Water Quenching Aluminum

Many different quenchant types and fluids are used for quenching high strength aluminum alloys. No universal quenchant is ideal for all situations. An optimum quench for one product/alloy/thickness/geometry configuration may be unacceptable for another. All of following quenchants have been used successfully on different aluminum products:
  • Cold Water Immersion
  • Hot Water Immersion
  • Water - Spray
  • Still Air
  • Air Blast
  • Polymers or Glycols
  • Liquid Nitrogen
  • Fast Quenching Oils
  • Brine Solutions
  • Combinations - Interrupted Quenching

 
WATER QUENCHING

The most common quenchant used for aluminum alloys is water. The main advantage of using a water quenchant is that water can provide the rapid quenching speeds necessary for achieving high properties in many different alloys. Water is also cheap and readily available. In addition, water is reasonably flexible in that its quenching characteristics can be changed somewhat by varying the temperature of the water.

 
COLD WATER QUENCHING


The use of ambient or room temperature water (commonly termed cold water quenching) is the most common practice for quenching aluminum alloys. The water bath is usually controlled over the temperature range of from 60-90° F. Most government and company specifications require that the temperature of the water bath be below 90° F at the start of the quench and not raise more than 10° F after the quench. This requirement governs the design of most quench tanks regarding the total volume of fluid in the tank. It also may limit the amount of material that can be loaded into the furnace, and is a critical factor in the design of fixtures, racks and baskets. If the combined weight of the rack and baskets is too heavy, the net load of parts could be severely restricted by the temperature rise criteria if the quench tank is too small.

Most product forms such as sheet, plate, extrusions and some forgings and castings are quenched in cold water. However, there are two major problems with water quenching these types of products: 

  1. Many parts will warp or distort during the quenching process and then require extensive check and straightening operations.
  2. Thicker parts can achieve a high level of residual stress.

 
HOT WATER QUENCHING

The problem of quenching distortion or warpage has been one of the major problems facing the aluminum heat treater for decades. Many approaches have been used to gain control over this distortion, each with varying degrees of success and each with its own limitations. In an attempt to achieve

(1) maximum distortion control in forgings, castings and thin sheet metal parts, and

(2) minimum residual stresses in forgings and castings,

hot water quenching, (particularly boiling water), is sometimes used with moderate success. The quenching characteristics of water are significantly changed by heating the water. To illustrate this fact, cooling rate curves for water quenching baths at different temperatures are shown in Figure 1 and cooling rates for various water temperatures for different thickness of metal are shown in Table 1. It can be seen that as the temperature of the water is increased, the quenching characteristics of the bath are significantly changed. At higher temperatures, much slower cooling rates are achieved. When the water temperature is raised above 160° F, the effectiveness of the water quench is drastically reduced as the quenching speeds are lowered. When the bath temperature reaches boiling, a very slow quench rate is achieved.

Figure 1. Cooling curves for water at different temperatures

As a result of slower cooling rates, increasing the water temperature can result in a significant loss in hardness and tensile properties when the part is later aged especially when heat treating the more quench sensitive 2000 and 7000 series alloys such as 7075, 7049 and 7178. Figure 2 illustrates this effect for one inch 7075 plate heat treated to the T6 temper. An 82,000 psi tensile strength was achieved in cold water while the strength dropped to 64,000 psi when boiling water was used - a loss of 25%. In general, significant loss in tensile properties may result when the water temperature is raised above 160° F because the cooling is not sufficient to freeze the solute atoms during the quench. Lower, but significant property losses are often observed in thicker products when the water temperature is increased above 120 F.

Figure 2. Effect of quench water temperature on the tensile strength of 1-inch 7075 aluminum plate.

Table 1: Typical Water Quenching Cooling Rates for Various Thicknesses of Aluminum Alloys


Because of the rapid change in the quenching characteristics of water above 160° F, control of quenching speeds at various locations in a quench tank is difficult when using a hot water quench. Precise temperature control and resulting quenching speeds at different locations within a given tank, in different tanks, and in sequential loads within the same tank is difficult to achieve in hot water production quenching operations. At higher water temperatures, to keep the water temperature constant and achieve consistent properties, close control of agitation is required.

 

BOILING WATER QUENCHING

Quenching in boiling water can be both beneficial and troublesome. It is definitely beneficial in that lowered residual stresses are achieved when compared to cold or hot water quenching. For instance, quenching 2014 forgings can achieve a 60-80% reduction in the stress level when compared to cold or hot water when quenched in boiling water. The disadvantage is that for most alloys, there is a a significant reduction in the strength level achieved due to the reduction in the cooling rate. In order to allow for the boiling water quenching of 2014 alloy, the design properties were reduced sufficiently to recognize the fact that lower properties would be achieved by the boiling water. Thus a special temper was defined. The T6 temper was used for cold or hot water quenching and the T61 temper is used when the products are quenched in boiling water. In instances where the boiling water quench is employed, the dimensional stability of the material is much more important than the strength level desired, so the strength has been sacrificed for dimensional stability.

Because the part is being quenched in a vapor pocket, the quenching in boiling water is sometimes uneven and uneven residual stress levels can be experienced. Experience has shown that sometimes quenching in high concentration glycol quenchants can show both greater and more consistent properties with consistently lower residual stress levels that realized from the boiling water.

One other problem can be experienced with boiling water quenching. The temperature at which water boils varies with the elevation of the plant. Thus water will boil at 212° F at sea level, but at a much lower temperature at 5000 feet elevation and can be as low as 203° F. One prime contractor had a requirement in their specification that the water quench tank had to be measured before each quench and could not be below 210° F. One production plant that was in Denver Colorado obviously could not get the tank to 210° F and thus could not manufacture the parts. Although no technical data was available to determine if there was any significant difference in the parts which were quenched in boiling water at 212° F or at 203° F, as the part is being completely quenched in a vapor, one doubts whether any significant difference would be observed.

There are safety problems that can be observed with a boiling water quench. If heavy loads are being quenched, the tank can be super heated and will literally explode when the parts are quenched. One casting foundry quenched their castings every morning at 7:00 AM from a drop bottom furnace. The operation became a spectacle for the whole plant as the operator would dress himself in a space man type thermal protection suit, push the quenching button, and run like crazy to get away from the quenching explosion that occurred when the parts hit the boiling water.

 

SPECIFIC REQUIREMENTS FOR DIFFERENT ALLOYS

Most specifications allow forging alloys to be quenched in hot water in order to achieve less distortion than would result from cold water quenching. In these cases, the forging material specification usually takes into account any loss in properties that may result. In practice, many aluminum parts (especially castings and forgings) which are prone to distortion, are quenched in either hot (140-160° F) or boiling (212° F) water although some forging alloys are quenched in 180° F water.

Many forgings are quenched in boiling water, but in the case of other wrought products such as plate, extrusions etc, only the less quench sensitive alloys (such as 6061) are quenched in boiling water. In some instances, specific tempers of other alloys such as 2014-T61 are required to be quenched in boiling water to achieve the highest level of dimensional stability. In these cases, it is recognized that for these specific applications, the dimensional stability is more important than the high tensile properties. As a result, the high strength is sacrificed somewhat in favor of a more dimensionally stable part.

Many aluminum casting alloys are quenched in boiling water because the quench rates developed are sufficient to achieve the required properties. However, in cases where premium properties are desired, (such as castings alloys A357 and A201), the use of colder water or polymers is recommended. The normally accepted water quenching practices for some of the more common aluminum alloys are shown in Table 2.


Table 2: Normally Accepted Water Quenching Practices for the Common Alloys



SPRAY OR FOG QUENCHING

Some distortion problems can be solved by using spray quenching techniques. The application of spray techniques used for aluminum alloys in the heat treating job shop varies from the use of a heavy volume of a high pressure spray to that of a light fog or mist. Thus the quenching characteristics of a particular spray quench facility can vary significantly depending upon the design and characteristics of the equipment being used.

When using spray quenching techniques, care must be taken to insure that the quenching procedure is adequate for achieving the required properties in all parts being heat treated. The use of spray quenching normally results in much lower quench rates when compared to cold water or glycol quenching. Depending upon the design of the spray quench system, the spray quenching system may be even slower than high concentration polymers as is shown in Figure 3. As a result, spray quenching can result in a significant loss in tensile properties when the part is later aged - especially when heat treating the more quench sensitive 2000 and 7000 series alloys. This property loss can be particularly high if the temperature of the water spray is raised above 160° F.


Figure 3. Cooling curve comparison of spray quenched parts versus other methods


If spray quenching is considered for aluminum heat treatment, a careful study should be made involving the thickness of the part, the alloy being heat treated, its quench sensitivity, and the shape or configuration of the part to insure that the quenching is adequate and that a degradation of properties will not occur. Pre-production qualification of the quenching procedure, including both destructive and non-destructive testing of heat treated parts, should be performed before a spray technique is used in production operations. If possible, cooling performance should be verified by cooling curves produced in the production chamber prior to quenching production parts. Once the technique is established, it must be rigidly followed regarding racking, placement of parts, nozzle type and size, placement of nozzles, pressure of the spray, water temperature etc. Also rigid maintenance procedures must be followed to ensure that nozzles do not get plugged or corroded. In one instance, inadequate maintenance of spray nozzles allowed clogging of the nozzles to occur over a period of time. As a result, millions of pounds of aluminum plate had to be rejected because the plates were not thoroughly quenched and soft spots were found throughout the plate in the areas that did not receive an thorough quench.

Most higher strength, highly quench sensitive alloys such as 7075 and 2024 are not spray quenched in heat treating plants except in special circumstances because slower quenching can result not only in lower properties but also a significant reduction in the inter-granular corrosion resistance. If a spray quench is used on these alloys, it must be a heavy volume spray which floods the part rapidly as it must extract a large amount of heat quickly. A light fog or a mist spray or quenching with a hand held hose is not adequate for quench sensitive alloys and for thicker parts of even the less quench sensitive alloys. Most heat treating specifications prohibit spray quenching unless it is specifically approved by the customer.

One of the major problems of spray quenching is assuring that all of the parts of the load are effectively flushed by the spray. Sometimes a heat treater will load parts in such a manner that those parts in the center of the load are not effectively quenched because they are obstructed from the spray by other parts on the load perimeter. Care must be taken to ensure that all of the parts of the load are effectively flushed by the spray.

In spite of its problems, spray quenching of 6061 sheet material is effectively performed by many job shops. When controlled properly, 6061 sheet can be effectively spray quenched because the alloy has a low quench sensitivity.


copyright © 2010

by Tom Croucher, a consultant to the metallurgy industry.



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