO 3. (2) Trade name.

3/214 4 qwertyuiopå äölkjh Breakdown potential (mv SCE ) Current density (µa/cm 2 ) Critical pitting temperature ( C) 1.E +4 1.E +3 1.E +2 1.E +1 1.E + 45 C, 5.5% NaCl 1.5% NaNO 3 8 C, 5.5 NaCl 1.E -1-1 -5 5 1 12 1 8 6 4 2 5.5% NaCl Potential (mv SCE ) 1.1% NaCl.3% NaNO 3 5.5% NaCl 2% NaNO 3 5.5% NaCl.5% NaNO 3 1 2 3 4 5 6 7 8 8 7 6 5 4 3 2 1 Temperature ( C) 1.1% NaCl 5.5% NaCl..2.4.6 NO 3 /Cl ratio 3.3% NaCl Figure 2 Use of polarization curves in acidified NaCl + NaNO 3 to evaluate critical temperatures for the transition between transpassive corrosion and pitting for 3 mm sheet of UNS S3211. RESULTS Electrochemical tests in nitrate inhibited solutions Polarization curves in acidified sodium chloride with and without additions of sodium nitrate are shown in Figure 2 and demonstrate the presence of a slight tendency to an active corrosion peak at ~-35mV SCE, which is depressed by nitrate additions. There is also an earlier onset of transpassive corrosion, at ~9 mv SCE in the presence of nitrate. If the breakdown potentials (defined as the potential at which the anodic current density exceeds 1 µa/cm 2 ) are plotted as a function of temperature it is seen that there is a sharp transition between the transpassive behavior at lower temperatures and the pitting behavior, typically at potentials of <3 mv SCE, at higher temperatures. This transition is denoted the critical pitting temperature (CPT) and plotted as a function of the NO3-/Cl- in the third diagram in Figure 2. The CPT shows a virtually linear increase as the level of nitrate is increased in the range.5 2%. Use of a more dilute solution of 1.1% NaCl +.3% NaNO 3 (i.e. with concentrations which were employed in [4] gives somewhat higher CPT, as is to be expected. The open circuit potential measured in the inhibited ferric chloride test (5 wt% FeCl + 1 wt% NaNO ) is close to 7 mv SCE, and it is seen from Figure 2 that this potential lies in the middle of the transition region and is thus well within the region in which the critical pitting temperatures is largely potential-independent [7]. Immersion testing in inhibited ferric chloride In Figure 3 the results of inhibited ferric chloride tests on a range of products of UNS S3211 and UNS S3234 are shown. The solid points represent temperatures at which the weight loss is >1 mdd, thus corresponding to a fail if the same acceptance criteria are applied as in ASTM A923. The open points represent a pass. The normal procedure used here is to test two specimens, but for clarity only a single point is given at each temperature and a pass given only if both specimens have a weight loss of 1 mdd or below. For UNS S3211 the thicker (3 mm) plate materials all pass the test at 3 C, although there are some cases in which the critical temperature may be higher. The 1.5 mm sheet material has an even more homogenous structure as a result of cold rolling and this give a slightly higher critical temperature of >4 C. After a holding time of 5 minutes at 7 C a drop in the critical temperature is discerned, with slightly more rapid sensitization being seen for the thinner 1.5 mm sheet material. The trends for UNS S3234 were somewhat less pronounced. The annealed states showed critical temperature ranging from 45 C for the 3 mm plate to 3 C for the 2 mm plate and 1.5 mm sheet. There was little effect of shorter sensitization times, but a clear drop was seen after >1 hour at 7 C. This is in agreement with data reported in the literature that for a significant level of sensitization of the UNS S3234 grade, sensitization times longer than 1 hours are usually needed.[8] In contrast to UNS S3211, the investigated sheet material showed slightly more rapid sensitization than the 3 mm plate, while the 2 mm plate showed very little effect. The challenge in trying to define an acceptance criterion for a qualification test is that all product forms must pass the test when in a correct mill-finished condition but at the same time fail if the material has been sensitized to a detrimental extent. The annealing temperature and cooling rate affect the phase ratio and also the partitioning of alloying elements between the phases, thus the corrosion resistance. In addition thick plate has a much coarser structure than cold rolled sheet and may show a great

3/214 5 degree of segregation, while bar material, and particularly rebar, may have undergone a process annealing step rather than a separate solution annealing stage. Corrosion results from all the product forms tested for UNS S3211 and UNS S3234 are shown in Figure 4 and indicate that for both grades an acceptance temperature of 2 or 25 C fills the specified requirements. For UNS S3211, the data presented in previous work indicated that good welds in 12 3 mm plate show a pass temperature in the range 35 45 C and can thus be treated in the same way as the base material. [3] For UNS S3234 even longer sensitization temperatures than 1 hours are really needed to see a clear degradation in corrosion properties, but it can be argued that this is hardly realistically likely to be encountered so this material can be regarded as fairly resistant to structural degradation caused by inappropriate heat treatment. 7 168 F 6 14 F S3211, 3 mm mdd<1 mdd>1 7 6 S3211, 12 mm mdd<1 mdd>1 7 6 S3211, 1.5 mm mdd<1 mdd>1 5 122 F 4 14 F 3 86 F 2 68 F 5 4 3 2 5 4 3 2 1 5 F 1 1 Annealed.1 1. 1. 1. Time at 7 C/1292 F (hours) Annealed.1 1. 1. 1. Time at 7 C/1292 F (hours) Annealed.1 1. 1. 1. Time at 7 C/1292 F (hours) 7 168 F 6 14 F S3234, 3 mm mdd<1 mdd>1 7 6 S3234, 12 mm mdd<1 mdd>1 7 6 S3234, 1.5 mm mdd<1 mdd>1 5 122 F 4 14 F 3 86 F 2 68 F 5 4 3 2 5 4 3 2 1 5 F 1 1 Annealed.1 1. 1. 1. Time at 7 C/1292 F (hours) Annealed.1 1. 1. 1. Time at 7 C/1292 F (hours) Annealed.1 1. 1. 1. Time at 7 C/1292 F (hours) Figure 3 Results of inhibited ferric chloride corrosion testing for different product forms of UNS S3211 (upper three diagrams) and UNS S3234 (lower three). Each set of vertical points to the left hand side of the diagrams represents a different variant of annealing. Test temoerature ( C) 7 168 F 6 14 F 5 122 F 4 14 F 3 86 F 2 68 F S3211 3 mm plate 12 mm plate 1.5 mm sheet 27x2 mm tube Bar/rebar Test temoerature ( C) 7 6 5 4 3 2 S3234 3 mm plate 2 mm plate 6 mm plate 1.5 mm sheet 1 5 F 1 Annealed.1 1. 1. 1. Time at 7 C/1292 F (hours) Annealed.1 1. 1. 1. Time at 7 C/1292 F (hours) Figure 4 Compilation of corrosion data for various product forms of UNS S3211 and UNS S3234. For annealed states the critical temperatures are arranged in descending order for clarity. An acceptance limit of 2 25 C appears appropriate.

3/214 6 Effect of test variables Any acceptance test needs to be examined in order to evaluate whether test variables have such significance that they need to be tightly controlled or whether a certain amount of variation can be permitted. Figure 5 shows the application of such sensitivity analysis to various variants of UNS S3211. Two effects appear very clearly: the starting temperature and the presence of weld oxides. If the specimen is placed directly into the preheated test solution, as is specified for the ferric chloride testing of duplex and superduplex grades in ASTM A923, then the critical temperature is lower than if the specimen is placed in a room temperature solution and then heated up to the test temperature. In the case shown, the latter leads to a pass at 6 C, while failures are seen at 35 C for the correct procedure. Allowing oxides, from annealing or welding, to remain on the specimen surface also had a critical influence. The presence of weld oxides caused failures due to the weight loss exceeding 1mdd for 6 mm UNS S3211, at temperatures as low as 2 C. However, there were no pits seen on these specimens, so it is apparent that the measured weight loss is related to the dissolution of the weld oxides in the ferric chloride solution rather than to actual corrosion attack. Not until 45 C was pitting attack observed. If the specimens were instead thoroughly cleaned, using sand blasting followed by laboratory pickling, failure was seen only at 55 C or higher. Tube material was tested both in the as-fabricated condition, as is permitted in ASTM A923, and after dry surface grinding to 12 mesh finish, which is required for mill products. The effect was minor to negligible, with failure in both cases occurring at 55 C. Likewise use of a newly-prepared test solution, or one which had been made up from a stock test solution prepared a week before, had only minor impact when applied to 12 mm UNS S3211. Cleaning specimens with acetone or a magnesium oxide paste had no significant effect on the weight loss when tested above the critical temperature Finally, two different test solutions were compared: the 5% FeCl 3 + 1% NaNO 3 which forms the basis of this work, and the 1% FeCl 3 +.3% NaNO 3 which has been investigated in other work. [4] It can be recalled from Figure 2 that a more dilute NaCl+NaNO 3 solution gave rise to a electrochemically evaluated critical pitting temperature which was some 1 C higher. However, the immersion testing actually yielded the same critical temperatures. This may reflect the lower degree of precision typically associated with immersion testing, but merits more extensive investigation. 7 S3211, 3 mm 7 S3211, 6 mm 7 S3211, 27x2 mm tube 6 6 6 5 4 3 2 1 Start at Start test temp. at RT mdd<1 pass mdd>1 fail 5 4 3 2 1 mdd 6.2 12. As Sand blast welded + pickled mdd<1 pass mdd>1 fail Pits 5 4 3 2 1 As rec. 12# mdd<1 pass mdd>1 fail 7 6 5 4 3 2 1 S3211, 12 mm Start at Start test temp. at RT mdd<1 pass mdd>1 fail 7 6 5 4 3 2 1 S3211, 12 mm 1682 3438 Acetone MgO mdd 8986 3768 7 6 5 4 3 2 1 S3211, 12 mm mdd<1 pass mdd>1 fail 5% FeCl 3 1% FeCl 3 +1% NaNO 3 +.3% NaNO 3 Figure 5 Effect of test variables on results from the inhibited ferric chloride test. Each marked point represents a single sample.

3/214 7 Inter-laboratory comparison A second aspect necessary to establish the robustness of a test method is to verify that the same, or sufficiently similar, results can be achieved at different laboratories. Three UNS S3211 materials were selected for a small Round Robin: 12 mm plate which had been laboratory annealed, a variant which had been sensitized for 5 minutes at 7 C and 27x2 mm tube material with both an as-received and 12 mesh ground surface. The results in Figure 6 show excellent agreement in the two latter cases even though there were some differences in test procedure. For example Lab A used the prescribed solution volume of 6 ml while Lab N used a smaller amount of 15 ml. The only discrepancy observed was for the laboratory annealed material and this was the subject of intense investigation, as witnessed by the large number of test points in the diagram. At Lab A the material passed at 4 C but showed some failures at 45 C, while at Lab N there were passes up to 6 C with only a single failure among sixteen tests conducted at 5 C. Lab D showed results intermediate between these two extremes. An exhaustive search of reasons behind this difference yielded only one tentative explanation: that it could be related to the presence of some remaining surface oxide or underlying depletion. As seen in Figure 5 oxides can have a large effect on the evaluated critical temperature, and it was found that Lab N removed much more material by grinding than did Lab A. There is thus a risk that there may be some surface effects remaining in the Lab A tests. 7 S3211, 12 mm. Lab annealed 7 S3211, 12 mm. Sense 7 C/5 min 7 S3211, 27x2 mm tube 6 5 4 3 2 mdd<1 pass mdd>1 fail 6 5 4 3 2 6 5 4 3 2 1 1 1 As-received 12# ground Lab A Lab D Lab N Lab A Lab N Lab A Lab N Lab A Lab N Figure 6 Results of inter-laboratory comparisons show some difference for solution annealed material (left) but good agreement for sensitized material and tube. Comparison with other corrosion testing methods As mentioned in the Introduction the use of ASTM A923 Method C for lean duplex grades results in impractically low testing temperatures, or at least the requirement of cooling baths which are not standard in many laboratories. The same applies to immersion testing in ferric chloride solution according to ASTM G48. [9] However, a limited comparison with the latter is included in Table 3 in order to put the present results into perspective. It is seen that the critical temperature according to ASTM G48E is 5 1 C, which is actually on the low side compared to normal values for the grade. [11] After sensitization for 5 minutes at 7 C pitting occurs at C. A comparison with results from electrochemical testing in 1M NaCl according to ASTM G15 [1] is also included in Table 3 and shows the same trend of a clear drop as the result of sensitization. The data for UNS S3234 indicates that the electrochemical testing even seems to be more sensitive to inappropriate heat treatment than immersion testing. However, this type of testing is impractical for qualification and acceptance purposes since the necessary equipment and electrochemical know-how are not so extensively available. Grade Condition 5% FeCl 3 + 1% NaNO 3 Max pass temp. (mdd < 1) 6% FeCl 3 +1% HCl ASTM G48 E 1M NaCl ASTM G15 Normal values 15 C 15 2 C UNS S3211 (12 mm) Lab annealed 4 C 5 C (visible pit) 1 C (mdd>1) 15.4 C 5 min 7 C 25 C C 9.6 C Normal values 11 2 25 C UNS S3234 (2 mm) Annealed 3 C 27 C 1 h, 7 C 25 C 6 C Table 3 Comparison between different corrosion test methods. 1 h, 7 C 25 C 2.8 C

3/214 8 Comparison to metallographic evaluation and impact toughness An extensive evaluation of the metallography of sensitization in the lean duplex grades UNS S3211 and UNS S3234 has been presented recently.[3] The 4% sodium hydroxide (NaOH) etchant specified in ASTM A923 was used, with an applied voltage of 2 V for 1 seconds (the specification in the standard is 1 3V for 5 6 seconds) and it was found that this did give some indication of detrimental phase boundary precipitates, but that such identification was challenging, as seen in Figure 7. An alternative method of electrolytic etching in 1% oxalic acid (C 2 H 2 O 4 ) at room temperature and 6 7 V for 1 seconds was found to be much more revealing of the fine carbides in the phase boundaries and nitrides within the ferrite phase, and to give a good first indication of sensitization. This etchant is therefore recommended as a preferable equivalent to ASTM A923 Method A for lean duplex grades. It should, however, be pointed out that this metallographic evaluation can be more of a challenge for thinner gauge materials and that very low but non-detrimental levels of precipitates are also often present in mill annealed material. The use of impact toughness as a qualification or acceptance test for lean duplex grades was also investigated in and it was concluded that a good sensitivity could be achieved by using a test temperature of 2 C for UNS S3211 and 2 C or perhaps even better -1 C for UNS S3234. [3] Two examples are shown in Figure 8 and indicate that a clear distinction between annealed and sensitized material can be detected. The impact toughness for 12 mm plate material of UNS S3211 has decreased from ~18J in the solution annealed condition to 5 6J after 5 minutes at 7 C. This degradation is also seen in the corrosion properties and in the oxalic acid etched microstructure. The corresponding curve for 3 mm plate of UNS S3234 indicates that the impact toughness drops after.5 to 1h to a level around 15J at room temperature or 1J at -1 C. However, the real drop in toughness for this grade is not really seen until sigma phase begins to precipitate at <1 hours, as indicated in Figure 1. Testing at -1 C appears to be slightly more sensitive to microstructural impairment for UNS S3234 but this slight advantage has to be weighed against the disadvantages of multiple testing temperatures and need for cooling specimens. Both corrosion and impact toughness testing seem to give similar results, in terms of the time at 7 C before a drop in properties is seen, although impact toughness may be marginally more sensitive. Both tests should nevertheless be interchangeable to use for approving or rejecting a lean duplex batch of material. A requirement should be placed on the number of specimens to be tested, and it would be appropriate to require a single pass, or a pass of both retested specimens in case of a first failure, as is specified in ASTM A923 today for standard and superduplex stainless steels. 7 6 S3211, 12 mm CV (J) mdd<1 2 Test temoerature ( C) 5 4 3 2 15 1 5 Charpy impact toughness (J) 1 (a) 4% sodium hydroxide. 2V, 1 seconds Annealed.1 1. 1. 1. Time at 7 C/1292 F (hours) 7 S3234, 3 mm 35 Test temoerature ( C) 6 5 4 3 2 CV (RT) mdd<1 CV (-1C) 3 25 2 15 1 Charpy impact toughness (J) 1 5 (b) 1% oxalic acid. 6 7V, 1 seconds Annealed.1 1. 1. 1. Time at 7 C/1292 F (hours) Figure 7 Microstructure of UNS S3211, 12 mm plate, sensitized for 5 minutes at 7 C showing the advantages of electrolytic etching in oxalic acid over sodium hydroxide Figure 8 Corrosion resistance and impact toughness for tested 12 mm plate of UNS S3211 and 3 mm plate of UNS S3234 showing how the impact toughness drop occurs marginally before the effect on corrosion resistance.

3/214 9 Future prospects The ASTM subcommittee A1-14 has recently established a working group to look into the question of standardization of acceptance testing for lean duplex grades. [12] This group will establish a proposal for a formal standardization of the test methods as well as guidelines for acceptance criteria for different alloys and products. The indications from the present work are that the lean duplex UNS S3211and UNS S3234 in the dimensions and products investigated here could be expected to pass an inhibited ferric chloride corrosion test at 2 25 C if correctly heat treated. The impact toughness values attainable seem to surpass the minimum values in the transverse direction specified in the materials standard EN 188-2/4 of 4J for UNS S3211 and 6J for UNS S3234. Thus it may be appropriate to specify somewhat higher limits based on the present data. Suggestions for limits and a comparison with the current acceptance criteria for duplex and superduplex grades according to ASTM A923 are given in Table 4. From the data presented here it seems that UNS S3211 reasonably attains 6 8J at room temperature, while the corresponding value for UNS S3234 may be in excess of 1J However, there are indications that -1 C might be a more sensitive impact test temperature for this steel. [3] Whatever criteria are set, the guiding principle must be that the proposed limits should be sensitive to different degrees of sensitization and be capable of clearly distinguishing materials with detrimental levels of carbides and nitrides from properly annealed states. Grade/condition A: Etching B: Requirements impact C: Requirements corrosion UNS S3225 base material 4% NaOH electrolytic 54J -4 C (4ft-lb -4 F) 6% FeCl 3,<1 mdd 25 C (77 F) ASTM A923 UNS S3225 weld metal 4% NaOH electrolytic 34J -4 C (25 ft-lb, -4 F) 6% FeCl 3 <1 mdd 22 C (72 F) UNS S3275 base material 4% NaOH electrolytic To be agreed 6% FeCl 3 <1 mdd 4 C (14 F) Grade/condition Proposal etching Proposal impact Proposal corrosion Lean duplex (this work and [3]) UNS S3211 base material UNS S3234 base material 1% C 2 H 2 O 4 6 8J, RT 1% C 2 H 2 O 4 1J, RT or 6 8J, -1 C 5% FeCl 3 + 1% NaNO 3 < 1 mdd, 2 25 C 5% FeCl 3 + 1% NaNO 3 < 1 mdd, 2 25 C Table 4 Acceptance criteria for UNS S3225 and UNS S3275 from ASTM A923 and comparison with proposed criteria for lean duplex grades UNS S3211 and UNS S3234 from this work.

3/214 1 Conclusions There are good possibilities to be able to define a qualification or acceptance test for the lean duplex grades UNS S3211 and UNS S3234 which parallel the methods specified in ASTM A923 for the higher alloyed duplex steels UNS S3225 and S3257. Immersion testing with weight loss evaluation in a 5% ferric chloride solution inhibited by the addition of 1% sodium nitrate is suitable for differentiation between acceptable solution annealed states and unacceptable detrimental sensitizing heat treatments. The proposed acceptance test temperature is 2 or 25 C. This has the advantage of avoiding the need for cooling baths for the sub-ambient temperatures which would be required in uninhibited ferric chloride. The test method has shown good inter-laboratory comparability and robustness in terms of surface grinding, cleaning and the way in which the solution is prepared. However, it is sensitive to the presence of residual weld oxides. It is also important that the specimen be placed in the preheated test solution to avoid false passing results which arise if the specimen is heated from ambient in the solution. There is good correlation with between loss of corrosion resistance and a drop in impact toughness. A screening evaluation based on electrolytic etching in 1% oxalic acid can be obtained to give a good first indication of a substandard material before proceeding to corrosion or impact toughness testing. Definition of acceptance limits requires further work, preferably under the auspices of the working committee established under ASTM A1.14. Acknowledgements Anette Wallin, ARC, Outokumpu Stainless AB is gratefully acknowledged for carrying out the immersion tests in FeCl 3 +NaNO 3 and Sukanya Mameng for the electrochemical work. Thanks are also expressed to James D. Fritz, TMR, for valuable discussions and to laboratories N and D for participating in Round Robin testing. References [1] ASTM A923-8 Standard Test Methods for Detecting Detrimental Intermetallic Phase in Duplex Austenitic/Ferritic Stainless Steels. [2] H Liu, P Johansson and M. Liljas: Structural evolution of LDX211 (EN 1.4162) during isothermal ageing at 6-85 C. Proc. 6th European Stainless Steel Conference, Helsinki (28), p555-56 [3] J. Y. Jonsson, C. Canderyd, R. Pettersson: Optimisation of a qualification test method for lean duplex stainless steels. Paper 28 presented at 7th European Stainless Steel Science and Market conference, September 211, Como, Italy [4] P. Boillot, R. Bergeron, J. Peultier, K. Wiegers and T. Ladwein: Investigations on standard corrosion test for quality control of lean duplex stainless steel. Proc. 8th Duplex Stainless Steels conference, Beaune (21), Beaune. [5] J. D. Fritz, P-E. Arnvig, J. Y. Jonsson, R. Pettersson and S. Randström: Evaluation of possible test methods for qualifying lean duplex stainless steel Proc. Stainless Steel World, Houston, Texas (21). [6] Arne Bergqvist, personal communication [7] R. Qvarfort: The Avesta cell a new tool for studying pitting. ACOM 1988 vol 2 3 p 2 5 [8] M. Liljas, P Johansson, H-P Liu, C-O Olsson: Development of a lean duplex stainless steel. Steel Research International. Vol. 79, no. 6, pp. 466 473. June 28 [9] ASTM G48-11 Standard Test Methods for Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by Use of Ferric Chloride Solution [1] ASTM G15-99 (21) Standard Test Method for Electrochemical Critical Pitting Temperature Testing of Stainless Steel [11] Outokumpu Corrosion Handbook, 1th Edition, 29, Outokumpu Oyj, Espoo, Finland [12] www.astm.org Reproduced with permission from NACE International, Houston, TX. All rights reserved. Paper No. C212-1527 presented at CORROSION/212, Salt Lake City, UT. NACE International 214.

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