Advances in Polymer Quenching Technology.pdf

8/10/2019 Advances in Polymer Quenching Technology.pdf

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ADVANCES IN POLYMER QUENCHING TECHNOLOGY

G.E. Totten1, B. Liscic

2, N.I. Kobasko

3, S.W. Han

4and Y.H. Sun

1

1. Union Carbide Corporation, Tarrytown, NY

2. University of Zagreb, Zagreb, Croatia

3. National Thermophysics Institute, National Academy of Sciences, Kiev, Ukraine4. Kum Won Industrial Company Ltd., Chungnam, Korea

ABSTRACT

Polymer quenchants are being used increasingly in the

heat treating industry. One reason for this increase is the

continual engineering advancements that facilitate their

replacement of quench oils and also water. In this paper,three technologies utilizing polymer quenchants will be

reviewed. These include: inverse hardening, intensive

quenching and immersion time quenching technology

(ITQT), a computer automated time quenching

technology.

INTRODUCTION

With the exception of water, oil quenchants have

traditionally been the most commonly used quenching

media in the heat treating industry, particularly for crack-

sensitive steels such as bearing and tool steels.[1]

However, with increasing environmental, disposal, safetyand toxicological concerns, there is an increasing interest

in the potential use of alternative quenching technologies.

One of the most commonly considered alternatives to

quench oils are aqueous solutions of water-soluble

polymers such as poly(alkylene glycol). In addition to

providing substantially greater safety with respect to fire

and disposal, polymer quenchants have been shown to

provide more uniform heat removal during quenching

resulting in reduced thermal gradients and reduced

distortion.[1,2]

In this paper, three applications of recent polymerquenching technology research will be discussed: 1.)

inverse hardening, 2.) intensive quenching and 3.)

immersion time quenching technology (ITQT). Each

discussion will include a brief introduction, description of

the technology and an application example.

DISCUSSION

A. Inverse Hardening Technology

Shimizu and Tamura were the first to report on the inverse

hardening phenomenon where the hardness at the core ofa round bar was greater than the surface.[3] Inverse

hardening was reported to be dependent on steel

hardenability, cross-section size and the on quenching

conditions.

Loria showed that contrary to expectations from Jominy

hardenability data and CCT diagrams, low-alloy steel

plates of 25.4-31.8 mm thickness can be quenched after as

much as a 120 second delay and still produce the same or

higher hardness than by direct quenching.[4] This was

reported to be due to the incubation process reflected by

the displacement of the transformation start curves in a

CCT diagram when using air cooling times in the A3to A1temperature range prior to water quenching.[4]

At the same time Shimizu and Tamura provided a

theoretical explanation for inverse hardening. Distribution

after quenching. [5] They also found that the pearlitic

transformation behavior with cooling rates

discontinuously changed during continuous cooling was

different than that predicted from the usual CCT diagram.

This transformation was related to the incubation time

consumed before changing the cooling rate.

In the case of delayed quenching, some of the incubation

time is consumed at the surface, while not at the center.The incubation period at any given temperature is the time

until transformation starts (Z), while (X) is the incubation

period consumed before the discontinuous change of the

cooling rate has taken place. Figure 1A, which is the

schematic illustration of delayed quenching, shows that at

time (t1) and temperature T1(at point P) , a discontinuous

change of cooling rate occurred. Up to this moment, the

surface of the specimen has consumed a portion (X) of the


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total incubation time (Z), but the center has not because

the at the moment t1, the center had a temperature above

A1. Further cooling below the point (P) has proceeded

with substantially increased cooling rate. Starting for the

center from temperature A1 at zero time, further cooling

below the point (P) has proceeded with substantiallyincreased cooling rate changing the transformation start

curve as shown in Figure 1B. In this way, the cooling

curve for the center which does not intersect any pearlitic

region, results in higher hardness than the cooling curve

for the surface which has started from point (P) and

intersected a portion of the pearlitic region.

Figure 1 Schematic illustration showing how delayed

quenching results in inverse hardening.

Every delayed quenching process is accompanied by adiscontinuous change in heat transfer at the surface of the

quenched part and thus a discontinuous change in cooling

rate in the subsurface region. Relatively high

concentrations of aqueous poly(alkylene glycol) - PAG -

polymer solutions provide a predetermined and

controllable delay in quenching by variation of the

polymer concentration.[6] To understand the possibility of

a controllable delay upon immersion in an aqueous PAG

quenchant, it is necessary to understand the heat transfer

mechanism.

When austenitized steel is first immersed in an aqueous

polymer solution, it is surrounded by a vapor film which

is encapsulated by a mobil hydrated polymer film asshown in Figure 2. [1] Cooling is very slow in this region

(film boiling). When the thermal energy is sufficient to

rupture the film, heat transfer will be facilitated by a

nucleate boiling process. This is the point where

discontinuous heat transfer may occur. One of the most

critical regions with respect to facilitating inverse

hardening is the initial film boiling region. The film

boiling process is controlled by the thickness of the film

(polymer concentration), agitation rate, bath temperature

and the film strength of the polymer (molecular weight

and polymer type). To date, only one aqueous PAG

polymer quenchant has been observed to provide

controllable delayed quenching behavior. [6]

Figure 2- Illustration of the quenching mechanism of an

aqueous PAG quenchant solution.

Controllable delayed quenching (CDQ) means that there

is a predetermined change in heat transfer on the cooling

surface during quenching which influences the dynamics

of heat extraction. The heat extraction rate at the surface

of a cooling specimen and any point through the radius tothe core can be determined using the Liscic-Nanmac

probe and the Temperature Gradient Method. [1,7,8]

The time where the maximum heat flux density occurs

(tqmax) is the real measure of delayed quenching. Figure

3A shows the temperature-time profile when quenching

the Liscic-Nanmac probe in mineral oil at 20OC and no

agitation. Figure 3B shows the calculated heat flux

between 1.5 mm below the surface and the surface itself

versus time. In this case, the heat flux maximum occurred


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at 12 seconds after immersion and there is no indication of

a discontinuous change in cooling rate.

Figure 4 illustrates the temperature-time profile for the

same probe quenched in a 25% aqueous solution of a

PAG quenchant [6]. In this case, the maximum heat fluxdensity occurred at 70 seconds after immersion and a

discontinuous heat transfer condition was observed.

Figure 3- A. Cooling curve for the Liscic-Nanmac probe

(50 mm Dia x 200 mm cylinder); B. Heat flux density

between Ttand TS versus time when quenching in mineral

oil at 20OC, no agitation.

Figure 5 illustrates the normal hardness distribution

measured across the section of a 50 mm round bar of AISI

4140 quenched in the mineral oil (Figure 3) and the

aqueous PAG polymer quenchant (Figure 4). This figure

clearly illustrates the inverse hardening effect attainable

with the PAG quenchant.

Recently, a study was conducted to evaluate the potentialeffect of inverse hardening on the fatigue strength of AISI

4140. The results obtained, see Figure 6, showed that after

quenching and tempering, an increase in bending fatigue

life factor of approximately 7 and an impact energy

increase of approximately 7% relative to mineral oil

quenched specimens with a normal hardness distribution.

Figure 4 - A. Cooling curves measured by the Liscic-

Nanmac probe (50 mm Dia x 200 mm); B. Heat flux

density between Ttand TS versus time when quenching in

an aqueous PAG quenchant at 25%, 40OC and 0.8 m/s.

Figure 5 - Normal hardness distribution (1) after

quenching in mineral oil with no agitation at 20OC;

Inverse hardness distribution after quenching in an

aqueous PAG polymer solution at 25%, 40OC and 0.8 m/s

agitation.


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Figure 6- Test results of quenched and tempered AISI 4140 test specimens prepared with normal and inverse

hardness distribution.

B. Intensive Quenching

Intensive quenching refers to quenching processesutilizing very strong agitation, typically involving high-

pressure sprays. Intensive quenching is usually typified

by Grossmann quench severity factors (H) 5.0 as

shown in Table 1. [10,11]

Table 1

Classification of the Severity

of Different Quenchant Media

Cooling Rate

Classification

Quenchant Type and

Agitation

Quench

Severity (H)

Slow CoolingOil - None

Oil - Violent

0.2

0.7

Fast CoolingBrine - None

Brine - None

2.0

5.0

Intensive

Cooling

Brine or Aq. Polymer-

High Pressure Spray5

One of the oldest known examples of intensive

quenching was the production of the rear axles for theFord Model T automobile using AISI 1035

modified.[10] Today automotive axles, punches, and

other parts produced from shallow-hardening steel

continue to utilize intensive quenching (shell

hardening) processes. [10,12]

Intensive quenching produces high residual compressive

stresses in the surface of the part. [12,13] The reasons

for using intensive quenching processes include:[10]

1. To achieve maximum martensitic microstrucures in

the surface of a part.

2. To develop maximum surface compressive residual

stresses. [12,13]3. To achieve optimal quench uniformity.

4. To achieve maximum depth of hardening with a

given alloy content to reduce production costs and

to take advantage of available steels.

5. To insure maximum hardening in critical areas of

parts such as in the root fillets of a gear.

6. To minimize heat treating distortion and maximize

uniformity.

7. To obtain optimal physical properties such as

tortional fatigue. [10]


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Although intensive quenching processes are known, this

is not a widely applied technology. However, because of

the cost savings offered and potential improvement in

the steel properties using low alloy and carbon steels,

this technology is being used increasingly in thecountries of the Former Soviet Union (FSU). For

example in one bearing plant, the lifetime of dies like

those illustrated in Figure 7 which were constructed from

IIIX-15 steel (0.9501.10 C, 0.2-0.4 Mn, 0.15-0.35 Si,

1.30-1.65 Cr) was evaluated. Tests showed that

intensive quenching provided 1.5-2 times longer

lifetimes than obtained with an oil quenching process.

Figure 7 - Illustration of a die that was intensivelyquenched.

The most severe operating conditions and most

expensive processing conditions are encountered when

thermal processing high alloy tool steels. Typically, parts

produced from these steels are heated in salt pots and

quenched in brine or alkali. Any simplification of this

process is of great practical interest.

In 1990, Bulgaria liscensed an intensive quenching

process and began commercially producing punches and

pelt strippers. The shape and sizes of these parts isillustrated in Figure 8. The use of the intensive cooling

process increased the lifetime of the tools by 1.5 - 2

times. [15]

Figure 8- Illustration of punches manufactured using an

intensive quenching process.

Analogous results were obtained with dies used for

growing artificial diamonds (see Figure 9). In both cases,

aqueous solutions of calcium chlorise and magnesium

chloride containing corrosion inhibitors were used as the

quench media.

Figure 9- Illustration of a die used for growing artificial

diamonds.

Despite positive results, intensive quenching conducted

with a bischofite (chloride) solution has not been readily

accepted because of the associated corrosion problems.

However, it is now possible to obtain the same result

with aqueous PAG polymer quenchants with the

corrosion problems that accompany the use of chloride.


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For quenching high-speed steel, aqueous solutions of

PAG were used with intensive quenching. Tool life

increased significantly.[15] This shows that it is possible

to to combine intensive steel quenching methods and use

PAG quenchants.

Intensive quenching technologies in use in Ukraine and

Bulgaria, and Russia are described in Reference 16. The

use of aqueous chloride solutions as quenchants for

intensive quenching are described in Reference 17. The

use of PAG quenchants for intensive quenching is

currently being studied in more detail.

C. Immersion Time Quenching Technology - ITQT

When polymer quenchants are used with crack-sensitive

steels, it may be necessary to minimize cooling in the

martensitic transformation (Ms) region to minimize both

transformational and thermal stresses during quenching.For polymer quenchants, this may be accomplished by

increasing the polymer quenchant concentration and/or

increasing the bath temperature. Alternatively, quench

severity can be reduced by decreasingthe agitation rate.

Of these variables, only agitation rate can be controlled

during the time cycle of the quenching process.

Although time quenching is a well known and traditional

heat treating process, the ability to continuopusly control

agitation during the quenching process is a more recent

development.[18] Recently, commercial examples of

continuous agitation control using a computerized

Immersion Time Quenching System - ITQS. ManyITQS system variations , both batch and continuous, are

being used in Korea at the present time.[18,19,20] Two

examples will be provided here.

The first example to be discussed is the batch production

of roller bearings. A PAG quenchant (8% UCON

Quenchant A) has been successfully used to replace a

brine quench for the production of carburized (AISI P2)

roller bearings with the following dimensions: 44 mm

OD, 35 mm ID and 35 mm long. Each bearing weighed

454 grams. The total load size was 130 kg, including the

weight of the fixture. The batch ITQS tank contained a

total of 4700 liters of quenchant which was agitatedusing a single draft-tube which encased a 540 mm

impeller mixer driven by a 10 Hp motor.

Traditionally, the bearings were pack-carburized in a

compound of charcoal and barium carbonate at 950-

980OC for 24 hours in a pit furnace. The case depth was

2.5 mm. The load was reheated in a 5-10% sodium

cyanide salt bath at 830OC for 25 minutes at which time

they were brine quenched at ambient temperature.

For the batch ITQS procedure, the same carburizing

process was used. However, the reheating process was

conducted at 900OC for 60 minutes in a neutral

benzene/methanol (20/1.5 liters respectively)

atmosphere. The load was then quenched in 8% UCON

Quenchant A at 27-30O

C for 10 seconds at 1.2 m/sfollowed by a slower quenching step by reducing the

agitation rate to 0.2 m/s.

A comparison of the metallurgical results for quenching

processes is provided in Table 2.[19] The results show

that the ITQS polymer quench system will: provide a

substantial reduction in bearing distortion, reduce

grinding depth, eliminate the sodium cyanide effluent

problem and reduce heat treating cost including total

energy, processing chemicals and water contamination.

Table 2Comparison of Brine and Batch ITQS Polymer

Quench Processes for Roller Bearing

ResultBrine

Quench

Batch ITQS

Polymer Quench

Hardness (Rc) >60 >60

Case Depth (mm) 2.5 2.5

Distortion (mm) 0.25 0.067

Grinding Depth (mm) 0.5 0.2

Track links (AISI 15B37) such as those illustrated in

Figure 10 were prepared from a direct forge condition

using a continuous ITQS process. In this case the aquousPAG quenchant that was used was UCON Quenchant

E. at 10-12%. The bath temperature was 35-40OC and

the time on the conveyor in the first step (high agitation

rate) was 10 seconds. The target hardness was Rc 52-55.

The results obtanied are shown in Figure 11. In addition

to reduced cracking, more uniform microstructure and

substantial cost reduction was achieved.

Figure 10 - Track links quenched from direct forge

condition using a continuous ITQS system.


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Figure 11 - Hardness distribution for track links

produced by the continuous ITQS process.

CONCLUSIONS

Three advances in quenching technology, although not

all new, reported here provide the heat treater with

substantially broader possibilities with respect to the

quality and properties of the heat treated parts being

produced. All illustrate the importance of quenchant

agitation and control and all illustrate that polymer

quenchants are capable of providing properties that are

superior to those attainable with quench oil.

REFERENCES

1. G.E. Totten, C.E. Bates and N.A. Clinton,Handbook of Quenchants and Quenching

Technology, 1993, ASM International, Materials

Park, OH, p. 161-190.

2. H.M. Tensi, A. Stich and G.E. Totten,

Fundamentals of Quenching,Metal Heat

Treating,1995, Mar./Apr., p. 20-28.

3. N. Shimizu and I. Tamura, An Examinationof the

Relation Between Quench-Hardening Behavior of

Steel and Cooling Curve in Oil, Transactions ISIJ,

, 1978, 18, p. 445-450.

4. E.A. LoriaTransformation Behavior on Air Cooling

Steel in A3-A1 Temperature Range, Metals

Technology, 1977, October, p. 490-492.

5. N. Shimizu and I Tamura, Effect of Discontinuous

Change in Cooling Rate During Continuous Cooling

in Pearlite Transformation Behavior of Steel,

Transactions ISIJ, 1977, 17, p. 469-476.

6. This quenchant is UCON Quenchant E which is

marketed by Union Carbide Corporation, Danbury,

CT.

7. B. Liscic, S. Svaic and T. Filetin,

Workshop Designed System for Quenching

Intensity Evaluation and Calculation of Heat

Transfer Data, in Quenching and the

Control of Distortion, Ed. G.E. Totten,

1993, ASM International, Materials Park,

OH, p. 17-26.

8. B. Liscic, H.M. Tensi and W. Luty, Eds.,

Theory and Technology of Quenching,

1992, Springer-Verlay, Berlin, Germany, p.

243-246.

9. B. Liscic, V. Grubisic and G.E. Totten, Inverse

Hardness Distribution and Its Influence on

Mechanical Properties, in 2nd International

Conference on Quenching and the Control of

Distortion, Eds. G.E. Totten, M.A.H. Howes, S.J.

Sjostrom and K. Funatani, 1996, ASM International,

Materials Park, OH, p. 47-54.

10. R.F. Kern, Intensive Quenching,Heat Treating,

1986, September, p. 19-23.


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11. M. Daming, Intense Quenching Method for

Preventing Cracking, Heat Treatment and

Technology of Surface Coatings: New Processes

and Application Experience, Proc. of 7thInt.

Congress on Heat Treatment of Materials Vol. II,Dec. 11-14, 1990, p.62-71.

12. N.I. Kobasko, 10.4 Intensive Steel Quenching

Methods, in Theory and Technology of

Quenching, Eds. B. Liscic, H.M. Tensi and W.

Luty, 1992, Springer-Verlag, Berlin, p. 367-389

13. R. Liss, C. Massleon, and A. McCloskey, The

Development of Heat Treat Stresses and Their

Effect on the Fatigue Strength of Hardened Steel,

SAE Mid-Year Meeting, Chicago, May 1965.

14. N.I. Kobasko, Technological Aspects ofQuenching,Metallovedenie i

Thermicheskaya Obrabotka Metallov, 1991,

No. 4, p.2-8.

15. G.M. Webster, G.E. Totten, S.H. Kang and

S.W. Han, Successful Use of Polymer

Quenchants With Crack-Sensitive Steels,

2nd

International Conference on

Quenching and the Control of Distortion,

Eds. G.E. Totten, M.A.H.Howes, S.J.

Sjostrom and K. Funatani, 1996, ASM

International, Materials Park, OH, p. 509-

515.

16. N.I. Kobasko, Method of Quenching Steel Parts

Made from High-Alloy Steels, Ukraine Patent

Appl. 4448, April 14, 1983, S.U. No. 3579858.

17. N.I. Kobasko and V.I. Grankin, Quenchant

for Quenching Steel Parts, Patent Appl.

4005February 2, 1986, S.U. No. 4020742.

18. S.W. Han, S.G. Yun and G.E. Totten, Continuously

Variable Agitation, in Quenching and Distortion

Control, Ed. G.E. Totten, 1992, ASM International,

Materials Park, OH, p. 119-122.

19. S.W. Han, S.H. Kang, G.E. Totten and G.M.

Webster, Immersion Time Quenching,Adv. Mat.

And Proc., 1995, September, p. 42AA-42DD.

20. G.M. Webster, G.E. Totten, S.H. Kang and S.W.

Han, Successful Use of Polymer Quenchants with

Crack-Sensitive Steels, 2nd

International

Conference On Quenching and the Control of

Distortion, Eds. G.E. Totten, M.A.H. Howes, S.J.

Sjostrom, and K. Funatani, 1996, ASM

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Advances in Polymer Quenching Technology.pdf