MODELLING OF CHEMICAL COMPOSITION OF WELD POOL metal IN ARC METHODS OF WELDING

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1 MODELLING OF CHEMICAL COMPOSITION OF WELD POOL metal IN ARC METHODS OF WELDING V.V. GOLOVKO and L.A. TARABORKIN E.O. Paton Electrc Weldng Insttute, NASU 11 Kazmr Malevch Str., 368, Kev, Ukrane. E-mal: Development of complex calculaton algorthm based on system approach for numercal predcton of formaton and growth of non-metallc nclusons n a weld metal s one of the mportant tasks n present tme. One of sgnfcant blocks of t s a calculaton estmaton of weld metal chemcal composton n arc methods of weldng, whch s of nterest and beng studed n present work. A procedure was proposed for calculaton of content of weld pool melt n arc weldng. The developed procedure s based on modellng of thermodynamcs of nterface nteracton n metal slag gas-vapor phase system n a temperature range typcal for weld pool exstence n arc methods of weldng. Predcted content of metal melt can be a bass for modellng of content, sze, morphology and chemcal composton of non-metallc nclusons n weld metal. 11 Ref., 3 Tables, 3 Fgures. Keywords: arc weldng, weld pool, melt, thermodynamcs, nclusons, slag, chemcal composton, modellng, predcton Development of hgh-strength low-alloy steels has become an outstandng achevement of ferrous metallurgy. Optmum combnaton of mechancal propertes and effcency of such steels producton promoted ther rapd development and wde mplementaton [1, 2]. Applcaton of processes of refnng and sparse alloyng [3], n partcular, provdes for the possblty of achevement of such combnaton. The man efforts of metallurgsts were drected on producton of metal wth more refned gran structure, snce gran sze s one of the most sgnfcant factors determnng servce propertes of low-alloy steels [4]. The experence of works on nvestgaton of peculartes of effect of non-metallc nclusons (NMI) on condtons of formaton of structure of low-alloy steels and level of ther servce propertes, whch resulted n development of ndustral technologes of producton of current structural steels, ndcate the relevance of nvestgaton of effect of NMI on structure and mechancal propertes of metal of welds from low-alloy steels of ncreased and hgh strength [5, 6]. In connecton wth t development of complex calculaton algorthm based on system approach for numercal predcton of formaton and growth of NMI n a weld metal s mportant and nterestng problem. One of sgnfcant blocks of t s a calculaton estmaton of weld metal chemcal composton n arc methods of weldng, representng absolute ndependent nterest and beng consdered n ths work. Submerged arc weldng s one of the comprehensve processes for complex descrpton of pattern of cause-and-effect relatonshp n composton structure propertes system. All know n present tme aggregatve states of the materal,.e. sold state, lqud, gas phase and plasma, partcpate n formaton of weld metal. Metallurgcal reactons n these phases and at nterface take place n the temperature range approxmately from 1 to 1, K. Duraton of ndcated reactons makes from 1 6 s to several seconds. Content of components n separate phases can sgnfcantly change n process of weldng. Presence of hgh-gradent temperature and concentraton feld, typcal for weldng processes, make mpact on character of nteractons. Addtonal dffcultes, related wth modellng of the processes of NMI formaton, are nduced by the fact that the man mass of nclusons s formed n molten metal of the weld pool [7], whle nformaton on chemcal composton of soldfed metal s used for calculatons of ther characterstcs. At that, content of alloyng and mpurty elements n steel, appearng * Ths ssue represents several papers (pp ), prepared by members of the PWI department «Physcal-chemcal processes n weldng arc» devoted to the memory of developer and permanent chef of the Department Prof. Igor K. Pokhodnya. The papers hghlght development of some scentfc deas and technologcal approaches to soluton of problems for provdng hgh qualty of weldng consumables, whch are typcal for developments of the PWI. These drectons reman prorty topcs of the department. V.V. GOLOVKO and L.A. TARABORKIN, ISSN X THE PATON WELDING JOURNAL, No. 1, 216

2 as a result of nteracton between metallc and slag phases n a course of melt exstence, are not consdered. It can sgnfcantly effect content and chemcal composton of nclusons [8]. Calculaton estmaton based on data on weld pool,.e. on content of metal n a temperature range above the temperature of start of ts soldfcaton, s necessary n order to get objectve nformaton on content, morphology and sze of NMI. Step-by-step method s used to study the metallurgcal peculartes of submerged arc weldng for the purpose of smplfcaton of stated problem. Condtonal schematc dvson of weld metal formaton process on three zones, namely reacton zone at droplet stage (from ron bolng temperature 3134 to 1 K); reacton zone n area of hgh temperatures (from 25 to 3134 K); and reacton zone n area of low temperatures (from 18 to 25 K), s taken for ths. Three man calculaton schemes were formed n accordance wth ths dvson and used for nvestgaton of metallc, slag and gas phase, respectvely. Today the sngle generally avalable method for calculaton of actvtes n metallc phase, whch s provded by numercal values of parameters for the most elements appled n metallurgy, s Wagner (decomposton) method [9], therefore t was taken as basc one. However, Wagner method does not nclude thermodynamc requrements to state equaton. At the same tme, a theory of subregular solutons, representng suffcently smple model, satsfyng specfc thermodynamc requrements, and havng sgnfcant advantages n descrpton of multcomponent systems, s not provded wth parameter numercal values. Jont applcaton of theory of subregular solutons wth calculaton of coeffcents of dstrbuton on parameters of Wagner nteracton n thermodynamc model of dstrbuton of elements between metal, slag and gaseous phase provdes for hgher correlaton of calculaton and expermental data n all area of compostons. Model of collectve electrons of Ponomarenko seems to be the best for calculaton of actvtes n slag phase. It consders slags as a soluton, components of whch are the perodc table elements, that allows calculatng ther actvtes ndependently. Therefore, calculaton modellng of metal slag system uses a procedure for evaluaton of thermodynamc functons of the slag as phase wth collectve electrons (method of collectve electrons) [1], provded wth all necessary numercal parameters and allowng calculatng actvty of the slag phase components as well as, n the most general case, takng nto account nonstochometry of all phases. ISSN X THE PATON WELDING JOURNAL, No. 1, 216 Thermodynamc equlbrum reactor (TER) was used for calculaton of gaseous phase. It s desgned for calculaton of chemcal equlbrum n mult-component heterogeneous systems [11]. A bass of algorthm of TER program s a general prncple of entropy maxmum S for calculaton of chemcal and phase composton and correspondng program works n Chemcal WorkBench program package (Knetc technologes) Company, Moscow, Russa). Accordng to ndcated prncple, an equlbrum state s characterzed by unform dstrbuton of thermodynamc parameters n studed volume, and chemcal composton corresponds to maxmum of possblty of dstrbuton of energy levels for macropartcles: S = S max at M j = const; U = const; v = const, where M j s the weght of j-th chemcal element; U s the functon of nternal energy; v s the specfc volume. Correspondng state equatons for calculaton parameters of thermodynamc equlbrum are embedded based on the fact that entropy of multcomponent system conssts of entropy of dfferent components and phases,.e. components of neutral gas and ons type, havng deal gas propertes, and components whch formed pure condensed phases (sold or lqud state) and condensed solutons. In partcular, entropy of gaseous phase s determned on formula k RT S = ln, g S R M M v = 1 where S s the standard absolute entropy; M s the number of moles of -th component per 1 kg of system; p = RTM /v of pressure of -th component. Entropy of components, formng separate pure condensed phase n accordance wth addtvty concept, equals N c S =, c SM n n n = 1 where S s the entropy of condensed phase on 1 mole n of a substance; M n s the number of moles per 1 kg of the condensed phase; N s the total number of separate condensed phases. Parameters of equlbrum state are found as values of all varables of the studed system, ncludng number of moles of components, under condton of entropy maxmum and addtonal lmtatons on parameters mposed by mass conservaton law. Procedure for calculaton of target parameters s based on method of Lagrange usng Newton Raphson method for solvng a set of non-lnear equatons [11]. 13

3 Fgure 1. Block-dagram of algorthm of program for calculaton of chemcal composton of weld pool metal n arc methods of weldng Developed calculaton scheme allows numercal evaluaton of weght fracton of alloyng elements n the weld pool and weld metal n submerged arc weldng based on data on chemcal composton of flux, wre and steel to be welded. The followng values are used as nput parameters: chemcal composton of flux, presented n oxde-slat form, moreover t s supposed that ntal composton of the slag system matches wth flux composton and contans all or some of mentoned components: SO 2, Al 2 O 3, CaF 2, MgO, MnO, TO 2, CaO, sum of percent content of whch shall equal 1 %; weght fractons (%) of alloyng components S, T, Mn, Al as well as oxygen n steel and weldng wre; porton of base metal n weld pool metal; temperature characterstcs of studed process. Correspondng output (resultant) varables of calculaton are as follows: weght fracton of oxygen n weld pool; weght fracton of alloyng elements S, T, Mn, Al n weld pool; weght fracton of alloyng elements S, T, Mn, Al n weld metal. Thus, the algorthm of calculaton evaluaton ncludes the followng steps (Fgure 1): 1. determnaton on calculaton TER of oxygen content n gaseous phase, whch s entered as a result of reacton between slag and weldng arc. A pecularty of ths stage s the fact that Chemcal WorkBench program, among formally thermodynamcally probable products of chemcal reactons n droplet stage, provdes for sgnfcant number of such, whch on practce (due to extraordnarly small correspondng contents) can be elmnated n further calculatons. Therefore, receved lst of reacton products s subjected to revsons and correspondng sgnfcant reducton; 2. calculaton of content of oxygen [O] n the weld pool, for whch purpose to receved n tem 1 value, frstly, t s necessary to add fracton, comng from wre (obtaned ntermedate sum corresponds to oxygen content n droplet), and then add to computed value content of oxygen n base metal takng nto account porton of base metal n weld pool formaton. As a rule, resultant [O] value does not exceed.1 wt.% and, therefore, t can be equaled to oxygen actvty [a O ] n the weld pool; 3. calculaton of oxygen content n slag, for whch t s suffcent to calculate oxygen actvty (a O ) n the slag on FEM accordng to work [1]; 4. determnaton of coeffcent of oxygen dstrbuton L O between slag and weld pool metal by formula L O = (a O )/[a O ]; 5. calculaton of actvty of alloyng elements [a Me ], passng from slag n weld pool metal by formula [a Me ] = (a Me )/L Me, where for smplcty t can be taken that coeffcents of dstrbuton L Me of alloyng elements between slag and weld pool approxmately equals to found n tem 4 coeffcent L O ; 6. calculaton of content of each alloyng element S, t, Mn, Al (wt.%) n weld pool and weld as sum of ts contents n droplet, n base metal (takng nto account ts porton) and alloyng elements, passng from slag phase (see tem 5). Computer realzng of descrbed algorthm was carred out n object-orented meda for vsual programmng Delph 7. Developed computer program works under the operatng system Wndows XP. After launchng the program dsplays a form (Fgure 2) whch contans felds and tables for nput and output of data, provded n algorthm descrpton. The program allows carryng out mult-choce calculatons for fxed set of values of nput varables, snce allows performng new calculaton wthout complete cleanng of all form,.e. after change of some (even 14 ISSN X THE PATON WELDING JOURNAL, No. 1, 216

4 Table 1. Composton of fluxes used n nvestgatons and ther bascty, wt.% Flux number MgO Al 2 O 3 SO 2 CaF 2 BI one) of entered values. Check of adequacy of the developed calculaton evaluaton was carred out n arc weldng usng agglomerated fluxes, content of whch was bult on MgO SO 2 Al 2 O 3 CaF 2 system. scheme of weld formaton, used for evaluaton of porton of base metal n weld pool, s gven n Fgure 3. Oxygen potental of fluxes was changed due to change of MgO/SO 2 relatonshp for nvestgaton of possblty of predcton of ts effect on condtons of NMI formaton. Submerged arc weldng was carred out n combnaton wth weldng wre Sv-8GA of 4 mm dameter n accordance wth the requrements of ISO 14171:2. Butt jonts from low-alloy steel of 1KhSND grade of 25 mm thckness wth 6 bevelng and 2 mm gap n weld root were produced n course of experments. Templates for producton of mrcosectons for metallographc nvestgaton were cut out from metal of the last layer, whch was located n the mddle of the upper layer. Metallographc nvestgatons were carred out on transverse mcrosectons, cut out from welded jonts. Quanttatve analyss of NMI and determnaton of general contamnaton of the weld wth nclusons was performed wth the help of optcal mcroscope Neophot-3 (Carl Zess Jena, Germany), equpped wth hgh resoluton dgtal camera. In partcular, dstrbuton of nclusons on sze was determned usng mages of pxels sze. Calculaton of amount of ncluson n each specmen on sze group,.e. from mnmum to maxmum sze, was carred out on set program. Analyss of NMI chemcal composton were performed on electron mcroscope JSM-35 (Japan) wth the help of energy dsperson spectrometer INCA-35 (Great Brtan) «pont by pont» wth the purpose of elmnaton of background radaton. Fgure 2. Wndow of program for numercal modellng of transfer of alloyng elements n molten pool and weld n submerged arc weldng Fgure 3. Scheme of weld formaton Compostons of expermental fluxes (Table 1; n general t was 2 varants) were calculated n accordance wth optmum mathematcal expermental plan. Values of ther bascty were calculated by formula MgO+CaF 2 BI =. SO +.5Al O Table 2 provdes for the results of correlaton of calculated contents of oxygen and alloyng elements n the weld metal wth data on ther determnaton n specmens of deposted metal obtaned n weldng usng plot fluxes. Content of these elements s gven n Table 3. Observed dfferences between calculaton and expermental data are caused by processes of formaton of NMI n wed metal n two-phase zone, located n nterdendrtc volumes at temperatures below equlbrum soldfcaton temperature. Besdes, they one more tme underlne that predcton of content and composton of NMI n metal of formed weld can not Table 2. Calculaton (n weld pool) and expermental (n weld metal) content of elements Flux Expermental data, wt.% Calculaton data, wt.% number O Mn Al S O Mn Al S ISSN X THE PATON WELDING JOURNAL, No. 1,

5 Table 3. Content of oxygen and alloyng elements n base metal and wre Materal Content, wt.% O Mn Al S Base metal Wre be based on data on ts chemcal composton, beng fnal product of all processes and reactons whch took place. Applcaton of calculaton data on composton of «vrtual» weld pool s more adequate for ths purpose. The smlar approaches were performed for other alloyng elements (T, S, Al). Thus, the calculaton data on content of alloyng elements and oxygen n weld metal, obtaned wth the help of descrbed model, can serve as ntal values for modellng ntegral composton of NMI, such as morphology as well as composton of separate phases, formng these nclusons, based on objectve nformaton on ntal condtons of ther formaton. Proposed approach and developed calculaton scheme can be dstrbuted on other methods of arc fuson weldng (wth coated electrodes, flux-cored wre), whch ncludes metallc, vapor-gas and slag phases. Conclusons The method was proposed for calculaton of composton of weld pool metal melt. Developed procedure s based on modellng of thermodynamcs of nterface nteracton n metal gas vapour phase system n a temperature range typcal for exstence of weld pool n arc methods of weldng. The predcted composton of metal melt can serve as ntal base for further modellng of content, sze, morphology and chemcal composton of non-metallc nclusons n weld metal. 1. Weng, Y. (23) Mcrostructure refnement of structural steel n Chna. ISIJ Int., 43(11), Borovkov, A.V. (23) Producton of straght-seam large-dameter ppes made of steel of strength class X8. Metallurgst, Vol. 47, Km, Y.M., Km, S.K., Lm, Y.J. et al. (22) Effect of mcrostructure on the yeld rato and low temperature toughness of lne ppe steels. ISIJ Int., 42(12), Shukla, R., Das, S.K., Kumar, B.R. et al. (212) Ultralow carbon, thermomechancally controlled processed mcroalloyed steel: mcrostructure and mechancal propertes. Metallurg. and Mater. Transact. A, 43(12), Park, J.S., Lee, C., Park, J.H. (212) Effect of complex ncluson partcles on the soldfcaton structure of Fe N Mn Mo alloy. Ibd., B, 43(12), Sarma, D.C., Karasev, A.V., Jonson, P.G. (29) On the role of nonmetallc nclusons n the nucleaton of accular ferrte n steels. ISIJ Int., 49(7), Babu, S.S. (29) Thermodynamc and knetc models for descrbng mcrostructure evoluton durng jonng of metals and alloys. Int. Materals Rev., 6, Znngrebe, E., Van Hoek, C., Vsser, H. et al. (212) Incluson populaton evoluton n T-alloyed Al-klled steel durng secondary steelmakng process. ISIJ Int., 52(1), Jung In-Ho, Decretov, S.A., Pelton, A.D. (24) Computer applcaton of thermodynamc databases to ncluson engneerng. Ibd., 44(3), Grgoryan, V.A., Stomakhn, A.Ya., Ponomarenko, A.G. (1989) Physcal-chemcal calculatons of electrc steelmakng processes. Moscow: Metallurgya. 11. (27) CHEMICAL WORKBENCH vers. 3.5: Descrpton of reactor models. Moscow: Knetc Technologes. Receved ISSN X THE PATON WELDING JOURNAL, No. 1, 216