Uphill Quenching

A SUMMARY OF THE UPHILL QUENCHING PROCESS

By

Tom Croucher

(last revised 2010)

Uphill quenching is not new. It was developed as a process to relieve quenching stresses by Alcoa engineers (Reference 1) in the 1950's but never has reached its true potential. Misundersood by most engineers, it is practiced successfully very little and only in some unique heat treating plants such as Newton Heat Treating in The City of Industry California.  Today, what is making the process much more attractive is its potential to be integrated into the systems approach for achieving the minimum stresses in heat treated high strength aluminum components. By considering all the techniques that the heat treater has at his disposal, (i.e. polymer quenching, uphill quenching, interim thermal treatment, and proper selection of machining configuration), a specialist can now develop a specific plan whereby the optimum part can be easily produced in production operations.

It is the purpose of this article to briefly review each of these practices, and provide specific success examples where this approach has been successfully used. Emphasis will be placed on the uphill quench practice of the system, since other practices have been previously discussed.

The polymer quenchants and the application of uphill quenching has finally provided tools by which this problem can be solved and high strength parts, with both higher properties and improved dimensional stability can be produced.

If the manufacturing procedure is planned correctly, many parts can be finish machined prior to heat treatment thus reducing set up costs. Also, as the section thickness of the part is greatly reduced during heat treating (i.e. 5-inches reduced to 3/4"-inch), the finished part is stronger and theoretically will have improved fatigue properties as it contains less residual stress than in the original part heat treated by the mill.

This approach is applicable to all heat treatable aluminum alloys, and is especially beneficial to the higher strength alloys such as the 2000 and 7000 series which are more sensitive to different quenchants while attempting to achieve full properties.




SUMMARY

The use of uphill quenching combined with polymer quenching allows for the first time a truly engineered heat treat system for manufacturing complex aluminum parts. Through the advantage of being able to pre-select a desired cooling rate based on quench sensitivity and geometry, desired engineering properties can be achieved. Quenching stresses are further reduced by the proper use of uphill techniques. In the final analysis, even the most complex parts can be produced with the following advantages:

* Improved mechanical properties

* Significant reduction in distortion.

* Reduction in residual stresses.

* Reduced need for spray-quenching.

* Elimination of preheated quench tanks. Slower cooling can be achieved by       increasing the concentration.

* Elimination of machining distortion

* Reduced manufacturing costs.

* Reduced labor intensive check and straightening operations.

One point must be re-emphasized. Polymer quenchants and uphill quenching are effective tools for achieving a better heat treated part. However, a thorough understanding of the technology of the quenchants must be understood and coupled with a suitable engineering and shop practice in order to avoid costly mistakes and to achieve maximum benefit of this method. Thus, results not achieved previously in the aluminum heat treat industry can be attained. One additional caution. All polymer quenchants are not the same. Their quenching characteristics and resulting degree of effectivity can vary significantly depending upon the product type. The type quenchant, concentration and quenching operating parameters should be selected very carefully.


THE NEED FOR UPHILL QUENCHING

High strength aluminum alloys attain their strength through heat treatment. While the specific treatment depends upon the particular alloy being treated, most parts are quenched in water from a solution heat treating temperature, usually in the range of from 870-1000EF. During the quench, the part surfaces cool faster than the part interior. Thermal gradients are created causing different areas of a part to contract at different rates. During the later stages of cooling, these gradients disappear, but their presence sets up an uneven distribution of residual stresses. These residual quenching stresses are the major cause of part instability during subsequent machining operations and can often cause stability problems later while the part is in service.

In the ideal case, these residual stresses are compressive on the part surface, which cools first, and tensile in the slower cooling interior. The magnitude and distribution of the final stress varies with the particular alloy, the thickness of material and especially the severity of the quench. However, when quenching actual production parts, the surface stresses are not always in compression. Stresses at the surface can be both compressive or tensile depending upon the geometry of the part, how the part was racked and the effectiveness of the quenching fluid to extract heat at each location on the part.

After the quench, the tensile and compressive stresses present in the part are completely balanced resulting in a total net stress = 0 for the entire part. The compressive stresses and the tensile stresses must balance each other. If the net stress is not zero, the part would have to move in the direction of the larger stress until a complete balance is obtained and the part achieved equilibrium.

It is not readily apparent by just viewing a part whether or not it may contain a high level of residual stress. Larger parts, because of their total mass, may not have even distorted during the quench. Thus heat treated parts, although visibly acceptable, may contain an unacceptable level of residual stress and the part may be just waiting for some unsuspecting machinist to remove some material so that it can warp or twist to relieve the internal stresses. During machining, removal of metal disturbs the original distribution and a new distribution is established. This re-orientation of the stress pattern normally results in part distortion which allows for the net stress on the part to remain at a zero value. This warpage can significantly increase the machining costs and can also lead to high rejection rates due to a parts failure to meet dimensional tolerances.

Stress relieving of most steel parts is easily achieved by thermal treatment. Unfortunately, unlike steel, there is no standard thermal stress relief technique which will make aluminum parts dimensionally stable. Thus to protect against the problems of excessive warpage and premature part failure, heat treated aluminum parts must be processed in some other manner to achieve a low level of residual stress. This processing will vary depending upon the alloy, strength required, quenching needed to achieve properties and level of dimensional stability needed. It has been shown that the "Uphill Quenching" process can be one of the more effective tools used to achieve the desired level of dimensional stability if the process is understood and applied correctly.

 

THE PRINCIPLE OF THE "UPHILL QUENCH"

The technique of "Uphill Quenching" has seen limited use over the past thirty years. In its initial form, it was known as "deep freezing" or "tri-cycle stress relieving". In its early use, the parts were usually immersed in dry ice followed by an immersion in boiling water. Reports as to the success or failure of the technique to achieve effective stress relief varied until the late 1950's when Alcoa metallurgists (Reference 1) studied the process in detail and further developed it using a liquid nitrogen/steam approach. They reasoned that since the residual quenching stresses result from thermal gradients which are induced when the part is being cooled, it should be possible to develop residual stresses of an opposite nature by subjecting a cold piece to rapid heating; i.e, by an "uphill quench." For maximum effectiveness, the uphill quench would have to develop more severe temperature gradients (ΔT) than were obtained by the conventional deep freeze approach. Furthermore, such a treatment could not involve temperatures high enough to affect the tensile properties.

An extensive investigation was then conducted to evaluate and compare different methods of uphill quenching and to develop the most effective method for use in stress relieving complex heat treated aluminum components. The technique found to provide the optimum stress relief involved the use of liquid nitrogen and high pressure steam.

Figure 1 - Effectiveness of Various Uphill Techniques in Reducing Stresses


Other techniques showed various degrees of effectiveness as shown in Figure 1. These other techniques all resulted in much lower stress relief than the liquid nitrogen/high velocity steam technique. Because the treatment was thermal in application while the residual stress relief was accomplished by mechanical plastic deformation, they termed the method "thermo-mechanical."

 

APPLICATION OF THE UPHILL QUENCH PROCESS

A short summary of the multi-step process is as follows:

1. After quenching, and before appreciable natural aging has occurred, the parts are cooled to a sub-zero temperature by immersing them in liquid nitrogen. If the parts cannot be immediately processed, they must be stored in a ice box at a temperature lower than zero degrees Fahrenheit to retard any room temperature aging which can reduce the effectiveness of the process.

2. The parts are maintained at the sub-zero temperature in order to insure that all areas of the part are at equilibrium.

3. The part is transferred from the cooling medium to the steam chamber and immediately heated to an elevated temperature. The most effective heating medium is a high velocity steam blast.

4. The part is aged in a conventional manner for the alloy and temper involved.

A detailed step by step process of the uphill quenching process as is practiced in our facilities is shown below:

Step 1:  Effectively quench the parts after solution heat treating. Select the quenchant which will provide the optimum quenching power to achieve the desired properties.

Step 2:  Cool the parts in Liquid Nitrogen.

Step 3:  Let stabilize at temperature in the Liquid Nitrogen.

Step 4:  Transfer the parts to the steam chamber. The transfer time should be kept to a minimum avoiding any ice build-up on the parts and the part should be steamed on all surfaces.

Step 5:  Uphill Quench with high velocity steam. The duration of the steam application is dependent upon the thickness of the part, but should not be excessive.

Step 6:  Immediately measure the part temperature to insure that it has reached the proper temperature during the steaming operation.

Step 7:  Measure the residual stress of the part, preferentially with the x-ray diffraction method.

 

EFFECTIVENESS OF UPHILL QUENCHING

In order to effectively determine the need for stress relieving parts after quenching, it is necessary to measure the residual stress levels that result from water quenching different thickness materials. Table 1 (Reference 1) illustrates the different stresses that result when 1/4 to 2 inch thickness material is water quenched using different water temperatures. A high residual stress of approximately 33,500 psi was measured for all thicknesses of materials above 1/2 inch when a cold water quench was used. The thinner 1/4 inch material exhibited a lower stress level because a smaller temperature differential (ΔT) was established between the surface and center. When the 1/4 inch material was quenched in 150EF water, the residual stresses measured were lower. This data illustrates that the greatest dimensional stability problems are encountered when thicker materials are water quenched, especially when cold water is used. The necessity of creating the greatest (ΔT) during the uphill process is shown in Figure 2

Figure 2 Effect of ΔT During Uphill Quenching on The Reduction of Residual Stress


As we previously discussed and shown in Figure 1, there are a number of different uphill practices currently followed in the aluminum heat treating industry. The effectiveness of a particular technique is a direct function of the ΔT that is developed during the uphill quench. The Alcoa data showed that the largest ΔT achieved, (371EF, which resulted in the greatest stress relief), was only achieved with the liquid nitrogen and the high velocity steam. (See Figure 1). The stress relief achieved with this procedure reached a level of 82%. With the dry ice and steam, the ΔT developed was only 290EF and the effectiveness of the stress relief was lowered by almost one-half the LN2/ high velocity steam value. The effectiveness of the liquid nitrogen and the low velocity steam was comparable to the dry ice and the high velocity steam.


                        Table 1:



Also, the low velocity steam was not nearly as effective as the high velocity steam. Thus, for maximum stress relief, the mere presence of steam is not sufficient to insure a good stress relief. The steam velocity must be extremely rapid in order to get the maximum effect.

 

When boiling water was used rather than high velocity steam, (with either LN2 or dry ice), the maximum residual stress relief achieved was a low 19%, so the use of boiling water with uphill quenching should only be considered when a minimum degree of stress relief is acceptable.

 

As aluminum alloys naturally age, resulting in an increase in their yield strength at room temperature, it is necessary to determine how effective is the uphill quenching operation if performed at different times after quenching. Table 2 (Reference 1) also shows this effect. In the case of the part with no uphill treatment, a differential stress of 24,000 psi was present. When the uphill quench was performed within an hour after the initial quench, the stress relief was extremely effective as the stresses were reduced from 24,000 psi to 4,000 psi. However, if a part was allowed to remain at room temperature for three hours, the stress relief was not as effective with a resulting value of 7,000 psi. When the parts were exposed for a total of 8 hours, the stresses were still higher reaching a value of 10,000 psi. After a full days exposure, the final residual stresses were 14,000 psi.

            Table 2:


When the technique was performed after the material had been aged to the T6 temper, the parts exhibited no degree of stress relief whatsoever. Thus it is imperative that the stress relief be conducted as soon as possible after the quenching operation and that using the process in any form to relieve quenching stresses in the T6 temper is, in most cases, a waste of time.

 

DISADVANTAGES OF UPHILL QUENCHING

In spite of the proven effectiveness of the uphill quenching technique for achieving dimensional stability in many parts, the process is not applicable for all situations and has some distinct disadvantages. As the most common cause of dimensional instability in aluminum alloys is the quenching operation, careful selection of a boiling water or glycol quench can (in some instances) achieve dimensionally stable parts without the need for an uphill quench. Also, test data has shown that uphill quenching may actually result in an increase in the stress level and part instability for parts that have quenched extremely slowly. As uphill quenching is a thermo-mechanical process, applying it to parts that already exhibit a low level of residual stress, may cause a stress reversal.

Thermo-mechanical stress relieving only works when temperature gradients are present. As a result, the process is not particularly effective in relieving fabrication stresses that result from severe forming or welding operations. These stresses should be relieved by a thermal process, preferably before a solution heat treating operation.

You are invited to contact us to see how uphill quenching can benefit you.

copyright © 2010

by Tom Croucher, a consultant to the heat treating industry.


References:

1. "The Thermo-Mechanical Method For Relieving Residual Quenching Stresses in Aluminum Alloys" by H. N. Hill, R.S Barker and L.A Willey: Published in Transactions of the ASM, Vol 52, 1960

2. "A Systems Approach to Quenching Aluminum Aids Dimensional Stability" by Tom Croucher’; HEAT TREATING, January 1983.

3. "How to Achieve Dimensional Stability in Aluminum Alloys" by Tom Croucher and Denny Butler: HEAT TREATING, September 1980

4. "Using Uphill Quenching to Effectively Stabilize Machined Aluminum Parts" by Tom Croucher, TOM CROUCHER AND ASSOCIATES, October 2006

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