Soumya S. Mohapatra

Satya V. Ravikumar

Department of Chemical Engineering,

Indian Institute of Technology,

Kharagpur 721302, India

Ravi RanjanDepartment of Metallurgical

Materials Engineering,

Indian Institute of Technology,

Kharagpur 721302, India

Surjya K. PalDepartment of Mechanical Engineering,

Indian Institute of Technology,

Kharagpur 721302, India

Shiv Brat SinghDepartment of Metallurgical

Materials Engineering,

Indian Institute of Technology,

Kharagpur 721302, India

Sudipto Chakraborty1

Department of Chemical Engineering,

Indian Institute of Technology,

Kharagpur 721302, India

e-mail: sc@che.iitkgp.ernet.in

Ultra Fast Cooling and ItsEffect on the MechanicalProperties of SteelThe objective of this work is to study about the ultrafast cooling of a hot static 6 mm thicksteel plate (AISI-1020) by air assisted spray cooling. The study covers the effect of airflow rate and the water impingement density on the cooling rate. The initial temperatureof the plate, before the cooling starts, is kept at 900 �C. The spray was produced from afull cone high mass flux and low turn down ratio air atomizer at a fixed nozzle to platedistance. The cooling rate shows that low turn down ratio air atomized spray can gener-ate ultra fast cooling (UFC) rate for a 6 mm thick steel plate. After cooling, the tensilestrength and hardness of the cooled steel plate were examined. The surface heat flux andsurface temperature calculations have been performed by using INTEMP software. Theresult of this study could be applied in designing of fast cooling system especially for therun-out table cooling. [DOI: 10.1115/1.4025638]

Keywords: atomized spray, UFC, tensile strength, hardness, transition, nucleate

1 Introduction

Modern steels used in the manufacturing of ship, buildings,automobile industries, and large gas pipe lines require high tensilestrength and toughness combined with good weldability which aredirectly related to the microstructure of the steel. The final micro-structure of the steel is, in turn, strongly dependent on the coolingrate which the plate/strip is subjected to immediately after rolling[1–3].

In steel industry, in a typical hot rolling mill, steel plate iscooled on the run-out table which is fitted with different types ofcooling systems. The conventional laminar cooling system pro-duces slow cooling rate (30 �C/s �80 �C/s) which is not adequatefor the production of high tensile strength steels. The biggest chal-lenge for achieving high cooling rate at high surface temperatureis the Leidenfrost phenomena [4]. Due to this, a vapor layer cov-ers the cooling surface and as a result the heat transfer ratedecreases drastically [5]. Hence, obtaining high cooling rate athigh surface temperature is the main thrust for the development ofUFC technology. Depending upon the plate thickness, the coolingrate may be 300 �C/s for a 4 mm thick strip [6–8]. For thickerplates, Cornet and Herman [9] made an effort to define ultrafastcooling in terms of multiplication factor of strip thickness andcooling rate. According to their study, ultrafast cooling is said tobe achieved when the product of plate thickness (mm) and coolingrate ( �C/s) is preferably greater than 800. In practice, irrespectiveof thickness, cooling rate obtained by ultrafast cooling technologyis always higher in magnitude than that of achieved by using anyconventional cooling technique.

In air atomized spray cooling, fine water droplet is sprayed byusing compressed air on the surface to be cooled and especially inevaporative quenching; droplets on the heated surface evaporateand do not merge in a water film [10–12]. Other advantages of airatomized spray cooling are: (1) uniform cooling is produced byair atomized spray, (2) during air atomized spray cooling, highvolumetric flow of air sweeps the partially evaporated dropletsfrom the hot surface thereby preventing film boiling, (3) atomizedspray produces finer droplets, and (4) at high initial surface tem-peratures, the heat transfer coefficient in atomized spray coolingcould be three times higher than conventional spray cooling[13,14]. The relationship between the heat transfer characteristicsof an air atomized spray with droplet distribution and its dynamicshave also been reported by Refs. [15–17]. The authors in their ear-lier work studied different cooling methods (water jet, waterspray, and air atomized spray) employed for a hot steel plate andit was observed that the air atomized spray produces the highestcooling rate [18].Therefore, air atomized spray cooling at high ini-tial surface temperatures has been found to be a promisingquenching technology, and hence employed in the current work.

It has been observed that the droplet diameter [19] and sprayimpingement density play significant role in the case of air atom-ized spray cooling. The experimental study by Oliveria and Sousa[20] on air atomized spray behavior at low impingement density(8 kg/m2 s) and at very low air/water ratio reveals that the surfaceheat flux is independent of spray impingement density due to theonset of film boiling at high surface temperature (�700 �C).Puschmann and Specht [13] studied air-atomized spray cooling byan internal mixing air blast atomizer but at very low impingementdensity of 4 kg/m2s and at high initial surface temperature(�900 �C). They found that surface heat flux removal rateincreased with impingement density at high air pressure. Thework reported by Al-Ahmadi and Yao [14], says that the heat flux

1Corresponding author.Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL

OF HEAT TRANSFER. Manuscript received March 29, 2012; final manuscript receivedAugust 8, 2013; published online November 28, 2013. Assoc. Editor: Wei Tong.

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is strongly affected by air flow rate even at high impingement den-sity and surface temperature. In addition to the above, the infor-mation provided by Alam et al. [21] concludes that due to highvolumetric flow of air (low turn down ratio nozzle, Lechler156.103) the surface heat flux removal rate increased up toimpingement density of 33 kg/m2s. However, the maximum sur-face temperature considered by these workers was 510 �C only.Moreover, in open published literature, the information on airatomized spray cooling at high surface temperature (�900 �C)and high impingement density is not available.

The current study covers ultra fast cooling of a hot stationarysteel plate at an initial surface temperature of 900 �C by an air-assisted spray with the maximum air/water ratio of 15� 105,which is very high in comparisons to the air water ratio of0.5� 105 m3/s and 2.16� 104 m3/s reported by the previousresearchers [21] and [22], respectively. In addition to the above,in the current work, the maximum impingement density is 400 kg/m2 s, which is also very high in comparison to the maximumimpingement density of around 33 kg/m2s reported by otherresearcher [21]. As each experiment was conducted at high air/water ratio, the effect of superposed air is a dominating factoreven at high impingement density. Hence, in the current workauthors expect that high volumetric flow of air at high impinge-ment density will produce high droplet renewal rate because ofthe effect of superposed air flow on the hot surface. Moreover,this droplet renewal rate is one of the important criteria for theproduction of fast cooling rate.

2 Experimental Setup

2.1 Measurement Apparatus. In the current work, all theexperiments were conducted on a 6 mm thick (100� 100 mm2)square shape AISI-1020 steel plate. For the measurement of tem-perature during experimentation, in the plate three K-type of ther-mocouples having diameter 3 mm (TC1, TC2, and TC3) wereinserted parallel to the quenching surface to avoid the errorinduced due to the thermocouple hole diameter [23]. The locationsof the thermocouples inside the steel plates are X1¼ 20 mm,Y1¼ 3 mm for thermocouple TC1; X2¼ 50 mm, Y2¼ 3 mm for ther-mocouple TC2 and X3¼ 70 mm, Y3¼ 3 mm for thermocouple TC3,respectively. The time-temperature histories were collected with thehelp of a data acquisition system (DAS) (NI-USB-6210 fromNational Instrument, USA) at a sampling rate of 10 data per second.

The air atomized spray was produced from a full cone internalmixing air assisted atomizer (Lechler: 170.801) from a fixed nozzleto plate distance (60 mm) and it was kept vertically down ward toget the maximum gravity effect. The atomizer was connected withthe air and water supply and these supply lines consist of rotametersand regulators to get the exact amount at the desired pressure.

The plate was initially heated up to a temperature of 1050 �C ina muffle furnace. After proper soaking of the steel plate inside themuffle furnace at 1050 �C, pump and air compressor were

switched on to make the spray ready. The flow rate of water andair were controlled by valves, and measured by the rotameters.Initially, the spray was covered and during this the hot steel platewas taken out from the muffle furnace and placed on the coolingpad. The spray cover was then removed and the cooling experi-ment was started. The entire temperature history during theexperiment was recorded by the DAS and saved in the personalcomputer for further analysis. The schematic diagram of the ex-perimental set-up is shown in Fig. 1.

2.2 Nozzle Characteristics and Droplet Diameter. For thecurrent research, an internal mixing air atomizing nozzle havinglow turn down ratio was used (Lechler 170.801). Table 1 showsthe different operational conditions for successful atomization ofwater by compressed air as found in the data sheet from the sup-plier of the nozzle. It has been noticed that the air flow rate is veryhigh and due to this a strong superposed flow of air is generatedduring cooling.

The variation of volumetric mean diameter of droplet with airflow rate was calculated using the data sheet provided by the noz-zle supplier and the results are presented in Fig. 2. It can be seenthat the droplets become gradually finer with the increasing airflow rate. The maximum droplet diameter of 150 lm is achievedat air flow rate of 20 N m3/h, whereas the minimum diameter of33 lm is obtained at air flow rate of 120 N m3/h.

2.3 Estimation of Surface Heat Flux by INTEMP Soft-ware. For the calculation of surface heat flux and surface temper-ature, an inverse heat conduction software (INTEMP) developedby Trujillo [24–26] has been used. For the current work usingINTEMP, a 2D planer model has been employed with 6 mm thick-ness and 100 mm length corresponding to the actual steel plateused in the experiments. Total 3340 quadratic elements with fournodes per element has been used to discretize the plate geometry.

Except the impinging surface (top surface), the remaining threesides are assumed to be adiabatic. The cooling surface (top sur-face) is divided into three constant heat flux zones which includeall type of heat losses (convective, conductive and also the

Fig. 1 Schematic diagram of the experimental set-up

Table 1 Prescribed operating conditions of the nozzle

Air flow rate (Fa) at the indicated pressure(N m3/h)

Water flow rate(Fw) (m3/s)� 10�5

Pa¼ 0.1MPa

Pa¼ 0.2MPa

Pa¼ 0.3MPa

Pa¼ 0.4MPa

3.33 30 55 77 1056.67 25 52 72 9610 22 46 67 9013.3 20 44 66 8216.7 18 45 64 79

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radiation heat transfer) from the impinging surface. The first zoneis extending from x¼ 0 to 35 mm (Z1); second is fromx¼ 35–65 mm (Z2); and third is from x¼ 65–100 mm (Z3).

The data recorded by the three thermocouples are used as theinput to the software as the temperature-time histories at thenodes, corresponding to the location of the thermocouples. Thenodes are numbered as 1715 at position X1¼ 20 mm, Y1¼ 3 mmfor thermocouple TC1; 1765 at position X2¼ 50 mm, Y2¼ 3 mmfor thermocouple TC2; and 1797 at position X3¼ 70 mm,Y3¼ 3 mm for thermocouple TC3, respectively. Figure 3 showsthe computational domain as described above.

2.4 Tensile and Hardness Testing. Specimens for tensiletest were prepared according to ASTM-E8 standard as shown inTable 2. After the cooling experiments, the plates were cut usingbandsaw and machined to the required shape as shown in Fig. 4.Tensile test was carried out in universal tensile testing machine(INSTRON 8862) having maximum load capacity of 100 kN andat a cross-head speed of 2 mm/min.

Hardness test was carried out using a Vickers hardness testingmachine (UHL-VHMT 001). The steel samples for hardness ex-amination were sectioned to 10 mm square piece and the top sur-face (the surface directly cooled by air atomized spray) waspolished using different grades of emery papers. Final polishingwas performed by using 1 lm diamond compound on disc polish-ing machine. During testing, the full load of (100 gf) was nor-mally applied for 10–15 s. The two diagonals of the indentationleft on the surface of the material after the removal of the loadwere measured using a microscope and their average was consid-ered for calculation of hardness.

2.5 Material Characterization. The composition of the steelplate used for the experiments was examined by an optical emis-sion spectroscope (Model No: ARL 3640) and the obtained chem-ical analysis is given in Table 3. The carbon percentage was foundto be 0.18% (by weight). The carbon percentage and other

elements present in the current steel plate conform to AISI-1020standard.

The material properties are listed in Table 4. The average mate-rial properties (density, specific heat, and thermal conductivity)have been taken from the previous research [27]. Before theexperimentation, the tensile strength and the hardness of the mate-rials were measured and they are also shown in Table 4.

3 Results and Discussions

In the current research, water flow rate and air flow rate havebeen considered as the operating variables and the experimentaldesign is shown in Table 5. All the variables were fixed at 4 equalintervals within the range of maximum level and minimum level

Fig. 2 The variation of droplet diameter with air flow rate

Fig. 3 Computational domain of the steel plate for INTEMP

Table 2 Dimensions of flat tensile specimen as specified inASTM E-8

Parameter Dimension (mm)

Gauge length (G) 25.0Width (W) 6.0Thickness (T) 6.0Radius of fillet (R) 6.0Overall length (L) 100Length of grip section (B) 30.0Width of grip section (C) 18.0

Fig. 4 Sketch of tensile specimen (ASTME-8; Table 3)

Table 3 Chemical compositions (wt. %) of steel plate used inthe experiments

Element C Si Mn S P Ni

(Wt. %) 0.18 0.26 0.57 0.08 0.15 0.02

Table 4 Material properties of steel

Density(kg/m3)

Thermalconductivity

(W/m K)

Specificheat

(J/kg K)Poisson

ratio

Tensilestrength(MPa)

Vickershardness

(0.1 kgf/mm2)

7858 51.9 486 0.27–0.30 440 172

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shown in Table 5 so that a total of 16 experiments were conductedfor the current set.

Mechanical properties of steel are strongly dependent on theirmicrostructure which in turn depends on the cooling rate it hasbeen subjected to after hot rolling. In the current study, the airflow rate and water flow rate were varied to obtain different cool-ing rates. Thereafter, the microstructure, tensile properties, andhardness of the cooled steel plate were examined to study theaffect of cooling rate on these properties.

3.1 Impingement Density. Spray water impingement densitywhich is defined as the amount water impinging per unit area perunit time was measured using an indigenously designed and fabri-cated Patternator for all the 16 experimental conditions studied.To observe the effect of air flow rate on impingement density andits local variation along the radial direction, the measuredimpingement density at different air flow rate for a fixed waterflow rate was analyzed and the results are shown in Fig. 5. It canbe seen that variation of impingement density along the radialdirections from the spray axis in both the direction decreases forall the cases. As the air/water ratio increases the spray spreadsradially and the local impingement density at the spray axisdecreases which is confirmed by the work of earlier researchers[13]. It is presumed that at high air/water ratio the superposed airflow is stronger than that at low air/water ratio and this makes theimpingement density to be more uniform along the radial direc-tion. In addition to the above, the effect of water flow rate onspray impingement density at constant air flow rate but at differentwater flow rate has been studied, Fig. 6. Here also it is noticedthat the local impingement density is maximum at the spray centerand decreases along the radial direction. However, it is observedthat local impingement density increases with increasing waterflow rate as expected.

The variation of average water impingement density with waterflow rate and air flow rate is shown in Fig. 7. The average waterimpingement density increases with the increasing water flow rateat constant air flow rate. However, at a constant water flow rate,

the average impingement density decreases with the increasing airflow rate due to the increasing formation of fine droplets in the re-sultant air atomized spray.

A mathematical co-relation among the average impingementdensity, air flow rate, and water flow rate has been developed andit is presented in Eq. (1). The determination coefficient (R2) of thedeveloped correlation is 0.98.

Id ¼ 238:34þ 6079� 105 � FW � 76:33� Fa þ 1015 � F2w

� 19:21� F2a þ 180� 105 � Fa � Fw (1)

In the current work, different combination of air flow rate andwater flow rate produces same water impingement density. But,the cooling rate and surface heat flux are affected by the air flowrate due to the superposed flow effect of air. For the sameimpingement density, at different air flow rates, the observed cool-ing rates and the surface heat fluxes are different. Therefore, thecooling rates and the surface heat flux have been presented as afunction of air and water flow rate instead of water impingementdensity.

3.2 Cooling Curves. As per the experimental designdescribed earlier a total of 16 numbers of experiment were con-ducted and the time-temperature history measured by the thermo-couple was recorded for each of these experiments. From these

Table 5 Experimental design

Name ofthe variable

Maximumlevel

Minimumlevel

No oflevel

Air flow rate(N m3/h)

40 25 (25,30,35, and 40)

Water flow rate(m3/s)

6.67� 10�5 16.37� 10�5 (6.67,10,13.33, and16.67)� 10�5

Fig. 5 The variation of local spray density at constant waterflow rate of 6.67 3 1025 m3/s

Fig. 6 The variation of local spray density at constant air flowrate of 40 N m3/h

Fig. 7 The variation of average water impingement densitywith air and water flow rates

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time-temperature history, the surface heat flux and surface tem-perature were calculated for each case with the help of INTEMPsoftware. Only one such result is presented here as a representa-tive case to understand the cooling behavior of the steel plate dur-ing the experiments. The remaining results have however beenused to analyze the effect of air and water flow rate on the coolingrate, illustrated later.

The time-temperature history recorded by the thermocouples asthe cooling progressed is shown in Fig. 8 at different locations ofthe plate. During this experiment, the water flow rate and the airflow rate were maintained at 13.37� 10�5 m3/s and 35 N m3/h,respectively. Before the atomized spray was turned on, the threethermocouples show the same temperature which confirms theproper thermal soaking of the steel plate inside the furnace result-ing in uniform temperature. The initial 33.5 s of cooling is due tothe natural convection and thereafter the atomized spray coolingstarts, which is evident from the sharp fall of temperature at allthe three locations. It is observed that the temperature at locationscorresponding to thermocouples TC2 and TC3 decrease more rap-idly than thermocouple TC1. To understand this cooling behavior,photographs were taken during the experimentation and one suchphotograph which was taken just after the spray touches the hotplate has been shown in Fig. 9. It shows that a dark circular areaappears around the spray center, and moreover, the radius of thisarea is equal to the radius of the circular area produced by the

direct impinging droplets. The dark circular area is produced dueto the forced convection cooling by the impinging droplets. Here,it is seen that thermocouple TC2 under the forced convectioncooling area and as a consequence TC2 exhibit the higher temper-ature drop than the thermocouple TC1 and TC3. In addition to theabove, thermocouple TC3 shows higher drop than TC1 becausebetween TC1 and TC3, TC3 is the nearest thermocouple from thespray center.

The experimental temperature data corresponding to Fig. 8have been used as input to the INTEMP software for estimation ofsurface heat flux and surface temperature and their variation withtime is shown in Figs. 10 and 11, respectively. The point at whichatomized spray cooling starts in Fig. 8 has been considered as theinitial time (t¼ 0) for both the figures.

The initial surface heat flux removal rate increases with time upto 4.5 s. This is due to the onset of transition boiling within thetime period of 0–4.5 s. At t¼ 4.5 s, the surface heat flux removalrate reaches a maximum value of 2.65 MW/m2 and thereafter thenucleate boiling starts and as a result the surface heat fluxdecreases with time. The same trend was observed by Al-Ahmadiand Yao [14].

The variation of calculated surface temperature with timeshown in Fig. 11 shows that the surface temperature decreasessharply in the time period of t¼ 1.3–3.2 s due to cooling in thetransition boiling regime and within this short time period the sur-face temperature decreases from 900 �C to 600 �C.

Fig. 8 Variation of temperature at different locations duringatomized spray cooling

Fig. 9 Photograph taken during experimentation

Fig. 10 Variation of surface heat flux with time as calculatedusing INTEMP

Fig. 11 Variation of surface temperature with time

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3.3 Transient Effect of Water Flow Rate on Surface HeatFlux. The variation of surface heat flux with time at differentwater flow rates is shown in Fig. 12. All the surface heat flux re-moval curves in Fig. 12 show the same trend; initially it increaseswith time due to the onset of transition boiling and thereafter,because of nucleate boiling surface heat flux removal ratedecreases. The variation in surface heat flux removal rate at differ-ent water flow rate is very small in the transition boiling regimewhereas the maximum variation is observed in nucleate boilingregime. The reason for above is that water flow rate/impingementdensity plays a more effective role in nucleate boiling regime thanthe transition boiling regime [18]. Moreover, it is observed that onincreasing the water flow rate there is a significant improvementin the critical heat flux.

The effect of air flow rate on the surface heat flux has also beeninvestigated. It is noticed that with high air flow rate an air atom-izing nozzle always produces fine water droplets along with asuperposed air flow on the hot surface. Fine droplets enhance thesurface heat removal rate by producing high contact area for heattransfer. In addition, the superposed air flow sweeps the partiallyevaporated droplets from the hot plate, preventing film boiling.However, if the droplets become very fine, the impingement den-sity decreases in the presence of superposed air flow (Sec. 3.1).By contrast, the impingement density increases in the absence ofsuperposed air flow on the hot plate but the vapor film appears athigh initial surface temperature due to the accumulation of waterwhich hinders surface heat flux. Hence, there are two opposingfactors which influence heat flux and surface cooling. Dependingon the experimental conditions, one factor dominates over theother and the cooling is affected accordingly.

The transient effect of air flow rate on the surface heat flux re-moval rate is shown in Fig. 13. It is observed that the surface heatremoval rate increases with time initially and thereafter decreasefor all the cases. This is because of the onset of early transitionboiling. In addition to the above, a huge difference is observed inthe initial surface heat flux and the critical heat flux which directlyaffect the cooling rate. Here, it is seen that the air flow rate of30 N m3/h produces five times higher initial surface heat flux and1.25 times larger critical heat flux than the other two cases withFa¼ 25 and 40 N m3/hr. The reason is that, in case of air flow rateof 30 N m3/h, superposed air flow plays an important role withmoderate impingement density. On the other hand, even thoughthe effect of superposed flow is present at an air flow rate 40 Nm3/h, the impingement density of water decreases significantly(Fig. 5, Sec. 3.1). Due to this the spray becomes dilute whichmight not be sufficient for rapid cooling. The same phenomenon(superposed air flow with less impingement density) was alsoobserved by Puschmann and Specht [13]. Hence, surface heat flux

increases with air flow rate up to 30 N m3/h, and thereafter itdecreases.

3.4 Effect of Water and Air Flow Rate on Average SurfaceHeat Flux and Cooling Rate. For ultra fast cooling operation,attention has been given to the surface temperature range of900–600 �C, because in low carbon steels, austenite becomes met-astable at around 900 �C and, at appropriate cooling rates, maytransform to ferrite-pearlite in this temperature range. Moreover,in steel industries after hot rolling, the steel plate is typicallycooled on the run-out table in this temperature range to control itsmetallurgical and mechanical properties. Hence, in the currentresearch the average surface heat flux and the average cooling ratehas been considered in this temperature range and their variationwith water flow rate at different air flow rate is shown in Figs. 14and 15, respectively.

The average heat flux in the said temperature range has beencalculated and shown in Fig. 14. Here, it is found that average sur-face heat flux increases with water flow rate for all levels of airflow rate due to the increasing impingement density. Moreover, atconstant water flow rate, the average heat flux increases with theincreasing air flow rate up to 30 N m3/h due to the superposedflow effect of air at moderate impingement density, and thereafterit decreases. In the current research, the maximum achieved sur-face heat flux of 2.7 MW/m2 is obtained at an air flow rate of 30 Nm3/h and water flow rate of 16.67� 10�5 m3/s.

A correlation (Eq. (2)) has been developed for average surfaceheat flux with the air and water flow rate by using the Design

Fig. 12 Variation of surface heat flux with water flow rate (Fw)/impingement density (Id)

Fig. 13 Variation of surface heat flux with air flow rate(Fw 5 6.67 3 1025 m3/s)

Fig. 14 Variation of average heat flux with water flow rate

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Expert Software version 7.0. The determination coefficient forthis correlation is 0.98 which shows close fitting of this correlationto the current experimental data.

qs ¼ 2:58þ 22:8� 105 � Fw þ 0:12� Fa � 792� 1010 � F2W

� 0:24� F2a þ 1:38� 105 � Fa � Fw (2)

The variation of average surface cooling rate with the waterflow rate and air flow rate is shown in Fig. 15. The greatest changein the cooling rate at different air flow rates is observed when thewater flow rate increases from 6.67� 10�5 m3/s to 10� 10�5 m3/s.This may be due to the two different spray characteristics at theaforesaid said water flow rates. The spray produced at water flowrate of 6.67� 10�5 m3/s is dilute spray which might not be suffi-cient for fast cooling whereas after flow rate of 10� 10�5 m3/s itbecomes dense. A correlation which relates cooling rate with waterflow rate (Fw) and air flow rate (Fa) is given Eq. (3). The determi-nation coefficient (R2) of the developed correlation is 0.97.

Z ¼� 55þ 708� 105 � FW þ 9:2� Fa � 2268� 1010 � F2w

� 0:153� F2a þ 6� 105 � Fa � FW (3)

3.5 Effect of Cooling Rate on Microstructure. Metallo-graphic samples (heat treated steel) were prepared from the spray-cooled plates by following the standard polishing methods. Thepolished samples were etched with 2% nital solution and driedusing a hot drier. The X-ray diffraction analysis was conductedwith a step size of 0.02 deg (at time per step of 17 s) to examinethe presence of retained austenite. The optical micrographs alongwith diffraction pattern are shown in Figs. 16 and 17,respectively.

The optical micrograph of the as-received material shown inFig. 16(a) confirms the ferrite- pearlite microstructure of the start-ing material. The microstructure of the spray-cooled samples(Figs. 16(b)–16(d)) consisted of lath martensite and as expectedfor a low carbon steel, retained austenite was not observed in anysample. The absence of retained austenite is confirmed by the X-ray diffraction patterns shown in Fig. 17 which show martensitepeaks only.

3.6 Effect of Cooling Rate on Hardness and TensileStrength. To determine the hardness of steels, a Vickers microhardness tester with the load of 100 gf was used. Seven differentindentations were made on the surfaces of steel plate and their av-erage has been considered.

It can be observed from Fig. 18 that the hardness value isincreasing with the increase in cooling rate, and it is expected tobe associated to the refinement of the martensite laths at highercooling rates. The maximum hardness of 520 HV0.1 is achieved atcooling rate of 166 �C/s. The maximum achieved hardness valueis thrice that of as-received steel. Moreover, the hardness data arecorrelated with cooling rate and a correlation (Eq. (4)) has beendeveloped.

H¼ 349þðð22890=ð105�ffiffiffiffiffiffiffiffip=2

pÞexpð�2ððC�181:3Þ=104:6Þ2Þ

(4)

Figure 19 shows that on increasing cooling rate the tensilestrength of AISI-1020 steel increases, which could be because ofthe refinement of martensite laths. At cooling rate of 166 �C/s, the

Fig. 15 Cooling rate versus water flow rate at different air flowrates

Fig. 16 Optical micrographs of the (a) as-received plate and(b), (c), and (d) after atomized spray cooling at a cooling rate of130 �C/s, 145 �C/s, and 160 �C/s, respectively

Fig. 17 X-ray diffraction pattern of the steels

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maximum ultimate tensile strength of 960 MPa is achieved(Fig. 19). Moreover, by using regression analysis a correlationbetween the tensile strength of steel and cooling rate has beendeveloped and it is given in Eq. (5). The determination coefficient(R2¼ 0.98) shows that the correlation is in good agreement withexperimental data.

Y ¼ 470 expðC=307Þ þ 148 (5)

3.7 Comparative Study. The hardness calculated by usingthe correlation developed by Maynier et al. [28] and the correla-tion developed by current research (Eq. (5)) are shown in Fig. 20.The agreement between the two results is very good (the maxi-mum difference is 20 HV0.1 only), considering the scatter in thehardness data which were used to develop these correlations.

4 Validation

The cooling rates calculated by the INTEMP software havebeen validated against experimental results obtained in the presentwork. The temperature calculated by INTEMP software at node

Fig. 18 Effect of cooling rate on the hardness of atomizedspray cooled material

Fig. 19 Effect of cooling rate on the tensile strength of atom-ized spray cooled material

Fig. 20 Comparative study with previous data

Fig. 21 Validation of estimated temperatures with themeasured

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number 1765 corresponds to the location of the thermocouple T2has been validated by comparing with the measured thermocoupledata during the experiment. The comparison (Fig. 21) shows thatthere is a close match (error< 5%) between the temperature esti-mated by INTEMP and the data measured by the thermocouple.

5 Measurement Uncertainty

In the present case, the measured variables are temperature(inside temperature) water flow rate and air flow rate. The mainsources of uncertainty in measured temperature data are tempera-ture fluctuations due to noise and uncertainty in the exact locationof the thermocouples. All the measurements are taken thrice andthe average of them is considered for final calculation. From theaverage data, the maximum uncertainty is found to be 69%. Incase of water flow rate measurement 6 0.5% uncertainty is found.

6 Conclusion

The ultra fast cooling of a hot steel plate by low turn down rationozzle has been studied. The results show that due to the super-posed air flow on the hot plate the heat dissipation rate signifi-cantly increases at high air/water ratio. As a result, the estimatedcooling rate obtained for a 6 mm thick plate falls in the ultra fastcooling regime.

The variation in cooling rate with air flow rate shows that thecooling rate increases gradually up to air flow rate of 30 N m3/h andthereafter it decreases. Increase in air flow rate up to a definite limitcauses decrease in size of the droplets moderately so that higherevaporation takes place which causes increase in ultrafast coolingrate. At very high air flow rate, the droplets become very fine. Thiscauses decrease in impingement density because the droplets areblown away from the surface by superposed air before it reachesthe hot plate. The maximum cooling rate of 176 �C/s is achieved at30 N m3/h air flow rate and 16.67� 10�5 m3/s of water flow rate.

The hardness and tensile strength properties of AISI-1020 steelincrease with cooling rate presumably due to the increasing fine-ness of martensite lath. The maximum ultimate tensile strength(UTS) and micro hardness of the steel obtained are 980 MPa and540 HV0.1, respectively. Hence, from the obtained cooling rate andsubsequent mechanical properties, it can be concluded that airatomized spray with low turn down ratio can be used in steel indus-tries for achieving ultra fast cooling rate on run out table (ROT) forthe production of high tensile strength steel.

Nomenclature

B ¼ length of grip section (mm)C ¼ width of the grip section (mm)

Fa ¼ air flow rate (N m3/h)Fw ¼ water flow rate (m3/s)G ¼ gauge length (mm)H ¼ Vicker hardness (HVO)Id ¼ water impingement density (kg/m2 s)L ¼ overall length (mm)qs ¼ surface heat flux (MW/m2)R ¼ radius of the fillet (mm)t ¼ time (s)

T ¼ temperature (�C)TC1 ¼ temperature measured by thermocouple 1 (�C)TC2 ¼ temperature measured by thermocouple 2 (�C)TC3 ¼ temperature measured by thermocouple 3 (�C)

X ¼ direction along the length of the plate (mm)Y ¼ direction along the thickness of the plate (mm)Y ¼ tensile strength (MPa)Z ¼ cooling rate (�C/s)

Z1 ¼ zone 1 (mm)Z2 ¼ zone 2 (mm)Z3 ¼ zone 3 (mm)

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