STABILITY OF COMPLEX NITRIDES IN HEAT RESISTANT STEELS. Vlastimil VODÁREK, Jan HOLEŠINSKÝ

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1 STABILITY OF COMPLEX NITRIDES IN HEAT RESISTANT STEELS Vlastimil VODÁREK, Jan HOLEŠINSKÝ VŠB-Technical University of Ostrava, Ostrava-Poruba, Czech Republic, Abstract The effect of two levels of niobium (0.1 and 0.3wt.%Nb) in AISI 316LN austenitic steels and intentional additions of nickel (0.5 and 1.15wt%Ni) to 12CrMoVNbN martensitic steels on precipitation reactions was studied. Minor phase evolution at temperatures C was studied on creep ruptured specimens with times to rupture extending up to 220,000 hours. Thermodynamic and dimensional stability of nitrogen rich Z- phase and M 6 X (Cr 3 Ni 2 SiX type) precipitates in dependence on chemical composition of steels and the temperature of exposure was investigated. Keywords: heat-resistant steels, precipitation, nitrides, TEM. 1. INTRODUCTION Dislocation creep in metallic materials is controlled by an evolution of dislocation substructure 1. However, changes in dislocation arrangement are strongly affected by dislocation interactions with precipitates. That is why both thermodynamic and dimensional stability of individual minor phases play a crucial role in the field of dislocation creep. The role of precipitates in the achievements of good creep properties of steels has been extensively studied for a long time. Many minor phases are now well documented 2. However, it is not true in the case of some interstitial phases in nitrogen-bearing steels. A limited amount of information is available about such nitrogen rich minor phases as Z-phase and M 6 X. Z-phase, a complex NbCrN nitride, was firstly detected in 1950 s in austenitic steels 2. Raghavan et al. 3 suggested that some carbon could be dissolved in this phase. Z-phase is seldom reported, even in alloys liable to form it on creep/aging. This could be a result of its composition and its general features of formation, which are not very different from those of MX precipitates. It has the tetragonal unit cell of dimensions a= nm, c= nm 4. The metal atom arrangement is characterized by double layers of similar atoms alternating along the c axis of the unit cell to give an AABBAABB... sequence. In austenitic steels particles of this phase in the shape of short rods usually form from the solid solution. The kinetics of Z-phase precipitation is generally fast 2,5. The solvus temperature of NbCrN nitrides in austenitic steels was reported to be between 1250 and 1350 C, depending on the steel composition 2. Fine Z-phase particles in austenitic heat resistant steels have frequently been credited with beneficial strengthening effects during creep 6. On the other hand, in martensitic (9-12)%Cr steels niobium in Z-phase is partially substituted by vanadium and this results in a reduction of the tetragonal unit cell of this modified Z-phase: a=0.286 nm and c=0.739 nm 7. The kinetics of (Nb,V)CrN precipitation in tempered martensite is slow. An important role of MX (NbX and (V,Nb)X) particles in the Z-phase formation was reported [8]. In situ transformation of fine (V,Nb)X particles to the Z-phase structure was proposed and experimentally proved. At early stages of the Z-phase formation, when chromium atoms diffused into (V,Nb)X precipitates, an FCC unit cell with the lattice parameter of a=0.405 nm was detected and this was at longer exposures gradually transformed into the above tetragonal cell 9. These results suggest that the FCC precursor at early stages of in situ transformation of (V,Nb)X phase to modified Z-phase is related to imperfect ordering of metal atoms along the c axis 10. Due to a very small directional misfit in the 001 and (001) Z planes the growth rate of plate-

2 like modified Z-phase particles in the ferritic matrix is fast. The solvus temperature of the modified Z-phase in martensitic steels was reported to be approximately 800 C 11. Studies on 12CrMoVNbN steels exhibiting a dramatic drop in creep strength during long-term testing revealed that precipitation of modified Z-phase occurred concurrently with breakdown in creep strength 7,12. M 6 X has a diamond cubic structure ( -carbide, FCC, space group Fd3m) and it refers to a phase of very variable composition from M 3M3X to M M 2SiX 3, where M and M indicate substitutional elements, while X specifies an interstitial element, such as N and/or C 13. Time-temperature parameters of its precipitation in steels are variable because its appearance is strongly linked to that of other minor phases. It has been proved that nitrogen stabilizes this minor phase and its composition in nitrogen-bearing steels is usually referred as Cr 3 Ni 2 SiX, although its actual composition can include substantial amounts of molybdenum and iron 14. The lattice parameter of Cr 3 Ni 2 SiX phase varies with its chemical composition (a= nm) and is very close to that of M 23 C M 6 X often nucleates on particles of other minor phases. Furthermore, dimensional stability of M 6 X particles in both austenitic and martensitic steels is generally poor and that is why the positive effect of this phase on long-term creep properties of heat resistant steels is not expected 2. The effect of the above multi-component minor phases on long-term creep properties of heat resistant steels is the subject of permanent interest. Conditions of their formation are not very clear and even less understood is their relative stability. Missing thermodynamic data about these minor phases complicate reliable numerical simulations of microstructural evolution in heat resistant steels 16. This paper deals with the stability of Z-phase and M 6 X, forming during long-term aging/creep exposure in both austenitic and martensitic heat resistant steels. 2. MATERIALS AND EXPERIMENTAL PROCEDURES Investigations on minor phase evolution in austenitic AISI 316LN+Nb steels and martensitic 12CrMoVNbN steels were carried out on creep ruptured specimens. Chemical compositions of AISI 316LN steels with additions of 0.1 and 0.3wt.%Nb are given in Table 1. Solution annealing of Casts A and B was carried out at 1050 and 1120 C, respectively. Microstructure in the as-received state was fully austenitic. Long-term creep rupture tests were carried out in air at temperatures of 600 and 650 C. Chemical compositions of 12CrMoVNbN steels are shown in Table 2. Both casts had a common base composition in which different amounts of nickel were added. Quality heat treatment of Casts C and D consisted of normalizing and tempering at 650 and 675 C, respectively. In the heat treated condition particles of primary NbX, M 23 C 6 and M 2 X phases were present in tempered martensite of both casts. Furthermore, a small number density of (V,Nb)X particles was detected in Cast D. Creep rupture tests were carried out in air at 475, 550 and 600 C for times to rupture extending up to 100,000 hours 12. Table 1 Chemical compositions of AISI 316LN+Nb steels, wt.% Cast C N Mn Si Cr Ni Mo B Nb A B Table 2 Chemical compositions of 12CrMoVNbN steels, wt.% Cast C Si Cr Mo V Nb Ni N C D Detailed microstructural studies were performed on heads of creep ruptured specimens using analytical transmission electron microscopy. The electron microscopy was carried out on carbon extraction replicas using a Philips CM20 TEM fitted with an EDAX SAED and EDS techniques were used for the

3 identification of minor phases. Quantification of EDS spectra was performed using PM THIN software, the results being normalized to 100%. Diagrams showing minor phase evolution in individual casts were constructed using data of microstructural investigations on at least 5 specimens with prolonging time of exposure at the given temperature. 3. EXPERIMENTAL RESULTS 3.1 Austenitic AISI 316LN+Nb steels The following minor phases were identified in the AISI 316LN+Nb steels after long-term creep exposure at 600 and 650 C: Z-phase, M 6 X (Cr 3 Ni 2 SiX type), -Laves (Fe 2 Mo type) and -phase, Figs. 1a and 1b. Primary Z-phase particles, about 100nm in size, were present in both casts after solution annealing. Secondary Z-phase particles in the form of short rods, which precipitated during ageing/creep exposure, were very dimensionally stable 5. Most secondary Z-phase particles in Cast A after exposure 600 C/223,603 hours were finer than 10nm. High dimensional stability of Z-phase particles was also observed in other austenitic steels 6. These particles nucleated directly from the solid solution, mainly on dislocations. a) b) Fig. 1 Minor phase evolution in Casts A (a) and B (b) at 650 C At short times of creep exposure a small amount of M 23 C 6 particles formed in Casts A and B at grain boundaries and incoherent twin boundaries. This phase was gradually replaced by M 6 X phase (Cr 3 Ni 2 SiX type) at longer exposures 17. Chromium and nickel in this phase were partially substituted by molybdenum and iron, respectively. Only a small amount of niobium was dissolved in this phase. The kinetics of Z-phase precipitation was faster than that of M 6 X. However, particles of M 6 X coarsened much faster than Z-phase particles. After long-term creep exposure at 650 C the typical size of secondary Z-phase particles was less than 20 nm, while particles M 6 X reached up to 1 m 5. Both minor phases coexisted in Casts A and B even after the longest exposures at both 600 and 650 C. It proves that thermodynamic stability of both Z-phase and M 6 X in nitrogen-bearing austenitic steels is high. Chemical compositions of Z-phase and M 6 X particles in Cast A after creep exposure at 600 C for 223,603 hours are shown in Table 3. In nitrogen-bearing austenitic steels M 6 X phase is regarded to be a nitride and its precipitation in these steels might be very intensive 15. However the fact that M 6 X gradually replaced M 23 C 6 particles suggests that some carbon is also dissolved in this phase. Studies on the effect of niobium additions to AISI 316LN steels revealed that the growing niobium content strongly reduced the minimum creep rate and prolonged the time to the onset of the tertiary stage of creep

4 5. This can be attributed to intensive precipitation of fine secondary Z-phase particles on dislocations. However, the enhanced creep resistance of niobium-bearing AISI 316LN steels in the primary and secondary stages has not been accompanied by the longer creep life that might have been expected 5,17. The positive effect of niobium on the creep resistance was gradually surpassed by its effect on acceleration of the -phase, M 6 X and -Laves formation. Coarse -phase and M 6 X particles facilitated the formation of creep cavities 5. These results demonstrate that fine intragranular Z-phase particles can have a very positive effect on the minimum creep rate but the final effect on long-term creep properties will also depend on coexisting minor phases in the austenitic matrix. Table 3 Z-phase and M 6 X compositions in Cast A after exposure 600 C/223,603h., wt.% Phase Si Cr Fe Ni Nb Mo prim. Z-phase ± ± ± ±0.5 M 6 X 4.3± ± ± ± ± ± Martensitic 12CrMoVNbN steels Both casts of martensitic 12CrMoVNbN steels exhibited sigmoidal creep rupture behaviour which was associated with marked softening of tempered martensite due to microstructural degradation effects occurring during the creep process 12. Minor phase evolution in Casts C and D during long-term exposure at 550 and 600 C is summarised in Figs. 2 and 3. At the beginning of creep testing the material was precipitation strengthened by the combined effects of finely dispersed M 2 X and secondary MX ((V,Nb)X) precipitates within the matrix together with M 23 C 6 particles at prior austenite grain and lath boundaries. With progressive aging/creep exposure at 550 and 600 C, dissolution of fine M 2 X and secondary (V,Nb)X precipitates occurred with the simultaneous precipitation of modified Z-phase. Metallic composition of this phase in Casts C and D was variable but approximately conformed to the ratio: 50 at% (Cr+Fe) and 50 at.% (V+Nb). Compositions of Z-phase particles in specimens tested at 550 C are shown in Table 4. Precipitation of Z-phase was also accompanied by partial dissolution of primary NbX particles. Nucleation of Z-phase particles on NbX was often observed. The kinetics of Z-phase precipitation was slow but the growth of particles was fast. The typical size of Z-phase particles in specimens tested at 600 C reached several hundreds of nanometres. Such particles did not contribute to precipitation strengthening. The preferential growth of Z-phase in the form of thin plates took place on cube planes of the ferritic matrix. Furthermore, minor -Laves (Fe 2 Mo type) precipitation was identified in Cast C, especially after exposure at 550 C. Table 4 Chemical compositions of modified Z-phase in 12CrMoVNbN steels at 550 C, wt.% Cast Time h. V Cr Fe Ni Nb Mo C 100, D 24, Table 5 Minor phases in 12CrMoVNbN steels after long-term exposure at 475 C Cast Time h. NbX M 23 C 6 M 2 X (V,Nb)X Z-phase -Laves M 6 X C 83,929 Y Y Y D 39,287 Y Y Y Y = yes Table 6 Chemical compositions of M 6 X in Cast D at 550 and 475 C, wt.% Exposure Si V Cr Fe Ni Nb Mo 550 C/24,024h C/39,287h

5 In the nickel rich Cast D dissolution of finely dispersed M 2 X and secondary MX ((V,Nb)X) particles at temperature of 550 C was accompanied by the formation of both Z-phase and M 6 X particles. At the test temperature of 475 C fine M 2 X and secondary MX precipitates in Cast D were solely replaced by M 6 X phase, Table 5. No evidence of Z-phase was found at this low test temperature in both casts investigated. Chemical composition of M 6 X phase corresponded to Cr 3 Ni 2 SiX, Table 6. Chromium and nickel in this phase were partly substituted by vanadium, molybdenum and iron, respectively. Niobium content in M 6 X phase was low. a) b) Fig. 2 Minor phase evolution in Cast C, a) 600 C, b) 550 C a) b) Fig. 3 Minor phase evolution in Cast D, a) 600 C, b) 550 C In 12CrMoVNbN steels the driving force for precipitation of M 6 X phase increases with increasing the nickel content. Similarly like in the case of Z-phase, precipitation of M 6 X is accompanied by dissolution of fine M 2 X and secondary (V,Nb)X precipitates. Precipitation of M 6 X in Cast D was preferred to Z-phase at temperatures below 550 C. Dimensional stability of M 6 X particles was poor. A similar effect of nickel content on the relative stability of M 2 X and M 6 X phases was also observed in 12CrMoV steels 18.

6 4. CONCLUSIONS Thermodynamic stability of both Z-phase and M 6 X (Cr 3 Ni 2 SiX type) in austenitic AISI 316LN+Nb steels is high. Both minor phases coexisted in the casts investigated even after the longest exposure times at 600 and 650 C. The kinetics of Z-phase formation during creep exposure is faster than that of M 6 X. Dimensional stability of Z-phase particles is excellent, M 6 X particles coarsen fast. Thermodynamic stability of modified Z-phase ((V,Nb)CrN) in martensitic 12CrMoVNbN steels is high but the kinetics of its formation is slow. The formation of Cr 3 Ni 2 SiX phase (M 6 X) in 12CrMoVNbN steels was detected only in the cast containing 1.15wt.%Ni. Precipitation of M 6 X was preferred to Z-phase at temperatures below 550 C. Dimensional stability of both modified Z-phase and M 6 X particles in 12CrMoVNbN steels is poor. ACKNOWLEDGEMENTS The authors wish to acknowledge the financial support from the project No. CZ.1.05/2.1.00/ "Regional Materials Science and Technology Centre" within the frame of the operation program "Research and Development for Innovations". REFERENCES [1] Cadek, J. Creep of Metallic Materials, first ed., Elsevier, Amsterdam, [2] Sourmail, T. Mater. Sci. Technol. 17 (2001) [3] Raghavan, A., Klein, C.F., Marzinsky, C.N. Metall. Trans. A 23 (1992) [4] Jack, D.H. Jack, K.H. JISI 209 (1972) [5] Vodárek, V. Mater. Sci. Eng. A 528 (2011) [6] Evans, N.D., Masiasz, P.J. Shingledecker, J.P., Pollard, M.J. Metall. Mater. Trans. A, 2010, vol. 41, pp [7] Strang, A., Vodárek, V. Mater. Sci. Technol. 12 (1996) [8] Cipolla, L., Danielsen, H.K., Venditti, D., Di Nunzio, P.E., Hald, J., Sommers, M.A.J. Acta Mater. 58 (2010) [9] Danielsen, H.K., Hald, J., Grumsen, F.B., Somers, M.A.J. Metall. Mater. Trans. A 37 (2006) [10] Danielsen, H.K., Hald, J. Somers, M.A.J. Scripta Mater. 66 (2012) [11] Danielsen, H.K., Hald, J. Energy Materials 1 (2006) [12] Strang, A., Vodárek, V. Proc. Conf. Microstructural Stability of Creep Resistant Alloys for High Temperature Plant Applications (ed. Strang A. et al.), IOM, London, 1998, pp [13] Stadelmaier, H. H. Developments in the Structural Chemistry of Alloy Phases, Plenum Press, 1969, pp [14] Jargelius-Petterson, R.F.A. Scripta Metall. Mater. 28 (1993) pp [15] T. Sourmail, T., Bhadeshia, H.K.D.H. Metall. and Mater. Trans. A 36 (2005) pp [16] Shim, J.-H., Kozeschnik, E., Jung, W.-S., Lee, S.-C., Kim, D.-I., Suh, J.-Z., Lee, Y.-S., Cho, Y.W. CALPHAD 34 (2010) pp [17] Vodárek, V. Proc. 9 th Liege Conference on Materials for Advanced Power Engineering (ed. Lecomte-Beckers J. et.al.), Forschungszentrum Jülich, 2010, pp [18] Vodárek, V. Strang, A. Scripta Materialia 38 (1998)