A CORROSION MANAGEMENT AND APPLICATIONS ENGINEERING MAGAZINE FROM OUTOKUMPU 3-4/2013. The two phased optimization of duplex stainless steel

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1 A CORROSION MANAGEMEN AND APPLICAIONS ENGINEERING MAGAZINE FROM OUOKUMPU 3-4/2013 he two phased optimization of duplex stainless steel

2 2 he two phased optimization of duplex stainless steel Presenter: Jan. Y. Jonsson, Alexander hulin,sukanya Hägg Outokumpu Stainless AB, Avesta Research Centre, Avesta, Sweden. Rachel Pettersson, Jernkontoret, Swedish Steel Producers Association, Stockholm, Sweden. Abstract Duplex stainless steels are well known for high strength in comparison to their austenitic counterparts. hey also have good cost efficiency providing required properties without the level of exposure to nickel price volatility seen for many austenitic grades. he development of these two phased grades is a continuous process and a natural focus is to optimize the composition to obtain the maximum possible benefit from the alloying elements. One proven way of doing this is to decrease the content of nickel while increasing the amount of other austenitizing elements such as nitrogen and manganese. Nitrogen has a strong beneficial influence on both strength and corrosion resistance. his alloying concept has been used successfully in the lean duplex grades, which have corrosion resistance on a par with standard austenitic grades. he most recent contribution based on this concept is the lean duplex grade LDX 24 with enhanced strength but a corrosion resistance which is still close to that of the standard duplex grade 05. When testing the corrosion resistance in the duplex grades it is important to consider whether there is an imbalance in the corrosion resistance of the individual phases. he development of the super duplex grade 27 was reported to be based on the concept that the grade exhibits optimal pitting corrosion resistance when annealed at a temperature where the localized corrosion resistance equivalent is equal in both phases. Such an optimization naturally also involves other concepts such as phase balance elemental partitioning and structural stability. his paper aims to take such considerations a step further, using an analysis which acknowledges the different performance of the austenite and ferrite phases in duplex grades and addresses the possibility that they do not need to have the equal PRE to give the alloy a maximum critical pitting temperature, CP. his concept has proven useful in duplex alloy development especially for molybdenum alloyed grades where the resistance against pitting corrosion is high. he paper exemplifies these considerations both by examining a number of different variants of the standard duplex grade 05 and by evaluation of a series of model alloys. he partitioning of alloying elements is determined by EDS/WDS analysis and correlated to predictions using the thermodynamic software hermo-calc. he location of pitting attack is evaluated in the different cases and the discussion focuses on the possible mechanisms behind the observed results. Introduction Duplex stainless steels are well known for high strength in comparison to their austenitic counterparts. hey also have good cost efficiency, providing required properties without the level of exposure to nickel price volatility seen for many austenitic grades. he development of these two phased grades is a continuous process and a natural focus is to optimize the composition to obtain the maximum possible benefit from the alloying elements. he term maximum possible benefit can be interpreted in many ways; one may consider that a low price is beneficial while another may think that e.g. higher impact toughness is more beneficial. hese different interpretations can be met by varying the chemical composition of the steel melts. he duplex grade 05 is a good example of this; Outokumpu Avesta Works has 4 different melt codes of the 05. One is the Outokumpu standard while the others fulfill three specific requirements; lower price, higher impact toughness or lower ferrite content. his paper focuses on the partitioning of the alloying elements and the effect that this has on the pitting resistance of each phase. he partitioning of alloying elements is determined by EDS analysis and correlated to predictions using the thermodynamic software hermo-calc. he duplex grades LDX 24 and EDX 24 M together with a set of experimental alloys are used to exemplify the alloying concept.

3 3 Materials and experimental procedure Materials he investigated materials are 05 as well as the leaner duplex steel grades; LDX 24 and EDX 24 M, the chemical composition of the grades are presented in able 1. All materials has been examined after a solution annealing in 10 C followed by 15s in air and a final water quench. he pitting resistance equivalent (PRE) is commonly used to estimate the corrosion resistance of stainless steel grades. here are different opinions on the exact formulation of the PRE formula, as discussed in a review paper (Pettersson & Flyg, 2004) and this work uses PRE(NMn) where the positive effect of nitrogen and a slight negative effect of manganese is taken into account. It can be noted that the overall effect associated with manganese alloying can nevertheless be positive, since this element increases the nitrogen solubility. he PRE(NMn) formula is given below able 1 and it contains a coefficient of for nitrogen and -1 for the manganese content. he most common PRE formula, PRE(N), uses a factor 16 or for nitrogen and it does not take into account any effect of manganese. his common formula is used in various situations where the general pitting resistance of a specific material needs to be evaluated, a customer specification can e.g. contain a requirement that sets a lower limit of PRE(N) that the material needs to fulfill. In addition to the commercially available alloys above, a set of small (0 g) experimental alloys were used to show how the optimization concept taken from the commercially available materials can be used on a real case. hese materials are shown in able 2. As for the commercial alloys, these materials were investigated after solution annealing at 10 C followed by 15 seconds in air prior to water quenching. Metallographic examination he microstructure was examined after a chemical etching in a modified Beraha II solution (ml HCl, 100ml H g K 2 SO 5 ). he ferrite contents were evaluated with light optical microscopy (LOM) using image analysis according to ASM E 12. he same etchant was also used for detecting pit initiation sites after corrosion testing. For comparison of the pitting resistance in each phase using the individual phase PRE the chemical composition of the austenite and the ferrite was analyzed with scanning electron microscope with energy dispersive spectroscopy (SEM-EDS). Calibration was performed using actual plate material as reference sample. he N-content was analyzed using wavelength dispersive spectroscopy, WDS. In this case a set of stainless steel materials with known N-contents was used a reference material. Corrosion testing he CP-testing has been performed according to ASM G1 in a 1M NaCl solution for the commercial alloys 05, LDX 24 and EDX 24. A 1 cm 2 test area was used for the commercial steels and 10 cm 2 for the set of laboratory materials. Materials Plate thickness (mm) Chemical composition (wt.%) PRE(NMn) * 05 alloys Cr Ni Mo N Mn Alloy A Alloy B Alloy C Alloy D Alloy E LDX EDX 24 M able 1 Chemical composition in % by weight of the duplex stainless steel materials investigated. *Pitting resistance equivalent. PRE(NMn) = %Cr+3.3%Mo+%N-%Mn. Materials Chemical composition (wt.%) PRE(NMn) * Experimental alloys Cr Ni Mo N Mn Alloy I Alloy J Alloy K Alloy L able 2 Chemical composition in % by weight of the duplex laboratory materials. * Pitting resistance equivalent. PRE(NMn) = %Cr+3.3%Mo+%N-%Mn.

4 4 Results and Discussions he commercial duplex alloys he chemical analyses of the alloy and specifically of the austenite and the ferrite are shown in able 3 together with the corresponding PRE(NMn) and the CP results. It is noted that CP increases with the PRE(NMn) in all cases except for alloy E which has a lower CP in spite of the higher PRE(NMn). he majority of the nitrogen found in a duplex alloys is located in the austenite phase, which contains approximately 10 times the nitrogen in the ferrite phase. It is noted that the addition of nitrogen in duplex grades has many benefits, such as higher strength as for LDX 24, but the resistance towards pitting is not in general favored by this addition if the only other adjustment is a decrease in nickel content, in part exemplified by alloys A-C. An increase in nitrogen can improve the austenite PRE but primarily increases the austenite fraction and will have to be balanced with higher levels of ferrite stabilizers such as chromium and molybdenum. Lowering the nickel content will lower the PRE of the already weakest phase, the ferrite, even though nickel is not a factor in the PRE formula, because of its influence on phase balance and elemental partitioning. his is exemplified in Figure 1 below for an alloy space around 05. Looking at initiation sites of the corrosion samples it can be seen that in samples from alloy E the austenite phase is predominantly attacked and this the weaker phase, see Figure 2. he ferrite phase is seen to be the weaker phase in all other tested alloys. he results have further been illustrated in Figure 3 to Figure 6 which show the results from able 3 graphically. he figures show the CP as a function of the PRE(N) for the general composition, which is the commonly used procedure, as well as the PRE(NMn) in the individual phases; the austenite phase, the ferrite phase and the weakest phase, which in the case of Alloy E is the austenite but in all other steels is the ferrite. Materials Chemical composition (wt.%) PRE(NMn) CP ( C) 05 alloys Cr Ni Mo N Mn Alloy A BCC FCC Alloy B BCC FCC Alloy C BCC FCC Alloy D BCC FCC Alloy E BCC FCC LDX BCC FCC EDX 24 M BCC FCC able 3 Chemical composition % by weight in austenite and ferrite analysed by SEM-EDS with WDS-analysis for N.

5 Investergation: 05CPN100 (MLR) Contour Plot = 1100, Cr =.5, Mo = 3 PREFCC PREFCC 0.23 3.7 36.5 36.5 0.23 36 36.5 37 0.21 0.21 N 0.19 34.5 34 N 0.19.5 0.17 33.

Figure 2 Micro photos of surface sections of polished and etched CP samples indicating pit initiation in the ferrite to the left for sample A and in the austenite

5 5 Investergation: 05CPN100 (MLR) Contour Plot = 1100, Cr =.5, Mo = 3 PREFCC PREFCC N N Ni Ni PRE ot Ferrit N 0.19 N Ni Ni Figure 1 Example of influence of Ni and N on ferrite content and PRE(NMn). Figure 2 Micro photos of surface sections of polished and etched CP samples indicating pit initiation in the ferrite to the left for sample A and in the austenite to the right for sample E (see arrow).

6 Critical pitting temperature G1 ( C) R 2 = PRE(N) Critical pitting temperature G1 ( C) R 2 = PRE(N) Figure 3 CP vs. PRE(N) of the general chemical composition. PRE(N)=%Cr+3,3%Mo+16%N. Figure 4 CP vs. PRE(NMn) of the austenite phase Critical pitting temperature G1 ( C) R 2 = PRE(N) Critical pitting temperature G1 ( C) R 2 = PRE(N) Figure 5 CP vs. PRE(NMn) of the ferrite phase. Figure 6 CP vs. PRE(NMn) of the weaker phase. red dot represent Alloy E. In Figure 3 it is indicated quite strongly that the overall PRE(NMn) formula only moderately describes the resistance towards pitting corrosion for the examined materials. Figure 4 indicate no direct correlation between the CP and the PRE(NMn) in the austenite phase while Figure 5, on the contrary, shows quite a good correlation. However, Figure 6 clearly shows that the PRE(NMn) of the weaker phase correlates very well with the CP results from this investigation. he PRE(NMn) in each individual phase and the resulting CP are further graphically shown in Figure 7. his view shows quite well that the CP follows the PRE(NMn) of the ferrite phase except for Alloy E where the CP drops somewhat in spite of an increase in the PRE(NMn) in both phases. As indicated by the metallographic investigation, the austenite phase is the weaker phase in Alloy E and the PRE(NMn) of this phase correlate very well with the slightly lower CP. It is important to note that nickel and nitrogen contents are the predominant variables in the alloys. he nickel content governs the alloying distribution between phases and thus the PRE of each phase, the higher the nickel content, the higher fraction of chromium and molybdenum in the ferrite. his is quite clear when looking at the higher alloyed grades in Figure 7: all have quite similar PRE in the austenite phase while the ferrite phase PRE is gradually increasing, notably because of the increasing nickel content in the alloys, able 1. he CP also increases and correlates quite well with the PRE except for Alloy E as shown in Figure 5 and 6. PRE (NMn) EDX 24 (34.2 C) Ferrite phase Austenite phase CP LDX (42.3 C) Alloy A (52.9 C) Alloy B (53.3 C) Alloy C (56.4 C) Alloy D (58.8 C) Alloy E (57.1 C) Figure 7 PRE(NMn) in ferrite phase and austenite phase from EDS analysis and CP. he result indicate that a switch from ferrite to the austenite as the weak phase does not occur until the local corrosion resistance in the austenite is a certain degree lower than that of the ferrite. It is not seen as soon as the PRE of the austenite falls below that of the ferrite. he conclusion is therefore that the pitting resistance of the austenite is better than the PRE suggests, or conversely that the pitting resistance of the ferrite is lower than CP (NMn)

contribute to the formation, maintenance and repair of the passive film on fcc and bcc substrates.

7 7 the PRE would indicate. he reasons for this are open to speculation but it is hardly surprising that such a simplified, generalized expression as PRE neglects to take into account the way in which alloying elements can contribute to the formation, maintenance and repair of the passive film on fcc and bcc substrates. For example, it is conceivable that the nitrogen in ferrite has no positive effect, or that molybdenum is more efficiently utilized in fcc. In this context it should, however, be borne in mind that AES studies (Olsson, 1996) have shown good lateral mobility of elements forming the passive film. A further consideration is the phase ratio: Alloy E has the lowest ferrite content of the 7 alloys tested. In purely statistical terms a higher percentage of austenite increases the risk for the weakest link to be found in the austenite if the two phases are equally resistant to pitting. If there is a protective function of the ferritic phase this should also be lower with less ferrite. Looking at the leaner duplex steels, they all have a weaker ferritic phase which follows which follows the general trend. See example in Figure 8. Concept of optimized alloying of a duplex steel With the previous results in mind one strategy to optimize the alloying content of a duplex grade is to aim to achieve the highest pitting resistance for a certain overall PRE. he results from the 05 variants indicate that the alloying content should be designed for the ferrite to only just be the weak phase, without any unnecessary over alloying of the austenite. It seems that if the PRE in the ferrite phase is 1.5 to 2.5 units higher than that of the austenite then the ferrite is still the weak phase but if the difference is larger the austenite becomes the weak phase, this will also result in a drop in pitting resistance. It should however be pointed out that this difference for 05-type grades, and the result may not be directly applicable to other alloy systems and duplex grades. Using hermo-calc, software for thermodynamic calculations, examination to optimize an alloying window can be performed. As an example optimizing Cr, Ni, Mo and N for ferrite content as well as a specific difference between the PRE value in the ferrite and the austenite have been done around the alloying range of 05. he results indicate that the window for optimization is quite narrow, see Figure 9. Comparing hermo-calc and SEM-EDSvalues a small difference can be seen but the overall trend is very similar. Laboratory melts he results have been used as a basis for preparation of a set of laboratory alloys with somewhat higher total PRE(NMn) than 05. he different PRE(NMn) values and the resulting pitting temperatures can be seen in Figure 10. he results indicate that pitting corrosion initiates in the austenite for alloy I and J and in the ferrite for alloy K and L, see Figure 11. he earlier interpretations of the 05 pit initiation sites are thus reinforced with these observations. A clearly lower PRE(NMn) in the austenite phase than in the ferrite phase make initiation take place in the austenite. By comparing the corrosion result with phase PRE(NMn) it can be concluded that an optimized alloy in this design window seems to need a PRE(NMn) difference between phases of between 2 and 6. Additional melts are needed to achieve a more precise definition of the optimization window, and the difference in corrosion resistance between alloy K and L merits further elucidation. Figure 8 Micro photos for surface sections of polished and etched CP tested samples indicating initiation in the ferrite (1) for EDX 24 M and (2) for LDX 24. PRE (Mn) Alloy I (49 C) Ferrite phase Austenite phase CP Alloy J (62 C) Alloy K (76 C) Alloy L (76 C) Figure 10 PRE(NMn) in ferrite phase and austenite phase from EDS analysis for four laboratory alloys. he results show quite clearly that the traditional PRE formula should generally only be used as a first approximation of pitting resistance. It can however without much change be used as a good tool for optimization in a local alloying window and for best use also by considering the resistance of each phase. CP ( C)

8 Investergation: 05CPN100 (MLR) Sweet Spot = Femite ( ) PREFCC (34 ). PREBCC (,5 ). DiffPRE (5 25) Sweet Spot Criteria met 3 Criteria met 2 Criterion met 1 Mo = 3.8.8.8 N = 0.

25 Ni = 6 Figure 9 Example of optimization using hermo-calc where green areas represent alloys with difference in phase PRE(NMn) of max 2.

8 8 Investergation: 05CPN100 (MLR) Sweet Spot = Femite ( ) PREFCC (34 ). PREBCC (,5 ). DiffPRE (5 25) Sweet Spot Criteria met 3 Criteria met 2 Criterion met 1 Mo = N = N = N = Ni = 4,5 Ni = 5.25 Ni = 6 Figure 9 Example of optimization using hermo-calc where green areas represent alloys with difference in phase PRE(NMn) of max 2.5 (higher PRE in the ferrite phase) and austenite PRE(NMn) of min 34 and a ferrite PRE(NMn) of min.5. Figure 11 Micro photos for surface sections of polished and etched CP tested samples indicating initiations in the ferrite to the left for sample I and in the austenite to the right for sample K.

9 9 CONCLUSIONS o optimize an alloy towards pitting corrosion a better tool than using PRE-formula for the overall composition is to use a PRE-formula for each phase. A key in optimization of duplex steel towards pitting corrosion is to find the change from ferritic to austenitic pit initiation. REFERENCES Olsson, C.-O. A. (1996). Analysis by AES and XPS of the influence of nitrogen and molybdenum on the passivation of 05 austenot-ferritic stainless steels. Acciaio Inossidabile. Pettersson, R., & Flyg, J. (2004). Electrochemical evaluation of pitting and crevice corrosion resistance of stainless steels in NaCl and NaBr. Acom. his article was first published in the Proceedings of the Stainless Steel World Conference & Expo 2013, 12th - 14th November, 2013, Maastricht, he Netherlands KCI Publishing, 2013.

10 Working towards a world that lasts forever EN-GB, Art 58, 12, 13. Outokumpu works with its partners to create long lasting solutions for the tools of modern life and the world s most critical problems: clean energy, clean water and efficient infrastructure. Our goal is a world that lasts forever. Information given in this brochure may be subject to alterations without notice. Care has been taken to ensure that the contents of this publication are accurate but Outokumpu and its affiliated companies do not accept responsibility for errors or for information which is found to be misleading. Suggestions for or descriptions of the end use or application of products or methods of working are for information only and Outokumpu and its affiliated companies accept no liability in respect thereof. Before using products supplied or manufactured by the company the customer should satisfy himself of their suitability. research.stainless@outokumpu.com outokumpu.com

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