GRAIN SIZE PREDICTION OF STEEL P92 BY HELP OF NUMERICAL SIMULATIONS. Jaromír MORAVEC, Josef BRADÁČ, Heinz NEUMANN, Iva NOVÁKOVÁ

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1 GRAIN SIZE PREDICTION OF STEEL P92 BY HELP OF NUMERICAL SIMULATIONS Jromír MORAVEC, Josef BRADÁČ, Heinz NEUMANN, Iv NOVÁKOVÁ Technicl University of Liberec, Studentská 2, Liberec, Škod Auto University, Tř. V. Klement 869, Mldá Boleslv, Technicl University of Liberec, Studentská 2, Liberec, Technicl University of Liberec, Studentská 2, Liberec, Abstrct New nd more complex industril pplictions lso cll for the development of new mterils with specil properties. Het-resistnt steel P92 cn be clssed s one of these mterils. To predict such mterils behvior by different running conditions or by defined het tretment it is importnt to cquire sufficient number of dequte input prmeters in temperture dependence. The submission dels with the nlysis of the temperture impct on the grin size, which is n importnt input prmeter for numericl simultions. This prmeter hs direct connection with the resulting mechnicl properties of given mteril. Different types of the evlution of the grin size ccording to ČSN EN ISO 643 stndrd re lso presented, both for thermlly unexposed specimen s well s for thermlly exposed specimens during different length of time. The submission further shows how the resulting dt cn be used in numericl simultions for the grin size prediction fter welding or het tretment. Key words: Grin Size, Steel P92, Numericl Simultions, Sysweld Progrm 1. INTRODUCTION Appliction of New nd progressive mterils nd mnging their technologicl processes is ccompnied by n effort to use simultion computtions, which cn substntilly eliminte the risks linked to the occurrence of uncceptble defects. This is lso vlid for the ppliction of numericl simultions in welded constructionl units. The Sysweld simultion progrm is one of such progrms used in welding processes. Input dt for therml-metllurgicl nd mechnicl nlyses re decisive for the credibility of simultion computtions results. In order to predict the structure nd resulting properties in weld joints re it is necessry to know the grin size, not only for the mteril s initil stte but lso for the processes of chnges which occur during welding or the subsequent het tretment. Within the TA project the reserch ws lso focused on the determintion of input prmeters for grin size in numericl simultions by different types of bsic mterils. Methodicl procedures nd experimentl results concerning grin size determintion by steel P92 re lso presented in the submission. 2. STEEL P92 CHARACTERISTICS Over long period power engineering hs been trying to increse energy efficiency nd operting time of energetic units s well s to reduce CO 2 emissions. These objectives cn be primrily chieved by incresing the stem temperture nd pressure. The development tendencies of stem prmeters nticipte tht the temperture could increse to 700 C nd the pressure to 35 MP. Some prognoses count even with higher tempertures. This increse of prmeters would trigger n introduction of new mterils on the Ni bsis. It is ssumed tht the increse of the overll efficiency of the equipment will up to 50% [1]. For the temperture rnge of C nd pressure of bout 26 MP, the modified 9-12 % of Cr steel ppers promising. P92 steel belongs to this kind of steel. P92 steel is ferritic steel (9% Cr, 1,75% W,

2 0,5% Mo) lloyed with vndium nd niobium nd with controlled mount of boron nd nitrogen. This steel is the result of Jpnese reserch nd its originl nme ws NF 616, fter the producer Nippon Steel Corportion. According to ASTM it is mrked s P92 steel. Due to the high mount of chromium nd the inherence of other lloying elements by unmodified (9-12)% Cr steels, the curves in CCT digrms re shifted to long times nd tht is why these steels re hrdening lso in gret thickness by cooling in the ir. Consequently these steels hve mrtensitic structure in wide rnge of cooling rtes. Mteril in the form of tube with ø 38 mm nd wll thickness of 6 mm ws used for grin size prediction nd nlysis. The chemicl composition nlysis is seen in tble 1. Figure 1 shows the initil stte of the tested mteril microstructure. Tb. 1 Chemicl composition of tested mteril X10CrWMoNb9-2 (P92) C Mn Si Cr Mo W V Nb N wt. % Fig. 1 Initil stte microstructure of P92 steel (Villel Bin 200 x) 3. GRAIN SIZE COMPUTATION IN SYSWE PROGRAM The thermodynmic driving force for the grin s growth is the reduction of Gibbs free energy of the surfce. The grin growing process results in the reduction of the grin surfce s boundry nd thus to decrese of free energy. The Grin size is importnt in two spects. It is importnt in the light of mechnicl properties of the mteril (e.g. brittleness properties), but lso with respect to the influence on the trnsformtion running processes, s it ws stted before. The chnge of the grin size is relted to temperture chnge nd is crucil for the proportionl mount of the relevnt phse. The following bsic rules re pplied for the grin growth: The grin grows becuse its boundries shift nd not becuse they grow together. The movement of grin size is interrupted nd the movement direction could suddenly chnge. The grin could growth into nother one to the detriment of its volume. The Grin consumption often increses, when the grins re lmost consumed. The Curved boundry usully migrtes to its curvture centre. If the grin boundries of one phse meet together in ngles different thn 120 grdes, grins with the cute ngle boundry re consumed so tht the ngle pproches 120 grdes.

3 The First models build on the physicl bsis were developed t the beginning of 1950s (Smith 1952, Burke Turnbul 1952). However there ws considerble difference between the theoreticl nd experimentl determined vlues. In 1980 new pproch bsed on numericl simultions ws introduced. On the bsis of simultion computtion it ws lso possible to monitor processes which were difficult to observe experimentlly (e.g. the volume chnge rte of individul grins). Over the yers number of simultion methods were developed, while the most used one is the Monte Crlo Potts Model method. A Description of this method is given for exmple in [2, 3]. An Idel rule for the grin growth is given by eqution (1). Constnt K is expressed by eqution (2) [4]. D D 0 K t (1) K Q RT K 0 e (2) D grin dimeter, D 0 initil grin size, Q ctivtion energy (J mol -1 ), R gs constnt (J K -1 mol -1 ), T temperture (K), coefficient (-). It ws estblished by experiments, tht m coefficient vlues move between 2 nd 5. Vlue =2 pplies, if the grooving process is solely controlled by diffusion. Vlue =4 is determined in cse of precipittion nd diffusion long the grin boundry. Grin grooving is however influenced by other fctors, for exmple heting rte nd grin growth brriers. The computtion of grin size proceeds in the Sysweld simultion progrm on the bsis of eqution (3) which expresses the growing rte of the grin size. D Q C exp R T (3) Constnt C (mm s) is usully mm 4. s -1. Stisfctory results by computtions re reched through the rtio Q/R = This computtionl eqution is designed for cses, when the mount of ustenite is constnt or is decresing. If the mount of ustenite increses, two developments re observed: The size of existing grins is incresing. Genertion of new grins is undergoing by zero initil grin size. By generliztion of the conventionl eqution, in order to determine the grin size, we rrive to the following eqution (4). Q D R T exp for >0 ustenite is creted (4) D C In this eqution the λ vlue expresses the ustenite proportion while the vlue show the trnsformtion rte of this phse. If the ustenite is not creted, i.e. 0 is expressed by the eqution (3).

4 4. EXPERIMENTAL DETERMINATION OF GRAIN SIZE For simultion computtions in Sysweld progrm it is importnt to know the rel conditions linked to the therml exposition of the mteril. Tht is why the grnt project tests the grin size chnges by therml exposition. The tested smples were 12 mm long rings cut from the given tube on the Delt AbrsiMet equipment. They were thermlly exposed in n oven t 1000 C, 1100 C for 4 nd 8 hours nd subsequent cooling down in. Other specimens were exposed t 1179 C for 1 nd 4 hour with subsequent cooling down in s well s in the ir. By heting to the tested temperture the heting rte for ll specimens ws 420 C/hour. Specimens for the structure vlidtion were prepred under stndrd metllogrphic procedure. Villel Bin enchnting ws used for highlighting the structure. The grin size ws evluted ccording to CSN EN ISO 643 stndrd. This stndrd specifies the microgrphic determintion method of ferrite nd ustenite grins in steels. It describes the possibilities of determining the verge size of grin in cse of specimens with equl size distribution. The Grin size is determined by microscopic observtion of metllogrphic specimen prepred by suitble method ccording to the steel type. The Averge size of grin is chrcterized either by the grin size number nd/or by the verge vlue of the cught grin sections. Within the experiment the grin size ws expressed prtly by the intersectionl method nd prtly by the plnimetric method. On the bsis of the estblished dt n verge mount of sectors per millimeter of the mesuring line N L, n verge length of the cptured sector l nd middle grin surfce nd the corresponding number of grin size G were determined. The Grin size number is mrked by G nd is defined with the eqution (5), while m is the number of grins per squre millimeter of the tested specimen in the evluted surfce. The Grin middle surfce is determined by the eqution (6) nd the Middle grin dimeter d is lso determined by the eqution (7). However, this pproch is bsed on wrong presumption tht grin hs squre shpe nd tht is why it cnnot be recommended. log m 3 log 2 G (5) 1 m (6) 1/ 2 d (7) Determined vlues of the grin size for individul experiment conditions re shown in tble 1. The middle grin dimeter vlue is lso mentioned there. 5. SIMULATION COMPUTATIONS OF GRAIN SIZE Experimentlly determined vlues of grin size were used s input dt for grin size clcultion in the Sysweld simultion progrm. The computtions of this progrm re bsed on the equtions (3) nd (4). The vlue of the Activtion energy Q cn be set by the definition of the computtion model using the positive power constnt nd the constnt C. On the bsis of long time experience it is recommended to keep the constnt vlue C=0, mm 4. s -1. When testing the computtionl model, it ws find out tht the best mtch with relity is chieved through the positive constnt vlue =4. The min output prmeter (prt from the grin size determintion by experiments) is the ctivtion energy vlue Q, or the rtio Q/R. When defining the computtion model, the model is first tested on simultions of experiments done during the mesurement of grin size nd compred with the rel-life estblished vlues of grin size for thermlly untreted specimen s well s for thermlly exposed specimens. These dt re used s the bsis for estblishing the ctivtion energy vlue Q, or rtio Q/R. In cse of P92 steel this rtio is Q/R=72800 for 1000 C, Q/R=69800 for 1100 C nd Q/R=68000 for 1179 C.

5 Such checked model is consequently used for the clcultion of the grin size on the bsis of nonsttionry temperture fields ppering during welding. Figure 2 shows the result of the numericl computtion of the grin size in cse of tube heterogeneous weldment from mterils P92/S304H. The Weld ws mde by n utomtic mchine using ISO method. The performed weld ws used to prepre specimens for mtllogrphicl nlysis, to determine the grin size in the het ffected zone to compre the results ginst the results from simultion computtion. Tb. 1 Determined vlues of grin size [5] Specimens - P92 N L l G1 Initil stte 1000 C 4 hours 1000 C 8 hours 1100 C 4 hours Number of grins in 1mm 2 Middle grin surfce [mm 2 ] G2 Middle grin dimeter [mm] C hours N L verge number of grins gripped to the line unit length, l verge length of liner sector, G1 grin size number evluted with intersectionl method, G2 grin size number evluted with plnimetric method, longitudinl direction, trnsversl direction Fig. 2 The result of numericl computtion of grin size for heterogeneous weldment P92/S304H [6]

6 Following the rel-life welding experiment the computed dt of the grin size nd the rel-life estblished grin size in HAZ were compred, see tble 2. Nmely they were plces with the distnce from the melting boundry (MB) of 0.5; 1.0; 1.5; 2.0; 2.5; 3.0 nd 5.0 mm. Tb. 2 Comprison of the grin size results computed numericlly nd estblished experimentlly Distnce from the MB [mm] Midle grin size [mm] Number of grins in 1 mm Grin size number G G5 G5 G6 G6 G6 G7 G11 Midle grin size - computtion [mm] Number of grins in 1 mm 2 comp Grin size number G computtion G3-4 G3-4 G4 G4-5 G6 G7 G11 6. COCLUSION Mteril properties depend on the grin size nd tht is why nowdys there is higher demnd for the grin size prediction by help of numericl simultions. The Computtionl progrm unit for the grin size determintion in the Sysweld progrm ws developed s mrginl unit, minly intended for generl purposes. Tht is why one of the ims of TA project is the vlidtion nd, if necessry, modifiction of this progrm unit. When compring the simultion computtion results nd the grin size vlues estblished experimentlly it ws found tht the simultion results did not fully correspond to the relity. From tble 2 it is evident tht the grin size predicted by the Sysweld progrm reflects the relity only in the distnce of prox. 2,5 mm from the melting boundry, i.e. outside of the het ffected zone. The discrepncy between relity nd the computtion is likely to hve been cused by more fctors which need to be further nlyzed nd which cll for n djustment of the computtionl progrm unit. The methodology of the grin size determintion (different size of grins in HAZ) could lso be one of the possible cuses. Although the surfce for the determintion of the middle grin size is very smll, the resulting ccurcy cn be ffected by it. Within the experimentl tests dt bout the grin size of selected steel types for energetic industry were obtined. These dt could be useful for clibrtion of the computtionl progrm unit in the Sysweld simultion progrm. ACKNOWLEDGEMENT The rticle ws prepred within the scope of the TA grnt project. REFFERENCES [1] MÄENPÄÄ, L., KLAUKE, F., TIGGES, K. D. Mteril spects of 700 C power plnt. Hitchi Power Systems, Ltd. Duisburg. 2010, 7 p. [2] ZÖLLNER, D., STREITENBERGER, P. Three-dimensionl norml grin growth: Monte Crlo Potts model simultion nd nlyticl men field theory. Script Mterili , p [3] ZÖLLNER, D., STREITENBERGER, P. Monte Crlo Potts Model Simultion nd Men-Field Theory of Norml Grin Growth. Shker Verlg GmbH, Germny, 2006, 170 p. [4] KHZOUZ, E. Grin Growth Kinetics in Steels. A Mjor Qulifying Project Report. Worcester polytechnic institute, pril 2011, 144 p. [5] MORAVEC, J., NOVÁKOVÁ, I. Mesurement of ustenitic grin size by P92 mteril. Technicl report TUL-Z-12- VZ , 8 p. [6] SLOVÁČEK, M., TEJC, J., KOVAŘÍK, J., BEJVL, J., RICHTER, T., VANĚK, M.: Vývoj vlidce mteriálových modelů. Rozbor stávjícího stvu. Technicl report Mecs ESI M_Z_12_024_R p.