Materials Science Forum. Online: 2016-11-15. ISSN: 1662-9752, Vol. 879, pp 1963-1968 doi:10.4028/www.scientific.net/MSF.879.1963. © 2017 Trans Tech ...
Materials Science Forum ISSN: 1662-9752, Vol. 879, pp 1963-1968 doi:10.4028/www.scientific.net/MSF.879.1963 © 2017 Trans Tech Publications, Switzerland
Online: 2016-11-15
Effect of Tempering on Microstructure and Creep Properties of P911 Steel Evgeniy Tkachev*, Marina Odnobokova, Alla Kipelova, Andrey Belyakov and Rustam Kaibyshev Laboratory of Mechanical Properties of Nanostructured Materials and Superalloys, Belgorod State University, Pobeda 85, Belgorod 308015, Russia Keywords: high-chromium creep resistant steel, tempering, electron microscopy, Nb(C,N) carbonitrides, creep behavior.
Abstract. The microstructure and creep properties of a P911-type steel normalized at 1060°C and then subjected to one-step tempering at 760°C for 3 h or two-step tempering at 300°C for 3 h + 760°C for 3 h were examined. The transmission electron microscope (TEM) observations showed that the tempered martensite lath structure (TMLS) with a lath thickness of 340 nm evolved after both tempering regimes. High dislocation densities of 3×1014 or 5×1014 m-2 retained after one- and two-step tempering respectively. M23C6 carbides with a mean size of 120 nm and V-rich MX carbonitrides having a “wing” shape with an average length of about 40 nm precipitated on highand low-angle boundaries and within ferritic matrix, respectively. A number of Nb-rich M(C,N) carbonitrides with a mean size of 20 nm precipitated on dislocations during low temperature tempering. The creep tests were carried out under constant load condition at 650°С at applied stresses of 100 and 118 MPa. Analysis of creep rate versus time curves showed that the use of twostep tempering decreases the minimum creep rate providing an increase in the creep strength in long-term conditions. 1. Introduction Extensive efforts have been devoted to the development of 9%Cr ferritic steel with high creep resistance [1, 2]. During the last twenty years, three such steels, P91 (9Cr-1Mo-V-Nb), P911 (9Cr1Mo-1W-V-Nb) and P92 (9Cr-0.5Mo-1.8W-V-Nb), were qualified for commercial use. 9Cr-1Mo1W-V-Nb steel developed in Europe in parallel to the grade P92 and designed as E911 is used for tubing, headers and piping, as well as for large forgings. The maximum service temperature determined by stability of tempered martensite lath structure (TMLS) under creep conditions is limited to 610°C. The creep strength of this steel can be improved by the formation of optimal dispersion of secondary phase particle under tempering. The precipitation strengthening effect of nanoscale Nb(C,N) and V(C,N) carbonitrides is important because their coarsening rate is small [1, 3]. Boundary M23C6 carbides exerting a high Zener drag force provide stability of TMLS [4]. It is well known that the TMLS is formed after normalizing at 1050-1100oC followed by tempering at 750-780°C and its stability is controlled by a dispersion of M23C6 carbides and MX carbonitrides [1-7]. There is limited information on effect of tempering on mechanical properties of 9%Cr steels [8-10]. The aim of the present work is to study the microstructure and creep behavior of P911 steel subjected to two step tempering procedure, which consists of tempering at 300°C followed by tempering at 760°C, with a reference to standard heat treatment, in order to explore a possible way for improving the exploitation properties of widely used material. 2. Experimental A P911-type steel (Fe-0,12C-0,36Mn-0,06Si-9,8Cr-0,2Ni-1,01Mo-0,93W-0,2V-0,05Nb-0,003B all in mass %) was fabricated by Chelyabinsk Metallurgical Plant (Chelyabinsk, Russia). The steel was subjected to solution treatment at 1060°C followed by air cooling and then subjected to one-step tempering at 760°C for 3 h or two-step tempering at 300°C for 3 h and at 760°C for 3 h. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.ttp.net. (#71026964, Belgorod State University, Belgorod, Russian Federation-03/11/16,16:22:27)
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The microstructural characterizations were carried out by using JEOL JEM–2100 transmission electron microscope (TEM) operating at a 200 kV accelerating voltage with an INCA energy dispersive X-ray spectroscope. The extraction replicas of the heat treated materials were prepared by depositing a thin carbon film on a polished and etched surface. Creep tests were carried out until rupture at 650°C with stress values of 100 and 118 MPa using an ATS2330 lever arm machine. The experiments were conducted on standard flat specimens with a gauge length of 25 mm and a cross section of 7×3 mm2. 3. Results 3.1 Microstructure observations Figure 1 (a, b) shows the representative TEM images of the TMLS in the P911 steel after normalizing along with a detailed image of the second phase precipitates revealed by extraction replica.
Figure 1. TEM micrographs of P911 steel structure after normalization at 1060°C: (a) lath structure; (b) dispersed particles. TMLS consists of prior austenite grains (PAG), packets, blocks and dislocation laths [1, 11] with thickness of about 200 nm. M3C carbides and relatively coarse Nb(C,N) carbonitrides with the mean size of about 85 nm were found within martensitic matrix. In addition, some fine Nb(C,N) precipitates, which presumably appeared on dislocations during cooling from the normalizing temperature of 1060°C, are observed. After tempering at 300°C the dispersion of secondary phase particles is shown in Fig. 2. Average size of Nb(C,N) is ∼45 nm; length of M3C carbides insignificantly increases from 60 nm to 80 nm. Figure 3 shows the bimodal distribution of Nb(C,N) carbonitrides after low temperature tempering. Coarse Nb(C,N) particles are considered to be primary precipitates, while the fine particles precipitated under auto-tempering and low temperature tempering of 300°C (Fig. 2).
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Figure 2. TEM micrograph of normalized and tempered at 300°C P911 steel showing Nb(C,N) particles on carbon replica.
Figure 3. Size distribution of the Nb(C,N) particles in P911 steel after tempering at 300°C for 3h. TEM micrographs of the specimen structures after one-step tempering at 760°C and two-steps tempering at 300+760°C are shown in Figs. 4 (a, b) and (c, d) respectively.
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Figure 4. TEM micrographs of P911 steel structures after normalization at 1060°C and one-step tempering at 760°C (a, b) or two-step tempering at 300+760°C (c, d); (a, c) lath structure; (b, d) dispersed particles. Numerous fine particles of V(C,N) carbonitrides with longitudinal dimension of ∼35 nm are observed after one-step tempering at 760°C and two-step tempering at 300+760°C. An example of V(C,N) particle is indicated by arrow in Fig. 4d. It is worth to note that the average diameter of Nb(C,N) particles is about 25 nm in the sample subjected to two step tempering, whereas that of 40 nm is evolved by one-step tempering (Table 1). In addition to V(C,N) and Nb(C,N) particles, the precipitation of boundary M23C6 carbides with the mean size of about 120 nm takes place. The M3C carbides are not detected after high temperature tempering. It is supposed that the precipitation of more stable M23C6 carbides results in replacement of less stable M3C carbides, which mainly precipitate during cooling from the normalizing temperature of 1060°C [9]. TMLS tends to be coarse under the tempering [10, 12]. + 20% increase in the lath thickness takes place after low temperature tempering at 300°C, whereas +70% increase in laths thickness occurs after one-step tempering at 760°C and two-step tempering at 300+760°C (Table 1). Table 1 also lists the results for mean sizes of the precipitate phases in the studied steel specimens tempered for 3h at different temperatures. The sizes of the precipitates were measured in the TEM micrographs of the carbon replicas. Table 1. Some microstructural parameters for the P911 steel after tempering at different temperatures. Tempering Normalized 300 760 300+760 temperature, °C Martensitic lath 200±10 230±10 340±20 340±15 width, nm Dislocation 9.2±1.8 13.1±3.4 3.0±0.6 5±1.0 density, ×1014 m–2 Average 60x15 80x15 size of M3C, nm Average size 125±10 115±15 of M23C6, nm Average size of 85±10 45±5 40±5 25±5 Nb(C,N), nm 40x20 35x20 Average size of V(C,N), nm
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3.2. Creep behavior The creep rate vs. creep time curves that were obtained at 650°C under initial stresses of 100 MPa and 118 MPa for samples subjected to one-step tempering (760°C 3 h) and two-step tempering (300°C 3 h +760°C 3 h) are shown in Fig. 5.
Figure 5. Creep rate versus creep time curves for P911 steel samples subjected to one-step tempering (760°C 3h) and two-step tempering (300°C 3h +760°C 3h). The creep tests were carried out at 650°C under 118 MPa (a) or 100 MPa (b). The creep rate vs. creep time curves consist of a transient creep region, where the creep rate decreases with creep time, steady creep regime and tertiary or acceleration creep region, where the creep rate increases with creep time after reaching a minimum creep rate, as shown in Fig. 5. As can be seen in Fig. 5a, the time to rupture is nearly the same for the steel samples subjected to one-step or two-step tempering under applied stress of 118 MPa. In contrast, the lower minimum creep rate and longer time to rupture is recorded by the creep tests under 100 MPa for the steel subjected to two-step tempering as compared to the steel subjected to one-step tempering (Fig. 5b). The times to rupture after creep tests under 100 MPa are 1820 h and 4982 h for the samples after one-step tempering (760°C, 3 h) and two-steps tempering (300°C, 3 h + 760°C, 3 h), respectively. Apparently, the finer MX carbonitrides in steel subjected to two-step tempering extend duration of transient creep and retard the onset of acceleration creep. In addition, these carbonitrides retard recovery of TMLS, thereby leading to the high creep strength. Summary The effect of two-step tempering at 300°C for 3 h and at 760°C for 3 h on the microstructures and creep behavior was studied in the P911-type steel. The microstructures after one-step (760°C, 3 h) and two-step tempering are characterized by almost the same structural parameters, i.e., the lath sizes, the dislocation densities, the sizes and distribution of M23C6 carbides. The results showed that two step tempering decreases remarkably the size of Nb(C,N) leading to the improvement of the creep resistance in long-term conditions. This suggests that fine Nb(C,N) carbides are effective for the stabilization of tempered martensite lath structure and shifting of the transition from the transient to the acceleration creep region to high times.
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Acknowledgements The study was financially supported by the Russian Science Foundation, Project No. 14-29-00173. Authors are grateful to staff of the Joint Research Center, Belgorod State University, for their assistance with structural characterizations. References [1] F. Abe, T.-U. Kern, R. Viswanathan, Creep-resistant steels, Woodhead Publishing, Cambridge (2008). [2] R.O. Kaybyshev, V.N. Skorobogatykh, I.A. Shchenkova, New martensitic steels for thermal power plant: Creep resistance, Phys. Met. Metall. 109 (2010) 186-200. [3] F. Abe, Creep rates and strengthening mechanisms in tungsten-strengthened 9Cr steels, Mater. Sci. Eng. A319–321 (2001) 770–773. [4] A. Fedoseeva, N. Dudova, R. Kaibyshev, Creep strength breakdown and microstructure evolution in a 3%Co modified P92 steel, Mater. Sci. Eng. A654 (2016) 1–12. [5] M. Taneike, K. Sawada, F. Abe, Effect of carbon concentration on precipitation behavior of M23C6 carbides and MX carbonitrides in martensitic 9Cr steel during heat treatment, Metall. Mater. Trans. A 35A (2004) 1255–1261. [6] K. Maruyama, K. Sawada, J. Koike, Strengthening Mechanisms of Creep Resistant Tempered Martensitic Steel, ISIJ Int. 41 (2001) 641–653. [7] A. Kipelova, M. Odnobokova, A. Belyakov, and R. Kaibyshev, Effect of Co on Creep Behavior of a P911 Steel, Metall. Mater. Trans. A 44 (2013) 577–583 [8] N. Dudova, R. Mishnev, R. Kaibyshev, Effect of Tempering on Microstructure and Mechanical Properties of Boron Containing 10%Cr Steel, ISIJ Int. 51: (2011) 1912–1918. [9] S.S. Wang, D.L. Peng, L. Chang, X.D. Hui, Enhanced mechanical properties induced by refined heat treatment for 9Cr–0.5Mo–1.8W martensitic heat resistant steel, Mater. Des. 50 (2013) 174-180. [10] A.Yu. Kipelova, A.N. Belyakov, V.N. Skorobogatykh, I.A. Shchenkova, R.O. Kaibyshev, Tempering-induced structural changes in steel 10Kh9K3V1M1FBR and their effect on mechanical properties, Met. Sci. Heat Treatm. 52 (2010) 100-110. [11] H. Kitahara, R. Ueji, N. Tsuji, Y. Minamino, Crystallographic features of lath martensite in low-carbon steel, Acta Mater. 54 (2006) 1279–1288. [12] A. Kipelova, R. Kaibyshev, A. Belyakov, D.A. Molodov, Migration of Dislocation Boundaries in a Modified P911 3%Co Heat Resistant Steel during Tempering, Ageing and Creep, Materials Science Forum 715-716 (2012) 953-958.
