Quenching Fundamentals Quenching of Aluminum Importance of Uniform Surface Rewetting

8/3/2019 Quenching Fundamentals Quenching of Aluminum Importance of Uniform Surface Rewetting

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QUENCHING FUNDAMENTALS

QUENCHING OF ALUMINUM: IMPORTANCE OF UNIFORM

SURFACE REWETTING

By

H.M. Tensi1, P. Stitzelberger-Jakob

1and G.E. Totten

2

1. Technical University of Munich, Munich, Germany2. Union Carbide Corporation, Tarrytown, NY USA

INTRODUCTION

The cooling process of age-hardenable aluminum alloys are affected by material

properties such as: strength, ductility, and thermal stresses. Thermal stresses are

minimized by reducing the cooling rate from the solution heat treatment temperature.

However, if the cooling rate is too slow, undesirable alloy segregation at the grain

boundaries will result. Conversely, if the cooling rate is too fast an increased tendency for

distortion may result. [1,2]

When water or water-soluble polymers are used to quench age-hardenable aluminum

alloys, heat transfer from the workpiece to the quenchant is determined by three boiling

phases: film boiling, nucleate boiling and convective heat transfer. Film boiling occurs

upon initial immersion. This is a slow cooling process because the hot surface is

surrounded by a vapor blanket. As the part cools, the vapor blanket collapses and

nucleate boiling results. The transition temperature between film boiling and nucleate

boiling is called the Leidenfrost temperature. Heat transfer is fastest during nucleate

boiling. The heat transfer coefficient,, for nucleate boiling is about 100 times the value


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of for film boiling. [8,9] When the part has cooled below the boiling point of the

quenchant, slow cooling occurs by a convective heat transfer process.

Since aluminum solution heat treatment temperatures are significantly higher than the

Leidenfrost temperature, film boiling is to be expected initially when quenching into

water or aqueous polymers. [3,4] If the surface temperature at any given point on the

workpiece is less than the Leidenfrost temperature, stable wetting and nucleate boiling

will occur at that point. [5,6,7]

Figure 1a illustrates a typical wetting sequence during cooling of a cylindrical specimen

quenched in distilled water at 40C. The simultaneous presence of different boiling

phases with the heat transfer coefficient of one surface region greater than 100 times the

other causes extremely uneven cooling of the workpiece.

Figure 1b illustrates the rewetting sequence of a cylindrical aluminum (AlMg 5, 15 mm

dia x 45 mm) quenched into distilled water at 80C with the percentage of the wetted

surface ( A = A(t) ) and with the change in temperature at the geometric center of the

cylindrical workpiece ( T = T(t) ) during cooling.

The consequences of uneven cooling include:

Regions where the film boiling persists longer experience greater separation of the

super-saturated mixed crystal structure than regions of faster cooling (low tS). During

age-hardening, these regions (high tS) experience a smaller increase in hardness and


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also exhibit greater potential for intergranular corrosion than neighboring regions

with faster fooling.

Volume regions where film boiling persists exhibit a much lower higher-temperature

yield point during the cooling process than regions with shorter film-boiling phases.

The non-uniformity of this process results in significant plastic deformation due to

increased thermal stresses and increased distortion.

Therefore, determination and characterization of the cooling processes involved in

quenching is critically important, especially with water quenching. To satisfy this need, a

process to quantitatively measure the rewetting process of standard quenched aluminum

probes was developed. [10,11]

DISCUSSION

A. Surface Rewetting Measurements

Temperature variation with time during the cooling process was measured by a

thermocouple inserted to the geometric center of a cylindrical probe. Rewetting

kinematics was determined by measuring the change in conductance between the probe

and a counterelectrode during the transition from film boiling to nucleate boiling as

shown in Figure 2.

If the heated probe is completely surrounded by a vapor blanket, the electrical resistance

between the probe surface and the counterelectrode is high because of the insulating

effect of the vapor blanket. As the vapor blanket collapses, the quenching fluid wets the


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probe causing the electrical resistance between the probe surface and the counterelectrode

to decrease and the conductance to increase. Using calibration curves, the percentage of

the surface (A) from the conductance (G) and the wetting kinematics ( dA/dt) may be

determined from the change in conductance with respect to time (dG/dt).

B. Rewetting Kinematics from Quenched Aluminum Bodies

Cooling behavior of aluminum during quenching is affected by: quenchant composition

(type and concentration of a polymer quenchant), bath temperature, temperature of the

aluminum being quenched, and surface condition of the aluminum.

Aluminum rewetting behavior when quenched into distilled water is strongly affected by

bath temperatures above 40C. Figure 3 shows that the temperatures for the beginning

of the rewetting process (TS) and for the end of the process (Tf) and the elapsed time

between tf and tS within which the two boiling phases, film boiling and nucleate boiling,

appear simultaneously, as a function of bath temperature. Up to bath temperatures of

40C, the probe surface wets very quickly (< 1 s ) and the rewetting process always

began at the lower surface. At temperatures above 60C, the rewetting time, the time

when the entire probe surface is wetted with the quenchant (tf tS) increases sharply with

increasing bath temperature and the temperature distribution in the probe becomes

extremely uneven. This means that with increasing bath temperature, the precipitation

kinetics become increasingly non-uniform in the longitudinal direction. For example, if

aluminum castings are quenched in water at 60C to reduce distortion and residual

stresses, the opposite results will be achieved. [13,14]


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These difficulties can be avoided by using water-soluble polymers as aluminum

quenchants. [15 - 18] These polymers are usually used at bath temperatures of 20-35C.

The quenching characteristics of these polymers in this temperature range are practically

independent of bath temperature. [18] Aqueous polymer quenchants provide a less

severe quench than cold water and the quench severity can be varied by varying

quenchant concentration. [17-20]

Figure 4 shows the effect of polymer concentration on the wetting behavior of a

cylindrical ALMgSiCu probe quenched in a commercial polymer quenchant. The time

until unitial wetting of the probe surface increases with polymer concentration which

means that film boiling persists for longer periods of time with increasing polymer

quenchant concentration. This behavior appears in Figure 4 as a decline of the starting

temperature for wetting (TS) measured at the geometric center of the probe. The rewetting

time increases only about 2 seconds with increasing quenchant concentration.

The cooling time from the solution treating temperature where the mixed crystal phase

begins is very important for age-hardened Al alloys. [2] Therefore, the effect of water

temperature and the effect of polymer quenchant concentration on cooling time was

compared. Figure 5 shows that the cooling time varies between 2 s and 16 s in both cases;

however, the effect achieved by varying the polymer quenchant concentration was more

favorable. [Note: The temperature was measured in the center of the probe which reveals

nothing about rewetting behavior or about the temperature distribution within the probe.]


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In addition to bath temperature and polymer concentration, agitation is also an important

variable which will be reported in a subsequent report. [20-21].

The aluminum surface condition during the quenching process also exhibits a large

effect on quenching performance. Surface characteristics can be greatly altered by

various factors such as variations in the duration of solution treatment. [22] Figure 6

shows the cooling and rewetting behavior of ALMgSiCu cylindrical probes that were

solution-treated for variable lengths of time then quenched in distilled water (TB = 25C).

With increasing duration of the solution treatment time from 1 minute to 180 minutes

which results in corresponding increases in the depth of the surface oxide layer, the

conductance-time curves and temperature-time curves clearly show the retardation of

cooling, i.e., the prolongation of wetting time with increasing duration of heat treatment

(Figure 6a). The cooling rate decreases with increasing duration of solution heat

treatment (Figure 6b).

C. Comparison of Quenching Characteristics of Silver and Aluminum Probes

Silver probes are often used to evaluate quench severity exhibited by different

quenchants. [23,24] Because of the similarity of the thermal characteristics of silver and

aluminum and because of the significantly lower oxidation tendency for silver relative to

aluminum, the cooling behavior of AlMgSiCu and a silver (99.5 %) probe was

compared.


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Thermal conductivity (a) and specific heat capacity of various materials are provided in

Table 1. Thermal conductivity, a, is a measure of the rate of propagation of

temperature change in a body and is related to the specific heat capacity:

=

Cp

Where: Cp is the specific heat capacity, is the thermal conductivity and is the density.

Figure 7 shows the cooling curves recorded during quenching of an aluminum

(AlMgSiCu) and a silver specimen (Ag 99.5) in a water-soluble polymer. The aqueous

polymer quenchant concentration was 10% by volume and the bath temperature was

25C. The temperature of both materials when quenched was 520C. Both probes were

cleaned with 600 grit abrasive paper before each test.

The polymer film surrounding the probe surface ruptured simultaneously around the

entire surface, also called explosive rewetting, for both probes and the rewetting times

MaterialThermal Conductivity

(m2s

-1)

Specific Heat capacity

(kJ k-1

K-1

)

Aluminum 99.5 95 x 10-6 0.896

Silver 99.5 174 x 10-6 0.235

Nickel 14 x 10-6 0.448

Cr Ni Steel* 4 x 10-6 0.477

INCONEL 600** 4 x 10-6 0.465

* Austentic stainless steel SAE 30304

** Ni-based alloy

Thermal Conductivity and Specific Heat Capacity

for Different Materials

Table 1


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(tf tS) were extremely short. However, a stable film-boiling range lasting about 4 s was

observed for the silver probe which was not observed for the aluminum probe. The

centerline probe temperature was about 440C for silver and 500C for aluminum. The

reason that the rewetting of the silver occurs about 4 s later for the silver probe is the

greater oxidation resistance of silver. (The ratio of heats of formation of Ag2O2 and Al3O

is 0.05.) When the quenching temperature of the silver probe is increased to 800C,

considerable stabilization of the film-boiling occurs as observed in Figure 8a. Wetting

now starts at about 260C after 24 s compared to the start of wetting of the AlMgSiCu

probe after 1 s at about 500C.

The main reason for this stabilization of the film-boiling phase of the entire surface of the

silver probe is the reduction of the silver oxide at the higher temperature. The high-

temperature annealing of the silver probe removes the oxide and leaves a bare metal

surface, resulting in stabilization of film boiling during quenching, especially in water-

soluble polymers. Accordingly, the maximum cooling rate of the silver probe is not

reached until a centerline probe temperature of 200C as shown in Figure 8b.

If distilled water at room temperature is used as the quenchant instead of an aqueous

polymer, the silver and the AlMgSiCu probes show almost identical cooling behavior

with coinciding rewetting kinematics as shown in Figure 9. This means that the

quenching behavior determined in water with silver probes can be safely compared with

those obtained for aluminum probes. However, when polymer solutions are used, there

are clear differences, especially with respect to initial wetting.


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SUMMARY

Cooling rate and wetting behavior have very strong effects on the properties of quenched

aluminum alloys. Age-hardening and quench distortion are determined, to a large extent,

by the variations in the duration of the film boiling to nucleate boiling transition. The

rewetting process can be determined exactly by measuring conductance and temperature

during the quenching process. The duration of the film boiling and rewetting kinetics

during quenching of aluminum alloys can be systematically influenced by varying the

bath temperature when water is used as the quenchant or by varying the aqueous polymer

quenchant concentration when a polymer quenchant is used. Surface oxidation of

aluminum alloys and the behavior of silver oxide at high temperature also affect the

rewetting process. Therefore, the cooling behavior of silver and aluminum probes is

comparable only when both metals have comparable states of surface oxidation.

REFERENCES

1. W. Kster and G. Hofmann, The Effect of Quenching Rate on the Kinetics of ColdAge Hardening of an Aluminum-Zinc Alloy with 10% Zinc, Z. Metallknd., 1963,

Vol. 54, p. 570-575.

2. C.E. Bates, Selecting Quenchants to Maximize Tensile Properties and MinimizeDistortion in Aluminum Parts, J. Heat Treating, 1987, Vol. 5, p. 27-40.

3. O. Zach, Experimental Investigation of the Cause of the Variation of the LeidenfrostPoint, Institute of Nuclear Energetics, IKE 2-34, Stuttgart, 1976, 78 pages.

4. Y. Kikuchi, T. Hori, and J. Michiyoshi, Minimium Film Boiling Temperature forCooldown of Insulated Metals in a Saturated Liquid, Int. J. Heat Mass Transfer,

1985, Vol. 28, No. 6, p. 1105-1114.


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5. Th. Knzel, Effect of Rewetting on Allotropic Modification of Quenched MetalBodies, Ph.D. Thesis, Technical University of Munich, Munich, Germany, 138pages.

6. K.J. Baumeister and F.F. Simon, Leidenfrost Temperature Its Correlation for

Liquid Metals , Cryogens, Hydrocarbons and Water, J. Heat Transfer, 1973, p. 166-173.

7. R.E. Henry, A Generalized Correlation for the Minimum Point in Film Boiling, 14thNational Heat Transfer Conference, AlCHE-ASME, August, 1973, Atlanta, Georgia.

8. D. Hein, Model Concepts on Rewetting by Flooding, Ph.D. Thesis, TechnicalUniversity of Hanover, Hanover, Germany, 1980, 182 pages.

9. R. Jeschar and R. Maass, Determination of Heat Transfer During Quench Hardeningof Metals in Water, Gas Wrme Intern., 1985, Vol. 34, No. 9, p. 348-354.

10.W. German Patent No. 3,538,807.2, 1987.

11.H.M. Tensi, Th. Knzel, and P. Stitzelberger, Wetting Kinetics as an ImportantHardening Characteristic in Quenching:, Hterei-Tech. Mitt., 1987, Vol. 42, No. 3, p.

125-132.

12.Th. Knzel, H.M. Tensi and G. Welzel, Rewetting Rate The DecisiveCharacteristic of a Quenchant, Tagungsband 5

thIntern. Congress on Heat Treatment

of Materials, Budapest, 20-24 October 1986, p. 1806-1813.

13.H. Bomas, Quenching Rate of AlMgSi Alloys Affects the Strength Values,Maschinemarkt, 1982, Vol. 88, p. 1220-1222.

14.T. Croucher, Water Quenching Procedure for Aluminum Alloys, Heat Treating,1982, Vol. 14, No. 9, p. 18-19.

15.E.H. Burgdorf, Properties and Uses of Synthetic Quenching Solutions, Z. Wirtsch.Fertig., 1979, Vol. 74, p. 431-436.

16.H. Beitz, Uses and Limitations of Synthetic Aqueous Quenching Liquids inHardening Technology, Hrterei-Tech. Mitt., 1979, Vol. 34, No. 4, p. 180-188.

17.Totten, G.E., C.E. Bates and L.M. Jarvis, "Type I Quenchants for Aluminum HeatTreating",Heat Treat., 1991, December, p 16-19.

18.Bates, C.E. and Totten, G.E., "Procedure for Quenching Media Selection toMaximize Tensile Properties and Minimize Distortion in Aluminum - Alloy Parts",Heat Treat. of Metals, 1988,4, p. 89-98.


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19.H.M. Tensi and E. Steffen, Measuring the Quenching Effect of Liquid HardeningAgents on the Basis of Synthetics, Steel Research, 1985, Vol. 56, No. 9, p. 489-495.

20.H.M. Tensi and P. Stitzelberger-Jacob, Convection in Quenching Baths DifferentWays of Determining the Effects of Convection, Eingereicht bei Materials Science

and Technology, October, 1987.

21.H.M. Tensi, P. Stitzelberger-Jacob and Th. Knzel, Laboratory Test for Assessingthe Cooling Characteristics of Polymer Quenchants, Draft International Standard,Technical University of Munich, Munich, Germany, Inst. Of Material and Processing

Sciences, 1987, 28 pages.

22.K. Wefers, Properties and Characteristics of Surface Oxides on Aluminum Alloys,Aluminum, 1987, Vol. 57, p. 722-726.

23.J. Wnning and D. Leidtke, Tests for Determining Heat Flux Density During Steel

Quenching in Liquid Quenchants by the QTA Method, Hrterei-Tech. Mitt., 1983,Vol. 38, No. 4, p. 149-155.

24.M. Tagaya and I. Tamura, Studies on Quenchants. Part 8: Effects of the Dimensionsof the SilverSample on the Quenching Process, Hrterei-Tech. Mitt., 1963, Vol. 18,

No. 4, p. 63-76.

LIST OF FIGURES

Figure 1: a. Wetting sequence for a cylindrical specimen quenched in distilled water at

40C; b. Temperature TZ (measured in the center of the specimen) and wetted specimensurface as a function of time during cooling of an AlMg5 sample in distilled water at

80C.TZ and tZ: Temperature in the in the center of the specimen when it is immersed

into the quenchant

and the time of immersion.

TS and tS: Temperature in the specimen center and time when wetting starts.Tfand tf: Temperature in the specimen center and time when the wetting process

is concluded.

Figure 2: Schematic illustrating the measuring principle for determining the percentage

of wetted sample surface of rewetted immersion-cooled specimens.


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Figure 3: Dependence of temperatures TS and Tf (measured at the geometric center of

the probe) and wetting time ( tf tS ) on bath temperature during immersion cooling of acylindrical AlMgSiCu probe ( dia. 15 x 45 mm) in distilled water.

Figure 4 - Dependence of temperatures TS and Tf (measured at the geometric center of

the probe) and wetting time ( tf tS ) on polymer quenchant concentration duringimmersion cooling of a cylindrical AlMgSiCu probe ( dia. 15 x 45 mm) in a water-

soluble polymer at 25C.

Figure 5 Time required for cooling a cylindrical AlMgSiCu probe ( dia. 15 x 45 mm)

from solution treating temperature (temperature at which the probe was immersed) to

180C.; a. Cooling in distilled water at varying bath temperatures and b. Cooling in awater-soluble polymer quenchant at varying quenchant concentrations and 25C.

Figure 6: Cooling process of a cylindrical AlMgSiCu probe ( dia. 15 x 45 mm) annealedin air for different periods of time: quenchant = distilled water, 25C, a. changes in

temperatures and conductivity as a function of time; b. cooling rate as a function of thetemperature at the geometric center of the probe.

Figure 7: Comparison of the cooling processes of a cylindrical AlMgSiCu probe ( dia. 15

x 45 mm) with a silver probe of the same dimensions: probes quenched into a 10%

solution of a water-soluble polymer at 25C (temperatures at the geometric center of theprobe); solution treating temperature: 520C (AlMgSiCu probe); annealing temperature

520C for the Ag probe.

Figure 8: Comparison of the cooling processes of a cylindrical AlMgSiCu probe ( dia. 15

x 45 mm) with those of a silver probe; cooled into a 10% solution of a water-solublepolymer at 25C (temperatures recorded at the geometric center of the probe); solution

treating temperature is 520C for the AlMgSiCu probe; annealing temperature is 800C

for the silver probe; a. changes in temperature and conductivity as a function of time andb. cooling rate as a function of temperature.

Figure 9: Comparison of cooling processes of a cylindrical AlMgSiCu probe ( dia. 15 x

45 mm) with those of a silver probe; probes quenched into distilled water at 25 (probetemperatures recorded at the geometric center); solution treating temperature is 520C for

the AlMgSiCu probe; annealing temperature is 520C for the silver probe; a. changes in

temperature and conductivity as a function of time and b. cooling rate as a function oftemperature.


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Symbol Unit Meaning

A mm Specimen surface area

G S Electrical conductance

TBoC Bath temperature

TfoC

Temperature in the center of the specimen at the conclusion of the

wetting process (final temperature of the wetting)

TsoC

Temperature in the center of the specimen at the beginning of the

wetting process (starting temperature of the wetting)

TzoC Temperature in the center of the specimen

k/s Cooling rate

a m/s Temperature conductivity

Cp J/kg Specific heat capacity

tf s Time at the conclusion of the wetting process (final time of wetting)

ts sTime at the beginning of the wetting process (starting time of wetting

with respect to immersion of the specimen into the fluid)

w/m2/K Heat-transfer coefficient

w/m/K Thermal conductivity

kg/m3 Density

Definition of Symbols

T


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Figure 1


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Figure 2


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Figure 3


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Figure 4


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Figure 5


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Figure 6


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Figure 7


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Figure 8


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Figure 9

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Quenching Fundamentals Quenching of Aluminum Importance of Uniform Surface Rewetting

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