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Polyalkylene glycol quenchants were introduced into the aluminum heat treating industry in the mid 1960's and immediately became the most important tool for reducing quenching distortion in sheet metal alloys.  Later, the feasibility and benefit of glycol quenching thicker products, mainly forgings (which were normally quenched in hot or boiling water), was proven.  Specifications were changed to permit glycol quenching of forgings with certain limitations based on test programs conducted by AMEC with decisions based on a zero delta approach.  Since that time, many test programs have shown that much thicker parts, particularly forgings produced from lower quench sensitive alloys can be successfully glycol quenched with acceptable results.  In some cases, specific parts were allowed to be quenched by drawing callout.  However, little research was conducted into the understanding of many of the anomalies that occurred with different programs that appeared to conclude that much thicker parts could be successfully processed.  As a result, there is a significant lack of understanding regarding glycol quenching technology and the full benefit of using these products is not currently being realized.  As forged aluminum parts become ever thicker, higher residual stresses are being imparted to parts.  The benefits of glycols being able to reduce residual stress and machining costs is not being realized due to the reluctance of many engineers to recognize the benefits of expanding the limits of the technology to achieve results not heretofore achievable.   

It is the purpose of this paper to review the basic concepts regarding glycol quenching of high strength aluminum alloys and to present recent data and analysis to show that quenching thick section forgings using polyalkylene glycols is technically feasible and actually provides a more consistent product than hot water quenching.  The influence of the vapor pocket in reducing the effectiveness of the hot water technique is discussed as is the influence of agitation rate in the quench tank on the various quenching methods. 

The author would appreciate any comments on this material  and a dialogue with others in the industry who have had unique experiences in this field.  He may be reached by email at <tom@croucher.us>, or by phone at his office at 888-502-8488.

  .


Polyalkylene glycol quenchants were introduced into the aluminum heat treating industry in the mid 1960's and immediately became the most important tool for reducing quenching distortion in sheet metal alloys.   After the initial test programs at Boeing [1] [2] and Northrop [3] proved successful, other companies [4] [5] [6] [7] [8]  tested the product and installed glycol quenching systems to reduce heat treat distortion.  The realized savings in check and straightening costs were huge.  At the time, most production tanks were charged with a 28-40% glycol /water mix and the controlling heat treating specifications were changed to permit many sheet metal alloys to use this method.  However, there was an early prevailing opinion that because the water/glycol products reduced cooling rates compared to the cold water quenching method, aluminum forgings would probably never be allowed to be quenched in glycol/water solutions. 

This author, [9] [10] [11] first conducted cooling rate and tensile property studies on thicker sections, comparing them to hot water quenched products.  Under sponsorship of the SAE AMEC committee, further studies were conducted on 7075 alloys with different glycol products to determine if glycol quenching parameters for highly quench sensitive, forged alloys could be added to their heat treat specifications (AMS 2770) [12] for aluminum alloy parts.  Tensile tests were performed on different forged alloys from different suppliers quenched in different glycol concentrations. 

Based on the test data, the committee then developed glycol quenching parameters for forgings of different alloys by using the "zero delta" approach.  This concept allowed the glycol quenching of parts produced from forgings if, when comparing the hot water quenching method directly with the glycol method, there was no significant difference (termed zero delta) between the properties achieved (within variations from normal testing procedures) between the hot water and the glycol methods.  Based on this concept, the committee then added allowances for quenching some forgings using the water/glycol method. 

Subsequence experience has shown that AMEC's zero delta approach was valid, but extremely conservative.  For instance, glycol quenching of forgings of much thicker dimensions using higher glycol concentrations was not allowed by most specifications.  However, since that time, it has been proven to be a viable alternative for achieving dimensionally stable forgings with superior properties.   One aircraft prime for many years used a 28% Ucon®-A concentration to achieve minimum warpage in some large 7075 forgings instead of the 16% permitted by specification.  They verified every lot with tensile tests and never had a failure.  During the 1970's, many test programs conducted by Anderson/Schuler [13] Lauchner [14], Alesh [15] Harvey Aluminum [16] and French [17] showed that much thicker parts, particularly forgings, could be successfully glycol quenched with acceptable results.  A summary of these efforts was presented to the SAE AMEC committee in February 1988 [18] and will be discussed later in this paper. 

Later, work by Torgerson and Kropp [19] [20] extended the glycol concentration limits for 7050 forgings to 4.0 inches to reduce the level of residual stress.  Again tensile and fracture toughness coupons were used to verify the results, again with no reported failures.

Later unpublished work by this author has shown that in production quenching, equivalent tensile and fracture toughness properties were achieved by quenching a 7-inch thick 7050 forging in a 25% glycol solution when compared to quenching in a production hot water tank.  This in spite of the fact that most specifications limit the thickness to 4-inches being quenched in a 12% concentration.  Seven inch thick forgings and 25% concentrations are not allowed to be used.  This work will also be discussed later in this paper. 

Over the past 25 years, specific production forgings were allowed to be glycol quenched using higher concentrations than allowed by specification.  This situation was permitted by specific drawing callout, but little research or data analysis was undertaken to resolve the anomalies that occurred with different test programs that appeared to conclude that much thicker parts could be successfully processed.  The prevailing opinion of the specification writers was that glycol quenchants still provided reduced cooling performance to hot water which precluded their extended use. 

It is the purpose of this paper to review some of the test programs undertaken in the past, and to develop and present a basic technical understanding of why polyalkylene glycol quenching provides a superior quenching method for processing thicker forged aluminum parts than the hot water technique. 


QUENCHING FORGINGS IN HOT/BOILING WATER

The early mistaken opinion by many that glycol quenching of forgings could be a problem was based on the fact that quenching in water/glycol solutions provided slower cooling than was achieved by cold water cooling.  What was being overlooked was that the normal quenching of forged products was accomplished by hot water, not by cold water.  By quenching in hot (140-180°F) or boiling water, the cooling rates were greatly reduced.  This is illustrated by the cooling curves shown in Figure 1 [21].  There is a serious degradation of cooling performance as the temperature of the water is raised.  Alcoa data shown in Figure 2 showed that the tensile properties of 3-inch cylinders for three different high strength alloys were seriously reduced as the water temperature exceeded 150°F.  This author showed the same effect[22] when testing one inch aluminum 7075 plate product,  shown in Figure 3.  

                                   Figure 1 -    Cooling Rate Curves for ½ inch Plate at different Temperatures [21].

Figure 2-    Effect of Quench Water Temperature on Strength Properties of Three Different Alloys.  Tests Conducted on Cylinders 3" Dia x 9" Long.




Figure 3-     Effect of Quench Water Temperature on the Tensile Strength of 1-inch 7075 Plate. [22]




In attempting to determine why so much data seems to indicate that polyalkylene glycols can be successfully used as a replacement for quenching most high strength aluminum forgings, (in spite of the original concepts) several items need to be reviewed.  These are (1) the characteristics of the hot/boiling water quenching process, (2) a review of recent test programs, (3) the parameters of the AMEC test program used to originally create the “zero delta” values that  were placed into the specification (4) the impact of the newer, less quench sensitive alloys such as 7050 on the problem and (5) a cooling rate analysis of glycol quenchants at different concentrations when compared to hot water quenching, particularly when quenching thicker product.   

THE THREE PHASES OF QUENCHING

(1) Effect of The Vapor Blanket Stage - "A" PHASE.
 
This is the first stage of cooling which occurs just after a part is immersed in the quenching fluid.  When an aluminum part at a temperature of 870-1000 °F is quenched into a tank of cold water, the water that is in contact with the hot part instantly boils and thus quickly turns into a vapor.  A vapor blanket or pocket then forms around the part due to the vaporization of the fluid as it contacts the hot surface.  This vapor blanket formation is shown in Figure 4,  which was taken by a high speed camera during the quenching of a 1" x 1" x 12",  6061 aluminum alloy bar. 








                        Figure 4-     Vapor Pocket Being Formed When Quenching a 6061 1-Inch Bar in Cold Water. 

The movie showed evidence that there is a continual buildup and collapse of the vapor pocket.  As the hot liquid first contacts the surface of the heated part, the liquid is vaporized, and then backs away due to the formation of the vapor pocket.  The entire process is repeated as fresh liquid again contacts the part, is vaporized and forms a new blanket around the part.  The process continues until the part temperature is lowered sufficiently or there is sufficient agitation of the quenching fluid to keep fresh liquid contacting the hot part without forming the vapor pocket.  During the "A" phase of quenching, the cooling rate of the part is extremely slow.  The strength of the aluminum at high temperature is very low.  The formation and collapse of the vapor pocket is believed to cause supersonic cavitation which can distort the part in a random manner.

(2)  The Nucleate Boiling or Vapor Transport Phase - "B" PHASE.
 
During the second stage of cooling, called the "B" or the nucleate boiling phase, the vapor film has completely collapsed and there is rapid heat transfer along with a violent boiling as continued fresh, cool liquid contacts the surface of the part.  Earlier researchers termed this the vapor transport phase.  During this "B" phase of cooling, actual quenching of the component takes place.  It is in this phase where control of the rate of quench for achieving properties and minimizing distortion and residual stress is important.  The quench rates obtained during the "B" phase of cooling are usually extremely rapid.  The distortion which occurs during this phase is repetitive.  Parts tend to distort toward the thicker sections since the thicker cross section cools last and “pulls” the material towards it.  

(3)  The Liquid Cooling Stage - "C" PHASE.

In this phase of cooling, also called the convection phase, the surface of the metal temperature has been reduced to near the boiling point of the quenching fluid, so that no further boiling of the quenching fluid occurs.  During this phase of cooling, the rate of heat extraction is again reduced to a rate much slower than that measured in the "B" stage.  This phase of quenching is sometimes termed the "tail-out".

The photo shown in Figure 4 was taken with cold water.  The effect of the vapor pocket is much more pronounced as the water temperature is raised.  As the temperature approaches boiling, the cooling process is almost entirely by vapor phase cooling, and thus is very slow.  That is why the tensile properties are reduced so drastically above 150°F as was shown in Figure 2.  Also, the eventual complete collapse of the vapor pocket is dependent upon the level and frequency of the agitation in the tank and about the part.  This fact also leads to distortion considerations as parts with greater sectional thicknesses change from “A” phase cooling to the “B” phase cooling at different times and thereby cause large differential contraction rates and resulting distortion of the part.  The high speed movie that Figure 4 was taken from clearly depicted the continuing collapse of the vapor pocket which lasted for 6-8 seconds when the bar was water quenched.  When the same bar was quenched in 20% glycol, the vapor pocket lasted less than two seconds due to the coating of the bar with concentrated polymer. 


    REVIEW OF PAST TEST PROGRAMS

A literature search was made by this author of past test programs and was  presented to the AMEC committee in 1988 [13].  Each of these efforts showed data that would indicate that the quenching of forgings using the water/glycol method had advantages over the hot water method. 

1)    7075-T73 Test Program - Anderson and Schuler [13] conducted tensile tests on 3 and 4 inch 7075 glycol quenched, hand forgings which were heat treated to the T73 temper and compared their results to a hot water quench.  Their results are shown in Figures 5 and 6.    For both thickness levels, the yield strengths were significantly higher at the 5, 10, and 15% glycol concentration levels and roughly equivalent at 20%.  What is still an anomaly and not yet  understood, is that although the yield strengths tended to increase as the glycol concentration increased from 5-15%, the tensile strengths tended to decrease.  However, in both cases, the strengths achieved were well above specification minimums.  

3)     In Lauchner’s [14] effort, 7075-T73 forgings from two different suppliers were tested.  In the first case, shown in Figure 7, both tensile and yield strengths for the glycol quenched forgings were approximately equivalent to the hot water quenched material.  In the second case, Figure 8, the tensile properties of the glycol quenched product was clearly superior.   

4)    Alesch [15] conducted a study where a 3.0-inch 7075-T73 forging was quenched in both hot water and 18% glycol and tensile tested all three directions.  A summary plot of his data is shown in Figure 9.  Again the properties of the glycol quenched samples were equivalent to the hot water samples.


   


                Figure 5 - Mechanical Property Comparisons of             Figure 6 - Mechanical Property Comparisons
                               3-inch 7075-T73 Forgings [13] [18]                               of 4-inch 7075-T73 Forgings [13][18]




                Figure 7 - Mechanical Property Comparisons of               Figure 8 - Mechanical Property Comparisons
                                2.5-inch 7075-T73 Forgings [14][18]                   of 2.5-inch 7075-T73 Forgings [14] [18]
.




                       Figure 9 - Mechanical Property Comparisons of             Figure 10 - Mechanical Property Comparisons
                                       3.0-inch 7075-T73 Forgings [15][18]              of 7075-T73 Forgings [16] [18]





                     Figure 11 - Mechanical Property Comparisons           Figure 12 - Mechanical Property Comparisons
                                     of 3.0-inch 7049-T73 Forgings [17][18]                of 3-inch 7049-T73 Extrusions [17][18] .



5)    This author obtained unpublished data from Harvey Aluminum Inc. [16] during the 1970's.  In their test program, they quenched die forgings of different thicknesses, (1, 2 and 3 ½ inch) in 120°F warm  water and 22% glycol.   A summary of their tensile test results is shown in Figure 10.  They concluded that the glycol quench provided equivalent properties to the 120°F warm water quench. 

6)    French [17] conducted two studies.  He quenched 3-inch thick extruded and forged product of alloy 7049-T73 in hot water, cold water and 12% and 18% glycol.  A summary of his results are shown in Figures 11 and 12.  In both cases, the 3-inch product quenched in 18% glycol was equivalent to the hot water quenched material. 

7)    Torgersen & Kropp [19] [20]  were the first to study in detail the quenching of thicker 7050 forgings.  It was their effort that allowed the SAE AMS 2770 glycol specification limits for 7050 forged alloys to be increased to a thickness of 4 inches. 

8)    In unpublished work, this author conducted a program on 2-inch thick 2618 forged rings, 30 inches in diameter.  In this effort, a direct comparison was made between forged rings that were hot water quenched and then compression stress relieved with those that were only quenched in 36% glycol with no compression stress relief.  The results showed that the tensile properties of the glycol quenched rings were equivalent to the rings that were hot water quenched and compression stress relieved.  Also, when the rings were machined, the dimensional stability of both methods was equivalent.  Maximum movement of the rings during the machining sequence was +.003 inches.  The final results of this program is shown in  Figure 13.  When the 36% glycol quenching was used, compressive stress relieving was not necessary to achieve dimensional stability.

9)     Recent unpublished work by this author has shown that in production quenching, equivalent properties were achieved by quenching a 7-inch thick 7050 forging in a 25% glycol solution when compared to quenching in a production hot water tank.  The results of this test program are discussed in detail later in this paper.  In addition, Timko [21]  reported data on 7-inch thick 7050 quenched in 11% glycol solution.  These results are also discussed later.




                Figure 13-     Schematic and Test Results Comparing 36% Glycol
                                    Quenched Ring with one that was Water Quenched
                                    and Compression Stress Relieved.


EARLY AMEC TEST PROGRAMS REGARDING FORGINGS

Quench Tank Agitation Effect

In the test program conducted by the SAE AMEC committee regarding the quenching of forgings, test samples up to 3.0 inch thick were evaluated.  The evaluation was conducted by quenching into a 3'x3'x3' quench tank that was vigorously agitated using a lightening mixer located in one corner of the tank as shown in the sketch in Figure 14.  It was also noted that because of the circular motion of the agitation, that the agitation level was not consistent throughout the tank.  Agitation was much higher near the outside of the tank, and much less near the center.  The placement of the test sample in the tank varied during the test program and thus in addition to trying to compare the effects of different quenchants, there was an additional agitation variable which was not taken into account.  At the time, the effect of this agitation variable on the test results was not recognized .



Figure 14 - Schematic of AMEC Quench Tank Showing Circular Agitation.


Later work by Bates [23] using a quench factor approach showed that this agitation variable would have a significant effect on the hot water results, but that the effect was much less pronounced when comparing the results of hot water quenched product to that which was quenched in glycol.  See Figure 15. 



                Figure 15-     Effect of Agitation Velocity on the Quench Factor of Aluminum Alloys. [23]


Also, in reviewing the vigorous agitation level that was used in the AMEC program, a survey of a number of job shop heat treaters concluded that the agitation level used in this test program did not represent the level that was being used in most production heat treating facilities.  In production quenching tanks, the agitation level is much less than “vigorous”.  Most surveyed tanks showed a moderate agitation level, and some, particularly those with circular agitation provided by pumps, were extremely varied, from moderate to low.  There is no specific requirement for a specified level of agitation in the heat treating specifications, only that agitation be provided.  A later test program was conducted by AMEC to resolve some of these anomalies, but the  results were never finalized due to continued anomalies in the data which were never resolved.

   
ALLOY DEVELOPMENT - REDUCING QUENCH SENSITIVITY   

Another factor influencing this discussion is the more recent development of less quench sensitive alloys, which has allowed the application of much thicker high strength aluminum forgings than were used in the past.  Prior to the stress corrosion cracking problems that the aerospace industry faced in the 1960's and 1970's, the predominant forged alloy for thicker parts was 7079-T6.  This alloy was used in most applications in the thickness range of 3-6 inches.  In order to achieve the high properties required by most designs, this alloy had to be cold water quenched.  When the stress corrosion cracking problems were identified with this material, extensive research was funded by the U.S. Navy to develop a substitute for this material with the goal of finding an alloy that could be used in thicker sections and that had much lower quench sensitivity.  A lower sensitivity to cooling rates would allow the product to be quenched slower to reduce the residual quenching stresses but still would have resistance to stress corrosion cracking.  The results of this effort was the development of the 7050 alloy, and the development of other high strength alloys by different aluminum companies has followed.   Unfortunately, the understanding that these materials can be quenched somewhat differently has eluded the specification writers for a long time.  Only recently, has it been shown that these newer alloys can be produced in much thicker sections and quenched in a manner to not only achieve higher strength values but at the same time reduce the residual stresses that are a constant problem.  


7050 TEST PROGRAMS

When Torgersen and Kropp [19] [20] conducted their 7050 work, they only quenched product up to a 4.0 inch thickness level.  The unanswered questions were (1) that with the reduced quench sensitivity of this alloy being established, could material thicker than 4 inches be processed, and if so, (2) what glycol concentrations would achieve acceptable results particularly when comparing data to the uneven quenching being produced by hot water in thick sections.  Data from three different  programs are presented in this regard. 

Croucher Effort

In 1991, we conducted a glycol quenching test program in conjunction with a major prime and a job shop heat treating company.  The program was carried out in a production drop bottom furnace having a quench tank that measured 4' x 6'x 8' deep.  The program stemmed from problems encountered during the production of a 7050-T74 flap hinge being used on a large aircraft.   The part had a center cross section of 7-inch in thickness, but there were many areas that were much thinner, some less than one inch thick.  When quenched in hot water, a severe distortion problem was encountered in the thinner sections causing many schedule delays because of the extensive check and straightening operations required.  The test program was conducted in order to determine if acceptable properties could be achieved by quenching the part in glycol, while at the same time minimizing the severe distortion that was being encountered.  The goal was to determine what effect the glycol quench would have in possible reduction of properties when compared to the required 140-160°F hot water quench which was causing the problem. 

Work Preformed

Aluminum alloy 7050 hand forgings, 7" x 7" x7" inch in thickness were procured from two different forging suppliers.  The forgings were solution heat treated and aged to the T74 temper.  Four different quenching variables were tested - 150°F water, and 14, 20 and 25% glycol.  Ucon® Quenchant A meeting the requirements of SAE AMS 3025, Type 1 was selected as the glycol product.  Tensile properties and fracture toughness tests were made from each block.  Tensile tests were taken in all three directions, longitudinal, long transverse and short transverse.  Fracture toughness tests were taken also from each block.  The sectioning of the block to obtain the tensile and fracture toughness test samples is shown in Figure 16. 





                Figure 16-    Sectioning Schematic for Tensile and Fracture Toughness Specimens.


Test Results

The tensile test results are tabulated in Table 1.  The longitudinal test results are plotted in Figure 16, the long transverse in Figure 17 and the short transverse in Figure 18.  The test results showed that the glycol method, even at 25% concentration, showed equivalent properties to the hot water quenching method.  The data showed that the quenching of 7050, 7-inch forgings in glycol concentrations from 14-25% resulted in above minimum properties for the T74 condition for both lots of material for all directions tested.

The fracture toughness results are shown in Table 2.  All values were well above the minimums required.  There was again an anomaly with slightly lower values at the 14% concentration, but this effect was attributed to larger residual stresses in the test specimen due to the more rapid quench which the testing procedure did not take into account.  At the time, the influence of residual stress on fracture toughness values was not well understood, but currently the effect is well recognized and provisions are now recommended to take their influence into account when testing.  

The conclusions of the program were as follows:

1.    Within the limits of good forging and heat treat practice, quenching of 7-inch 7050 forgings in Ucon® A quenchant from 14-25% when compared to hot water quenching results in properties that are comparable to hot water quenching.

2.    The variation in properties achieved by Ucon® quenching production quenching 7-inch, 7050-T74 forgings using concentrations of from 14-25%, when compared to hot water quenching, does not result in a significant change in the tensile or fracture toughness properties.   The effect of other forging, heat treating and testing variables were more significant to the final properties than the effect of the glycol quench.

3.    All future hinge flap forgings were allowed to be quenched in 22-25% glycol rather than the normal 140-160°F water quench.









Figure 16     Longitudinal Mechanical Property Test Results for 7-Inch Forgings Quenched in Hot Water and Three Different Glycol Concentrations.
.
Figure 17    Long Transverse Mechanical Property Test Results for 7-Inch Forgings Quenched in Hot Water and Three Different Glycol Concentrations





Figure 18    Short Transverse Mechanical Property Test Results for 7-Inch Forgings Quenched in Hot Water and Three Different Glycol Concentrations.





In an unpublished work [24], Timko has recently conducted two test programs involving the quenching of thick hand forgings in glycol.  In the first effort, he quenched a 7-inch x 34inch x 67 inch, 7050 hand forging weighing 1650 pounds in an 11% glycol tank and conducted tensile, fracture toughness and stress corrosion tests on the material and compared the results to minimum values specified in the material specification. 

The tensile property results are shown in Table 4.  All properties well exceeded the minimums required by the material specification.  The fracture toughness tests showed a K1c value of 31.6 which far exceeded the minimum value required of 25.0.  The material also successfully passed a stress corrosion test at a stress level of 35,000 psi for 20 days with no failures. 




In his conclusion, Timko concluded the following:

“Certain alloys, such as 7050, are capable of meeting strength requirements as well as secondary mechanical property requirements such as corrosion resistance, when glycol is utilized.  Higher concentrations and higher load weights need to be explored, but the 3" thickness limit imposed by Boeing and 4" limit imposed by AMS 2770 can safely be increased.” 

In the second program, he tested four large 7050-T7452 hand forgings which were 5 3/4 inch thick, weighing 4600 pounds each,  were quenched in 15% glycol.  The test results, shown in Table 5, showed that the tensile properties were well above the minimums specified in the required material specification.  Further effort is planned in this program. 






ANALYSIS OF COOLING RATE DATA COMPARING HOT WATER QUENCHING TO THE GLYCOL METHOD

In some of the earlier test programs, there appeared to be anomalies.  Some as referenced here showed that significant benefits could be achieved by choosing glycol quenching over hot water quenching when heat treating high strength aluminum forgings.  This was particularly true when quenching the newer alloys that definitely possess lower quench sensitivity characteristics. Other programs appeared to show that hot water quenching resulted in higher properties.  The question becomes, what is causing all the earlier anomalies and whether or not they were really anomalies or is something occurring regarding the quenching fundamentals that is being overlooked?  Could the prevailing opinion that glycols slow down cooling rates when compared to hot water quenching be incorrect, especially in the case of thicker forgings? 

In order to investigate this problem, it was felt that the best approach was to apply the cooling rate analysis that was originally put forth by Hunsicker [25].  In his work, he showed that the cooling rates achieved when quenching aluminum plates by different water quenchants followed a logarithmic relationship by the formula
 
                log rt = log r1- k log t
 
where r1 equals the cooling rate at 1 inch thickness,  k equals a constant, and log rt equals the cooling rate at any other thickness. 

The cooling rate graph developed by Hunsicker is shown in Figure 19.  The cooling rate characteristics when plotted in this manner are straight line relationships.  This author previously showed in Reference 24 that the glycol quenching rates were also straight line relationships when plotted in a semi-log manner as shown in Figure 20. 

Cooling curve analysis has also shown a distinct difference in the cooling path resulting from attempting to control cooling rates by temperature with hot water vs controlling the rates using glycols by concentration. It must be noted that when quenching in hot water, as the temperature is increased, the “A” or vapor phase is extended greatly as the temperature of the water is increased, thereby causing increasing the effect of the vapor phase on the cooling process.




        Figure 19-    Effect of Different Water Temperatures on the Cooling Rates of Different Thickness Plates [25]


This fact is illustrated by the cooling curves for 3-inch thick material shown in Figure 21.  As the product gets thicker, the “A” phase of quenching extends out significantly, while with the glycol quenched samples, the “A” phase breaks down quickly.





        Figure 20-    Effect of Polymer Concentration on the Cooling Rate of Different Thickness Plates. [26]







                                Figure 21-  Cooling Curves for 3-inch Aluminum Plate.        


It can be hypothesized that as aluminum forgings get thicker (3-8 inches thick) and their mass becomes greater, when quenching in 140-160°F degree water, the excessive mass of heat being extracted from the material during cooling tends to create an extended vapor phase around the part which is more difficult to break down, particularly with the agitation level of normal production quench tanks.  Thus the cooling rates of the thicker parts being quenched are much slower than realized due to the large mass of the part. 

If this hypothesis is correct, then a cooling rate analysis conducted according to the method of Hunsicker [25] should show that as the product being cooled gets thicker, then the cooling characteristics of the glycol products should be approaching those of hot water at 150°F.  This conclusion is based on the fact that the glycol products are not nearly as affected by the vapor phase due to the coating action of the polymer during the quenching process.  Thus, although a wide variation in quenching capability may be observed with thinner product, i.e. one inch thick and less, the capability of the glycols should improve as the products get thicker. 

To determine if this is the case, an analysis of cooling curves using plate products was made using the method of Hunsicker.  Data for 1-3 inch plates was plotted and then extrapolated using the straight line method to determine what effect the quenching of hot water vs glycol method would have on quenching rates for larger forgings.  This data is plotted in Figure 22. 

The data clearly shows (1) that for thicknesses above 3-inches, the quenching performance of 18-24% glycol is nearly identical to 150°F water and as the  thickness of the material increases beyond 3-inches, the cooling performance of the glycol quenchants (18-24%) is equivalent to the performance of the 150°F water.  It had already been shown previously that the quenching characteristics of 60% glycol provides the same cooling performance of boiling water. 

To put this data in perspective, Figure 23 shows the data of Figure 22, and adds the data for cold, 180°F and boiling water to the chart.  The difference in the relative cooling performance becomes readily obvious.   







                Figure 22-    Cooling Rate Data Plotted and Extrapolated According to Hunsicker's Method







            Figure 23-     Cooling Performance of Water at Four Different
                                Temperatures Compared to Two Glycol Concentrations. 


The Case For 2014 Forgings.

In order to justify the glycol quenching of 2014 alloys, it is necessary to first of all ascertain the current hot water quenching requirements for the alloy, and then determine comparable quenching rates for different glycol solutions.  It should be noted that most specifications specify the normal quench water temperature for 2014 forgings to be 140-180°F, compared to 140-160°F for other forged alloys.  See Table 6.  Thus 2014 is much less sensitive than other alloys, and a high level of properties are achieved at much slower quenching rates.  Also, in some cases, 2014 is routinely quenched in boiling water, which is a very slow quench.  It used to be that the boiling water quenched material had a specific temper designation, T61, to note that it was boiling water quenched, but I can find no current reference to that temper.   


Comparable quenching rates for different glycol quenching of different thickness material are shown in Table 7 and plotted in Figure 23.  It can be seen that a four inch forging is being quenched 6 times faster in 20% glycol than in 180°F water and two times faster in 40% glycol than the 180°F water.

Currently two companies allow glycol quenching of 2014 forgings.  Bell Helicopter in BPS 4139 allows quenching of this product in 22% glycol up to 2.0 inches thick.  Airbus allows the quenching of any alloy or form up to a thickness of 0.250-inch in glycol concentrations up to 40%. 

Table 7
Measured Quench Rates for Different Quenchants used for Quenching 2014 Alloy
    (Quench Rates Measured in °F/second between 750 and 500°F)

Limited mechanical property data for the quenching of 2014 forgings can be found.  Some pertinent data is below: Data for one lot of 2014 die forgings, shown in Figure 24 which was 1.515 inches thick, is shown in the illustration in Table 8.  When quenched in the 22% glycol product, the tensile results were 12,000 psi above the specification minimum for ultimate strength, 16,000 psi above the minimum for yield strength and 4% above the minimum specified for elongation. 

A second set of data involves two lots of 2014 forgings which were heat treated to the T4 temper using a glycol quench.  The data is shown in Table 9.  As in the case of the T6 temper, all mechanical properties easily exceed the minimums required.

A third set a data involved a short test program we conducted to determine the residual stress levels achieved by a glycol quench at 22% and compare it to a normal water quench at 150° and 175°F.  The results are shown in Table 10.  Only hardness and conductivity values were taken, but the results showed that the hardness and conductivity values were equivalent to the both water quench temperatures. 





                Figure 24-     Photo of a 1.515 Inch 2014-T6 Forging Quenched in 22% Glycol

Table 8
2014 Forging Test Results

    Table 9

   



The impact of glycol quenching 2014 forged wheels can possibly be understood more clearly by the following story.  About 1973, while I was managing a commercial heat treating facility and conducting early research into the effects of glycols on forgings, I was approached by the forging metallurgist, at Harvey Aluminum in Torrance California, about the possibility of glycol quenching 2014 forged aircraft wheels.  Prior to this time, as a result of our research, we had been performing the first production quenching of 7075 and 2024 forgings for both Harvey and Alcoa, long before glycol quenching of forgings was addressed by any specification.  At the time, Harvey had been quenching all 2014 forged wheels in boiling water, and were still having random problems with residual stress and sometimes meeting required tensile properties.  After researching the problem, and reviewing comparable cooling rates achieved for a glycol quench versus a boiling water quench, we determined that a 60% glycol would achieve about the  same cooling rates as boiling water.  (Note the curves in Figure 25).  When we first told the Harvey metallurgist what we had found out and wanted to try a 60% Ucon concentration, he thought we were crazy.  After reviewing our quenching data with him, he agreed to try.  We set up a small 60% tank, and quenched out a number of wheels in the 60% concentration and returned the wheels to him for final aging and testing.  The results were outstanding.  He reported back that the tensile properties were slightly higher than he was obtaining with the boiling water quench, and the residual stresses were more consistent and slightly lower.  I never did receive the actual data, but he put it into a report which he submitted to the wheel company indicating that they were ready to start quenching their wheels in glycol, instead of the boiling water.  When the metallurgists at the wheel manufacturer reviewed that data, they first were extremely enthused about the possibility of achieving consistently lower stresses with better properties.  However, they did not understand the basics of controlling quench rates with different concentrations, and as their specification only allowed the quenching of forgings in a 16% glycol concentration, they insisted that the 16% be used by Harvey, not the 60% we had used in our experiments.  A load of forged wheels was thus quenched in their required 16%, and as we expected from our data, significantly higher residual stresses resulted than were being seen from the boiling water quench.  The resulting conclusion by the wheel company was that glycol quenching provided no benefit.  Thus the project was cancelled and the process returned to the boiling water quench with all its problems.  Here we are, 38 years later, and most personnel still do not understand the fundamentals of when and how to apply the process correctly.



            Figure 25-     Cooling Curves For ½ Inch Aluminum Plate Showing
                                That 60% Glycol Cools In A Similar Manner To 200°F Water


The Case For Alloy 2219

Alloy 2219, as is the case for most 2000 series alloys, is not as quench sensitive as the higher strength 7000 series alloys.  Richard Chou at Boeing Huntington Beach conducted extensive studies in various lots regarding the quenching of 2219 2.35-inch plate in 15-17% glycol solutions.  As has been the case for all other situations contained in this report, all mechanical properties tested were well above the minimums required by specification even in plate product.  See Table 11.  Although no specific forging data could be found, It would be even more appropriate for quenching 2219 forgings as the specification minimums would reflect the slower quenching rates achieved by the hot water quenchants. 

The Case For Alloy 6061

Alloy 6061, is not nearly as quench sensitive as the higher strength 2000 and 7000 series alloys. If fact, most specifications even allow spray quenching of this alloy in section thickness up to 0.125-inch, because of this reduced sensitivity.  This technique is not allowed for any 2000 or 7000 series alloy because of their increased sensitivity to cooling rate reductions.  In fact, the alloy is less quench sensitive than 7050, so it can be assumed that any thickness that will achieve acceptable properties while glycol quenching 7050, would also achieve similar results in 6061.  Thus glycol quenching, because of it’s ability to achieve cooling performance similar to water quenchants at somewhat elevated temperature are ideal for use in quenching 6061 alloy.   We have not found any available specific tensile property for 6061, but over the years, have quenched many part up to 7-inch in thickness, and never had a rejection.




GENERAL DISCUSSION

It is important to understand that the polyalkylene glycols control the quenching rates by the concentration of the polymer.  The most important characteristic of these glycols used for quenching is the characteristic of inverse solubility.  As the temperature of the water/glycol mix is raised, the solubility of the glycol in water, and at a temperature of 165°F, there is complete separation.  Thus, as the hot part enters the fluid, a coating of concentrated polymer coats the part and the thickness of that coating is a function of the concentration of that fluid.   As was shown by Bates [23], as long as the temperature of the fluid is at room temperature, the cooling performance is not significantly affected by agitation.  The coating of the part by the polymer significantly reduces the vapor pocket that forms as cooling rates are controlled through the polymer film that now coats the part.  It is this fact that allows for greater control of distortion of a part that has large thickness transitions, (thin areas vs thick areas) which are common in aerospace parts.   Contrary to this scenario, quenching in hot water allows for early break down of the vapor phase in thinner areas, rapid cooling and shrinkage of these areas, resulting in excessive distortion and a higher build up of residual stress in the thicker areas.   

It is also important to understand that the critical temperature range when quenching high strength aluminum is in the temperature range that allows for significant super saturation while at the same time providing rapid diffusion rates for the hardening elements to move during the cooling process.  This was shown by Fink and Willey [27] in their “C” curve analysis.  Based on this premise, they concluded that critical temperature range is in the range of 750-550°F as shown in Figure 26.  Others have deduced that this range to be is 800-500°F, or 750-500°F.  Irregardless, it is definitely recognized that in this area, if cooling is too slow, a significant loss of properties can be experienced.  This cooling effect is due to the fact that the solute atoms which were dissolved during solution heat treating, tend to precipitate from solution during the cooling process.  During quenching, these atoms do not know which fluid they are being quenched in.  They are only reacting to the time/ temperature phenomena to which they are subjected.  The media to which they are subjected to has nothing to do with the precipitation process and a resulting change of properties. 

In the hot water quenching of thicker forgings, one does not know how much “A” phase cooling is present in different facilities due to the large mass of the part which allows for diffusion of the hardening solutes and lowering the optimum properties.  The advantage of using the glycols in this manner is that the “A” phase cooling is significantly reduced when compared to hot water quenching which is clearly shown in Figure 21 when quenching 3-inch plates.  Experience has shown also that glycol quenching in a specific concentration continually results in more repeatable results than is experienced when the hot water method is employed, particularly in thicker forgings. 
 


                                Figure 26-     Effect of Time and Temperature in Interrupted
                                                   Quenching Tests on the Strengths of 7075 Alloy [24]


One other misconception needs to be addressed.  It has been concluded by some that slower cooling rates results in lowered fracture toughness values.  This again has led to the misconception that glycol quenching slows down cooling rates and thus would lower fracture toughness values.  While it true that large differences in cooling rates, such as cold water quenching vs boiling water or hot salt cooling does show a lowered fracture toughness value as was shown by Robinson and Tanner [28], it cannot be concluded that quenching in glycols results in lowered fracture toughness values particularly when comparing the glycol technique to hot water quenching.  The data shows just the opposite.  Fracture toughness values for glycol quenched material were just as high when thicker parts were glycol quenched using appropriate concentrations as when they were quenched in hot water. 

CONCLUSIONS

1)    Glycol quenching of thicker high strength aluminum forgings is an effective alternative to the hot water quenching method.  The quench rates achieved are equivalent to the hot water method, and provide more consistent property results due to better control of the vapor phase prevalent in the hot water technique.  Equivalent and sometimes improved mechanical property results can be expected with a large reduction of distortion and residual stress problems. 

2)    Fracture toughness values are not reduced by substituting the glycol technique for the hot water method.  Both techniques achieve acceptable fracture toughness values. 

3)    Allowances should be made for alloy 2014 forgings to be quenched in high concentration glycols.   Current specifications allow the starting water temperature to be as high as 180°F, which means that most of the quenching would be “A” phase quenching.  Quenching of thicker forgings of this material in water/glycol mixes would allow higher properties to be obtained, while providing relief from residual stresses.  Glycol limitations in current specifications for this material are completely unrealistic. 

4)    When quenching forging alloys that are less quench sensitive, such as 7050, 6061, and 2219, the glycol specification allowable concentrations values should be expanded greatly, particularly for thicker forgings that repeatedly exhibit problems of high residual stresses when hot water quenched. 

5)    As forged alloys continue to get thicker, quenching in appropriate glycol solutions shows distinct advantages over hot water - equivalent properties with less distortion and lower residual stresses. 

6)    Further research needs to be conducted to understand the fundamentals of different quenching methods in order to reap the optimum economic benefits of reducing distortion and residual stresses in complex forgings and machined parts. 



References

1. Lauderdale, R. H., “Evaluation of Quenching Media for Aluminum Alloys,” MDR 6-18002, Boeing, March 1967.
2. Lauderdale, R.H., "A New Quenchant for Thin Gage Aluminum;" Metal Progress, December 1967.
3. Lauchner, E. A. and Smith, B. O., “Evaluation of Ucon® Quenching,” NOR 69-65, Northrop Corporation, May 1969.
4. Kesler, R. D., and Gerde, D. O., “Distortion Control of 7075 Aluminum Forgings During Heat Treatment, Utilizing a Synthetic Quenching Medium  Phase 1 ,” MP-572-2-7, General Dynamics Convair Division; August 1970.
5. Scott, B. L., “Evaluation of Ucon® A,” FMR12-1812, General Dynamics, Forth Worth, Jan. 1970.
6. Jamieson, J. L., “Evaluation of Ucon® Quenchant A,” Report S. O. 152034-139, Douglas Aircraft Co.  no date.
7. Morse, A. E., “Evaluation of Ucon® Quenchant A for Distortion Control of Thin Gauge Aluminum,” MET 9-8-12, North American Aviation, Los Angeles, Sept. 1968.
8. Scott, J. K., “Organic Quenchant Aids Heat Treatment of Dip-Brazed Aluminum Parts,” Metal Progress, Vol. 65, 1969, pp. 80–82.
9. T.R. Croucher and M.D. Schuler, "Distortion Control of Aluminum Products Using Glycol Quenchants", Metals Engineering Quarterly, 1970, August, p. 14-18.
10. T. Croucher, "Synthetic Quenchants Eliminate Distortion", Metal Progress, 1973,  p. 52-55.
11. T. Croucher, Polymer Quenchants: “Their Advantages for Aluminum Alloys”, Heat Treating, November 1982.  Volume XIV, No.11,  pages 18-19
12. SAE AMS 2770H, "Heat Treatment Of Wrought Aluminum Alloy Parts", SAE International, 400 Commonwealth Drive, Warrendale, PA 15096-0001, August, 2006
13. T.O. Anderson & M.D. Schuler, "Polymer Quenching of Heavy Gage 7075 Hand Forgings," Hughes Aircraft, Ground Systems (December 1972).
14. E.A. Lauchner, “Glycol Quenching of 7075 Aluminum up to 3 Inches Thick, "NOR 72-46, Northrop Corporation, (March 1972).
15. Alesch, C.W.,  “Quenching of ADA 7050 Parts in 18% Ucon Quenching Solution, “ Letter Report to W.E. Service, Geneal Dynamics Convair Division, 7 July 1971   
16. Unpublished data obtained from Harvey Aluminum Inc. by T.R. Croucher
17. French J.C,  "Quenching and Stress Relieving of Aluminum Forgings;"  Presented at SME Heat Treating Conference, June 10-12, 1980.  
18. Croucher, T.R.,  “Trend Analysis to Determine Polymer Quenchant Equivalents for Hot Water Quenchants for Aluminum Alloy Forgings” Submitted to SAE AMEC Committee, February 1, 1988.
19. R.T. Torgerson and C.J. Kropp; "Properties and Processing of 7050 Aluminum Alloy"  General Dynamics Convair; Report No CASD-ERR-75-044; October 28, 1976.         
20. R. T. Torgersen, C. J. Kropp; "Improved Heat Treat Processing of 7050 Aluminum Alloy Forgings Using Synthetic Quenchants;" General Dynamics Convair Division; Presented at SAMPE National Spring Meeting, April 26-28, 1977; San Diego California.
21. Thomas R. Croucher. "Applying Synthetic Quenchants to High Strength Alloy Heat Treatment;"  Presented October 1970 at the ASM National Metal Congress, Cleveland, Ohio.  Published by ASM in Metals Engineering Quarterly; May 1971; pp 6-11
22. Tom Croucher and Denny Butler. "Quenching: Major Stress Source in Heat Treated Aluminum Alloys"  HEAT TREATING, Vol XII, No. 10. October 1980; pp 34-37.
23. C.E. Bates; “A Recommended Approach to Quenchant Evaluation;” Southern Research Institute Report;  Presented to the SAE AMEC Committee, May 20, 1987.  
24. M. M. Timko;, “Glycol Quench Experiment on 7-inch Thick 7050 T7452 Aluminum Hand Forgings”, Unpublished Company Report, Weber Metals.  Feb. 8, 2008.
25. H. Y. Hunsicker, “Chapter 5, The Metallurgy of Heat Treatment” Aluminum Vol 1; American Society for Metals; 1967,  pp 136-137. 
26. T. Croucher;  "Polymer Quenchants: Their Advantages For Aluminum Alloys"; HEAT TREATING, November 1982.  Volume XIV, No.11,  pages 18-19
27. W. L, Fink and L.A. Willey, Transactions AIME, 175, 1948; pp 414-427.
28. Robinson, J. S. and Tanner, D. A, “Reducing Residual Stress in 7050 Aluminum Alloy Die Forgings by Heat Treatment”; Journal of Engineering Materials and Technology, July 2008, Vol 130.

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