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University of Miskolc
FACILITY OF MECHANICAL ENGINEERING AND INFORMATICS
INSTITUTE OF MATERIALS SCIENCE AND TECHNOLOGY
UNIVERSITY OF MISKOLC
MSc Thesis
Investigation on the efficiency parameters ofsynthetic quenehant solution as a function of the
concentration
By:Mohannad Aldaher
MSc StudentNeptun code: CLL8BW
Supervisor:Dr. László KuzsellaAssociate professor
Consultant:
Bassei AlsalamahPhD Student
Miskolc, 2020
University of MiskolcFaculty of Mechanical Engineering andInformaticsH-3515 Miskolc-Egyetemváros
Mechanical engineer (MSc)CAD/CAM Specialísation, full-time
INSTITUTE OF MATERIALSSCIENCE AND TECHNOLOGY
http://www.met.uni-miskolc.hu
MSc Degree ThesisNumber: MN-CC-01l2020
DEGREE THESIS
Mohannad Al DaherNeptun code: CLL8BW
Topic:Title:
Mechanical technologies - Heat treatmentInvestigation on the efficiency parameters of synthetic quenehantsolution as afunction of the concentrationDr. László Kuzsella, associate professorBasseI Alsalamah, PhD student
Supervisor:Consultants:
Deadline: 20th November 2020
Objectives:
1. Prepare a literature overview about the quenehing of steels, especially analyse thedifferent quenchants media!
2. Prepare a detailed literature overview of the immersion quenehing of steels!
3. Define the main parameters of quenchants special attention on theirs efficiency!
4. Evaluate the results of a measurement series made on water based polymer addedsolutions!
5. Describe the effect of the polymer concentration on the efficiency pararneters ofquenchants!
Miskolc, 12th October, 2020
professor, director of the institute
Instructor:
Modification needednot necessary
date supervisor
The Degree Thesis can be submitted:
date consultant( s) supervisor( s)
The Degree Thesis includes:.................... pages,.................... drawings,.................... design documents.
Appendix:
date supervisor
The Degree Thesis is suggestedis not suggested
for submiuing to revision.
Reviewer(s): .
date director of the institute
Grade of Degree Thesis:Reviewer's proposal: .Propo sal of the Institute: .Decision of the State Examination Committee: .
Miskolc, .
head of the State Examination Committee
PLAGIARISM DECLARATION
I Mohannad Aldaher; Neptun-code: CLL8BW, the student of the Faculty of MechanicalEngineering and Information Sciences, University of Miskolc, being fully aware of my legalliability, hereby confirm, declare and certify with my signature, that the assignment, entitled:Quenehing Media Management
- except where indicated by referencing -, is my own work, is not copied from any otherperson's work, and is not previously submitted for assessment at University of Miskolc orelsewhere, and all sources ( both the electronic and printed literature, or any kind) referred toin it have been used in accordance with the rules of copyright.
I understand that a thesis work may be considered to be plagiarised if it consists of
• Quoting word by word or referring to literature either with no quotation marks or noproper citation;
• Referring to content without indicating the source of references.• Representing previously published ideas as one's own.
1, hereby declare that 1have been informed of the term of plagiarism, and I understand that incase of plagiarism my thesis work is rejected.
Miskolc, ... C.~ ....(dd) ... ~~ .. (mm) ..l.~.2.(year).,., 1,..../
Student's signature
2
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Contents 1. Introduction ......................................................................................................................................... 6
2. Literature Summary ............................................................................................................................. 7
2.1. Quenching .................................................................................................................................... 7
2.3. Steps of quenching ...................................................................................................................... 7
2.4. Quenching Media ......................................................................................................................... 8
2.4.1 Air ............................................................................................................................................... 8
2.4.2. Oil .............................................................................................................................................. 8
2.4.3. Water ......................................................................................................................................... 8
2.4.4. Brine .......................................................................................................................................... 8
2.5. Spray Quenching .......................................................................................................................... 9
2.6. Water Quenching Heat Transfer ................................................................................................ 13
2.6.1. Immersion Quenching ............................................................................................................. 13
2.6.2. Spray Quenching ..................................................................................................................... 14
2.6.3. Jet Quenching .......................................................................................................................... 15
3. Critical Cooling Curve, Temperatures and Rates ............................................................................... 18
3.1. Grossman Quench Severity Factor (H-Value) from Cooling Curve Data .................................... 20
3.2. Tamura’s V-Value ....................................................................................................................... 22
3.3. Master Cooling Curve ................................................................................................................. 25
3.4. QTA Method ............................................................................................................................... 27
3.5. Cooling Rate Area ....................................................................................................................... 28
3.6. Quench Uniformity ..................................................................................................................... 30
3.7. Castrol Index ............................................................................................................................... 31
3.8. Heat Transfer Coefficient Correlation ........................................................................................ 33
3.9. Critical Heat Flux Density ............................................................................................................ 38
3.10. Another quenchant efficiency parameters (TCP, TVP, HP) ......................................................... 41
4. Used Measurement Technique and Quenchant ............................................................................... 42
4.1. Test procedure ........................................................................................................................... 43
4.2. The Studied Quenchant .............................................................................................................. 45
4.3. Aqua-Quench .............................................................................................................................. 45
4.3.1 Description ............................................................................................................................... 45
4.3.2 Application ............................................................................................................................... 45
4.3.3 Features and Benefits ............................................................................................................... 46
5. Measurement Results ....................................................................................................................... 47
5.1. Results of Concentration Change ............................................................................................... 47
4
6. Conclusion ......................................................................................................................................... 54
7. References ......................................................................................................................................... 55
5
List of Figures
Figure 1: Cooling rates for spray, polymer, and oil immersion quenching ............................... 9
Figure 2: Hardness distributions for spray, polymer, and oil immersion quenching ............... 10
Figure 3: Heat transfer versus surface temperature .................................................................. 11
Figure 4: Heat transfers in the vapour blanket portion of a spray quench [12] ........................ 12
Figure 5: The three main water-cooling configurations on run out tables. From left to right:
spray, laminar, and water curtain [24] .............................................................................. 14
Figure 6: Water-mass-flux effect on heat-transfer coefficient [26] ......................................... 15
Figure 7: Different heat flow paths .......................................................................................... 16
Figure 8: Two bubble growth-termination scenarios. (a) Dynamic equilibrium. (b) Thermal
equilibrium [31] ................................................................................................................ 18
Figure 9: Critical cooling curve parameters [34] ..................................................................... 20
Figure 10: Correlation between the heat transfer coefficient (α) with different wetting phases
........................................................................................................................................... 21
Figure 11: Tamura‘s steel classification system for different types of steels [40] ................... 23
Figure 12: Illustration of the methodology of determining the film-boiling to nucleate boiling
transition [40] .................................................................................................................... 24
Figure 13: Calculation methodology for V-values from different steel classifications [41] ... 24
Figure 14: Master cooling curves for different quenching media [43] .................................... 26
Figure 15: Illustration of Tamura‘s method of correcting cooling curve shape to account for
the heat of transformation [43] ......................................................................................... 26
Figure 16: Parameters for the QTA model [50] ....................................................................... 28
Figure 17: Thelning‘s area under the cooling rate curve [51] .................................................. 29
Figure 18: Calculation of VS/VC Value to quantify quench uniformity [54] ........................... 31
Figure 19: Wolfson Inconel 600 probe [56] ............................................................................. 32
Figure 20: Cooling power of various quenchants and heat transfer coefficients at a probe
temperature of 500 °C [57] ............................................................................................... 35
Figure 21: Schematic illustrations of The Liscic-Nanmac probe and Nanmac thermocouple
arrangement [69] ............................................................................................................... 40
Figure 22: Equipment used in my measurements .................................................................... 43
Figure 23: Houghton, Aqua-Quench 320 AQ EC/WE ............................................................. 45
Figure 24: Cooling rate at 300 °C as a function of concentration change ............................... 47
Figure 25: The maximum cooling rate as a function of concentration change ........................ 48
Figure 26: Temperature for maximum cooling rate ................................................................. 49
Figure 27: Time corresponding to the maximum cooling rate ................................................. 49
Figure 28: Apparent transition temperature between boiling and convection phase ............... 50
Figure 29: Apparent transition temperature between steam and boiling stage ........................ 51
Figure 30: Refrigerant training performance ........................................................................... 52
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1. Introduction
Steels and other metallic alloys may exhibit a wide variety of properties depending on
composition, phases and micro constituents. The metallographic structure, and thus properties
of the workpiece are varied by heat treatment. Heat treatment refers to the process of heating a
part to a defined temperature, holding the part at this temperature for a specific time and then
cooling quickly to form the desired microstructure. The cooling rates necessary to achieve a
de-sired microstructure mainly depend on the composition of the alloy, hardenability and
geometry and thickness of the part to be quenched. Cooling rates can be changed in hardening
practice by the type of quench ant, the selected quenching conditions and arrangement of the
pieces in the quench tank. Most quench ants used today are vaporizable liquid media such as
water, oil and aqueous polymer solutions. Quenching techniques are typically immersion
quenching and spray quenching.
Optimal quench uniformity is essential if the potential for cracking, distortion, residual stress
and spotty hardness is to be minimized. This means that heat transfer during the film boiling
(vapor blanket) and nucleate boiling processes during heat transfer in vaporizable liquids such
as water, oil and aqueous polymers must be as uniform as possible throughout the quenching
process. One of the most important factors affecting quench uniformity is the design of the
quench system. Deficiencies in system design have frequently been inadequately addressed by
both heat treating engineers and equipment suppliers, often with disastrous results. With the
exception of a few little-known company specifications, there are no industry-wide guidelines
for quench system design. Therefore, there is no extensive compilation of state-of-the-art
design criteria to assist the engineer in the design and construction of a quenching system that
will provide optimal heat transfer and quench uniformity
Agitation is one of the most critical areas of system design. The effect of agitation on the
performance of various quench oils has been studied in detail. The ability to through harden
AISI 4135 steel in conventional quench oil increased with increasing agitation. Although
decreasing oil temperature provided some improvement in through hardening, it was
considerably less effective than increasing agitation rate. It is well known that increasing
agitation, increasing cooling rates and the through-hardening ability of both oil and water
quenchants. And it‘s proved that agitation of quench oil was necessary to destabilize film
boiling and nucleate boiling processes if uniform heat transfer throughout the quenching
operation was to be achieved. Adapted from ―Spray Quenching‖ in Handbook of Quenchants
and Quenching Technology [1]. The term spray quenching refers to a wide variety of
quenching processes that involve heat removal facilitated by the impingement of a quenchant
medium on a hot metal surface. Some of these processes have obvious differences, while
others are similar and differ only in degree. Examples include the addition of droplets of
water (or other volatile liquids) to a gas quenching stream in fog quenching [1]; quenching
with water or water/air streams [2]. Sprays of volatile liquid quenchants other than water [3]
and high pressure jets of an oil [4]. Water [5, 6, 7], or aqueous polymer solution [8].under the
liquid level in a bath.
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2. Literature Summary
2.1. Quenching
In materials science, quenching is the rapid cooling of a workpiece in water, oil or air to
obtain certain material properties. A type of heat treating, quenching prevents undesired low-
temperature processes, such as phase transformations, from occurring. It does this by reducing
the window of time during which these undesired reactions are both thermodynamically
favourable, and kinetically accessible; for instance, quenching can reduce the crystal grain
size of both metallic and plastic materials, increasing their hardness.
In metallurgy, quenching is most commonly used to harden steel by inducing a martensite
transformation, where the steel must be rapidly cooled through its eutectoid point, the
temperature at which austenite becomes unstable. In steel alloyed with metals such as nickel
and manganese, the eutectoid temperature becomes much lower, but the kinetic barriers to
phase transformation remain the same. This allows quenching to start at a lower temperature,
making the process much easier. High-speed steel also has added tungsten, which serves to
raise kinetic barriers, which among other effects gives material properties (hardness and
abrasion resistance) as though the workpiece had been cooled more rapidly than it really has.
Even cooling such alloys slowly in the air has most of the desired effects of quenching; high-
speed steel weakens much less from heat cycling due to high-speed cutting.
Quenching is a type of metal heat treatment process. It involves the rapid cooling of a metal to
adjust the mechanical properties of its original state. To perform the quenching process, a
metal is heated to a temperature greater than that of normal conditions, typically somewhere
above its recrystallization temperature but below its melting temperature. The metal may be
held at this temperature for a set time in order for the heat to ―soak‖ the material. Once the
metal has been held at the desired temperature, it is quenched in a medium until it returns to
room temperature. The metal also may be quenched for an extended period of time so that the
coolness from the quenching process is distributed throughout the thickness of the material.
Quenching is defined as the rapid cooling of the work material in order to acquire certain
mechanical properties. This is followed by any heat treatment process. The high temperature
material is soaked in any quenching medium like air, water, oil or polymer where there is an
instant change in state from austenite to martensite. It is related to the change in crystalline
structure or phase transformation of the material which affects its hardness and other
properties [4].
2.3. Steps of quenching
There are three main steps of quenching.
1. Heating up steel till the appropriate temperature.
2. Tempering. It is the austenitization step. The more open structure of the austenite is
then able to absorb carbon from the iron-carbides in carbon steel. An incomplete
initial austenitization can leave undissolved carbides in the matrix.
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3. Quenching. I involves the rapid cooling of a steel. Steel is soaked in any quenching
medium like air, water, oil or polymer where there is an instant change in state from
austenite to martensite.
2.4. Quenching Media
There are a variety of quenching media available that can perform the quenching process.
Each media has its own unique quenching properties. Considerations for the type of media
use include quenching speed, quenching media environmental concerns, quenching media
replacement, and quenching media cost. Here are the main types of quenching media:
Air
Oil
Water
Brine
2.4.1 Air
Air is a popular quenching media used to cool metals for quenching. Affordability is one of
the main benefits of air; its affordability is a result of its profusion on earth. In fact, any
material that is heated and then allowed to cool to room temperature simply by being left
alone is considered to have been air quenched. Air quenching is also more intentionally
performed when it is compressed and forced around the metal being quenched. This cools the
part more rapidly than still air, although even compressed air may still cool many metals too
slowly to alter the mechanical properties.
2.4.2. Oil
Oil is able to quench heated metals much more rapidly than compressed air. To quench with
oil, a heated part is lowered into a tank that is filled with some type of oil. The oil can also be
flushed through the part. Different types of oil are often used depending on the application
because of their varying cooling rates and flash points.
2.4.3. Water
Water is able to quench heated metals rapidly as well. It can cool a metal even faster than oil.
In a fashion similar to oil quenching, a tank is filled with water and the heated metal is
submerged in it. It can also be flushed through a part. One benefit of water is that
flammability of the media is not a concern.
2.4.4. Brine
Brine is a mixture of water and salt. Brine cools faster than air, water, and oil. The reason for
this is that the salt and water mixture discourages the formation of air globules when it is
placed in contact with a heated metal. This means that more of the surface area of the metal
will be covered with the liquid, as opposed to air bubbles.
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2.5. Spray Quenching
One advantage of spray quenching, relative to other quench methods, is that a large and
adjustable range of cooling rates is achievable by simple changes in flow rates and pressures.
The high rates of heat extraction possible with sprays are critical for attaining good depth of
hardness, Segerberg has examined this and illustrated the results with the cooling curves
shown in Fig. 1 [5].
Figure 1: Cooling rates for spray, polymer, and oil immersion quenching
Immersion quenching was performed using oil and a polymer solution. The water flux for the
spray quench was chosen to give a cooling curve with a vapour film of the same duration as
that for the oil immersion quench.
The higher cooling rates achieved by the immersion quench in the poly (alkylene glycols)
PAG solution and by the water spray quench account for the greater depth of hardness shown
in Fig. 2.
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Figure 2: Hardness distributions for spray, polymer, and oil immersion quenching
Spray quenching is used to optimize heat transfer for hardening while simultaneously
developing the desired distribution and level of stress [5]. With spray quenching, the heat
transfer coefficient from the part to the quenching medium is directly related to the flow rate,
turbulence, and impingement pressures of the quenchant at the hot surface. It is possible to
adjust these parameters during the quench to yield a cooling profile achievable in no other
way [5].
The next section of this article, ―Water Quenching Heat Transfer,‖ reviews heat-transfer
characteristics of immersion quenching and spray quenching with water. Liquid quenching
provides high cooling rates required for the steel hardening process. Water is the most
commonly used liquid in quenching because it is readily available, easy to pump, no toxics,
and inflammable. Quenching can be carried out by directly submerging the part in a quench
bath, bottom relooking, falling film, sprays, and impinging jets. Impinging jets are used to
remove extensive amounts of heat locally, while sprays are used to achieve less but more
uniformly distributed cooling rates.
Concepts of Spray Quenching: when quenching a hot metal part in a water bath, the cooling
mechanisms vary according to regions, as illustrated in Fig. 3. [9, 10] showing the heat
transfer from the part as a function of its surface temperature. Fig. 3 also relates the surface
temperature and heat-transfer coefficient to the mechanism of heat removal. Upon immersion,
the part will first be surrounded by a vapour blanket, which will collapse as the part cools.
The heat transfer through this vapour blanket is poor, and the part will cool slowly in this
region.
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The second region of the cooling curve is called the nucleate boiling region and corresponds
to rapid heat transfer caused by direct contact of the part with the water. In this region, the
part is still very hot and the water will boil vigorously. The high heat of water vaporization
accounts for the very rapid heat transfer. In the third, or convective, cooling region, the
surface of the part has cooled to a temperature below the boiling point of water. Only
convective heat transfer occurs in this region.
Figure 3: Heat transfer versus surface temperature
Cooling by the impact of a stream of droplets or liquid in spray quenching is also represented
by Fig. 3. Fluid flow is used to accelerate the quench by rupturing the vapour blanket. The
high agitations rates inherent in a spray quench also accelerate cooling in the nucleate boiling
and convective cooling portions of the quench. Based on the mathematics needed to describe
the spray systems, one author has stated that ―spray cooling is also assort of turbulent flow
cooling‖ in all three regions of the cooling curve [11]. In fact, it is useful to use the same
terms to describe the heat-transfer mechanisms in both spray and immersion quenching.
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The heat transfer process for this state can be pictured as shown in Fig. 4. [2, 12] The heat
transfer rate through the vapour film is relatively slow; therefore, there is a relatively slow
decrease in the temperature of the plate. As the temperature falls, the thickness of the vapour
film will decrease [11]. At a characteristic temperature, which is dependent on the quench
conditions, the droplets with higher than-average kinetic energy will penetrate the vapour
film, contact the plate, and begin to spread out on the surface. The temperature at which this
occurs is called the Leiden frost point. At this temperature, regions of the plate will be
observed to be ―wetted‖ by the quenching.
Figure 4: Heat transfers in the vapour blanket portion of a spray quench [11]
The increased contact of the droplets with the metal surface will result in a corresponding
increase in the heat transfer from the plate, causing the plate temperature to decrease more
rapidly. A larger number of droplets will penetrate the film, and heat transfer will increase
until the entire plate is wet with boiling liquid. At this point, the heat-transfer process is
characterized by nucleate boiling. Further reduction of the plate temperature leads to an end of
boiling, and the heat-transfer mechanism becomes dependent on convective cooling. In
practice, the Leiden frost temperature is found to be a function of the surface physical
properties [13, 14]. As well as its temperature [15, 16]
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2.6. Water Quenching Heat Transfer
2.6.1. Immersion Quenching
Oils and water are the most widely used liquids in steel quenching processes. Water, although
most commonly used, causes cracks and distortion for quenched parts. Uniformity in the heat
transfer coefficient in the film and nucleate boiling regimes is required to abate such problems
[19]. The heat-transfer coefficient in water immersion quenching depends on the surface
temperature of the part, the thermo physical properties of the part, and the initial water bulk
temperature, Bamberger developed the following correlation of the heat-transfer coefficient in
immersion quenching [20].
Investigating the heat transfer during quenching operations experimentally has many
difficulties associated with it: measuring the temperature of the quenched part surface,
determining the cooling rate, and quantifying the heat transfer coefficient, especially with the
two phase nature of the flow. Recently, enormous attention has been given to modelling the
heat transfer coefficient in water immersion quenching, taking into consideration the boiling
phase change. Srinivasan. [17, 18] developed a numerical code for immersion quenching
processes. They used the most widely and commonly used relations to estimate the heat
transfer coefficient: Nussle‘s method for film condensation to model film boiling, Zuber‘s
relation for the critical heat flux, and Rohsenow‘s correlation for nucleate boiling. Their
results matched the experimental data, except for some deviations in the case of complex
shape quenching in the transition boiling regime.
The Effect of Quenching Agitation: when the part is first immersed in the water, the adjacent
water layer evaporates rapidly, forming a vapour layer with low heat-transfer coefficient. Film
boiling dominates until the surface Temperature falls to the Leiden frost point or the vapour
layer breaks at higher temperature. Agitation helps in breaking the vapour layer. The collapse
starts from the corners of the part toward the center, augmenting the heat-transfer coefficient
and increasing the cooling rate. Quenchant stirring, quenchant circulation, and submerged
jet/spray mixing are the most commonly used techniques in quench tank agitation [21].
Hernandez Morales [22] compared the heat transfer rate in quenching of a steel disk in
stagnant water and in agitated water.
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2.6.2. Spray Quenching
Spray quenching is superior to immersion quenching in that the cooling rate range can be
adjusted by changing the flow rate and liquid pressure. Different spray and jet types can be
used, as shown in Fig. 5. Spray quenching follows the boiling cure.
Figure 5: The three main water-cooling configurations on run out tables. From left to right:
spray, laminar, and water curtain [23]
The water droplet accelerates the quenching process by rupturing the vapour layer. The
vapour layer is formed when water droplets from the spray touch the hot surface, and the
liquid in contact with the plate evaporates, leaving the droplet in its spherical shape with a
miniature contact with the surface. McGinnis and Holman [23] quantified the maximum heat
flux from a single droplet on a heated surface. The maximum heat flux existed because of two
opposing effects of the increasing temperature gradient and the weaker contact between the
surface and the droplet with time. The vapour layer dramatically decreases heat removal from
the surface. Because the droplets from the spray have high momentum, they break the vapour
layer and wet the surface.
Sozbir compared low mass flux spray jets with air jets and found that the heat-transfer
coefficient increases with the increase of the water mass flux [24], between 0 and 7.67
kg/m2/s, for the mist jet, as shown in Fig. 6. They pointed out that the increase in the heat flux
was due to the fact that the droplet hits the same place on the surface. They indicated that the
droplet velocity has a minor effect on the heat transfer coefficient augmentation. High water
velocity results in distortion of the droplet when it hits the surface.
15
Figure 6: Water-mass-flux effect on heat-transfer coefficient [25]
Sengupta found that vapour film boiling in steel continuous casting is the dominant heat-
transfer mode, and it is preferred because its heat-removal rate is mass flux dependent [25].
They established an empirical correlation for water heat-transfer coefficient with the
following spray nozzle characteristics: type, nozzle separation, distance of the nozzle from the
surface, and water-mass flux. Ciofalo carried out a transient study of the effect of different
nozzles on swirl spray cooling [26]. Their results are limited to the confections they used. The
reported heat flux is higher than the other reported heat fluxes for the same water-mass flux.
Most of the empirical models developed for cooling steel plates cannot be applied for other
conditions than the ones they have been developed for Bamberger and Prinz carried out
experiments on spray cooling using water pressure between 0.12 and 0.5 MPa (0.017 and 0.07
ksi) and related the spray heat transfer coefficient to the immersion heat transfer.
2.6.3. Jet Quenching
Because the water jet is not uniformly spread over the quenched part and it is more
concentrated over a narrow region, there is a hydrodynamic variation in the flow, which
results in a non-uniform cooling pattern. However, impinging jets produce significantly high
cooling rates. Several studies have been carried out investigating cooling rates in quenching
processes using impinging jets.
Modelling of jet cooling includes global and mechanistic forms. Global Modelling is one of
the work cited in the previous two sections involved the develop mend of an empirical
correlation of the heat flux to the jet parameters and surface superheat. Although these kinds
16
of relations are easy and fast to apply, they are limited to the conditions for which they have
been developed. Omar developed an empirical analytical model to predict the stagnation heat
flux from a free planar jet impinging on a horizontal flat surface [27]. With the assumption of
enough liquid superheat around the bubbles in the layer, bubbles departed the surface and
then collapsed in the layer above, at the saturation temperature. Additional disturbances are
induced due to bubble dynamics, which result in an enhancement of the heat transfer. Such
enhancement has been represented by an additional diffusion term in the momentum and
energy equations. The total diffusivity term in the momentum and energy equations is
assumed to be the sum of the molecular and the proposed bubble-induced diffusivity
By solving the equations in a no dimensional form, they derived to calculate the nucleate
boiling heat flux at the surface [28].
Mechanistic Modelling of Jet Cooling on the contrary to global models, mechanistic models
have the ability to quantify total heat flux, and each component of heat flux independently,
based on the sub models or experimental observations for bubble maximum diameter, release
frequency, and density of the nucleation sites. One of the oldest wall flux partitioning models
was proposed by Griffith [28] when he noticed a sub cooled boiling region with low void
fraction and a less sub cooled boiling region with high void fraction. In the first region, the
scattered static bubbles acted as surface disturbances, and the heat flux components are due to
the single-phase flow and boiling, while in the second region the heat is assumed to transfer to
the liquid through condensation of the bubbles.
Figure 7: Different heat flow paths
Based on an earlier work of flow boiling heat transfer by Base [29] Omar [30] recently
applied the wall heat flux portioning concept to the stagnation zone heat flux produced by an
17
impinging jet. Omar assumed that the total heat flux is the sum of three partitions due to
forced convection, evaporation, and transient conduction heat fluxes, as shown in Fig. 7.
Omar found a closure for his mechanistic model by developing sub models for the bubble
growth diameter and bubble growth terminal scenario. He assumed that after the bubble
grows, it may slide or collapse in place, based on whether the dynamic equilibrium or the
thermal equilibrium condition would be reached first Fig. 8. Applying these equilibrium
conditions, Omar calculated the maximum bubble diameter. He used high-speed imaging and
an intrusive optical probe to collect information about bubble dynamics (diameter, frequency,
and number).
To find closure for his model, Omar also modelled the onset of nucleate boiling, bubble
frequency, and nucleation-site density. Onset of nucleate boiling was correlated as a function
of the film velocity, which is a function of the jet velocity. Using the optical probe, Omar
measured the bubble frequency as a function of the dimensionless flow parameters. The
model was validated using experimental data and was found to be 30% accurate.
18
Figure 8: Two bubble growth-termination scenarios. (a) Dynamic equilibrium. (b) Thermal
equilibrium [30]
3. Critical Cooling Curve, Temperatures and Rates
To describe cooling performance of a quenching, it is important to completely describe the
cooling time and rate properties during each heat transfer stage throughout the cooling
process. For example, the time required to complete the nucleate boiling stage and the cooling
rate during the nucleate boiling process is critical in quantifying the hardening capabilities of
the quenching. Also, the cooling rate during convective cooling is important in assessing the
potential for cracking of the steel being quenched.
The procedure used by Hilder to describe the cooling performance of a quenching is to
determine the maximum cooling rate (CRmax), and the cooling rate at 300 °C (CR300) [32-33].
The maximum cooling rate will occur at the beginning of the nucleate boiling phase and is
typically inversely proportional to the time required to progress from the nucleate boiling
phase to the convective cooling phase. The cooling rate at 300 °C provides a measure of the
cooling rate during convective cooling and is valuable since 300 °C is typically within the
martensitic transformation range for steel.
It may be critically important. However, to obtain a more completed assessment of the
cooling properties of a quenching. For example, some quenchants may exhibit extended film
boiling properties which may exhibit dramatic effects on as-quenched properties such as
hardness and in other cases, no film boiling may occur at all. Also, monitoring a single
cooling rate during the convective cooling process does not permit an adequate
characterization of potential variation of cooling properties in this region.
19
To address this problem, Tensi [33].have recommended the reporting of additional cooling
parameters as shown in Fig. 9. These parameters include:
I Film boiling to nucleate boiling transition time (s) (tA–B)
II Temperature of film boiling to nucleate boiling transition (ºC) (TA–B)
III Film boiling to nucleate boiling transition cooling rate (ºC/s) (CRDHmin)
IV Cooling rate at 700 ºC (CR700)
V Maximum cooling rate (ºC/s) (CRmax)
VI Temperature of the maximum cooling rate (ºC) (TCRmax)
VII Cooling rate at 300 ºC (CR300)
VIII Time to cool to 300 ºC (t300)
IX Cooling rate at 200 ºC (CR200)
X Time to cool to 200 ºC (t200)
Parameters I–III are related to the full-film boiling (vapour blanket cooling) to nucleate
boiling transition time, temperature and the cooling rate at critical temperatures Cooling rate
at 700 ºC, parameter IV, is measured since it is usually desirable to maximize this cooling rate
to avoid the steel pearlite transformation region. Parameters V and VI are the maximum rate
of cooling and the temperature where this occurs. Generally, it would be desirable to
maximize CRmax whether TCRmax is maximized or minimized depends on the steel alloy being
hardened. Low hardenability steels require a short a film-boiling region as possible and for
higher hardenability steels, decreasing the TCRmax will typically result in reduced thermal
stresses which is an important and desired effect. Rate of cooling at temperatures such as
200 ºC and 300 ºC, parameters VII and IX, are also determined since they are related to the
potential for steel cracking and distortion. To minimize these problems, it is desirable to
minimize cooling rates in this region. Parameters VIII and X are related to region of the
martensite transformation. It is generally desirable to reduce cooling rates in this region to
reduce transformational stresses which contribute to unacceptable distortion control and
possibly even cracking.
20
Figure 9: Critical cooling curve parameters [34]
Utilization of these parameters will more fully quantify the characteristic cooling properties
throughout full film boiling, nucleate boiling and convective cooling of quenchant.
3.1. Grossman Quench Severity Factor (H-Value) from Cooling Curve Data
A classic method of relating the ability of a quenching to harden steel (hardenability) is to
determine the Grossman H-value [34, 35]. The H-value is defined as,
(1)
Where:
α: is the average heat transfer coefficient at the surface of the part.
λ: is the thermal conductivity of steel.
This shows that the Grossman H-value provides a method of interrelating the heat transfer
properties of the quenchant with the thermal properties of the steel. Although a common and
relatively simple method of classifying quench severity, the H-value calculated in this way
does not account for temperature variation of the heat transfer coefficient as illustrated in Fig.
21
10. Nevertheless, this is method of quantifying quench severity continues to be widely used
today.
Monroe and Bates developed a method of estimating the H-value from the cooling rate at 704
°C at the center of cylindrical type 304 stainless steel probes of different cross-section sizes
13, 25, 38, 50 mm [32]. From the cooling rate data originally reported using type 304 stainless
steel probes, Dakins, developed an equation to calculate the Grossman H-value [37].
Figure 10: Correlation between the heat transfer coefficient (α) with different wetting phases
Occurring simultaneously over the probe surface during the quenching process due to the
non-stationary wetting front. In this case, the bottom of the probe cools first and the wetting
process ascends (upward) as the quenching process proceeds [38].
22
(2)
Where:
X: is the cooling rate at 704 °C and A, B, C and D are constants taken from Table 1.
Table 1: Parameters for Dakins H-Value Equation
Type 304 Stainless Steel
Probe Diameter
mm in A B C D
13 0.5 0.002802 0.1857 10-7
1.201 2.846
25 1.0 0.002348 0.2564 10-9
1.508 4.448
38 1.5 0.002309 0.5742 10-9
1.749 5.076
50 2.0 0.003706 0.03456 10-10
1.847 6.631
When using the H-value to characterize quenchants, it is important to understand that H is a
constant and does not characterize the non-steady state heat transfer conditions occurring on
the surface of a workpiece quenched into a vaporizable quenchant. It is well known that the
heat transfer coefficient (α) varies substantially with the three different cooling stages; fulfill
boiling, nucleate boiling and convective cooling and also varies with actual position of the
moving wetting front during the quenching process see Fig. 10. Thermal conductivity (λ) of
steel is also dependant on the both the temperature and microstructure of the steel. Another
limitation of Grossman H-values is that the mass of the part being quenched is not accounted
for when the H-value is determined from cooling curves with a center thermocouple. Cooling
curves measured at the center do not reflect the heat flux at the surface due to the damping
effect of the thermal conductivity (λ) of the probe material.
3.2. Tamura’s V-Value
Tamura performed a comparison of the CCT and TTT curves for a series of different steel
alloys and found that they could be characterized by one of the following four categories [40].
Ferrite, pearlite and bainite-type (F-P-B-Type) steel such as AISI 1045.
Pearlite-type (P-Type) such as AISI 1085 steel.
Pearlite and bainite-type (P-B-Type) such as AISI 52100 steel.
Bainite-Type (B-Type) such as AISI 4135 and 8620 steel.
These comparisons are shown in Fig. 11. Where X is the temperature range where rapid
cooling is required in order to achieve the desired microstructure and properties. This steel
classification allows the determination of the value of ―X‖ from TS (the start of the critical
hardening transformation) and Tf (the finish temperature for the critical transformation).
23
Figure 11: Tamura‘s steel classification system for different types of steels [39]
Using the quenchant cooling curve characterization parameters illustrated in Fig. 12
[39].Tamura was then able to interrelate the cooling properties exhibited by the quenchant
with those required by the steel alloy to achieve the desired hardening results by calculating a
so-called V-value using the scheme shown in Fig. 13 [40].The V-value is defined as the ratio
of the temperature range of the nucleate boiling stage of the quenchant to the temperature
range of the steel which requires the most rapid cooling.
24
Figure 12: Illustration of the methodology of determining the film-boiling to nucleate boiling
transition [39]
TC: nucleate boiling transition temperature.
Td: cooling transition temperature.
Figure 13: Calculation methodology for V-values from different steel classifications [40]
25
Tamura‘s work showed that the V-value was a better criterion for quench hardening for given
steel than was the Grossman H-value. However, Tamura only illustrated this for oil
quenchants. Results for aqueous polymer quenchants were not reported.
3.3. Master Cooling Curve
Cooling curve behaviour of unagitated quenchants is not only dependent on the quenchant
being analysed but on other parameters including the material, shape and dimensions [41].
Tokihiro and Tamura built on earlier work reported and developed a ―Master Cooling Curve
Method‖ which permits the comparison of cooling curves obtained with thermocouple
instrumented probes of different sizes and materials [42, 43, 44]. This research showed that
cooling curve time was only affected as the size and material were varied according to:
m
L
n
n
aS
DWkt
(3)
Where:
t: cooling time in the center of the probe during the quenching process.
W/S: ratio of the probe volume to surface area per unit of size.
a: thermal diffusivity.
nL , nm: experimental constants.
The master cooling curve is prepared by calculating the k value in the above equation for each
temperature data point on the x-axis versus temperature on the y-axis as shown in Fig. 14. For
normalized silver, SK6 and SUS27 steel probes [45]. In Tamura‘s work, the experimental
values of nL and nm were found to be 1.3 and -0.215 respectively.
The steel probes used by Tamura exhibited a martensitic phase transformation during the
quenching process Fig. 15. Illustrates Tamura‘s method of converting cooling curves
exhibiting exothermic behaviour into cooling curves where no transformation effects are
observed which allows quenching of different steels to be compared on the same basis [41].
26
Figure 14: Master cooling curves for different quenching media [42]
That was obtained by Tamura which are normalized for size, shape, material and for a
thermocouple located at the geometric center.
Figure 15: Illustration of Tamura‘s method of correcting cooling curve shape to account for
the heat of transformation [42]
27
At approximately the same time as Tamura was developing his Master Cooling Curve
methodology which permitted the direct comparison of cooling curve data obtained by any
probe, material, shape size and thermocouple location, Kobasko was developing a more
fundamental numerical process related to thermal properties of materials and understanding of
their quenching behaviour [46]. Wang developed experimental and numerical methodology to
convert cooling curves obtained with a silver probe into analogous cooling curves for steel or
other materials [47] Although excellent results were obtained by these early and relatively
simple methods, there are more advanced computational procedures that can be utilized at the
present time that will further minimize errors related to the temperature dependence of the
thermal properties of the materials involved such as heat capacity and thermal diffusivity.
3.4. QTA Method
The QTA method developed by Wünning incorporates three model parameters, Q, T and A
[49].
48Q: is the average heat flux density on the surface of the quenched part until it cools to
the temperature at which the vapour blanket collapses i.e. the wetting temperature TL.
Since TL values are generally unavailable. Wünning recommends the use of a constant TL
value of 500 °C. To determine the value of Q for each particular quenching process, a
standard steel probe with a known hardenability is used.
T: is the temperature at which the extrapolated line of the heat flux density in the
nucleate boiling stage intersects the temperature axis as shown in Fig. 16. [50]. this
temperature (T) approximately corresponds to the boiling temperature of the
quenchant (Tb) and does not depend on the cooling conditions (T ≈ Tb).
A: is the average heat transfer coefficient of the convective cooling stage and can be
calculated from known heat transfer relationships.
Surface heat transfer described by the QTA model is illustrated in Fig. 16. During the film
boiling stage, the temperature variations are calculated using the heat flux density Q. The
parameter Q depends on the properties of the fluid (thermal conductivity, viscosity, etc.), sub
cooling and flow conditions and geometry of the part. The assumption of constant average
heat flux density is valid because the cooling course in the film boiling stage is usually linear.
At the Leidenfrost, or wetting temperature (TL), transition from film boiling to nucleate
boiling occurs.
The heat transfer coefficient increases by up to two orders of magnitude so that heat
extraction from the surface can be calculated using the first boundary condition assuming that
the surface temperature drops immediately to temperature T. To dampen this infinitely high
heat transfer rate, the temperature T is arbitrarily assigned the distance r‘ on the sample
surface. According to Wünning, r‘ is 1 mm when quenching in water and 2 mm when
quenching in oil.
28
Figure 16: Parameters for the QTA model [49]
The transition from nucleate boiling to convective cooling occurs when the heat flux densities
becomes equal. The temperature (T) where this occurs is approximately equal to the boiling
temperature of the quenchant (T = Tb).
The value A depends on the agitation rate and the dimensions of the workpiece. Although A-
values can be calculated from the physical properties of the quenchant and the T-value can be
determined from a laboratory test, the Q-value must be measured under workshop conditions.
Winning reports that only one hardness measurement is necessary to define the Q-value of a
of a steel part quenched under known conditions, provided that a calibration curve is available
describing the relationship between different Q-values and the resulting as-quenched
hardness.
3.5. Cooling Rate Area
One of the methods that have been reported to determine a single number from a cooling
curve which would provide a useful correlation between cooling rates and as-quenched
hardness was to integrate the area under the cooling rate curve. Thelning performed a study
where he integrated the area under cooling rate curves obtained using the 12.5 mm dia x 60
mm Inconel 600. Wolfson probe utilized by ASTM D6200, D6482 and D6549 [50]. This
study showed that cooling rate during nucleate boiling and convective cooling provided a
reliable measure of the cooling capacity of a quenchant and exhibited the greatest impact on
as-quenched hardness. As a result of this work, it was concluded measurement of the area
under the cooling rate curve between two critical temperatures (T1 and T2) as shown in Fig.
29
17 [50]. These critical temperatures are dependent on steel alloy chemistry and temper.
However, Thelning reported that the area under the cooling rate curve between 600-300 °C
provided the best general temperature range.
Figure 17: Thelning‘s area under the cooling rate curve [50]
Another similar value that has been reported is the mean reduced cooling rate (νred) [51, 52].
red
m
red
Tv e
(4)
Where: ΔT is the theoretical temperature range over which the area is being summed and τred
is the reduced cooling time and is calculated from.
tot
reda
S2
(5)
Where: S is the area under the cooling rate curve defined by ΔT, atot is the total area under the
cooling rate curve. The mean reduced cooling rate is related to Thelning‘s value [50, 52].
Except it is normalized to the total area under the cooling rate curve
30
3.6. Quench Uniformity
One of the primary causes for the occurrence of cracking and poor distortion control resulting
from the quenching process is non-uniform cooling. One method that has been reported for
quantifying the degree of non-uniform cooling is to calculate the cooling rate difference from
cooling time-temperature curves for the surface (VS) and center (VC) of a probe at 300 and
200 °C [53]. These temperatures were selected as being representative of the MS and Mf of
many steels. The VS and VC values are calculated as illustrated in Fig. 18. [53]. The VS and
VC values are calculated from the surface and center cooling curves by dividing the
temperature difference (300–200 °C) by the times to cool to these temperatures ( t300 – t200 )
from the appropriate cooling curve, for example VS is calculated from:
s
stt
CCV
300200
200300
(6)
The ratio of VS / VC can be shown to be:
s
c
c
s
tt
tt
V
V
300200
300200
(7)
The uniformity of cooling improves as the VS / VC ratio approaches 1 which means that the
cooling rate at the center is the same as cooling rate at the surface and is therefore uniform
across the section of the steel in the critical martensitic transformation region. When this
occurs, the degree of surface layer and core deformation will be the same which will preclude
first order stresses that may lead to cracking [53].
31
Figure 18: Calculation of VS/VC Value to quantify quench uniformity [53]
3.7. Castrol Index
Deck and Demay have developed the Castrol Index (CI), a method of calculating the so-called
―hardening power‖ of a quench bath based on cooling curves obtained with either a 16 mm
dia x 48 mm cylindrical AFNOR NFT 60178 silver probe or the Wolfson probe shown in Fig.
19. Which was made upon the ASTM D6200, ISO 9950, D6482 or D6549 and Chinese
National Standard JB/T 7951—2004 standards [54].
32
Figure 19: Wolfson Inconel 600 probe [55]
bathTT
VKCI
max
max
(8)
Where:
Vmax: is the maximum cooling rate (°C/s).
Tmax: is the temperature (°C).
Tbath: is the quench bath temperature (°C).
K‘: is probe-dependent equipment constant.
The equipment constant is calculated from the CI equation using cooling curve data for either
a neutral grade 150 pure mineral oil as a reference for cold quenching oil and 600 neutral oil
as the reference for hot oil. By convention, a CI value of 12 is assigned for a neutral grade
150 pure mineral oil and a CI value 11.5 for 600 neutral oil. A comparison of the K‘ value
calculated in this way for cooling curves obtained on a neutral grade 150 pure mineral oil
using a silver and the Wolfson probe [54].
33
3.8. Heat Transfer Coefficient Correlation
The Grossman value (H) discussed earlier H=α/2λ is equal to the interfacial heat transfer
coefficient divided by 2 times the thermal conductivity. When the H-value is multiplied by
the diameter of the body (D), the product corresponds to the dimensionless Biot number (Bi)
which relates to the resistance to heat transfer both at the surface and inside of a body:
RBHD i
(9)
The physical significance of Biot number can be understood by considering the heat flow from
a hot metal to the quenchant during the cooling process. There are factors inhibiting heat
flow:
Heat flow from the metal to the surface.
Heat flow from the surface into the quenchant.
This equation means that heat transfer is proportional to section size (thickness) of the metal
being quenched and that the heat transfer coefficient at the interface between the cooling
metal and the quenchant is inversely proportional to the thermal conductivity of the metal.
Rose showed that cooling rate curves versus temperature completely and accurate describe the
cooling capacity of quenchants. However, measured cooling rates also depend on the size,
shape and thermal properties of the steel. To quantify the cooling properties of a quenchant
that are independent of the testing conditions, Rose proposed deriving the heat transfer
coefficient from the cooling rate data obtained from cooling curve analysis [56]. Rose made
the assumption that the temperature being measured within the probe used to obtain the
cooling curves (in this case a spherical silver probe) is equivalent to the interfacial
temperature at the probe surface/quenchant interface which could be described by the
equation:
ATTdt
dQcp
(10)
Where:
α: is the heat transfer coefficient.
TP: is the temperature of the probe.
TC: is the temperature of the quenchant.
A: is the surface area of the probe.
Q: is the amount of heat transferred and is assumed equal to:
34
cGTQ p
(11)
Where:
G: is the mass of the probe.
C: is the specific heat capacity of the probe material.
Rose‘s solution for the heat transfer coefficient (α):
ATT
cG
dt
dT
cp
p
(12)
Using this simplified calculation for α, Rose constructed Fig. 20, which illustrates the cooling
capacities of various quenchants obtained from heat transfer coefficients at 500 °C, which was
the temperature of the fastest transformation rate for most grades of steel available at that time
[56]. A 20 mm silver probe was used by Rose whose cooling behaviour corresponds to
approximately 10 mm section of steel. Therefore, Fig. 20 shows a critical cooling rate which
will just completely harden 10 mm of steel. However, as illustrated in Fig. 20 Rose‘s
methodology, while relatively simple to perform, does not account for the varying heat
transfer coefficient occurring on the steel surface throughout the quenching process.
35
Figure 20: Cooling power of various quenchants and heat transfer coefficients at a probe
temperature of 500 °C [56]
To more accurately relate the Biot number to size and shape, the generalised Biot criterion BiV
is calculated from:
V
SK
VL
VBiv
(13)
Where:
L: characteristic length, which is commonly defined as the surface area of the body (S)
divided by the volume of the body (V)
K: Kondratyev form coefficient (shape factor). A limited number of these values
is provided in Table 2 [57].
36
Table 2: Equations for calculation of Kondratyev shape factor (K) for simple shapes
Shape of body K S/V
Parallelepiped with sides L1, L2, L3 (L12 + L22 + L32) / π2 2(L1-1 + L2-1 + L3-1)
Cylinder of infinite size with height Z (5.783R-2 + 9.87 Z-2)-1 2(R-1 + Z-1)
Sphere R2/π2 3/R
The generalized Biot criterion (BiV) is related to the temperature difference within the probe
and the cooling metal surface to the quenchant by the dimensionless Kondratyev number (Kn)
There is universal correlation between Kn. and generalized Biot number BiV and it is useful for
any configuration of steel part. The Kondratyev number may be calculated as follows or it
may be obtained from Table 3 [58, 59].
21
2 1437.1
vv
vvn
BiBi
BiBiK
(14)
Ψ: is the temperature field non-uniformity criterion which is equal to:
mv
msf
TT
TT
(15)
Where:
Tsf: is the average temperature of the surface of the component being quenched,
Tm: is the temperature of the quenchant
TV: is the average temperature over the volume of the component.
The value ψ can also be defined in terms of the generalised Biot criterion BiV:
21
22 1437.1
1
vv BiBi
(16)
37
Table 3: Summary of generalized Biot numbers, ψ-values, and Kondratyev numbers (Kn)
BiV ψ Kn f(BiV) 1/ψ (1/ψ )-BiV
0.00 1.000 0.000 1.000 1.000 1.00
0.01 0.993 0.010 0.005 1.010 1.00
0.10 0.931 0.093 0.040 1.074 0.97
0.20 0.868 0.174 0.070 1.152 0.95
0.30 0.811 0.243 0.092 1.23 0.93
0.40 0.759 0.304 0.11 1.32 0.92
0.50 0.713 0.356 0.124 1.40 0.90
0.60 0.671 0.403 0.135 1.49 0.89
0.70 0.633 0.443 0.146 1.58 0.88
0.80 0.599 0.479 0.154 1.67 0.87
0.90 0.568 0.511 0.160 1.76 0.86
1.00 0.539 0.539 0.170 1.85 0.85
1.50 0.430 0.646 0.190 2.32 0.82
2.00 0.386 0.712 0.210 2.81 0.81
2.50 0.304 0.760 0.213 3.29 0.79
3.00 0.264 0.792 0.220 3.79 0.76
4.00 0.210 0.839 0.230 4.76 0.76
4.50 0.190 0.854 0.231 5.26 0.76
5.00 0.174 0.869 0.233 5.75 0.76
6.00 0.148 0.889 0.236 6.76 0.76
7.00 0.129 0.903 0.240 7.75 0.75
9.00 0.1026 0.929 0.240 9.75 0.75
10.00 0.0931 0.951 0.240 10.74 0.74
20.00 0.0482 0.965 0.240 20.75 0.75
50.00 0.0197 0.986 0.240 50.76 0.76
100.00 0.0099 0.993 0.240 100.30 0.76
∞ 0.0000 1.000 0.240 ----- -----
38
Kobasko [59], later Fernandes and Prabhu [60] used these relationships to calculate the heat
transfer coefficient from the two temperatures (T1 and T2) corresponding to the times (t1 and
t2) taken from quenchant cooling rate curve. The cooling rate (m) is then calculated from:
12
21 )ln()ln(
tt
TTTTm mm
(17)
The Kondratyev number (Kn) is calculated from the cooling rate (m):
KmK n
(18)
For a cylindrical probe, the Kondratyev form factor (K) is R2/5.783. The heat transfer
coefficient for film boiling (αFB) is calculated from:
AK
VBiv
FB
(19)
In addition to simplified methods for the calculation of heat transfer coefficient such as those
described here, one may also use inverse analysis such as that described by Beck [61]. This
methodology is commonly performed on cooling curve data [62]. And have a number of
advantages over those methods described above since surface heat flux can be calculated from
known positions of the heated probe [63].
3.9. Critical Heat Flux Density
The total amount of heat transferred to the quenchant is denoted by the symbol (Q). When
heat is released from the hot body to the cooler surroundings, Q is negative. The heat flow per
unit time (heat transfer rate) is measured in watts and is quantitatively defined as:
(20)
Heat flux is defined as the heat transfer rate per unit of cross-sectional area (q) is measured as
watts per square meter and is determined experimentally by measuring the temperature
difference over a material with a known thermal conductivity. This is analogous to
measurement of an electric current by the voltage drop over a known resistor which possesses
both magnitude and direction.
39
It is being increasingly recognized that the determination of critical heat flux densities are a
vitally important means of characterizing the overall quenching process [64, 65]. As discussed
throughout this thesis, there are three modes of heat transfer commonly observed during
quenching and all three are related to the first critical heat flux density (qcr1) which is not
observed by conventional cooling curve analysis. Upon immersion, the initial heat flux (q)
may be in different ranges relative to qcr1 [65]:
The first case, q >> qcr1 and full-film boiling will occur.
The second case, q ≈ qcr1 and transition boiling occur.
The third case, q << qcr1 and nucleate boiling occurs without full-film boiling. The
value of q can be less qcr1 since temperature gradient at the beginning of nucleate
boiling may be less than the temperature for the qcr1 transition range.
Initial heat flux density (q) can be calculated by using CFD modelling [66]. Or by solving
hyperbolic equation [67]. Since all three cooling mechanisms may potentially occur on a
surface simultaneously, for a given heat transfer coefficient, only by determining the critical
heat flux densities is it possible to fully assess the potential for uniform heat transfer on the
metal surface during the quenching process.
Kobasko reports that heat flux (qcr2) can be calculated from the following equation using
cooling rate data obtained from a 20 mm spherical silver probe [66]:
vS
VCq pcr 2
(21)
Where:
CP: specific heat capacity of the silver probe.
ρ: density of the silver probe.
V: volume of the probe.
S: surface area of the probe.
: average cooling rate during the transition from film boiling to nucleate boiling
The value for qcr2 is determined from the minimum value of q and then qcr1 is calculated from
[66]:
2.0
21
crcr
(22)
Using the Liscic-Nanmac thermal gradient probe shown in Fig 27 [69] calculated qcr2 from
[65]:
40
(23)
Where:
αFB: heat transfer coefficient for the film boiling stage.
Ttrans: surface temperature of the Liscic-Nanmac probe at the time of transition from
the film boiling stage to the nucleate boiling stage.
TS: the saturation temperature (boiling point) of the quenchant. (Saturation
temperature is the temperature at a given pressure where a liquid boils into its vapour
phase. Saturation means that the liquid is saturated with thermal energy and any
additional thermal energy will result in a phase transition.)
Once qcr2 is determined, the value for qcr1 is then easily obtained from qcr2/qcr1 = 0.2.
Figure 21: Schematic illustrations of The Liscic-Nanmac probe and Nanmac thermocouple
arrangement [68]
(A) is the Liscic-Nanmac probe used for measurement of surface temperature gradients during
quenching and (B) is Nanmac thermocouple arrangement for surface temperature
measurement.
41
3.10. Another quenchant efficiency parameters (HP, TCP, TVP)
Bodin and Segerberg developed HP formulas to rank the curing performance of polymer and
oil quenchant. Segerberg expressed HP correlations by regression analysis of the recorded
cooling curves of different quenchants with the equipment defined in ISO 9950 and the
surface hardness values of non-alloy steel specimens hardened in the same quenchant. The
correlations include the characteristic points of the cooling rate curve. For polymer and oil-
based quenchant, the formulas are as follows [59] [60].
HPolaj = 91.5 + 1.34 TVP + 10.88 CRmax – 3.85 Tcp
(24)
HPpolimer = 3.45 CRf + 12.3 CRm – 3.85 Tcp
(25)
Where:
CRf: is the cooling rate on the cooling curve at the temperature (550 ° C) determined
by the 'peak' of the ferrite transformation curve C on the isothermal transformation
diagram of 0,45% carbon-containing non-alloy steel.
CRm: is the cooling rate for the initial temperature (325 ° C) of the martensitic
conversion of non-alloy steel containing 0,45% carbon.
Tcp: apparent transition temperature between boiling and convection phase.
Tvp: apparent transition temperature steam and boiling stage.
CRmax: is the maximum cooling rate.
42
4. Used Measurement Technique and Quenchant
The test was performed using a standard method, the aim of which is to record the heat
removal of the medium and to characterize its cooling capacity.
The device background and use of the qualification method are set out in the following
standards:
ISO 9950: 1995, "Industrial quenching oils - Determination of cooling characteristics -
Nickel-alloy test method"
ASTM D 6200-01, ―Standard Test Method for Determination of Cooling
Characteristics of Quenching Oils by Cooling Curve Analysis‖
ASTM D 6482-06, ―Standard Test Method for Determination of Cooling
Characteristics of Aqueous Polymer Quenchants by Cooling Curve Analysis with
Agitation (Tensi Method)‖
A method according to ISO 9950: 1995 based on a cooling curve analysis was used to test the
hardening fluid. The thermocouple used in the measurement series:
ø12.5x400mm size
Inconel 600 material quality
850 ° C Heated
r=0 mm, l=300 mm placed in position
For each series of measurements, the thermocouple sample is immersed in a liquid refrigerant
until the data is stored at a sampling frequency.
43
Figure 22: Equipment used in the measurements
The registered data:
The heat transfer coefficient.
The temperature characteristic of the cooling kinetics.
The time characteristic of the cooling kinetics.
The values of the proportion of the tissue element.
Values of hardness distribution.
The Fig. 22 shows the measuring equipment and the assembly of the equipment.
4.1. Test procedure
I first heated the thermocouple rod to 850 ° C in a resistance heating furnace specially
designed for this task and then immersed it in room temperature (since the measurements
were made in the summer the media were around 28 ° C) 1.5-litre refrigerant, which was at
the lowest level. Circulating, thus facilitating the cooling process. Using SQ Integra target
software, the following components are required to perform the test.
One minute after immersion, we recorded the temperature data, which were evaluated with
the IVF SmartQuench software and saved on a computer. Seven cooling characteristics were
determined and used to characterize the concentration dependence of the polymer additive.
44
I examined four concentrations and performed three parallel measurements for each
concentration to make the results statistically process able. The concentrations tested were as
follows:
Hand meter
Specimen
Oven
Mixer
Data transmission unit
1.5 litre test crucible
The measurement result may be affected if the surface of the specimen is exposed to dirt
during the measurement, so the surface of the specimen must be cleaned before the start of
each heating cycle.
Beside of the several parameters can be used to describe the efficiency of the queinchants, in
this thesis seven parameters were used.
Table 4: The used parameters to describe the efficiency of the quenchants
Notation Unit of measure Name
CR300 [°/s] Cooling rate at 300 ° C
CRmax [°/s] Maximum cooling rate
TCrmax [°C] Temperature for maximum
cooling rate
tCrmax [s] The time corresponding to
the maximum cooling rate
TCP [°C] Apparent transition
temperature between boiling
and convection phase
TVP [°C] Apparent transition
temperature between steam
and boiling stage
HP [-] Refrigerant training
performance
45
4.2. The Studied Quenchant
Figure 23: Houghton, Aqua-Quench 320 AQ EC/WE
4.3. Aqua-Quench
An oil-free, non-flammable, water soluble synthetic quenchant for the hardening of ferrous
and non-ferrous alloys
4.3.1 Description
Aqua-quench is an oil, non-flammable, water soluble. Synthetic quenchant for the hardening
of ferrous alloys and solution treatment of aluminium alloys.
4.3.2 Application
Aqua-quench is a versatile water soluble polymer quenchant for use in both ferrous and
aluminium alloys.
46
Aqua-quench is used for the hardening of ferrous forgings, castings and stamping. It can be
used in open quench tanks, continuous furnaces and selective surface hardening equipment.
The flexibility of quenching speed and uniform heat transfer characteristics of Aqua-Quench
eliminates many of the disadvantages of water or mineral oil based quenchants.
Aqua-Quench can also be used for the solution treatment of aluminium alloys. Aqua-Quench
complies with aerospace material specification AMS 3025.
4.3.3 Features and Benefits
Uniform quenching eliminates steam pockets and formation of soft spots associated
with water quenching.
Flexibility of quenching speed helps to minimize distortion and provide excellent
dimensional control.
Water based quenchant to eliminate fire hazard and smoke associated with quenching
oils along with cleaner parts.
Health and Safety
47
5. Measurement Results
The main interest of this thesis to investigate the effect of the polymer concentration on the
efficiency parameters of polymer added quenchants. During an experimental series, the
cooling strengths for 5 different concentrations (0%, 5%, 10%, 15%, and 20%) of polymer
additive quenchant were investigated. For each concentration, 5 different measurements were
performed. All the measurements were made by the supervision of Dr. László Kuzsella.
5.1. Results of Concentration Change
Based on the recorded cooling curves of the measurements, seven efficiency parameters of the
five quenchants were calculated. These were the followings, CR300, CRmax, TCRmax, tCRmax,
TCP, TVP, HP. In each case of every concentrations the cooling curve measurements were
repeated five times. Therefore statistical evaluation can be made on the results. The following
figures contains bar diagrams, where the high of the bars shows the average and on the top the
stick shows (error bars) the deviation from the average.
Figure 24: Cooling rate at 300 °C as a function of concentration change
The results of the cooling rate at 300 °C as a function of concentration change are shown in
Fig. 24.
0 5 10 15 20
0
20
40
60
80
100
120
CR
30
0,
°/s
Concentration, V%
48
Figure 25: The maximum cooling rate as a function of concentration change
As the concentration increases, the cooling rate at 300 °C decreases significantly. The results
of the maximum cooling rate as a function of the change in concentration are shown in Fig.
25.
0 5 10 15 20
0
50
100
150
200
250
C
Rm
ax,
°/s
Concentration, V%
49
Figure 26: Temperature for maximum cooling rate
Figure 27: Time corresponding to the maximum cooling rate
0 5 10 15 20
300
350
400
450
500
550
600
650
700
750
TC
rma
x,
°C
Concentration, V%
0 5 10 15 20
4
6
8
10
12
14
16
t Cr
max, s
Concentration, V%
50
Based on Fig. 62 and Fig. 62, both the maximum cooling rate and the associated temperature
show an increase with increasing concentration.
Figure 28: Apparent transition temperature between boiling and convection phase
And Fig. 28 shows that the higher the temperature in the convection stage gets the higher the
concentration reaches, so when temperature reaches TCP=450 °C the concentration raises up
to V=20%.
0 5 10 15 20
0
100
200
300
400
500
600
TC
P,
°C
Concentration, V%
51
Figure 29: Apparent transition temperature between steam and boiling stage
However, it is not the same case in the boiling stage, where the temperature keeps the same
value with the increasing of the concentration till one point that after the concentration starts
to decrease, as Fig. 29 suggests
0 5 10 15 20
0
200
400
600
800
1000
TV
P,
°C
Concentration, V%
52
Figure 30: Refrigerant training performance
In full accordance with the findings in the literature, as the concentration increases, the
hardening performance of the refrigerant decreases, as shown next in Fig. 33.
0 5 10 15 20
400
600
800
1000
1200
1400
1600
1800
2000
HP
, -
Concentration, V%
53
The measurement results are tabulated in Table 4.
Table 5: Effect of increasing concentration on characteristics affecting cooling strength
Concentration changing CR300
⁄ CRmax
⁄ TCRmax TCRmax S TCP TVP HP -
0%
Measurement 1 91.00 212.27 606.81 5.20 125.77 844.85 1891.16
Measurement 2 97.97 230.45 623.97 421 121.51 849.66 1929.28
Measurement 3 100.25 240.78 645.15 3.96 143.45 850.59 1972.94
Measurement 4 99.92 217.51 618.13 4.34 119.90 849.60 1939.39
Measurement 5 100.15 212.78 634.73 4.71 122.26 845.72 1911.66
Average 97.86 225.25 625.56 4.48 126.58 848.08 1914.49
Scattering 3.95 12.87 15.13 0.48 9.67 2.60 57.77
REL. Scattering % 4.03 5.71 2.42 10.78 7.64 0.31 3.02
5%
Measurement 1 97.28 217.02 631.26 3.96 149.88 850.61 1867.54
Measurement 2 96.75 214.25 641.12 4.34 15.23 849.88 1852.61
Measurement 3 95.08 218.62 651.55 3.72 129.99 850.90 1844.64
Measurement 4 94.99 215.84 654.08 4.09 129.64 850.31 1863.38
Measurement 5 93.88 216.28 647.63 4.46 130.13 849.95 1821.89
Average 95.60 216.40 649.13 4.11 137.97 850.33 1844.61
Scattering 1.39 1.60 5.03 0.30 11.03 0.43 17.13
REL. Scattering % 1.46 0.74 0.78 7.19 7.99 0.05 0.93
10%
Measurement 1 99.26 182.51 621.12 6.94 152.15 807.28 1807.80
Measurement 2 103.74 208.51 677.48 3.59 143.12 855.31 1913.89
Measurement 3 93.76 206.94 649.53 3.84 156.45 851.26 1789.75
Measurement 4 94.36 204.84 661.37 4.09 156.43 850.21 1791.86
Measurement 5 90.52 201.74 651.03 4.56 134.03 849.83 1728.73
Average 96.33 200.91 652.11 4.58 148.44 842.78 1806.41
Scattering 5.19 10.60 20.60 1.36 9.72 19.96 67.23
REL. Scattering % 3.39 5.27 3.16 29.57 6.55 2.37 3.72
15%
Measurement 1 65.43 140.56 611.05 10.29 193.15 743.89 1285.27
Measurement 2 69.77 188.70 704.62 3.47 495.26 855.06 1322.44
Measurement 3 64.23 183.75 701.63 3.59 475.09 851.47 1227.57
Measurement 4 64.74 183.98 687.04 4.21 466.47 849.50 1244.76
Measurement 5 65.45 181.89 694.44 4.21 481.23 849.47 1209.76
Average 65.92 175.78 679.76 5.15 422.26 829.88 1257.96
Scattering 2.21 19.85 39.01 2.89 128.50 48.12 45.62
REL. Scattering % 3.35 11.29 5.74 56.10 30.43 5.80 3.63
20%
Measurement 1 64.28 135.04 666.99 7.94 523.04 787.59 992.11
Measurement 2 71.22 178.20 722.51 3.34 538.07 854.02 1060.92
Measurement 3 62.18 94.39 593.45 13.15 508.95 696.65 1024.94
Measurement 4 58.29 72.73 408.50 17.12 184.08 496.58 1006.29
Measurement 5 63.24 95.63 610.39 12.90 507.46 701.07 981.06
Average 63.84 119.20 600.37 1089 452.32 707.18 1013.60
Scattering 4.71 37.43 118.66 5.33 150.45 134.66 31.39
REL. Scattering % 7.37 31.40 31.40 48.95 33.26 19.04 3.10
54
6. Conclusion
The quenching process involves two fundamental branches of science; metallurgy and heat
transfer. Metallurgical properties of the metal are dependent on specified heating temperatures
and cooling times and rates which are provided by the quenchant which acts as the heat
transfer media. Characterization of quenchants by cooling curve analysis is currently the
method of choice for characterization of the heat transfer properties of a quenchant. An
introduction to cooling curve data acquisition, and cooling curve shape and cooling
mechanisms illustrates the importance of adequately quantifying cooling curve behaviour has
been provided. Various approaches that have been used to quantify not only the temperature-
time behaviour but also a general quantitative assessment of relative quench severity
exhibited by the quenching media being analysed are described the reasons for their
utilization are discussed.
During the series of measurements, I examined the effect of the change of the concentration
on the refrigerant and the characteristics influencing the cooling strength and found an inverse
relationship between the concentration and the cooling rate at certain temperature. The same
thing applies for the relationship between the concentration and, the hardening performance of
the quenchant.
55
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