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

Abstract

In the present work, wetting kinetics, kinematics and heat transfer characteristics of mineral oils having varying 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 low viscosity and surface tension on Inconel substrate indicated improved wettability and fast spreading kinetics while mineral oils of high viscosity and surface tension showed reduced wettability and slower spreading kinetics. Further, the spreading behavior of mineral oils of lower viscosity and density showed the absence of viscous regime. During rewetting, formation of double wetting fronts and more uniform nature of wetting front were observed with mineral oils of high viscosity and flash point whereas no additional wetting 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 boiling and lower during liquid cooling stage. Further, highly viscous mineral oils showed uniform heat transfer compared to mineral oils having low viscosity.

1. Introduction

Quenching is a critical step of heat treating process which defined as rapid cooling of components in-order to form martensitic/bainitic microstructure and to avoid the transformation of pearlite/ferrite in the case of steel. The cooling conditions during quenching play an important role on phase transformation and development of mechanical properties of the components.¹⁾ Factors that influence the cooling conditions of component during quenching are categorized into three groups (i) workpiece characteristics (composition, mass, geometry, surface roughness and condition) (ii) quenchant characteristics (density, viscosity, specific heat, thermal conductivity, boiling temperature) and (iii) quenching facility (bath temperature, agitation rate, flow direction). Of these, the quench medium used to extract the heat from the hot component at particular rate is a significant factor for heat treating engineer to alter/achieve the desired cooling condition of components in both technical and economical consideration.²⁾

Different liquid quench media used in heat treating industries are water, brine, aqueous polymer and mineral oil quenchants. Even though mineral oil possesses several environmental problems and fire risks, still it is most widely (almost 85%) used quench medium in heat treating industries. This is due to its better aging stability, thermal stability and oxidation resistance. Further, mineral oil quenching resulted in more uniform cooling, reduced distortion and cracking of steel components.¹⁾ The cooling behavior of mineral oil during quenching is same like water which involves three stages of cooling namely, vapour blanket, nucleate boiling and convective cooling stages. However, the cooling performance of mineral oil is much lower than water. Further, formation of double wetting fronts on quench probe was reported for mineral oil quenching.^3,4)

Mineral oils are petroleum byproducts, generally mixtures of chemical structures with a range of molecular weights and do not contain any fatty components. Generally the mineral oils are distilled from the C26 to C38 fraction of petroleum and composed of branched paraffins (C_nH_2n+2) and cycloparaffins (C_nH_2n) together with a small amount of aromatics (benzene ring and its derivatives). Within an individual molecule, there are some cycloparaffin rings, aromatic rings and the necessary paraffin and olefin side or connecting groups.⁵⁾ The wetting agents, accelerators and anti-oxidant may be added to achieve specific quenching characteristics of mineral oils. Mineral oils can be grouped into distinctive groups based on the composition, the presence of additives and application temperature. They are classified as conventional oils, fast or accelerated oils, martempering or hot quenching oils. Conventional quenching oils are usually composed of paraffinic and naphthenic fraction with a viscosity ranging from 100 to 110 SUS (Saybolt Universal Seconds) at 40°C (some oils may have viscosities of upto 200 SUS at 40°C). These oils may contain antioxidants to reduce the rates of oxidative and thermal degradation but do not contain additives to increase the cooling rate. The conventional quenching oil tends to show a prolonged vapor blanket stage, a short nucleate boiling stage and finally a very slow cooling of convective stage. Fast quenching oils have low viscosities in the range of 50 and 110 SUS at 40°C. These oils contain one or more additives to enhance the wetting and quenching speed and often contain antioxidants. The fast quenching oil shows a high quenching speed during the vapor blanket stage and in some situations approaching the initial speed of water followed by a moderately fast cooling rate in the nucleate boiling range. The cooling rate in convection stage is usually about the same as provided by conventional quenching oils. However, some fast quenching oils containing special additives provide faster cooling rates in convection stage. Martempering or hot quenching oils are solvent-refined paraffin-base mineral oils with good thermal stability and oxidation resistance. They are used at temperatures between about 95°C and 230°C. They may also contain antioxidants to improve their aging stability.^4,6,7)

Totten et al.⁸⁾ discussed the importance of chemistry of quench oil and its significance on heat transfer performance during quenching. Ma et al.⁹⁾ investigated the performance of a series of mineral oil based quenchants having viscosities ranging from 10.4 to 120 mm²/s using medium carbon alloy steel (AISI 4140) probe of 9.5 mm diameter by 38.1 mm length. They observed that peak cooling rate and heat transfer coefficient obtained for the probe were increased with decrease in viscosity of quenchant. Similarly, the hardening power of the quenchant was increased with a decrease in viscosity of quenchant. Asada and Fukuhara¹⁰⁾ observed that the length of vapour stage during quenching was influenced by the viscosity and molecular weight of the mineral oil. The higher viscosity and molecular weight of the mineral oil resulted in early collapse of vapour film at higher temperature. Yokota et al.¹¹⁾ investigated mineral oil based quenchants having identical viscosities and additives formulation but different types of mineral base stocks. They observed that cooling of 0.45% C steel (especially in the temperature range 350–300°C) was significantly influenced by difference in the base stocks of quenchants. Fernandes and Prabhu¹²⁾ showed that the blending of palm oil with mineral oil increased the spreading rate as well as the quench severity. The literature showed wide range of quenching performance of mineral oil can be obtained through careful formulation and blending.

The mineral oils from the different producers are formulated to obtain different chemical structures with different additives. The varying chemical compositions of mineral oil quenchants have significant influence on its cooling performance and wetting behavior during quenching. A detailed understanding of wetting and cooling behavior of mineral base oil quenchants is therefore necessary for judicious selection of quenchant to obtain superior properties of components with reduced distortion and cracking. The present work is aimed at the study of wetting kinetics, kinematics and cooling performance of mineral oils sourced from different suppliers and assessment of the suitability of these oils for industrial heat treatment.

2. Experimental

In 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 oil quenchants were obtained from four different suppliers. The mineral oils, MQ-1, MQ-5 and MQ-7 were procured from one supplier. Similarly, MQ-3, MQ-4 and MQ-8 were procured from one supplier. The remaining mineral oils, MQ-2 and MQ-6 were procured from different sources. Among the mineral oils, MQ-7 and MQ-8 are classified as hot oils whereas MQ-4, MQ-5 and MQ-6 are classified as accelerated/fast oils. Remaining mineral oils, MQ-1, MQ-2 and MQ-3 are classified as normal/bright oils.

The viscosity and thermal conductivity of quenchants were measured using Brookfield LVDV-IIIU Rheometer and KD2 Pro thermal property analyser respectively. Weight displacement method was used determine the density of the fluids. The fire and flash points of quenchants were determined using Cleveland open cup apparatus. Pendant drop method was to determine the surface tension of quenchants. A 2.5 ml syringe with 0.9 mm diameter needle having a precision flow control valve was used for this purpose. For spreading studies, a droplet of quenchant was dispensed on to the Inconel 600 substrate. The spreading phenomenon was recorded using dynamic contact angle analyzer FTA 200 (First Ten Angstroms, USA) equipment. Captured images were analyzed using the FTA image analysis software to determine the interfacial tension, contact angle, droplet base diameter and spread area. The surface texture of the Inconel 600 substrate was similar to the Inconel 600 probe used for quenching experiments. The experiments were carried out at an ambient temperature of 30°C.

For cooling curve analysis, two quench probes of 12.5 mm diameter and 60 mm length were prepared from Inconel 600 material. To assess the axial variations in heat flux transients, holes of 1 mm diameter were drilled at different heights located at 2 mm from the surface (probe I) as shown in 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, 52.5 and 40 mm ± 1 mm from the top surface of the quench probe respectively. For determination of heat flux variations in the radial direction, holes of 1 mm were drilled at different azimuth angles to a depth of 30 mm ± 1 mm and were located at 2 mm from the surface (probe II) as shown in Fig. 1(b). These holes, designated as R1, R2, R3, R4, R5, R6, R7 and R8, were located at angles of 0°, 45°, 90°, 135°, 180°, 225°, 270° and 315° respectively. Holes of diameter 1 mm (A9 for probe I and R9 for probe II) were drilled at geometric centers of both probes. Quench probes were conditioned by heating and quenching in quench oil for several times in-order to obtain reproducible results. Calibrated K-type Inconel thermocouples were inserted into the quench probe. The other ends of thermocouples were connected to a PC based temperature data acquisition system (NI 9213). Vertical tubular electric resistance furnace open at both ends was used to preheat the quench probe to 850°C. The heating zone of the furnace was 80 mm in diameter and having a length of 190 mm. During heating, the top and bottom parts of the furnace were covered with insulating blanket. A quench tank of internal diameter of 115 mm and length of 210 mm with 2000 ml of quenchant was kept below the furnace during quenching. The quench probe support operated through guide pins was designed such a way that, the probe was positioned at centre of heating zone during heating and at 50 mm from the bottom surface of quench tank during quenching. Once the probe attained the preheating temperature, it was directly quenched into fluid without any significant time delay (<0.35 s). The probe temperatures were recorded at a time interval of 0.1 s during quenching. The schematic of the experimental setup is shown in Fig. 2. A high performance smart camera (NI 1774C) was used for online video monitoring of the quenching process. The scanning rate was 3 images per second.

Fig. 1.

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

Fig. 2.

Schematic of experimental setup.

The metal/quenchant interfacial heat flux transients were estimated from the measured temperature histories and thermo-physical properties of probe material by solving the inverse heat conduction problem (IHCP). The equation that governs the two-dimensional transient heat conduction is given below.

For axial location:   

$$\frac{1}{r}\frac{\partial }{\partial r}\left(\lambda r\frac{\partial T}{\partial r}\right)+\frac{\partial }{\partial z}\left(\lambda \frac{\partial T}{\partial z}\right)=\rho \hspace{0.17em}{C}_p\frac{\partial T}{\partial t}$$(1)

For radial location:   

$$\frac{1}{r}\frac{\partial }{\partial r}\left(\lambda r\frac{\partial T}{\partial r}\right)+\frac{1}{r^2}\frac{\partial }{\partial \phi }\left(\lambda \frac{\partial T}{\partial \phi }\right)=\rho \mathrm{\hspace{0.17em} }{C}_p\frac{\partial T}{\partial t}$$(2)

The above equations were solved inversely with the following initial and boundary conditions using the finite element based TmmFE inverse solver software (TherMet Solutions Pvt. Ltd., Bangalore, India), for estimating metal/quenchant heat flux transients.

Initial condition   

$$T(r,z)=T_i\hspace{1em}\text{at t}=0$$(3)

and boundary conditions   

$$-\lambda \frac{\partial T}{\partial r}{n}_r-\lambda \frac{\partial T}{\partial z}{n}_z=q_k(r,z,t)\mathrm{\hspace{0.17em} } \text{on}\mathrm{\hspace{0.17em} }{\mathrm{\Gamma }}_K;k=1,2,\mathrm{....},p\mathrm{....},l$$(4)

  

$$-\lambda \frac{\partial T}{\partial r}{n}_r-\lambda \frac{\partial T}{\partial z}{n}_z=0\mathrm{\hspace{0.17em} }\text{on}\mathrm{\hspace{0.17em} }{\mathrm{\Gamma }}^{\iota }$$(5)

  

$$-\lambda \frac{\partial T}{\partial r}{n}_r-\lambda \frac{\partial T}{\partial z}{n}_z=h(T-T_{\infty })\mathrm{\hspace{0.17em} }\text{on}\mathrm{\hspace{0.17em} }{\mathrm{\Gamma }}^{\iota \iota }$$(6)

The mathematical description of the serial solution to IHCP is given in Ref. 13).¹³⁾ Figure 3(a) shows the solution domain of half symmetrical shape of the quench probe I used for estimation of heat flux components in the axial direction. The geometry was discretized using four node quadrilateral and four side linear, uniform mesh. The total number of elements used was 3000 (25 × 120). Figure 3(b) shows the solution domain of the quench probe II used for estimation of heat flux components in the radial direction. The geometry was discretized using three node triangle and three side curved, uniform mesh. The total number of elements in this case was 5000. The thermo-physical properties of the probe material used in the inverse model are given in Table 1.¹⁴⁾ For both probes the surface in contact with the liquid was divided into eight segments which were assigned an unknown heat flux boundary. The convergence limit for Gauss-Siedel iterations was set at 10^–6.

Fig. 3.

Solution domain of (a) quench probe I and (b) quench probe II used in IHCP.

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)	8400	8370	–	8340	––	8300	–	8270	–	8230	8190	8150	8100	8060

3. Results and Discussion

The measured thermo-physical properties of quenchants are presented in Table 2. The density, thermal conductivity, surface tension, viscosity, flash point and fire point of the mineral oil quenchants obtained from the different sources were 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 mineral oils, MQ-8 showed higher values of thermal conductivity, surface tension, viscosity, density, fire point and flash point while lower values were obtained with MQ-4.

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

Quenchant	Density (kg/m3)	Thermal conductivity (W/mK)	Flash point (°C)	Fire point (°C)	Surface tension (mN/m)	Viscosity at 30°C (cP)	Remarks
MQ-1	848	0.125	160	175	33.1	23.56	Normal mineral oil
MQ-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	14.36	Fast mineral oil
MQ-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	Hot mineral oil
MQ-8	885	0.135	255	281	42.28	189.87

3.1. Contact Angle and Spreading Behavior

Figure 4 shows images of mineral oils droplet on an Inconel 600 substrate during spreading. A droplet form a triple phase contact point, also known as the contact line front/advancing front, upon dispensing which starts to move from its initial position as spreading proceeds. The movement of advancing front was fast in the initial stage and slows down in the later stage before attaining equilibrium. The relaxation of droplet spreading was presented by using the time dependence of contact angle and spread area and is shown in Fig. 5. All quenchant droplets showed similar relaxation of contact angle and spreading behavior over the Inconel substrate. The contact angle decreases from the initial high value while spread area increases with time. The decrease in contact angle value and increase in spread area were rapid during initial stage and become gradual as the system approached equilibrium. The equilibrium contact angle θ_e (defined as the value of θ beyond which $\frac{d\theta }{dt}\le 0.001$ °/ms) for all quenchant spreading was determined from the plot of contact angle relaxation. The θ_e values of 13.79°, 18.05°, 20.84°, 11.18°, 15.85°, 16.27°, 25.71° and 26.07° were obtained for MQ-1, MQ-2, MQ-3, MQ-4, MQ-5, MQ-6, MQ-7 and MQ-8 respectively. Higher contact angle and lower spreading area were obtained for MQ-8 while lower contact angle and higher spreading area were obtained for MQ-4. This is due to the difference in the physical properties of oil. The high viscosity and surface tension of oil offers higher resistance to flow resulted in high contact angle and low spreading area. On the other hand, the low viscosity and surface tension of oil offers lower resistance to flow resulted in low contact angle and high spreading area. The high equilibrium contact angle and low spread area of oil is indication of lower wettability and slower spreading of oil on Inconel substrate. Further, oil having high density showed higher transition contact angle (i.e., change of contact angle from the rapid initial stage). For example, MQ-8 having high density showed transition contact angle of 41.04° while MQ-4 having low density showed transition contact angle of 19.42°. Figure 6 shows the plot of natural logarithm of drop base diameter vs natural logarithm of relaxation time. It was observed that mineral oils having high viscosity and density showed all three regimes of spreading namely capillary, gravity and viscous regimes. On the other hand, spreading behavior of mineral oils having low viscosity and density on Inconel substrate consisted of 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 was almost completed. The absence of viscous regime in spreading of MQ-1, MQ-2, MQ-4, MQ-5 and MQ-6 indicates that spreading of oil was still active. The results clearly indicate that improved wettability and fast spreading kinetics for mineral oils of low viscosity and surface tension while reduced wettability and slow spreading kinetics for mineral oils of high viscosity and surface tension.

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-7 and (h) MQ-8 spreading on Inconel 600 substrate.

Fig. 5.

(a) Contact angle relaxation and (b) spread area of quenchants droplet during spreading on Inconel 600 substrate.

Fig. 6.

Plot of ln (droplet base radius) verses ln (time) during spreading of oils.

3.2. Wetting Behavior

The video images taken during the quenching of hot Inconel probe in mineral oils are shown in Fig. 7. Due to the dark nature of color, video imaging of the mineral oils, MQ-2, MQ-5 and MQ-6 were not possible. The thermal histories of Inconel probe measured at various axial and radial locations during quenching in different mineral oils were used to determine nature of wetting front, wetting front velocity and rewetting temperature for all quenchants. Figure 8 shows typical time temperature data of Inconel probe measured during quenching in mineral oil. All mineral oils show the formation of stable vapour film around the quench probe surface. Rewetting of the fluid begins after a time period at the bottom surface of quench probe and resulted in formation 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 quenching oils (MQ-4, MQ-5 and MQ-6) show less time to start the rewetting than conventional quenching oils. However, it was observed that hot oils show short time to start rewetting than fast and conventional oils. The rewetting times at different radial locations were determined to assess the nature of wetting front. Figure 9 shows the variation of rewetting time at 30 mm in different radial locations of probe. It indicates that nature of wetting front was not uniform over the probe surface. To assess the nature of the wetting front, a wetting front uniformity parameter was defined as the difference between the maximum rewetting time and minimum rewetting time measured at 30 mm in different radial locations of probe. 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-8 respectively. A lower value of the wetting front uniformity indicates more uniform of nature of the wetting front. The nature of the wetting front was more uniform with mineral oils of high viscosity and surface tension while less uniform with mineral oils of low viscosity and surface tension. For example, MQ-8 having higher viscosity (189.87 cP) and surface tension (42.28 mN/m) showed wetting front uniformity value of of about 0.4 s while MQ-1 having lower viscosity (23.56 cP) and surface tension (33.1 mN/m) showed rewetting time variation of about 2.6 s. The wetting front uniformity decreased monotonically with increase in the surface tension and viscosity of oil. The low viscosity and surface tension of mineral oils offer improved wettability and faster spreading which expected to collapse of vapour film easily resulted in less uniformity in nature of wetting front than higher viscosity and surface tension of mineral oils which offer greater resistance to flow and collapse of vapour film. The video imaging of quenching process showed formation of the additional wetting front at the top surface of quench probe and started moving downwards for MQ-3, MQ-7 and MQ-8. On the other hand no additional wetting front was observed for MQ-1, MQ-4. The rewetting times (transition from vapour to nucleate boiling stages) at different axial locations and the corresponding temperatures (rewetting temperature) of the probe were measured. The wetting front velocity was calculated by dividing the axial location by rewetting time at that location. Figure 10 shows variations of wetting front velocity and rewetting temperature on axial locations of quench probe for all mineral oils. The velocity of the wetting front increases with distance from the bottom surface of probe. In the case of MQ-3, MQ-6, MQ-7 and MQ-8, the maximum wetting front velocity was obtained at intermediate locations of probe surface. This is due to the formation of additional wetting front at the top of quench probe. The heat extraction at the metal/quenchant interface is by thermosyphon effect in the case of low viscosity quenching oil whereas by heating of thin layer of oil at the part surface in the case of high viscosity of quench oil.⁷⁾ The mineral oils, MQ-1, MQ-2, MQ-4 and MQ-5 were low viscosity quenching oils and flash/fire points of these oils were also low. Heat extraction at the probe surface after collapse of vapour film in these mineral oils are expected by thermosyphon effect which resulted in strong convection at the metal quench interface. This causes continuous movement of wetting front. This is also possible reason for less uniformity of nature of wetting front in low viscosity oil. Further, these mineral oils showed improved wettability and fast spreading. Thus no additional wetting front formation in these mineral oils. On the other hand, MQ-3, MQ-6, MQ-7 and MQ-8 were high viscosity quenching oils and flash/fire points of these oils were also high. Heat extraction at the probe surface after collapse of vapour film in these mineral oils is expected by heating of thin layer of oil at the part surface. This causes slow movement of wetting front from the bottom of quench probe and more uniform nature of wetting front. At that same time, the increase in quenchant temperature resulting in lower viscosity of oil. This causes the formation of additional wetting front at top of quench probe which started moving downward. The schematic of the two wetting phenomena are shown in Fig. 11. The values of wetting front velocity and rewetting temperature measured at different axial locations (Fig. 10) were used to calculate the average 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-8 respectively. The corresponding average rewetting temperatures were found to be 543, 625, 611, 676, 695, 708, 726 and 705°C respectively. Fast quenching oils showed higher wetting front velocities than convention and hot quenching oils. This is due to the presence of accelerating additives (such as calcium naphthenate, alkenyl succinimide and sodium sulfonate etc.) in the fast mineral oils. However, among the convectional/fast/hot mineral oils, higher wetting front velocity were obtained for low viscosity oil while lower wetting front velocity was obtained for high viscosity oil. Further, the rewetting of the fluid occurs at higher temperature with increase in viscosity of the mineral oil.

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 different (a) axial locations and (b) radial locations during quenching in MQ-1.

Fig. 9.

Variation of rewetting time at different radial locations of probe during quenching in various mineral oils.

Fig. 10.

Variation of (a) wetting front velocity and (b) rewetting temperature on axial locations of quench probe surface during quenching in various mineral oils.

Fig. 11.

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

3.3. Cooling Behavior

Figure 12 shows cooling curves obtained at geometric centers during quenching of quench probe I and probe II respectively in the reference fluid before and after quenching experiments with mineral oils. Maximum cooling rate differences of 3 and 5°C/s were observed for probe-I and probe-II respectively. The results confirm the repeatability of experiments with Inconel probe. In order to compare the cooling performance of mineral oils, the thermal histories measured at geometric center of Inconel probe were plotted. Figure 13 shows cooling and cooling rate curves measured at the geometric center of quench probe for all mineral oils used in the present study. Cooling curves showed three stages of cooling namely vapour blanket, nucleate boiling and convective cooling for all the quench media. Cooling rate was significantly higher in nucleate boiling stage. The presence of accelerating additives in the fast quenching oils resulted in short duration of vapour film compared to normal quenching oils. However, hot oils show even less duration of vapour film than fast oils due to its higher flash points. On the other hand normal quenching oils show longer vapour film stage due to its lower flash points. The duration 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 the plots, the critical cooling curve parameters such as peak cooling rate (CR_peak), temperature of the peak cooling rate (T_CRpeak), time to cool from 730 to 260°C (t_730–260), cooling rates at 705°C (temperature at which austenite transformation starts to occur for the most of the carbon steels), 550°C (temperature which is at or near the nose of TTT curves for many steels), 300°C and 200°C (temperatures which are in the region of the martensitic transformation for many steels) (which were denoted as CR₇₀₅, CR₅₅₀,CR₃₀₀ and CR₂₀₀ respectively) were determined and are presented in Table 3. Higher cooling rates at critical temperatures and lower t_730–260 were obtained for fast quenching oils than hot oils and conventional quenching oils which indicating the fast cooling performance. Hot quenching oils show higher cooling rates at peak cooling and 705°C than conventional quenching oils. The cooling rates at 550°C, 300°C and 200°C of hot oils and conventional quenching oils were comparable. Further, it was observed that temperature at which peak cooling occurs and time to cool from 730 to 260°C of hot oils were higher than fast and conventional quenching oils. However, among the convectional/fast/hot mineral oils, oils having high viscosity showed lower cooling rates at critical temperatures and higher time to cool from 730 to 260°C while higher cooling rates at critical temperatures and lower time to cool from 730 to 260°C were observed for low viscosity oils. Hardening power (HP) of oils were determined using the following equations⁶⁾   

$$HP=91.5+1.34T_{VP}+10.88CR-3.85T_{CP}$$(7)

where

T_VP – temperature of film boiling to nucleate boiling transition (°C)

CR – cooling rate over the temperature range of 600 to 500°C (°C/s)

T_CP – temperature of nucleate boiling to convective cooling (°C)

Fig. 12.

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

Fig. 13.

Cooling curves (thin line) and cooling rate curves (thick) line of quench probe at the geometric centre during quenching.

Table 3. Cooling curve parameters determined for various quenchants.

Critical cooling parameters	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

The HP values of 522, 589, 345, 1067, 800, 619, 156 and 44 were obtained for MQ-1, MQ-2, MQ-3, MQ-4, MQ-5, MQ-6, MQ-7 and MQ-8 respectively. Even though hot quenching oils showed higher cooling rates at peak cooling and 705°C than conventional quenching oils, lower values of HP were obtained due to its high viscosity. Higher value of HP was obtained for low viscosity oil while lower value of HP was obtained for high viscosity oil.

3.4. Metal/quenchant Heat Flux

The thermal histories measured at various axial and radial locations and thermo-physical properties of quench probe were input to the inverse heat conduction problem (IHCP) to estimate the spatial dependence of metal/quenchant heat flux transients for all mineral oils quenching. The time temperature data measured at different axial locations except at A8 (40 mm from the top surface) were input to the inverse program and the temperatures measured at A8 location was used to compare the estimated temperatures at the same location. Figure 14 shows a good agreement between the measured and estimated time temperature data at the A8 location. In the case of radial locations, all thermal histories measured were input to the inverse program. The overall error in estimated temperatures for the whole domain was calculated using the equation:¹³⁾   

$$\begin{array}{l}\%Error in Estimated Temperatures=\\ \left|\frac{1}{n}\sum _{i=1}^n{\left[\frac{T_{measured}-T_{estimated}}{T_{measured}}\times 100\right]}_i\right|\end{array}$$(8)

where ‘n’ is the number of unknown heat fluxes assigned at the quench probe surface. Figure 15 shows overall % error in the estimated temperatures obtained from the solution of IHCP. The maximum overall % errors in the estimated temperatures of axial and radial locations were found to be less than 5%.

Fig. 14.

Measured and estimated temperature profile of quench probe-I at A8 location.

Fig. 15.

Overall % error in estimated temperatures of (a) probe-I and (b) probe-II.

Figure 16 shows typical spatially dependent metal/quenchant interfacial heat flux transients estimated at axial and radial locations of the quench probe surface. Heat flux curve showed initial peak followed by decrease in heat flux to a value. This is due to initial wetting of liquid and subsequent vaporization resulted in formation of vapour around the probe surface. Heat flux value then increased sharply to higher value due to collapse of vapour film and formation of bubble boiling on the quench probe. Thereafter, it starts to decrease with surface temperature of the probe. Tables 4 and 5 show the estimated peak heat fluxes during vapour and nucleate boiling stage respectively in the axial as well as radial locations. The average peak heat flux values of 623, 482, 597, 614, 549, 519, 713 and 545 kW/m² were obtained for MQ-1, MQ-2, MQ-3, MQ-4, MQ-5, MQ-6, MQ-7 and MQ-8 respectively during film boiling. The corresponding peak heat flux values during nucleate boiling were found to be 1292, 1309, 1233, 2138, 2031, 1899, 1735 and 1506 kW/m² respectively. Higher values of peak heat flux were obtained for fast quenching oils and lower values of peak heat flux were obtained for conventional quenching oil. Hot oils show intermediate peak heat flux values.

Fig. 16.

Estimated metal/quenchant heat flux transients at (a) axial and (b) radial locations of probe surface during quenching in MQ-5.

Table 4. Estimated multiple peak heat flux components during vapour blanket stage.

Probe	Quench medium	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	607	Normal mineral oil
	MQ-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	518	Fast mineral oil
	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	Hot mineral oil
	MQ-8	501	492	488	517	498	540	424	379
Probe-II (radial variation)	MQ-1	604	665	638	651	570	553	573	603	Normal mineral oil
	MQ-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	580	Fast mineral oil
	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	Hot mineral oil
	MQ-8	683	595	593	603	622	601	608	572

Table 5. Estimated multiple peak heat flux components during nucleate boiling stage.

Probe	Quench medium	qnucleate (kW/m2)								Remarks
		q1	q2	q3	q4	q5	q6	q7	q8
Probe-I (axial variation)	MQ-1	1423	1548	1172	1313	976	1452	1319	1174	Normal mineral oil
	MQ-2	1181	1264	1271	1383	1173	995	1290	1494
	MQ-3	1005	1509	1336	1195	1454	1290	1183	1115
	MQ-4	2107	2090	1960	2416	1729	2593	2320	2237	Fast mineral oil
	MQ-5	2122	2410	1772	2408	1608	2343	1713	2073
	MQ-6	2118	1960	1647	1455	2078	1427	2275	1900
	MQ-7	1874	1915	2127	1763	1754	1869	1927	1379	Hot mineral oil
	MQ-8	1972	1360	1735	1623	1574	1414	1734	1667
Probe-II (radial variation)	MQ-1	1266	1236	1243	1045	1285	1378	1346	1493	Normal mineral oil
	MQ-2	1349	1349	1353	1347	1485	1315	1402	1290
	MQ-3	1067	1498	1142	1196	1004	999	1341	1385
	MQ-4	2501	2237	2196	2030	1993	1942	1886	1978	Fast mineral oil
	MQ-5	1714	2361	2211	2062	1913	1826	2000	1959
	MQ-6	2092	1954	2103	1993	1819	1792	1898	1870
	MQ-7	1926	1828	1521	1681	1476	1542	1633	1553	Hot mineral oil
	MQ-8	1192	1442	1353	1420	1466	1429	1449	1266

The variations of heat flux values at critical temperatures 705, 550, 300 and 200°C at different locations of quench probe surfaces for all quenchants were determined. Figure 17 show typical spatially dependent heat flux at critical temperatures in axial and radial locations on the probe surface during quenching. The plots show heat transfer during quenching was not uniform over the surface of probe. For example, standard deviation of heat flux values at 705°C in axial 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 flux variations at axial locations were found to be higher than at radial locations for all quenchants. The heat flux variations at 705 and 550°C were found to be higher than at 300 and 200°C. It indicates that heat transfers during liquid cooling stage was more uniform than film and nucleate boiling stage. Further, no definite relation between heat flux variation and properties of oil was found. However, lower standard deviations of heat flux values were observed for hot oils having high viscosity indicating more uniform heat transfer than conventional and fast mineral oils. The amount of heat removed during quenching was determined by plotting the integral heat flux curve for all quench media. The amounts of heat extracted by the quenchants to cool the probe 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 of heat removal during quenching in mineral oils. The time for known fraction of heat removal was lower for fast quenching mineral oils indicating superior heat extracting capability compared to conventional and hot quenching oils. The hot oils quenching showed lower time up to about 65% heat removal and higher time for remaining percent heat removal compared to conventional oil. Cooling curve analyses parameters also showed higher cooling rates at peak cooling and 705°C and comparable cooling rates at 550°C, 300°C and 200°C for hot oils compared to conventional quenching oils. Further, the times to cool from 730 to 260°C were higher for hot oils. This is the reason for low HP values of hot oils quenching though it showed higher peak heat flux values and peak cooling rates than conventional oil. It indicates that heat extracting capability of the hot oil was faster during vapour and nucleate boiling and slower during liquid cooling stage.

Fig. 17.

Typical spatially dependent heat flux at critical temperatures in (a) axial and (b) radial locations on the probe surface during quenching in MQ-5.

Fig. 18.

Time for specified heat removal from the quench probe.

To quantify the effect of cooling performance of quench media on the formation of micro-constituents and hardness in steel, cooling curves measured at geometric centre of the probe were superimposed on the CCT diagram. It should be noted that cooling curve analysis was carried out using Inconel 600 probe. However, superimposition of cooling curve on the steel CCT diagram can be used to quantify the effect of cooling performance of quench media. Figure 19 shows the CCT diagram of AISI 1040 obtained using JMatPro software (Sente Software Ltd., UK). The resultant micro-constituents of steel are given in Table 6. Fast mineral oil quenching resulted in higher amount of transformed products (bainite and martensite) than normal and hot oils. Further, normal oil quenching resulted in higher amount of transformed product than hot oils. It indicates that higher quenching severities for fast mineral oils, lower quench severities for hot oils and intermediate quench severities for normal oils. With varying cooling rates obtained with mineral oil quenchants used in the present work, the resultant hardness of the AISI 1040 steel estimated from the CCT diagram was found to be 277, 271, 266, 288, 289, 273, 268 and 261 HV for MQ-1, MQ-2, MQ-3, MQ-4, MQ-5, MQ-6, MQ-7 and MQ-8 respectively.

Fig. 19.

Experimentally measured cooling curves of mineral oils superimposed on CCT curve of AISI 1040 steel.

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

Micro constituent	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

4. Conclusions

The 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 tension whereas mineral oils of low viscosity and surface tension showed lower contact angle. It indicates that wettability of mineral oil improved with a decrease in viscosity and surface tension of mineral oil. Similarly, relaxation of contact angle and spreading area of droplet on Inconel substrate indicated faster spreading kinetics for mineral oils of low viscosity and surface tension and slower spreading kinetics for mineral oils of high viscosity and surface tension.

(2) Spreading behavior of mineral oils having high viscosity and density showed all three regimes of spreading namely capillary, gravity and viscous regimes. On the other hand, spreading behavior of mineral oils having low viscosity and density on Inconel substrate consisted of capillary, gravity regimes and absence of viscous regime.

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

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

(5) Mineral oils of high viscosity and flash point showed the formation of additional wetting front at the top of quench probe which started moving downward during rewetting of fluid. On the other hand, no additional wetting front was observed for mineral oils of low viscosity and flash point

(6) Among the convectional/fast/hot mineral oil, higher wetting front velocity was obtained for low viscosity oil while lower wetting front velocity was obtained for high viscosity oil. Further, the rewetting of the fluid occurs at higher temperature with an increase in viscosity of the mineral oil.

(7) Higher cooling rates at critical temperatures and lower t_730–260 were obtained for fast quenching oils than hot oils and conventional quenching oils. Hot quenching oils showed higher cooling rates at peak cooling and 705°C than conventional quenching oils. The cooling rates at 550°C, 300°C and 200°C of hot oils and conventional quenching oils were comparable.

(8) Higher values of peak heat flux were obtained for fast quenching oils and lower values of peak heat flux were obtained for conventional quenching oil. Hot oils show intermediate peak heat flux values.

(9) Among the conventional/fast/hot oils, oil having higher viscosity showed more uniform heat transfer than low viscosity oil.

(10) Fast quenching mineral oils showed faster heat extracting capability compared to conventional and hot quenching oils. The cooling performance of the hot oil was better during vapour and nucleate boiling stages and slower during liquid cooling stage compared to conventional quenching oils.

Acknowledgement

One of the authors (KNP) gratefully acknowledges the financial support provided by the Science and Engineering Research Board (SERB), Department of Science and Technology (DST), Government of India, New Delhi, India under a R&D project.

References

• 1)   B.  Liscic,  H. M.  Tensi,  L. C. F.  Canale and  G. E.  Totten: Quenching Theory and Technology, CRC Press, Boca Raton, FL, (2010).
• 2)   B.  Liscic: Quenching and Carburising, ed. by P. D. Hodgson, The Institute of Materials, London, (1993), 1.
• 3)   H. M.  Tensi,  A.  Stich and  G. E.  Totten: Steel Heat Treatment, ed. by G. E. Totten, CRC Press, Boca Raton, FL, (2006), 540.
• 4)   H. E.  Boyer and  P. R.  Cary: Quenching and Control of Distortion, ASM International, Materials Park, OH, (1988).
• 5)   S.  Ma: PhD Thesis, Worcester Polytechnic Institute, Worcester, USA, (2002).
• 6)   G. E.  Totten,  C. E.  Bates and  N. A.  Clinton: Handbook of Quenchants and Quenching Technology, ASM International, Materials Park, OH, (1993).
• 7)   C. E.  Bates,  G. E.  Totten and  R. L.  Brennan: ASM Handbook - Heat Treatment, Vol. 5, ASM International, Materials Park, OH, (1991), 160.
• 8)   G. E.  Totten,  H. M.  Tensi and  L. C. F.  Canale: Proc. of 22nd Heat Treating Society Conf. and 2nd Int. Surface Engineering Cong., ASM International, Materials Park, OH, (2003), 141.
• 9)   S.  Ma,  M.  Maniruzzaman and  R. D.  Sisson, Jr.: Proc. of 1st ASM Int. Surface Engineering and 13th IFHTSE, ASM International, Material Park, OH, (2000), 281.
• 10)   S.  Asada and  K.  Fukuhara: Proc. of 20th Heat Treating Society Conf., ASM International, Materials Park, OH, (2000), 833.
• 11)   H.  Yokota,  H.  Hoshino,  S.  Satoh and  R.  Kanai: Proc. of 20th Heat Treating Society Conf., ASM International, Materials Park, OH, (2000), 827.
• 12)   P.  Fernandes and  K. N.  Prabhu: J. Mater. Sci. Eng. B, 3 (2013), No. 2, 90.
• 13)   T. S. P.  Kumar: Numer. Heat Tr. B-Fund., 45 (2004), 541.
• 14)   R. N.  Penha,  L. C. F.  Canale,  G. E.  Totten,  G. S.  Sarmiento and  J. M.  Ventura: J. ASTM Int., 3 (2006), No. 5, JAI13614.