外文翻译（英文）.pdf

Development of a high temperaturemoldprocess for sand casting witha thin wall and complex shape Eun Hee Kim a Guen Ho Choa Yoon Suk Ohb Yeon Gil Jungaa a School of Advanced Materials Engineering Changwon National University Changwon Gyeongnam 641 773 Republic of Korea b Engineering Ceramic Center Korea Institute of Ceramic EEG Sol gel reaction NaOR H2O ROH NaOH 1 RO 3S O Si OR 2 nOR n 2 H2O n 1 SiO2 2 n 2 ROH 2 where ROH NaOH and RO 3Si O Si OR 2 nOR denote alcohol sodium hydroxide and alkyl silicate respectively NaOR is hydrolyzed to form NaOH by Eq 1 and RO 3Si O Si OR 2 nOR is hydrolyzed to form SiO2and ROH by Eq 2 Heat treatment reaction 2NaOH SiO2 Na2O SiO2 H2O 3 The sodium silicate Na2O SiO2 of glass phase is produced by the reac tion between NaOH and SiO2at a temperature of about 1000 C whichcan give a strength to the mold During the above reactions the organic Thin Solid Films 620 2016 70 75 Corresponding author E mail address jungyg changwon ac kr Y G Junga http dx doi org 10 1016 j tsf 2016 08 069 0040 6090 2016 Elsevier B V All rights reserved Contents lists available at ScienceDirect Thin Solid Films journal homepage binder coated on the particle surface is substituted by the glass phase synthesized from the inorganic precursor which is called an organic inorganic conversion process Therefore the inorganic precursor should behomogeneouslycoatedonthesurfacesofparticlesandtheconversionef fi ciency of inorganic precursor to glass phase should be optimized to pro vide a suffi cient strength to the mold resulting in maintaining the mold shape during the casting In this study the mold process was developed for preparing a mold that sustained a shape at a temperature of 1000 C and then a real product a 250 mm impeller was cast with the actual mold The relationship be tween fracture strength and preparation process is discussed 2 Experimental procedure 2 1 Preparation of mold samples The inorganic binder precursor used for preparing a mold sample in the thin wall casting was composed of tetraethyl orthosilicate TEOS Sigma Aldrich Korea Yongin Korea as a SiO2precursor and sodium methoxide NaOMe Sigma Aldrich Korea as a Na2O precursor The bead powder nominal particle size 0 15 mm Itochu Ceratech Corpora tion Seto Japan was used as a starting material for preparing the mold sample The chamotte sand nominal particle size 0 15 mm Namgan Casting Materials Pusan Korea wasused asa startingmaterialtoprepare the comparison sample Basic formulations and experimental ranges for preparing molds through the organic inorganic conversion process are shown in Table 1 The size of the mold sample used as a starting substrate was 10 mm 10 mm 50 mm which was determined and prepared by a commercial casting company Jinsung Metal Gimhae Korea The bead powder coated with the organic binder through a resin coated sand RCS process was stuffed into a forming mold made of metal and then heat treated at 200 C for 1 h The chamotte sand mixed with water glass sodium silicate Na2O SiO2 53 hydrated H2O Sigma Aldrich Korea Yongin Korea was stuffed into the forming mold and then hardened for 10 s under a CO2atmosphere The mold samples prepared with the bead powder and the chamotte sand were dipped into the inorganic binder precursor at room temper ature Thedippedsamplesweredriedat80 Cforaperiodof1hforelim inating H2O and ROH synthesized during the sol gel reactions and then heat treated at 1000 C with a dwell time of 1 h in the atmosphere for the glassifi cation of the inorganic precursors of Na2O and SiO2 Cooling processtoroomtemperaturefollowedaftereachprocess Adetailedexpla nation of the organic inorganic conversion process is shown in Fig 1 2 2 Preparation of actual mold and casting In this work an impeller with 250 mm diameter which is presently used in fuel gas desulfurization system of power plants was manufactured as an actual cast product Therefore iron with high chro mium contenttoincreasethecorrosion resistanceandantiabrasion was used as a base metal for the actual casting The composition of the base metal is presented in Table 2 First the wooden forming mold for prepar ing an actual mold was designed and made through reverse engineering Sungil Turbine Co Busan Korea The bead powder coated with the or ganic precursor through the RCS process was stuffed into the wooden forming mold and the green mold was heat treated at 200 C for 10 h The prepared actual mold was dipped in the inorganic precursor solu tionwiththe same composition usedinpreparing the moldsample Theac tual mold coated with the inorganic precursor was dried at 80 C for a period of 24 h and then heat treated at 1000 C for 1 h Cooling process toroomtemperaturefollowedaftereachprocess Themoltenmetalmelted Table 1 Basic formulations and experimental ranges to prepare sand mold samples in this work ComponentInorganic precursor g Starting particle g Condition ContentTEOS 38 NaOMe 56 Bead powder 100 Dry process 80 C 24 h Heat treatment 1000 C 1 h 1 Starting powder coated with organic binder 2 Dipping of sand mold in the inorganic precursor 3 Starting powder with inorganic precursor coated on the organic binder 4 Dry and heat treatment processes 5 Starting powder coated with only inorganic binder 2 3 1 4 5 Inorganic binder Organic binder Starting powder Fig 1 Schematic diagram of the organic inorganic conversion process Table 2 Composition of base metal with high content of Cr for the actual casting product ElementCSiMnPSNiCrMoCuVAl 2 820 620 790 010 0140 1526 50 010 050 020 03 BS EN 12513 EN GJN HB555 XCr23 71E H Kim et al Thin Solid Films 620 2016 70 75 at 1600 C in an induction heating furnace was poured into the actual mold at room temperature in the atmosphere and cooled to room tem perature Finally mechanical machining for the cast product was conduct ed as a post processing 2 3 Characterization The morphology and size of starting powders and the fracture surfaces of prepared mold samples were observed using a scanning electron microscope SEM Model JSM 5610 JEOL Tokyo Japan The element analysis on the particle surfaces in the mold samples after heat treatment was conducted using an energy dispersive spectrometer Oxford Instruments Oxford UK The phase of the virgin and recycled Table 3 Comparison of properties in various starting powders used in fabricating sand mold 15 PropertiesCerabeadsZirconChromiteSilica RefractorinessSK37SK37SK39SK33 1825 C1825 C1880 C1730 C Bulk density g cm3 1 692 952 811 58 pH7 25 77 96 6 Thermal conductivity W m K 0 2230 3050 2580 255 Thermal expansion 0 030 180 261 50 Heat transfer W m K 0 560 800 580 71 Chemical composition Al2O3 61 SiO2 37 ZrO2 66 SiO2 33 Cr2O3 45 Fe2O3 25 Al2O3 61 MgO 10 SiO2 99 Major mineral compositionMulliteZirconChromiteQuartz Fig 2 Microstructure of bead particles a virgin and b recycled Each number indicates the microstructures of particles before dipping into the precursor solution the low and high magnifi cation atinterface of particles respectively Dottedarrows indicate the glass phase synthesized at theinterface between particles Element analysisresultsare insertedineachfi gure Fig 3 XRD analysis results for bead particles a virgin and b recycled 72E H Kim et al Thin Solid Films 620 2016 70 75 beadpowderswasanalyzedusinganX raydiffractometer XRD X pert MPD Model PW3040 Philips Eindhoven Netherlands The toxic gas testsforthepreparedmoldsampleswereperformedusingaTruSpecEl emental Analyzer LECO Co St Joseph MI USA and SC 432DR Sulfur Analyzer LECO Co The fracture strengths of the mold samples before and after heat treatment were measured using a universal testing ma chine Instron 5566 Instron Corp Norwood MA USA in the 4 point bending mode at a rate of 0 5 mm min 1 Tests were carried out at room temperature and fi ve runs were performed to determine the standard deviation of the fracture strength The thermal expansion of the mold samples was measured with a heating rate of 5 C min 1up to 1000 C using a dilatometer Model 500S Mac Science Tokyo Japan 3 Results and discussion Conventionally themoldemployed inthesandcastinghasbeen pre paredwithnaturalsand anartifi cialsand achamotte achromite etc In this study the bead powder with a mullite phase was used as the starting material instead of conventional sand powders because the bead powder has an excellent thermal resistance and a low thermal expansion to molten metal at high temperature as indicated in Table 3 15 However the heat resistance of traditional sand powders such as SiO2 chamotte and zircon continuously decreases with increasing temperature In addition the burnt layer by metal penetration begins to be generated at the interface of the mold and the molten metal in traditional sand molds and then the molten metal rapidly spreads farther and farther out into the interior of the mold Eventually it becomes a full mold made of the burnt layer and the sand mold is easy to collapse during the casting It was reported that thethicknessoftheburntlayercanbereducedbyemployingthebeadpow der even in traditional sand mold due to the high heat resistance 15 In thiswork theorganicbinder resin usedinthenewprocesswassubstitut ed with the inorganic binder glass phase during heat treatment improv ing surface properties Therefore the burnt layer was not formed in the mold prepared by the new process The particle morphology interface microstructure and detected ele ment at the interface in the mold samples prepared with the virgin and recycled bead powders are shown in Fig 2 Recycled bead powder is the powder of the mold used in sand casting Each number in Fig 2 a and b indicates the microstructures of particles before dip ping in the inorganic precursor and the low and high magnifi cation at the interface between particles after heat treatment respectively The recycled powder showed nearly the same spherical shape and size as the virgin powder as seen in Fig 2 b 1 The glass phase which is indi cated with dotted arrows in Fig 2 made from the inorganic precursor was generated at the interface between individual particles in the mold prepared with recycled powder as well as virgin powder Fig 2 a 2 and b 2 In addition the elements of Al Si and Na from the starting powder and inorganic precursor were detected together on both particle surfaces Fig 4 Fracture strength of mold samples prepared with the CO2and RCS processes using the chamotte sand and bead powder respectively Fig 5 Thermal expansion of mold sample prepared with bead powder Table 4 Toxic gases generated in combustion tests for molds SamplesCarbon C Hydrogen H Nitrogen N Sulfur S Conventional sand mold1 930 060 13N D Mold prepared with inorganic binder0 030 0520 12N D N D Not Detected below 0 0001 Sample condition Dried sample 73E H Kim et al Thin Solid Films 620 2016 70 75 XRD peaks of the virgin and recycled bead powders are shown in Fig 3 Only the peaks of the mullite phase were detected This result means that the mullite is not affected in the extreme circumstances basic solution of inorganic precursor and heat treatment at 1000 C Namely it demonstrates that the bead powder is stable and reusable to prepare the sand mold as shown in Figs 2 and 3 Therefore the bead powder is determined as the optimum starting material with price competitiveness in spite of bead powder having a higher price than conventional disposable sand powders The fracture strength values of the mold samples prepared through two types of processes CO2and RCS processes and starting powders chamotte and bead are presented in Fig 4 which were measured for the mold samples before and after heat treatment The mold samples prepared by the CO2process showed a green strength of 5 6 0 5 MPa mean standard deviation and a fi ring strength of 8 3 0 3MPa TheCO2processusingthechamottesand whichisconventional ly applied to manufacture large impellers was prepared by the same company for comparison purposes In the samples prepared through the RCS process the strengths were determined to be 7 3 2 5 and 8 7 1 4 MPa for the green and fi ring strengths respectively In the CO2process the green strength of the mold sample is from the water glass The water glass can be well and homogeneously mixed with the chamotte sand due to the liquid phase However in the RCS process thegreenbodyisformedbypolymerizationreactionbetweenvariousresins phenol and isocyante and so on Therefore 100 polymerization and the uniform degree of polymerization are very diffi cult issues in the RCS pro cess Consequentially the standard deviation in the strength of the mold sample prepared through the RCS process is larger than that with the CO2 process However the water glass used in the CO2process generates the glass phase as well as H2O and Na2CO3during the mold preparation and the by products H2O and Na2CO3 can induce defects in the mold which caused the relatively low green strength in the mold prepared by the CO2 process Meanwhile the bead powder can be well rearranged during the forming process due to the network and entanglement of the polymer with long chains resulting in the relatively high green strength However the inorganic precursors applied in the new mold process are converted into the glass phase during the sol gel reaction and heat treatment induc ing the fi ring strengths in the molds prepared through both processes 16 18 To do that the inorganic precursors of a liquid status were used to do homogeneous coating of the inorganic precursors on the surfaces of particles Therefore the new mold process is effi cient method in improving mold strength in both processes Inaddition theshaperetentionofthemoldduringthecastingisalso amajorfactortogetherwiththestrength Thethermalexpansion related to thedimensional stability ofthe mold wasmeasured ata low value of about 0 23 up to a temperature of 1000 C as shown in Fig 5 The con centrationoftoxicgasesincombustiontestsforthemoldpreparedwith the inorganic binder which can be generated during the casting is sig nifi cantly lower than that for the conventional sand mold as shown in thetoxic gas analysis of Table 4 Therefore it shows thatuse of theinor ganic precursor and bead powder is a good choice for fabricating the mold for sand casting Photographs of the wooden mold for preparing the actual casting mold the castproduct and the productafter post processing are presented inFig 6 Thesandmoldforcastingmetalscanbepreparedwiththeforming mold made of metal or wood However most of sand molds are fabricated using the wooden forming mold In the case of the wooden forming mold Fig 6 a the smooth and clean surface in the green body is hard to be prepared because it does not apply pressure The metal forming mold can be used to improve the surface condition in the green body by applying a pressure while the production coat is higher than the wooden forming mold The surface of the cast product Fig 6 b is not clear since the prod uct was just cast using the mold prepared with the wooden forming mold which was improved with a post processing Fig 6 c Finally the cast product with the thin wall and complex shape could be successfully fabri cated by applying the new mold process 4 Conclusions An inorganic precursor and bead powder have been used to over come the drawbacks of conventional sand molds such as the frequent collapse phenomenon and severe defects during the casting The inor ganic precursor wasvery eco friendly aswell asbeinguseful in improv ing the strength of the mold The shape and size of the recycled powder were not changed compared with those of the virgin powder The mold sample prepared through the RCS process using the bead powder showed higher green strength than those through the CO2process using the chamotte sand However the fi ring strengths were improved and showed similar values by introducing the new mold process using the inorganic binder indicating that the new mold process is effi cient method in improv ingmoldstrengthinbothprocesses Inaddition themoldpreparedthrough the new mold process showed the dimensional stability of 0 25 up to a temperature of 1000 C The cast product an impeller having the thin wall and complex shape could be successfully fabricated by applying the newmoldprocess Therefore thenewmoldprocessusingtheinorganicpre cursorandthebeadpowdercanbeappliedasthefabricationmethodofthe mold for the thin wall casting of a complex shape Acknowledgements This work was supported by a Korea Institute of Energy Technology Evaluation and Planning KETEP grant numbers 20142020103400 2013 101010 170C funded by the Korean Government Ministry of Trade Industry and Energy and by a National Research Foundation of Korea NRF grant number 2011 0030058 funded by the Korean Government References 1 Y A Meng B G Thomas Modeling transient slag layer phenomena in the shell mold gap in continuous casting of steel Metall Mater Trans 34B 2003 707 725 2 M im ir L C Kumruo lu A zer An investigation into stainless steel structure alloy steel bimetal produced by shell mould casting Mater Des 30 2009 264 270 Fig 6 Actual mold and products a wooden mold b cast impeller after casting and c cast impeller after post processing 74E H Kim et al Thin Solid Films 620 2016 70 75 3 H Saridikmen N Kuskonmaz Properties of ceramic casting molds produced with two different binders Ceram Inter 31 2005 873 878 4 J Jiang X Y Liu Dimensional variationsof castingsand mould