Materials and Design

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1 Material and Deign 35 (2012) Content lit available at SciVere ScienceDirect Material and Deign journal homepage: Shrinkage poroity criteria and optimized deign of a 100-ton 30Cr 2 Ni 4 MoV forging ingot Jiaqi Wang, Paixian Fu, Hongwei Liu, Dianzhong Li, Yiyi Li Shenyang National Laboratory for Material Science, Intitute of Metal Reearch, Chinee Academy of Science, 72 Wenhua Rd, Shenyang , China article info abtract Article hitory: Received 14 Augut 2011 Accepted 25 September 2011 Available online 29 September 2011 Keyword: A. Ferrou metal and alloy C. Cating E. Shrinkage The heavy 30Cr 2 Ni 4 MoV forging ingot have been extenively ued to fabricate the low-preure rotor, which play a crucial role in the large nuclear power plant. However, the quality of the heavy ingot ha been commonly affected by variou inner defect. One of the mot typical defect i the centerline hrinkage poroity, which ha been found to be alway accompanied by incluion and macroegregation. Within thi context, a 100-ton 30Cr 2 Ni 4 MoV ingot wa cut and the axle plane of it wa analyzed to oberve the ditribution and ize of the hrinkage poroity in the ectioned urface. On bai of thee experimental reult, through the FEM (Finite Element Method) imulation everal criteria of the centerline hrinkage poroity have been clarified. Of them, the bet criterion ha been found to be the condition under G=R 0:5 <2.5 C 0.5 mm 1.5 (G and R are the temperature gradient and olidification rate, repectively). With thi criterion, the ditribution and ize of the imulated centerline hrinkage poroity agree well with the experimental reult oberved by the ectioning and ultraonic teting. Subequently, by utilizing the optimized deign, the imulated reult uncovered that the centerline hrinkage poroity could be nearly eliminated. Thereby, offering a poibility by ignificantly improving the performance for the nuclear power low-preure rotor. Ó 2011 Elevier Ltd. All right reerved. 1. Introduction Correponding author. Tel.: ; fax: addre: dzli@imr.ac.cn (D. Li). The ability to manufacture high-quality heavy cating and forging i not only a ymbol of a developed heavy indutry but alo ha a fundamental influence upon the further development of the country becaue of their wide application in aeropace, hipbuilding, electric power, petrochemical and machinery manufacturing indutrie. A a raw material of heavy forging, the heavy teel ingot determine the quality of product. Of all the factor that deteriorate the propertie of the heavy teel ingot, incluion, macroegregation and hrinkage poroity are the mot ignificant one ince it i highly difficult to reduce thoe defect by ubequent thermo-mechanical treatment [1]. A the computational technology develop rapidly, olidification, egregation and even microtructure evolution in the cating proce can be now imulated uccefully. In uch a way, the cating ha already advanced from a non-viible to a viible proce. Therefore, the deign baed on the computational imulation and the real-time radiography technology i highly attractive becaue it would have many advantage, i.e., to improve quality, to reduce cot and to horten preproduction time, etc. [2 4]. From the traditional point of view, one often ignored the influence of ome defect (i.e., the centerline hrinkage poroity) in heavy teel ingot. The reaon i that the forward forging proce wa thought to have the capability of cloing up the entire hrinkage poroity defect. However, if the poroitie in the heavy teel ingot are too big, it i difficult to healing all the poroity jut by the forward forging proce. Indeed, it turn out to be that the hrinkage poroity affect urely the quality of the product, epecially, for the ingot ued in high-level requet product, uch a nuclear power low-preure rotor. A a reult, it i ignificant to theoretically imulate hrinkage poroity accurately for heavy teel ingot and alo to find the effective way to eliminate hrinkage poroity in real practice. Although hrinkage formation involve heat flow, ma flow and ome other complicated phenomena, a generally accepted practice to imulate hrinkage poroity i purely from thermal calculation [5]. Through thi implet way, hrinkage can be recognized in term of olidification pattern. It i defined that hrinkage appear in a pot within the cloed loop formed by the iochronal contour of olidification time. Thi mean a complete eparation of the cating from the rier, imultaneouly, accompanied by the diappearance of the feeding path [6]. More pecifically, there are many hrinkage poroity criteria for cating, uch a the temperature gradient criterion (G), critical olid fraction criterion and famou Niyama criterion (G/L 0.5 < contant, where L i cooling rate) [7,8]. In 1988, Hanon alo propoed a hrinkage criterion GL 0.25 U 0.5, where U i the feeding flow velocity [9]. In addition, Minakawa reearch demontrated that the /$ - ee front matter Ó 2011 Elevier Ltd. All right reerved. doi: /j.matde

2 J. Wang et al. / Material and Deign 35 (2012) parameter G/R (where R i the olidification rate) can erve a the bet criterion to indicate the onet of poroity in plate cating [10]. Jia and Liu further dicued the theoretical derivation of Niyama criterion G/L 0.5 < contant [11]. However, for heavy teel ingot there i till not a precie criterion to predict the occurrence, ditribution and ize of the centerline hrinkage poroity. In thi work, through FEM imulation in combination with the experimental ectioning invetigation of 100-ton ingot, a criterion which can be ued to reproduce preciely the experimental ize and ditribution of the hrinkage poroity in heavy teel ingot ha been uccefully propoed. Baed on thi propoed criterion the technique were further redeigned and optimized to fabricate the heavy ingot with an aim of improving the quality by eliminating the hrinkage poroity. 2. Deign 1: A preliminary deign 2.1. Numerical imulation FEM model The mold filling and olidification procee of the 100-ton 30Cr 2 Ni 4 MoV ingot were imulated uing the commercial FEM cating imulation oftware ProCat package. Before calculation, ome aumption were made: (1) the incoming liquid metal wa evenly ditributed jut below the whole free urface to imulate the mooth filling proce; (2) the liquid metal wa incompreible Newton fluid; (3) the convection wa driven by thermal buoyancy; (4) the olutal convection wa ignored; and (5) the heat tranfer wa coupled with the mold filling. In the preent tudy, the imulation model adopt the ma, momentum and energy equation during the olidification proce. The natural convection of the liquid metal ha been decribed by the continuity and the Navier Stoke equation a follow: Continuity equation r ð ~ VÞ¼0 Navier Stoke þ r ~ VUÞ ¼r ðlruþ rp þ qg Furthermore, baed on the claical iotropic Fourier conduction law the heat tranfer ha been governed by: qc dt dt ¼ r ðkrtþþ _ Q where q i the denity, t i the time, l i the dynamic vicoity, P i the preure, g i the gravitational acceleration component, ~ V i the velocity vector, U i the velocity component, T i the temperature, k i the thermal conductivity, c i the pecific heat and _ Q i an internal power ource. The conervation equation are olved numerically uing a fully implicit time-tepping cheme and a tandard finiteelement formulation [12]. Detail of the numerical model are given in the uer manual of ProCat [13]. The finite element meh of the mold and ingot conit 151,360 node and 770,163 tetrahedral element (a hown in Fig. 1). The meh wa elected baed on everal meh refinement. Fig. 2 how the geometry and boundary condition of the model. A it can be een, an inulation condition wa aigned over the melt urface at the hot top a the melt wa covered by the inulation material and covering flux. A natural air cooling convection wa et over the mold body. Initial value of the interfacial heat tranfer coefficient between metal/inulation and metal/mold were et a 200 and 1000 W/m 2 /K repectively. The compoition and main thermophyical parameter of the ingot and mold are lited in Table 1 and 2 (the thermophyical ð1þ ð2þ ð3þ Fig. 1. The finite element meh of the mold and ingot. Fig. 2. The geometry and boundary condition of the model. parameter of the ingot were calculated by ProCat). The initial proceing parameter ued in the imulation include that (1) the pouring temperature i 1565 C, (2) the filling time i 29 min and (3) the pre-heated hot-top mold temperature i 80 C Shrinkage poroity model [13] Baed on ProCat Uer Manual, there have been three available option to analyze the hrinkage poroity in a cating. They are (1) Temperature field and olid fraction field, (2) Shrinkage poroity field and (3) Specific RGL criteria. (1) Temperature field and olid fraction field: Through looking at the temperature and olid fraction field in the cut-off mode (which provide a convenient option to viualize the inide of the cating during the olidification), it i eay to oberve the poroity which i primarily due to encloed pocket of liquid. (2) Poroity: When POROS model i activated, it can viualize the contour called Shrinkage poroity. Value correponding to a level below 0.01 hall be conidered a microporoity and thoe above 0.01 are conidered a macroporoity.

3 448 J. Wang et al. / Material and Deign 35 (2012) Table 1 Chemical compoition of the ingot and mold (wt.%). Material C Si P S Mn Cr Ni Mo V Ingot (30Cr 2 Ni 4 MoV) Mold (Eutectic Gray Iron) Table 2 Thermophyical parameter of the ingot and mold ued in imulation. Property Ingot (30Cr 2 Ni 4 MoV) Mold (eutectic gray iron) Liquidu temperature ( C) Solidu temperature ( C) Denity (kg/m 3 ) Thermal conductivity (w/m/k) The default hrinkage poroity model in ProCat account for coupled micro and macroporoity, a well a pile hrinkage. During the olidification of a cating, pocket of liquid are created, urrounded by a muhy zone and then a olid hell. Accordingly, the cating i divided into region in which the olid fraction i lower than that in the region that are bounded by wall (or ymmetry plane). A olidification proceed and the geometric hape of the cating become more complex, the number of region may increae. A region can thu be plit in more region. A region can diappear when all node have completely olidified. When a region i cooling down, ome hrinkage occur under the condition that the denity increae with decreaing temperature (the uual cae for mot alloy). The detailed cenario of hrinkage and microporoity calculation are given in the uer manual of ProCat [13]. (3) Specific RGL criteria: Per requirement, it ha the choice to define a cutomized criterion function with the RGL option in ProCat. Function RGL i able to calculate the olidification rate R, the temperature gradient G and the cooling rate L. R correpond to the velocity of a given iotherm wherea G i calculated at a given temperature which ha to be the ame temperature a the one ued for the olidification rate. The definition of G read: ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi G ¼ 2 A a linear interpolation between two temperature, the cooling rate L i defined a: L ¼ T upper T lower t upper t ð5þ lower ð4þ where T i the temperature and t i the time. In ProCat, there are way to combine the R, G and L variable together o a to obtain a combined criterion function (called Mapping factor) a follow: M ¼ ar b Gc L d where the coefficient a, b, c and d hould be pecified. When a = 1.0, b =0, c = 1.0 and d = 0.5 are ued, Niyama criterion can be obtained a: M ¼ G=L 0:5 < contant where the contant i dependent on the material and ize of the cating [14] Experiment The 100-ton 30Cr 2 Ni 4 MoV teel ingot wa pouring in vacuum condition (top filling). The ingot wa cut along the axle plane and machined by milling. The milled urface wa then prepared for metallurgical analyi by grinding and polihing procee. The urface wa etched by the dilute nitric acid to diplay the microtructure of the ingot. The ultraonic teting wa ued to detect the ditribution and ize of the hrinkage poroity. Fig. 3 compile the photo of the 100-ton ingot (panel (a)), it ectioned urface (panel (b)), and the magnified centerline hrinkage poroity (panel (c)) a well a the macro-examination of the etched ingot (panel (d)), repectively Reult and dicuion Fig. 4 compile the filling and olidification equence of the preliminary deign of the 100-ton ingot with the melt flow rate ð6þ ð7þ

4 J. Wang et al. / Material and Deign 35 (2012) Fig. 3. Photo of the 100-ton 30Cr 2 Ni 4 MoV ingot (a), it ectioned urface (b), the magnified centerline hrinkage poroity (c), and the macro-examination of the etched ingot (d). In (b) the red circle denote the region of the typical centerline hrinkage poroity. (For interpretation of the reference to colour in thi figure legend, the reader i referred to the web verion of thi article.) Fig. 4. Ditribution of imulated olid fraction during the filling and olidification of the preliminary deign of the 100-ton ingot at: (a) 134, (b) 1124, (c) 1709, (d) 9800, (e) 32,750, (f) 61,100, (g) 74,600 and (h) 89,450.

5 450 J. Wang et al. / Material and Deign 35 (2012) Fig. 5. The imulated reult of variou criteria to predict hrinkage poroity of the ingot: (a) the default hrinkage poroity model in ProCat, (b) the default hrinkage poroity model in Procat but viewed with lice, (c) the temperature gradient criterion, (d) Niyama criterion(g/l 0.5 ), (e) G=R 0:5 <2.5 C 0.5 mm 1.5 criterion and (f) the experimentally ectioning and ultraonic teting of the ingot. of 3200 kg/min. The panel (a c) demontrate the filling proce. In order to better fit to our experimental proce a dicued above, here a top filling proce wa activated. The advantage for uch a deign i to guarantee that incoming ma can be evenly ditributed jut below the whole free urface, thereby imulating the mooth filling proce of the large ingot. On the other hand, Panel (d h) in Fig. 4 how that the olidification proce from which the hrinkage poroity mechanim that Koichi TASHIRO propoed can be imulated. During the initial period of olidification (namely, 15 h after pouring time), the hrinkage poroity hardly form. Table 3 The parameter of the preliminary and optimized deign of the ingot. Fig. 6. Scheme of the preliminary and optimized deign of the ingot: (a) preliminary deign and (b) optimized deign. Improved method Preliminary deign Optimized deign Quality of the inulation material General Good Hollow inulation brick in the hot top No Ye mold Preheat temperature of the hot top 80 C Much higher mold Hot top taper 42% 15.8% Ma ratio of the hot top of the ingot 15.6% 22.0% Ma ratio of the tail cone of the ingot 3.96% 2.47%

6 J. Wang et al. / Material and Deign 35 (2012) The reaon can be attributed to the fact that in the initial period the U-haped contour of the olidification front, induced by the obviou cooling effect from the bottom of the mold, provide ufficient feeding channel from the hot top, a illutrated by Fig. 4d and e. However, in the middle of the olidification period, the cooling effect from the bottom of the mold decreae rapidly and vertical olidification progree due to the effect of lateral olidification. Thi ituation reult in the ignally accelerated vertical olidification velocity. The contour of the olidification front i a harp Vhaped framework, a hown in Fig. 4f. Therefore, many hrinkage poroitie occur during thi period. The reaon i probably due to that, a the V-haped contour become harper, the formation of the dendritic crytal become eaier and the feeding melt from the hot top into the dendritic pace become more difficult. In addition, the formation of the hrinkage poroity in the olidification acceleration area i related with the ditance of range where feeding from the hot top i difficult. When olidification proceed near the hot top, the contour of the olidification front appear a V hape with a wider angle (a hown in Fig. 4g). Thi fact make the melt feeding from the hot top eaier, and thereby the formation of hrinkage poroitie decreae [15]. The completed olidification time i about 24.8 h (a hown in Fig. 4h)), which i in good agreement with the experiment reult (24 h). Fig. 5 how the imulated reult through the application of ome criteria for the prediction of hrinkage poroity in the ingot. Fig. 5b and f confirmed that the imulated poition of hrinkage pipe in the hot top of the ingot i in good agreement with the experimental reult, which alo indicated the accuracy of our FEM model. It ha been concluded that the criterion G=R 0:5 <2.5 C 0.5 mm 1.5, where G and R are the temperature gradient and olidification rate, repectively, can erve a the bet criterion to predict the centerline hrinkage poroity for the heavy teel ingot (a hown in Fig. 5e). A R repreent the feeding requirement and G the feeding eae, their ratio indeed demontrate a revered likelihood of hrinkage formation: the larger G=R 0:5 i, the maller the likelihood i and vice vera [5]. Furthermore, the ditribution and ize of the hrinkage poroity agree well with the experimental reult obtained by the ectioning and ultraonic teting detection, a hown in Fig. 5f. Within the context, both the imulated and experimental reult reveal conitently eriou centerline hrinkage poroity defect in the 100-ton 30Cr 2 Ni 4 MoV teel ingot. Therefore, it would be reaonable to expect that the correponding forging proce could not compenate all thoe defect. That i the reaon a to why the optimized deign i further dicued in the following chapter. Fig. 7. Comparion of the hrinkage poroity imulation through improving variou technique in the optimized deign under the ame G=R 0:5 <2.5 C 0.5 mm 1.5 criterion: (a) the preliminary deign, (b) good inulating hot top material, (c) preheating the hot top mold, (d) the optimized mold, (e) the optimized mold + good inulating hot top, (f) the optimized mold + preheating the hot top mold, (g) the optimized mold + hollow inulation brick in the hot top mold and (h) a whole of optimized condition by combining all thee term (including good inulating hot top, preheating the hot top mold, the optimized mold and hollow inulation brick).

7 452 J. Wang et al. / Material and Deign 35 (2012) Deign 2: An optimized deign 3.1. The propoal of a package of optimized technique In order to eliminate the centerline hrinkage poroity, it i important to apply an optimized deign during the proce. Koichi Fleming ha treed that to deign any hot top the ideal way i not to put heat into it, but rather to prevent heat from leaving and the efficiency of the exothermic material can be greatly improved by urrounding it with inulating material [16]. TASHIRO ha repreented that the geometry of hot top and the mold deign play the mot important role in the formation of central poroitie and looe tructure in heavy forging ingot [15]. Scepi ha put forward ome option to improve the quality of heavy forging ingot. For intance, (1) H/D hould be never more than 1.3, (2) the ma ratio of the hot top of the ingot hould be no le than 23%, and (3) the pouring time hould be decreaed wherea the pouring temperature hould be increaed, and o on [17]. Kermanpur uggeted that the inner defect and the rier efficiency can be greatly improved by pouring the melt under a contant rate, reducing the mold lenderne ratio, and uing a proper deign for the hot top iolate [18]. In our 100-ton ingot, the H/D and the taper of the preliminary ingot are 1.04 and 8.5%, repectively, which are proper according to the widely-accepted cenue. A a reult, the middle mold deign with the ame H/D and taper can be adopted (a hown in the preliminary deign). The optimized deign mainly focue on the hot top and the tail cone of the ingot. Here, it i propoed that, in order to reduce the inner defect of the ingot, a erie of technique hould be adopted through (1) utilizing better thermal inulation material, (2) adding a circle of hollow inulation brick in the hot top mold, (3) preheating the hot top mold, (4) decreaing the taper Fig. 9. Influence of the tapper of the hot top on the hrinkage poroity of the ingot (the optimized deign): (a) 15.8% and (b) 28%. Fig. 8. Influence of the preheat temperature of the hot top mold on the hrinkage poroity of the ingot (the preliminary deign): (a) 80 C (b) 200 C (c) 350 C (d) 500 C (e) 650 C (f) 800 C (g) 950 C and (h) 1150 C.

8 J. Wang et al. / Material and Deign 35 (2012) of the hot top, (5) increaing the ma ratio of the hot top, and (6) finally optimizing the tail cone deign. The difference for the deign between the preliminary and optimized procee for the 100-ton ingot were further compared in Fig. 6. The detail on the deign were lited in Table 3. Furthermore, the influence of each term of the optimized deign wa alo invetigated a to how it affect the centerline hrinkage poroity of the ingot under the ame condition of G=R 0:5 < 2.5 C 0.5 mm 1.5 criterion. It ha been found that each term exhibit a good effect to ignificantly reduce the hrinkage poroity in the ingot, a illutrated in Fig. 7a g. In particular, through combining all thee optimized technique a one whole of deign, the imulated reult exhibit that the hrinkage poroity could be nearly eliminated a evidenced in Fig. 7h Reult and dicuion Thi part decribe how each of the propoed technique affect the centerline hrinkage poroity of the ingot Influence of the inulation material and the hollow inulation brick in the hot top mold A hown in Fig. 7a, b, d and e, the influence of the inulation material demontrate that the better inulation material i the Fig. 10. Influence of the height of the hot top on the hrinkage poroity of the ingot (optimized deign): (a) 600 mm (b) 800 mm and (c) 1000 mm. Fig. 11. The flow vector of the half-filling ingot: (a) the preliminary tail cone and (b) the optimized tail cone.

9 454 J. Wang et al. / Material and Deign 35 (2012) le likelihood of the centerline hrinkage poroity i, however, the effect of inulation material on reducing the centerline hrinkage poroity i not very ignificant. It i eay to image that better inulation material can bring better inulation effect in the hot top. Therefore, the olidification in the hot top will be delayed and the liquid metal in the hot top will have more chance to feeding the inner ingot. However, the inulation material of the preliminary deign i good (k inulation brick 0.9 W/m/K, k inulation board 0.4 W/m/K, where k i the coefficient of thermal conductivity), and jut uing the better inulation material (k better inulation brick 0.6 W/m/K, k better inulation board 0.3 W/m/K) ha no appreciable influence on the olidification pattern of the inner ingot body. The reaon i that rather than by emiion from the outer urface of the hot top, the heat in the hot top i aborbed by the cat iron and the inulation material of the hot top becaue of their large heat capacity. In the ame way, adding hollow inulation brick (k hollow inulation brick 0.5 W/m/K) in the hot top can reduce the centerline hrinkage poroity of the ingot to ome extent(a illutrated by Fig. 7d and g), but it would not have ignificant effect. A dicued above, it can be demontrated that jut uing better inulation material or adding a layer of hollow inulation brick in the hot top mold cannot obtain ignificant effect to reduce the centerline hrinkage poroity of the heavy ingot. The hape of the ingot and the hot top i more important. However, the factory can till adopt the two economical way to improve the inulation effect of the hot top and to obtain better quality ingot Influence of the preheat temperature of the hot top mold Fig. 8 how the influence of the preheat temperature of the hot top mold on the centerline hrinkage poroity of the ingot. It can be demontrated that the higher preheat temperature of the hot top mold i, the le centerline hrinkage poroity i. A dicued above, the mot heat in the hot top i aborbed by the cat iron and the inulation material of the hot top due to their large heat capacity, o it i ignificant to decreae the whole heat capacity of the hot top mold by preheating it. If the preheating temperature of the hot top mold i low (namely, C, a hown in Fig. 8a c), the effect of decreaing the centerline hrinkage poroity i not ignificant. The reaon i that the low preheating temperature cannot compenate the large heat capacity of the hot top mold. However, if preheating the hot top mold at a much higher temperature (namely, C, a hown in Fig. 8d g), the effect of decreaing the centerline hrinkage poroity i much tronger. At thi time, the heat capacity of the hot top mold i decreaed ignificantly; meanwhile the inulation effect of the hot top mold i alo trengthened ignally. Therefore, it i more beneficial to the ingot equential olidification and there will be much more chance for Fig. 12. Ditribution of imulated olid fraction during filling and olidification of the optimized deign of the 100-ton ingot at: (a) 139, (b) 1154, (c) 1819, (d) 7405, (e) 32,605, (f) 57,805, (g) 80,205, and (h) 158,605.

10 J. Wang et al. / Material and Deign 35 (2012) the melt of the hot top to feeding the lower part of the dendritic tructure of the inner ingot. ingot (uch a the ton ingot) to produce high-quality heavy cating and forging Influence of the tapper of the hot top A hown in Fig. 9, the influence of the tapper of the hot top demontrate that, the larger the tapper i, the more likelihood of the centerline hrinkage poroity i. The reaon i that the larger trapper will caue narrower and deeper unolidified molten pool during the middle of the olidification period (about h after pouring). Conequently, the formed dendrite will caue bridging and then the feeding channel of the liquid metal into the inner ingot mut be blocked ignificantly. However, maller tapper of the hot top can provide wider unolidified molten pool than that of the larger tapper one. Therefore, the feeding channel into the lower part of the dendritic tructure hould be much wider, and it will have the effect of le likelihood of the formation of the centerline hrinkage poroity Influence of the height of the hot top Fig. 10 how the influence of the height of the hot top on the centerline hrinkage poroity of the ingot. It demontrate that the higher the hot top i, the le likelihood of the centerline hrinkage poroity i. The reaon i that the higher hot top mean the larger heat capacity; meanwhile, the feeding preure of the liquid metal to the center part of the ingot i alo increaed. Then it i more beneficial to the ingot equential olidification and the feeding channel to the center ingot are moother. However, it i not neceary to increae the height of the hot top infinitely, a it decreae the yield of the large ingot and it cot a lot Influence of the tail cone deign The imulation reveal that the optimized tail cone a hown in Fig. 6b can make the liquid melt run along the edge of the cone, effectively eliminating the turbulence and eddy from the bottom of the ingot. However, if uing the preliminary deign a illutrated in Fig. 6a the liquid melt run unmoothly, away from the edge of the cone due to the obviou occurrence of the turbulence and eddy from the bottom of the ingot induced by the improper deign of the tail cone. Their behavior related to two deign of the tail cone are further evidenced by the flow vector with the half filling ingot in Fig. 11, from which it can be een that the optimized tail cone make the flow vector much more table than that in the preliminary deign. Conequently, it i more beneficial to enhance the incluion floating and reduce it aggregation in the bottom of the ingot. Finally, uing the bet combined condition (a hown in Fig. 7h), the ditribution of imulated olid fraction during filling and olidification of the optimized deign ingot i hown in Fig. 12. It ha been found that the olidification proce of the optimized ingot i much better in comparion with thoe in the preliminary deign (a hown in Fig. 12d h and Fig. 4d h). Fig. 12f and 4f how the middle-term olidification period (namely, h after pouring) of the preliminary and optimized deign of the ingot. It can be een from the figure that the V-haped contour of the olidification front of the optimized deign i much wider than that of the preliminary one. A dicued above, the harp V-haped framework caue bridging, and then the feeding channel are blocked by the interection of the dendritic crytal. Thi i the main caue of the centerline hrinkage poroity. However, with the combined technique, the V-hape i much wider (a hown in Fig. 12f); the formation of the dendritic crytal and bridging i harder; and the feeding melt from the hot top i much eaier. Therefore, the formation of the hrinkage poroity i much more difficult. It i believed that the technique that have been propoed will be ueful not only for thi 100-ton ingot, but alo for the larger 4. Concluion In order to effectively control the hrinkage poroity of the heavy forging ingot, through the three-dimenional finite element method the flow and olidification characteritic of a 100-ton ingot were imulated. The imulated reult have been confirmed by our ectioning experimental reult. The combined theoretical and experimental effort further reult in the propoal of the bet criterion to predict the hrinkage poroity for heavy forging ingot. In addition, the optimized deign to eventually eliminate the hrinkage poroity ha been propoed to improve the quality of the ingot. The obtained reult are ummarized a follow: (1) The criterion of G=R 0:5 <2.5 C 0.5 mm 1.5, where G and R are the temperature gradient and olidification rate, repectively, can erve a the bet criterion to predict the centerline hrinkage poroity of the heavy teel ingot. The theoretically imulated ditribution and ize of the hrinkage poroity agree well with the experimental reult of the ectioning invetigation. (2) In order to obtain high quality heavy ingot, the geometric deign of the mold and hot top i fairly ignificant. Baed on the proper H/D and taper of the ingot, it i propoed to increae the ma ratio of the hot top, to decreae the taper of the hot top, to ue better thermal inulation material, to preheat the hot top mold, to add a circle of hollow inulation brick in the hot top mold, and to optimize the tail cone deign of the ingot. With the optimized deign, the imulated reult revealed that the hrinkage poroity could be nearly eliminated. Acknowledgement Thi reearch i upported by Chinee National Key Technology R&D Program (2009ZX ). The author greatly appreciate the language editing work and valuable dicuion with Profeor Xing-qiu Chen from Intitute of Metal Reearch, Chinee Academy of Science. Reference [1] Baoguang Sang. Numerical imulation and experimental tudy of incluion and macroegregation in the bottom-filled heavy teel Ingot: [diertation]. Shenyang: Intitute of Metal Reearch, Chinee Academy of Science; [2] Campbell J. Cating. 2nd ed. Oxford: Butterworth; [3] Li DZ, Campbell J, Li YY. Filling ytem for invetment cat Ni-bae turbine blade. J Mater Proce Technol 2004;148: [4] Hu BH, Tong KK, Niu XP, Pinwill I. Deign and optimiation of runner and gating ytem for the die cating of thin-walled magneium telecommunication part through numerical imulation. J Mater Proce Technol 2000;105: [5] Niyama Eiuke. Computer-aided deign for metal cating. Annu Rev Mater Sci 1990;20: [6] Henzel J G, Keverian J. Comparion of calculated and meaured olidification pattern in a variety of cating. Am Foundrymen Soc 1966(1):86. [7] Niyama E, Uchida T, Morikawa M. Predicting hrinkage in large teel cating from temperature gradient calculation. Int Cat Met J 1981;6: [8] Niyama E, Uchida T, Morikawa M. A method of hrinkage prediction and it application to teel cating practice. Int Cat Met J 1982;7: [9] Hanen PN, Sahm PR. How to model and imulate the feeding proce in cating to predict hrinkage and poroity formation. Model Control Cating Weld Procee 1988;4: [10] Minakawa S, Samaraekera IV, Weinberg F. Centerline Poroity in plate cating. Metall Tran B 1985;16: pffiffiffi [11] Jia BQ, Liu BC. Theoretical derivation of criterion G= _T for prediction of hrinkage cavitie and poroitie in cating. Acta Metall Sin 1996;9:310 2.

11 456 J. Wang et al. / Material and Deign 35 (2012) [12] Baoguang Sang, Xiuhong Kang, Dianzhong Li. A novel technique for reducing macroegregation in heavy teel ingot. J Mater Proce Technol 2010;210: [13] ProCat Uer Manual Verion ESI group. The virtual try-out pace company; [14] Fu PX, Kang XH, Ma YC, Liu K, Li DZ, Li YY, et al. Centrifugal cating of TiAl exhaut valve. Intermetallic 2008;16: [15] Tahiro K, Watanabe S, Kitagawa I, Tamura I. Influence of mould deign on the olidification and oundne of heavy forging ingot. Tran Iron Steel Int Jpn 1983;23: [16] Fleming MC. Principle of control of oundne and homogeneity of large ingot. Scand J Metall 1976;5:1 15. [17] Scepi M, Andreoli B, Baevi S, Giorgetti AL. Thermal and metallurgical control of the efficiency of ingot mould for forging ingot. Boll Tec Finider 1981;391: [18] Kermanpur A, Ekandari M, Purmohamad H, Soltani MA, Shateri R. Influence of mould deign on the olidification of heavy forging ingot of low alloy teel by numerical imulation. Mater De 2010;31: