EVALUATION OF MICROSTRUCTURAL STABILITY OF CREEP-RESISTANT STEELS WELD JOINTS ON THE BASIS OF A COMPUTATIONAL MODELING

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1 EVALUATION OF MICROSTRUCTURAL STABILITY OF CREEP-RESISTANT STEELS WELD JOINTS ON THE BASIS OF A COMPUTATIONAL MODELING Pavel ŠOHAJ a, Vít JAN a, Ondřej DVOŘÁČEK a a VUT, FSI, UMVI, Technická 2896/2, Brno, Česká republika, Pavelsohaj@seznam.cz Abstract The paper presents results of study of structural stability of creep-resistant steels P24, P22, P91 and their weld joints P24/P22 and P22/P91. The microstructural changes during annealing at temperatures of C and hours were examined. During high temperature exposure, carbon diffuses in the direction of its chemical potential gradient and carbon enriched and carbon depleted zones are formed in the weld interface area. In the case of P24/P22 joint, the P22 steel is carburised, in case of P22/P91 joint, P91 steel is carburised. Redistribution of carbon causes changes of mechanical properties in the weld interface area through changes of phase composition. Calculations of phase diagrams shape, phases composition, weight and volume fractions of minor phases and temperature dependence of carbon activity were performed using the Thermo-Calc software. The DICTRA software was used for simulations of diffusion redistribution of carbon across the welds. These calculations were used for theoretical predictions of expectable behaviour of investigated materials and welds during their long time high temperature exploitation. These were consequently verified by structural and phase analysis of investigated samples. The computed results were compared with published data with satisfactory agreement. Keywords: Microstructural stability, creep-resistant steel, weld joint, Thermo-Calc 1. INTRODUCTION At present time, large number of power generating facilities undergo reconstruction and refitting. The most widely used materials in these applications are creep-resistant steels which are often connected by dissimilar weld joints. Generally, the necessary high creep-strength of these materials and their joints is ensured by microstructural stability. In this work a microstructural stability of creep-resistant steels P24, P22, P91 and their dissimilar welds is examined. As a suitable tool for evaluation of microstructural stability, computational modelling of phase composition at thermodynamical equilibrium was selected. This modelling approach forms nowadays one of the standard tools of material designing process. The Thermo-Calc software, which uses the CALPHAD method [1], [2] presents a generally accepted standard software used for computational phase equilibria determination. By use of the add-on package DICTRA [3], it is possible to simulate diffusion and its effects on chemical and phase composition. Thermodynamic database STEEL12 [4] and kinetic database Dif were used in the calculations. 2. EXPERIMENTAL P24, P22 and P91 steels of standard purity were used as experimental material. The chemical composition of the used steels is in Table 1. The steels were supplied in heat-treated state. Cylindrical samples with one polished basis were machined out of the materials. The samples were resistance-welded to form the experimental welds of P24/P22 and P22/P91. These were subsequently annealed in evacuated glass capsules at temperatures C, for hours (see Table 2). After annealing the samples were rapidly cooled down in water. Samples were cut up from the heat treated samples perpendicularly to the 1

2 weld interface. Metalographical evaluation of microstructure, microhardness measurements and chemical analyses were performed on the samples across the weld interface. Table 1 Chemical composition of the used steels Table 2 Annealing conditions of the experimental joints Sample joint type annealing conditions Sample joint type annealing conditions 1 P22/P C/16h 6 P24/P C/16h 2 P22/P C/160h 7 P24/P C/160h 3 P22/P C/320h 8 P24/P C/320h 4 P22/P C/1000h 9 P24/P C/1000h The microstructure of samples was evaluated by means of light and SEM microscopy. The carbon concentration profiles perpendicularly to the weld interface were measured using WDX method on samples 3 and RESULTS AND DISCUSSION 3.1 Phase diagram calculations of the base materials The P24 steel The P24 steel is a low alloyed steel with chemical composition 0,5Cr-0,5Mo-0,25V. The phase diagram of the steel calculated in Thermo-Calc is in the Fig. 1. According to the phase diagram, MC carbines are stable in matrix up to 900 C. The M 2 C carbides are stable up to 680 C; up to 730 C a small amount of M 7 C 3 carbide is present in the matrix. Fig. 1. Phase diagram and weigth fraction of minor phases for steel P24 (Thermo-Calc calculation) The matrix of the microstructure of the experimental samples consisted of ferrite and bainite mixture. The largest fraction of minor phases was found in samples annealed at 500 C. Very fine particles dispersed on grain boundaries were observed and more particles in clusters inside grains were observed. With increasing temperature the amount of observed particles decreased and their inter-particle distance increased. A small amount of fine particles was found in the microstructure undissolved also in samples annealed at 900 C (MC carbides). 2

3 The P22 steel The P22 steel is a low alloyed steel of the 2,25Cr- 1Mo type. Fig. 2 shows the phase diagram of the steel calculated in Thermo-Calc. According to the calculated diagram, the M 7 C 3 carbide is the main minority phase in this material. At 520 C the molybdenum rich M 6 C carbide starts to precipitate Fig. 2. Phase diagram and weigth fraction of minor phases for steel P22 and is stable to 720 C. At (Thermo-Calc calculation) the usual operating temperatures of 550 C the undesirable Laves phase is also stable. The microstructure of all the samples was formed by bainitic matrix. The highest fraction of minority phases was found at the 500 C temperature. The volume of the minority phases decreased with increasing temperature of annealing. Particles on grain boundaries were found stable up to higher temperatures. At 625 C the matrix was partially recrystallized. The P91 steel The P91 steel belongs in the group of 9-12%Cr steels and it is regarded as a distinctive step in creep resistant steels development [5]. According to computed diagrams in Fig. 3, the material is hardened by M 23 C 6 carbides and MX carbonitrides which are present in the martensitic matrix. Up to 620 C the Laves Fig. 3. Phase diagram and weigth fraction of minor phases for steel P91 phase is also stable. The (Thermo-Calc calculation) calculations predicted also the presence of nitridic Z-phase instead MX carbonitrides. According to previously published results [6], the Z-phase is thermodynamically more stable than MX carbonitrides at temperature around 800 C. Precipitation of this phase starts in the steel P91 at 650 C after hours [6] and generally is regarded to cause a loss of creep strength. The presented calculated phase diagram is consistent with phase state of the material in the beginning of the operation time. The microstructure of all samples of the P91 steel had fine-grained martensitic matrix. The highest fraction of the minor phases was found at 500 C. Very fine particles were dispersed in the matrix. The particles on the grain boundaries formed almost a continuous network. Particle size and interparticle distance grew at 575 and 625 C. A small amount of undissolved precipitates was kept even at 900 C. 3

4 3.2 The weld joints Considering the high values of carbon mobility at the usual operating temperatures of creep resistant steels ( C); carbon redistribution can be expected during high-temperature exposure of heterogeneous welds of the materials in question. Carbon will diffuse in the direction of its thermodynamical activity gradient - disregarding the direction of the concentration gradient - and carbon enriched (CEZ) and carbon depleted zones (CDZ) will be formed in the weld interface area [7]. The temperature dependences of carbon thermodynamical activity of used steels calculated in Thermo-Calc were used for the formulation of basic expectations on the high temperature behaviour of the investigated steels (Fig. 4). Simulations of the diffusional processes were performed in DICTRA module afterwards. P24 P22 P91 Fig. 4. Temperature dependence of carbon activity of the investigated steels (Thermo-Calc calculation) The P24/P22 joint Higher carbon activity in the P24 steel compared to steel P22 can be derived from the computed dependences of carbon activity on temperature (Fig. 4.). Diffusion of carbon from the P24 steel to the P22 steel causes formation of CDZ and CEZ by which the joint can be gradually weakened. This presumption was confirmed by simulation in DICTRA software and by examination of the annealed samples. Changes of microstructure were found at 575 and 500 C. At 575 C (sample 8) the precipitates partly dissolved on the side of P24 and the amount of precipitates was increased on the site of P22. Maximum value of carbon concentration profile measured in the CEZ in P22 steel was 0,15 mass %. This concentration declined steeply with the distance from weld interface and at 200 μm from weld interface, original concentration of carbon was measured again. The P24 half of the joint was partially recrystallised and intensive precipitation occurred in the steel P22 in 10 μm thick zone from the interface. The average decrease of hardness in CDZ of P24 steel was HV 0,1, growth of hardness in CEZ of P22 steel were HV 0,1, 60 HV 0,1 in case of sample 9. The P22/P91 joint The carbon activity values in the P22 steel are two orders of magnitude higher when compared to the P91 steel (Fig. 4.). Therefore strong redistribution of carbon by diffusion at elevated temperatures can be expected. Also distinctive zones of changed microstructure are formed (CEZ and CDZ). Again, this was confirmed by combination of DICTRA simulations and experimental evaluation of the samples. The highest gradient of carbon activity across the weld interface correspons to temperature of 575 C (sample 3). Considerable carbon redistribution was also well observed on samples annealed at 625 C. In these cases, almost complete decarburisation of the P22 steel was measured in a zone 140 μm wide from 4

5 the weld interface. Simultaneously, the P91 steel was carburised in the width of 140 μm from interface. The changes were very distinctive even after relatively short time of annealing, which in this case was 320 hours. The maximum concentration of carbon measured in the CEZ was 0,91 mass % (Fig. 5.). P22 P91 Fig. 5. Carbon redistribution in the P22/P91 weld joint after annealing 575 C/320h. Fig. 6. Weld interface in P22/P91 joint after annealing 575 C/320h. Fig. 7. Microhardness values profile across weld interface of P22/P91 joint expected for that reason. The carbon redistribution was followed by microhardness changes in the vicinity of the weld. The decarburizing of P22 steel led in all cases to dissolution of carbides, hence lower dispersion hardening of the matrix. Dissolution of carbides in P22 also enabled grain coarsening at 575 C and at 625 C re-crystallisation of main part of the bainitic ferrite. These microstructural changes had direct effect on the reduction of hardness in CDZ (by HV 0,1). The fraction of carbidic phases increased dramatically in the P91 steel as result of carburizing. At 625 C the carbides underwent marked coarsening, whereas at 575 C stayed the particles size remained small and the carbides were finely dispersed in the matrix. The coarsening of the carbidic particles strongly suppresses the effect of dispersion strengthening, thus no intensive increase of hardness was measured at 625 C in the CEZ, even though considerable carburising was identified. On the other hand at temperature 575 C, where the carbides did not coarsen, the hardness was increased due to carburizing for 60 HV 0,1 in CEZ (Fig. 7.). The values of chemical profiles of carbon content across the weld interface measured by WDX corresponded to calculations results from DICTRA software and also compared to previous published results [8 11] with good agreement. One exception in this is the comparison of calculated chemical composition of MX phases compared to published data. MX composition show inaccuracies larger than the measurement error usually is [12],[13]. The reason lies within the fact that MX carbonitrides do not have stoichiometric composition, since they contain a large amount of vacancies [14]. The proportion of metallic elements to carbon and nitrogen can be larger than 5

6 4. CONCLUSION The dissimilar weld joints P24/P22 and P22/P91 are unstable at temperature C from microstructural point of view. During their high-temperature long-term exposition, carbon diffuses in the direction of its chemical potential gradient and CDZ and CEZ are formed in the weld interface area (usually "up hill" diffusion occurs). The steel with larger Cr amount is carburised. Microstructural instability of the examined joints reaches a critical point at temperatures near 575 C, when mechanical properties of the materials forming the weld joints are significantly degraded. ACKNOWLEDGEMENT This work was supported by GP106/07/P198 funded by GAČR and FSI-J funded by FSI VUT Brno. REFERENCES [1] Saunders, N. Miodownik, A.P. CALPHAD, Elsevier Science, Amsterodam, 1998, ISBN [2] ThermoCalc User`s Guide, Div. of Comput. Thermodynamics, Dept. of Mater Science and Engineering, Royal Inst. of Technology, Stockholm, 1998 [3] DICTRA User`s Guide, Div. of Comput. Thermodynamics, Dept. of Mater Science and Engineering, Royal Inst. of Technology, Stockholm, 1998 [4] Kroupa, A. et al. Journal of Phase Equilibria, 22, 2001, 312 [5] Vodárek, V. Fyzikální metalurgie modifikovaných (9-12)%Cr ocelí. VŠB Technická Univerzita Ostrava, Ostrava, 2003, 163 s. ISBN [6] Danielsen, H.K. Hald, J. A thermodynamic model of the Z-phase Cr(V, Nb)N, Computer Coupling of Phase Diagrams and Thermochemistry 31, 2007, [7] Pilous, V. Stránský, K. Strukturní stálost návarů a svarových spojů v energetickém strojírenství, Academia, Praha, 1989, ISBN [8] Sopoušek, J. Foret, R. Jan, V. Simulation of dissimilar weld joints of steel P91, Science and Technology of Welding and Joining, 9, 2004, [9] Jan, V. Sopoušek, J. Foret, R. Weld joint simulations of heat-resistant steels, Archives of Metallurgy and Materials, 49, 2004, [10] Sopoušek, J. Foret, R. More sophisticated thermodynamic designs of welds between dissimilar steels, Science and Technology of Welding and Joining, 13, 2008, 24 [11] Hodis, Z. Difúze uhlíku a dusíku ve svarových spojích žáropevných feritických ocelí, Brno: Vysoké učení technické v Brně, Fakulta strojního inženýrství, 2009, 88 s. [12] Abe, F. Kern, T.U. Viswanathan, R. Creep-resistant steels, Woodhead publishing, Cambridge, 2008, ISBN [13] Thomson, R.C. Bhadeshia, H.K. Changes in chemical composition of carbides in 2.25Cr-1Mo power plant steel, Material Science and Technology, 10, 1994, [14] Sopoušek, J. Foret, R. Carbon and nitrogen redistribution in weld joint of ion nitrided 15CrMoV and advanced P91 heatresistant steels, Journal of Phase Equilibria and difusion, 27, 2006,