Prediction of Reoxidation Inclusion Composition in Casting of Steel

Transcription

1 Prediction of Reoxidtion Inclusion Composition in Csting of Steel LIANG WANG nd CHRISTOPH BECKERMANN A model is developed to clculte the composition of reoxidtion inclusions tht form during pouring of steel cstings. The softwre pckge Thermo-Clc is used to obtin the inclusion phse frctions nd compositions s function of the temperture nd oxygen content of the steel. Oxygen is ssumed to be continully bsorbed by the steel until the liquidus temperture is reched. Both lever rule nd Scheil-type nlyses re performed. The model is pplied to reoxidtion of two crbon steels, one low-lloy steel nd one high-lloy steel. The effects of vritions in the steel composition nd the oxygen bsorption rte on the inclusion composition re investigted in prmetric study. The mss frction of bsorbed oxygen is determined by mtching predicted with previously mesured reoxidtion inclusion compositions for the vrious steels. Good greement is obtined for most phses present in the inclusions. Interestingly, the greement in the inclusion compositions occurs for ll steel grdes when the percentge of bsorbed oxygen is equl to 0.9 wt pct. This vlue is explined using seprte model for the rte of oxygen bsorption t the steel-tmosphere interfce. Vrious scenrios re outlined tht llow for the 0.9 wt pct of bsorbed oxygen to be chieved. The model is then used to clculte the mount of lloy elements consumed nd inclusions formed s function of the oxygen boundry lyer thickness in the tmosphere nd the integrted free surfce re of the liquid steel during pouring. It is found tht for unprotected liquid steel trnsfer opertions, such s tpping nd ldle filling, the integrted free surfce re nd exposure time product cn rech vlues of the order of 100 m 2 s per ton of steel, nd tht the ir-to-steel volume rtio during pouring cn be s lrge s 40. It is concluded tht, in order to crete comprehensive tool for simulting reoxidtion formtion, more detiled models re needed for the externl oxygen trnsfer in the tmosphere, the flow of the liquid steel during pouring, nd the internl trnsport nd rections of chemicl species in the steel. I. INTRODUCTION INCLUSIONS re mjor cost fctor in the production of steel cstings. Removing inclusions nd refilling the defect res with the weld metl ccount for pproximtely 20 pct of the direct cost of steel cstings. [1] Reoxidtion inclusions re the most common form of inclusions tht re found in steel cstings. Bsed on survey of steel foundries, they ccount for pproximtely 83 pct of the inclusions in crbon nd low-lloy steel cstings nd pproximtely 48 pct of the inclusions in high-lloy nd stinless steel cstings. [2] Reoxidtion is defined s the rection of elements in steel with oxygen during pouring of the (deoxidized) steel from the ldle into the mold. Oxygen my come from the surrounding tmosphere, refrctories, slg, or the snd mold. [3] The tmosphere is generlly believed to be the primry source of oxygen, nd limiting the exposure of the steel to the tmosphere is n importnt considertion in designing pouring systems. Lrge reoxidtion inclusions often ccumulte on the cope surfce of steel cstings, where they my be visible to the nked eye. They hve detrimentl effect on mchining nd mechnicl performnce of steel cstings nd my cuse csting rejection. An imge of typicl reoxidtion mcroinclusion cut from low-lloy steel csting is shown LIANG WANG, Grdute Reserch Assistnt, nd CHRISTOPH BECKERMANN, Professor, re with the Deprtment of Mechnicl nd Industril Engineering, The University of Iow, Iow City, IA Contct e-mil: becker@engineering.uiow.edu Mnuscript submitted September 20, in Figure 1. [2] The inclusion hs dimeter of bout 2 mm nd consists of 34 wt pct Al 2 O 3, 46 wt pct SiO 2, nd 20 wt pct MnO. Its globulr shpe indictes tht reltively lrge frction of the oxide mixture ws in liquid form during formtion of the inclusion. Inside the inclusion, smller prticles tht were presumbly solid during formtion re visible s well. The objective of the current study is to predict the composition of reoxidtion inclusions in the csting of steels in snd molds. This study is prt of lrger project to model the entire process of reoxidtion inclusion formtion nd trnsport in steel csting. [4] It is shown tht the inclusion composition is very sensitive to the steel composition nd the mount of oxygen bsorbed t the steel-tmosphere interfce. Avilble thermochemicl nlysis softwre is used, which enbles prmetric studies to be conducted tht would be difficult to perform experimentlly. Reoxidtion of liquid steel hs been investigted extensively in the pst. [5 14] All of these studies conclude tht the oxidtion rte is primrily controlled by the rte of oxygen trnsfer through the tmosphere to the surfce of the steel. Only for very low lloy element concentrtions, or in the presence of stble oxide films, do internl diffusion processes become importnt. [6,7,9 11] The fct tht the oxidtion rte is limited by tmospheric trnsport of oxygen is estblished in the bove references by performing experiments where the O 2 gs prtil pressure in the tmosphere is vried. The experiments show tht the consumption of oxidizing elements is linerly proportionl to the O 2 prtil pressure. Only few studies combine experiments with thermochemicl modeling to predict oxidtion of liquid METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 37B, AUGUST

2 iron-bse lloys (e.g., References 9 nd 10). The use of thermochemicl nlysis softwre in modeling oxide inclusion formtion hs become possible only within the pst decde. For exmple, Hsieh et l. [15] nlyzed inclusion formtion in low-lloy steel welds s function of the weld metl composition using Thermo-Clc. [16] Similrly, Eijk et l. [17] predicted the evolution of inclusion composition in Ti deoxidized steel. Neither of the ltter two studies considered reoxidtion, where oxygen is continully trnsferred to the liquid steel. The composition nd other chrcteristics of reoxidtion inclusions in steel cstings hve been studied in severl projects conducted under the uspices of the Steel Founders Society of Americ (SFSA). [1] Lymn nd Boulger [18] mesured inclusion compositions in crbon steel cstings s function of the Al content nd the Mn/Si rtio. Griffin nd Btes [19,20] investigted reoxidtion inclusions in both lowlloy nd high-lloy steel cstings. Extensive use is mde of these dt in the current study in order to vlidte the predictions. In ddition, the mtching of mesured nd predicted inclusion compositions llows for considerble insight into the inclusion formtion processes in steel csting, including the bsorption of oxygen. The following section briefly reviews the modeling of oxygen bsorption t the steel surfce. Section III explins the method used to predict the inclusion composition in the presence of oxygen bsorption. Section IV presents the results of the predictions nd the comprisons with mesurements for crbon, low-lloy, nd high-lloy steels. These results re discussed in Section V. The dependence of the reoxidtion rte on the free surfce re nd the exposure time is exmined in Section VI. The conclusions of the current study re summrized in Section VII. II. REVIW OF THE MODELING OF THE OXYGEN ABSORPTION RATE Reoxidtion of the liquid steel during pouring is ssumed to be controlled by the trnsfer of O 2 gs from the tmosphere to the tmosphere-steel interfce. [5 14] There, the oxygen rects with the elements in the steel to form reoxidtion inclusions. As illustrted in Figure 2, the oxygen bsorption rte cn be modeled s [11] dn o dt 5 2r A s M D o2 d o ð C o 2 2 C o 2 s Þ [1] where n O is the number of moles of oxygen bsorbed by the molten steel (mol), t is time (s), r is the density of the tmosphere (g/cm 3 ), A s is the surfce re of the molten steel (cm 2 ), M is the moleculr weight of the tmosphere (g/mol), D o 2 is the mss diffusivity of O 2 gs in the tmosphere (cm 2 /s), d o 2 is the thickness of the concentrtion boundry lyer of O 2 gs in the tmosphere (cm), C o 2 is the mole frction of O 2 gs in the tmosphere fr from the tmosphere-steel interfce (mol/mol), nd C o 2 s is the mole frction of O 2 gs in the tmosphere t the tmosphere-steel interfce (mol/mol). The fctor of 2 in Eq. [1] stems from the fct tht 1 mole of bsorbed O 2 decomposes into two moles of O in the steel. The boundry lyer thickness, d o 2, ccounts for the trnsport properties nd flow conditions of the tmosphere (s subsequently discussed); for exmple, lrger reltive velocity between the steel nd the tmosphere would result in smller boundry lyer thickness nd, therefore, ccording to Eq. [1], lrger oxygen bsorption rte. The rtio D o 2 =do 2 (cm/s) cn lso be interpreted s convective mss trnsfer coefficient. Eqution [1] cn be rewritten in terms of prtil pressures, by employing the idel gs lw, s dn o dt D o 2 5 2A s d o 2 ðp o 2 po 2 s Þ RT where p o 2 is the prtil pressure of O 2 gs in the tmosphere fr from the tmosphere-steel interfce (P), p o 2 s is the prtil pressure of O 2 gs in the tmosphere t the tmospheresteel interfce (P), R is the universl gs constnt [R P cm 3 /(mol K)], nd T is the bsolute temperture (K). The prtil pressure of O 2 gs in the tmosphere t the tmosphere-steel interfce, p o 2 s, is much lower thn p o 2 [11] nd cn be sfely neglected. Then, it cn be seen tht the oxygen bsorption rte is linerly proportionl to the [2] Fig. 1 Opticl microgrph of reoxidtion inclusion cut from steel csting; its bulk composition is 34 wt pct Al 2 O 3, 46 wt pct SiO 2, nd 20 wt pct MnO. [2] Fig. 2 Schemtic illustrtion of oxygen bsorption from the tmosphere nd the oxygen boundry lyer thickness during reoxidtion of steel. 572 VOLUME 37B, AUGUST 2006 METALLURGICAL AND MATERIALS TRANSACTIONS B

3 prtil pressure of O 2 gs in the tmosphere, s mentioned in Section I. For the purpose of the current study, it is necessry to convert the number of moles of bsorbed oxygen into n oxygen mss frction (per mss of steel). For smll oxygen mss reltive to the mss of steel, the mss frction, or simply concentrtion, of bsorbed oxygen, C O (g/g), is given by C o ¼ no M o [3] m s where M O is the tomic weight of oxygen (g/mol). The mss of steel, m s, is s of now undefined. However, for the preceding definition of mss frction to be useful, it must refer to tht portion of the steel where the oxygen concentrtion is uniform. Obviously, the oxygen does not need to be evenly distributed in the steel during pouring, nd m s could be less thn the totl mss of steel poured. This issue is discussed further in Section V. After substitution of Eq. [3], Eq. [2] becomes [11] dc o dt 5 2A sm o m s D o2 d o 2 p o 2 RT [4] In order to demonstrte the vlidity of Eq. [4] nd to illustrte the effect of the boundry lyer thickness, Eq. [4] is compred in the following to the experimentl results of Ssi nd Mizukmi. [11] The experiments involved the oxidtion of vrious iron melts tht were either in still stte or stirred by n electromgnetic field. The melts were contined in crucibles nd exposed t the top surfce to vrious Ar-O 2 tmospheres t 1 tm. Both pure Fe melts nd Fe-Al melts were investigted. For the Fe-Al melts, 0.1 wt pct luminum ws dded to the electrolytic iron. In some experiments, 0.1 wt pct sulfur ws dded to the Fe nd Fe- Al melts. For the experiments in the still stte, 500 g of iron ws melted in cylindricl lumin crucible, hving n inside dimeter of 4 cm nd height of 15 cm. The prtil pressure of O 2 gs in the Ar-O 2 tmosphere ws vried over the rnge of 5 to 50 kp. For the experiments in the stirred stte, 70 to 80 kg of iron ws melted in cylindricl mgnesi crucible, hving n inside dimeter of 22 cm nd height of 40 cm. Here, the prtil pressure of O 2 gs ws vried from 4 to 20 kp. Other detils of the experimentl setup cn be found in Reference 11. For the purpose of compring the mesured oxygen bsorption rtes to Eq. [4], the entire mss of the melt is used for m s, implying tht the oxygen is ssumed to be well mixed inside the crucibles. The mss diffusivity of O 2 in Ar t 1600 C is equl to 4.37 cm 2 /s. [21] Figure 3 shows the plots of the oxygen bsorption rte s function of the prtil pressure of O 2 gs in the tmosphere. These plots re similr to Figures 7 nd 8 in Reference 11. It cn be seen from the log-log plots tht the mesured oxygen bsorption rte (symbols) indeed vries linerly with the O 2 prtil pressure, s predicted by Eq. [4]. This is true for the experimentl dt shown in Figure 3() for the still stte nd the dt in Figure 3(b) for the stirred stte. Not included in Figure 3() re the dt from Reference 11 for luminum-killed molten steel in the still stte. For tht cse, Ssi nd Mizukmi [11] found stble Fig. 3 Vrition of the oxygen bsorption rte in vrious Fe melts with the prtil pressure of O 2 gs in the tmosphere: () still stte nd (b) stirred stte. oxide film to be present on the surfce of the steel. Their nlysis shows tht the oxidtion rte is then controlled by diffusion of oxygen in the oxide film. Stirring of the melt immeditely broke up the oxide film, such tht the oxidtion rte ws gin controlled by trnsport of O 2 in the tmosphere. This indictes tht stble oxide films tht limit the oxidtion rte re unlikely to be present during typicl steel pouring opertions, since the free surfce flow during pouring is usully highly turbulent nd unstedy. Figure 3 lso shows tht the mesured oxygen bsorption rtes re not ffected by the ddition of Al or S to the iron melts; hence, neither internl diffusion processes or interfcil chemicl rections control the oxidtion rte. [11] In order to directly compre the predictions from Eq. [4] to the mesured dt, the boundry lyer thickness, d o 2, must be known. Unfortuntely, it is difficult to estimte d o 2 from the informtion provided in Reference 11. Therefore, d o 2 ws vried until the predictions fit the dt (this corresponds to n up or down shift of the stright lines in Figure 3). Using this method, the boundry lyer thickness ws found to be equl to 7.9 cm for the experiments in the still stte nd 0.98 cm for the stirred stte. As expected, METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 37B, AUGUST

4 stirring the melt gretly decreses the boundry lyer thickness in the tmosphere becuse of the incresed reltive velocity between the melt nd the tmosphere. This decrese in d o 2 cuses n pproximtely eightfold increse in the oxygen bsorption rte due to stirring. A detiled discussion regrding clcultion of boundry lyer thicknesses cn be found in Section V. The vlues obtined previously for d o 2 re relistic in tht (1) even in the presence of strong stirring, d o 2 is unlikely to be smller thn ;0.1 cm (Figure 19); nd (ii) in the bsence of stirring, d o 2 cn be expected to be of the sme order of mgnitude s the dimeter of the crucible (4 cm) used in Reference 11. III. CALCULATION OF REOXIDATION INCLUSION COMPOSITION The composition of reoxidtion inclusions s function of steel composition nd oxygen bsorption rte is clculted in the current study using the softwre Thermo-Clc, Version P. [16] This widely used nd crefully verified thermochemicl nlysis softwre for multiphse equilibrium clcultions in multicomponent systems requires dtbses for the free energy functions nd other bsic dt. The thermodynmic dt for steel ws tken from the TCAB steels/lloys dtbse (TCFE) nd tht for the oxides from the SGTE substnces dtbse (SSUB). It is beyond the scope of the current study to provide n independent ssessment of the ccurcy of the Thermo-Clc predictions for the chemicl systems being studied; however, the comprisons of mesured nd predicted inclusion compositions presented below my shed some light on its ccurcy or potentil shortcomings. Thermo-Clc is used in the current study in conjunction with n lgorithm tht steps down the temperture of the system nd dds oxygen t specified rte, s explined in the following discussion. This wy, the cooling of the liquid steel nd the bsorption of oxygen from the tmosphere during pouring re pproximted. Both full equilibrium ( lever rule ) nd Scheil-type nlyses re performed. For given temperture, T, pressure, P, nd mss frction of element i in the system, C i, Thermo-Clc clcultes, mong other quntities, the mss frction of ech phse k in the system, f k, nd the mss frction of ech element i in phse k, C i k, ssuming complete thermodynmic equilibrium. This reltion between the input nd output prmeters cn be written s ðt; P; C i Þ!ðf k ; Ci k ;...Þ [5] where the superscript * denotes n equilibrium vlue. Note tht C i ¼ + f k Ci k : The clcultions re strted t k temperture T 0 tht corresponds to the pouring temperture of the steel (e.g., 1600 C). The initil vlues for C i re given by the composition of the steel exiting from the ldle, nd the pressure P is lwys kept t 1 tm. After completing the clcultions t the temperture T 0 nd outputting the phse frctions nd compositions, the temperture is decresed by smll step, DT (less thn 5 K), nd the Thermo-Clc clcultions represented by Eq. [5] re performed gin t the new temperture. This procedure is repeted until room temperture (20 C) is reched. The bsorption of oxygen is included in the Thermo-Clc clcultions in the following mnner. It is ssumed tht certin totl mss of oxygen per mss of steel, DC O,is bsorbed during the time intervl between the initil temperture, T 0, nd the liquidus temperture, T l, of the steel. This totl mount cn be obtined by integrting the oxygen bsorption rte given by Eq. [4] over the time intervl the steel cools from T 0 to T l. However, in Section IV, DC O is simply treted s n djustble prmeter (lso specified subsequently in units of weight percent insted of g/g). If there re n temperture steps between T 0 nd T l, the concentrtion of oxygen, C O, is then incresed t ech step by n mount equl to DC O /n. Hence, s first pproximtion, it is ssumed tht the oxygen bsorption rte is constnt during pouring. The initil oxygen concentrtion is ssumed to be negligibly smll, nd no oxygen is dded fter the liquidus temperture is reched. Below the liquidus temperture, solid shell forms tht protects the steel from further fst oxidtion. The preceding procedure constitutes the complete lgorithm for the so-clled lever rule nlysis. In the lever rule pproch, diffusion is ssumed to be infinitely fst, such tht the chemicl species re well mixed within ech phse, nd complete equilibrium is mintined t ll times. As consequence, previously formed phses cn redily dispper t lower temperture where they my not be thermodynmiclly stble. The ssumption of infinitely fst diffusion cn become prticulrly inpproprite once the steel solidifies, becuse the mss diffusivities of the vrious elements in steel re usully severl orders of mgnitude lower in the solid phse thn in the liquid phse. Hence, s n lterntive pproch, stndrd Scheil nlysis is lso performed. In the Scheil pproch, diffusion in the solid is ssumed to be negligibly smll, while the liquid is still tken to be well mixed. Therefore, previously formed solid phses cnnot dissolve. The clcultion procedure for the Scheil nlysis is similr to tht for the lever rule method, except tht t the beginning of ech temperture step, the concentrtions, C i, re reset to the concentrtions in the liquid steel, C i0 l, from the previous temperture step (s denoted by the prime). Also, the new phse frctions re not simply the equilibrium vlues, f k, but must be clculted from f k ¼ f 0 k 1 f l f k : These nd other procedures involved in the Scheil nlysis re further explined in the Thermo-Clc mnul. [16] The results presented in the next section illustrte the differences between the lever rule nd Scheil pproches, s well s their effect on the prediction of reoxidtion inclusion composition. IV. RESULTS The results of the reoxidtion inclusion composition clcultions re presented in this section seprtely for crbon steel nd low- nd high-lloy steels. The effects of the oxygen bsorption rte, DC O, nd the steel composition re investigted, nd the predicted inclusion compositions re compred with mesured dt. A. Crbon Steel The clcultions for crbon steel re intended to correspond to the mesurements of reoxidtion inclusion 574 VOLUME 37B, AUGUST 2006 METALLURGICAL AND MATERIALS TRANSACTIONS B

5 Tble I. Nominl Compositions of the Steels Investigted in the Present Study (Weight Percent) Steels C Si Mn S P Al Cr Ni Mo High-Al crbon steel Low-Al crbon steel Low-lloy steel High-lloy steel, CF8M composition by Lymn nd Boulger. [18] In the experiments by Lymn nd Boulger, pproximtely 400 crbon steel plte cstings were poured from 62 experimentl hets. The cstings rnged in size from 41 to 113 kg (90 to 250 pounds). A green snd mold ws used for most of the cstings. The pouring temperture ws tken s 1600 C. Lymn nd Boulger investigted both high-al (0.15 wt pct) nd low-al (0.05 wt pct) crbon steels. The nominl steel compositions re provided in the first two rows of Tble I. In some experiments, the Mn content ws chnged from the nominl vlues in Tble I in order to investigte the effect of the Mn/Si rtio on inclusion composition; the Mn content ws vried from 0.4 to 3.0 wt pct. 1. High-Al crbon steel Figure 4 shows the evolution of the oxide mss percentges (per mss of steel) s function of temperture for the high-al crbon steel (Tble I), clculted using the lever rule method. For the purpose of this figure, the mss frction of bsorbed oxygen, DC O, is tken to be 0.9 wt pct (i.e., g/g). It cn be seen tht the totl mss of ll oxides increses lmost linerly until the liquidus temperture, T l, is reched (Figure 4()). The liquidus tempertures indicted in this nd subsequent figures re those predicted by Thermo-Clc. The increse in the totl oxide mss is not completely liner, even though the oxygen bsorption rte is ssumed to be constnt, becuse of the different moleculr weights of the vrious oxides tht form. Below the liquidus temperture, the totl oxide mss is pproximtely constnt, since no more oxygen is dded, except for smll vritions due to the continully chnging composition of the oxides (Figure 4(b)). Focusing first on the temperture rnge bove liquidus (Figure 4()), it cn be seen tht the first oxide to form is solid lumin (Al 2 O 3 ). After the entire Al is consumed, Si begins to oxidize (strting t pproximtely 1590 C). At first, Si is oxidized in rection tht consumes the lumin to form solid mullite (3Al 2 O 3 _2SiO 2 ). After the lumin is completely converted, solid cristoblite (SiO 2 ) strts to form. At pproximtely the sme temperture (;1585 C), crbon monoxide gs (CO) lso ppers. Finlly, t pproximtely 1560 C, Mn begins to oxidize by forming liquid tephroite (Mn 2 SiO 4 ), while the mullite nd cristoblite mss percentges sty reltively constnt. At the liquidus temperture, the oxide mixture consists of liquid tephroite (Mn 2 SiO 4 ), solid mullite (3Al 2 O 3 _2SiO 2 ), solid cristoblite (SiO 2 ), nd CO gs; the exct percentges re listed in Tble II. Obviously, the CO gs will occupy very lrge volume nd cn be expected to escpe from the liquid steel, but the present method trets the system s closed other thn for the oxygen ddition. The fct tht more thn hlf of the oxides (excluding CO gs) re in liquid stte grees qulittively Fig. 4 Evolution of the mss percentge of the vrious oxide phses in high-al crbon steel clculted using the lever rule method (DC O wt pct). () Temperture rnge: 1600 C to 1480 C. (b) Temperture rnge: 1600 C to 20 C. with the observtion mde in connection with Figure 1 regrding the globulr shpe of reoxidtion inclusions. For completeness, Figure 4(b) shows the predicted evolution of the oxide mss percentges ll the wy down to room temperture. The trnsformtions below the liquidus temperture re unlikely to occur to completion in relity, becuse of the slow rte of mss diffusion in the solid nd the fct tht the CO gs will escpe from the system. Therefore, the detils of the trnsformtions re not discussed here. Nonetheless, it is noteworthy tht two iron contining oxides (Fe 2 O 2 _SiO 2 nd Al 2 FeO 4 ) re predicted to be METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 37B, AUGUST

6 Tble II. Predicted nd Equivlent Oxide Percentges for High-Al Crbon Steel DC O = 0.9 wt pct Lever Rule Approch Scheil Approch Predicted oxide mss per mss of steel t the liquidus temperture (Wt pct); in prenthesis: percent of totl Equivlent oxide percentge in inclusions, excluding CO gs (wt pct) crbon monoxide (g) (CO) (20.6 pct) (20.1 pct) lumin (s) (Al 2 O 3 ) 0 (0 pct) (15.4 pct) cristoblite (s) (SiO 2 ) (15.6 pct) (19.6 pct) mullite (s) (3Al 2 O 3 _2SiO 2 ) (19.0 pct) 0 (0 pct) tephroite (1) (Mn 2 SiO 4 ) (44.8 pct) (44.9 pct) totl (100 pct) (100 pct) Al 2 O SiO MnO totl thermodynmiclly stble t room temperture. As shown in Figure 4(), no iron contining oxides re predicted to exist t the liquidus temperture (for DC O wt pct). It is emphsized gin tht the results in Figure 4 re only the equilibrium oxide phse frctions s function of temperture (nd oxygen content), s predicted by Thermo-Clc using lever rule nlysis. Figure 5 shows the corresponding results using the Scheil pproch. As in Figure 4, the first oxide to form is solid lumin (Al 2 O 3 ). However, once the entire Al is consumed (t bout 1590 C), the Al 2 O 3 phse frction remins constnt, i.e., the lumin is not consumed in other rections. The inbility of lredy formed oxides to be consumed in other rections is the min consequence of the Scheil nlysis, s noted in Section III. As consequence, Si oxidizes (strting t 1590 C) by directly forming cristoblite (SiO 2 ), insted of first consuming the lumin to form mullite. This constitutes the min difference to the lever rule predictions. The ensuing sequence of C nd Mn oxidtion is very similr to tht observed in Figure 4. At the liquidus temperture, the oxide mixture is predicted by the Scheil nlysis to consist of liquid tephroite (Mn 2 SiO 4 ), solid cristoblite (SiO 2 ), solid lumin (Al 2 O 3 ), nd CO gs; gin, the exct percentges re listed in Tble II. No chnges in the oxide phse percentges re predicted by the Scheil nlysis for tempertures below liquidus. This illustrtes limittion of the Scheil nlysis, since t lest some further trnsformtion of the liquid tephroite cn be expected to occur t lower tempertures. The oxide mss frctions predicted by the lever rule nd Scheil methods t the liquidus temperture re compred in Tble II. There is difference of bout 20 pct in the totl mss of the oxides (per mss of steel) obtined by the two pproches. The individul oxide mss frctions re lso different, but when viewed s percent of the totl mss of oxides formed (vlues in prenthesis in Tble II), the similrities become pprent. The CO gs nd liquid tephroite (Mn 2 SiO 4 ) percentges for the lever rule nd Scheil methods re nerly identicl. Furthermore, by dding the mullite (3Al 2 O 3 _2SiO 2 ) percentge (i.e., 19 pct) to the two constituent oxide percentges, ccording to their moleculr weights, the Al 2 O 3 nd SiO 2 percentges for the lever rule pproch lso become nerly identicl to the ones for the Scheil nlysis. The sme comprison for the other steel grdes investigted in the current study reveled similr greement between the oxide percentges t the liquidus temperture (not shown here). Fig. 5 Evolution of the mss percentge of the vrious oxide phses in high-al crbon steel clculted using the Scheil method (DC O wt pct). As next step, the predicted oxide percentges t the liquidus temperture re used to estimte the composition of the reoxidtion inclusions, using the following conversion method. First, the mss frctions of the complex oxides (e.g., Mn 2 SiO 4 nd 3Al 2 O 3 _2SiO 2 ) t the liquidus temperture re converted into equivlent mss frctions of simple oxides (e.g., SiO 2, MnO, nd Al 2 O 3 ) using the chemicl blnces shown in Tble III. It is such simple oxides tht were mesured in the experiments of Lymn nd Boulger [18] nd Griffin nd Btes. [19,20] Then, the equivlent oxide mss frctions in the system (i.e., per mss of steel) re converted to oxide mss frctions in the inclusion, where inclusion is defined s the sum of ll oxides except the CO gs. The CO gs is excluded from the inclusion composition clcultions, since it cn be ssumed to hve escped from the inclusion. The results of these conversions re shown in Tble II for the Thermo-Clc predictions in Figures 4 nd 5. It cn be seen tht the equivlent inclusion oxide percentges from the lever rule nd Scheil nlyses re very close to ech other. Numerous dditionl clcultions showed tht the inclusion composition predictions from the lever rule nd Scheil nlyses re nerly identicl for ll steels investigted in the current study. Therefore, only results from the lever rule pproch re presented in the reminder of this rticle. This ws done for convenience only, becuse the lever rule computtions re somewht fster. Both the lever rule nd the Scheil nlyses hve serious shortcomings or uncertinties, s 576 VOLUME 37B, AUGUST 2006 METALLURGICAL AND MATERIALS TRANSACTIONS B

7 discussed bove. Since the present conversion method yields the sme inclusion composition for both pproches, more complex model of the diffusion processes during inclusion formtion does not pper to be wrrnted. In Figures 4 nd 5, the mss percentge of bsorbed oxygen, DC O, ws tken to be 0.9 wt pct. The effect of the oxygen bsorption rte on the oxide phse formtion nd the inclusion composition is investigted in Figure 6. Figure 6() shows tht the totl mss (per mss of steel) of ll oxides increses lmost linerly with the mount of oxygen bsorbed. However, the percentges of the individ- Tble III. Chemicl Equtions Used to Convert between Predicted nd Equivlent Oxide Frctions 3Al 2 O 3 _2SiO 2 5 3Al 2 O 3 + 2SiO 2 Mn 2 SiO 4 5 SiO 2 + 2MnO Fe 2 SiO 4 5 SiO 2 + 2FeO Al 2 SiO 4 5 SiO 2 +Al 2 O 3 MnO_Al 2 O 3 5 MnO + Al 2 O 3 Cr 2 FeO 4 5 FeO + Cr 2 O 3 Fig. 6 Effect of the mss percentge of bsorbed oxygen on the oxide mss percentges t the liquidus temperture nd on the inclusion composition for high-al crbon steel. () Mss percentge of oxide phses t the liquidus temperture (1528 C, lever rule). (b) Inclusion composition (numbers next to symbols indicte number of smples). ul oxides t the liquidus temperture, s predicted by the lever rule method, vry considerbly with DC O. For DC O, 0.4 wt pct, no CO gs nd liquid tephroite re predicted to form, i.e., ll oxides re in solid stte. For lrger oxygen bsorption rtes, the mss of liquid nd gseous oxides continully increses. For DC O. 1.2 wt pct, two Fe contining oxides form: solid Al 2 FeO 4 nd liquid fylite (Fe 2 SiO 4 ). According to the preceding discussion in connection with Figures 4 nd 5 nd Tble II, similr results cn be expected for the Scheil pproch. The vrition in the inclusion composition with DC O, clculted using the bove conversion method, is plotted in Figure 6(b). It cn be seen tht for very low oxygen bsorption rtes (i.e., DC O, 0.1 wt pct), the inclusions re predicted to consist of Al 2 O 3 only. With incresing DC O, first SiO 2, then MnO, nd finlly FeO re predicted to be present in the inclusions. At the DC O, where new oxides re formed, the percentges of the previously formed oxides decrese, since the inclusion oxide percentges must lwys totl 100 pct. The results in Figure 6(b) re now compred to the experimentl mesurements of Lymn nd Boulger. [18] For the high-al crbon steel cstings with the nominl Mn content of 0.8 wt pct (Tble I), the mesured inclusion composition, verged over 28 seprte smples, is indicted s symbols in Figure 6(b). Unfortuntely, mesurement uncertinties were not reported by Lymn nd Boulger, [18] nd the spred in the mesured compositions mong the smples is not known. The symbols were shifted long the horizontl xis until the mesured nd predicted Al 2 O 3 nd SiO 2 contents greed. As cn be seen in Figure 6(b), n excellent mtch for both contents occurs for DC O wt pct. For smller DC O, the Al 2 O 3 nd SiO 2 contents would be overpredicted, nd the reverse would be true for lrger DC O. The MnO content is overpredicted nd the clculted FeO percentge is zero for DC O wt pct; however, note tht the sum of the mesured MnO nd FeO percentges grees very well with the predicted MnO content t DC O wt pct. It is resonble to dd the MnO nd FeO percentges becuse (1) once FeO forms it does so t the expense of MnO only (Figure 6(b)), nd (2) FeO could form s the liquid tephroite (Mn 2 SiO 4 ) is trnsformed during cooling to room temperture, s mentioned previously in connection with Figure 4(b). In summry, fter dding the mesured MnO nd FeO percentges, ll mesured nd predicted inclusion oxide percentges mtch for DC O wt pct. This greement provides some confidence in the present method for clculting inclusion compositions, despite the uncertinty ssocited with the FeO content. The significnce of the 0.9 wt pct vlue for the mss percentge of bsorbed oxygen, DC O, obtined by mtching the mesured nd predicted inclusion compositions, is discussed in Section V. Figures 7 through 10 explore the effect of vritions in the steel composition on oxide formtion nd inclusion composition. In these figures, the mss percentge of bsorbed oxygen is kept constnt t DC O wt pct. In Figure 7, the Mn content is vried from 0.1 to 3.6 wt pct. The resulting effect on the oxide percentges t the liquidus temperture (predicted by the lever rule method) is shown in Figure 7(). The totl mss of ll oxides (per mss of steel) increses, up to Mn content of 2.3 wt pct, even though the mount of bsorbed oxygen is constnt. As METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 37B, AUGUST

8 Fig. 7 Effect of the Mn content on the oxide mss percentges t the liquidus temperture nd on the inclusion composition for high-al crbon steel (DC O wt pct). () Mss percentge of oxide phses t the liquidus temperture (1528 C, lever rule). (b) Inclusion composition (numbers next to symbols indicte number of smples). expected, the percentge of Mn contining oxides, in prticulr liquid tephroite, increses with incresing Mn content. The percentge of CO gs decreses with incresing Mn content nd no CO gs is formed for Mn content bove bout 3.0 wt pct. The predicted inclusion composition is plotted s function of the Mn content in Figure 7(b). For Mn content in the steel of up to bout 1.6 wt pct, the percentge of MnO in the inclusions increses strongly with incresing Mn content, while those of SiO 2 nd Al 2 O 3 decrese proportionlly. Beyond 1.6 wt pct Mn, the inclusion composition is predicted to be pproximtely independent of the Mn content in the steel. Also included in Figure 7(b) re the inclusion compositions mesured by Lymn nd Boulger [18] for Mn contents of 0.8 wt pct (nominl), 1.6 wt pct, nd 3.0 wt pct (for the high-al crbon steel of this subsection). As in Figure 6(b), the mesured MnO nd FeO contents re summed up. While the mesured nd predicted inclusion compositions gree well for the nominl Mn content of 0.8 wt pct, s lredy observed in Figure 6(b), some disgreement is Fig. 8 Effect of the Si content on the oxide mss percentges t the liquidus temperture nd on the inclusion composition for high-al crbon steel (DC O wt pct). () Mss percentge of oxide phses t the liquidus temperture (1528 C, lever rule). (b) Inclusion composition. present t the two higher Mn contents. The MnO content in the inclusion is predicted to increse more strongly with incresing Mn content in the steel thn mesured. Even though there is reltively poor greement t 1.6 wt pct Mn, the mesured trend of the SiO 2 content in the inclusion decresing for n increse in the Mn content from 0.8 to 3.0 wt pct is cptured correctly by the clcultions. The higher vlue of the mesured Al 2 O 3 percentge t 3.0 wt pct Mn, reltive to the ones t the lower Mn contents, is difficult to explin since the Al content in the steel ws presumbly held constnt nd, from the predictions in Figure 7(b), the Al 2 O 3 content is not expected to be strong function of the Mn content in the rnge of interest. Note tht, since the oxide inclusion percentges must dd up to 100, higher percentge in one oxide will necessrily cuse other percentges to be smller. Hence, this could explin the reltive lrge disgreement between the mesured nd predicted MnO contents t 3.0 wt pct Mn. One possible explntion for the higher Al 2 O 3 percentge mesured t 3.0 wt pct Mn my be tht more Al 2 O 3 deoxidtion prticles re cptured by the liquid tephroite, since the 578 VOLUME 37B, AUGUST 2006 METALLURGICAL AND MATERIALS TRANSACTIONS B

9 Fig. 9 Effect of the Al content on the oxide mss percentges t the liquidus temperture nd on the inclusion composition for high-al crbon steel (DC O wt pct). () Mss percentge of oxide phses t the liquidus temperture (1528 C, lever rule). (b) Inclusion composition. Fig. 10 Effect of the C content on the oxide mss percentges t the liquidus temperture nd on the inclusion composition for high-al crbon steel (DC O wt pct). () Mss percentge of oxide phses t the liquidus temperture (1528 C, lever rule). (b) Inclusion composition. tephroite percentge increses strongly with incresing Mn content (Figure 7(b)). It is lso possible tht the oxygen bsorption rte chnges with the Mn content in the steel, perhps due to chnges in the CO gs formtion, s shown in Figure 7(). Figure 8 shows the vritions in the predicted oxide percentges t the liquidus temperture (Figure 8()) nd inclusion composition (Figure 8(b)) with the Si content incresing from 0.1 to 1.0 wt pct. In this nd severl subsequent figures, the nominl lloy element content is indicted s verticl dshed line; thus, t this weight percentge, the predicted nd mesured inclusion compositions gree. As opposed to the effect of Mn in Figure 7(), it cn be seen from Figure 8() tht the totl mss of oxides (per mss of steel) decreses slightly with incresing Si content. This cn be ttributed to the moleculr weight of Mn being lmost twice tht of Si. The percentges of CO gs nd liquid tephroite (Mn 2 SiO 4 ) decrese with incresing Si content, nd bove bout 0.8 wt pct Si, ll oxides re predicted to be in the solid stte t the liquidus temperture. The clculted inclusion compositions in Figure 8(b) show tht the increse in the SiO 2 content with incresing Si is fully t the expense of the MnO content, while the Al 2 O 3 percentge is reltively constnt. Above 0.8 wt pct Si, MnO is completely bsent from the reoxidtion inclusions, such tht they consist of SiO 2 nd Al 2 O 3 only, nd the inclusion composition becomes independent of the Si content. Figure 9() investigtes the effect of the Al content on oxide formtion. As expected, the percentge of lumin (Al 2 O 3 ) increses with incresing Al content. All other oxide phse frctions decrese slightly, since the totl mount of oxygen bsorbed is fixed. The plot of the inclusion composition in Figure 9(b) indictes tht the SiO 2 content is reltively unffected by the Al content, nd tht the increse in the Al 2 O 3 content is mostly ccommodted by decrese in the MnO percentge. Finlly, Figure 10 illustrtes the effect of the crbon (C) content. As the C content increses from 0.1 wt pct the totl mss of the oxides (per mss of steel) first decreses nd then reches constnt vlue t bout 0.7 wt pct C (Figure 10()). The liquid tephroite percentge t the liquidus temperture decreses steeply with incresing C content METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 37B, AUGUST

10 nd vnishes completely t bout 0.7 wt pct C. No other liquid phse is present t higher C contents. The CO gs percentge increses lmost linerly with incresing C content, until it reches n pproximtely constnt vlue t round 1.0 wt pct C (Figure 10()). The CO gs percentge increses t the expense of the solid nd liquid oxides, since the totl mss of the oxides decreses or remins constnt. This indictes tht the mss of the (solid) inclusions tht re finlly formed decreses strongly with incresing C content (up to bout 1.0 wt pct), since the CO gs will escpe. The clculted inclusion composition s function of the C content is shown in Figure 10(b). Below 0.7 wt pct C, the MnO percentge decreses nd the SiO 2 nd Al 2 O 3 percentges increse with incresing C content. Between 0.7 nd 1.3 wt pct C, the inclusion consists of SiO 2 nd Al 2 O 3 only. However, the SiO 2 content decreses in this rnge, nd bove 1.3 wt pct C, the inclusion consists of Al 2 O 3 only. This is possible becuse most of the bsorbed oxygen rects with the crbon (Figure 10()). Obviously, such high C contents re not used in cst steels, but the trends re nonetheless interesting. 2. Low-Al crbon steel A seprte set of experiments were conducted by Lymn nd Boulger [18] for low-al crbon steel, where the Al content ws only 0.05 wt pct (Tble I). The nominl Mn content ws 0.4 wt pct, s opposed to the 0.8 wt pct for the high-al steel of Section IV A 1. Figures 11() nd (b) show the predicted vrition of the oxide mss percentges t the liquidus temperture nd the inclusion composition, respectively, s function of the percentge of bsorbed oxygen, DC O. This figure is nlogous to Figure 6 for the high-al crbon steel. Figure 11(b) is used to determine the mss of bsorbed oxygen, DC O, by mtching the mesured nd predicted inclusion compositions. In this cse, the dt points were shifted long the horizontl xis until the mesured SiO 2 content fell exctly on the curve representing the predicted SiO 2 content. Interestingly, the sme DC O wt pct vlue is obtined s for the high-al crbon steel (Figure 6(b)). With tht vlue for DC O, the mesured nd predicted Al 2 O 3 nd MnO 1 FeO contents lso gree very well. Note tht the Al 2 O 3 content in the inclusion is much lower for the present low-al crbon steel thn for the high- Al crbon steel (Figure 6(b)). The mtch in Figure 11(b) is unique in tht no other DC O would result in mtch of ll inclusion oxide percentges. Figure 12 investigtes the effect of the Mn content in the low-al crbon steel on the oxide formtion nd inclusion composition, with DC O kept constnt t 0.9 wt pct. This figure is similr to Figure 7 for the high-al crbon steel. The inclusion composition mesurements of Lymn nd Boulger [18] re indicted s symbols in Figure 12(b) nd correspond to Mn contents of 0.4 wt pct (nominl), 0.8 wt pct, 1.6 wt pct, nd 3.0 wt pct. Overll, the trends observed in the mesured dt re predicted correctly. The MnO content increses, the SiO 2 content decreses, nd the Al 2 O 3 content is pproximtely constnt with incresing Mn content. Above 1.6 wt pct Mn, the inclusion composition becomes independent of the Mn content. Some disgreement between the mesured nd predicted inclusion compositions exists t the two higher Mn contents (1.6 nd 3.0 wt pct), s lredy observed in Section IV Fig. 11 Effect of the mss percentge of bsorbed oxygen on the oxide mss percentges t the liquidus temperture nd on the inclusion composition for low-al crbon steel. () Mss percentge of oxide phses t the liquidus temperture (1528 C, lever rule). (b) Inclusion composition (numbers next to symbols indicte number of smples). A 1 for the high-al crbon steel. As discussed in connection with Figure 7(b), no definite conclusions cn be drwn with respect to the origin of the differences. Some of the disgreement my be ttributed to uncertinties in the experimentl dt. For exmple, with incresing Mn content, the mesured SiO 2 content in the inclusions first decreses, then increses (t 1.6 wt pct Mn), nd finlly decreses gin. If one ignores the three smples t 1.6 wt pct Mn, the trends re in much better greement. Despite these uncertinties, the comprisons provide dditionl confidence in the present method for clculting the reoxidtion inclusion composition. B. Alloy Steel Griffin nd Btes [19,20] opticlly nd chemiclly nlyzed mcroinclusions from both low-lloy nd high-lloy steel cstings. The mesurements of reoxidtion inclusion composition used in the following re from test pltes tht were cst in both snd nd specil cermic molds. The geometry of the test plte is shown in Figure 13. The pouring weight 580 VOLUME 37B, AUGUST 2006 METALLURGICAL AND MATERIALS TRANSACTIONS B

11 Fig. 13 Geometry of the low- nd high-lloy steel test plte cstings; [19,20] the unit for ll lengths is inches. Tble IV. Mesured Mtrix Phse nd Bulk Inclusion Compositions for Low-Alloy Steel (Weight Percent) [20] Fig. 12 Effect of the Mn content on the oxide mss percentges t the liquidus temperture nd on the inclusion composition for low-al crbon steel (DC O wt pct). () Mss percentge of oxide phses t the liquidus temperture (1528 C, lever rule). (b) Inclusion composition (numbers next to symbols indicte number of smples). of the plte ws pproximtely 90.7 kg (200 lb) nd the csting weight ws bout 43.1 kg (95 lb). The pouring temperture ws bout 1600 C. From mong the steels investigted by Griffin nd Btes, [19,20] one low-lloy nd one high-lloy steel were selected for nlysis here. Their nominl compositions re provided in Tble I. The high-lloy steel designtion is CF8M (Alloy Csting Institute). The reoxidtion inclusion compositions mesured by Griffin nd Btes [19,20] for these two steels re summrized in Tbles IV nd V. For the low-lloy steel (Tble IV), both mtrix phse nd bulk inclusion compositions re provided. They differ primrily in the Al 2 O 3 content. For the mtrix phse, the Al 2 O 3 content is reltively constnt for ll eight specimens, with the verge equl to 22 wt pct; this mtrix phse is likely to be the ctul reoxidtion product. On the other hnd, the bulk Al 2 O 3 contents in Tble IV vry strongly nd re, on verge, much higher (bout 60 pct). The higher bulk Al 2 O 3 content my be explined by the presence of Al 2 O 3 deoxidtion prticles inside the reoxidtion inclusions. These deoxidtion inclusions re lredy present in the steel in Specimen Mtrix Phse Bulk Inclusion Al 2 O 3 SiO 2 MnO CO Al 2 O 3 SiO 2 MnO CO Averge Tble V. Mesured Inclusion Compositions for High-Alloy Steel, CF8M (Weight Percent) [19] Specimen SiO 2 Cr 2 O 3 MnO FeO TiO 2 Al 2 O Averge the ldle. They my be cptured by the rpidly moving, prtilly liquid reoxidtion inclusions during filling. [4] The strong vritions in the bulk Al 2 O 3 content my be due to the eight specimens being from different test pltes tht were poured under different conditions nd from hets with different deoxidtion prctices. This issue is further METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 37B, AUGUST

12 ddressed subsequently. No such distinction between mtrix nd bulk inclusion compositions is necessry for the high-lloy steel. [19] The compositions of the ten inclusion specimens listed in Tble V for the CF8M steel re ll reltively similr. These inclusions re believed to be reoxidtion products. [19] 1. Low-lloy steel Figure 14 shows the effect of the mss of bsorbed oxygen, DC O, on oxide formtion nd inclusion composition for the low-lloy steel. This figure is similr to Figures 6 nd 11 for the crbon steels, nd the reder is referred to in Section IV A for more detiled discussion. The min difference from these erlier figures is tht for the low-lloy steel Cr contining oxide forms for DC O greter thn bout 1.1 wt pct (Figure 14()). In the sme mnner s in Section IV A, Figure 14(b) is used to determine the mss of bsorbed oxygen, DC O, by mtching the mesured nd predicted inclusion compositions. According to the preceding discussion, the verge mtrix phse composition in Tble IV represents the mesured reoxidtion inclusion composition. Note tht no FeO ws detected in the inclusions (Tble IV); hence, the MnO nd FeO percentges do not need to dded s ws done for the crbon steels. The mesured CO content (3 wt pct) is neglected. Figure 14(b) shows tht DC O wt pct provides good mtch between the mesured nd predicted inclusion compositions. The mtching ws performed bsed on the SiO 2 nd MnO contents only. The mesured Al 2 O 3 content of 22 wt pct ws primrily used to determine the Al content in the steel, becuse the Al content ws not provided by Griffin nd Btes. [20] The resulting vlue of 0.2 wt pct Al (Tble I) is relistic. The good mtch observed in Figure 14(b) provides incresed confidence in the present method for determining the inclusion composition, since there is no uncertinty due to the FeO content s for the crbon steel. Interestingly, the 0.9 wt pct vlue for the mss of bsorbed oxygen is the sme s ws determined for the crbon steels. With n Al 2 O 3 content of 22 pct in the mtrix phse creted by reoxidtion, the mesured Al 2 O 3 bulk inclusion percentges cn be used to bck out the mss percentge of deoxidtion inclusions for ech of the specimens in Tble IV. The result is shown in Figure 15. It cn be seen tht between 0.5 nd 4 wt pct Al 2 O 3, deoxidtion inclusions must be present in the steel to explin the high vlues of the mesured Al 2 O 3 bulk inclusion percentges. For exmple, mesured Al 2 O 3 content of 60 wt pct in the bulk inclusion corresponds to 2 wt pct Al 2 O 3 in the steel before ny oxygen is dded. Such high deoxidtion inclusion weight percentges re not relistic. The high vlues imply tht the solid Al 2 O 3 deoxidtion inclusions re not uniformly distributed in the liquid steel. Most likely, deoxidtion inclusions re cptured by the prtilly liquid reoxidtion inclusions during filling, s mentioned previously. This effect is not esily incorported in the present model. Nonetheless, it is useful to see from Figure 15 tht the mesured vritions in the SiO 2 nd MnO bulk inclusion percentges cn be explined by the presence of Al 2 O 3 deoxidtion inclusions. Fig. 14 Effect of the mss percentge of bsorbed oxygen on the oxide mss percentges t the liquidus temperture nd on the inclusion composition for low-lloy steel. () Mss percentge of oxide phses t the liquidus temperture (1528 C, lever rule). (b) Inclusion composition (numbers next to symbols indicte number of smples). Fig. 15 Effect of the mss percentge of Al 2 O 3 deoxidtion inclusions on the bulk composition of the reoxidtion inclusions in low-lloy steel. 582 VOLUME 37B, AUGUST 2006 METALLURGICAL AND MATERIALS TRANSACTIONS B