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ISIJ International, Vol. 54 (2014), No. 6, pp. 1426–1435

Wetting and Cooling Performance of Mineral Oils for Quench Heat Treatment of Steels

Gopalan RAMESH and Kotekar Narayan PRABHU*

Department of Metallurgical and Materials Engineering, National Institute of Technology Karnataka, Srinivasnagar, Mangalore,575025 India.

(Received on December 20, 2013; accepted on February 19, 2014)

In the present work, wetting kinetics, kinematics and heat transfer characteristics of mineral oils havingvarying thermo-physical properties sourced from different suppliers were investigated using contact angle,online video imaging and cooling curve analysis techniques. The relaxation behavior of mineral oils of lowviscosity and surface tension on Inconel substrate indicated improved wettability and fast spreading kinet-ics while mineral oils of high viscosity and surface tension showed reduced wettability and slower spread-ing kinetics. Further, the spreading behavior of mineral oils of lower viscosity and density showed theabsence of viscous regime. During rewetting, formation of double wetting fronts and more uniform natureof wetting front were observed with mineral oils of high viscosity and flash point whereas no additionalwetting front was observed for mineral oils of low viscosity and flash point. Among the convectional/fast/hot mineral oils, higher wetting front velocity and cooling rate were obtained for low viscosity mineral oil.The heat extracting capability of high viscosity mineral oils was higher during vapour and nucleate boilingand lower during liquid cooling stage. Further, highly viscous mineral oils showed uniform heat transfercompared to mineral oils having low viscosity.

KEY WORDS: quenching; mineral oils; wetting; contact angle; cooling curve analysis; heat flux transients.

1. IntroductionQuenching is a critical step of heat treating process which

defined as rapid cooling of components in-order to formmartensitic/bainitic microstructure and to avoid the transfor-mation of pearlite/ferrite in the case of steel. The coolingconditions during quenching play an important role onphase transformation and development of mechanical prop-erties of the components.1) Factors that influence the coolingconditions of component during quenching are categorizedinto three groups (i) workpiece characteristics (composition,mass, geometry, surface roughness and condition) (ii) quen-chant characteristics (density, viscosity, specific heat, ther-mal conductivity, boiling temperature) and (iii) quenchingfacility (bath temperature, agitation rate, flow direction). Ofthese, the quench medium used to extract the heat from thehot component at particular rate is a significant factor forheat treating engineer to alter/achieve the desired coolingcondition of components in both technical and economicalconsideration.2)

Different liquid quench media used in heat treating indus-tries are water, brine, aqueous polymer and mineral oilquenchants. Even though mineral oil possesses several envi-ronmental problems and fire risks, still it is most widely(almost 85%) used quench medium in heat treating indus-tries. This is due to its better aging stability, thermal stabilityand oxidation resistance. Further, mineral oil quenchingresulted in more uniform cooling, reduced distortion andcracking of steel components.1) The cooling behavior ofmineral oil during quenching is same like water whichinvolves three stages of cooling namely, vapour blanket,

nucleate boiling and convective cooling stages. However,the cooling performance of mineral oil is much lower thanwater. Further, formation of double wetting fronts on quenchprobe was reported for mineral oil quenching.3,4)

Mineral oils are petroleum byproducts, generally mix-tures of chemical structures with a range of molecularweights and do not contain any fatty components. Generallythe mineral oils are distilled from the C26 to C38 fractionof petroleum and composed of branched paraffins (CnH2n+2)and cycloparaffins (CnH2n) together with a small amount ofaromatics (benzene ring and its derivatives). Within an indi-vidual molecule, there are some cycloparaffin rings, aromaticrings and the necessary paraffin and olefin side or connectinggroups.5) The wetting agents, accelerators and anti-oxidantmay be added to achieve specific quenching characteristicsof mineral oils. Mineral oils can be grouped into distinctivegroups based on the composition, the presence of additivesand application temperature. They are classified as conven-tional oils, fast or accelerated oils, martempering or hotquenching oils. Conventional quenching oils are usuallycomposed of paraffinic and naphthenic fraction with aviscosity ranging from 100 to 110 SUS (Saybolt UniversalSeconds) at 40°C (some oils may have viscosities of upto200 SUS at 40°C). These oils may contain antioxidants toreduce the rates of oxidative and thermal degradation but donot contain additives to increase the cooling rate. The con-ventional quenching oil tends to show a prolonged vaporblanket stage, a short nucleate boiling stage and finally avery slow cooling of convective stage. Fast quenching oilshave low viscosities in the range of 50 and 110 SUS at 40°C.These oils contain one or more additives to enhance the wet-ting and quenching speed and often contain antioxidants.The fast quenching oil shows a high quenching speed duringthe vapor blanket stage and in some situations approaching

* Corresponding author: E-mail: prabhukn_2002@yahoo.co.inDOI: http://dx.doi.org/10.2355/isijinternational.54.1426

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the initial speed of water followed by a moderately fastcooling rate in the nucleate boiling range. The cooling ratein convection stage is usually about the same as provided byconventional quenching oils. However, some fast quenchingoils containing special additives provide faster cooling ratesin convection stage. Martempering or hot quenching oils aresolvent-refined paraffin-base mineral oils with good thermalstability and oxidation resistance. They are used at temper-atures between about 95°C and 230°C. They may also con-tain antioxidants to improve their aging stability.4,6,7)

Totten et al.8) discussed the importance of chemistry ofquench oil and its significance on heat transfer performanceduring quenching. Ma et al.9) investigated the performanceof a series of mineral oil based quenchants having viscosi-ties ranging from 10.4 to 120 mm2/s using medium carbonalloy steel (AISI 4140) probe of 9.5 mm diameter by38.1 mm length. They observed that peak cooling rate andheat transfer coefficient obtained for the probe wereincreased with decrease in viscosity of quenchant. Similarly,the hardening power of the quenchant was increased with adecrease in viscosity of quenchant. Asada and Fukuhara10)

observed that the length of vapour stage during quenchingwas influenced by the viscosity and molecular weight of themineral oil. The higher viscosity and molecular weight ofthe mineral oil resulted in early collapse of vapour film athigher temperature. Yokota et al.11) investigated mineral oilbased quenchants having identical viscosities and additivesformulation but different types of mineral base stocks. Theyobserved that cooling of 0.45% C steel (especially in thetemperature range 350–300°C) was significantly influencedby difference in the base stocks of quenchants. Fernandesand Prabhu12) showed that the blending of palm oil withmineral oil increased the spreading rate as well as thequench severity. The literature showed wide range ofquenching performance of mineral oil can be obtainedthrough careful formulation and blending.

The mineral oils from the different producers are formu-lated to obtain different chemical structures with differentadditives. The varying chemical compositions of mineral oilquenchants have significant influence on its cooling perfor-mance and wetting behavior during quenching. A detailedunderstanding of wetting and cooling behavior of mineralbase oil quenchants is therefore necessary for judiciousselection of quenchant to obtain superior properties of com-ponents with reduced distortion and cracking. The presentwork is aimed at the study of wetting kinetics, kinematicsand cooling performance of mineral oils sourced from dif-ferent suppliers and assessment of the suitability of theseoils for industrial heat treatment.

2. ExperimentalIn the present work, different kind of mineral oils were

used as quench media and denoted as MQ-1, MQ-2, MQ-3,MQ-4, MQ-5, MQ-6, MQ-7 and MQ-8. The mineral oilquenchants were obtained from four different suppliers. Themineral oils, MQ-1, MQ-5 and MQ-7 were procured fromone supplier. Similarly, MQ-3, MQ-4 and MQ-8 were pro-cured from one supplier. The remaining mineral oils, MQ-2and MQ-6 were procured from different sources. Among themineral oils, MQ-7 and MQ-8 are classified as hot oilswhereas MQ-4, MQ-5 and MQ-6 are classified as acceler-ated/fast oils. Remaining mineral oils, MQ-1, MQ-2 andMQ-3 are classified as normal/bright oils.

The viscosity and thermal conductivity of quenchantswere measured using Brookfield LVDV-IIIU Rheometer andKD2 Pro thermal property analyser respectively. Weight dis-placement method was used determine the density of thefluids. The fire and flash points of quenchants were deter-mined using Cleveland open cup apparatus. Pendant dropmethod was to determine the surface tension of quenchants.A 2.5 ml syringe with 0.9 mm diameter needle having a pre-cision flow control valve was used for this purpose. For

spreading studies, a droplet of quenchant was dispensed onto the Inconel 600 substrate. The spreading phenomenonwas recorded using dynamic contact angle analyzer FTA200 (First Ten Angstroms, USA) equipment. Capturedimages were analyzed using the FTA image analysis soft-ware to determine the interfacial tension, contact angle,droplet base diameter and spread area. The surface textureof the Inconel 600 substrate was similar to the Inconel 600probe used for quenching experiments. The experimentswere carried out at an ambient temperature of 30°C.

For cooling curve analysis, two quench probes of 12.5 mmdiameter and 60 mm length were prepared from Inconel 600material. To assess the axial variations in heat flux tran-sients, holes of 1 mm diameter were drilled at differentheights located at 2 mm from the surface (probe I) as shownin Fig. 1(a). Holes designated as A1, A2, A3, A4, A5, A6,A7 and A8 were located at 7.5, 15, 22.5, 30, 37.5, 45, 52.5and 40 mm ± 1 mm from the top surface of the quench proberespectively. For determination of heat flux variations in theradial direction, holes of 1 mm were drilled at different azi-muth angles to a depth of 30 mm ± 1 mm and were locatedat 2 mm from the surface (probe II) as shown in Fig. 1(b).These holes, designated as R1, R2, R3, R4, R5, R6, R7 andR8, were located at angles of 0°, 45°, 90°, 135°, 180°, 225°,270° and 315° respectively. Holes of diameter 1 mm (A9 forprobe I and R9 for probe II) were drilled at geometric cen-ters of both probes. Quench probes were conditioned byheating and quenching in quench oil for several times in-order to obtain reproducible results. Calibrated K-type Inc-onel thermocouples were inserted into the quench probe.The other ends of thermocouples were connected to a PCbased temperature data acquisition system (NI 9213). Verti-cal tubular electric resistance furnace open at both ends wasused to preheat the quench probe to 850°C. The heatingzone of the furnace was 80 mm in diameter and having alength of 190 mm. During heating, the top and bottom partsof the furnace were covered with insulating blanket. Aquench tank of internal diameter of 115 mm and length of210 mm with 2 000 ml of quenchant was kept below the fur-nace during quenching. The quench probe support operatedthrough guide pins was designed such a way that, the probewas positioned at centre of heating zone during heating andat 50 mm from the bottom surface of quench tank duringquenching. Once the probe attained the preheating temper-ature, it was directly quenched into fluid without any signif-icant time delay (<0.35 s). The probe temperatures wererecorded at a time interval of 0.1 s during quenching. Theschematic of the experimental setup is shown in Fig. 2. Ahigh performance smart camera (NI 1774C) was used foronline video monitoring of the quenching process. The scan-ning rate was 3 images per second.

The metal/quenchant interfacial heat flux transients wereestimated from the measured temperature histories and ther-mo-physical properties of probe material by solving theinverse heat conduction problem (IHCP). The equation thatgoverns the two-dimensional transient heat conduction isgiven below.

Fig. 1. Schematic of (a) quench probe I and (b) quench probe II.

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For axial location:

........... (1)

For radial location:

........ (2)

The above equations were solved inversely with the follow-ing initial and boundary conditions using the finite elementbased TmmFE inverse solver software (TherMet SolutionsPvt. Ltd., Bangalore, India), for estimating metal/quenchantheat flux transients.Initial condition

......................... (3)

and boundary conditions

..... (4)

.................. (5)

........... (6)

The mathematical description of the serial solution to IHCPis given in Ref. 13).13) Figure 3(a) shows the solutiondomain of half symmetrical shape of the quench probe Iused for estimation of heat flux components in the axialdirection. The geometry was discretized using four nodequadrilateral and four side linear, uniform mesh. The totalnumber of elements used was 3 000 (25 × 120). Figure 3(b)shows the solution domain of the quench probe II used forestimation of heat flux components in the radial direction.The geometry was discretized using three node triangle andthree side curved, uniform mesh. The total number ofelements in this case was 5 000. The thermo-physical prop-erties of the probe material used in the inverse model aregiven in Table 1.14) For both probes the surface in contactwith the liquid was divided into eight segments which wereassigned an unknown heat flux boundary. The convergencelimit for Gauss-Siedel iterations was set at 10–6.

3. Results and DiscussionThe measured thermo-physical properties of quenchants

are presented in Table 2. The density, thermal conductivity,surface tension, viscosity, flash point and fire point of themineral oil quenchants obtained from the different sourceswere found to be in the range of 848–885 kg/m3, 0.125–0.135 W/mK, 33.10–42.28 mN/m, 14.36–189.87 cP, 156–255°C and 172–281°C respectively. Among the mineraloils, MQ-8 showed higher values of thermal conductivity,surface tension, viscosity, density, fire point and flash pointwhile lower values were obtained with MQ-4.

3.1. Contact Angle and Spreading BehaviorFigure 4 shows images of mineral oils droplet on an Inc-

onel 600 substrate during spreading. A droplet form a triplephase contact point, also known as the contact line front/advancing front, upon dispensing which starts to move from

Fig. 2. Schematic of experimental setup.

Fig. 3. Solution domain of (a) quench probe I and (b) quench probeII used in IHCP.

1

r rrT

r z

T

zCp

T

t

∂∂

∂∂

⎛⎝⎜

⎞⎠⎟ +

∂∂

∂∂

⎛⎝⎜

⎞⎠⎟ =

∂∂

λ λ ρ

1 12r r

rT

r r

TCp

T

t

∂∂

∂∂

⎛⎝⎜

⎞⎠⎟ +

∂∂

∂∂

⎛

⎝⎜

⎞

⎠⎟ =

∂∂

λϕ

λϕ

ρ

T r z Ti( , ) = =at t 0

−∂∂

−∂∂

= =λ λT

rn

T

zn q r z t k p lr z k K( , , ) ; , ,...., ...., onΓ 1 2

−∂∂

−∂∂

=λ λ ιT

rn

T

znr z 0 on Γ

−∂∂

−∂∂

= − ∞λ λ ιιT

rn

T

zn h T Tr z ( ) on Γ

Table 1. Thermo-physical properties of Inconel 600 used in IHCP.14)

Temperature (°C) 50 100 150 200 250 300 350 400 450 500 600 700 800 900

Thermal conductivity (W/mK) 13.4 14.2 15.1 16 16.9 17.8 18.7 19.7 20.7 21.7 – 25.9 – 30.1

Specific heat (J/kgK) 451 467 – 491 –– 509 – 522 – 533 591 597 597 611

Density (Kg/m3) 8 400 8 370 – 8 340 –– 8 300 – 8 270 – 8 230 8 190 8 150 8 100 8 060

Table 2. Thermo-physical properties of various mineral oils used in the present study.

Quenchant Density (kg/m3) Thermal conductivity(W/mK)

Flashpoint (°C)

Firepoint (°C)

Surface tension(mN/m)

Viscosity at30°C (cP) Remarks

MQ-1 848 0.125 160 175 33.1 23.56Normal

mineral oilMQ-2 868 0.135 200 230 38.74 40.5

MQ-3 875 0.132 238 250 33.53 56.73

MQ-4 848 0.128 156 172 32.92 14.36Fast

mineral oilMQ-5 864 0.128 200 213 40.8 34.22

MQ-6 872 0.126 220 240 39.44 50.08

MQ-7 882 0.134 234 261 40.27 116.54 Hotmineral oilMQ-8 885 0.135 255 281 42.28 189.87

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its initial position as spreading proceeds. The movement ofadvancing front was fast in the initial stage and slows downin the later stage before attaining equilibrium. The relax-ation of droplet spreading was presented by using the timedependence of contact angle and spread area and is shownin Fig. 5. All quenchant droplets showed similar relaxationof contact angle and spreading behavior over the Inconelsubstrate. The contact angle decreases from the initial highvalue while spread area increases with time. The decreasein contact angle value and increase in spread area were rapidduring initial stage and become gradual as the systemapproached equilibrium. The equilibrium contact angle θe

(defined as the value of θ beyond which °/ms)for all quenchant spreading was determined from the plot ofcontact angle relaxation. The θe values of 13.79°, 18.05°,20.84°, 11.18°, 15.85°, 16.27°, 25.71° and 26.07° wereobtained for MQ-1, MQ-2, MQ-3, MQ-4, MQ-5, MQ-6,MQ-7 and MQ-8 respectively. Higher contact angle andlower spreading area were obtained for MQ-8 while lowercontact angle and higher spreading area were obtained forMQ-4. This is due to the difference in the physical proper-ties of oil. The high viscosity and surface tension of oiloffers higher resistance to flow resulted in high contactangle and low spreading area. On the other hand, the lowviscosity and surface tension of oil offers lower resistanceto flow resulted in low contact angle and high spreadingarea. The high equilibrium contact angle and low spreadarea of oil is indication of lower wettability and slowerspreading of oil on Inconel substrate. Further, oil havinghigh density showed higher transition contact angle (i.e.,change of contact angle from the rapid initial stage). Forexample, MQ-8 having high density showed transition con-tact angle of 41.04° while MQ-4 having low density showedtransition contact angle of 19.42°. Figure 6 shows the plotof natural logarithm of drop base diameter vs natural loga-rithm of relaxation time. It was observed that mineral oilshaving high viscosity and density showed all three regimesof spreading namely capillary, gravity and viscous regimes.On the other hand, spreading behavior of mineral oils hav-ing low viscosity and density on Inconel substrate consistedof capillary, gravity regimes and absence of viscous regime.The presence of viscous regime in spreading of MQ-3, MQ-7 and MQ-8 indicates that relaxation of contact angle wasalmost completed. The absence of viscous regime in spread-ing of MQ-1, MQ-2, MQ-4, MQ-5 and MQ-6 indicates thatspreading of oil was still active. The results clearly indicatethat improved wettability and fast spreading kinetics formineral oils of low viscosity and surface tension whilereduced wettability and slow spreading kinetics for mineraloils of high viscosity and surface tension.

3.2. Wetting BehaviorThe video images taken during the quenching of hot Inc-

onel probe in mineral oils are shown in Fig. 7. Due to the

Fig. 4. Images showing contact angle relaxation in (a) MQ-1 (b) MQ-2 (c) MQ-3 (d) MQ-4 (e) MQ-5 (f) MQ-6 (g) MQ-7and (h) MQ-8 spreading on Inconel 600 substrate.

d

dt

θ≤ 0 001.

Fig. 5. (a) Contact angle relaxation and (b) spread area of quen-chants droplet during spreading on Inconel 600 substrate.

Fig. 6. Plot of ln (droplet base radius) verses ln (time) duringspreading of oils.

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dark nature of color, video imaging of the mineral oils, MQ-2, MQ-5 and MQ-6 were not possible. The thermal historiesof Inconel probe measured at various axial and radial loca-tions during quenching in different mineral oils were usedto determine nature of wetting front, wetting front velocityand rewetting temperature for all quenchants. Figure 8shows typical time temperature data of Inconel probe mea-sured during quenching in mineral oil. All mineral oils showthe formation of stable vapour film around the quench probesurface. Rewetting of the fluid begins after a time period atthe bottom surface of quench probe and resulted in forma-tion of wetting front. The wetting front then ascends to top.The time taken to start the rewetting was about 6.0, 4.4, 3.8,2.7, 3.2, 3.8, 1.1, and 2.6 s for MQ-1, MQ-2, MQ-3, MQ-4,MQ-5, MQ-6, MQ-7 and MQ-8 respectively. Fast quench-ing oils (MQ-4, MQ-5 and MQ-6) show less time to start therewetting than conventional quenching oils. However, it wasobserved that hot oils show short time to start rewetting thanfast and conventional oils. The rewetting times at differentradial locations were determined to assess the nature of wet-ting front. Figure 9 shows the variation of rewetting time at30 mm in different radial locations of probe. It indicates thatnature of wetting front was not uniform over the probe sur-face. To assess the nature of the wetting front, a wettingfront uniformity parameter was defined as the differencebetween the maximum rewetting time and minimum rewet-ting time measured at 30 mm in different radial locations ofprobe. Wetting front uniformity parameters of about 2.6,1.2, 1.6, 1.6, 1.7, 0.6, 0.9 and 0.4 s were obtained for MQ-1, MQ-2, MQ-3, MQ-4, MQ-5, MQ-6, MQ-7 and MQ-8respectively. A lower value of the wetting front uniformityindicates more uniform of nature of the wetting front. Thenature of the wetting front was more uniform with mineraloils of high viscosity and surface tension while less uniformwith mineral oils of low viscosity and surface tension. Forexample, MQ-8 having higher viscosity (189.87 cP) andsurface tension (42.28 mN/m) showed wetting front unifor-mity value of of about 0.4 s while MQ-1 having lower vis-cosity (23.56 cP) and surface tension (33.1 mN/m) showedrewetting time variation of about 2.6 s. The wetting frontuniformity decreased monotonically with increase in the

surface tension and viscosity of oil. The low viscosity andsurface tension of mineral oils offer improved wettabilityand faster spreading which expected to collapse of vapourfilm easily resulted in less uniformity in nature of wetting

Fig. 7. Photographs of Inconel probe heated to 850°C quenched in different mineral oils.

Fig. 8. Typical thermal histories of quench probe measured at dif-ferent (a) axial locations and (b) radial locations duringquenching in MQ-1.

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front than higher viscosity and surface tension of mineraloils which offer greater resistance to flow and collapse ofvapour film. The video imaging of quenching processshowed formation of the additional wetting front at the topsurface of quench probe and started moving downwards forMQ-3, MQ-7 and MQ-8. On the other hand no additionalwetting front was observed for MQ-1, MQ-4. The rewettingtimes (transition from vapour to nucleate boiling stages) atdifferent axial locations and the corresponding temperatures(rewetting temperature) of the probe were measured. Thewetting front velocity was calculated by dividing the axiallocation by rewetting time at that location. Figure 10 showsvariations of wetting front velocity and rewetting tempera-ture on axial locations of quench probe for all mineral oils.The velocity of the wetting front increases with distancefrom the bottom surface of probe. In the case of MQ-3, MQ-6, MQ-7 and MQ-8, the maximum wetting front velocitywas obtained at intermediate locations of probe surface.This is due to the formation of additional wetting front at thetop of quench probe. The heat extraction at the metal/quen-chant interface is by thermosyphon effect in the case of lowviscosity quenching oil whereas by heating of thin layer ofoil at the part surface in the case of high viscosity of quenchoil.7) The mineral oils, MQ-1, MQ-2, MQ-4 and MQ-5 werelow viscosity quenching oils and flash/fire points of theseoils were also low. Heat extraction at the probe surface aftercollapse of vapour film in these mineral oils are expected bythermosyphon effect which resulted in strong convection atthe metal quench interface. This causes continuous move-ment of wetting front. This is also possible reason for lessuniformity of nature of wetting front in low viscosity oil.Further, these mineral oils showed improved wettability andfast spreading. Thus no additional wetting front formation inthese mineral oils. On the other hand, MQ-3, MQ-6, MQ-7and MQ-8 were high viscosity quenching oils and flash/firepoints of these oils were also high. Heat extraction at theprobe surface after collapse of vapour film in these mineraloils is expected by heating of thin layer of oil at the part sur-face. This causes slow movement of wetting front from thebottom of quench probe and more uniform nature of wettingfront. At that same time, the increase in quenchant temper-ature resulting in lower viscosity of oil. This causes the for-mation of additional wetting front at top of quench probewhich started moving downward. The schematic of the twowetting phenomena are shown in Fig. 11. The values of wet-ting front velocity and rewetting temperature measured atdifferent axial locations (Fig. 10) were used to calculate theaverage wetting front velocity and rewetting temperature.Averages wetting front velocities of 7.85, 2.34, 3.88, 9.98,15.68, 11.98, 9.61 and 6.97 mm/s were obtained for MQ-1,MQ-2, MQ-3, MQ-4, MQ-5, MQ-6, MQ-7 and MQ-8respectively. The corresponding average rewetting tempera-tures were found to be 543, 625, 611, 676, 695, 708, 726 and705°C respectively. Fast quenching oils showed higher wet-ting front velocities than convention and hot quenching oils.

This is due to the presence of accelerating additives (suchas calcium naphthenate, alkenyl succinimide and sodiumsulfonate etc.) in the fast mineral oils. However, among theconvectional/fast/hot mineral oils, higher wetting frontvelocity were obtained for low viscosity oil while lowerwetting front velocity was obtained for high viscosity oil.Further, the rewetting of the fluid occurs at higher temper-ature with increase in viscosity of the mineral oil.

3.3. Cooling BehaviorFigure 12 shows cooling curves obtained at geometric

centers during quenching of quench probe I and probe IIrespectively in the reference fluid before and after quench-ing experiments with mineral oils. Maximum cooling ratedifferences of 3 and 5°C/s were observed for probe-I andprobe-II respectively. The results confirm the repeatabilityof experiments with Inconel probe. In order to compare thecooling performance of mineral oils, the thermal historiesmeasured at geometric center of Inconel probe were plotted.Figure 13 shows cooling and cooling rate curves measuredat the geometric center of quench probe for all mineral oilsused in the present study. Cooling curves showed three stag-es of cooling namely vapour blanket, nucleate boiling andconvective cooling for all the quench media. Cooling ratewas significantly higher in nucleate boiling stage. The pres-ence of accelerating additives in the fast quenching oils

Fig. 9. Variation of rewetting time at different radial locations ofprobe during quenching in various mineral oils.

Fig. 10. Variation of (a) wetting front velocity and (b) rewettingtemperature on axial locations of quench probe surfaceduring quenching in various mineral oils.

Fig. 11. Schematic of rewetting phenomena in (a) low viscosityand (b) high viscosity mineral oils quenching.

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resulted in short duration of vapour film compared to nor-mal quenching oils. However, hot oils show even less dura-tion of vapour film than fast oils due to its higher flashpoints. On the other hand normal quenching oils show lon-ger vapour film stage due to its lower flash points. The dura-tion of vapour film was found to be 8.00, 8.40, 6.90, 5.80,5.20, 6.30, 2.70 and 4.80 s for MQ-1, MQ-2, MQ-3, MQ-4,MQ-5, MQ-6, MQ-7 and MQ-8 respectively. From theplots, the critical cooling curve parameters such as peakcooling rate (CRpeak), temperature of the peak cooling rate(TCRpeak), time to cool from 730 to 260°C (t730–260), coolingrates at 705°C (temperature at which austenite transforma-tion starts to occur for the most of the carbon steels), 550°C(temperature which is at or near the nose of TTT curves formany steels), 300°C and 200°C (temperatures which are inthe region of the martensitic transformation for many steels)(which were denoted as CR705, CR550,CR300 and CR200respectively) were determined and are presented in Table 3.Higher cooling rates at critical temperatures and lower t730–260 were obtained for fast quenching oils than hot oils and

conventional quenching oils which indicating the fast cool-ing performance. Hot quenching oils show higher coolingrates at peak cooling and 705°C than conventional quench-ing oils. The cooling rates at 550°C, 300°C and 200°C ofhot oils and conventional quenching oils were comparable.Further, it was observed that temperature at which peakcooling occurs and time to cool from 730 to 260°C of hotoils were higher than fast and conventional quenching oils.However, among the convectional/fast/hot mineral oils, oilshaving high viscosity showed lower cooling rates at criticaltemperatures and higher time to cool from 730 to 260°Cwhile higher cooling rates at critical temperatures and lowertime to cool from 730 to 260°C were observed for low vis-cosity oils. Hardening power (HP) of oils were determinedusing the following equations6)

........ (7)whereTVP – temperature of film boiling to nucleate boiling transi-tion (°C)CR – cooling rate over the temperature range of 600 to500°C (°C/s)TCP – temperature of nucleate boiling to convective cooling(°C)

The HP values of 522, 589, 345, 1 067, 800, 619, 156 and44 were obtained for MQ-1, MQ-2, MQ-3, MQ-4, MQ-5,MQ-6, MQ-7 and MQ-8 respectively. Even though hotquenching oils showed higher cooling rates at peak coolingand 705°C than conventional quenching oils, lower valuesof HP were obtained due to its high viscosity. Higher valueof HP was obtained for low viscosity oil while lower valueof HP was obtained for high viscosity oil.

3.4. Metal/quenchant Heat FluxThe thermal histories measured at various axial and radial

locations and thermo-physical properties of quench probewere input to the inverse heat conduction problem (IHCP)to estimate the spatial dependence of metal/quenchant heatflux transients for all mineral oils quenching. The time-temperature data measured at different axial locationsexcept at A8 (40 mm from the top surface) were input to theinverse program and the temperatures measured at A8 loca-tion was used to compare the estimated temperatures at thesame location. Figure 14 shows a good agreement betweenthe measured and estimated time temperature data at the A8location. In the case of radial locations, all thermal historiesmeasured were input to the inverse program. The overallerror in estimated temperatures for the whole domain wascalculated using the equation:13)

.......... (8)

Fig. 12. Comparison of cooling curves of (a) probe-I and (b)probe-II against reference oil before and after quenchingexperiments with mineral oils.

Fig. 13. Cooling curves (thin line) and cooling rate curves (thick)line of quench probe at the geometric centre duringquenching.

Table 3. Cooling curve parameters determined for various quen-chants.

Critical coolingparameters

Normal mineral oil Fast mineral oil Hot mineral oil

MQ-1 MQ-2 MQ-3 MQ-4 MQ-5 MQ-6 MQ-7 MQ-8

CRpeak (°C/s) 71 74 58 108 94 92 90 74

TCRpeak (°C) 526 552 532 577 623 634 680 614

CR705 (°C/s) 29 22 29 31 69 41 85 45

CR550 (°C/s) 61 74 57 104 81 76 60 52

CR300 (°C/s) 8 6 7 11 11 7 5 7

CR200 (°C/s) 4 3 4 4 4 4 4 3

t730–260 (s) 20.94 22.75 29.12 15.05 14.76 22.33 28.22 34.87

HP T CR TVP CP= + + −91 5 1 34 10 88 3 85. . . .

%Error in Estimated Temperatures

n

T T

Tmeasured estimated

measu

=

−1

rred ii

n

×⎡

⎣⎢

⎤

⎦⎥

=∑ 100

1

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1433 © 2014 ISIJ

where ‘n’ is the number of unknown heat fluxes assigned atthe quench probe surface. Figure 15 shows overall % errorin the estimated temperatures obtained from the solution ofIHCP. The maximum overall % errors in the estimated tem-peratures of axial and radial locations were found to be lessthan 5%.

Figure 16 shows typical spatially dependent metal/quen-chant interfacial heat flux transients estimated at axial andradial locations of the quench probe surface. Heat flux curveshowed initial peak followed by decrease in heat flux to avalue. This is due to initial wetting of liquid and subsequentvaporization resulted in formation of vapour around theprobe surface. Heat flux value then increased sharply tohigher value due to collapse of vapour film and formationof bubble boiling on the quench probe. Thereafter, it startsto decrease with surface temperature of the probe. Tables 4and 5 show the estimated peak heat fluxes during vapourand nucleate boiling stage respectively in the axial as wellas radial locations. The average peak heat flux values of

623, 482, 597, 614, 549, 519, 713 and 545 kW/m2 wereobtained for MQ-1, MQ-2, MQ-3, MQ-4, MQ-5, MQ-6,MQ-7 and MQ-8 respectively during film boiling. The cor-responding peak heat flux values during nucleate boilingwere found to be 1 292, 1 309, 1 233, 2 138, 2 031, 1 899,1 735 and 1 506 kW/m2 respectively. Higher values of peakheat flux were obtained for fast quenching oils and lowervalues of peak heat flux were obtained for conventionalquenching oil. Hot oils show intermediate peak heat flux

Fig. 14. Measured and estimated temperature profile of quenchprobe-I at A8 location.

Fig. 15. Overall % error in estimated temperatures of (a) probe-Iand (b) probe-II.

Fig. 16. Estimated metal/quenchant heat flux transients at (a) axialand (b) radial locations of probe surface during quenchingin MQ-5.

Table 4. Estimated multiple peak heat flux components duringvapour blanket stage.

Probe Quenchmedium

qvapour (kW/m2)Remarks

q1 q2 q3 q4 q5 q6 q7 q8

Probe-I(axial

variation)

MQ-1 628 615 650 619 602 625 762 607Normalmineral

oilMQ-2 404 511 502 575 416 410 498 485

MQ-3 559 537 540 560 585 476 574 498

MQ-4 562 563 577 579 549 539 543 518Fast

mineraloil

MQ-5 569 586 587 555 527 528 535 526

MQ-6 515 469 513 456 530 461 506 515

MQ-7 651 450 523 830 687 825 645 756 Hotmineral

oilMQ-8 501 492 488 517 498 540 424 379

Probe-II(radial

variation)

MQ-1 604 665 638 651 570 553 573 603Normalmineral

oilMQ-2 489 487 485 477 503 500 499 468

MQ-3 607 621 612 674 754 692 630 639

MQ-4 556 576 600 680 828 797 776 580Fast

mineraloil

MQ-5 524 584 529 486 549 578 606 520

MQ-6 557 537 552 555 541 530 520 544

MQ-7 726 680 870 837 810 733 683 698 Hotmineral

oilMQ-8 683 595 593 603 622 601 608 572

© 2014 ISIJ 1434

ISIJ International, Vol. 54 (2014), No. 6

values.The variations of heat flux values at critical temperatures

705, 550, 300 and 200°C at different locations of quenchprobe surfaces for all quenchants were determined. Figure17 show typical spatially dependent heat flux at critical tem-peratures in axial and radial locations on the probe surfaceduring quenching. The plots show heat transfer duringquenching was not uniform over the surface of probe. Forexample, standard deviation of heat flux values at 705°C inaxial locations of the probe were found to be 162, 138, 147,236, 274, 197, 93, 81 kW/m2 for MQ-1, MQ-2, MQ-3, MQ-4, MQ-5, MQ-6, MQ-7 and MQ-8 respectively. Heat fluxvariations at axial locations were found to be higher than atradial locations for all quenchants. The heat flux variationsat 705 and 550°C were found to be higher than at 300 and200°C. It indicates that heat transfers during liquid coolingstage was more uniform than film and nucleate boilingstage. Further, no definite relation between heat flux varia-tion and properties of oil was found. However, lower stan-dard deviations of heat flux values were observed for hotoils having high viscosity indicating more uniform heattransfer than conventional and fast mineral oils. The amountof heat removed during quenching was determined by plot-ting the integral heat flux curve for all quench media. Theamounts of heat extracted by the quenchants to cool theprobe from 850 to 200°C were found to be 9.45, 9.49, 9.75,9.43, 9.41, 9.46, 9.44 and 9.57 MJ/m2 for MQ-1, MQ-2,MQ-3, MQ-4, MQ-5, MQ-6, MQ-7 and MQ-8 respectively.Figure 18 shows the average time for the known fraction ofheat removal during quenching in mineral oils. The time forknown fraction of heat removal was lower for fast quench-ing mineral oils indicating superior heat extracting capabil-ity compared to conventional and hot quenching oils. Thehot oils quenching showed lower time up to about 65% heatremoval and higher time for remaining percent heat removalcompared to conventional oil. Cooling curve analysesparameters also showed higher cooling rates at peak coolingand 705°C and comparable cooling rates at 550°C, 300°Cand 200°C for hot oils compared to conventional quenching

oils. Further, the times to cool from 730 to 260°C were high-er for hot oils. This is the reason for low HP values of hotoils quenching though it showed higher peak heat flux val-ues and peak cooling rates than conventional oil. It indicatesthat heat extracting capability of the hot oil was faster dur-ing vapour and nucleate boiling and slower during liquid

Fig. 17. Typical spatially dependent heat flux at critical tempera-tures in (a) axial and (b) radial locations on the probe sur-face during quenching in MQ-5.

Fig. 18. Time for specified heat removal from the quench probe.

Fig. 19. Experimentally measured cooling curves of mineral oilssuperimposed on CCT curve of AISI 1040 steel.

Table 5. Estimated multiple peak heat flux components duringnucleate boiling stage.

Probe Quenchmedium

qnucleate (kW/m2)Remarks

q1 q2 q3 q4 q5 q6 q7 q8

Probe-I(axial

variation)

MQ-1 1 423 1 548 1 172 1 313 976 1 452 1 319 1 174Normalmineral

oilMQ-2 1 181 1 264 1 271 1 383 1 173 995 1 290 1 494

MQ-3 1 005 1 509 1 336 1 195 1 454 1 290 1 183 1 115

MQ-4 2 107 2 090 1 960 2 416 1 729 2 593 2 320 2 237Fast

mineraloil

MQ-5 2 122 2 410 1 772 2 408 1 608 2 343 1 713 2 073

MQ-6 2 118 1 960 1 647 1 455 2 078 1 427 2 275 1 900

MQ-7 1 874 1 915 2 127 1 763 1 754 1 869 1 927 1 379 Hotmineral

oilMQ-8 1 972 1 360 1 735 1 623 1 574 1 414 1 734 1 667

Probe-II(radial

variation)

MQ-1 1 266 1 236 1 243 1 045 1 285 1 378 1 346 1 493Normalmineral

oilMQ-2 1 349 1 349 1 353 1 347 1 485 1 315 1 402 1 290

MQ-3 1 067 1 498 1 142 1 196 1 004 999 1 341 1 385

MQ-4 2 501 2 237 2 196 2 030 1 993 1 942 1 886 1 978Fast

mineraloil

MQ-5 1 714 2 361 2 211 2 062 1 913 1 826 2 000 1 959

MQ-6 2092 1954 2103 1993 1819 1 792 1 898 1 870

MQ-7 1 926 1 828 1 521 1 681 1 476 1 542 1633 1553 Hotmineral

oilMQ-8 1 192 1 442 1 353 1 420 1 466 1 429 1 449 1 266

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cooling stage.To quantify the effect of cooling performance of quench

media on the formation of micro-constituents and hardnessin steel, cooling curves measured at geometric centre of theprobe were superimposed on the CCT diagram. It should benoted that cooling curve analysis was carried out using Inc-onel 600 probe. However, superimposition of cooling curveon the steel CCT diagram can be used to quantify the effectof cooling performance of quench media. Figure 19 showsthe CCT diagram of AISI 1040 obtained using JMatPro soft-ware (Sente Software Ltd., UK). The resultant micro-constituents of steel are given in Table 6. Fast mineral oilquenching resulted in higher amount of transformed prod-ucts (bainite and martensite) than normal and hot oils. Fur-ther, normal oil quenching resulted in higher amount oftransformed product than hot oils. It indicates that higherquenching severities for fast mineral oils, lower quenchseverities for hot oils and intermediate quench severities fornormal oils. With varying cooling rates obtained with min-eral oil quenchants used in the present work, the resultanthardness of the AISI 1040 steel estimated from the CCT dia-gram was found to be 277, 271, 266, 288, 289, 273, 268 and261 HV for MQ-1, MQ-2, MQ-3, MQ-4, MQ-5, MQ-6,MQ-7 and MQ-8 respectively.

4. ConclusionsThe following conclusions were drawn based on the

results and discussion.(1) Higher contact angles on Inconel substrate were

obtained for mineral oils of high viscosity and surface ten-sion whereas mineral oils of low viscosity and surface ten-sion showed lower contact angle. It indicates that wettabilityof mineral oil improved with a decrease in viscosity and sur-face tension of mineral oil. Similarly, relaxation of contactangle and spreading area of droplet on Inconel substrateindicated faster spreading kinetics for mineral oils of lowviscosity and surface tension and slower spreading kineticsfor mineral oils of high viscosity and surface tension.

(2) Spreading behavior of mineral oils having high vis-cosity and density showed all three regimes of spreadingnamely capillary, gravity and viscous regimes. On the otherhand, spreading behavior of mineral oils having low viscos-ity and density on Inconel substrate consisted of capillary,gravity regimes and absence of viscous regime.

(3) Hot oil quenching showed less time delay to startrewetting phenomenon while longer time was taken to startrewetting with conventional oil quenching. Fast oils quench-ing showed intermediate time to start rewetting of the fluid.

(4) The nature of the wetting front was more uniformwith mineral oils of high viscosity and surface tension whileless uniform with mineral oils of low viscosity and surfacetension.

(5) Mineral oils of high viscosity and flash pointshowed the formation of additional wetting front at the top

of quench probe which started moving downward duringrewetting of fluid. On the other hand, no additional wettingfront was observed for mineral oils of low viscosity andflash point

(6) Among the convectional/fast/hot mineral oil, higherwetting front velocity was obtained for low viscosity oilwhile lower wetting front velocity was obtained for highviscosity oil. Further, the rewetting of the fluid occurs athigher temperature with an increase in viscosity of the min-eral oil.

(7) Higher cooling rates at critical temperatures andlower t730–260 were obtained for fast quenching oils than hotoils and conventional quenching oils. Hot quenching oilsshowed higher cooling rates at peak cooling and 705°C thanconventional quenching oils. The cooling rates at 550°C,300°C and 200°C of hot oils and conventional quenchingoils were comparable.

(8) Higher values of peak heat flux were obtained forfast quenching oils and lower values of peak heat flux wereobtained for conventional quenching oil. Hot oils showintermediate peak heat flux values.

(9) Among the conventional/fast/hot oils, oil havinghigher viscosity showed more uniform heat transfer thanlow viscosity oil.

(10) Fast quenching mineral oils showed faster heatextracting capability compared to conventional and hotquenching oils. The cooling performance of the hot oil wasbetter during vapour and nucleate boiling stages and slowerduring liquid cooling stage compared to conventionalquenching oils.

AcknowledgementOne of the authors (KNP) gratefully acknowledges the

financial support provided by the Science and EngineeringResearch Board (SERB), Department of Science and Tech-nology (DST), Government of India, New Delhi, Indiaunder a R&D project.

REFERENCES

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Table 6. Resultant micro constituents (in volume%) of AISI 1040steel for mineral oils quenching.

Microconstituent

Normal mineral oil Fast mineral oil Hot mineral oil

MQ-1 MQ-2 MQ-3 MQ-4 MQ-5 MQ-6 MQ-7 MQ-8

Ferrite 16 17 18 13 13 16 17 19

Pearlite 01 03 05 01 01 02 04 07

Bainite 83 80 77 85 85 82 79 74

Martensite 00 00 00 01 01 00 00 00