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Surface Hardening of a Gray Cast Iron Used for a DieselEngine Cylinder Block Using High-Energy Electron BeamIrradiationSEONG-HUN CHOO, SUNGHAK LEE, and SOON-JU KWONThis study investigates the effects of high-energy electron-beam (1.4 MeV) irradiation on surfacehardening and microstructural modification in a gray cast iron currently used for a diesel enginecylinder block. The gray cast-iron samples were irradiated in air using an electron accelerator.Afterward,theirmicrostructure,hardness,andwearpropertieswereexamined.Theoriginalmicrostruc-ture, which contained graphite flakes in a pearlitic matrix, was changed to martensite, ledeburite, andretained austenite, along with complete or partial dissolution of the graphite. This microstructuralmodification occurred only when the surface was irradiated with an input-energy density over 1.1kJ/cm2, and it greatly improved the surface hardness and wear resistance. In order to investigate thecomplex microstructures, thermal analysis and simulation testing were also carried out. The resultsindicated that the irradiated surface was heated to the austenite-temperature region and then quenchedtoroomtemperature,whichwasenoughtoobtainsurfacehardeningthroughmartensitictransformation.The thermal analysis results matched well with the microstructures of the thermally simulated samples.I.INTRODUCTIONof these advantages, little attempt has been made to applyit to industrial practice.SINCEgray cast iron contains a high percentage ofThe present study aims to improve the durability of auto-flake graphite, it has special properties such as excellentmotive parts by applying high-energy electron-beam irradia-machinability, the ability to resist galling with restrictedtion to the gray cast iron used for an automotive diesellubricant, and excellent vibration damping.14Cylinderengine cylinder block and, thereby, modifying its surfaceblocks andheads are someof theways in whichthe dampingstructure. To attain the maximum hardened depth and thecapacity of gray cast iron is utilized. As the demand forpeak hardness without surface melting, it was essential tohigh performance and durability in automobiles has recentlyestablish the optimum irradiation conditions. Surface hard-soared, strenuous efforts have been made to improve theness and wear properties can be enhanced by the phasewear resistance of engine parts by hardening their surfaces.transformation from pearlite to martensite, while flakeNotably, the surface modification of cast irons by high-graphites remain and play their own distinctive roles.2,3energy beams such as the laser beam and electron beam hasThe surface hardening mechanism was clarified by investi-been studied extensively by many investigators.511Sincegation of the microstructural modification and the phasean electron beam has an energy range from 50 to 200 keV,transformationbothbeforeandafterirradiation.Correlationsit usually requires a vacuum chamber, which is one of thebetween process parameters, microstructures, and surfacemany limitations of an electron beam as compared to thepropertieswerealsoinvestigated.Furthermore,thermalanal-laser beam.11,12,13ysis was conducted to calculate the temperature distribution,Recently, a high-energy electron-beam (energy range: 0.5including peak temperature, cooling rate, and irradiationto 1.5 MeV) irradiation technique, which can be extractedinto air, has been developed.14,15,16A high-energy electrondepth. To verify the applicability of this model, a thermalbeam can penetrate up to several tens of centimeters of air.simulation test was conducted, and the results were analyzedBecause the depth of the hardened surface layer is propor-in comparison to those of actual irradiation.tional to the electron penetration depth, a layer depth ofabout 1 mm can be hardenedby a high-energy electron beamwithout surface melting, which is almost impossible by thelaser technique. Furthermore, many overlappings are notII.EXPERIMENTAL PROCEDUREneeded to harden a broad area, because the scanning widthA. Materialof an electron beam can be easily controlled up to severaltens of centimeters by changing the magnetic field. BlackingThe material used in the present study was the gray castis not necessary in electron-beam surface hardening. In spiteiron (FCH2D) commercialized for a diesel engine cylinderblock and was a product finished in its final form throughcasting. Its chemical composition is Fe-3.54C-2.21Si-SEONG-HUNCHOO,ResearchAssistant,andSUNGHAKLEE,Profes-0.67Mn-0.025Cr-0.013Cu-0.056P-0.031S(wtpct).Thesor, are withthe Center for Advanced AerospaceMaterials, Pohang Univer-sityofScienceandTechnology,Pohang,790-784Korea.SOON-JUKWON,samplewasabout19-mmthick,themaximumpossiblethick-Associate Professor, is with the Department of Materials Science and Engi-ness that can be obtained from the cylinder block. It wasneering, Pohang University of Science and Technology, Pohang, 790-machined to a size of 30 3 30 3 19 mm to prepare wear-784 Korea.Manuscript submitted September 30, 1997.test specimens.METALLURGICAL AND MATERIALS TRANSACTIONS AVOLUME 30A, MAY 19991211B. Irradiation with a High-Energy Electron Beamscanning width, which can be adjusted by varying the elec-tromagnetic field, was fixed at 3.2 cm, a little larger than theThe irradiation with a high-energy electron beam wassample size, so that the sample surface could be sufficientlyconducted at the Budker Institute of Nuclear Physics (Novo-irradiated. When the beam traveling speed (v) and the scan-sibirsk, Russia) using an electron accelerator (Model ELV-ning width (l) are decided, the input-energy density is calcu-6). The energy of this electron accelerator ranges from 0.5lated in proportion to the beam power as follows:14to 1.5 MeV, and the maximum beam current and power are70 mA and 100 kW, respectively. The irradiation conditionsW 5(1 2 f)Pvl4are determined by process parameters such as the beamcurrent and beam traveling speed and by material constantswhere f is the electron reflectivity from the sample surface,such as the density, thermal conductivity, and thermal diffu-and P is the beam power. The reflectivity varies with thesivity. The electron beam size (d) varies with the distanceelectron energy and the atomic number. When an electron(h) from the final diaphragm to the sample surface and isbeam ranging from 1 to 2 MeV is irradiated on iron or steel,related experimentally to the following equation, in the casethe reflectivity is about 40 pct.20,21For convenience, theof the ELV-6 electron beam accelerator:14irradiatedsampleswerenamedsamplesAthroughE,respec-tively, depending on the beam current used. The calculatedd 5 0.1 1 0.055h 1 0.0075!1.4 (MeV)U (MeV)h21values of input-energy density are listed in Table II.Here, the electron beam size is calculated to be 1.02 cm,because the electron energy (U) is 1.4 MeV and the valueC. Microstructural Analysis and Hardness Testingof h is 8 cm. The beam-current density has a GaussianThe central region of the irradiated samples was sectioneddistribution, showing an exponential decrease as it gets far-perpendicular to the irradiation direction. To minimize thether away from the central axis of the beam. If the maximumloss and deformation to the samples, they were sectionedcurrent density at the central axis is J0, the beam currentby an electrodischarge machine. They were polished, etcheddensity (J(r) along the distance from the central axis (r)innital,andexaminedbyanopticalmicroscope.Quantitativecan be expressed by the following equation:17analyses of retained austenite, cementite carbides, martens-ite, and ferrite were done using Mo ssbauer spectroscopy.J(r) 5 J0exp12r2r2b22Because the Mo ssbauer spectroscopy provides informationon the local environment of iron atoms only,22flake graph-where rbis the beam radius (d/2). If scanning is carried outites cannot be analyzed. A 20mCi Co57gamma ray was usedwith the beam fixed, the heat input irradiated on the sampleas a source, and disc specimens of about 10 mm in diametersurface becomes inhomogeneous due to the difference inand about 100mm in thickness were obtained from thecurrent density between the central and the edge regions. Inirradiated surface layer. Also, the microhardness of thethis study, to expand the beam-treated area while avoidingmatrix only, excluding flake graphites, was measured alongthe inhomogeneity of heat input, the beam was scanned at athe depth from the surface by a Vickers harness tester, underdeflectionangleofabout610deginthehorizontaldirection.a load of 50 g.The electron range (S), i.e., the penetration depth of theelectrons, varies, as in the following formula, in the case ofD. Friction Wear Testan electron-beam energy of over 1 MeV:18,19The wear test was conducted using a friction wear testerS 1r(5.1 3 1027U 2 0.26)3(Model EFM-III-EN/F, Orientec Co.) of ring-on-disc type.An SUS 420 J2 stainless steel was used for the upper ring,which had a 25.6 mm o.d. and a 20 mm i.d. The lowerBecause the density (r) of the gray cast iron is about 7.04g/cm3, the electron range is calculated to be about 640mm.disc, measuring 30 3 30 3 5 mm, was prepared from theirradiated surface. Wear arises from friction with the upperThe process conditions were calculated in the aforemen-tioned manner, the results of which are listed in Table I,ring in a pressurized state while the lower disc rotates. Thewear test was conducted for 5 hours at room temperaturetogetherwithirradiationconditionsandmaterialconstants.1In the present study, the input-energy density (W), i.e., theunder a 50 kgf applied load, a 67 rpm rotating speed, anda1500mslidingdistance,withoutusinganylubricants.Wearinput energy per unit area, was varied, with all other processparameters beingfixed at theoptimum conditions.The beamresistance was evaluated from the weight loss after testing.Table I.Irradiation Conditions of High-Energy Electron Beam and Material Constants of the Gray Cast IronProcess ParameterMaterial ConstantElectron energy1.4 MeVthermal diffusivity, k0.12 cm2/sBeam current4.35 to 8.35 mABeam traveling speed1 cm/sthermal conductivity, K0.502 Watt/cm 8CBeam diameter1.02 cmScanning width3.2 cmelectron reflectivity, f0.4Distance from diaphragm to specimen8 cmInput energy density, W1.14 to 2.19 kJ/cm2electron range, S640mm1212VOLUME 30A, MAY 1999METALLURGICAL AND MATERIALS TRANSACTIONS ATable II.Input Energy Density, Depth of Hardened Layer, and Maximum Hardness of the Gray Cast Iron Samples Irradiatedby High-Energy Electron BeamSampleABCDEBeam current (mA)8.357.356.355.354.35Input energy density, W (kW/cm2)2.191.931.671.411.14Depth of hardened layer (mm)2050 (900/850/300)*7004502500Maximum hardness (VHN)860830810790300 (matrix)*Depths of the melted, the transformed, and the partially transformed regions, respectively.E. Thermal Simulation TestIn order to understand the phase transformation behaviorof the irradiated samples, the thermal simulation test wasconducted using a dilatometer. The specimens used were around tubular type of 1-mm thickness, 5-mm o.d., and 10-mm length to achieve fast heating and cooling rates. Theywere abruptly heated at 100 8C/s, held at the peak tempera-ture (Tp) for the shortest possible time, and then rapidlyquenched at 130 8C/s. The Tpwas changed in the range from700 8C to 1160 8C. Here, 1160 8C is the approximate meltingtemperature of the gray cast iron. After the test, the micro-structure was examined by an optical microscope, and theresults were compared to those of the actually irradiatedsamples.III.RESULTSA. Preirradiation MicrostructureFigures 1(a) and (b) are optical micrographs of the unirra-diated gray cast iron. Flake graphites, pores, and MnS parti-cles are observed in the microstructure of the unetchedsample, as shown in Figure 1(a). The size and distributionof flake graphites show deviations in the thickness direction,because the cooling rate differs in locations and becausegraphites float up to the surface during casting. Since allanalysesweredoneatthesamelocationofthesurfaceregion,the distribution effect of graphites was not taken intoaccount. Pores are typical casting defects, and a few MnSparticles hardly affect the properties of the cast iron.3Thenital-etched micrograph of Figure 1(b) shows that the matrixof the gray cast iron is mainly composed of pearlite, togetherwith a small amount of eutectic structure of iron and ironphosphide, viz., steadite, as indicated by arrows. These stea-dites are solidified in the temperature range from 954 8C to980 8C and are reported as hard and brittle compoundsFig. 1(a) and (b) Optical micrographs of the gray cast iron used for asegregated mainly in the interfacial regions of solidifica-diesel engine cylinder block. (a) and (b) are nonetched and nital-etchedtion cells.3,23microstructures,respectively.Notephosphorus constituentsteaditesasindi-cated by arrows in (b).B. Postirradiation MicrostructureFigures 2(a) through (d) are low-magnification opticalthe transformed region. At the interface between the unal-tered and the transformed regions, a heat-affected zonemicrographs which illustrate the microstructural modifica-tion asthe irradiationcondition changes.In sampleA, irradi-(HAZ) is formed, which does not differ much from theunaltered region, and there is no clear demarcation betweenated with an input-energy density of 2.19 kJ/cm2, themicrostructure is modified to the depth of about 2,000mm,the regions. Based on these observations, the microstructureof sample A can be divided into four regions, i.e., the meltedas shown in Figure 2(a). The melted region extends to adepth of 900mm from the surface. Below the melted region,region,transformedregion,partiallytransformedregion,andunaltered region, as marked by arrows in Figure 2(a).flake graphites are left almost intact, and only the matrixpearlite structure is transformed as the irradiated thermalFigure 2(b) is an optical micrograph of sample B, irradi-ated with an input-energy intensity of 1.93 kJ/cm2. Here,energy is transmitted into the interior. Thus, it can be calledMETALLURGICAL AND MATERIALS TRANSACTIONS AVOLUME 30A, MAY 19991213Fig. 2Optical micrographs of the gray cast iron samples irradiated with high-energy electron beam: (a) sample A, (b) sample B, (c) sample C, and (d)sample D.the melted region is not as visible as it was in sample A,graphites and the matrix reaches a sufficient level (4.3 pct)to cause the eutectic reaction and to form ledeburites. Theseand the transformed region only is formed to a depth ofabout 700mm. Samples C and D, irradiated with 1.67 andledeburites are mostly retained in the final microstructurebecause of the rapid quenching. However, the matrix is1.41 kJ/cm2, respectively, show similar microstructures tothat of sample B (Figures 2(c) and 2(d), but the depth ofprimarily composed of martensite due to the rapid coolingrate, unlike the pearlitic matrix of a typical white cast iron.the irradiated layer is reduced to about 450 and 250mm,respectively. In the case of sample E, irradiated with 1.14Figure 3(b) shows the interfacial area between the meltedand the transformed regions. In the transformed region justkJ/cm2, there is no sign of microstructural modification.These results indicate that the microstructural modificationbeneath the interface, the pearlitic matrix is transformed intomartensite, with flake graphite remaining. Melting does notoccurs only when the surface is irradiated with input-energydensity over 1.14 kJ/cm2.occur because the temperature does not rise as high as inthemeltedregion.AusteniteisrapidlyquenchedtotransformFigures 3(a) through (d) are higher-magnification opticalmicrographs of sample A. The melted region consists of ato martensite, or part of it is retained until room temperatureis reached. As a result, coarse plate martensite is observeddendritic solidification structure (Figure 3(a). The micro-structure of the gray cast iron is modified into one similarinside the retained austenite. This plate martensite becomesfiner and more densely populated as it gets into the interior,to that of a white cast iron via the process of melting andrapidsolidification.Aledeburiteeutecticstructurecomposedand the fraction of retained austenite tends to be reduced,as shown in Figure 3(c). In the lower part of the transformedofg1 Fe3C is formed24and grows sidewise as well asedgewise, resulting in a typical structure of white cast ironregion (Figure 3(d), steadites are observed, as marked byarrows. This indicates that the temperature was not raisedcomposed of an interdendritic ledeburite eutectic struc-ture.25As the temperature was raised above the meltinghigh enough to dissolve steadites; thus, this particular regioncan be called the partially transformed region.point, the carbon content in the interface between the flake1214VOLUME 30A, MAY 1999METALLURGICAL AND MATERIALS TRANSACTIONS AFig. 3Optical micrographs of sample A irradiated with highest input energy density, showing (a) the melted, (b) the melted/transformed interfacial, (c)the transformed, and (d) the partially transformed regions.C. Mo ssbauer Spectroscopyretained austenite, while the transformed region consistsof martensite and retained austenite, corresponding to theThe phases present inside the unaltered, transformed, andmicrostructures of Figures 3(a) and (c).melted regions of sample A were quantitatively analyzedby Mo ssbauer spectroscopy, and the results are shown inFigures 4(a) through (c) and in Table III. The unalteredD. Hardnessregion is composed of 82 pct ferrite and 18 pct Fe3C, as canbe seen in Figure 4(a), which is consistent with the fact thatSince the most-critical factor affecting wear resistance ofthe gray cast iron is hardness, Vickers microhardness valuesthe matrix is made up of pearlite. Because the gray cast ironcontains about 2 pct of Si, ferrite peaks are displayed inofthefourirradiatedsamplesweremeasuredalongthedepthfrom the surface, and their results are shown in Figures 5(a)three areas, as marked by II, III, and IV in the figure. PeaksII and III are shown by ferrites containing Si, whereas peakand (b). Vickers hardness number (VHN) microhardnessvalues in the melted and the transformed regions of sampleIV is shown by pure ferrite. The transformed region consistsof 76 pct martensite, 10 pct austenite, and 14 pct Fe3CA (Figure 5(a) range from 600 to 850 VHN and show adouble to triple increase over the unaltered matrix hardnesscarbides (Figure 4(b). There are four martensite peaks pres-ent in the transformed region, among which peaks IV andof about 300 VHN. The hardnesses stay constant at around800 VHN to the depth of about 900mm. Where the trans-V are formed by martensite containing Si, while peaks VIand VII are formed by martensite without Si. The meltedformed region starts, the hardness is radically reduced to600 VHN, gradually rises again up to an 1800mm depth,region is composed of 46 pct martensite, 5 pct austensite,and 49 pct Fe3C carbides (Figure 4(c). The number of peakswhere it reaches 850 VHN, and then falls again up to a 2100mm depth, where the unaltered region appears. These threeof martensite is the same (four) as in the transformed region.The melted region consists of a eutectic ledeburite structureregions correspond, respectively, to the melted, transformed,and partially transformed regions in Figure 2(a). A lowmade up of carbides, martensite, and a small amount ofMETALLURGICAL AND MATERIALS TRANSACTIONS AVOLUME 30A, MAY 19991215Table III.Quantitative Results of the Mo ssbauerSpectroscopy for the Unirradiated and the IrradiatedSurface Layers for Sample A without Taking Account ofGraphites (Error Range: 6 2 Percent)UnalteredTransformedMeltedPhaseRegionRegionRegionFerrite82Martensite7646Austenite105Fe3C181449(a)(a)(b)(c)(b)Fig. 4Mo ssbauer spectra of (a) the unaltered, (b) the transformed, andFig. 5Vickers microhardness vs depth from the irradiated surface of (a)(c) the melted regions in sample A.sample A and (b) sample B through D.hardness at the start of the transformed region is associatedwith the extreme formation of retained austenite aroundcontributes to a hardness as high as twice that of the matrixharness. In the transformed region, the hardness increasescoarse plate martensite. The presence of martensite here1216VOLUME 30A, MAY 1999METALLURGICAL AND MATERIALS TRANSACTIONS Athe irradiated samples are greatly improved by increasingthe input-energy density and the layer depth. This wearresistance indicates a three- or fourfold enhancement and,thus, matches well with the hardness results of Figure 5(b).F. Microstructure of Samples Subjected to ThermalSimulationFigures 7(a) through (e) are optical micrographs of thethermallysimulatedsamples,whichweresubjectedtoabruptheating at Tpvalues of 700 8C, 800 8C, 900 8C, 1000 8C,and 1130 8C, respectively, and then quenched. At a Tplowerthan 800 8C, no microstructural changes are observedbecause flake graphites and steadites (marked by arrows)remain in the pearlitic matrix, as can be seen in Figures 7(a)and(b).AlthoughtheA3transformationtemperatureisabout723 8C, it can rise upon abrupt heating.5,26,27Accordingly,ata fastheatingrate (about1008C/s),the Ac3transformationtemperature might rise over 800 8C, thereby causing nophase transformation under 800 8C. When Tpis 900 8C, onlythe matrix is transformed to fine plate martensite, leavingflake graphites and steadites undissolved (Figure 7(c).When Tpreaches 1000 8C, martensitic transformation in theFig. 6Friction wear resistance test data for the unirradiated sample andmatrix occurs overall, but steadites are not found becausethe irradiated samples B through D. Wear resistance was evaluated fromthey are all decomposed (Figure 7(d). In the case of Tp5the weight loss of the wear specimen after testing.1130 8C, flake graphites remain, plate martensite becomescoarse, and retained austenite starts appearing, as displayedin white in Figure 7(e).Figures 8(a) through (c) are optical micrographs takenas it gets into the interior, because of a decreasing amountwhen Tpwas raised to the melting point of 1160 8C. Some-of retained austenite and of further refinement and densifica-what different structures can be observed, depending on thetion of plate martensite (Figures 3(b) and (c). The intervallocation. This is because there is some difference in Tpduefrom a 1400 to 2100mm depth is the partially transformedto local inhomogeneity during heating and cooling. Figureregion (Figure 3(d), where martensite is transformed par-8(a) shows the formation of a ledeburite eutectic structuretially within cementite lamellae and shows a deviation ofdue to complete melting. On the other hand, Figure 8(b)hardnessdependent onthe location.The regionbeyond 2100shows a mixture of dissolved flake graphites, ledeburite,mm in depth is the HAZ, through which the irradiated heatplate martensite, and retained austenite due to incompleteis transferred and which, accordingly, has the same hardnessmelting. Around the dissolved flake graphites, a largelevel as in the matrix.amount of ledeburite is formed, whereas, in the region farIn sample B, the hardness shows an increase from 600 tofrom them, coarse plate martensite and a considerable800 VHN and is then nearly constant to a depth of 600mmamount of retrained austenite tend to be formed. Figure(Figure 5(b), corresponding to the transformed region of8(c) shows the unmelted region, where flake graphites areFigure2(b).Thisresultmightbefurtherevidencethatmicro-partially dissolved while maintaining their forms.structuralmodificationincludesthephasechangefrompearl-ite to a mixture of martensite and retained austenite. A rapiddecrease of hardness occurs between the transformed andIV.DISCUSSIONthe unaltered regions, indicating the existence of a partiallytransformed region. In samples C and D, the same trend asWhen the gray cast iron is irradiated by a high-energyin sample B can be observed, but their hardened depthselectron beam, the microstructure is modified according todecreaseto500and200mm,respectively,asshowninFigurethe input-energy density. With a very small input-energy5(b). The maximum hardness and the hardened depth ofdensity, the effect of the temperature rise on the microstruc-each sample are shown in Table II.tural change is negligible, whereas, with a very large input-energy density, the surface layer may melt due to an extremetemperature rise. Thus, the optimum input-energy densityE. Wear Resistanceconditions should be obtained from investigations based onthe thermal history.Figure 6 shows the wear-test results of the irradiated sam-ples. The evaluation was based on the weight loss after theThe depth of the hardened layer increases with an increas-inginput-energydensity,asshowninTableII,andisgraphedwear test for sample B through D, as compared to that ofthe unirradiated sample. Sample A was not tested here,in Figure 9. It is almost linearly proportional to the input-energydensity. Ifthis relationis extrapolatedtothe initiationbecause it did not meet with the objective of the presentstudy to use samples without further machining after irradia-of hardening, it can be found that an input-energy densityabove 1.1 kJ/cm2is required to harden the surface by elec-tion. The irradiated samples have better wear resistance thanthe unirradiated one. The hardness and wear resistance oftron-beam irradiation. By applying this relation between theMETALLURGICAL AND MATERIALS TRANSACTIONS AVOLUME 30A, MAY 19991217Fig. 7Optical micrographs of the gray cast iron samples thermally simulated at the peak temperatures (Tps) of (a) 700 8C, (b) 800 8C, (c) 900 8C, (d)1000 8C, and (e) 1130 8C.input-energydensityandthehardeneddepth,themicrostruc-Since flake graphites and the pearlitic matrix have differentthermal properties, the process of heat transfer displays ature and the hardened depth can be controlled.An understanding of the heat-transfer and phase transfor-complex mode. Due to the lack of time for carbon atoms tobe sufficiently diffused during the thermal cycle of abruptmation processes upon irradiation is essential in establishingthe optimum microstructure and the process parameters.heating and cooling, the local inhomogeneity of the carbon1218VOLUME 30A, MAY 1999METALLURGICAL AND MATERIALS TRANSACTIONS AFig. 9Hardened case depth vs input energy density, showing that mini-mum input energy density required for the case hardening of the gray castiron irradiated with high-energy electron beam is about 1.1 kJ/cm2.content affects the phase transformation process to a greatextent.To understand the mechanism of microstructural modifi-cation upon irradiation, information on input-energy densityand the peak temperature is most essential. The applicationofthe followingthermal-transfer modelis usefulinthe inter-pretation of the effects of the process conditions on themicrostructural changes. The temperature variation as afunction of the depth and the time of irradiation can becalculated using the following equation,28taking into con-sideration the beam-current density, material constants, andprocess parameters: T t5 K2T x21(1 2 f)J(r)SF1xS25where k refers to the thermal conductivity, f is the electronreflectivity on the material surface, S is the electron range,and J(r) is the beam-current density along the distance fromthe central axis (r) The second column on the right-handside of Eq. 5 shows the power of the absorbed electronbeam per unit area. The term F(x/S) is a geometrical factorindicating the change of heat source with the depth and isexpressed in terms of the ratio of S to depth (x), as follows:F1xS25 0.74 1 4.71xS22 8.91xS221 3.51xS236Equation 6 assumes that the heat input is dissipated onlyby heat conduction through the material itself via self-quenching, without considering heat convection.Fig. 8Optical micrographs of the gray cast iron samples thermally simu-Because of the complexity of solving nonlinear secondarylated at the peak temperatures (Tp) of 1160 8C, showing (a) the ledeburiteformation in the completely melted region; (b) the complicatedly mixeddifferential equationslike Eq.5 byan analyticalprocedure,microstructure containing dissolved flake graphites, ledeburite, plate mar-a finite-difference method, specifically, the implicit methodtensite, and retained austenite in the partially melted region; and (c) theof backward difference, is used. This method provides stabledissolution of flake graphites in the martensitic matrix in the trans-values irrespective of the mesh size.29Assumptions wereformed region.made that the initial condition, i.e., the sample temperaturebeforeirradiation,isconstantat208Candthatthereisnoheatdissipation due to either radiation or convection on both thesurface and the bottom of the sample. These assumptions onMETALLURGICAL AND MATERIALS TRANSACTIONS AVOLUME 30A, MAY 19991219Table IV.Calculated Peak Temperatures from theThermal Transfer Model (Equation 5)InputCalculatedBeamBeamEnergyPeakCurrent, IPower, PDensity, WTemperature,Sample(mA)(kW)(kJ/cm2)Tp(8C)A8.3511.72.191290B7.3510.31.931138C6.358.91.67986D5.357.51.41834E4.356.11.14687the boundary conditions were made because there is no timeavailable for irradiated heat input to dissipate on the surfaceand bottom boundaries, due to therapid heat transfer into theinterior. The assumptions can be expressed as follows.Initial condition: T(x, 0) 5 20 8C7Boundary condition: T(x, t)x5 0 at x 5 0 and x 5 1.9(a)Here, 1.9 refers to the sample thickness. The peak tempera-tures calculated in the aforementioned manner are listed inTable IV, and the temperature variations vs the time and thedepth from the surface are graphed in Figures 10(a) and (b).The surface temperature abruptly reaches Tpin 1 second andthen rapidly drops to about 300 8C in 4 to 6 seconds due tothe rapid cooling by the self-quenching of the material itself(Figure10(a).Thistendencyiscommontoallsamples,withthetemperaturevariationinTpdependingontheinput-energydensity.InthecaseofsampleA,Tpis12908C,whichishigherthanthemeltingpoint,therebycausingmelting.TheTpvaluesof samples B through D are calculated as 1138 8C, 986 8C,and8348C,respectively,causingmartensitictransformation.In sample E, irradiated with 1.14 kJ/cm2, microstructuralchange does not take place, with the calculated Tpbeing687 8C. Figure 10(b) illustrates the temperature variationalongthedepthatTpandshowsadecreasingtemperaturepro-portional to the depth beyond 640mm, which is the electronrange.TheseTpvaluescorrespondwelltothemicrostructuralmodification of the actually irradiated samples. These find-ingsalsoconfirmtheusefulnessofthethermal-transfermodelin understanding the microstructural modifications.(b)Thethermal-analysisdatacanberelatedtothoseofthether-Fig. 10Temperature variations as a function of (a) time and (b) depthmal simulation test.When Tpis lower than 8008C, no micro-from the surface for samples A through E irradiated with high-energystructural modification arises (Figures 7(a) and (b),electron beam.indicating that the Ac3transformation temperature is above8008C.Interpretingthisintermsofthemicrostructuralmodi-fication in sample D (Tp5 834 8C), where martensitic trans-formationhasoccurred,itisfoundthattheAc3transformationgraphites, and the transformed region, is formed. The ther-mallysimulatedmicrostructuresmatchwellwiththoseofthetemperature lies somewhere between 800 8C and 834 8C.WhenTpis9008C,thematrixistransformedtomartensite,butactually irradiated samples. However, in the thermally simu-lated samples, partially dissolved flake graphites exist nearsteaditesremain.Whenitrisesto10008C,steaditesdisappear(Figure7(d).Thisiscongruentwiththereportonthedissolu-themeltingpoint.Thisdiscrepancyisassociatedwiththedif-ference in holding time at Tp; 1 secondin irradiation vs 3 to 4tion of steadites at the temperature range from 954 8C to980 8C.3As Tpincreases to 1130 8C, plate martensitesecondsinthermalsimulation.Theheatinputmusthavebeenlargerinthelattercase.Thetimeheldattheaustenitetempera-becomescoarserwithincreasing amountsofretainedausten-ite, and flake graphites start dissolving. At 1160 8C, close totureregionisalsoextended,whichactivatesthecarbondiffu-sion and the graphite dissolution (Figures 8(b) and (c).themeltingpoint,acomplexmicrostructure,composedofthemelted region with ledeburite, the partially melted region,The thermal analysis results via the thermal-transfermodel and the thermal simulation test are quite consistentwith ledeburite, martensite, austenite, and dissolved flake1220VOLUME 30A, MAY 1999METALLURGICAL AND MATERIALS TRANSACTIONS Awith microstructural observations of the irradiated samplesconfirming the usefulness of these thermal analyses in theinterpretation of microstructural modification.and prove to be very significant in clarifying the temperaturevariation and phase transformation in the process of thermaltransfer. For a more-accurate thermal analysis, however, theACKNOWLEDGMENTSlatentheataccompanyingphasetransformationandheatcon-This work was supported by the 1996 Ministry of Educa-vection within the melting pool should be taken into consid-tion Research Fund for Advanced Materials and Kia Motorseration. Since the thermal properties of the gray cast ironCorp.TheauthorsthankProfessorsNackJ.KimandYangmoare different in flake graphites and in the pearlitic matrix,Koo and Dr. Dongwoo Suh, POSTECH, and Drs. Won Suktheir respective characteristics should also be considered.Cho and Sang Ho Kim and Mr. Sung Mu Choi, Kia MotorsThe results of this study provide important experimentalCorp., for their helpful discussion on microstructural analy-data which illuminate the mechanisms of phase transforma-sis. Use of the electron accelerator of the Budker Institutetion, melting, and solidification upon irradiation. In casesof Nuclear Physics is also gratefully acknowledged.where high wear resistance is demanded, not only is the sur-facehardnessgreatlyimprovedbythemartensitictransforma-REFERENCEStion, but the wear resistance of engine parts is also enhanced1. H.T. Angus: Cast Iron; Physical and Engineering Properties, 2nd ed.,to a great extent by controlling the hardened case depth. IfButterworth and Co., London, 1976, pp. 161-76.the input-energy density is high, as in sample A, the melted-2. R. Elliott: Cast Iron Technology, Butterworth and Co., London, 1988,surface region is changed to awhite cast iron with high hard-ch. 1.3. W.F. Smith: Structure and Properties of Engineering Alloys, 2nd ed.,ness and about 2 mm of the surface layer is hardened. How-McGraw-Hill, Inc., New York, NY, 1993, ch. 8.ever, in this case, the surface inhomogeneity arising from4. I. Minkoff: The Physical Metallurgy of Cast Iron, John Wiley & Sons,surface melting requires further machining. When surfaceLtd., New York, NY, 1983, ch. 3.hardening is achieved by martensitic transformation without5. N. Za rubova , V. Kraus, and J. Cerma k: J. Mater. Sci., 1992; vol. 27,surface melting by using a lower input-energy density, as inpp. 3487-96.6. C.H. Chen, C.J. Altstetter, and J.M. Rigsbee: Metall. Trans. A, 1984,sampleB,thehardenedcasedepthisabout1mm.Becauseofvol. 15A, pp. 719-28.the unoxidized uniform surface, it can be used as a finished7. F.FouquetandE.Szmatula:Mater.Sci.Eng.,1988,vol.98,duct without additional machining.8. C.W. Draper and P. Mazzoldi: Laser Surface Treatment of Metals,NATO ASI Series E, No. 115, NATO, Martinus Nijhoff, Boston, 1986,pp. 413-33.9. I.L. Pobol: Met. Sci. Heat Treatment, 1991, vol. 32 (78), pp. 520-27.V.CONCLUSIONS10. I. Atsushi: Bull. Jpn. Soc. Prec. Eng., 1984, vol. 18 (3), pp. 219-24.Based on investigations of the microstructure and wear11. D.N.H. Trafford, T. Bell, J.H.P.C. Megaw, and A.S. Bransden: Met
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