APPLIED PRESSURE CONTROL RISERING OF A DUCTILE IRON SAND CASTING. Claudia FLORES, Eudoxio RAMOS, Marco RAMÍREZ and Carlos GONZÁLEZ.

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1 APPLIED PRESSURE CONTROL RISERING OF A DUCTILE IRON SAND CASTING Claudia FLORES, Eudoxio RAMOS, Marco RAMÍREZ and Carlos GONZÁLEZ. Department of Metallurgical Engineering, Facultad de Química, Universidad Nacional Autónoma de México (UNAM, Edificio D Circuito de los Institutos s/n, Cd. Universitaria, México D.F., México. carlosgr@servidor.unam.mx Abstract Experimental trials were carried out using a green sand mold test casting for ductile iron (DI designed according to pressure control risering method (PCR. The system is composed by a gating system connected to a blind riser feeding a cubic casting through a rectangular neck. The system was used to explore the effect of pouring temperature and C and Si content on the production of castings free of shrinkage defects. An induction furnace was used to prepare DI liquid alloys from the same base metal and the burdens were adjusted in order to obtain melts with selected C and Si content allowing the production nearly eutectic alloys with different C/Si content ratios. The sandwich method was used for the nodulization treatment and inoculation and post-inoculation were used in all cases. The alloys were cast into the test molds using two different pouring temperatures. The castings were macrographically analyzed in order to detect the presence of shrinkage defects. Results suggest that control of pouring temperature and C/Si ratio is necessary to avoid presence of shrinkage defects. Process conditions and PCR system design and dimensions obtained from the test casting were applied to castings of different geometries and similar solidification modulus. Results shows that the test casting and methodology proposed in this work can provide useful information to smooth the progress of on-line implementation of successful gating/risering PCR systems. 1. INTRODUCTION Shrinkage defects in ductile iron castings obtained in green sand molds can take several forms: pull-downs, suck-ins, macro-shrinkage and micro-shrinkage. Feeding techniques and DI processing variables affects shrinkage occurrence in DI castings [1]. Pouring temperature and chemical composition have been identified as important parameters defining shrinkage cavity formation[2-4].. When DI solidifies there are two main solidification products, austenite and graphite and owing to the differences in density of liquid DI, austenite and graphite, during solidification it can be a volume expansion which can eventually be used to compensate liquid and solidification contraction of the melt. In order to use expansion to avoid shrinkage defects in DI castings several special feeding methods, known as applied risering methods [5], have been developed. Pressure control risering is an applied risering method used in production of DI castings in green sand moulds. The technique is intended [6] to control the level of expansion pressure in the cooling liquid during eutectic solidification so that it is always under positive pressure in relation to atmospheric pressure but never as high as to cause permanent deformation of the green sand mould.applied gating/risering system is designed to ensure solidification of the gates as soon as possible after filling of the mould and includes the presence of one or several top blind risers, depending on the casting design. During liquid contraction stage of the system, the blind riser compensates for liquid contraction of the casting cavity. Thus a void is created inside the riser. As the liquid in the casting cavity starts to expand, pressure increase in the liquid in the casting is initially avoided since the pressure

2 energy drives liquid from the casting cavity back into the riser which is partially or completely refilled. The transfer of metal to the riser stops and the liquid in the casting/riser system experiences a moderate pressure increase which continues to the end of the expansion. Provided the positive pressure in the liquid is greater than the pressure decrease resulting from final volume contraction of the last liquid to solidify, a secondary shrinkage will not be formed [6]. The risers should be able to provide sufficient molten metal to feed the liquid contraction of the casting. The function of the neck, i.e. the connection between riser and casting, is of great importance because it must allow the flow of the liquid to the riser, driven by the expansion pressure, to avoid plastic deformation of the mould. In addition, the neck must be closed rapidly so that the residual pressure in the liquid in the casting inhibits formation of defects associated to the final contraction stage. The main problem lies in the fact that optimum dimensions of gating system, riser and riser neck depend on metallurgical quality of the melt and on the green mold properties and hence must be determined by trial and error in each foundry. This is an expensive and time consuming process. In a previous work[ 8], experiments performed using a cubic test casting have suggested that for this test casting, which includes a PCR risering system, shrinkage defects are avoided for slightly hypoeutectic and eutectic compositions. In this work a test casting and methodology are proposed in order to obtain design parameters, such as neck and riser dimensions, and processing conditions for specific casting characteristics under local metallurgical quality and molding conditions, avoiding shrinkage related defects in the solidified castings. This test casting is used to explore the effect of relative amounts of carbon and silicon present in nearly eutectic DI alloys on the presence of shrinkage defects in experimental castings. The effect of pouring temperature was also under study. Finally the risering system and process conditions obtained from test casting are applied to other castings of different geometry but thermally equivalents, in order to explore the applicability of the information generated to other similar castings. 2. EXPERIMENTAL A green sand mold test casting for ductile iron (DI was designed according with pressure control risering method (PCR. The system is composed by a gating system connected to a blind riser feeding a cubic piece of 8cm side through a rectangular neck. The riser and its connection to the casting were dimensioned according with the heat balance and considerations shown in the appendix. The gating system was dimensioned following the procedure outlined in [6,7]..Experimental ductile irons were produced using gray and ductile iron scrap, ferroalloys, inoculants (75 % ferro-silicon powder, a 6% Magnesium nodulizing agent and low carbon steel scrap. The nodularization was obtained using the sandwich method. The chemical composition of the molten metal was measured by optical emission spectrometry. Composition was adjusted with coke and ferro-silicon in order to obtain the expected carbon and silicon contents. The chemical compositions of the experimental melts are shown in Table I.

3 Table I. Chemical composition of experimental melts. Effect of the C/Si ratio in nearly eutectic ductile iron. Alloy C Si Mn P C/Si Mg C.E Fe bal bal bal In order to explore the performance of the proposed PCR test mould for near eutectic DI with different C/Si content ratios, PCR test moulds were filled with experimental molten iron at two different pouring temperatures, 1370 and 1270 o C. Control of pouring temperatures was performed using K-type thermocouples protected whit zirconia paint and connected to a data acquisition system. The thermocouples were immersed into the liquid melt just prior to pouring. The experimental castings were cut transversely and metallographically prepared in order to detect the presence of shrinkage defects. Using pouring temperature and chemical composition giving the best results obtaining the casting free of shrinkage defects during experiments described above, the gating and risering system were applied to other castings of different geometry but similar solidification modulus. (a (b (c Fig. 1 Transverse section of test casting produced with nearly eutectic DI, Tp= 1370 oc and : (a C/Si = 1.1 ; (bc/si = 1.6 and (bc/si = RESULTS AND DISCUSSION Fig. 1 shows the effect of C/Si content in nearly eutectic ductile irons on the presence of shrinkage defects in the casting. It can be seen that low C/Si content ratio promotes the presence of shrinkage defects apparently due to the presence of a minor amount of graphite formed during eutectic solidification. It can be seen that there is an interval of C/Si values that can avoid the presence of shrinkage defects for a given casting obtained using a properly designed PCR risering system.

4 (a (b Fig. 2 (a A typical as cast test casting and (b Transverse section of test castings produced with nearly eutectic DI C/Si = 1.6 and two different pouring temperatures, right Tp = 1350oC and left Tp= 1270 oc. Fig. 2 shows a typical as cast test casting and the results for the same experimental nearly eutectic melt, with C/Si = 1.6 and two pouring temperatures. The casting obtained using the high pouring temperature did not show presence of contraction defects while the casting obtained from liquid at the low pouring temperature shows the presence of a swollen casting connected to a partially refilled riser. Apparently with a lower pouring temperature the neck has finished its solidification too early not allowing the refilling of the riser during the expansion stage of the solidifying casting, causing mold deformation and an oversized casting as can be observed in Fig.2(b. Fig. 3.-Transverse section of thermally equivalent castings (a plate and (b cylindrical casting using PCR system and process conditions obtained from proposed test casting. Fig.3 shows the transverse sections of two thermally equivalent castings obtained using the PCR system and process conditions obtained from the test casting trials. Here it can be seen that these castings were obtained sound, without the presence of shrinkage defects. This result suggests that dimensioning of the feeding system obtained from the test casting can be extended to other, thermally equivalent, castings.

5 CONCLUSIONS The test casting and methodology proposed in this work can be used to dimension PCR feeding systems and to identify process conditions enabling the production of ductile iron castings free of contraction defects, smoothing the progress of the on-line implementation of successful gating/risering PCR systems. APPENDIX When ductile iron cools and solidifies in sand moulds, due to its low heat diffusivity, the main resistance to heat flow is within the mould. During cooling the heat flux into the mould is given by: q = ( kρcp πt S ( T T 0 (A.1 The solidification time t C of a casting originally superheated to ΔT =T p -T EU, where T p is the pouring temperature and T EU is the eutectic temperature, with a volume V and transferring heat to the mould through a metal/mold interface of area S, is given by: t C ωc M Cρ m ( CpmΔT + H F = ( kρcp S (A.2 2 ( TEU T 0 π In Ec. A.2 M C =V/S is a geometrical modulus and ω C a form factor that takes into account the effect of the contour of the mould wall on the cooling kinetics. Subscripts m and s correspond to the metal and the sand mould, respectively, and k, ρ and C p are the thermal conductivity, density and heat capacity, respectively. H f is the latent heat of solidification and T 0 is room temperature. The time necessary to reach the range of safe expansion pressure t S in the casting equals the time of liquid permeability of the riser t T. Applying Eq. A.2 to the casting and to the riser, considering that liquid permeability in the riser stops at a solid fraction of 0.75 and that safe expansion pressure is reached in the casting at a solid fraction x, the modulus of the riser can be obtained from: M T ωc = ω T M C H F ( ΔT + x Cpm H F ( ΔT Cp m (A.3 Using H F = J/Kg and Cp m =920 J/Kg o C and considering a pouring temperature of 1400 o C and T eu =1150 o C, ω C = 0.75 ( cubic casting and ω T = 0.75 (cylindrical riser: 1 + x M T = Mc (A The two limit values of x are 0, when all transfer is accomplished in the liquid state, which corresponds to good metallurgical quality, and x = 0.75 solid fraction assumed to correspond to the limit of permeability and to a bad metallurgical quality of the melt.

6 The neck, which is commonly of rectangular section, is dimensioned as a function of the selected transfer modulus. The modulus of the neck, commonly of rectangular section with a height twice the width was calculated as M N = 0.7M T. ACKNOWLEDGEMENTS The authors acknowledge DGAPA, UNAM for financial support (Project IN and A. Amaro, C. Atlatenco, and I. Beltran for their valuable technical assistance. LITERATURE REFERENCES [1] SHOLAPURWALLA A., SCOTT S.. Capturing the Complexities of Ductile Iron Solidification Through Simulation AFS Transactions American Foundryman Society Paper (01. [2] T. KANNO. Effect of pouring temperature, composition,mould strength and metal flow resistance on shrinkage cavities in spheroidal graphite cast iron International Journal of Cast Metals Research 2008 VOL 21 NO 1 4,pp 2-6. [3] KANNO T., KANG I., FUKUDA Y, MIZUKI T.AND KIGUCHI S, AFS Transactions, Vol.114, (2006, p [4] QIMING CHEN. Influence of C, Si on volume change of SG iron during solidification computer modelling for hypereutectic and hypoeutectic compositions. JOURNAL OF MATERIALS SCIENCE LETTERS 16 ( [5] KARSAY S. I., Ductile Iron III Gating and Risering, (1981 QIT-Fer et Titane [6] CORLETT G.A AND ANDERSON J.V., AFS Transactions Vol.91 (1983 p [7] GERHARDT, P C JR Computer Applications in Gating and Risering System Design for Ductile Iron Castings AFS Transactions 1983, Vol. 91,pp [8] C. GONZALEZ-RIVERA ET AL. On the testing of shrinkage tendency of ductile iron prior to pouring. From Conference Proceedings METAL2007 Hradec nad Moravici TANGER, 2007,