THROUGH COOLING ANALYSIS

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1 I: Joural of Machie ad Formig Techologies ISSN: Volume 3, Number 1/ Nova Sciece Publishers, Ic. The exclusive licese for this PDF is limited to persoal website use oly. No part of this digital documet may be reproduced, stored i a retrieval system or trasmitted commercially i ay form or by ay meas. The publisher has take reasoable care i the preparatio of this digital documet, but makes o expressed or implied warraty of ay kid ad assumes o resposibility for ay errors or omissios. No liability is assumed for icidetal or cosequetial damages i coectio with or arisig out of iformatio cotaied herei. This digital documet is sold with the clear uderstadig that the publisher is ot egaged i rederig legal, medical or ay other professioal services. THE ASSESSMENT OF HOT FORGING BATCHES THROUGH COOLING ANALYSIS Edilso Guimarães de Souza, Wyser José Yamakami*, Alessadro Roger Rodrigues, Miguel Âgelo Meezes, Juo Gallego, Vicete Afoso Vetrella ad Hidekasu Matsumoto UNESP - Uiv Estadual Paulista, Egieerig Faculty of Ilha Solteira, Av. Brasil Cetro 56, Zip Code: , Ilha Solteira - SP, BRAZIL. Abstract Usig the available thermal eergy i hot forgig through cotrolled coolig as soo as the part is forged allows cost reductio by avoidig post-forgig thermal treatmets. This paper assesses, through trasiet aalysis of the coolig process, the batch size of cylidrical hot forged parts that must be subjected to cotrolled coolig i the furace i order to provide a microstructure similar to that obtaied by ormalizig. Thus, the first forged part of the batch must preset a miimum surface temperature that keeps it i austeitic coditio whe cotrolled coolig started. Therefore, by assessig the time that the first forged part takes to reach the miimum temperature ad measurig the productive flux, it is possible to calculate batch size for the subsequet cotrolled coolig i the furace. The size of the batches obtaied for the aalyzed parts was small, idicatig the eed for cotiuous forgig ad cotrolled coolig process. It was demostrated that forged parts that have larger volume/area ratio require larger batches sice the forgig time is the same for all parts. Keywords: batch size, cotrolled coolig, hot forgig, coolig aalysis. Itroductio Several research works cocered with hot forgig followed by cotrolled coolig have bee performed (Rasouli et al., 2008; Kazemiezhad & Karimi Taheri, 2003; Rog Zhou et al., 2001; Bakkaloglu, 2002; Liu et al, 1998; Kaspar et al, 1997). Other works correlated to this subject, such as developmets metallurgic of alloys, that permit the direct coolig (Li, 1994), ad the applicatio of the thermomechaical treatmet (Ghosh et al, 2003) have allowed to obtai more adequate microstructures ad mechaical properties required i forgig applicatios, thus eablig the reductio of productio cycle ad eergy costs (Luo et al., 2009). I covetioal hot forgig processes, the batch of forged parts is cooled to room temperature ad the heated i a cotiuous furace for additioal ormalizig, commoly used after hot forgig. Thus, it is observed that the covetioal procedure

2 138 Edilso Guimarães de Souza,Wyser José Yamakami et al. used i hot forgig wastes the thermal eergy available i the hot forged parts, hece requirig their reheatig for subsequet ormalizig. O the other had, obtaiig a ormalized microstructure by coolig i furace the forged parts immediately after the hot forgig for a give temperature ad time lower tha that used i the ormalizig ca simultaeously represet lower thermal eergy ad time costs, whe compared to the covetioal process. I hot forgig tests followed by furace coolig, the forged parts are immediately put i the furace to avoid excessive loss of thermal eergy to the eviromet, hece avoidig ay udesired microstructural trasformatios. Each part is idividually moitored for a more precise cotrol, thus avoidig sigificat parameter variatios as time, temperature ad coolig rates defied for each testig coditio. Lou et al. (2009) showed the formatio of differet microstructures for a same coolig coditio, accordig to the chemical compositio of the hot forged steel. I the same way, obtaiig a ormalized microstructure for a particular steel requires determiig the temperature ad coolig rates from the momet the forged parts are obtaied up to their isertio i the furace i order to avoid that microstructural trasformatios take place out of the furace. However, durig forgig the movemet of parts i the productio steps geerally occurs such as batches ad ot idividually. Thus, depedig o the productive flux ad the mass of the forged parts, the air coolig times that the first ad last forged parts from a give batch were submitted will lead to quite differet coolig rates ad fial temperatures, which ca prevet obtaiig a totally ormalized microstructure through the furace coolig of this batch. Aalyzig the thermal eergy trasfer pheomea is importat for determiig the best coditios for the cotrolled coolig process to occur ad to defie the amout of time that the first forged part ca stay out of the furace without ay microstructural trasformatio. I practical situatios i which the complexity does ot allow a aalytical approach, as for istace, i the aalysis of forged parts of complex geometries, umerical simulatios ca assist i solvig these problems. The aim of this work was to apply the cocepts of heat eergy trasfer to determie the size of hot forged part batches i order to allow the formatio of a ormalized microstructure similar to the real parts i terms of microstructure ad hardess usig the coolig furace. Materials ad Procedures AISI 1008, 1042 ad 5132 steels were selected for air coolig simulatios due to the similarities of their thermal properties with steels employed by the automotive idustry. The cylidrical geometry of forged parts amed after FOR 1 ad FOR 2 preset legth ad diameter defied so that their masses are equal to billets used i actual forged gears productio. The former hot forged has 144 mm legth, 88.9 mm diameter, 6.99 kg mass ad the latter has 101 mm legth, 63.5 mm diameter ad 2.51 kg mass. The chemical compositio rage of steels is show i Table 1.

3 The Assessmet of Hot Forgig Batches through Coolig Aalysis 139 Cotrolled Coolig after Hot Forgig The hot forgig followed by cotrolled coolig proposed i this research was idealized accordig to the Figure 1. The billets were heated betwee 1473 ad 1523 K (T b ) i a cotiuous furace, forged i eccetric-shaft press with 1000, 2000 ad 3000 to (depedig o forged part dimesios) ad stored i a cotaier ear presses. After forgig the first part ( 1 ) uder temperature T 1 = T i, the iitial time t = 0 is defied. As forged parts are put i cotaier, they start to cool due to thermal eergy loss by radiatio ad covectio. Table 1. Chemical compositio rage of steels employed i this work AISI Grade Elemet (% weight) C M P S Si Cr (max) (max) (max) (max) (max) 0.04 (max) Figure 1. Hot forgig followed by cotrolled coolig i furace. I order to allow the formatio of pearlite ad ferrite similar to that obtaied by ormalizig i idustrial process, it is relevat to determie the coolig time ct that forged part 1 spet to attai the critical temperature T c so that the material has a austeitic microstructure whe put ito furace of cotrolled coolig. O the other had, the determiatio of temperatures, rates ad coolig times of first forged parts exposed to the eviromet is ecessary i order to avoid some austeitic trasformatio i other microcostituets before parts beig put ito furace of cotrolled coolig. The trasformatio diagrams for cotiuous coolig extracted from Atkis (1980) predict the formatio of pearlite/ferrite for air cooled bars with 75 mm medium diameter. I additio, the iitiatio of austeitic-ferritic trasformatio for these diameters takes place at 1108 K, 997 K ad 1005 K, for AISI 1008, 1042 ad 5132 steels, respectively. The ormalizig of steels employed i idustry, which air coolig simulatios are the aim of the preset work, is performed i a coveyor type furace at 1123 K for 2 hours, after hot forged parts are air cooled up to room temperature. This process aims to promote a microstructure of pearlite ad ferrite evely distributed alog forged parts with a hardess ragig from 163 to 187 Briell.

4 140 Edilso Guimarães de Souza,Wyser José Yamakami et al. The pearlitic-ferritic microstructure predicted by the trasformatio diagram i cotiuous coolig for the air coolig coditio does ot cotai the same characteristics ad hardess through ormalizig. The cotrolled coolig of forged parts i furace at 873 K for 20 miutes carried out i laboratory immediately after their formig allowed obtaiig a similar microstructure to that from idustrial ormalizig with hardess levels specified for this thermal treatmet. The temperature Ti adopted i simulatios was 1473 K owig to it is the miimal value of billet after it is removed from the furace ad before forgig. From this temperature the steel is cooled to the room temperature. The critical temperature T c = 1200 K, assumed i simulatios, is above the temperature required for ferrite formatio iitiatio for assurig the existece of a uique austeitic phase i materials. Besides, this temperature allows time to itroduce a cotaier with a batch of forged parts ito the coolig furace, sice this movemet would ot be performed i a istataeous way after coolig time ct is reached. The umber of forged parts of a batch () ca be determied through mea time ft to forge the parts (time/forged part) ad coolig time ct (Figure 1) that the first forged part takes to reach the critical temperature T c, see Equatio (1). ct 1 (1) ft Estimate of the Coolig Time To estimate coolig time, the spatial distributio of the temperature as a fuctio of time must be kow. Thus, the heat diffusio equatio must be solved. Therefore, the equatio for a cylider with iitial ad boudary coditios idepedet of the agular coordiate is give by Equatio (2): 1 T kr r r r The boudary coditios are as follows: T T k c (2) z z t T k r rr0 h T r 0, z, t T (3) T k z z2l h T r,2l, t T (4) T k 0 (5) z z0 Ad its iitial coditio is:

5 The Assessmet of Hot Forgig Batches through Coolig Aalysis 141 T r z,0 Ti, (6) Where T is the temperature; t is the time;, c, k are the desity (kg/m 3 ), specific heat (J/kg. K) ad the thermal coductivity (W/m. K), respectively; T i is the iitial temperature (K); h is the geeralized heat trasfer coefficiet (W/m 2. K) ad 2L (m) is the total legth of billet. The geeralized heat trasfer coefficiet is foud by addig the covective heat trasfer coefficiet, h c (W/m 2. K), with the radiatio heat trasfer coefficiet, h r (W/m 2. K): The coefficiet h r is obtaied as followed: h r h h r h c (7) 2 2 T T T T (8) Where is the surface emissivity, adopted as 0.8 sice it is a more adequate value for oxidized surfaces (Kaviay, 2002); is the Stefa-Boltzma costat ( W/m 2. K 4 ); T is the surface temperature ad T sur is the surroudig temperature. The coefficiet h c ca be estimated through correlatios, although, i this paper the typical maximum value of 25 W/m 2. K for free covectio i gaseous eviromet was adopted, redraw from Icropera et al (2007). Equatios 2, 3, 4 ad 5 are solved aalytically by the multidimesioal effects method. For this purpose, Equatios 7 ad 8 are used. The solutio is give by: sur sur r, z, t T T z, t T T r, t T Ti T 0 z 2L 0 r r0 t 0 T T i. T T T Where T is the room temperature (K). T(z,t) ad T(r,t) are solutios for oedimesioal problems, plae wall ad ifiite cylider, respectively. The solutio for plae wall is give by Equatio (10): i (9) T z, t T T T. i 1 4. se 2. se 2 t exp. 2. 2L 2 z.cos. 2L (10) The values of are the positive roots of: h2l. ta (11) k Ad the solutio for ifiite cylider is give by Equatio (12):

6 142 Edilso Guimarães de Souza,Wyser José Yamakami et al. T r, t T T T. i 2 J. 2 J J t 1 2.exp 2.. J r r0 r (12) Where J 0 ad J 1 are zero-order ad oe-order Bessel fuctio of the first kid, respectively. The values of are positive roots of: hr k J1 0. (13) J 0 A algorithm i the Pytho programmig laguage was developed to speed up the calculatios owig to the fact that Equatios (11) ad (13) are trascedetal. The coolig time was obtaied umerically by cosiderig Equatio (9) equal to the critical temperature T c, r equal to r 0 ad z equal to 2L, due to the heat trasfer rates are more itese i these poits (r 0, 2L) at the border. Results ad Discussio The average forgig time (ft) for forged parts FOR1 ad FOR2 was 15.9 ad 10.1 s, respectively. It is oted that heavier parts are more difficult to be hadle ad cosequetly waste loger time to be obtaied from the billet. Figure 2 shows the temperature distributio i a cross sectio of AISI 1008 forged part with 88.9 mm diameter at 46 secods after forgig. The critical temperature, T c =1200 K, was reached at the borders, which cofirms that these poits (r 0, 2L) are those that cool more rapidly. The critical temperature was selected for a value at which the materials still remai i the austeitic coditio. The coolig rates o surface ad closed regios are larger tha that provided to forged parts ier ad give the microstructure formatio with morphology ad hardess distict alog material eve they cotai qualitatively oly pearlite ad ferrite. Figure 2. Temperature distributio i the AISI 1008 forged part at 46 secods after forgig ad with iitial temperature of 1473 K.

7 The Assessmet of Hot Forgig Batches through Coolig Aalysis 143 This differece i coolig rates betwee the surface ad iterior of forged part, as well as i microstructure ad hardess, was observed by Luo et al. (2009) for differet steels employed for mold applicatios used i plastic ijectio. The chemical compositio ad the thermomechaical processig of the steels are importat for austeite trasformatio, which have usually show to affect microstructure-mechaical properties relatioships of the hot forged steel parts. Cosiderig the chemical compositio ad carbo cotet i AISI 1008, 1042 ad 5132 steel grades, i additio to the chrome cotet i this latter, this is the critical elemet for the A3 ad eutectoid trasformatio temperatures. For a medium diameter of 75 mm, Atkis (1980) foud for AISI 1008, 1042 ad 5132 steels coolig rates of 45, 41 ad 39 K/mi, respectively. It has bee oticed that for these coolig rates, which are ormally applied i idustrial forgig, there is a ferritepearlite microstructure formatio i all these steels. The icrease i the amout of carbo has cotributed to reduce the temperature of austeite decompositio, effect that was reiforced by the presece of chrome i the AISI 5132 steel - a elemet cosidered a very strog ferrite stabilizer (Bhadeshia ad Hoeycombe, 2009). The morphology of ferrite ad pearlite could be affected for the coolig coditios which are imposed by the idustrial process. Higher coolig rates have ot cotributed to the solute redistributio by diffusio ad beyod icrease the dislocatio desity i the ew microcostituets. As cosequece, ferrite grai size after trasformatio has showed to decrease, as well as the iterlamellar spacig of pearlite (Zhao et al., 2003). Both effects cotribute to a sigificat icrease i stregth ad toughess of the forged steels. Figure 3 ad 4 show the evolutio of the temperature as a fuctio of time. The curves were obtaied from a poit at the border of the forged part with 88.9 ad 63.5 mm diameter, respectively. It is kow that thermal diffusivity measures the ability of a material to coduct thermal eergy relative to its ability to store thermal eergy (Icropera et al., 2007). Figure 3. Reductio of temperature for forged part with 88.9 mm diameter.

8 144 Edilso Guimarães de Souza,Wyser José Yamakami et al. Figure 4. Reductio of temperature for forged part with 63.5 mm diameter. Materials of large thermal diffusivity respod quickly to chages i their thermal eviromet, while materials of small thermal diffusivity respod more slowly, takig loger to reach a ew equilibrium coditio (Icropera et al., 2007). Figure 5 shows the thermal diffusivity (ratio betwee the thermal coductivity ad the product of the specific heat by desity) for the three materials aalyzed i this work as a fuctio of temperature. The data was obtaied from thermal coductivity ad specific heat values collected from ASM Iteratioal (ASM, 1995). This figure shows that the values of diffusivity have the same magitude for all materials. The average values betwee 900 ad 1300 K are 4.3 x 10-6, 4.4 x 10-6 ad 3.9 x 10-6 m 2 /s for the steels AISI 1008, 1042 ad 5132, respectively. Figure 5. Thermal diffusivity variatio with temperature. Owig to the fact that the differece betwee the thermal diffusivity values for the three materials was egligible, the curves show i Figure 3 ad 4 remaied practically superposed.

9 The Assessmet of Hot Forgig Batches through Coolig Aalysis 145 Table 2 shows the coolig time for both forged parts. Despite the fact that the coolig time for forged part with 63.5 mm diameter was shorter tha that with 88.9 mm diameter, the batch for the former is larger due to the shorter forgig time. Table 2. Batches sizes of forged parts FOR1 ad FOR2 for all materials Coolig time ct/ft Batches [s] [-] [-] AISI FOR FOR FOR GRADE FOR1 is the forged part with 88.9 mm diameter; FOR2 is the forged part with 63.5 mm diameter. ct is the coolig time ad ft is the forgig time. The size of the batches obtaied were quite small as observed i Table 2 ad that occurred because the forged part was left to cool freely, i.e., there was o cocer i keepig its iteral eergy by placig it i a thermal box that could reduce the covectio, radiatio ad coductio heat trasfer rates. Like the aalyzed forged parts had a small batch size, a forgig process ad cotiuous cotrolled coolig seemed to be a better optio for the process. Figure 6 shows the batch size behaviour as a fuctio of the forgig time for cylidrical forged parts with diameter equal to height. The material is the AISI 5132 steel ad it ca be observed that the forged with larger ratio betwee volume ad area preset larger batches. The reaso for this behaviour is that the larger this ratio, the smaller the heat chagig area with the eviromet ad, therefore, loger is the time that the first forged part ca stay outside before beig put ito the cotrolled coolig furace. This tedecy is well proouced for small forgig times. Thus, the reductio of productio time leads to a icrease i the batch size, especially for large parts. Figure 6. Size of the batches as a fuctio of the forgig time for AISI 5132 steel. D is the diameter ad V/A is the ratio betwee volume ad area.

10 146 Edilso Guimarães de Souza,Wyser José Yamakami et al. Coclusio The three steels employed i air coolig simulatios (AISI 1008, 1042 ad 5132) preseted similar properties ad thermal behavior betwee 1200 ad 1473 K. The umber of forged parts from each batch submitted to the cotrolled coolig was: 4 parts with 6.99 kg for the three steels, 5 parts with 2.51 kg for AISI 1042 steel ad 6 parts with 2.51 kg for AISI 1008 ad 5132 steels. Forged parts with greater or smaller masses preseted similar batch sizes due to the fact that larger forged parts, eve possessig slower coolig, required loger forgig time whilst smaller forged parts (which cool faster) are forged faster. The size of the batches obtaied for the aalyzed parts was small, idicatig the eed for cotiuous forgig ad cotrolled coolig process. The cotrolled coolig may miimize the cosumptio of electrical eergy ad processig time for forgig ad ormalizig commoly used i the automotive idustry. For the same forgig time, the forged parts of larger volume/area ratio preseted greater batch size compared to the smaller ratio. The higher the forgig time, the lower the differece amog batches sizes. For small forgig times, this differece is large. Ackowledgmets The authors would like to thak FUNDUNESP (Foudatio for the Developmet of UNESP) for the fiacial support. Refereces ASM Iteratioal: The Materials Iformatio Society, (1995), ASM Hadbook Vol. 1: Properties ad Selectio: Iros, Steels, ad High Performace Alloys, 10ª Ed., Carbo ad Low-Alloy Steels, pp. 149, 179, Atkis, M., (1980), Atlas of Cotiuous Coolig Trasformatio Diagrams for Egieerig Steels. America Society for Metals. Metals Park, τhio. British Steel Corporatio. Sheffield, Eglad, 260 p. Bakkaloglu, A., (2002), Effect of processig parameters o the microstructure ad properties of a Nb microalloyed steel, Materials Letters, 56, Bhadeshia, H. K. D. H., Hoeycombe, R., (2009), Steels: Microstructure ad Properties, 3 rd editio. Elsevier Ltd., Oxford Ghosh, A., Das, S., Chatterjee, S., Mishra, B. ad Rao, P. R., (2003), Ifluece of thermo-mechaical processig ad differet post-coolig techiques o structure ad properties of a ultra low carbo Cu bearig HSLA forgig, Materials Sciece ad Egieerig A, 348,

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